Do you typically have to maintain an aileron deflection in a constant-heading full-rudder sideslip in a Cessna 152 or 172? If so, in which direction? And why -- what is the source of the roll torque that needs to be countered>
Note that aileron deflection is not the same thing as bank angle.
Yes. If you apply full rudder with no aileron, you initially just get a flat skid, but then differential lift starts to have an effect and the inside wing drops because that wing has slowed down, the fuselage is obstructing some of the flow on that side, and if there is wing dihedral, it creates a geometric angle of attack differential. One or more of these factors generate the rolling torque. It's called "roll/yaw couple".
Some airplanes roll with rudder-only input stronger than others. On swept wing airplanes the effect is intensely magnified due to the wing sweep and they roll really hard once the yaw angle exceeds 10 or 20 degrees. Pilots doing jet type ratings discover this right away on their first V1 cut (engine failure) on take off when they don't correct the yaw from asymmetric thrust fast enough and it's taking full aileron just to keep from flipping over. Side slips are generally not done in swept wing airplanes except in some extreme circumstance.
So the amount of opposite aileron to maintain a constant heading side slip is a function of how strong the airplane's roll/yaw couple is. A 172 or 152's roll/yaw couple is moderate and you will need some opposite aileron just to keep the wings level. Plus, to prevent a skidding turn, the amount of aileron has to be more than the minimum required to keep the wings level because some opposite bank is also required to keep the heading constant or else the skid itself starts to pull the nose around. So you end up with quite a lot of aileron in, maybe one third to one half of the available travel, with the wing banked perhaps 10 to 20 degrees, to keep from turning.
There is a lot of variation from airplane type to airplane type and on some the roll/yaw couple is quite weak and it takes only light aileron inputs to hold heading in a skid. On any airplane you are flying for the first time, this is something you experiment with to find out how strong the roll/yaw couple is and how much counter aileron is required in a side slip.
Say you are sideslipping so you have a clear view of the runway thru the pilot side window. You turn the yoke left, so left aileron up/right aileron down, that would start a left banked turn. Then you kick in right rudder to force the tail to move left and counter the left turn of the ailerons. I do that all the time in my 177, it's a fun way to lose some altitude quick when you realize you're too high on final and don't want to float way down a short runway.
The amount of sideslip that results will depend on how much aileron and how much rudder is used.
Amount of aileroninput depends on the amount of slip desired. Straighten the ailerons and rudder and the plane straightens out. So yes, it must be maintained.
Comments have referenced a technique involving holding a constant rudder input and working the ailerons as needed to stay on heading or on course.
Holding a constant rudder input and working the ailerons as needed to stay on the runway centerline is EXACTLY what I often do when landing a rc model airplane in a crosswind. Heading does not stay exactly constant, but close enough to avoid a groundloop. The ailerons are generally deflected against the rudder, but sometimes they may be briefly deflected in the same direction as the rudder during a major course correction involving a large reduction in bank angle or even at times a brief shallow bank in the SAME direction as the rudder is deflected.
Sitting in the real thing, making corrections in turbulent air to stay exactly on centerline and exactly on heading tends to require simultaneous changes in the deflection of the rudder and the deflection of the ailerons. If the controls are worked in together in a smooth manner to make corrections early, it will be rare that the position of the controls will have to modified so much (from their AVERAGE position during the slip) that the ailerons will ever be deflected in the SAME direction as the rudder. Certainly the aircraft will never be allowed to get so off course that it will be necessary to actually bank it in the SAME direction as the rudder is deflected to straighten things out.
So back to basics-- in smooth air, with no need for corrections, what aileron input will be needed to maintain constant-heading slip? That is what the question intended to ask and my answer is that an aileron input AGAINST the rudder input will be required.
This may be helpful to keep in mind: there is no conflict whatsoever between these three statements:
A) One way (not necessarily the best way) to execute a sideslipping approach is to maintain a constant rudder deflection and adjust the ailerons as needed for course corrections-- and sometimes the ailerons may be briefly deflected in the SAME direction as the rudder as the aircraft is banked/ turned back onto the desired course via a reduction in the average bank angle being maintained during the slip.
B) In the same aircraft, when the rudder is deflected, the dihedral wing geometry, high wing configuration, sweep, etc generate a roll torque in the "downwind" direction, i.e. in the same direction as the yaw string deflects, i.e. opposite to the direction that the slip-skid ball is displaced, i.e. opposite to the direction that the rudder is deflected. This roll torque must be opposed with an aileron input opposite to the rudder input if net roll torque is to be zero.
C) During a course correction during a sideslip, a pilot might perceive that he is temporarily deflecting the ailerons in the SAME direction as the rudder is deflected, when he is actually just bringing the ailerons closer to centered to allow the aircraft to roll toward a shallower bank angle.
To answer your first question briefly: I asked a friend who flies a C172, he says that he needs a lot of aileron in the opposite direction of the rudder deflection. But he also mentioned that you don't need to slip the cessna, if you put flaps to full you have more drag than you need.
The reason why the aircraft wants to roll in a side slip has several causes, one is that the vertical stabilizer is above the center of gravity and even with the rudder deflected (which would counter the roll) it creates quite a bit of torque. If you do a sideslip and then apply full rudder against the sideslip you are going to roll further because the v-stab then creates even more lift and even more torque to roll the aircraft. Other factors are the fuselage blocking inflow of the trailing wing, the wing dihedral that causes leading wing to have a higher angle of attack, the slipstream of the propeller hitting only one wing and at idle it usually just causes turbulent flow and could disturb the trailing wing flow more which causes a loss of lift, the lift tends to wash towards the trailing portions of a wing (e.g. in swept wings you have a higher lift coefficient towards the wing tips), the fuselage shape plays a roll, and I can probably think of another 5 reasons. This all depends greatly on the aircraft design. Even the wing struts could have an affect here.
This maneuver, with constant heading and increased rate of descent to correct a high glide slope, is known as a "forward slip".
First, before attempting, speak with a qualified instructor familiar with the specific type of plane you are flying.
Second, practice cross controlled stalls at altitude to validate safe approach speed for that maneuver. The 172 is extremely forgiving, but this must not be done at marginally low approach speeds (if you are high, you will rarely be slow, I did mine at 65 knots).
Thirdly, be mindful that the POH does not recommend trying this at 30 degrees of flaps as a tail pitch oscillation will result, due to flap and slip induced turbulence on your tail.
With the 172, roll coupling with rudder is much less than at cruise speed, so:
You are coming in at 65 knots (trimmed), 20 degrees of flaps, throttle at idle, and looks like it will be past the first 1/3 of the runway, decision: 1. Go around 2. forward slip
If you are not too high and elect to do a forward slip in a 172, push the rudder all the way over. This would probably be lethal in a low wing, dihedral aircraft due to roll coupling, but the 172 yaws with much less roll.
You will be seeing your old aiming point moving up on your windscreen. The aileron is used to hold line with the runway, you just work it back and forth. Once your (shorter) aiming point is achieved, smoothly center ailerons and rudder first, then use pitch to break glide path and round out for flare and landing.
You should not need to stay cross controlled while applying elevator.
As far as how much aileron is needed in the maneuver, I do not recall it being close to what was available in either direction.
We want to SAFELY increase our rate of descent. Why full rudder deflection? To maximize drag while maintaining safe speed and AOA. Ailerons are used to counter the drifting and rolling tendency created by side force. This is a FORWARD SLIP. It could also be called a "flat or skid" slip. It works.
Now to consider the SIDE SLIP. Rather than increasing drag, we reduce VERTICAL lift component by rolling with ailerons. This can be done very gently to coax a plane floating in ground effect down to the runway.
Recovery from the forward slip uses the rudder, which is always functional, even when the wing is stalled.
