Slips by Type
In Canada they do three types of slips for the flight test.
Forward Slip
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.

Side Slip
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.

Slipping Turn
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.

Slip (Opinion)
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.

Revisiting the Slip (Opinion)
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.  

On 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" patterens 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. --
Highflyer

Airspeed 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 unport 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.

 
Continued on Stall Origins and Performance

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