Downwind Turns
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Keep it simple, guys! Of course it has nothing to do with your ground speed. It's the air speed that keeps you in the air, be it true or indicated or whatever...the motion of the aircraft in regards to the surrounding air. Now, I'm pretty shocked to see so many explanations from experienced people not being close to the truth. Try this in a simulator: W/V: 360/100kts. HDG:360. IAS: 100kts. This will give you GS:0kts. Then simply change the heading to 180, as you would in a turn, except that this happens instantly, not letting the aircraft accelerate. Of course it will fall out of the sky, because of negative airspeed.
How about it, Avi8tor? I hope at least you'll agree...
How about it, Avi8tor? I hope at least you'll agree...
Last edited by DJ Mixmaster; 26th Feb 2004 at 23:36.
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I have been resisting contributing towards this thread until now.
The problem is not with steady-state wind, as has already been pointed out by John Farley in an excellent post.
The problem is that there is no such thing as a steady wind.
If you are flying into a gusty wind, your groundspeed will not vary directly with the wind gusts because of your momentum. So turning downwind will affect your lift.
This is, basically, the same phenomenon that can trick people flying into a microburst. As you get near, the headwind increases massively and rapidly, but your groundspeed does not change much immediately because you have inertia providing momentum to be lost first. So people see their ASI getting all excited and cut power. Just at that moment, as their airspeed is adjusting itself naturally, the engines reduce thrust, overall energy decreases - and then the headwind drops, reducing airspeed further, and the downdraft hits them. Low engine power, low airspeed, low momentum and massive downdraft and the ground stops their fall.
You cannot consider instantaneous airspeed to be a simple function of grounspeed and windspeed as there are fluctuations due to gusts and the (sometimes) cushioning effect of your momentum. So when you turn downwind, a gust of wind up the chuff will rapidly reduce your airspeed.
The downwind turn phenomenon does exist - no matter how much common sense dictates that it should not.
The problem is not with steady-state wind, as has already been pointed out by John Farley in an excellent post.
The problem is that there is no such thing as a steady wind.
If you are flying into a gusty wind, your groundspeed will not vary directly with the wind gusts because of your momentum. So turning downwind will affect your lift.
This is, basically, the same phenomenon that can trick people flying into a microburst. As you get near, the headwind increases massively and rapidly, but your groundspeed does not change much immediately because you have inertia providing momentum to be lost first. So people see their ASI getting all excited and cut power. Just at that moment, as their airspeed is adjusting itself naturally, the engines reduce thrust, overall energy decreases - and then the headwind drops, reducing airspeed further, and the downdraft hits them. Low engine power, low airspeed, low momentum and massive downdraft and the ground stops their fall.
You cannot consider instantaneous airspeed to be a simple function of grounspeed and windspeed as there are fluctuations due to gusts and the (sometimes) cushioning effect of your momentum. So when you turn downwind, a gust of wind up the chuff will rapidly reduce your airspeed.
The downwind turn phenomenon does exist - no matter how much common sense dictates that it should not.
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DJ
I am not really sure what you are saying. In a sim if you did that turn (in a steady state wind) there would be no drop in IAS - I don't know what you mean by an instantaneous turn - they simply don't happen (you would need infinite energy to do that). The turn would proceed as per normal (in fact i have done this in the sim).
Capt Stable.
What you say in regards gusts is true, but it has nothing to do with turning downwind. The same can (and does) happen turning upwind, crosswind or not turning at all.
Gusts and IAS fluctuations is well known.
In regards turning downwind, even in a moderately stable wind there usually will be no change in IAS. The only change in IAS is due to a gust NOT the fact you are turning downwind.
I am not really sure what you are saying. In a sim if you did that turn (in a steady state wind) there would be no drop in IAS - I don't know what you mean by an instantaneous turn - they simply don't happen (you would need infinite energy to do that). The turn would proceed as per normal (in fact i have done this in the sim).
Capt Stable.
What you say in regards gusts is true, but it has nothing to do with turning downwind. The same can (and does) happen turning upwind, crosswind or not turning at all.
Gusts and IAS fluctuations is well known.
In regards turning downwind, even in a moderately stable wind there usually will be no change in IAS. The only change in IAS is due to a gust NOT the fact you are turning downwind.
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I've changed my mind. I agree with the minority now.
The downwind turn/loss of lift phenomenon does exist in gusty conditions.
But as cowabunga438 has indicated:
The crosswind turn/loss of lift phenomenon in gusty conditions also exists .
The upwind turn/loss of lift phenomenon in gusty conditions also exists.
The little bit crosswind and little bit downwind at the same time turn/loss of lift phenomenon in gusty conditions also exists.
I think licenses should be endorsed for "Flight Restricted To Calm Wind Conditions" until this topic is fully understood
FIS.
The downwind turn/loss of lift phenomenon does exist in gusty conditions.
But as cowabunga438 has indicated:
The crosswind turn/loss of lift phenomenon in gusty conditions also exists .
The upwind turn/loss of lift phenomenon in gusty conditions also exists.
The little bit crosswind and little bit downwind at the same time turn/loss of lift phenomenon in gusty conditions also exists.
I think licenses should be endorsed for "Flight Restricted To Calm Wind Conditions" until this topic is fully understood
FIS.
Do a Hover - it avoids G
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Field in Sight
Just one small point about your brave post (!). If we assume the gusts are mainly, repeat mainly, variations of wind speed rather than variations in wind direction the crosswind stuff hits you abeam and while you may have to work at the ailerons you do not expect to see the ASI going quite so wild. Which leaves gust effects appearing on the ASI during downwind and into wind turns.
If there is anything in my gut feel that while flying my toy R/C aeroplane gusts tend to arrive all of a sudden and fade away more slowly then this would account for why the gust as you turn into wind does not bother people but the one hitting you up the jacksi seems worse and has given rise to so much debate.
I really would like a met man to discuss (factually) the shape of typical speed changes in a gust.
Just one small point about your brave post (!). If we assume the gusts are mainly, repeat mainly, variations of wind speed rather than variations in wind direction the crosswind stuff hits you abeam and while you may have to work at the ailerons you do not expect to see the ASI going quite so wild. Which leaves gust effects appearing on the ASI during downwind and into wind turns.
If there is anything in my gut feel that while flying my toy R/C aeroplane gusts tend to arrive all of a sudden and fade away more slowly then this would account for why the gust as you turn into wind does not bother people but the one hitting you up the jacksi seems worse and has given rise to so much debate.
I really would like a met man to discuss (factually) the shape of typical speed changes in a gust.
