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Skids or Slips
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Skids and Slips; ...What Makes a Sideslip? ...Types of Slips; ...POH Advice; ...Slips and Dihedral; …Use of Slips; …The Why of Slips with Flaps; …Slips by Type; ...Slip; ...Revisiting the Slip…Airspeed Indications During Slips; ...Slipping Forward to Side Slips; ...Things to Know about Slips; ...
Skids and Slips
A slip is a turn that is stopped by use of the rudder. The forward slip has the aircraft moving toward the runway with the nose pointed at an angle to the runway. The side slip is used in crosswinds. It has the aileron holding a wing low while the rudder keeps the nose aligned parallel to the runway center line Any increase or decrease in aileron will cause the aircraft to slide to one side or the other of the center line. Both the skid and the slip are uncoordinated maneuvers. A skid, in which the turn is being 'helped' by the rudder is more dangerous. A skid has too much rudder used in the direction of bank.
A slip does not have enough rudder, or more likely rudder opposite to the direction of the bank. The airspeed indicator will be unreliable in either situation. A slip or skid away from the static port will give a high airspeed reading. The forward slip is used to lose altitude; the side slip is used for runway alignment. Both slips can be varied at will. The forward slip is best performed without power since it is using the fuselage to control the descent rate. Aircraft tend to be stable in the slip and easy to control as to airspeed. Inexperienced slippers tend to allow an increase in airspeed. The slip entry need not be abrupt. Lead with the aileron and use opposite rudder to prevent a turn. Incremental additions of aileron and rudder can increase the descent rate. The slip is very easy to control and can be gradually reduced to give a smooth transition into the flare.
In any uncoordinated stall, the rudder will be turned towards the slower wing. This wing will stall first. It is this factor that makes the slipping stall a safer stall. The rudder is toward the high wing that, when stalled must fall through level before dropping into the spin mode. This gives you additional time to correct the problem. In a skid the low wing stalls first. You don't get the time or attitude opportunity that a slip-stall gives before going inverted. Neither stall is something you would want to happen turning final.
Your eyes need to be outside the cockpit when you're on final. You should be trimmed for your approach speed on final. You should automatically apply forward pressure on the yoke in s slip. Being trimmed for my approach speed means that releasing the slip pressures will put you at the correct speed. In a slip the rudder may reduce the elevator authority by 'blanking' part of the airflow. A full flap slip may cause pitch oscillations on Cessnas. You should practice your slipping and skidding maneuvers at altitude.
Regardless of all the flight and aircraft variables at the time of stall break, spinning is prevented by rapid correct use of the rudder. This means rudder applied to prevent any yaw from precipitating into a spin as the nose is moved to break the critical angle of attack. A pilot learns the required rudder for spin recovery by doing the falling leaf-type maneuvers and oscillation stalls., and wild Dutch rolls.
In a skidding base to final turn the possibility of getting a stall spin is
because the excess rudder application causes the nose to drop while elevator
is used to keep it up. G-forces increase, angle of attack becomes greater,
stall speed rises and indicated airspeed drops--BANG..Stall and spin occur as
one. All the forces and factors for spinning are present in a skidding turn.
A slip is a stabilized maneuver in which the airplane is held with rudder, aileron, and elevator in steady flight with no change in attitude on any flight axis. The slip uses the fuselage as a speed brake and angles the wing to reduce lift and its angle of attack. We are being kept in the air by the fuselage's air resistance. Once the slip is set the yaw and roll forces are, unlike in a skid, in opposite directions. The more intense the slip the farther we fly away from any stall /spin situation. Slips are inherently stall/spin resistant unless controls are misused in such a way as to change a slip into a skid.
In a slip or skid the wings will not stall at the same time since one is wing is slower than the other. The slow wing will always stall first. In a skid the low wing will always will stall first. The stall will cause it to tuck under even more and initiate a 'spin.out the bottom'. Use of ailerons to raise the wing will only aggravate the spin..
In a slip, the high wing will stall first since it is moving more slowly.
When the high but slow wing of the slip stalls it must drop through level.
This breaks the stall and centering the rudder removes the lateral imbalance.
A stall in a slipping turn can become a spin if the wing is allowed to drop
through level. The resulting unexpected 'over the top' spin will get your
What Makes a Sideslip?
In a sideslip the relative wind has an influence from the side of the aircraft. This wind presses on all the vertical aircraft surfaces to one side. Because the vertical surfaces are not evenly distributed over the length of the aircraft, this relative wind from the side causes the aircraft to turn or yaw about its vertical axis. If the wings have dihedral this wind initiates a roll as well by giving one wing more lift than the other.
---A relative wind from the right causes the aircraft to swerve left
---The right yaw comes from the aircraft turning into the relative wind.
--- A roll is caused by dihedral
---A right sideslip causes the nose to pitch up due to propeller effect.
---A gust of wind from the side can cause all of these during the flare.
As part of your flight test you may be expected to both explain and demonstrate the slip. The forward slip in a C-150 is a power off, no flap, cross control, and low speed maneuver. (There is no logic to slipping to lose altitude with power on.) (Cross control means that you may be using right aileron to lower the right wing while applying left rudder to move the nose to the left.) The purpose of the slip is to produce a dramatic increase in the rate of descent. The plane is flown sideways to increase the downward slope of the glide path without increasing the airspeed. The slip does this by using the side of the aircraft against the direction of the relative wind as a brake. The performance of the slip requires the pilot to apply carburetor heat and take all the power off. Having power on during a slip for purposes of descent is self-defeating. The aircraft is trimmed for 60 kts with three down turns of the trim wheel. The wing is lowered into the wind and opposite rudder is applied. The yoke is pressed forward to maintain 60 kts. The further the wing is lowered with corresponding more opposite rudder, the more rapid will be the altitude loss. You can practice the skills required in side slipping by doing Dutch-rolls and forward slip skills at altitude with some power and following a ground reference line.
When a turn is initiated with inadequate or even opposite rudder the result is a slip. The amount of bank is commanding a turn greater than the nose is making. The aircraft is leaning and sinking, not turning. A slip is unpleasant in the way it makes passengers feel.
The slip causes errors in airspeed indications. Some of this is due to the angle of the pitot tube but most of it is due to the location of the static air hole. With the static port on the left and a slip to the left the static pressure (now partly dynamic) is increased. This causes a decrease in the indicated air speed. Your indicated air speed is lower than you are really going. A slip to the right reverses the effects and indications. Your indicated airspeed will be higher than you are really going. Be more aware of an adverse situation developing (Cross control stall) while slipping to the right. Always anticipate the decrease in indicated airspeed by applying forward yoke pressure to maintain at least 1.3 Vso. I suggest that this pressure not be trimmed off since you will be returning to coordinated flight shortly. If landing in a crosswind you should always initiate a forward slip into the wind and at approach speed. Equal effects of aileron and rudder cause the plane to move forward while flying sideways. This will make the transition into the crosswind sideslip easier with the correct wing already low and partial removal of rudder used to straighten the nose. The canted lift of the raised wing is used to offset the drifting effect of the crosswind.
At 1.3 Vso a full cross-controlled slip can be maintained without stalling. Do as many slips to the right as you do to the left and note the difference in airspeed indications. Flying indicated airspeeds when the pitot tube is not directly aligned with the direction of flight provides a margin of additional safety. Get used to the sound, feel and yoke pressure requirements of the slip. Maintain your ground track and use a simultaneous relaxation of rudder and aileron to set pitch for approach speed. Examiners like good slips.
The beauty of the slip is how easily it can be controlled. The angle of slip can be delicately controlled so as to affect the rate of turn and descent. The effect is real time and not delayed as with flaps and power. The aircraft is not flying normally nor as designed in a slip and will come out of the slip once pressures are removed. Yet, the aircraft is both stable and controllable in the slip.
The smoothest way to enter a slip is with an initial application or aileron soon followed by opposite rudder. The ailerons starts a turn, the rudder stops it. It is the side of the airplane that is turned toward the direction of flight. The side is not aerodynamically designed for flight and acts as a brake that both slows and causes a descent. When the rudder is applied so as to direct the nose parallel to a runway while preventing a turn initiated by the aileron we have a sideslip.
Coming out of a slip can be as smooth as is the entry. Such a slip can be carried to a height lower than would be used in a normal flare. By not leveling the wings until the last moment we can exercise greater control for the actual touchdown.
Types of Slips
Of the two different slips, the forward slip is used to augment any approach path available through the use of flaps. The sideslip is used to cancel out any crosswind effects to achieve and maintain runway alignment. Both of these slips cause the aircraft to descend more rapidly. The slip is a variable descent control.
Forward slips: There are two slips. The forward slip is used to lose altitude. It is used when the wind is nearly down the runway. Slipping uses the side of the fuselage to drag the plane into a steeper descent. The more the side is exposed to the relative wind by rudder application the steeper the descent angle. Smooth, simultaneous control inputs are the key to successful slips.
Most every aircraft will slip but some slip better than others do. This is one area in aviation where older tends to be better than younger. The best slip speed is critical and difficult to determine since in the slip the air strikes the pitot tube on an angle. At altitude, you should practice both right and left slips with full rudder applied and develop a chart of airspeed to rate of descent for use closer to the surface.
The slipping process requires a smooth application aileron followed by sufficient opposite rudder to keep the aircraft from turning. Continue the process until you have the descent rate desired or you run out of rudder authority. Like the entry, a smooth removal of aileron and rudder pressures will accomplish the slip recovery. Properly controlled the slip will transition directly into the landing flare at the appropriate altitude with required crosswind correction. The properly controlled slip to a landing is a thing of beauty.
The forward slip for purposes of losing altitude into a short field or over an obstruction is pretty much a lost art due to the existence of flaps. As later models of Cessna decrease their flap settings the forward slip once again becomes useful. Especially so if the windshield is obscured by ice or oil.
My students and I find great satisfaction in the controllability of a
smoothly performed slip. I personally prefer to slip all the way from abeam
the numbers around to the landing. Aircraft can be slipped nicely in a turn.
However, some examiners frown on such slips so I additionally instruct slips
that are performed only on final. Slips give beautiful visibility and control
down to the landing. It does take some experience and practice to time the
flare. When slips are performed without flaps as is often required by the POH
there may be considerable float. Timing this float for a precision touchdown
is not easy. The greater the headwind the better the forward slip in losing
altitude. The cross-controls are applied but the nose is held at an angle to
the runway for maximum altitude loss. The greater the control deflection the
greater the slip effect or descent rate. When the runway is made, cross
control application is removed to allow the aircraft to straighten to the
runway alignment. This sideslip is used in crosswind landings to align the
aircraft with the runway. It also continues the descent to landing.
The sideslip is used for runway alignment. The sideslip consists of half a Dutch roll. The rudder holds the nose straight while the wing is lowered to slide the aircraft to one side or the other to center on the runway center line. Skill in the sideslip is very important since most student pilots tend to land off the runway centerline.
The sideslip can be practiced by practice descents along straight lines. They can be practiced on long straight-in landing approaches. The most important phase of the sideslip can only be practiced during actual landings. Getting aligned with the centerline often requires that sideslips be performed close to the runway even when the runway is not in sight. It takes a special wide peripheral view over the nose to maintain alignment by watching the nose's relationship with the visible horizon to each side.
When an aircraft has a POH or a placard that says that slips with flaps should be avoided this means the same as prohibited to the FAA. If you are in a situation where a slip with flaps is required for a safe landing, do so only as an emergency procedure. Go-around is your best option and you need the practice. A well-controlled slip is a maneuver of joy. The sink rate is controlled at will. The transition from one slip to another requires different control pressures to assure that the nose attitude will give the required indicated airspeed for approach. The difference between the slips is the control pressures applied to achieve the intended purposes. A full forward slip will require application of controls at an indicated speed in the forties to achieve a maximum descent rate for ground covered. The rudder is used to hold the fuselage sideways while the aileron controls the flight direction. The most effective forward slip is with the nose at 90-degrees to the wind direction.
Slips and Dihedral
When we first read about dihedral we learn of the geometric kind where the tips of a flying surface is higher in a horizontal plane than the base. There is another kind of dihedral known as effective dihedral. Effective dihedral occurs when an aircraft is slipped with one wing low and opposite rudder. Effective dihedral differs between high and low wing aircraft by its effect on the roll response of the aircraft. Dihedral is said to have either a positive or negative effect on the roll of an aircraft. Positive dihedral effect causes an aircraft to want to level off out of the roll. Negative dihedral effect causes it to increase the roll. High winged aircraft are positive while low winged aircraft are negative. This is caused by the flow of the relative wind over the wings of the different types. Since the positive effect is more to be desired you will find that geometric dihedral occurs in more low-winged aircraft to offset its inherent negative dihedral effect.
A pilot in making a slip is faced with dihedral effect from all the flying surfaces. He uses only the accumulative effect to maintain a specific slipping condition. There is always sufficient elevator authority to over-control the rolling effect of dihedral.
Use of Slips
A slip is a turn that is stopped by use of the rudder. The forward slip has the aircraft moving toward the runway with the nose pointed at an angle to the runway. The side slip is used in crosswinds. It has the aileron holding a wing low while the rudder keeps the nose aligned parallel to the runway center line Any increase or decrease in aileron will cause the aircraft to slide to one side or the other of the center line.
The forward slip is used to lose altitude; the side slip is used for runway alignment. Both slips can be varied at will. The forward slip is best performed without power since it is using the fuselage to control the descent rate. Aircraft tend to be stable in the slip and easy to control as to airspeed. Inexperienced slippers tend to allow an increase in airspeed.
The slip entry need not be abrupt. Lead with the aileron and use opposite rudder to prevent a turn. Incremental additions of aileron and rudder can increase the descent rate. The slip is very easy to control and can be gradually reduced to give a smooth transition into the flare.
The Why of
"Slips with Flaps"
The 'Slips with Flaps' controversy stems for POHs that have a warning statement such as, "Steep slips should be avoided with flaps settings greater than 20 degrees due to a slight tendency for the elevator to oscillate under certain combinations of airspeed, sideslip angle, and center of gravity loadings."
This warning cannot be considered a prohibition else a pilot would not be allowed to make conventional crosswind landings with flaps extended. Earlier C-172s with 40-degrees flap extension tend to be more subject to this warning that do later models with the extended rudder faring. A pilot is well advised to used an aircraft specific operations manual. It is not a dangerous condition and is easily stopped by relaxing yoke pressure.
I and several other experienced pilots have experienced the nose down pitch
that occurs when the flaps blank out the airflow over the horizontal
This is abrupt but easily recovered from in a couple of hundred feet. Over the
years of Cessna aircraft production the Pilot Operating Handbook has grown
more and more extensive in advising on allowable flight procedures. This has
been a direct result of lawsuits and insurance settlements. One of these
additions to the POH has been regarding the use of flaps.
Prior to the invention of flaps, all aircraft would control their descent, along with the use of power and airspeed, through the use of slips. A slip is a cross-controlled right-left or left-right application of rudder and aileron that through its lack of coordination uses the side of the aircraft as an air brake. As the slip decreases forward motion, the aircraft's rate of descent can be increased dramatically. A slip is most often applied without flaps, but not necessarily so.
It was discovered that an occasional abrupt application of a slip when a high-wing Cessna had full flaps would cause a bobble or wavering of the nose. There has never been, as far as I can discover, an accident from this event. Never the less, Cessna Corporation in an effort to reduce the potential financial impact of an accident, appended to their POH a remark to the effect, "Slips with flaps not recommended."
Over the years a broad series of interpretations of this mild warning have led to exaggerated statements and even prohibitions. These have been spread by the uninformed, misinformed, deluded, and imaginative until it has become a matter of aviation folklore. The misconceptions arising from this folklore has led to a failure of many pilots to learn and use what can be a very useful aspect of aircraft control. The "Slips with Flaps' T-shirt is a small effort at facing up to the teaching and learning problem derived from the lawyer instigated statement. The number of my aircraft as it appears on the shirt is in recognition of my contribution to the teaching and learning of flying. I am honored.
Slips by Type
In Canada they do three types of slips for the flight test.
Increases rate of descent without increasing airspeed. This is an essential skill for losing excess altitude in aircraft and can usually be accomplished in any flap configuration. When performing the forward slip the longitudinal axis is at an angle to descent path. The side of the aircraft increases drag and gives higher rate of descent with the engine idling. You want to lose altitude without building up speed. The maneuver is best accomplished by lowering the upwind wing and at the same time applying opposite rudder. In unison the two controls are used to keep the plane's line of descent aligned with the runway. The plane's heading will veer towards the lowered wing so compensating rudder is used to maintain the desired descent line.
The primary use of the side slip is to establish and maintain alignment with runway in crosswind. The aircraft longitudinal axis is kept parallel with runway centerline. With change in altitude the crosswind effect varies so that the combined use of rudder and aileron are used to slide the aircraft side to side to counteract any crosswind changes. Parallel alignment is kept with the rudder while variations of bank angle are used to compensate for changes in crosswind effect. By lowering the upwind wing into the wind and using enough opposite rudder to maintain runway alignment very small adjustments will be needed. This particular slip requires very uncoordinated use of both the rudder and aileron. This segment of the slip is best practiced by doing Dutch rolls. If you need to think of what you are doing you won't do it easily or correctly. If you run out of rudder authority needed to keep the plane aligned with the runway you can increase the airspeed as required. At some point you may not be able to keep the nose aligned and centered. Go around and go elsewhere. You have exceeded your piloting skills and are probably exceeding the crosswind capabilities of the plane as well.
The slipping turn is a slip performed while turning. The desired turning path and rate of descent can be easily controlled through uncoordinated use of rudder and aileron. The slipping turn is an additional landing tool to be used in descending and arriving at a specific point prior to roundout, flare, and touchdown.
It is commonly used at unfamiliar small airports where poor visual factors or obstructions make it necessary to lose excess altitude. Landing in this type of situation can be practiced at large airports by requesting a short approach at every opportunity. This is especially important for IFR trainees out of the-circle-to-land approach.
A stall in a slipping turn will cause the high wing to drop quickly, this will simultaneously stop both the turn and the stall. The stall occurring in a skid will aggravate both the stall and the turn while entering a spin.
I usually use all three slips when descending in the pattern. By flying the indicated air speed in training Cessnas at 60kts. I have found that the impact angle of the relative wind is such that with that much indicated airspeed, I have never, repeat never, had any indication of a stall.
However, I once had a DE question the use of the slipping turn to a short approach landing of one of my PP applicants. Never did find out what his problem was. Of all the low altitude maneuvers the various slips are the most controllable and enjoyable of maneuvers.
For students who have trouble aligning with a runway from far out, I will have them do side slips back and forth to each side of the runway clear down to very short final. I do teach slips with flaps with recovery at roundout altitude. Abide by any POH that you find restricting the endurance time for holding a slip.
The airplane is not difficult to control. However, it may be difficult for you to control because, when you come out of a slip, you are doing several things at one time.
To enter a slip you start a bank, preferably with proper coordination. You know the coordination is proper because the airplane will bank to the angle you are trying for with the nose remaining on the same heading or spot on the horizon.
As soon as the bank is complete, when it is properly coordinated, the airplane will start to turn in the direction of the bank. When you are properly entering a slip as the bank reaches the desired angle you are putting in the opposite rudder. When it is done correctly the airplane will not turn at all and the nose will swing in the direction opposite the turn exactly enough to keep you flight track from changing. The tendency of most airplanes is to pitch up somewhat in a slip. This is countered by smoothly applying a bit of forward pressure as the nose starts to swing away from the bank direction. Apply just enough to maintain the pitch attitude constant.
Basically, to initiate a slip, you start a turn normally and then proceed
to do everything else just the opposite of what you would do in a turn. In a
normal turn you do NOT use rudder to move the nose in the wrong direction, and
you add a bit of BACK pressure to the controls to provide the increased lift
you require for a turn. This means that, in a slip you are making precise and
smooth control inputs that are the direct opposite of the habit patterns you
have carefully inculcated! That requires some thought and effort. :-)
Coming out of the slip the same is true, but in the reverse sequence. To exit a slip you ease off the forward pressure while easing the pressure on the rudder just sufficiently to bring the nose back in front of you. While the nose is returning you are removing the bank so that you will arrive back in wings level flight without changing the pitch attitude or wobbling the nose around. This requires a precise coordination of UNUSUAL movement of the controls.
In my instruction of crosswind landings I endeavor to do as many in the
left pattern as in the right. There is a madness in this method.
When the wind
is blowing you into the runway you can hold the turn all the way into the runway and put in opposite rudder for the sideslip to the runway.
the other hand if the crosswind is blowing you away from the runway you
will need to reverse the wings from high to low and low to high to set up the
required sideslip to establish runway alignment on final. Remember your sideslips are used to maintain runway alignment. Do some "handee" patterns while walking a pattern to see the maneuver differences required.
Eventually you will stop thinking about control movements or pressures and just do what you have to do to put the airplane in the position you want, without having to think about it. You will think about what you want the airplane to do and your hands and feet will carry out your orders without conscious intervention! Then you will not encounter that problem.
I suspect the problem you (the student) are having is making these
uncharacteristic control changes smoothly and with the proper amount of
"togetherness." :-) It will come. --
Indications During Slips
Excellent question. With the static air hole on only one side there will be indicated differences in identical slips to each side. The critical airspeed factor is when will the stall occur not what you have as airspeed. Perhaps you want to find out at altitude. With any yaw the indicator is somewhat inaccurate. The unknown factor is how much. Very important for you to realize that the air striking the pitot does not enter the pitot. It merely presses upon the air already in the tube. I do not know if any aircraft has been equipped so that airspeed in a slip/skid could be accurately measured. For practical purposes it is relatively unimportant. Suffice it to say, slips are good, skids are bad.
The key is what happens due the relative airspeed inaccuracy in a slip compared to a skid. In a skid the bottom wing tucks under when it stalls and puts you into a spin. (Think crosswind base to final turn.) However, in the slip the high wing will stall first. Meaning that your wings become level at least for a while. No immediate spin entry unless severely aggravated.
Perhaps the entire question could be resolved by during a series of slips and skids at various indicated air speeds and comparing these with GPS readings. Hey, you could write a significant aviation paper using the information.
Slipping Forward to Side Slips
--The good landing is near the centerline and in the first-third of the runway.
--The turning slip base to final gives better visibility, rate of descent and float control.
--The forward slip is used to increase the rate of descent into a shorter distance.
--The forward slip is made power off and putting the side of the aircraft into the wind.
--The side slip is half of a Dutch roll and used to slide an aircraft from either side of a runway into the center.
--A slip is a controlled uncoordinated turn that increases drag.
–A slip is entered by applying aileron for a banked turn that is prevented by rudder application.
--Some aircraft have slip restrictions due to un-porting of fuel lines or controllability problems.
--In a slip the high wing will stall first and allow time to recover.
--In a skid the low wing will stall first and will tuck under the aircraft into a spin.
–Airspeed will read lower when slip is away from the static port.
--Safety factor in a slip is to apply forward yoke pressure.
--Recovery from a slip requires only a smooth removal of rudder pressure.
--Passengers do not enjoy slips.
--The fastest descent is by maintaining a 45-degree banked dive without flaps.
Things to Know about Slips
---Watch your local airport landings, fewer than 30% land in the first third of the runway.
---When fast and high, slow down and slip power off into the prevailing wind for losing height.
---Slips are a more precise means of reducing energy.
---Slips are controlled and controllable uncoordinated turns
---Slips and non-use/misuse of them are in the top two reasons for checkride failures.
---The forward slip is used to lose altitude by using the side of plane to produce drag.
---Altitude loss is determined by wind velocity and direction, airspeed, bank and rudder authority.
---The drag produced by a slip is both variable and controllable.
---Use of power in a forward slip is counter productive to losing altitude.
---Use of power in a side slip increases rudder authority.
---The side slip to correct for crosswind drift correction.
---You do not know how to side slip if you must think about how and what to do.
---Cessnas may oscillate under certain conditions of slips with flaps.
---Prolonged slips may cause tank fuel pickups to un-port under certain low fuel conditions.
---Safety of a slip lies in the fact that at lower speeds a stall will first occur and drop the higher wing.
---Holding the rudder in a stall derived from a slip will result in an over-the-top spin entry.
---Relaxing the rudder in a stall derived from a slip will result in an immediate recovery from the stall.
---In a skid the low wing stalls first and gives an immediate spin entry.
---The slip is an additional tool in controlling a stable descent other than power and airspeed.
---Running out of rudder is the limiting slip control.
---The slip is one the best options for losing altitude in an emergency employed as with flaps or turning.
---The slip is not as fast as is a steep bank in a diving turn for losing altitude in a hurry. Use flaps, too.
---The slip is instantly recoverable but a smoother release of rudder is more comfortable.
---Slips into the static port gives a false high airspeed, away gives false low airspeed.
---Extreme and prolonged slips are uncomfortable to those not performing the slip.
---Improper use of slips is second most common reason for flight test failures. First is precision landings.
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