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Flying
Surfaces, Controls and More
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Contents:
…Wing; …Wing
Terminology; ...Controls and What They
Do; ...Horizontal Tail Surfaces; ...Rudder; …More
Rudder ; ...Using the Rudder (Guest
Opinion)…Use of Rudder;
…Flying with rudder; Unintentional
Pushing on a Rudder Pedal ...Frise
Aileron; ...Latches; ...Rig;
...Spiral Stability; ...Aircraft
Category; ...Lifting Surfaces; …L/D Ratio; ...Wing
Loading; ...Power Loading; ...Drag
& Performance; ...Parasitic Drag;
...Induced Drag; ...Boundary
Layer; ...Control Failure; ...Cables;
...Friction Test of Controls; ...Aircraft Aluminum; ...Corrosion
and Skin Condition; ...Hoses and Lines;
...Plastic Windows; ...Hobbs
Meter; ...Stall Warner; ...Spinner;
...Pitot Heat; ...Cabin
heat; ...Oxygen; ...Maintenance
Failures; …Paint; …Pilot Induced Oscillation; …The Propeller; …Constant
Speed Propeller Differences; ...Q-Tip
Propellers; …Over-Square Constant-Speed
Propeller Operation; …Using
Your Non-flight Instruments; …Angles;
…Stability; ...Static
Stability Test of Worst Case Aft CG Situation; ...…Flap
Chafe Caps; …Static Wicks;
…Window Care; …Flutter;
...Metal Fatigue; .Physical Forces of Flight; ...Hand
Propping; ...Vortex Generators; ...
Wing
The force that balances the airplane's weight in flight against
the pull of gravity is caused by the differences in pressures
between the upper and lower surfaces of the wing. A pure airfoil
creates lift without any down wash whatsoever. The down wash
from an airplane's wing is a by-product of lift (through the
tip vortices), not the other way around.
An airfoil in its purest form is a wing with an infinite wingspan.
The flow over the surface will be the same no matter where along
the span we measure it. If the wing has no tips, there can be
no tip vortices. An airfoil having no tip vortices will have
no induced drag since there is nothing to induce downwash.
In the real world of aircraft wings that have wing tips we
know that an angular span wise flow of air is drawn by the flow
of air off the wingtip. Only a portion of the air low pressure
air flowing over the wing can get to the wingtip before most
of it passes off the trailing edge as relatively low velocity
downwash. Even more air is being drawn from the high-pressure
bottom of the wing to flow from under the wing tip and into the
low pressure above the wingtip. Hence the flow of air above and
below the wing has different flow amount and direction coming
off the trailing edge the wing.
At the wingtip the high pressure air below the wing curls upward
into the lower pressure above the wing to create a vortex. The
airflow from the vortex whirls around, downward and inward above
and behind the wing. Downwash and induced drag are greatest where
the vortex rotates at the highest speed around the tip and over
the top of the wing.
The speed and pressure of airflow across the wing affects the (local) angle of attack. As the airflow moves toward the tip there is a lowering of the angle of attack, a decrease in lift, an increase in downwash, and an increase in induced drag. At the tip there is no lift, no vortex, no drag, and no downwash. Any increase in the angle of attack near the wingtip will increase the concentration of the vortex core hence increasing the lift, increasing the drag, and increasing the power of the downwash.
The velocity in the tip vortex is very high near the core, but decreases the further you move from the core to its outer rings. The core remains near the tip while the outer rings have an effect further inboard. The lift-induced speed of both inner and outer vortices is downwash, the downwash velocity is highest near the tips and lower at the root. Downwash reduces the angle of attack wherever it flows from the wing proportionate to its velocity. The amount of lift available at a given angle of attack will always be less where there is greater downwash.
Approximately 75% percent of lift generated by an airfoil are the result for differential pressures, and the other 25% are a result of Newton's third law of motion, "For every action, there is an equal and opposite reaction." This is most obvious while in ground effect.
Wing design plans for the stall to begin at the wing root.
This provides a gentler break as compared to a tip-break. Some
aircraft have stall strips on the leading edges to assure the
root stall before the tip stall. Most wings that are slightly
tapered have lift distributions that are close to elliptical.
An improperly designed wing may have the stall cause loss of
aileron control before the break. This is the reason why highly
tapered wings have a leveling twist called washout. By reducing
the wing incidence toward the tips, you lower the local angles
of attack and the lift near the tips. Then the stall moves toward
the rood and becomes less abrupt.
Wing Terminology
Draw the wing and use the words
as appropriate.
Mean Camber line
Upper Surface
Leading
Edge
Trailing edge
Lower Surface
Center of Lift
Aerodynamic center of lift is also called center of pressure
is located aft of the center of gravity. This location gives
airplanes a nose pitch down tendency. This is why the download is
required on the horizontal tail surfaces.
Chord Line
Imaginary line from the leading edge of the wing to the trailing
edge.
Wash-in; Wash-out
When a wing is manufactured it is mounted into a jig that holds
all the ribs and stringers into place while the
holes are aligned and drilled so that the skin can be riveted.
The outer half of the wing is often twisted slightly.1/8 to 1/4
of an inch. Depending on the twist it is called washout or wash-in.
Interestingly, if a Cessna were launched at cruise speed it would
fly quite well without the outer part of the wings but the ailerons
would be sorely missed. (8<))
The part of the wing outside the wing strut of Cessnas have this
slight twist that flexes depending upon the speed of the aircraft.
At slow speeds this twist gives a slightly better low speed handling
performance as needed in the flare for landing. I don't believe
it exists in short-wing Pipers but may be there in the longer
tapered wing.
Aspect Ratio
This is a method of describing a wing by creating a ratio
by dividing the wingspan by the average chord. You can use the aspect ratio fraction to find either average
width or length knowing either chord or span. A long wing has
a high aspect ratio giving lift with minimum drag. A low aspect
wing will be short and stubby. The determining factor is structural
strength. As engineering and materials allow we will have high
aspect wings on more aircraft.
Controls
and What They Do
The primary controls are the elevators, ailerons and rudder.
These provide primary movement around the axes of flight. In
combination, they give coordinated movement around the axes of
flight. Engine power is an additional primary control of pitch.
Again, in combination, it gives coordinated movement. No change
in one axis occurs without having some effect on the other axes.
Secondary controls include trim and flaps. Devices that augment
engine power and control operations, weight, center of gravity
and load factor have secondary effect on control. Complex aircraft
may have additional controls. The effect on all controls is dependent
on conditions of altitude, speed, temperature and weather.
Neutral pitch is engineered into the placement of engine, wings.
horizontal stabilizer and loading limits. The pitch is moderated
to a designed degree by elevator, engine power and trim. Any
change in elevator or engine power along with the rapidity of
change requires coordinated control movement in the other axes.
To change only pitch, by whatever means, some additional combination
of rudder and aileron is required. (Modified C-172 marked change)
Ailerons "control" bank angle, roll and roll rate but,
in combination with the other controls. On application of aileron
in a turn, rudder must be "coordinated" to keep the
tail behind the nose; elevator is used to counter loss of vertical
lift. Ailerons work in opposite directions, usually in differing
distance and with an effect called adverse yaw. The down aileron
gives lift and drag (induced). The drag resists the turn so that
rudder is applied for coordination.
Rudder is used most often in anticipation of known requirements
from the other controls. Rudder will induce roll as well as yaw.
The rudder can be used to raise a wing in a stall. Anticipatory
rudder is applied to counter the effects of power/pitch applications.
A rudder-applied yaw is used to make possible crosswind landings.
P-factor, torque, precession and slipstream all require use of
the rudder. Skillful rudder on the ball and in anticipation is
the distinctive mark of a good pilot.
Power is a pitch control. Just adding power (no other control
input) will cause the nose to rise and roll to the left. Speed
will decrease. In a turn, power will make the left turn possible
with little or no rudder but require rudder to "lead"
the right turn. There are countless cause/effects in the creation
and control of a given airspeed and pitch condition. If you are
ever asked about what controls airspeed and pitch, just say,
"The pilot".
Horizontal Tail Surfaces
These two surfaces, stabilizer and elevator or combined stabilator,
provide longitudinal stability and control of pitch. Even the
simplest of aircraft have a controllable pitch that can be adjusted
for hands-off flight using a trim control. Trim adjusts in several
different ways but the effect is always the same, stability of
flight. All horizontal tail surfaces use the movement of air
either relative, prop wash, or wing wash, to produce a down load
to counter-balance the engine on the other side of the center
of lift. The different washes affect pitch controls differently.
A T-tail does not get the same amount of prop wash as does a conventional
tail. Every tail design has positive and negative facets. Some
airplanes don't even need tails.
The size and location of the horizontal tail surfaces is usually
designed into the aircraft (T-tail exception) so that the air
flowing over the wing augments the normal slipstream. An additional
source of airflow will be the propeller wash or prop wash. Together
they give the download required to counter balance the weight
of the engine. If this flow over the wings is diverted in some
manner by a gust, ice, wind shear, or slip the downloading of
the tail surfaces may be reduced. In an incipient stall the first
effects you feel and hear are on the tail surfaces. This is buffeting
caused by the burbles of air breaking loose from the wing during
the incipient stall.
At the stall the wing loses its ability to sustain the weight,
gravitational and aerodynamic, of the aircraft. At the wing stall
reduced airflow over the horizontal tail surfaces reduces the
download capacity of the flight surfaces. The tail goes up and
the nose goes down. How much and how abruptly depends on the
extent and suddenness of change in the download. A tail surface
in the direct flow of prop wash may be overly affected by changes in
engine power. On takeoff this effect is to be desired as it enables
the nosewheel to be raised sooner. A pilot in flare will need
to use additional elevator to counter the decrease in prop wash
caused by power reduction. The airflow between the airplane and
the surface of the earth becomes significant when within half
a wingspan. This ground effect redirects the downwash off the
wing making it less effective over the horizontal stabilizer
and elevator thus requiring greater deflection to raise the nose for
touchdown. Now you know the 'rest' of the story.
Down load on the horizontal tail surfaces is required to hold
the nose up and the tail down about the aerodynamic center of
lift. When the download on the tail cannot hold the nose up the
nose will pitch down. Read Flight Training Handbook on page 277.
In normal flight, including stalls, the horizontal stabilizer
doesn't stall. With the elevator down, the effective angle of
attack of the tail goes up, just like a wing with flaps down,
and since the angle of attack is already high, pushing down forces
the tail past the maximum angle of attack. Then it stalls.
Icing of the tail can result in total loss of control. I think
that most turbo-props are operated with the c. g. behind the
aerodynamic center of the wing, so that loss of control of the
stabilizer causes the airplane to pitch up, eventually beyond
maximum lift, and without the tail to counter the pitch up, the
situation becomes unrecoverable.
Rudder
The function of a boat rudder, as is commonly perceived, does
not apply to airplanes in the air.
Addendum from Roy Smith:
You might be interested to know that it doesn't apply to a boat
either! The worst way to turn a boat is to use the rudder. Rudders
cause drag when moved off the centerline (induced drag) and slow
the boat down. All turns, as much as possible, are done by shifting
crew weight. The rudder is there for fine tuning, and to add
additional turning moment for very fast turns when just shifting
weight isn't enough.
As explained in taxiing, the rudder pedals and brakes are used
for ground steering. The purpose of the airplane rudder is to
keep the tail behind the nose, it is the first control to become
effective on takeoff. The rudder should keep the tail behind
the nose in level cruise configuration. This is built in or 'rigged'
and trimmed performance often by a small metal 'tab' on the rudder.
The 'ball' is centered. Changes in pitch, power, or bank
require rudder application to keep the ball centered and the
tail behind the nose.
The usual application of rudder begins with leg power which gives
a relatively course control. Smaller variations of rudder can
then be applied or removed by flexing the ankle. The cables and
pulleys of the rudder require relatively heavy control forces
when compared to that of the ailerons or elevator. The rudder
gives the directional stability to an aircraft that makes flying
pleasant.
Throughout aviation history the use and effect of the rudder
in the air has not changed. The rudder is a participant in every
slip, skid or bank. As an aircraft centerline is displaced in
level flight by just rudder application you have created 'yaw'.
The fact that adverse yaw is the cause of the off center ball
never gets mention. The ball merely reflects the effect of adverse
yaw. With the aircraft now flying at an angle to the relative
wind we have established a yaw or sideslip angle. Left alone
the aircraft will not fly with any sideslip angle.
The rudder effectiveness is enhanced by a slight displacement
of the engine, which allows the prop-wash to have greater effect
on the vertical control features than on the horizontal features
of the empennage. A hand-adjustable trim tab or a trim adjustment
wheel can be used to relieve rudder pressure.
More
Rudder
Aaron says that's a misleading statement from Roy.
Rudder
Displacing the rudder to the left, as if to enter in a coordinated
left turn causes a force which acts to push the tail to the right
(left yaw). However, since the rudder is located above the CG
the rudder also creates a moment about the longitudinal axis
which counteracts the ailerons (roll right instead of left) when
trying to maintain the coordinated turn to the left. If the aircraft
rudder was mounted underneath the CG, one could easily turn an
aircraft in a more coordinated manner, and it would turn like
a boat.
The primary difference between a boat rudder and a aircraft rudder
is the inefficient placement of the Rudder above the CG in an
aircraft which makes the analogy misleading. Because the rudder
in a boat is below the CG, the rudder also creates "horizontal
lift" acting on an arm which causes the boat to roll into
the turn. An aircraft rudder has the opposite effect. Displacing
the rudder to the left, as if to enter in a coordinated left
turn causes a force which acts to push the tail to the right
(left yaw). However, since the rudder is located above the CG
the rudder also creates a moment which counteracts the ailerons
(roll right) when trying to maintain the coordinated turn to
the left. If the aircraft rudder was mounted underneath the CG,
one could easily turn an aircraft in a more coordinated manner
like a boat.
But that isn't the optimal design. By mounting the rudder so
that there is equal surface areas above and below the a horizontal
line extended from CG, Every maneuver involving use of the rudder
(crosswind landings, stalls, slips, turns, et al.) could be done
in a much safer and efficient manner.
A problem arises in design constraints which prevent a down-rudder:
It would certainly cause a lot of problems in the landing flare
et al. There are certainly other differences between the A/C
rudder and boat rudder. Most of which are related more specifically
to fluid dynamics and the differences between the density/viscosity
of air and water. A rudder in a boat usually has at least 25%
of its surface area ahead of its axis of rotation, which alleviates
the need for much stronger power steering. Rudders do cause a
lot of drag when the turn is initiated, which can result in bent
rudder shafts in the test tank, but the naval architect never
lets that happen twice in his career. After the vessel is established
in the turn, the drag is minimized.
Aaron Prosser
Using
the Rudder
(Guest Opinion).
Rudder use is pretty straightforward. There are two different
reasons for using the rudders.
The first reason has to do with the way ailerons work. When you
want to change direction in an airplane you must create an unbalanced
acceleration in the direction of turn. Without this acceleration
you will proceed in a straight path. We do this by "banking"
the airplane. To bank the airplane for a turn you must deflect
the ailerons.
When you want to raise a wing, the aileron on that wing must
go DOWN. The aileron on the wing that is going lower goes UP.
The up aileron is hidden behind the bulk of the wing and adds
very little drag. The down-going aileron is like applying flaps and adds a considerable
drag increment. This difference in drag out at the ends of the
wings tends to swing the nose of the airplane in the WRONG direction.
This phenomenon is called "adverse yaw" and is much
more noticeable in older airplanes and sailplanes with longer
wings. You cancel this unwanted yawing moment by applying some rudder.
Generally, anytime the ailerons are deflected from neutral, so
should the rudder be deflected from neutral to maintain "coordination."
The other use of the rudder is when you want to be UN-coordinated.
You do this when you want the airplane to move through the air-mass
sideways, greatly increasing the drag of the airplane and bank angle of your flight
path. There are two times that we do this. One, of course, is
to increase the descent angle of our glide path to lose altitude more quickly.
This is the forward slip with the nose pointed away from the runway
The other, and very important one, it to align the airplane correctly
with the runway when the wind is blowing in a direction that
precludes a straight ahead landing. In this case you bank the
airplane so it will
slip sideways to remain aligned with the runway centerline while using
the rudders INDEPENDENTLY of the ailerons to keep the nose of
the airplane pointed at the FAR end of the runway. This makes
sure that you don't have any lateral velocity when you touch
down and that your wheels are pointed in the proper direction
when they start to turn. All very important for good directional
control in landing.
"Demonstrated cross wind" from the POH is not necessarily
a crosswind limitation of the aircraft. It basically represents
the strongest cross wind they landed in during the certification
process. I believe the
"demonstrated crosswind" for a Cessna 152 is an example
of a figure in the POH that represents a crosswind that is well
below the crosswind capability of the Cessna 152!
I would use the crosswind component that I was comfortable
with, whatever was given in the POH. It could be more, but more
likely will be less. :-) I promise you the DE won't flunk you
if you say, "The airplane may be able to handle that much
crosswind, but I don't feel comfortable with it myself."
:-)
Highflyer
Use
of Rudder?
Is there a 'proper' way to place and use one's feet on the
C-172 rudder/brake pedals?
In a way I can't believe I'm asking this. Not that I have a tremendous
amount of time (15 hours), but it seems like it is one of those
basic things that would have been settled by now. I have looked
for the answer in documentation and asked my instructor about
it, but I'm still unclear on the matter.
A little background: The older (1970's) 172's have pedals that
accept the entire foot. So the first few lessons, I put both
feet right on 'em. No problem in the air, but on the ground found
it a little clumsy separating turning action from braking action.
Seemed like no matter how much I tried, I always got brake action
even when I only wanted to turn. I thought it was just something
that would come with time, as I became more proficient.
Then one lesson we were on the takeoff roll and my instructor
noticed my whole foot on the pedal and admonished me to "stay
off the brakes." I asked him later what the proper technique
was, and he said, "To use the brake, lift your heel."
It seemed like that instruction could apply whether or not the
whole foot was placed on the pedal, but since he seemed to freak
at the sight of my whole foot there, I decided I'd keep my toes
off the top half of the pedal entirely.
From that lesson on, the normal position of my feet is heels
on the floor, toes on the bottom portion of the pedals. So when
I do want to use the brake, I not only have to lift my heel,
I have to reposition my foot (to get my toes to the top of the
pedal). Is this correct?
Where I need clarification is, do you place your entire foot
on the pedal and extend your knee to steer, keeping you ankle
joint/toes retracted unless you also want to brake, or do you
stay completely away from the top half of the pedal until you
want to hit the brake?
And is it true the newer 172's have only a half-size pedal? Which
half and how do you use that system?
Ken Wiebe, Student Pilot
Flying with Rudder
When you first try to fly with rudder, you will find that
the rudder affects the roll axis. Normal coordinated use of the
rudder and ailerons gives the expected yaw effect of the rudder.
Due to cross-axis coupling, the rudder is above the centerline of the
aircraft, use of the rudder alone gives roll
because of the rudder's position above the center of gravity.
Left rudder alone gives a left yaw and a right roll. For a given
rudder application more yaw than roll will occur because of the
relative moment arms.
The wing on the outside of a turn becomes a drag factor causing
adverse yaw. It is the difference of the drag between the raised
and lowered ailerons that produces the yaw away from the intended
direction of flight. This induced drag is mostly due to a greater
angle of attack rather than the higher airspeed. It is the downward
deflection of the ailerons that effectively increases the angle
of attack. More rudder is required entering a turn than when
the desired bank is achieved, because the ailerons are deflected
more when rolling in and out. The geometry of the aileron pulleys
allows relative up or down movement of the two ailerons to be
different. Some modification of yaw is possible by doing this.
At a 30-degree bank the ailerons are neutral. Rudder must be
applied to counter this adverse yaw. A reason very little rudder
is required in the left turn is because the yaw effect of the
ailerons is countered by the P-factor of the propeller. More
rudder is required in right turns since the P-factor is added
to the yaw of the ailerons.
The rudder's steering function in the air is only related to
a small degree to that on the ground. It is used to correct or
offset the undesired steering effects of propeller, propeller
slipstream, wing, and aileron. Pressures in level cruise flight
are reduced if the small trim tab on the rudder has been correctly
adjusted. As speed is reduced in level flight the nose must be
raised. More and more right rudder will be required. In descents
light left rudder might be required. The proof of poor rudder
use by a student can be demonstrated by having the student climb
in a given direction without reference to the panel. Over several
minutes the plane will begin its gradual left turn. A request
to get back to the assigned heading results is a great deal of
bank and little turn. As I instruct, I often find myself, unconsciously,
holding right rudder for my student. Aircraft except for the
training aircraft have a rudder trim to ease the amount of rudder
pressure required as when climbing to high altitudes.
The need for right rudder in the climb is due to several factors
the most obvious of which is the P-factor. That is the increased
thrust of the downward moving propeller blade brought about by
the raised pitch attitude. The most significant factor in these
changes is P-factor. P-factor is caused by the differential in
thrust between the rising and falling propeller blade. Any addition
of power or raising of the nose increases the differential and
causes the nose to pull to the left. Additional right rudder
must be applied to keep the ball centered. This rudder application
must be and not as an afterthought in reaction. Some flight situations,
such as slips, crosswind landings, Dutch rolls, and IFR approaches
require the rudder to be misused (abused) from its design intent.
Rudder applications are mostly required in making coordinated
rolling maneuvers where the rudder counter reacts to adverse
yaw. Rudder application is best used in anticipation of the roll
to come. Only the rudder can effectively stop the wing drop that
occurs in an uncoordinated stall. Opposite rudder lifts the wing
where aileron only makes things worse. In the making of crosswind
landings the authority of the rudder determines the pilot's ability
to keep the aircraft centerline parallel to the runway centerline.
Once a rudder is fully depressed more authority can be obtained
by increasing speed or propeller wash.
Use of the rudder to yaw the aircraft or even to counter any
yaw will cause a pitch change that must be anticipated by the
pilot. I usually teach Dutch rolls in a climb and the propeller
disk is tilted and left yaw is more evident. For this reason
and the slow speed involved when doing the Dutch rolls far more
right rudder is requires over longer periods. Some aircraft require
no right rudder at all. This propeller plane effect, P-factor,
is also evident when doing forward slips.
Unintentional Pushing on
a Rudder Pedal
This problem is usually caused by a pilot not sitting perfectly relaxed in the
seat. What this means simply is that unless you are flying in a relaxed state,
you might have a tendency toward placing unneeded forward pressure with your
feet against the pedals. If this happens and you're a bit twisted in the seat,
one leg can in effect be "longer" than the other.
A lot of pilots have a tendency to do this. Its not a big problem and you have
most of it solved by simply being aware of it and wanting to fix it.For
starters, concentrate on sitting absolutely straight in the seat. Then put the
full weight of your legs on your HEELS, using your ankle as a fulcrum for the
rest of your foot which should be resting extremely gently on the pedal. The
goal here is to have practically 0 pressure on either pedal unless turning the
airplane.
Think of having your hand on the yoke connected to your legs so that when in
normal level flight with the yoke centered and no pressure applied on it, your
legs are the same. Any pressure to either side with the yoke gets pressure on
a rudder pedal also.You don't apply pressure to one without the other, and
when not applying pressure, your feet are again in a relaxed state.
The bottom line on all this "fancy footwork" :-) is that what you
need to do is concentrate completely on flying the airplane with your body
relaxed at all times. Look at the nose. It will tell you immediately if you
are carrying a yaw input. If there's any doubt at all, just lift both feet and
reset them as I've described above. Personally, I don't think this is a huge
issue for you, and concentrating on flying a bit more relaxed and centered in
the airplane will solve it. All the best and let me know how things go for you
with this.
Dudley Henriques
Opinion
Yep, see it all the time. I did it, too. You can try just taking your foot
off the pedal and resting it on the floor, but that is only a temporary fix.
Ultimately you have to develop an eye and feel for coordinated flight. That is
when you really stop doing it. But keeping the left foot flat on the floor
most of the time can be a big help in training yourself not to tense up the
leg.
It is a more serious problem than you might think at first, beginning with the
fact that sooner or later you are going to blow out your left tire on landing.
Not helpful. Uncoordinated flight is also more dangerous than most people
realize.
I have also seen that people who do this also tend to subconsciously clench
their teeth. You might want to see a dentist about that.
CJCampbell
Opinion
Check your seat position to be sure it is comfortable and that you are
sitting centered. Don't be any closer to the panel than you need to be to be
able to get full rudder travel with you leg and also have full control on the
brakes with your ankle joint. Wear shoes that fit properly and wiggle your
toes. Use your toes on the rudders, not the whole foot. Use shoes with soles
that let you feel the pedals.
Wear good wool socks so your feet stay warm. Don't lean on the side of the
airplane, sit up straight.
Frise Aileron
Leading edge is in front of hinge line so that airflow reduces
control pressure required to hold deflection. This aileron is
the primary reason adverse yaw has been reduced in modern aircraft.
There is more down movement than up movement along with the descending
leading edge. The net result is that there is not so much rudder
requirement to correct for adverse yaw. This has effectively
inhibited flight instruction. Pilots can no longer feel the as
much yaw in a bank and thus do not use rudder correctly. Some
aircraft (Pipers) can be flown with feet on the floor if the
aileron movement is not abrupt. In any established maneuver,
such as an approach to landing, elevator will control the airspeed
and the angle of attack.
Aileron movement changes the pressure distribution over the ailerons
and thus changes the pressure over the wing chord ahead of the
aileron. This effect is true only before the stall burble. Once
the air breaks from the wing surface any movement of the aileron
will be ineffective on the wing pressure. It is because of this
airflow separation that the use of the ailerons is ineffective
until the angle of attack is reduced and the airflow reattaches
to the wing surface.
Ailerons can be designed so that they will move differentially,
that is, more up than they do down as a method of reducing drag
and thus adverse yaw. This is done by running the control cables
or rods to a bell crank with its arms of differing lengths or
angles to the actuator arm.
Latches
It is a shame, considering the cost of an aircraft, that
the latching mechanisms of doors and other such should be so
inferior in design and construction. If door locks do not work
properly, report it to maintenance. Do not tolerate a situation
that has caused so many needless accidents. It is not wise to
live with a known discrepancy. It is a violation of the FARs
to fly with a known discrepancy.
Rig
No two planes are exactly the same because of the infinite
variations of rigging. Rigging is the total alignment of the
parts of the aircraft, wings, flaps, controls, and fuselage.
You may have see a car go down the road without the back wheels
tracking behind the front. An out of rig airplane can fly much
the same way. There is no way to see this, but 'identical' airplanes
usually fly differently. The rigging of an airplane will not
perform as well as it should. It will not have the stability
that allows hands-off flight nor will it react in the stall and
spin as it should.
Some minor defects of rigging can be corrected with the fixed
tabs on rudder or ailerons. Some controls can be adjusted by
bungee cords or springs. Poor stall characteristics can be manipulated
by the use of leading edge stall strips or by slots in wings
or horizontal tail.
Spiral stability
Every time you bank an airplane you are changing the bank angle
and affecting the airplane's spiral stability. Most G.A. planes
will hold a 30-degree bank with less than a half-turn of the
trim wheel in cruise. In this situation the ailerons are neutral
and the yoke should be level with the instrument panel. At less
than 30-degrees the bank will tend to decrease and aileron must
(should) be held against the bank to offset any roll generated
by the yaw rate. At more than 30-degrees bank the bank will tend
to increase and aileron must be held against the increase in
bank to offset any roll generated by the yaw rate. All of these
flight conditions assume that coordinating rudder is applied.
A blending of pitch and roll attains a constant bank. The vertical
fin provides roll vertical stability.
In a bank we are dealing with four separate flight elements,
roll, yaw, lift, and stability. We want an IFR plane that will
not enter, on its own, into a divergent descending spiral, or
into convergent spiral especially if the entry is rather quick.
You should determine just how your plane flies in VFR by checking
how well a bank is maintained and held into a standard rate turn
and what it does in level flight hands-off. You need to find
out how the plane behaves and what you should do to make it behave.
The three axes of the aircraft are not equally stable. The pitch
axis has a trim control that lets the pilot adjust the pitch
stability for a desired flight condition. The vertical axis has
a trim control that allows the pilot to relax on the rudder.
However, it is the roll axis that is usually without such a control
with some exceptions.
The roll axis or lateral axis is not stable because the pilot
needs this instability to maneuver the aircraft. We can let go
of the yoke in pitch and yaw for a length of time. Try the same
thing with the bank and subtle differences arise. The ailerons
resist the initial change, then accept it and then maintain their
new position. The new position is maintained until the over-banking
tendency takes over. The tendency to roll on over is caused by
the outside wing moving faster in the turn and the propeller's
slipstream hits the rising wing the most. Flaps increase this
effect.
Countering these over-banking effects we have dihedral. Dihedral
is the up slope of the wing from the wing-root to the tip. The
shape of the wind and its sweep also make a difference. Low-wing
aircraft have more dihedral than high wing aircraft. The effect
of dihedral and wing shape will vary with the power and pitch
attitude of the aircraft.
The conflict between the instability and stability of the aircraft
explains why the aircraft can maneuver and will hold one position
only for a few moments. One of the roll stability demonstrations
I use with students is to put the aircraft into a 30-degree bank
and put in anywhere from 1/3 to 1/2 turn of trim and let go.
The aircraft will perform a turn that can be held with just light
touches of rudder. The usual design of light aircraft is to make
the 30-degree bank more stable than any other bank. Less than
30-degrees bank and the aircraft will strive to level off. More
than 30-degrees and the aircraft will eventually roll over.
Aircraft Category
Manufacturers have the FAA test a new plane model for production.
Engineering design and tests are given as evidence to justify
certification in one of three categories -- normal, utility,
or acrobatic
Requirements appear in FAR 23. Differences in the categories
are based on strength and spin recovery. Wing loadings are 3.8
G's for normal, 4.5 G's for utility, and 6 G's for acrobatic.
Negative G loadings are assigned as well. The structure must
be designed and tested to exceed the certified limits by 150%.
Certification can be held in more than one category depending
on the tests passed. The C-150 and C-152 are utility all the
time, a C-172 is either normal or utility depending on load carried.
The POH sets the requirements for each category. Spins are not
allowed in all utility aircraft.
General aviation categories are for aircraft less that 12,500
pounds gross weight. Normal, utility, and acrobatic correspond
to certification categories. The FARs describe general maneuvers
the aircraft is allowed to perform. Normal category aircraft
can do normal flying, stalls, lazy-eights, chandelles, steep
turns to 60 degrees. Utility aircraft can do all of the normal
maneuvers plus (approved) spins and 90-degree banks. Acrobatic
aircraft are unrestricted except for placarded maneuvers in operating
limitations
An aircraft may be certificated in more than one category. Exceeding
allowable gross can cause the structural load on the aircraft
to exceed its capability as in turbulence. Normal category aircraft
can withstand 3.8 positive G's. Over gross aircraft can be permanently
damaged in turbulence even if operated at maneuvering speed Va.
Categories have limit loads that are maximum anticipated. These
loads an aircraft can safely support in flight. The manufacturer
normally designs a 150% safety factor into the structure. At
a predicted ultimate load structural failure will occur. If flight
occurs with simultaneous pitch and roll, as in a bank, the limit
loads drop by over 30%. For this reason turns are not a good
idea in turbulence.
Normal, utility or acrobatic category airplane are required to
be controllable for landing by power and trim in the event of
total loss of elevator control. These categories must be able
to make a 2-degree climb in a go-around in their most "dirty"
configuration. Properly restrained passengers should survive
at 26-G .05 second deceleration without restraint failure. a
9-G forward, 3-G upward (Acrobatic 4.5) and 1.5 G side-load.
Part 23 aircraft designed in the 80s and later are not multi-category.
Single engine in all three categories in landing configuration
must be able to land at 61 knots CAS.
Correct control inputs in a stall must be able to prevent a spin
entry. The one-turn spin test required is really a test of recovery
from an abused stall. Normal category only aircraft are not approved
for spins.
Aircraft can be classified by weight and as of August 1996 the
classification table is for purposes of wake turbulence separation:
Small...less than 41,000 pounds (changed from 12,500 pounds)_
Large...greater than 41,000 but less than 255,000 pounds
Heavy... greater than 255,000
Lifting Surfaces
The rudder and vertical stabilizer form a variable airfoil. The elevator and horizontal stabilizer form a variable airfoil,
The wing by itself is an airfoil. The wing plus the aileron and
flaps is a variable airfoil. The span of any of these airfoils
is its length. The width is called chord.
Dividing the chord into the span gives the aspect ratio. Aircraft
with long wings have a high aspect ratio while jets will have
a low aspect ratio. Its aspect ratio and its airfoil determine
the aircraft's sensitivity to control input in various situations.
The airfoil develops lift by having different air pressure on ftop and
bottom.. The movement of the rudder will create a differential
in lifting forces on either side to move the tail left or right.
The elevator does the same to move the tail up and down. The
wing's differential pressures between the upper and lower surface
at different wing angles of attack cause to aircraft to go up,
down or fly level.
This pressure differential can be demonstrated very simply by
using pieces of paper. Hold a piece of binder paper upward by
the edges between both arms with the thumbs and forefingers of
each hand. Allow the paper to droop away from you. Allow it to
form a curved airfoil. Blow directly across the top of the curve
and you will be creating a pressure differential great enough
to cause the paper to rise. It will rise farther than it would
were you to blow directly across the bottom because the curve
creates a higher air speed and therefore a lower pressure. The
curve is a more efficient creator of low pressure.
This low pressure can be more dramatically demonstrated by holding
two pieces of binder paper from the top edge between thumb and
forefinger so that they hang parallel about three inches apart
facing each other. Hold them parallel to your line of sight so
you can see between the sheets. Now blow. The sheets come together
because relatively fast air between the sheets form a low-pressure
region. Nature does not like vacuums so, as with lifting surfaces,
the pull of the low pressure combined with the push of the adjacent
high pressure moves the intervening surface which in this case
is a piece of paper.
When an aircraft is in level un-accelerated flight the amount
of lift created by the wing will equal the weight of the aircraft,
the downward 'lift' of the tail surfaces used to counter the
weight of the engine and any drag however created. When an aircraft
is capable of such flight at one times the force of gravity (G-1)we
can determine its wing loading by averaging the number of aircraft
pounds supported for each square foot of wing surface. The higher
the pounds per square foot the more like a rock becomes the airplane.
L/D ratio
Typical aircraft has an L/D of 10 to 1. For every ten pounds
of weight there is one pound of drag. There are two kinds of
drag; induced is produced by surfaces producing lift and parasitic
which is produced by friction.
Drag varies with airspeed. Induced drag decreases with airspeed.
Parasitic drag increases with increased airspeed. Minimum drag
exists when the two forms of drag are equal. In most aircraft
this is very close to Vy in level flight. At Max L/D a 10% reduction
in weight will give the same 10% increase in mileage.
Fuel consumption is gauged by pounds of fuel per horsepower per hour. This value is normally
.5 lbs/hp/hr. Extra lean mixture
will give .4 while rich will give .7. TAS increases with altitude
but to obtain the same CAS more power is required with a higher
fuel flow rate. It is best to fly maximum efficiency by indicated
airspeed rather than by power settings.
Modifications on aircraft affect efficiency more than speed.
It is most important to keep the front half of the wing chord
clean and smooth as well as the front half of the propeller clean
and smooth. For a given weight CAS determines aircraft efficiency
regardless of altitude or temperature. The TAS to CAS ratio increases
by 1.7% per thousand feet of altitude increase. (TAS is 17% higher
than CAS at 10,000' --100 CAS =117 TAS). Aircraft efficiency
is best determined at a constant weight using calibrated airspeed.
Lower air density affects lift and drag the same as it affects
the indicated airspeed. This is why the same IAS is used for
all landings at all airports. Improving efficiency in the propeller,
fuel consumption, and aircraft efficiency by 5% each would amount
to a 15% improvement in performance. The optimum speed of most
aircraft for efficiency of fuel consumption, propeller, endurance
and time is about 10% faster than the Vy airspeed. This is the
Vz speed and should be flown in climb, cruise, and descent.
Wing Loading
Wing loading is a measure of how much weight the wing must
lift at gross weight expressed as pounds per square foot of wing
surface area. Wing loading affects stall speed, maneuvering speed,
and twitchyness in turbulence. The weight carrying ability of
the aircraft is a function of wing area. greater wing area lowers
the stall speed. The lower the weight per pound the better the
short field capability.
Power Loading
Power loading is the prime measure of climb performance.
The more power per pound of aircraft weight the better the climb.
Power must be increased dramatically by four times to double
the speed. The increase of power will reduce cabin load and increase
fuel consumption so as to reduce range and carrying ability below
practical considerations. I one flew a 180 h.p. Yankee that carried
14 gallons of fuel before being at refueling minimums. Aircraft
went 146 knots but you had to land every hour and a half for
fuel.
Drag
& Performance
--Parasitic drag is caused by the aircraft skin and protrusions.
It increases with speed.
--Induced drag is caused by lift, increases at slower speeds
and higher angles of attack.
--A fixed pitch propeller is most efficient at only one power
setting.
--Maximum range occurs in flight that has the highest ratio of
speed to fuel flow or at maximum lift over drag.
--Maximum range is achieved at the minimum power setting that
gives level flight.
Parasitic Drag
The structure of the aircraft that creates wind resistance
or any friction produces parasitic drag. It exists as with all
parasites as a constant fixed burden to its host. It affects
performance and efficiency.
Induced Drag
Induced drag exists in ever increasing amounts as the amount
of lift increases. The greater the lift requirement per unit
of wing area and thus the higher the induced drag. Long wings
have low induced drag, short wings have high-induced drag. The
proportion of the wing reduced by fuselage location and wing
tip vortices reduces wing efficiency. The slower you fly, the
higher the angle of attack, the greater the weight, the greater
the induced drag. The induced drag increases exponentially far
beyond the factor of decreased speed, increase in wing loading.
This is one more reason a pilot must, at altitude, determine
his skill limits at flying slow while maneuvering.
As the backside of the power curve is approached and the maximum
endurance airspeed is reached, airspeed can be controlled most
easily by pitch and altitude by power. This is also called the region of
reversed command. Once on the backside,
where no more power is available, the only available option is
to lower the nose and sacrifice altitude. You must lower the
nose or stall. In this situation at low altitude you have run
out of options.
Boundary Layer
As air flows over an air foil the thin layer of air right
next to the surface does not move relative to the surface because
of viscosity. A dusty wing stays dusty. This thin layer of air
is called the boundary layer. The subsequent layers of air increase
in speed until finally one layer acquires velocity sufficient
to create a pressure differential.
Control Failure
The rarest emergency is caused by control failure. Uncommon
and unmanageable unless you pre-plan possible scenarios. You
must take immediate action to keep the aircraft from getting
into an extreme attitude. Rudder can be used to control roll
in banks less than 15 degrees. It is a good idea to practice
flight with only the rudder. Rudder can be used to counter the
effects of a jammed aileron. An aircraft with a jammed aileron
can be landed in a slip preferably against a crosswind. Touch
down slowly and use the "crunch" of aircraft parts
to take the shock.
A jammed rudder could yaw you into a spin. Avoid climbing turns.
Use power and ailerons in small amounts. Get into a higher than
normal speed slip if the rudder is stuck deflected. Land crosswind
in a slip.
Jammed elevators can only be countered by power, C.G. changes
and trim tab position. A free-floating elevator is best controlled
by trim. A right rudder and left aileron slip may help lower
the nose. A left rudder, right aileron will raise the nose. Use
power all the way to the ground. Use a shallow approach to a
long runway. Remember, power raises the nose.
Asymmetric flap position is the only critical flap failure. Get
into a slip that will counter the yaw/roll effects of the split
condition. The wing with the least flap will stall first. This
condition seldom occurs but when it does it catches you by surprise.
Undo whatever you did first. Bring flaps up or down as the case
may be. Being low and slow is the worst possible situation. The
strength of the forces attempting to roll the aircraft can only
be partially countered by aileron and rudder. Any bank that you
allow will be too much.
These simulated control conditions can be practiced at altitude
by having the instructor lock a control while you regain control
with what is left. Get close to the ground before you attempt
any major changes. Don't touch down until you are as slow as
your controls allow.
Cables
Cables of 1/16" to 1/4" with 7 strands of 7 wires
(7 x 7) where strength is required and 7 x 19 where flexibility
is required (controls). The yoke rod has attached linkage to
the two sets of cables via pulleys under the cabin floor. These
pull on a bell crank attached to the 'axle' of the elevator.
Turning the yoke moves cables via pulleys to bell cranks in the
wings, which have short pushrods to the ailerons. By making the bell crank asymmetric the ailerons can be made to move more in
one direction than in the other. This is used to limit adverse
yaw in turns. Service manuals tell how much a given control should
be able to move in a given direction. Mooney uses push rods instead
of cables for its control operation. An unexpected control failure
can be mitigated if the aircraft is properly trimmed. This, alone,
is a good reason to always fly a trimmed aircraft.
Any binding or uneven movement should be checked. Listen for
cable sounds. Changes in the time flaps or gear take to move
should be checked. Only one rudder pedal should be able to be
moved at a time. The best time to check controls is when it is
very quiet and you can hear things that don't sound right. All
control cables require lubrication as do all hinges and pushrod
connections. Failure to lubricate opens parts to wear and corrosion.
Any part not lubricated can 'freeze' in position and cause the
cable to wear a flat spot to wear and eventually fray the cable.
Cable tension should be maintained and any sign of loose cables
should be reported as a maintenance item. Cable tension changes
with temperature conditions. Too loose or too tight is equally
dangerous. Unbalanced controls and loose cables cause control
flutter at high speeds.
Control damage can be caused over a period of time by such things
as corrosion, wind pressures, improper rigging, interior route
exposures, and pilot abuse. Use every preflight as an opportunity
to find potential damage. Elevator failure is most likely to
occur. 50% more likely as rudder failure; 200% more likely than
aileron failure. In the event of any control failure practice
at as much altitude as you can whether control exists at landing
speeds. Plan what to do according to what you determine.
Friction Test of Controls
---Trim for any level situation hands-off
---Single finger slow aircraft 10%, if done very little control friction.
---Repeat trim for same level situation hands-off
---Pitch to slow aircraft 10%, then slowly relax pressure to let nose lower to
level flight with speed increase
---Repeat trim to same level situation hands off
---Hand pitch aircraft nose down for 10% increase in speed.
---Relax nose down pressure to see if nose rises and speed decreases
---Any of the forces requires if that of lifting a gallon of water indicates
excessive control friction
---Lubrication and checking pulleys is fix for excessive friction
Aircraft Aluminum
All aluminum alloys have the same density, one size sheet
will weigh the same, regardless of the alloy.
Plain aluminum has about one forth the strength of 2024 alclad
aluminum but is the MOST resistant to corrosion
6061 has about 85 percent of the strength of 2024 alclad aluminum.
2024 is clad (alclad ) with a thin coating
( about .001 inch thick ) of pure aluminum for corrosion resistance.
The 2024 is alloyed with copper. The copper increases the strength
but makes it subject to corrosion that causes the sheet of metal
to thicken up and get spongy. 6061 is alloyed with zinc. It is
resistant like pure aluminum but as strong as 2024. The strength
differential of 2024 vs 6061 is that for the same strength given
by the use of 2024 alclad, 18% more aluminum sheeting would be
required using 6061. Using the stronger material for the same
mass is the best choice. Cessnas and Pipers are built from 2024.
Corrosion and Skin
Condition
Corrosion is ever with airplanes and us. It is a part of aging.
Preventive methods have improved but given the opportunity corrosion
always wins. If the aircraft environment contains moisture, oxygen,
carbon dioxide, hydrogen sulfide, salts, fungus, slime, chlorine,
and high temperatures corrosion is going to be a problem. Corrosion
will etch a surface and make it dull. Localized corrosion will
pit a metal surface. Where two different metals are alloyed we
can get inter-granular corrosion stress areas. Filiform corrosion
occurs beneath paint and will produce a blister where the surface
has been poorly prepared in its worse form it is called exfoliation. Under exfoliation the metal literally comes apart in layers.
Galvanic corrosion occurs where dissimilar metals meet, as may
occur around a rivet. The bending of a piece of metal may cause
an area of stress corrosion.
Skin cracks, corrosion, and loose rivets need to be sought below
the aircraft. Working rivets show gray streaks of aluminum dust.
Corrosion is an electrochemical process that destroys metal.
Corrosion usually requires contact between differing metals but
can occur between similar metals if moisture is allowed to accumulate.
Keeping moisture out prevents corrosion. Prevention, detection,
and elimination are required. Rivets are color-coded to limit
use where galvanic corrosion is likely to occur. Galvanic corrosion
is an exchange of electrons between dissimilar metals but it
can occur between differing alloys.
Fretting is a combination of fatigue and corrosion failure. It
occurs when two surfaces are vibrating together when aircraft
is in use. These movements are usually quite small with localized
damage and often lead to cracks and failure. Fretting appears
as gray dust or streaks in the vicinity of rivets or Dutz fasteners.
Improper cleaning or repair can accelerate fretting damage. Shot
peening is often used to slow damage that occurs in heat treatable
aluminum alloys. Leaving grease layers on components can serve
as protection against fretting.
Corrosion is aircraft cancer. It is often concealed by a seam,
joint or paint. Chemical corrosion comes from the atmosphere
or applied cleaners containing metallic salts. Once begun, its
growth rate increases. Corrosion begins to form when the oxides,
sulfates, or hydroxides formed from moisture take the place of
metal. Corrosion is a chemical change. In time corrosion spreads
and reduces the metal strength to zero and will cause blistering
of any paint covering. Corrosion forms in crevices, welds (engine
mounts). Smooth surfaces resist corrosion.
Uniform corrosion covers a wide area and occurs slowly. Filiform
corrosion is uniform corrosion that causes bubbles beneath pain.
concentrated corrosion is usually concealed in lap joints that
such that allow moisture to intrude. Pitting is a type that grows
from when paint or preventive means have been misapplied. When
this occurs between two surfaces it is called fretting.
Sealing the surface or sealing an adjoining surface can halt
corrosion. Cadium is often used as such a sealant. Surface oxidation
makes a seal over an aluminum surface. Paint is most common sealant.
Frequent flying is perhaps the overall best preventative.
Inspection for corrosion is simple. Visually inspect for grayish-white
powder on aluminum and red deposits on steel. Landing gear wells
are vulnerable due to moisture on wet runways. The best protection
is shelter. The worst location is coastal regions or big cities.
Corrosion can occur at any time, place, or climate. A product
knows as Boeshield is used to protect along with ACF-50 or Corrosion-X.
Coastal aircraft should be treated annually. See FAA AC 43-4A.
Hoses and Lines
Where pressure exists, a rigid line is better except when
flexing is likely to occur. Lines are designed to carry fluids
or gases of low, medium, and high pressure. Damage to lines occurs
normally through aging or exposure to corrosives or oxidation.
Misdirected lines, poor maintenance practices, and chafing accelerate
the process.
Most of the lines are routed so that ordinary pre-flight is unable
to catch anything but the obvious. 100 hour and annual inspections
are supposed to detect chafing, leaks, cracks, broken braiding,
twists, and kinks before they cause accidents. Stiffness and
damage to the cloth cover indicate pre-failure of hoses.
It is good aviation practice to change all lines at a minimum
of every five years regardless on condition. Changing all hoses
every few years is cheap safety insurance. More often may be
required where abuse has occurred. Hoses should be changed at
engine overhaul. Fuel lines should have fire resistant covers
installed.
Plastic Windows
Aircraft windows rank third in aircraft maintenance costs.
Must of this is due to improper cleaning or protection from the
elements. Repair may require use of a pressure chamber. Most
damage comes from inside the cockpit due to metallic contact.
Another major source of windshield and window damage is the reflected
heat from interior heat shields that protect the interior and
cook the plastic windows.
Damage consists of crazing, scratches, yellowing, delamination,
milkiness, and cracking. It is better to polish, stop drill,
cover, and repair a plastic window early than late. Repair can
be made at less than 50% of replacement cost.
Hobbs Meter
Hobbs is the manufacturer's name for the hour meter. The Hobbs
Meter is an electric clock that keeps time about 10 to 20% faster
(more) than does the tachometer. It is activated by an oil pressure
switch on the engine firewall. It runs whenever the engine is
running. It always runs at one minute per minute. It usually
reads in hours and tenths of hours, so you pay for your rental
in 6-minute intervals. When the engine runs the Hobbs meter runs. Hobbs
meters can be wired to run when master switch is on.
Tach time" is the time recorded on the tachometer. The Tach
is an RPM instrument and the "tach time" dial is like
the odometer on your recording engine revolutions. Most tachometers
are geared for 2400 revolutions per hour to equal one hour of
tachometer time. . The time during start, taxi and flight training
operations usually are at much lower rpm. The "tach time"
is used for maintenance, 100-hour inspections, etc.
Stall Warner
The Stall warner is placed on the leading edge of the left
wing for a purpose. The left wing is most likely to stall due
to insufficient right rudder application. There are several different
types of stall warners. They are activated by the low-pressure
(suction) airflow that occurs on the leading edge of the wing
at high angles of attack. The electric vane types can only be
checked with the master on. First covering the hole with a cloth
and using the mouth to gently suck can check the suction type.
Under no circumstances should you blow. The horn has several
sounds from a whimper to a raucous squawk. The volume increases
and the pitch lowers as the stall gets closer. There is usually
a 10-knot speed difference between the initial stall sound and
the actual occurrence of the stall.
Va varies with the weight of the aircraft. Control coordination
assures that the stall warner will go off before the stall. Normal
category stall horns must activate 10 knots prior to stall; aerobatic
5 knots.
Spinner
Spinners may have no authorized repair procedures. Spinners
must be balanced or centrifugal forces can cause it to shatter
or at best cause excessive vibration. Some aircraft are certified
with the spinner as a requirement to meet cooling specifications.
the most common cause of spinner damage is having someone move
the aircraft by pushing on the spinner. I once met a pilot pushing
on the spinner of 56K. I asked him why and his reply was that
his checkout instructor told him never to touch the propeller.
Spinners should always be checked for cracks, loose bolts or
'working' rivets. A defective spinner makes an aircraft unairworthy.
Pitot Heat
If there is an expectation of possible precipitation the
pitot heat should be checked for operation during the preflight.
It should be used on prior to entering any precipitation so that
it can get warm before it is needed. Pitot heat is an ice preventive
not anti-ice. A heater coil around the pitot air-intake will
warm air the airspeed indicator and prevent the formation of
ice. If not applied a pitot blockage can cause airspeed to drop
to zero. A properly operating airspeed indicator measures ram
air pressure against static air pressure. In conditions than
lead to a blocked pitot it is important to know your aircraft
performance and appropriate power settings without an airspeed
indicator. A blocked pitot can also show an increase in airspeed
as altitude is gained because it acts as an altimeter. The pitot
should be checked for IFR or rainy operations as part of the
preflight. On some aircraft (older C-182) the pitot heat also
heats the stall warner. Named after a French physicist/dentist
On a recent local ferry flight I found I had no indicated airspeed
at a point too late to abort the takeoff. At a few hundred feet
the airspeed began to function. I believe that during a three
day stop that ice had accumulated in the pitot tube. Next time
that it appears such ice is possible I will turn on pitot heat
during taxi and even activate alternate air to meet the possibility
of the static hole being frozen.
Cabin heat
The cabin heater is obtained from the same shroud around
the exhaust system that provides carburetor heat. Like the carburetor
heat the heat control is a diverter that allows selected amounts
of air passing over the heater muff to enter the cockpit. The
outside temperature, the engine temperature and the efficiency
of the ducting also affect the amount of heat. Cabin heat can
be maintained during descents by running the engine at reduced
power.
If the exhaust system should leak carbon monoxide (CO) into the
heater muff it could enter the cabin even without cabin heat
on. With cabin heat on the problem and effect would be compounded.
CO can enter the cabin through openings in the firewall or other
openings. If you should smell the engine odor in the cabin, immediately
let fresh outside air in and have the system checked for CO as
the first opportunity. Carbon monoxide detectors have a 3-month
life and should be replaced frequently.
During preflight check all air intakes and ducts that can be
made visible. When the shroud heat is not used in the cabin it
is available as carburetor heat and serves to cool engine parts
by removing engine heat. Heater and exhaust parts are going to
have a longer life if you avoid shock cooling of the engine and
its parts.
The air comes in through the nose of the aircraft as allowed
by the cowling baffling. The air is guided by a flexible
tube into the metal outer cover or shroud around the aircraft
muffler. The region between the muffler and the
shroud has a series of thin metal partitions that absorb the
heat from the muffler and allows the ram air to flow and
absorb the heat. This air is safely separated from the poisonous
carbon monoxide passing through the muffler and
exhaust pipe. When the cabin heat control is pulled it opens
a door that allows the heated air from the interior of
the shrouded area to enter the cabin by way of selected vents.
If a leak should occur, and it can, the use of the
cabin heater could allow carbon monoxide to enter the cabin and
incapacitate or kill the occupants. The odor of
other engine gases beside carbon monoxide are a clue that there
is a leak. Carbon monoxide has no odor.
Oxygen
Cryogenic oxygen is make by compressing air during which
process all carbon dioxide and water are removed. The air is
then cooled and liquefied to -200 C and gases other than oxygen
are distilled out. What remains is oxygen that is 99% purse with
less than 4 parts per million of other elements. Any contamination
is unlikely to occur in manufacture. Problems tend to arise at
the user level.
At altitude, you are under physiological stress over and above
all the other stresses of flying. Hypoxia gives you that 'good'
feeling that may be accompanies by dizziness, headache, sweat,
vision problems, and fatigue. The situation is even worse at
night. You lose 24% of your vision capability at 8000' and 50
% at 12,000. Only oxygen can give you normal vision.
Aviation grade oxygen is not supposed to have as much moisture
as medical oxygen but I have read that they are very much the
same and come from the same production system. Only in freezing
conditions should this difference be considered a problem.
One oxygen system, the continuous flow is good to 25,000'. The
system has a cylinder, a regulator, and individual re-breather
masks. The pure oxygen is mixed with the last breath exhaled.
The mask used determines the oxygen mix.
The diluter-demand system is very much the same but the mask
has its own regulator. The pilot can adjust the percent of oxygen
being used. The mask is more efficient and fits tighter.
Maintenance failures
--Rigging of ailerons backwards can result in reverse control
unless safety checked by the 'thumbs-up' control check.
--Loose objects in cockpit are common cause of jammed controls.
--Full flaps stuck down can be caused by electrical failure.
Aircraft can be flown under such conditions but only very
slowly and carefully.
--Wiggle elevators to feel looseness of hinge brackets.
--Control yoke failures of Cessnas make 100-hour inspections
necessary.
--A loud pop in the control system is usually indicative of a
cable failure.
Paint
Paint provides airplanes with appearance and protection.
FAR Part 43 considers paint as maintenance. As a minor repair
paint must be signed off by the person doing the painting in
the aircraft logbook. Rebalance of flight controls may be a consideration
after painting is completed. If the original paint is removed
and the plane repainted no weight and balance entries are required.
The quality of the paint job depends on the preparation. Chemical
stripping is traditional but is having environmental
problems. Media blasting using plastic beads both clean and remove
surface corrosion while they roughen the surface for better paint
adhesion. Traditional aircraft paint jobs were primed after stripping
with zinc chromate before being sprayed with synthetic enamel,
which cures, by oxidation. Aircraft come from the factory with
acrylic lacquer because it is quick and easy when applied over
a two-part wash primer. Polyurethane is a two-part application
that cures over a two-epoxy primer. Polyurethane dries with a
'wet' look. It is the most durable of all paint finishes.
Wing-walks, steps and some other special places require specialty
paints are available where needed. Low pressure
spraying is beginning to replace high pressures used in the past.
All paint is helped when kept clean. Aircraft quality
waxing is good for all aircraft surfaces and it makes for good
pilot exercise.
Pilot
Induced Oscillations
This is aircraft movement that are either initiated or increased
because of pilot response lag due to sensory perception and response
delays. With experience a pilot can inhibit his behavior to reduce
the oscillations. The classic situation is when a pilot lands
nose-wheel first and has his reactions accentuated by the nose-strut
and yoke movements.
Propeller
The propeller has the same capacity to kill as does the famous
'unloaded' gun. The most dangerous situation is the
malfunctioning magneto switch. This is where the key can be removed
from any position. In some cases the magneto switch fails to
ground the P-lead or the P-lead is broken. Some 'dieseling' can
occur if the spark plugs are badly fouled. A propeller that happens
to be at a critical firing point can be fired by just the wind.
It has happened.
Items:
1. Always walk around propellers.
2. Move the propeller backwards, if at all.
3. Never curl your fingers around the blade when moving.
4. Don't push/pull by the propeller.
5. Don't hand prop.
6. No pets or children allowed by propellers.
7. The magnetos are always ON.
As a metal, aluminum is very intolerant of abuse. Any nick that
you can catch with your fingernail is a potential stress riser
and breaking point of aluminum. All nicks should be removed by
a qualified mechanic. Every annual inspection and 100-hour must
include approval of the propeller for return to service.
Even minor repairs must be made by a mechanic. Major problems
require removal and repair by a certified propeller repair facility.
Internal damage to propellers can only detected by removal and
electronic analysis. While there is no FAA requirement, manufacturers
recommend a 2000-hour re-conditioning. Any change is pitch or
length is capable of changing the vibratory characteristics and
cause unanticipated failures.
Propeller care:
1. Minimum rpm over unimproved surfaces. Keep it moving if possible.
2. Start up over pavement or surface free of lose objects.
3. Make frequent visual inspections.
--Generally a two-bladed propeller is more efficient than the
three-bladed efficient at cruise.
--New technology has made three-bladed propellers equal to the
two-bladed.
--Three-bladed propellers have less vibration, more ground clearance
and lower tip speeds.
--Moving a propeller every week will redistribute oil and water
over the cylinders and reduce corrosion.
Constant
Speed Propeller Differences.
McCauley
Oil pressure failure - fine pitch.
Speeder spring failure - course pitch
Hartzell
Oil pressure failure - coarse pitch
speeder spring failure - fine pitch
There are two possibilities. In the event that the blades move
to fine position, this is OK as you just use the prop as per
a fixed pitch prop. If the engine is developing power there will
be a tendency to overspeed the prop. As long as I reduce MP quickly
this should be OK. Fly as normal and head for the nearest field.
If the prop moving to coarse pitch, should apply power here I
am going to overboost the engine (excessively high MP, low RPM).
If the engine is not failed already, I am going to risk it now.
What would my actions be here? I obviously need to reduce MP,
but will I have enough power to maintain level or even climb?
Do I head for a field or commence decent and initiate a forced
landing.
I realize there is another issue here, in that if my engine fails
and I loose oil pressure the Hartzell prop is going to produce
less drag and increase the glide distance. This is a good thing...
Oversquare
Constant Speed Propeller Operation
One analogy used in this type of operation is related to gearshift
operation in an automobile. With an over square setting of the
manifold pressure in inches at 25 and an rpm of 2200 you are
effectively trying to climb a hill in overdrive. You can't hear
pinging in an airplane.
Q-tip
Propellers
--The 90-degree bend keeps the airflow from going off the end
of the propeller blade.
--It reduces the diameter of the blades by about an inch.
--The propeller produces less noise.
--The propeller stirs up less dust.
Using
Your Non-flight Instruments
The health of your engine is reflected in the oil pressure,
oil temperature, fuel pressure and cylinder head temperature
instruments. Green is in the normal operating range; anything
else should get your attention. Any wavering or near the edge
indications need A&P attention.
At any loss of oil pressure, reduce power. Continental engines
take oil pressure measurements from the below the pump point
used by Lycoming. It is because of these different pressure points
that Lycomings tend to have higher pressure reading than Continentals.
If your aircraft has a thermocouple oil temperature gauge, temperature
fluctuation is probably gauge error. Likewise, a low pressure
without a high temperature is usually an instrument problem.
High temperatures can be the result to too much oil as well as
too little. Low octane fuel can cause higher operational temperatures.
Inadequate airflow over the cooling vanes of the cylinders will
immediately cause high cylinder temperatures to be followed by
hot oil readings.
The engine tachometer may be mechanical or electric to count
the propeller revolutions per hour. The numbers on the tach are
used to determine required maintenance but not hours of actual
operation. Tachometers can have errors in readings that require
adjustment. Running up in a crosswind can cause the tach reading
to vary.
Spark plugs are designed to be self-cleaning. Lead fouling occurs
when engine internal temperatures are too low to completely vaporize
the lead additives in the fuel. At some point small balls of
lead can short out the spark plug. This shorting results in rough
engine operation. The lead can be removed in most cases by increasing
engine speed while leaning the mixture to raise the internal
engine temperature sufficiently to vaporize the lead. Unleaded
fuel can leave additive deposits on the lower plugs as well but
these may be from difficult to impossible to remove by leaning.
Oil fouling will leave the plugs wet. This fouling is caused
by oil leaking past the rings.
--Fuel pumps come in several types. Only a few models of high-wing
aircraft have fuel pumps since gravity can do the job unless
the aircraft is capable of an extremely high climb angle.
--A fuel line break on carbureted aircraft may be indicated by
lost pressure restored by use of the fuel pump. This could mean
that you are spraying fuel throughout the engine compartment.
Shut off the fuel supply immediately.
--Absence of fuel is quickly shown by the fluctuations of the
fuel pressure gauge.
--A fuel line leak exists if there is a loss of power and drop
in fuel-pressure. Fuel-injected pressure gauges are really fuel
flow indicators.
--A carburetor float may stick and flood the engine with sufficient
fuel to make it stop. I had this happen once at low altitude.
I pulled the mixture and the engine restarted. Years later I
learned that this is the appropriate procedure. Worked for me.
The manifold gauge is an aneroid barometer that measures airflow
into the engine. At full power the pressure will approach the
ambient air pressure. A broken gauge or line will cause serious
erratic engine performance.
The Exhaust Gas Temperature gauge is a thermocouple attached
to one or more cylinders that will give a reading of temperatures
in the exhaust system. These temperatures can be used as a measure
of engine fuel efficiency. If a cylinder has a fouled plug of
stuck exhaust valve the EGT readout will tell you.
Angles
--Incidence is the angle from the horizontal fuselage that
the wing is mounted either directly or by strut.
--Decalage (French) is the angle of placement of the horizontal stabilizer
measured as the angular difference from that of the placement
of the wing. There is usually a download on the horizontal stabilizer
that counters the weight of the engine forward of the center
of gravity. Stall the horizontal stabilizer and the nose drops.
The engine or rudder of airplanes may be set at an angle to offset
the left turning tendency of the aircraft due to the engine power
or propeller thrust.
Stability
Of the three axes, it is the longitudinal or roll axis that
is the least stable. Your ability to turn the aircraft easily
depends on this lateral instability. The aircraft resists the
initial input of the ailerons and then will continue with little
or no resistance. This lack of resistance is because the outside
wing is moving faster and if the turn is to the left the power
facilitates the turn even more. Unplanned turns tend to be more
to the left than to the right. If the wings have dihedral the
turn is somewhat self-correcting and the aircraft will tend to
level itself. The Navion comes to mind as a very stable platform
for instrument flying.
The three axes of the aircraft are not equally stable. The pitch
axis has a trim control that lets the pilot adjust the pitch
stability for a desired flight condition. The vertical axis has
a trim control that allows the pilot to relax on the rudder.
However, it is the roll axis that is usually without such a control
with some exceptions.
The roll axis or lateral axis is not stable because the pilot
needs this instability to maneuver the aircraft. We can let go
of the yoke in pitch and yaw for a length of time. Try the same
thing with the bank and subtle differences arise. The ailerons
resist the initial change, then accept it and then maintain their
new position. That is the new position is maintained until the over-banking tendency takes over. The tendency to roll on over
is caused by the outside wing moving faster in the turn and the
propeller's slip-stream hits the rising wing the most. Flaps
increase this effect.
Countering these over-banking effects we have dihedral. Dihedral
is the up slope of the wing from the wing-root to the tip. The
shape of the wind and its sweep also make a difference. Low-wing
aircraft have more dihedral than high wing aircraft. The effect
of dihedral and wing shape will vary with the power and pitch
attitude of the aircraft.
The conflict between the instability and stability of the aircraft
explains why the aircraft can maneuver and will hold one position
only for a few moments. One of the roll stability demonstrations
I use with students is to put the aircraft into a 30-degree bank
and put in anywhere from 1/3 to 1/2 turn of trim and let go.
The aircraft will perform a turn that can be held with just light
touches of rudder. The usual design of light aircraft is to make
the 30-degree bank more stable than any other bank. Less than
30-degrees bank and the aircraft will strive to level off. More
than 30-degrees and the aircraft will eventually roll over.
Static
Stability Test of Worst Case Aft CG Situation
---Load inside CG range for most aft CG
---Trim for any level situation hands-off
---Slow plane by 10 or more knots
---Hand stabilize for the slower speed, release and count oscillations and
time required for level flight
---Repeat trim for any situation hands-off
---Dive plane to increase speed by 10 or more knots
---Hand stabilize for the faster speed, release and count oscillations and
time required for level flight
Flap
Chafe Caps.
A row of small (about 3/8 inch) holes running from the inboard
edge to the outboard edge, and located just aft of the inspection
covers located by the leading edge of the flap. There are
12 on the right wing and 14 on the left wing. Each hole has
a small plug (or cap) installed. They are to prevent metal to
metal chaffing which wears through the metal. The plastic comes
in between the flap and the flap well in the wing and prevents
metal to metal contact and wear.
Static
Wicks
Static wicks are the five inch plastic covered wires the
extend from the trailing edges of selected aircraft flying surfaces.
The function of a static wick is to allow removal of static electricity
from the plane. Static electricity accumulates on the aircraft
as it flies below a charged cloud, precipitation or dust. A build
up of this static electricity will affect your radio reception
or transmission as well as the ADF and other avionics.
The problem can appear as 'white noise' or hissing static. The
placement of the wicks and number of wicks is essential to correct
function. A broken wick should be replaced since it could cause
the a receiver to become desensitized. Aircraft flown in IFR
conditions should have wicks.
Window
Care
…Depth of ;cracks can be measured.
…Pledge works but must be applied often.
…Never use paper to wipe windows.
…Pilots are allowed to cut and fit side windows.
…Thicker windows cut sound.
…There are kits for refinishing.
Flutter
In the 1930's practically every non-war airplane movie I
saw as a child had to do with bad things that happen to airplanes
going fast. The difficulty had to do with harmonic vibration
of various parts of the airplane that
would cause the part involved to self-destruct. In time the problem
was discovered to be a matter of balance.
Every control must be balanced within certain limits in order
to avoid flutter. Even the painting of a control surface can
affect the weight and balance sufficient to create flutter.
When the original aircraft design is subject to flutter problems
the 'mass balance' is used to reduce or modify
the problem. The mass balance is a horn like protrusion usually
projected on both sides of the control with a lead weight at
the end of the horn forward of the hinge line. This device tends
to dampen any flutter tendency in a very simple way.
Flutter is relatively uncommon in general aviation aircraft
but can and does occur when flight loads are flown that exceed
design limits. The flexing of the empennage can be driven beyond
limits by vibration of any part of the tail section. Every part
of an airplane has a natural vibration frequency that once started
can be amplified to the point of destruction. Like a chain, the
part most susceptible will break first. Flutter is much like
a flag whipping in the breeze. Many 'pilot caused' destruction
may well be flutter related. It takes very complex instrumentation
to determine the flutter susceptibility of a given aircraft part.
Metal Fatigue
--Theory is that a metal part may be made to last forever.
--Repeated and variable stress loads fare below design loads can cause early failure.
--Such a failure is said to be caused by fatigue.
--Fatigue is progressive over time.
Physical Forces of Flight
- Lift Vs Weight
- Thrust Vs Drag
- P Factor
- Gyroscopic Precession (Left Turning Tendency)
- The Other Gyroscopic Precession (DG Drift)
- Compass Dip / Turning Errors / Deviation
- Torque
- Slipstream Rotation
- Lift Vs AOA
- Pitch Effects of lowering/raising flaps
- Right Turning Tendency power off
- Altimeter Vs Pressure Changes
- Ground Effect
- Mixture Vs Air Density
- Density Altitude (Coming soon to a hot desert near me...)
- Income Vs Training Expenses
What I'm beginning to realize is that pilots can't eliminate these forces but
rather we develop procedures and skills that allow flying and the laws of
physics to peacefully coexist!
Feel free to add more Key Matchups !
Jay Beckman
Hand Propping
---Do not lift leg when pulling prop down and through.
---Don’t wear gloves
---Do not hand prop if you cannot determine if your system has an impulse
coupler
---If you don’t know. Don’t.
Vortex Generator
----These Are two/three inch long and ½ inch high right angular bits of
metal or plastic strategically fastened to wing and tail surfaces by the dozen
to improve low-speed performance without affecting high speed performance.
---Alters air flow from one state to another
---It alters laminar flow close to the wing’s surface to a whirling and
turbulent motion
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