Why do we Lose Airspeed in a Turn and What Causes This?
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Do you require less thrust to fly east than west in still-air conditions on a rotating earth? The centrifugal force of an eastbound aircraft must counter and thus reduce the effect of gravity while spinning 'with' the earth as your rotational velocity with respect to an unaccelerated frame of reference is greater. Less 'gravity-like force'=>less 'weight'=>less lift=>less drag=>less thrust. It must be a minimal effect but it disproves those who suggest flying circles IMC without power changes shows that the airmass is the only valid frame of reference.
I'm assuming they launch space shuttles eastward though I've never flown over florida at the right time to see for myself.
I'm assuming they launch space shuttles eastward though I've never flown over florida at the right time to see for myself.
One of those subjects that generates real 'black and white' responses.
Regarding the 'earth reference point', consider a 50 kt Northerly wind air mass, with two gliders on reciprocal N/S tracks doing 50 kts KIAS (with lateral separation, before the smart arses say they collide!) As they land in a field they encounter a brick wall at the point of touchdown. The one heading North touches down gently from a hover and the pilot steps out unscathed from his intact glider. The southerly heading one disappears into a pile of glassfibre shards and all they find is the pilot's floppy sunhat.
Relevant? Maybe not, but don't tell me an aircraft's total energy is not related/referenced to the earth.
Following on from JF's theory on theories, I once read an article written by a USN Lt. Cmdr. that explained in great detail how it was impossible for a light SE aircraft to carry out a turnback from 500 ft following an EFATO. It had lots of calculations and diagrams to prove the point. Strangely enough, as a 40 hour Bulldog student, I was able to fly quite nice turnbacks from 450 ft and upwards, so I guess that theory (which is still widely held) was wrong!
Regarding the 'earth reference point', consider a 50 kt Northerly wind air mass, with two gliders on reciprocal N/S tracks doing 50 kts KIAS (with lateral separation, before the smart arses say they collide!) As they land in a field they encounter a brick wall at the point of touchdown. The one heading North touches down gently from a hover and the pilot steps out unscathed from his intact glider. The southerly heading one disappears into a pile of glassfibre shards and all they find is the pilot's floppy sunhat.
Relevant? Maybe not, but don't tell me an aircraft's total energy is not related/referenced to the earth.
Following on from JF's theory on theories, I once read an article written by a USN Lt. Cmdr. that explained in great detail how it was impossible for a light SE aircraft to carry out a turnback from 500 ft following an EFATO. It had lots of calculations and diagrams to prove the point. Strangely enough, as a 40 hour Bulldog student, I was able to fly quite nice turnbacks from 450 ft and upwards, so I guess that theory (which is still widely held) was wrong!
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212,
I don't think anyone here disputes that (?) - you are having a raging agreement here! BUT what some people are missing is that energy is relative to something - you can measure it against what you want and then argue that black is white.
Your gliders, relative to the air mass, have the same energy. Relative to the ground, they have different energies. It depends what you measure it against.
G
I don't think anyone here disputes that (?) - you are having a raging agreement here! BUT what some people are missing is that energy is relative to something - you can measure it against what you want and then argue that black is white.
Your gliders, relative to the air mass, have the same energy. Relative to the ground, they have different energies. It depends what you measure it against.
G
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This has me concerned, as I'm due to undertake a train trip soon. If I'm walking towards the back of a moving train, and then turn around and start walking towards the front, will I suddenly fall over?
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Ok, here's my stab at this.
I think MO's theory wrt conservation of energy is, at least partially, correct, but he is misattributing where the energy comes from. Take this example:
You are flying an aircraft at 100KIAS directly into a 100kt headwind. Relative to the ground, you have the same kinetic energy. Therefore, stating that 'your kinetic energy is zero' is in fact correct (in keeping with relativity). An object's kinetic energy will only become relevant if it meets another object with a different level of kinetic energy (imagine two bullets fired from guns next to each other - if they touch, the effects are negligible; they have massive amounts of kinetic energy due to their high speed, but roughly the same amount as each other. If you now fire them directly towards each other, and they make contact, there is a very large energy transfer and the effect IS relevant.)
Now, initate a 180deg turn so you are flying directly 'out' of the wind. In the time it takes to make the turn, the aircraft will accelerate (the quicker the turn, the greater the acceleration) from zero to 200kts groudspeed and both you, and the aircraft, will FEEL the acceleration - you will percieve it as an increase in 'g'. Ordinarily, when an aircraft turns, acceleration, or 'g', is produced and felt. But the energy to create this acceleration comes from aerodynamic sources - the wings are directing some of the lift force towards turning the aircraft, so to maintain constant IAS and height, some extra energy must be added (more thrust) to provide more lift at the same IAS - total energy is conserved.
In the case above, the energy to accelerate the aircraft (relative to the ground) comes NOT from the wings, but from the air (the air 'behind' the aircraft will slow down relative to the ground). Therefore, although you will feel 'g' during the acceleration, the aerodynamic forces produced by the wings will not change - there is no increase in wing loading, despite the 'g' that you are feeling - therefore there will be no effect on IAS. Energy is, once again, conserved.
In JF's hovering Harrier turn, and Prince William's air race crash, there is a difference - they were both DELIBERATELY trying to fly with reference to the GROUND. The fact that PW crashed was nothing to do with IAS loss in a downwind turn, but the fact that he must have overloaded and stalled his wings in a turn, in an attempt to make the 'gate'. Bringing Helos into the argument will always complicate matters, as, when hovering or flying slow and low, they are almost invariably flying with reference to the ground.
Does that make sense? Or am I talking bollox as well? Makes sense in my head.
I think MO's theory wrt conservation of energy is, at least partially, correct, but he is misattributing where the energy comes from. Take this example:
You are flying an aircraft at 100KIAS directly into a 100kt headwind. Relative to the ground, you have the same kinetic energy. Therefore, stating that 'your kinetic energy is zero' is in fact correct (in keeping with relativity). An object's kinetic energy will only become relevant if it meets another object with a different level of kinetic energy (imagine two bullets fired from guns next to each other - if they touch, the effects are negligible; they have massive amounts of kinetic energy due to their high speed, but roughly the same amount as each other. If you now fire them directly towards each other, and they make contact, there is a very large energy transfer and the effect IS relevant.)
Now, initate a 180deg turn so you are flying directly 'out' of the wind. In the time it takes to make the turn, the aircraft will accelerate (the quicker the turn, the greater the acceleration) from zero to 200kts groudspeed and both you, and the aircraft, will FEEL the acceleration - you will percieve it as an increase in 'g'. Ordinarily, when an aircraft turns, acceleration, or 'g', is produced and felt. But the energy to create this acceleration comes from aerodynamic sources - the wings are directing some of the lift force towards turning the aircraft, so to maintain constant IAS and height, some extra energy must be added (more thrust) to provide more lift at the same IAS - total energy is conserved.
In the case above, the energy to accelerate the aircraft (relative to the ground) comes NOT from the wings, but from the air (the air 'behind' the aircraft will slow down relative to the ground). Therefore, although you will feel 'g' during the acceleration, the aerodynamic forces produced by the wings will not change - there is no increase in wing loading, despite the 'g' that you are feeling - therefore there will be no effect on IAS. Energy is, once again, conserved.
In JF's hovering Harrier turn, and Prince William's air race crash, there is a difference - they were both DELIBERATELY trying to fly with reference to the GROUND. The fact that PW crashed was nothing to do with IAS loss in a downwind turn, but the fact that he must have overloaded and stalled his wings in a turn, in an attempt to make the 'gate'. Bringing Helos into the argument will always complicate matters, as, when hovering or flying slow and low, they are almost invariably flying with reference to the ground.
Does that make sense? Or am I talking bollox as well? Makes sense in my head.
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John F,
Being in the hover, into wind (say 10 knots), bang on the markers.
You then decide to turn downwind and, hey presto, you need a bunch of power to remain in the hover because you have just lost 10 knots worth of lift pixies (I think at lower speeds they a called a 'Clatter' of LP's, then a 'Gaggle' until about 600+ knots when they are referred to as a 'Philharmonic'), which is a fact! However, sitting in the hover on the markers, is, by it's very nature, referenced to groundspeed (0) and therefore, we have to make power adjustments to maintain our position on the markers....
Now, if we then take away the earth and we fly 0 IAS and maintain this whilst turning downwind without adjusting power, we will not fall out of the sky......if we then magically make the earth re-appear and look down......we will no longer be on the markers but will, in fact, be moving downwind.....and our net lift-pixies remain the same!
If you agree, then I mis-read your post and apologise! Easy:
Working Groundspeed = Be careful
Working Airspeed = Fill yer boots
Being in the hover, into wind (say 10 knots), bang on the markers.
You then decide to turn downwind and, hey presto, you need a bunch of power to remain in the hover because you have just lost 10 knots worth of lift pixies (I think at lower speeds they a called a 'Clatter' of LP's, then a 'Gaggle' until about 600+ knots when they are referred to as a 'Philharmonic'), which is a fact! However, sitting in the hover on the markers, is, by it's very nature, referenced to groundspeed (0) and therefore, we have to make power adjustments to maintain our position on the markers....
Now, if we then take away the earth and we fly 0 IAS and maintain this whilst turning downwind without adjusting power, we will not fall out of the sky......if we then magically make the earth re-appear and look down......we will no longer be on the markers but will, in fact, be moving downwind.....and our net lift-pixies remain the same!
If you agree, then I mis-read your post and apologise! Easy:
Working Groundspeed = Be careful
Working Airspeed = Fill yer boots
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This has me concerned, as I'm due to undertake a train trip soon. If I'm walking towards the back of a moving train, and then turn around and start walking towards the front, will I suddenly fall over?
Now, initate a 180deg turn so you are flying directly 'out' of the wind. In the time it takes to make the turn, the aircraft will accelerate (the quicker the turn, the greater the acceleration) from zero to 200kts groudspeed and both you, and the aircraft, will FEEL the acceleration
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I read a crash report that said that the aircraft completed a normal 30 degree banked turn to port, levelled out and flew level for a short time. The port wing then dropped suddenly, came level again and then dropped again hitting the ground. It was concluded that the aircraft was flying below the stall speed for its condition (nearly fully loaded).
What is going on here? Why should only one wing drop? Would the plane have levelled itself or could it be as a result of the pilot's actions?
What is going on here? Why should only one wing drop? Would the plane have levelled itself or could it be as a result of the pilot's actions?
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Gents,
We're all getting sidelined by physics:
Actually, everyone has a good point, but non yet have covered the 'common ground' Inertia.......
In the hover case, change in direction does indeed change your IAS, as the wind is unable to instantaeously speed you up.... ie headwind becomes tailwind.
In the 'Flying Case', all respondents are correct, but by different levels! In a FJ, it means nothing, in a Tucano it might be apparent in the finals turn, based on your gournd track (as stated). Helos and Hover Jets, I'm less sure, but the point is, that the 'Air' cannot move you straight away, and so there is a 'Relative' change in air motion over your craft. This you see as 'Sheer' or (If you are using a ground reference) IAS loss.
Basically. The 'AIR' has a momentum too, Oh and it's a lot bigger than yours!
Personally, I've only had a scary once with wind sheer, in the Falklands, and that was covered by the 10Kts I always held for 'Granny', Now I know why I did that.
Advo
We're all getting sidelined by physics:
Actually, everyone has a good point, but non yet have covered the 'common ground' Inertia.......
In the hover case, change in direction does indeed change your IAS, as the wind is unable to instantaeously speed you up.... ie headwind becomes tailwind.
In the 'Flying Case', all respondents are correct, but by different levels! In a FJ, it means nothing, in a Tucano it might be apparent in the finals turn, based on your gournd track (as stated). Helos and Hover Jets, I'm less sure, but the point is, that the 'Air' cannot move you straight away, and so there is a 'Relative' change in air motion over your craft. This you see as 'Sheer' or (If you are using a ground reference) IAS loss.
Basically. The 'AIR' has a momentum too, Oh and it's a lot bigger than yours!
Personally, I've only had a scary once with wind sheer, in the Falklands, and that was covered by the 10Kts I always held for 'Granny', Now I know why I did that.
Advo
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Surely it is simply due to the increased drag that you get when you turn that slows you down, assuming that you didn't loose any airspeed due this your IAS would remain the same no matter what the wind is doing, and anyway surely it is almost impossible to quantify or prove that airspeed is lost in a turn due to the wind, given that you would lose speed in an abrupt turn anyway.
Actually, thinking about it, the "Monentum" and "Kinetic Energy" crowd are correct. The vectors should resolve themselves even to an observer on the ground.
And they DO!
The mistake many people are making (Mike O of the impressive credentials chief amongst them) is to substitute Speed for Velocity. They are two very different things.
Speed is a scalar quantity and as such only has magnitude whilsy velocity is a vector quantity and thus has magnitude AND direction.
Turn an aircraft through 180deg at a constant speed in nil wind and you haven't changed it's speed, but you HAVE changed it's velocity.By how much? Well, do the vector diagram.
To change a vector of, say, 100kts due west to one of 100kts due east you must add a vector of 200kts to the East. You do this by tilting the lift vector, or reversing your harriers nozzels or however. The result is a delta vee of 200 kts.
Now lets introduce a head wind of fiftey knots. Your ground speed is fiftey knots. We introduce the same angular acceleration by tilting our lift vector or whatever in exactley the same way as nil wind.
Now we are heading due east with an airspeed of 100kts and a ground speed of 150kts.
Our velocity with reference to the ground has changed from 50kts to the west to 150kts to the east. A change of? 200kts to the east, exactley the same as in nil wind.
The "down wind turn" is a crock.
And they DO!
The mistake many people are making (Mike O of the impressive credentials chief amongst them) is to substitute Speed for Velocity. They are two very different things.
Speed is a scalar quantity and as such only has magnitude whilsy velocity is a vector quantity and thus has magnitude AND direction.
Turn an aircraft through 180deg at a constant speed in nil wind and you haven't changed it's speed, but you HAVE changed it's velocity.By how much? Well, do the vector diagram.
To change a vector of, say, 100kts due west to one of 100kts due east you must add a vector of 200kts to the East. You do this by tilting the lift vector, or reversing your harriers nozzels or however. The result is a delta vee of 200 kts.
Now lets introduce a head wind of fiftey knots. Your ground speed is fiftey knots. We introduce the same angular acceleration by tilting our lift vector or whatever in exactley the same way as nil wind.
Now we are heading due east with an airspeed of 100kts and a ground speed of 150kts.
Our velocity with reference to the ground has changed from 50kts to the west to 150kts to the east. A change of? 200kts to the east, exactley the same as in nil wind.
The "down wind turn" is a crock.
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I don't really understand this negative Airspeed business, if you hovering in a 10 knot headwind, you have a IAS of 10 knots, in order to maintain that you need some power, as you turn around you will gradually start moving relative to the ground, but a given power setting will always (all other things being equal) maintain that 10 Knot IAS, what has negative airspeed got to do with anything?
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I take it though that the instance of the helicopter only applies to helicopters, in the case of the harrier the thrust nozzles would be moving with the body of the aircraft and therefore would provide thrust in the same direction that the nose is pointing (provided you didn't adjust them) so a flat turn coming out of a headwind would not result in a negative airspeed. Is that correct?
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I don't really understand this negative Airspeed business, if you hovering in a 10 knot headwind, you have a IAS of 10 knots, in order to maintain that you need some power, as you turn around you will gradually start moving relative to the ground, but a given power setting will always (all other things being equal) maintain that 10 Knot IAS, what has negative airspeed got to do with anything?
Consider a Su-29 in a vertical dive at 250kts IAS which is rotated rapidly in pitch through 180deg to point vertically upwards. Does it retain 250kts IAS? Does it initially have any positive IAS? Nope, it tail slides at negative IAS. A nice airshow party piece!
Conceptually, that scenario is not materially different from a level flight rapid rotation in yaw through 180 degrees, and is independent of wind velocity (as long as it's constant). This is inertia at play (albeit, in the vertical plane, aided by gravity), eventually overcome by thrust.
Any clearer?
Last edited by Islander2; 18th May 2007 at 12:51.
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That sounds slightly different from the helicopter idea, where my understanding is that the body of the helicopter is now moving backwards relative to the rotors (hence negative airspeed). With the Su-29 it tailslides (also I understand negative airspeed) but that is due to its temporary loss of momentum.
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Sorry, but the Su-29 tailslide I described is because of its momentum with respect to the air mass (aided by gravity), not because of a loss of momentum. If you very rapidly yaw a helicopter (or a Harrier) that has some airspeed in level flight, the effect is the same ... not as severe, because you only have inertia, you don't have a force (gravity) attempting to maintain the previous flight path, but you for sure will initially experience a loss of airspeed.
And windspeed relative to the ground is immaterial. The effect is just the same in zero wind as it is in a steady 80kts howling gale.
And windspeed relative to the ground is immaterial. The effect is just the same in zero wind as it is in a steady 80kts howling gale.
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I have to admit that I don't really understand the finer detail of this, but just to be clear, I understand all these effects discussed more recently on the thread; Su-29 etc. are all by virtue of the effects experienced in a turn and nothing to do with this 'downwind turn' rubbish talked about earlier.
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Hurrah, it's me.
I'm back, and for those that were wondering, no I was not beaten down by the fierce banter nor did I need to take medication. I've been away on a cse without internet access and so am amazed to see the power of this thread. It is now attracting new posters who clearly have not read from the beginning and hence repeat old ground. This means that the thread has now reached a critical mass and has the capability of living forever, or never dying (depending on your point of view). Congratulations to the original poster.
You are in for a treat as I will contribute later, having clarified my thought processes in happy hour, but for starters, let me just say that if you do not see eye to eye with the likes of JF on this one, perhaps this issue isn't as black and white as you'd like to suggest!
FS Bell, (very young to be an SNCO!) before dismissing opinions contrary to your own as rubbish do a little research and compare John Farley's experience to your own.
As for trotting out my quals, that was merely to demonstrate that I am not actually retarded as suggested by, I believe, Deliverance. Hopefully we can discuss this at a slightly higher level than the 'that's rubbish', 'you're mental', 'your mum's fat' level.
TTFN
By the way, Wizo, can you tell your mum to give me back my slippers.
You are in for a treat as I will contribute later, having clarified my thought processes in happy hour, but for starters, let me just say that if you do not see eye to eye with the likes of JF on this one, perhaps this issue isn't as black and white as you'd like to suggest!
FS Bell, (very young to be an SNCO!) before dismissing opinions contrary to your own as rubbish do a little research and compare John Farley's experience to your own.
As for trotting out my quals, that was merely to demonstrate that I am not actually retarded as suggested by, I believe, Deliverance. Hopefully we can discuss this at a slightly higher level than the 'that's rubbish', 'you're mental', 'your mum's fat' level.
TTFN
By the way, Wizo, can you tell your mum to give me back my slippers.
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Oxmels,
To my knowledge I have not suggested that I disagree with anything that John has said, but you have admit, when faced with something said (even from someone of your calibre) that goes against everything you've ever been told one is bound to question it (and when I said rubbish I was merely quoting what an instructor had said to me when I asked him about the 'downwind turn')
To my knowledge I have not suggested that I disagree with anything that John has said, but you have admit, when faced with something said (even from someone of your calibre) that goes against everything you've ever been told one is bound to question it (and when I said rubbish I was merely quoting what an instructor had said to me when I asked him about the 'downwind turn')
By the way, Wizo, can you tell your mum to give me back my slippers.
lf you'd like, however, to point out the errors in any of my posts, please do so.
You might also note that John Farley has said that people more qualified than HIM have told him he's wrong...