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Lateral control at Stall

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Lateral control at Stall

Old 30th Nov 2020, 12:21
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Lateral control at Stall

Hello fellow PPruners

I would like very much to get your valuable inputs on the following question: do you agree with that statement: "At stall, to counter a wing-drop phenomenon, on an airplane with straight wings you use rudder, and for an airplane with stretch wings (or delta), you use ailerons" ? And of course agree or disagree, why?

Thanks a million as always,
Leo
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Old 30th Nov 2020, 17:23
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Neither as a test pilot nor as an instructor would I advocate *correcting* a wing drop in a straight wing aeroplane with rudder, unless there was some known unusual characteristic of the aeroplane. I would advocate using rudder to keep the aeroplane in balance, which should minimise wing drop, but it should not be "picked up" with anything. Whatever bank angle occurs should be picked up with aileron - co-ordinated as required with rudder, but only once the aeroplane is clearly unstalled.

Plenty of people here with several orders of magnitude more swept / delta time than I have. Whilst I *think* I know the answer, I'm going to wait for a grown up, then see if I was right.

G

Last edited by Genghis the Engineer; 30th Nov 2020 at 22:24.
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Old 2nd Dec 2020, 13:42
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On a swept wing aircraft it is exactly the same as described by Genghis.
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Old 2nd Dec 2020, 17:00
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I had a student (RAF Bulldog) who had previously done some private pilot training at a civvie flying school. He repeatedly used aileron to try to keep the wings level, even after feeling the stall buffet.

I eventually decided to show him (at a safe height) what could happen if he didn’t get out of this habit and if he ever did this in the finals turn. I got him to follow me through. As the aircraft hit the buffet a wing dropped and I tried to pick it up to wings level with a small amount of aileron, without removing the buffet. Nothing happened so I used slightly more aileron movement. The aircraft rolled rapidly inverted towards the down aileron and we lost a lot of altitude. Point made. He lost the habit. He went on to fly the Tornado.

Last edited by ShyTorque; 2nd Dec 2020 at 23:40. Reason: Grammar
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Old 2nd Dec 2020, 23:27
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ShyTorque, I blame the awful duo of the PA28 and C172, two easy, docile touring aeroplanes that suit the financial model of civilian flying schools very well. Unfortunately they are also far far too benign and forgiving to be good training aeroplanes, and have trained generations of pilots with insufficient respect for what happens if you abuse an aeroplane, particularly at high AoA. There was an issue a few years ago where EASA and FAA went, quite rightly, on a hunt to eliminate "powering out of the stall" from civilian schools where this had originated from a combination of a very dodgy interpretation of requirements in the PPL and CPL skill test / checkride requirements to minimise height loss in the stall recovery, and training aeroplanes sufficiently docile as to let them get away with this.

And it had been going on long enough that students who learned that way had become instructors in their own right - I think it's been stamped out now, but you don't need to think hard about the consequences of somebody trying to do that in a T67M260, or even a Bulldog or PA38, to realise how problematic that had become.

G
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Old 2nd Dec 2020, 23:49
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Genghis, strangely enough, I had previously done an RAF Flying Scholarship on the C150 and C152. In those days we were required to practice spin entry and recovery, which was later removed from the basic PPL syllabus so I’d say it was more a product of incorrect instruction than aircraft type. Possibly resulted in a generation of instructors who were reluctant to see anything more “exciting” than a few degrees of bank anywhere near the stall.
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Old 3rd Dec 2020, 09:15
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My PPL GFT was the reverse of ShyTorque’s story: having first been trained on Bulldogs to keep the aircraft balanced and then recover wings level only after unstalled, my examiner pulled me up on this and we had an inflight disagreement where he wanted the wing drop to be removed/lifted by rudder and wouldn’t accept that this could be a pro-spin input. I did it his way and passed the test, but remained concerned that this seems to be the civvy way.
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Old 3rd Dec 2020, 10:37
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Dan, that rather confirms my theory. I think the difference between what the military teach (or taught, it’s been a very long time since I moved on) is that the emphasis is slightly different. Perhaps some think of the stall and recovery as a tidy, set piece exercise, rather than what it’s really all about. Obviously an inadvertent stall is unlikely to occur in ideal, wings level circumstances (!) and the ingrained recovery technique should reflect that.
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Old 3rd Dec 2020, 11:41
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Present civilian teaching (at least the EASA CPL skill test and FAA CP checkride) does not take the aeroplane to the stall at-all, it is recovery at stall warning.

The PPL is supposed to include a stall, but when instructors have been trained for their CPLs not to ever take the aeroplane past stall warning, there is a clear direction of travel there.

Personally I am content seeing the spin left as theory only for the PPL, but when the CPL doesn't even include an aerodynamic stall, let alone a spin, there's stuff going on there with which I'm personally far from happy. (For completeness, it should be said that the instructor course both sides of the Atlantic does include demonstration of the spin, but no more than that, and surreally without any of the usual precautions - such as a parachute - that would be required for all other flying; the FAA has a specific exemption that parachutes need not be worn for spinning for instructor courses only. I freely admit to finding that a classic triumph of expediency over good practice.)

G
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Old 3rd Dec 2020, 20:50
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Originally Posted by Genghis the Engineer View Post
Present civilian teaching (at least the EASA CPL skill test and FAA CP checkride) does not take the aeroplane to the stall at-all, it is recovery at stall warning.
The PPL is supposed to include a stall, but ......
In Australia, CASA increased the suite of stall exercises plus various entries to incipient spins with their new Part 61 rules in 2014. Eventually saw that was not appropriate and issued Advisory Circular AC61-16 Spin avoidance and stall recovery training earlier this year. The Part 61 rules are yet to be amended. CASA's Flight Instructor Manual still as it was aeons ago with "full power is applied at the commencement of recovery", hopefully to be changed before long.

Yesterday I encountered a flight instructor who told me that the recovery from a stall in a climbing turn would require full rudder against the expected wing drop. I can't retire just yet.

Originally Posted by Genghis the Engineer View Post
(For completeness, it should be said that the instructor course both sides of the Atlantic does include demonstration of the spin, but no more than that, and surreally without any of the usual precautions - such as a parachute - that would be required for all other flying; the FAA has a specific exemption that parachutes need not be worn for spinning for instructor courses only. I freely admit to finding that a classic triumph of expediency over good practice.)

G
Australian flight instructors require an upright spin endorsement as a prerequisite. We generally don't use parachutes for basic spin and aerobatic training here.
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Old 4th Dec 2020, 14:39
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In the context of straight wing planes (I have no swept wing expereince);

I would advocate using rudder to keep the aeroplane in balance, which should minimise wing drop, but it should not be "picked up" with anything. Whatever bank angle occurs should be picked up with aileron - co-ordinated as required with rudder, but only once the aeroplane is clearly unstalled.
I agree with this. Bearing in mind that the design requirement states "normal use of the controls" during the approach to stall. I am not in favour of using the wrong control to correct a roll. I accept that there are some types for which the rudder is the preferred control for "picking up a wing" at approach to stall, okay, in that case, as state in the flight manual, the rudder would be the right control.

I like the idea of flying an airplane properly coordinated unless there is an intent to fly it uncoordinated. I don't like the idea of deliberately uncoordinating at at the approach to stall, as that is an invitation to a spin entry. I like to see pilot tune their "seat of the pants" and awareness to the ball, so as to keep a plane coordinated. Of course, reading the flight manual for the type is pretty important. An example in the light civil world is the Cessna Caravan, which recommends minimizing the use of ailerons at the approach to stall, as a large aileron displacement also extends that roll spoiler, which results in undesirable drag, and further loss of lift on that wing. For my experience, small aileron displacement, and coordinated use of the rudder works well while stalling a Caravan. Indeed that type is amazingly tolerant of abused approach to a stall.

I am not an advocate of powering out of stalls, and would never teach that. Of course, if you're in the recovery and minimizing altitude loss is important, than use some power - sparingly. It is a bad habit to depend upon sudden availability of power, particularly during unusual attitude maneuvering, and carburetted airplanes which do not have an accelerator pump (they tend to quit when you jam the throttle). Powering into what may become a dive, particularly in a C/S prop plane could result in an engine overspeed. And, I like the idea that if you have stalled a plane which is not under power, you still have muscle memory to recover it.

If you have entered a spin, the ailerons should generally not be used, until unstalled flight is resumed. The ailerons, on a stalled wing(s), are only adding to the aerodynamic confusion, not the recovery.

I'm an advocate of at least spin awareness (by instructor demonstration) if not training. We train spiral dive recovery (I hope?), a spin and a spiral dive are both loss of control maneuvers, for which the wrong recovery will make thing much worse quickly. I think that a pilot should be able to recognize stalled vs unstalled as they spiral down - the recovery will be different, and you should know which. Most STOL kit modified Cessnas will self recover a spin into a spiral dive on their own. It's useful to be able to recognize that this has happened, and apply the correct recovery.
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Old 5th Dec 2020, 23:30
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Originally Posted by leonard17F View Post
"At stall, to counter a wing-drop phenomenon, on an airplane with straight wings you use rudder, and for an airplane with stretch wings (or delta), you use ailerons" ?
Leo
This is not a simple subject! I consider that stalling is, perhaps, the most complex and potentially ambiguous aspect of flight testing that there is. One interesting aspect is how you define the 'stall' this being considerably different between what is taught in training, military specifications and part 23/25 specification definitions. Also, are we considering recovering at the point of aerodynamic stall (perhaps Cl max, perhaps maximum usable lift, however defined) or manoeuvring an aeroplane close to maximum usable AoA?

Let us consider consider two cases. First, a wing drop at the point of aerodynamic stall with a straight wing and, typically, this will be a rapid/sudden event. Using aileron to try to correct the wing drop may exacerbate it by increasing local AoA. However, part 23/25 certification states a maximum permissible wing drop and lateral and directional control may be used to contain it. Therefore, most, if not all, such aircraft will have ailerons that are still effective at the stall. However, one of the problems may be significant adverse aileron yaw such that co-ordinating rudder may also be needed in order to generate a rolling moment. Attempting to use rudder to generate a rolling moment with sideslip may well be effective if only small rudder inputs are used. However, if large rudder inputs are used then a departure and spin may result.

Secondly, swept wing aircraft. The Cl vs AoA relationship inevitably will not exhibit a clear break as seen with most straight wings and there are far greater variations in 'stall' characteristics than with a straight wing. Typically, aileron power will reduce markedly as AoA increases and lateral stability will increase. Therefore, the use of lateral stick may give a weak rolling moment in the conventional sense but the combination of adverse yaw and strong lateral stability may result in a roll rate in the opposite direction to the applied stick. However, a rudder input may result in a strong rolling moment in the same direction due to the strong lateral stability. The F4 and the F-14 are two types that demonstrated this well. To roll the F4 above 17 units AoA you kept the stick in the middle laterally and used the rudder.

Returning to straight wing aeroplanes, if a wing drop occurs at the stall then the recovery procedure must be to reduce AoA to less than the stall value with a forward stick input and then roll wings level using normal lateral stick inputs (with rudder co-ordination if appropriate). For safety of flight recoveries I would not recommend using the CS-23 certification technique of controlling the bank angle with ailerons and, if required, rudder before reducing the AoA. With respect to the use of power during recovery, consider carefully whether you are recovering at the full stall or at stall warning onset. If the latter case then the use of power to minimise altitude loss is most certainly appropriate.
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Old 5th Dec 2020, 23:55
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With respect to the use of power during recovery, consider carefully whether you are recovering at the full stall or at stall warning onset. If the latter case then the use of power to minimise altitude loss is most certainly appropriate.
If the application of power is being combined by deliberate reduction of AoA to prevent continuing into the stall.
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Old 6th Dec 2020, 08:30
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Originally Posted by Pilot DAR View Post
If the application of power is being combined by deliberate reduction of AoA to prevent continuing into the stall.
Absolutely. Thank you for the clarification. There have been issues in the past with commercial operation when recovery at stall warning did not stress the need to reduce AoA and had procedures that were akin to a wind shear recovery or a go-around.
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Old 6th Dec 2020, 09:31
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However, part 23/25 certification states a maximum permissible wing drop and lateral and directional control may be used to contain it.
Whilst I agree that is what the regulations say, I can't say I'm a great fan of seeing the skill of a highly experienced handling test pilot used to create a beneficial outcome that would not be seen if the event was encountered by an average line pilot, with their likely training and reaction times.

There have been issues in the past with commercial operation when recovery at stall warning did not stress the need to reduce AoA and had procedures that were akin to a wind shear recovery or a go-around.


Indeed, exacerbated, as I indicated above, by the use of training aeroplanes with exceptionally benign handling characteristics, not necessarily representative of the characteristics of aeroplanes that trainee pilots may fly later.

G
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Old 6th Dec 2020, 11:30
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Originally Posted by Genghis the Engineer View Post
Whilst I agree that is what the regulations say, I can't say I'm a great fan of seeing the skill of a highly experienced handling test pilot used to create a beneficial outcome that would not be seen if the event was encountered by an average line pilot, with their likely training and reaction times.
G
Genghis,
I agree with you and this is a classic case of a quantitative test technique not being role relatable.
Rgds L
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Old 7th Dec 2020, 12:47
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Thanks a million for your great inputs ! Let me review them carefully and thoroughly and I will get back to you ASAP. Really appreciate !!
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Old 15th Dec 2020, 06:37
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From the B-52 manual
Maintain wings level attitude with lateral control as the stall is approached. Fairly large lateral corrections may be necessary. Caution should be used because lateral control capability decreases rapidly as the stalling speed is approached. Rudder may be used to maintain heading; however, during low speed flight, a delay in aircraft response after control input of up to 3 seconds may exist before roll correction develops.

The stall characteristics of the aircraft will vary with wing flap extension and drop tank installation. The following stall characteristics can be expected: With flaps extended and no drop tanks installed, there is little or no stall warning speed margin. Buffet of the flaps when the stall is approached will mask aircraft buffet. At stall, the aircraft will tend to fall off on one wing. This fall-off can be stopped by applying forward pressure on the control column and by using whatever lateral control and rudder is required. As lateral control degrades rapidly at speeds below initial buffet. the use of rudder may be necessary to correct for bank angle.
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