Why do we Lose Airspeed in a Turn and What Causes This?
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Could someone please clear up this related issue....
I have read similar threads on this issue from back in 2000 and someone then claimed that at Boscombe Down in the 1960s they attributed the 'downwind turn' effect to the 'fact' that although we assume that the plane flies relative to the air around it, the plane's intertia is relative to the earth mass and therefore looses some when making a downwind turn, explaining the loss of airspeed.
Many people on this thread have denied that the above is true and until today I was inclined to agree with them, but now I'm not so sure.
I have read similar threads on this issue from back in 2000 and someone then claimed that at Boscombe Down in the 1960s they attributed the 'downwind turn' effect to the 'fact' that although we assume that the plane flies relative to the air around it, the plane's intertia is relative to the earth mass and therefore looses some when making a downwind turn, explaining the loss of airspeed.
Many people on this thread have denied that the above is true and until today I was inclined to agree with them, but now I'm not so sure.
Remain sure.
Once off the earths surface the only contribution the earth makes is gravity, which is a vertical acceleration.
If not, could someone please tell me at what height this mysterious force cuts out? 100 miles? 1000?
If i'm half way to Mars is my momentum still only relative to the earth?
Don't think so...
Once off the earths surface the only contribution the earth makes is gravity, which is a vertical acceleration.
If not, could someone please tell me at what height this mysterious force cuts out? 100 miles? 1000?
If i'm half way to Mars is my momentum still only relative to the earth?
Don't think so...
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I am really guessing now but perhaps they believed the relativity of intertia to the earth decreased gradually as you moved away fromt the earth?
Again I am inclined to agree with Wizofoz but when someone who is clearly an experinced helicopter pilot like Mike starts saying otherwise one starts to question one's existence.
Again I am inclined to agree with Wizofoz but when someone who is clearly an experinced helicopter pilot like Mike starts saying otherwise one starts to question one's existence.
F/sgt,
I've flown a glider which had a stalling speed of 32kts in a constant banked turn at an IAS of 40kts in a 35kt wind. The IAS didn't budge. The centre of my circle (and the thermal I was in) just drifted downwind at 35kts, without myself or the glider being aware of it.
Similarly I've been in a holding pattern with 120kts of wind blowing. If the downwind turn existed I'd currently be dead.
I've flown a glider which had a stalling speed of 32kts in a constant banked turn at an IAS of 40kts in a 35kt wind. The IAS didn't budge. The centre of my circle (and the thermal I was in) just drifted downwind at 35kts, without myself or the glider being aware of it.
Similarly I've been in a holding pattern with 120kts of wind blowing. If the downwind turn existed I'd currently be dead.
Fly Conventional Gear
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Agreed. But what Mike seemed to talking about earlier was that in a flat turn (or at least quite abrupt) you do get a 'downwind turn' effect; John F was talking about doing 180s in Harriers and the associated loss of IAS when turning out of the wind.
I think there must be some effect going here, but neither of the two have yet explained why they are putting this down to the wind and not the fact that they are conducting very high drag manoevres which will always loose airspeed.
Of course if you have a 40 knot IAS in a 40 headwind and turn round you will not have an 80 knot groundspeed immediately, because the plane will loose airspeed in the turn, and take time to put it back on. (to use an example used earlier)
What would be nice would be someone who advocates the 'downwind turn' effect proving that airspeed is being lost due to the wind, not the other effects of the turn.
I think there must be some effect going here, but neither of the two have yet explained why they are putting this down to the wind and not the fact that they are conducting very high drag manoevres which will always loose airspeed.
Of course if you have a 40 knot IAS in a 40 headwind and turn round you will not have an 80 knot groundspeed immediately, because the plane will loose airspeed in the turn, and take time to put it back on. (to use an example used earlier)
What would be nice would be someone who advocates the 'downwind turn' effect proving that airspeed is being lost due to the wind, not the other effects of the turn.
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What would be nice would be someone who advocates the 'downwind turn' effect proving that airspeed is being lost due to the wind, not the other effects of the turn.
An aeroplane in flight is completely oblivious to a steady wind ... it has not the slightest clue as to whether it's pointing upwind, downwind or sideways.
The pilot, however, isn't oblivious if he can see the ground, and therein lies the real danger of the low-level downwind turn!
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I thought so.
On a slightly different point though...
Some posters seem to be equating a turn out of a steady wind as being the same as windshear, i.e. the oncoming wind speed is suddenly very different, leading to a loss of airspeed. Would anyone care to clear this one up?
On a slightly different point though...
Some posters seem to be equating a turn out of a steady wind as being the same as windshear, i.e. the oncoming wind speed is suddenly very different, leading to a loss of airspeed. Would anyone care to clear this one up?
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Well, we're back to Su-29s!
An aeroplane cannot "turn out of a steady wind", assuming 'wind' is being used in its conventional sense of airflow across the earth's surface. Aerodynamically, the airmass is the aeroplane's inertial frame of reference. What that airmass is doing with respect to another frame of reference (the earth) is completely irrelevant. It doesn't know, and doesn't care.
True it does have an "oncoming wind speed"; however in level flight that wind speed is provided by its thrust, and is nothing to do with the movement of the air over the ground. The aeroplane has momentum through the air, which is proportional to the product of its mass and air velocity. Accordingly, if it points its nose very rapidly in a different direction, it initially maintains its original flightpath through the air, resulting in a loss of airspeed (which, in extremis, can even end up negative). It also has momentum across the ground;this is almost certainly a different value but that's because momentum and energy are both inertial frame of reference dependent.
You could say the loss of airspeed is because "the oncoming wind speed is suddenly very different", but you would be referring to "wind" as the speed of the air along the aeroplane's longitudinal axis, nose to tail, not in the conventional sense of the speed of the air over the ground (and the loss of airspeed is the same whether you're turning upwind or downwind).
Windshear, however, relates to a very different inertial frame of reference. Here you would be talking about a sudden change of wind speed (or direction) with respect to the earth's surface.
An aeroplane yawed rapidly anticlockwise does see the same effect on its airspeed as the wind veering sharply due to windshear. But this is not an effect the aeroplane sees in a balanced turn.
Bluntly put, I know, but those that equate normal turning with windshear are simply wrong. An aeroplane in a normal, banked level turn loses airspeed for one reason only - increased drag, as correctly stated by the first two respondents to 4L3X's request for help!
An aeroplane cannot "turn out of a steady wind", assuming 'wind' is being used in its conventional sense of airflow across the earth's surface. Aerodynamically, the airmass is the aeroplane's inertial frame of reference. What that airmass is doing with respect to another frame of reference (the earth) is completely irrelevant. It doesn't know, and doesn't care.
True it does have an "oncoming wind speed"; however in level flight that wind speed is provided by its thrust, and is nothing to do with the movement of the air over the ground. The aeroplane has momentum through the air, which is proportional to the product of its mass and air velocity. Accordingly, if it points its nose very rapidly in a different direction, it initially maintains its original flightpath through the air, resulting in a loss of airspeed (which, in extremis, can even end up negative). It also has momentum across the ground;this is almost certainly a different value but that's because momentum and energy are both inertial frame of reference dependent.
You could say the loss of airspeed is because "the oncoming wind speed is suddenly very different", but you would be referring to "wind" as the speed of the air along the aeroplane's longitudinal axis, nose to tail, not in the conventional sense of the speed of the air over the ground (and the loss of airspeed is the same whether you're turning upwind or downwind).
Windshear, however, relates to a very different inertial frame of reference. Here you would be talking about a sudden change of wind speed (or direction) with respect to the earth's surface.
An aeroplane yawed rapidly anticlockwise does see the same effect on its airspeed as the wind veering sharply due to windshear. But this is not an effect the aeroplane sees in a balanced turn.
Bluntly put, I know, but those that equate normal turning with windshear are simply wrong. An aeroplane in a normal, banked level turn loses airspeed for one reason only - increased drag, as correctly stated by the first two respondents to 4L3X's request for help!
Last edited by Islander2; 19th May 2007 at 07:26.
"In IMC I turn and make no changes in power. My airspeed remains the same. I have no idea what my groundspeed is at that time as the actual (as opposed to reported) wind could be doing anything.
Therefore Mike and John are talking a bunch of hoop.
Therefore the pixies win (barring false dichotomies).
QED."
Sorry to pick on you PTT, but I have to disagree. During my time in the world of aviation I have looked quite closely into this 'misunderstanding'. However, I can assure you that if you take a relatively small/light ac and try to fly a low IAS turn in a strong wind it is much harder than in calm coditions.
I managed to find a 130/150kt jet (no turbulence) and fly steep turns at 100knots in a Tincan. Control inputs/power changes needed all the time, but not when the exercise was repeated clear of the jet or at higher IAS.
This was all flown on the dials and not with reference to the gnd.
Furthermore, during many IRTs I have seen others battle with this as they attempt to fly level 180kt steepies!
The proof is in the pudding!!!
Therefore Mike and John are talking a bunch of hoop.
Therefore the pixies win (barring false dichotomies).
QED."
Sorry to pick on you PTT, but I have to disagree. During my time in the world of aviation I have looked quite closely into this 'misunderstanding'. However, I can assure you that if you take a relatively small/light ac and try to fly a low IAS turn in a strong wind it is much harder than in calm coditions.
I managed to find a 130/150kt jet (no turbulence) and fly steep turns at 100knots in a Tincan. Control inputs/power changes needed all the time, but not when the exercise was repeated clear of the jet or at higher IAS.
This was all flown on the dials and not with reference to the gnd.
Furthermore, during many IRTs I have seen others battle with this as they attempt to fly level 180kt steepies!
The proof is in the pudding!!!
Cobblers!
In a word.
Difficulty in flying low IAS turns in strong winds is all to do with associated gusts and turbulence, nothing to do with the 'downwind' nonsense.
As for 180 KIAS steep turns, difficulty is all to do with gyroscopic instrument turning errors.
In a word.
Difficulty in flying low IAS turns in strong winds is all to do with associated gusts and turbulence, nothing to do with the 'downwind' nonsense.
As for 180 KIAS steep turns, difficulty is all to do with gyroscopic instrument turning errors.
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Ok, but what would Peacock say to Wizofoz's earlier experience that flying in an 120 knot wind in the hold does not effect the airspeed?
A steady wind of 120kts at typical holding altitudes is as big a myth as the downwind turn! See BEagle's post (#145) - the real reason for the small changes in airspeed that Wizofoz failed to observe is the variability of the wind's velocity.
Don't just talk about it, get out there and do it. I've just been flying in a 35kt wind. A/c S and L, nicely trimmed for level flight indicating 80kts plus or minus 2 or 3, over the shoreline, wind off the sea, so no turbulence. Trimmed into a 15 deg turn, adjusted power to maintain height. A/c quite happily flew 360 deg turns with airspeed plus or minus 2 or 3 knots, which can be ignored for gusts. 
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I have to say Islander's description of windshear does seem to sort of vindicate what Mike and John have been saying all along, doing low level circuits in a helicopter with abrupt turns seems likely to cause a sort of 'windshear' effect.
Although to me why Mike thinks groundspeed has anything to do with the plane's momentum in terms of the air is still unclear.
Although to me why Mike thinks groundspeed has anything to do with the plane's momentum in terms of the air is still unclear.
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I have to say Islander's description of windshear does seem to sort of vindicate what Mike and John have been saying all along, doing low level circuits in a helicopter with abrupt turns seems likely to cause a sort of 'windshear' effect.
Although to me why Mike thinks groundspeed has anything to do with the plane's momentum in terms of the air is still unclear.
I know I won't be able to persuade him otherwise, but just maybe I can give an illustration that will help others that are less entrenched.
For this, we need to engineer an absolutely steady wind. So we are going to embark on HMS Ark Royal and go boating. But bear with me, some aviating comes later!
Let's clear the ship's hangar of all those nasty whirly and V/STOLy things, and put a proper aeroplane in place. We're going to fly it in the hangar, so it's going to have to be small. A model aeroplane. And perlease, don't throw Reynolds Number at me, that's completely irrelevant for this discussion.
Okay, let's close all the doors to the hangar so it's air tight, and ask the ship's captain to give us 35 kts. To eliminate any confusion, btw, we've chosen a day and location where there's zero real wind ... that took some finding, I can assure you! Inside the hangar we have perfectly still air, yet that air is moving over the earth's surface at 35kts. We've just created the perfect, never seen in practice, absolutely steady 35 kt wind!
Now let's fly our model. As we fly upwind at 35KIAS towards the ships stern, we have a groundspeed (earth reference) of zero. Now let's cross our fingers and risk one of those terrible downwind turn thingies. On completion of the turn our model is now headed towards the ship's bow with a groundspeed (earth reference) of 70kts.
By 'eck, says Mike, look at all that extra momentum (or he may say look at all that extra kinetic energy). That needs an energy input from somewhere, so extra power is required, and until it takes effect airspeed will decrease.
Flying the model, meanwhile, we leave the power unchanged and, aside from a tiny decrease in airspeed in each of the turns, which is promptly restored upon levelling the wings, the airspeed as measured by our super-accurate instrumentation remains absolutely constant regardless of the direction of travel.
Which leaves Mike somewhat bemused!!
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The airborne world would be a very bizarre place if it worked the way MO preaches:
Imagine BFM – rather than come in out of sun, you’d probably come in based upon the wind direction! That first, critical, 180 would be interesting as (all else (but the wind!) being equal) you’d get your nose on first - particularly if it was a real stonker of a wind ;-)!
I can hardly believe that there is an argument going on about this – but it’s dead amusing to watch.
And as for ‘The proof is in the pudding’ (H_P)
“Control inputs/power changes needed all the time”
That doesn’t say much!
If you can’t understand the logic, then the last hope is experimental evidence – objective assessment is what is needed, not subjective (“Control inputs/power changes needed”). Go up in an aircraft with suitably sensitive recording equipment and see what really happens.
Going ‘slow’ (for your particular aeroplane) might make the handling a little trickier in turns, but that’s not because of the steady wind!
Imagine BFM – rather than come in out of sun, you’d probably come in based upon the wind direction! That first, critical, 180 would be interesting as (all else (but the wind!) being equal) you’d get your nose on first - particularly if it was a real stonker of a wind ;-)!
I can hardly believe that there is an argument going on about this – but it’s dead amusing to watch.
And as for ‘The proof is in the pudding’ (H_P)
“Control inputs/power changes needed all the time”
That doesn’t say much!
If you can’t understand the logic, then the last hope is experimental evidence – objective assessment is what is needed, not subjective (“Control inputs/power changes needed”). Go up in an aircraft with suitably sensitive recording equipment and see what really happens.
Going ‘slow’ (for your particular aeroplane) might make the handling a little trickier in turns, but that’s not because of the steady wind!
Cobblers!
In a word.
As for 180 KIAS steep turns, difficulty is all to do with gyroscopic instrument turning errors.
In a word.
As for 180 KIAS steep turns, difficulty is all to do with gyroscopic instrument turning errors.
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Are you telling me that the possibility of a slow erection towards a false vertical datum is causing it!!!
I think Beagle may have been refering to Gimble Error. Noticeable in the Tincan and other aircraft with simple Gyro-compasses.
Or did I dream it?
Or did I dream it?