PPL PoF Question
(a) Zoom climb.
Not the best term ever, but clear enough. Pull back hard on the stick at a highish speed, and the first things that'll happen is the aeroplane pitches up, and as a result will climb.
Keep pulling, and the aeroplane will probably stall, but it'll be decelerating at the same time as approaching the g limit, so will meet the O-A curve at a speed lower than you started, and a g loading below the g limit. So, structural damage is pretty unlikely.
Whether thats the right answer in the reckoning of the aerodynamic ignoramises who set PofF exams at both PPL and CPL/ATPL level, who knows. But it's what will actually happen.
G
Not the best term ever, but clear enough. Pull back hard on the stick at a highish speed, and the first things that'll happen is the aeroplane pitches up, and as a result will climb.
Keep pulling, and the aeroplane will probably stall, but it'll be decelerating at the same time as approaching the g limit, so will meet the O-A curve at a speed lower than you started, and a g loading below the g limit. So, structural damage is pretty unlikely.
Whether thats the right answer in the reckoning of the aerodynamic ignoramises who set PofF exams at both PPL and CPL/ATPL level, who knows. But it's what will actually happen.
G
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I don't fully agree. The question is what happens if you apply the controls in a full and abrupt manner. I agree that during the application the aircraft will pitch up, but once you reach full application of the controls, the aircraft will stall and there will be no further pitch up forces couple.
Now how long does it take for the full application? Half a second? Even at 4G, half a second is not enough to overcome the inertia of the airframe and to pitch it up sufficiently for a zoom climb. Sure, it will pitch up a bit, but not sufficient for a zoom climb.
At least, if you define a zoom climb as something along the lines of "a climb entered at high velocity, where the aim is to convert this velocity (kinetic energy) into as much height as possible (potential energy)." A zoom climb to me happens with a seriously high pitch up attitude, and is not sustainable. Anything less than that is not a "zoom climb" in my vocabulary, but a regular climb.
I don't think you will reach that zoom climb attitude before the aircraft will stall, if you apply an abrupt and full control input. In fact, if you look at how an aerobatics aircraft performs a snap roll, hauling back abruptly and fully is exactly what you do. You then put in a bootful of rudder for the roll to happen. And snap rolls are performed without any significant altitude gain, and with just a minor pitch change. (The pitch has to change because the wings need to reach the critical AoA, obviously.)
Now how long does it take for the full application? Half a second? Even at 4G, half a second is not enough to overcome the inertia of the airframe and to pitch it up sufficiently for a zoom climb. Sure, it will pitch up a bit, but not sufficient for a zoom climb.
At least, if you define a zoom climb as something along the lines of "a climb entered at high velocity, where the aim is to convert this velocity (kinetic energy) into as much height as possible (potential energy)." A zoom climb to me happens with a seriously high pitch up attitude, and is not sustainable. Anything less than that is not a "zoom climb" in my vocabulary, but a regular climb.
I don't think you will reach that zoom climb attitude before the aircraft will stall, if you apply an abrupt and full control input. In fact, if you look at how an aerobatics aircraft performs a snap roll, hauling back abruptly and fully is exactly what you do. You then put in a bootful of rudder for the roll to happen. And snap rolls are performed without any significant altitude gain, and with just a minor pitch change. (The pitch has to change because the wings need to reach the critical AoA, obviously.)
Last edited by BackPacker; 15th Nov 2014 at 09:17.
Prior to the A300 fin loss accident, conventional wisdom within JAR ATPL/CPL teaching was that Va was the maximum speed at which abrupt full scale deflection would cause the aircraft to stall at the limiting load factor, so no permanent damage to the aircraft structure would occur.
After the A300 accident, the CAA made efforts to emphasize that although a single full scale deflection was OK, repeated rapid full scale deflections in opposite directions would cause overload damage.
All of the above overlooked the fact that Va is actually concerned only with the design strength of the flight control systems. It is not concerned with protection of the aircraft structure. After some discussion the JAA introduced the Operating Manoeuvre Speed Vo, to prevent overload of the structure.
I do not know whether or not this is a real EASA ATPL/CPL theory question, but whatever its origin, it looks to me like it is based on a misinterpretation risks associated with repeated full scale deflections. Of the two options listed, option a is the most correct. But as another poster has already said, I suspect that the correct answers is one of the missing ones (c or d).
After the A300 accident, the CAA made efforts to emphasize that although a single full scale deflection was OK, repeated rapid full scale deflections in opposite directions would cause overload damage.
All of the above overlooked the fact that Va is actually concerned only with the design strength of the flight control systems. It is not concerned with protection of the aircraft structure. After some discussion the JAA introduced the Operating Manoeuvre Speed Vo, to prevent overload of the structure.
I do not know whether or not this is a real EASA ATPL/CPL theory question, but whatever its origin, it looks to me like it is based on a misinterpretation risks associated with repeated full scale deflections. Of the two options listed, option a is the most correct. But as another poster has already said, I suspect that the correct answers is one of the missing ones (c or d).
I agree that during the application the aircraft will pitch up, but once you reach full application of the controls, the aircraft will stall and there will be no further pitch up forces couple.
So pitch up, as you say would be a better answer, but pitch up, and the next thing that happens is a conversion of speed into height. A stall (or structural damage in some circumstances) would occur about third.
G
Well if we are going to get picky then the first thing that happens is that the elevators deflect upwards. But that is not one of the options provided, and when taking multi-choice exams our choices are limited to the options available.
Of the two options listed in the OP, option a "enter a zoom climb" will always happen. But option b, "damage to the controls" will happen only if the designer has chosen a value for Va that is significantly greater than the legal minimum of Vs x root n.
So of the options available in the OP, option a is the most correct.
But as I said in my previous post, I suspect that the real answer is one of the missing options.
Of the two options listed in the OP, option a "enter a zoom climb" will always happen. But option b, "damage to the controls" will happen only if the designer has chosen a value for Va that is significantly greater than the legal minimum of Vs x root n.
So of the options available in the OP, option a is the most correct.
But as I said in my previous post, I suspect that the real answer is one of the missing options.
Last edited by keith williams; 15th Nov 2014 at 12:51.
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All of the above overlooked the fact that Va is actually concerned only with the design strength of the flight control systems.
I suspect that the correct answers is one of the missing ones (c or d)
Although interesting I do not think a detailed discussion of aerodynamics is required to answer the OP's question.
A written exam question would be looking for a basic understanding of a topic and the key (exam technique) to getting the correct answer is understanding what they are asking about and then selecting the 'most' correct answer.
This question is about Va.
The candidate would be expected to know that at speeds below Va full control deflection should stall the aircraft before overstressing it and that at speeds above Va there is a risk of overstress.
The question setters have removed the ambiguity of what would happen at exactly Va by not giving "the aircraft will stall" as an option.
So the question should be read as what would happen first if flying above Va.
Ignoring the fact that some would, in real life, overlap the order of events would be:
1) Full & Abrupt Elevator Deflection with risk of overstressing controls/control surfaces
2) Rapid Pitch Change with risk of overstressing aircraft
3) Aircraft will climb (but it will not be a very efficient climb - certainly not a "zoom")
The first event would be a risk of overstressing the controls and, although it says "will" rather than "may", answer b) matches this and is therefore the correct answer.
Last edited by Level Attitude; 15th Nov 2014 at 12:57.
Option b actually states "You will cause structural damage to the control surfaces".
If we ignore for a moment the possibility that Va may be greater than the limiting load factor stall speed, there is still a problem with setting the question at exactly Va. It is a bit like asking "at the time of your death are you dead or alive?" Before this time you are clearly alive and after this time you are clearly dead. But the actual time of your death is a transition point between being alive and being dead.
If we assume that Va is Vs x root n, then the aircraft will stall at its limiting load factor (n). The controls are designed to sustain loads up to the limiting load factor without permanent damage, but they will be damaged at any higher load factor. So the limiting load factor marks the transition from being damaged and not being damaged. We cannot therefor say that full control deflection at Va will damage the controls.
It does not help that many sources such as Wikipedia give definitions along the lines of "at speeds below Va full control deflection will not cause damage" or "at speeds close to or above Va full control deflection will cause damage".
The text below which was extracted from FAR 23 is more valid.
The question would be far less contentious had it said "at a speed slightly greater than Va". If the author of the question had done this then I am pretty sure that this thread would not have arisen.
I'm not sure that the term "zoom climb" necessarily implies any particular level of efficiency. My interpretation would be something along the lines of " a climb in which kinetic energy in the form of decreasing airspeed is converted into potential energy in the form of increased height." This would certainly happen if we were to simply pull the stick fully aft without increasing power setting.
If we ignore for a moment the possibility that Va may be greater than the limiting load factor stall speed, there is still a problem with setting the question at exactly Va. It is a bit like asking "at the time of your death are you dead or alive?" Before this time you are clearly alive and after this time you are clearly dead. But the actual time of your death is a transition point between being alive and being dead.
If we assume that Va is Vs x root n, then the aircraft will stall at its limiting load factor (n). The controls are designed to sustain loads up to the limiting load factor without permanent damage, but they will be damaged at any higher load factor. So the limiting load factor marks the transition from being damaged and not being damaged. We cannot therefor say that full control deflection at Va will damage the controls.
It does not help that many sources such as Wikipedia give definitions along the lines of "at speeds below Va full control deflection will not cause damage" or "at speeds close to or above Va full control deflection will cause damage".
The text below which was extracted from FAR 23 is more valid.
§23.423 Maneuvering loads.
Each horizontal surface and its supporting structure, and the main wing of a canard or tandem wing configuration, if that surface has pitch control, must be designed for the maneuvering loads imposed by the following conditions:
(a) A sudden movement of the pitching control, at the speed VA, to the maximum aft movement, and the maximum forward movement, as limited by the control stops, or pilot effort, whichever is critical.
Each horizontal surface and its supporting structure, and the main wing of a canard or tandem wing configuration, if that surface has pitch control, must be designed for the maneuvering loads imposed by the following conditions:
(a) A sudden movement of the pitching control, at the speed VA, to the maximum aft movement, and the maximum forward movement, as limited by the control stops, or pilot effort, whichever is critical.
Aircraft will climb (but it will not be a very efficient climb - certainly not a "zoom"
Last edited by keith williams; 16th Nov 2014 at 12:51.
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There's a lot of talk about climbing here. Any kind of climb is not assured from a full nose up application of pitch controls. A pitch up is likely, but if you were in a dive, doubtful you're going to climb at all, you may just dive less, or you may pitch up, stall, and continue in a descent. I did this many times at C 150 speeds in a C 150 Aerobat, you can get a heck of a high speed stall.
Think of Va as the speed given to the pilot as a simple tool to assure that pitch maneuvers can be carried out without wrinkling the aircraft. In a certified aircraft, full pitch up at a speed faster than Va may leave some expensive wrinkles in the wings, but they are very unlikely to fail such that you cannot safely land the plane. The dangerous part for you will be turning it back in on the ramp with stress wrinkles all over it.
If aircraft were all equipped with G meters, this would not be an issue. Acceleration is one of the few stated limitations for every aircraft, which you, the pilot have no real way of measuring in flight. So provide Va, and it should keep you safe.
If I were to be in a stupid fast dive, and I could not determine G loading on the plane, In smooth air, I would rather overspeed the plane, that risk overstressing it. Your speed you do know!
Va does not count for ailerons and rudder, just elevator. You are not protected against yaw or roll caused airframe damage by Va. For example, a snap roll is entered at slower than Va, but is risky for airframe damage.
My main interest in Va, is be slower than, if I enter severe turbulence. You are not pitching the plane up, you just instantly got a whole bunch more lift than you planned on. In that case, as undesirable as a stall may be, it is more desirable than wing damage. Happily, many light GA singles barely cruise at GA anyway.
Think of Va as the speed given to the pilot as a simple tool to assure that pitch maneuvers can be carried out without wrinkling the aircraft. In a certified aircraft, full pitch up at a speed faster than Va may leave some expensive wrinkles in the wings, but they are very unlikely to fail such that you cannot safely land the plane. The dangerous part for you will be turning it back in on the ramp with stress wrinkles all over it.
If aircraft were all equipped with G meters, this would not be an issue. Acceleration is one of the few stated limitations for every aircraft, which you, the pilot have no real way of measuring in flight. So provide Va, and it should keep you safe.
If I were to be in a stupid fast dive, and I could not determine G loading on the plane, In smooth air, I would rather overspeed the plane, that risk overstressing it. Your speed you do know!
Va does not count for ailerons and rudder, just elevator. You are not protected against yaw or roll caused airframe damage by Va. For example, a snap roll is entered at slower than Va, but is risky for airframe damage.
My main interest in Va, is be slower than, if I enter severe turbulence. You are not pitching the plane up, you just instantly got a whole bunch more lift than you planned on. In that case, as undesirable as a stall may be, it is more desirable than wing damage. Happily, many light GA singles barely cruise at GA anyway.
