1
$\begingroup$

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.

$\endgroup$
4
$\begingroup$

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.

$\endgroup$
  • $\begingroup$ Nitpicking: 10-20° is a lot, practically extreme. Much smaller angles will cause significant roll. And this is sideslip angle, not yaw (yaw the angle is the same as body heading and is measured from north). $\endgroup$ – Zeus Oct 30 '18 at 6:45
  • $\begingroup$ Well, the motion to get to the sideslip angle is still a yaw motion in this case, rotation about the yaw axis, as opposed to sideslip generated by bank. Yes you get really high roll rates from just a couple degrees of sideslip with swept wings. You can control a jet quite nicely in roll with little squeezes of rudder. My point is 10 to 20 of degrees of sideslip angle will cause a moderate roll in a straight wing airplane but will flip a jet on its back. $\endgroup$ – John K Oct 30 '18 at 11:31
  • $\begingroup$ I know you know, but you explicitly said "yaw angle exceeds...", which is technically wrong. Yaw angle can easily be 180°. No matter how achieved: bank, gust or rudder input (i.e. whether by yaw motion or not), it is the sideslip angle that matters here. $\endgroup$ – Zeus Oct 30 '18 at 23:19
1
$\begingroup$

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.

$\endgroup$
  • $\begingroup$ Thank you for answering. You might be able to expand your anwer just a bit more. The question has more to do with what aileron input has to be held once the slip is fully established, than what aileron input is need to roll the plane to the necessary bank angle. PS It may be the case that I have a pretty good idea as to how I would answer this question myself but am interested in how other people would explain the situation. $\endgroup$ – quiet flyer Oct 27 '18 at 18:59
  • $\begingroup$ It may be possible in a turning half circle to dump some altitude with ailerons, but holding it might generate too much of a change in relative wind (sink). Birds commonly rock back and forth to lose height. Coordinated S turns also work, especially if it is an engine out approach. The forward slip recovery is "flat", seems easier. $\endgroup$ – Robert DiGiovanni Oct 27 '18 at 21:39
  • $\begingroup$ So are you saying that holding a constant aileron input is not consistent with a constant-heading full-rudder-deflection sideslip? $\endgroup$ – quiet flyer Oct 28 '18 at 0:28
  • $\begingroup$ The rudder is constant (on the 172). With throttle idle and pitch trimmed to 65 knots, the only thing left to do is work the ailerons, so as needed to hold heading. $\endgroup$ – Robert DiGiovanni Oct 28 '18 at 0:41
  • $\begingroup$ Yes, as needed to holding. And in smooth air with no sudden gusts trying to lift one wing, what will be needed? $\endgroup$ – quiet flyer Oct 28 '18 at 14:07
1
$\begingroup$

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.

$\endgroup$
  • $\begingroup$ Appreciate the continuing work on this issue! Let's not forget one more aerodynamic force that will require a "cross controlled" aileron input. This will be a sideways drift caused by the relative wind striking the angled fuselage, wing, tail, landing gear of the aircraft, like wind into a sail. With the 172, this may be a significant portion of the aileron input. You've made your own cross wind, and correct with ailerons. Also important is with throttle idle, the sideways drift will be less. One for the test pilots to check. $\endgroup$ – Robert DiGiovanni Oct 28 '18 at 15:06
  • $\begingroup$ What you refer to as "sideways drift", I would call "sideforce". I would say it is the fundamental reason that the aircraft must be banked to hold heading while the rudder is deflected. Theoretically, a flying-wing aircraft with virtually no side area exposed to the airflow, could fly a constant-heading strongly-yawed (slipping) approach, to align the landing gear with the runway in a strong crosswind, with virtually ZERO bank angle. (In such a case, yaw/slip could be induced by a rudder on a very skinny tail boom, or by airbrakes that opened on one wingtip only.) $\endgroup$ – quiet flyer Oct 28 '18 at 15:26
  • $\begingroup$ And note that in the flying-wing case, to hold that near-zero bank angle, a strong aileron (elevon) deflection in the "upwind" direction might still be required, if the wing has dihedral or sweep. And conversely an aircraft with a flat (no dihedral) mid-mounted wing, and fuselage that has lots of side area, might need lots of bank to hold the heading constant during a full-rudder sideslip, yet little or no aileron input might be needed to maintain this bank angle. (Some r.c. model airplanes intended for "3D aerobatics" fit this description.) $\endgroup$ – quiet flyer Oct 28 '18 at 15:29
  • $\begingroup$ Xtra 300 might be one of them! $\endgroup$ – Robert DiGiovanni Oct 28 '18 at 16:06
0
$\begingroup$

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.

$\endgroup$
  • 1
    $\begingroup$ I forgot to mention spoilers! $\endgroup$ – Robert DiGiovanni Oct 29 '18 at 12:55
-1
$\begingroup$

Ok, welcome back! This maneuver, constant heading and losing altitude (increasing rate of descent) correcting 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 (changing AOA, you are doing an impersonation of Flipper due to flap and slip induced turbulence on your tail).

With the 172, roll coupling with rudder is much less than at cruise speed. To be honest with you, it was barely noticable, so:

You are coming in at 65 knots (trimmed), 20 degrees of flaps, throttle at idle, and: looks like its gonna be past the first 1/3, decision 1. Go around 2. forward slip

ok, never force a landing, go around!

But, 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.

Ok, throttle idle, rudder full stop, pitch set for 65 knots, you are seeing your old aiming point moving up on your windscreen. It's working! 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, the 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, well it's been a while, but I do not recall it being close to what was available in either direction.

Appreciate the more learned response from the writer considering the ENTIRE aircraft in evaluating aerodynamic forces! And we seem to be starting to speak the same language, so: Mira y Escucha, por favor.

Firstly, yes, that's what flaps are there for, no doubt. The scenario is 20 flaps in, throttle idle, 65 knots (not crazy fast), still high. Go around considered, enough runway left for safe landing attempt.

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. But if held too long, it will create a significant amount of sink. That, combined with the need to roll back out of it with ailerons, makes it slightly more dangerous. Recovery from a forward slip uses the rudder, which is always functional even when the wing is stalled.

The side slip resulted in the crash of one of the finest pilots of her age, Hanna Reitsch, when she tried to save a Komet that could not release its wheel dolly after taking off. The increased weight, combined with the interrupted airflow over the wing and change in relative wind when the sinking aircraft rolled out of its slip, caused it to stall.

So, with the forward slip, we use a lot, a little, some, about 1/2, etc. of our aileron to control and recover by centering ailerons and yawing nose back to straight with rudder. We also make sure this is done with ample speed margin and practiced at altitude first.

There are no liars here, just people sharing experiences and finding common ground.

$\endgroup$
  • $\begingroup$ "Second, practice cross controlled stalls at altitude to validate safe approach speed for that maneuver." -- So it IS in fact a cross-controlled maneuver? I.e. the ailerons ARE in fact deflected, if the heading is constant? $\endgroup$ – quiet flyer Oct 28 '18 at 0:24
  • $\begingroup$ Absolutely, yes. $\endgroup$ – Robert DiGiovanni Oct 28 '18 at 0:36
  • $\begingroup$ But-- you said before that there was little or no slip-roll coupling in a Cessna 152 or 172. I'm sorry Robert I think I had better break this off. Comments are not intended for this anyway. $\endgroup$ – quiet flyer Oct 28 '18 at 0:48
  • $\begingroup$ No problem, just very little. When I went from RC to full scale, one of the first things I did was push the rudder on the 172 and see what it did. Hardly anything! Very flat skid. There is some, you mentioned doing rudder turns back and forth, but the roll really needs to be helped with the ailerons, the plane is designed to self correct hands off, and to be turned coordinated. $\endgroup$ – Robert DiGiovanni Oct 28 '18 at 1:00
  • $\begingroup$ Related-- aviation.stackexchange.com/questions/48919/… .I was actually stretching the truth a little to say that I saw a video of this. Yes I saw the video, but I also filmed the video. Slip-roll coupling in Cessna 120, 150, 152, 172 is NOT trivial and is generated primarily by the high-wing configuration. Of this bunch I think the Cessna 172 has the most dihedral; the rest have very little. I''ve also done reversing turns though at least 35 dgrs bnk, usng the rdder pedals only, hands-off the yoke, in all these a/c. $\endgroup$ – quiet flyer Oct 28 '18 at 1:06

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.