Windshear question
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THUNDERTAILED
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From: L200
Windshear question
If a strong headwind shears to calm when an aircraft is on final approach, will it pitch up or down? It will undershoot the glideslope, yes, but which direction will it pitch?
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From: Escapee from Ultima Thule
Initial response will be a pitch nose down.
The a/c 'sees' an instantaneous loss of of IAS leading to it's inherant stability pitching down to regain trimmed speed. The loss of speed also gives a loss of lift leading to an initial undershoot.
Later, once equilibrium is regained then the loss of a headwind means that less power/thrust will be needed to maintain the approach profile.
The a/c 'sees' an instantaneous loss of of IAS leading to it's inherant stability pitching down to regain trimmed speed. The loss of speed also gives a loss of lift leading to an initial undershoot.
Later, once equilibrium is regained then the loss of a headwind means that less power/thrust will be needed to maintain the approach profile.
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From: UK
Elaborating a litle on Tinstaafl's reply...
The aircraft is trimmed for a particular AOA. The pitch stability maintains that AOA. When the IAS drops, the aircraft is short of lift and starts to accelerate downwards. If the flight path were to become a descent and the pitch attitude remained the same, the AOA would increase, so to maintain that AOA the trimmed aircraft pitches downwards too.
The aircraft is trimmed for a particular AOA. The pitch stability maintains that AOA. When the IAS drops, the aircraft is short of lift and starts to accelerate downwards. If the flight path were to become a descent and the pitch attitude remained the same, the AOA would increase, so to maintain that AOA the trimmed aircraft pitches downwards too.
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From: various places .....
The original question needs a little bit of qualification as the answer will depend on the resultant of all the things going on with the aircraft at that time .... i.e. there is no way we can answer the question without a lot more information... or putting a lot of qualifications in with any answer.
If the intent of the question is to ask something like "what is the pitching tendency of the aircraft, other things being unchanged" - which is what I think AfricanSkies was looking at - then the question revolves around longitudinal stability.
For a certifiable aircraft, if the speed is a little below trim speed (doesn't really matter how it got there) then the pilot must require a pull force to maintain that off-trim speed, i.e. the pitching moment (what the aircraft wants to do if you let go of the stick) is nose down.
For some aircraft, this presents a problem in some circumstances (commonly seen in the missed approach situation with small turboprops) if engine intake lip/prop forces provide a significant nose up pitching moment sufficient to overwhelm the basic aircraft's pitch stability. In this case, the aircraft would normally have one of a number of flight test fixes, commonly a stability augmentation system (SAS), to counter the problem caused by having the engine at high thrust/power with a significant nose up pitch angle.
Similarly, if the off-trim speed is a little fast, then the pilot must require a push force to maintain that off-trim speed, i.e. the pitching moment is nose up.
If the intent of the question is to ask something like "what is the pitching tendency of the aircraft, other things being unchanged" - which is what I think AfricanSkies was looking at - then the question revolves around longitudinal stability.
For a certifiable aircraft, if the speed is a little below trim speed (doesn't really matter how it got there) then the pilot must require a pull force to maintain that off-trim speed, i.e. the pitching moment (what the aircraft wants to do if you let go of the stick) is nose down.
For some aircraft, this presents a problem in some circumstances (commonly seen in the missed approach situation with small turboprops) if engine intake lip/prop forces provide a significant nose up pitching moment sufficient to overwhelm the basic aircraft's pitch stability. In this case, the aircraft would normally have one of a number of flight test fixes, commonly a stability augmentation system (SAS), to counter the problem caused by having the engine at high thrust/power with a significant nose up pitch angle.
Similarly, if the off-trim speed is a little fast, then the pilot must require a push force to maintain that off-trim speed, i.e. the pitching moment is nose up.
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From: UK
Hello BOAC- you are quite right, a tip stall on a swept wing would move the centre of pressure forward........but.......because a tip stall is such a dangerous phenomenon (especially if it is asymmetric- maybe due to sideslip/gust), it is designed out in a large transport. Hence tips tend to wash out and reduce angle of attack, leading edge profiles change near the tip, and/or sweepback reduces (most visible I think on the Victor). Look at wingtips of most large jets, you can usually see several features designed to ensure wingtips don't stall first !
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From: Dorset
The content (or rather lack of content) in this question suggests that it is from a JAR ATPL POF or Performance examination. Such questions often contain insufficient information to enable a single option to be selected. In this case the problem is made worse by the fact that no options have been given. I suspect that the question is intended to test the the effects of windshear and static longitudinal stability.
The sudden drop in headwind will cause a reduction in airspeed and dynamic pressure. But the inertia of the aircraft will (initially) hold ground speed constant. The reduced dynamic pressure will reduce the lift, thereby causing the aircraft to descend. The downward motion of the aircraft will give the relative airflow an upward component.
An aircraft that is longitudinaly stable will pitch into the relative airflow whenever the angle of attack is other than that for which it has been trimmed. Assuming that the aircraft was correctly trimmed prior to the wind change, its longitudinal stability wil cause it to pitch nose down, so that the original angle or attack tends to be restored.
When facing a question of this type in the JAR examinations the effect of stall and speed stability are best ignored unless there are specific clues to suggest that this is what the examiners are looking for.
The sudden drop in headwind will cause a reduction in airspeed and dynamic pressure. But the inertia of the aircraft will (initially) hold ground speed constant. The reduced dynamic pressure will reduce the lift, thereby causing the aircraft to descend. The downward motion of the aircraft will give the relative airflow an upward component.
An aircraft that is longitudinaly stable will pitch into the relative airflow whenever the angle of attack is other than that for which it has been trimmed. Assuming that the aircraft was correctly trimmed prior to the wind change, its longitudinal stability wil cause it to pitch nose down, so that the original angle or attack tends to be restored.
When facing a question of this type in the JAR examinations the effect of stall and speed stability are best ignored unless there are specific clues to suggest that this is what the examiners are looking for.
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From: UK
Notso
So why does the phenomenon of super stall exist if it has been designed out? And if the pilots would recognise this situation developing and react, why do aircraft sucseptible to this have stick pushers fitted as a mandatory requirement? Are you really suggesting that pilots are infallible? Lot of evidence to the contrarary I'm afraid.
Ah, don't you love the JAR POF exams.
So why does the phenomenon of super stall exist if it has been designed out? And if the pilots would recognise this situation developing and react, why do aircraft sucseptible to this have stick pushers fitted as a mandatory requirement? Are you really suggesting that pilots are infallible? Lot of evidence to the contrarary I'm afraid.
Ah, don't you love the JAR POF exams.
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From: UK
Witch- you're talking about something completely different. A superstall is where a higher-set tailplane at very low speed/full wing stall is 'blanked' by very disturbed airflow off the wing, and thus elevator authority is totally lost. This resulted in one of the early Tridents on Salisbury Plain being lost with the flight test crew, a BAC 1-11 similarly so, possibly the Staines Trident, and no doubt several others, along with a very frightened Notso in a VC10 on a CofA test over Anglia at 17,000'. I didn't want to do it or be there, but I was. We had an AofA meter fitted. It hovered at about 15 degrees with extensive airframe vibration and Captain holding wings level, then it jumps. I was told if it goes past 17 degrees, you're superstalled and you die. Pprune will be forever grateful it stopped at 17, I think I saw 95 knots ASI and Notso swore never again, under any circumstances, should he ever, whilst trying to bring up a family, be involved in such nonsense again, which is why to this day I do not parachute, hang glide, paraglide. Waterskiing and snowboarding has been an exception.
But to get back to the question, the phenomenon of superstalls exists because it does- you cannot ever have a wing that does not stall. It was recognised that someone, somehow, somewhere, would get into one in an aeroplane with a high tail, it's known that pilots are fallible, so stick pushers are there as a last resort. Recognising that all wings eventually stall, you must therefore ensure the last place to stall are the tips because:
1- as BOAC says, you will get a nose up moment which is disastrous and adds to your problems
2- if it is assymmetric for any reason such as sideslip or gust, you will suddenly find yourself upside down.
Examine closely wingtip areas in swept wing jets, even in photographs on !!!!!!!!!!!!!!. You will see the varying design tricks I outlined to delay tip stalling. Even on the 747-400 it is plainly visible. You will always be able to see reduced incidence and/or reduced sweepback.
But to get back to the question, the phenomenon of superstalls exists because it does- you cannot ever have a wing that does not stall. It was recognised that someone, somehow, somewhere, would get into one in an aeroplane with a high tail, it's known that pilots are fallible, so stick pushers are there as a last resort. Recognising that all wings eventually stall, you must therefore ensure the last place to stall are the tips because:
1- as BOAC says, you will get a nose up moment which is disastrous and adds to your problems
2- if it is assymmetric for any reason such as sideslip or gust, you will suddenly find yourself upside down.
Examine closely wingtip areas in swept wing jets, even in photographs on !!!!!!!!!!!!!!. You will see the varying design tricks I outlined to delay tip stalling. Even on the 747-400 it is plainly visible. You will always be able to see reduced incidence and/or reduced sweepback.
