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Downwind turn Myth

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Old 11-22-2010, 10:58 AM
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highhorse
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Default Downwind turn Myth

Another thread contains a postulation that a model crashed due to the wind direction relative to the models flight path as it topped a loop. In other words, the model might not have crashed if the nose had been pointed into the wind over the top. (sigh)

Most of you learned better a long time ago, but this myth just keeps hanging in there. It's persists (sadly) even in the lower rungs of full scale aviation, and among pilots who have had enough training to know better. Lets set the record straight once and for all please.

It does seem counter-intuitive, but here is the truth:

Once an a/c has broken ground, steady state winds have no effect on airspeed (hence, lift) whatsoever, and airspeed does not change simply because one is flying upwind, downwind, crosswind, or even when alternating between any combinations of the above. Period. That is the beginning, middle, and end of the story.


As an aid to understanding this seemingly counter-intuitive fact, imagine yourself boating in a wide river with a 10 knot currrent. If you are putting along at 5 knots indicated speed, the water does not come crashing over the side simply because you are traveling from one bank to the other perpendicular to the current. It does not wash over the stern when headed down stream, as if you were suddenly traveling at a speed of five knots negative.

Or go for a swim in the ocean where the current is flowing parallel to the beach.

Or go scuba diving.

You will be carried along with the current, but not feel it, no matter which way you face or swim.

Or note that airliners don't fall out of the sky when making a 180 degree turn from a 150 kt headwind to a 150 tailwind, even though that net 300 kt difference in the wind is TEN TIMES the typical stall speed margin at the altitudes where such winds are encountered.

Birds fly just fine downwind, and don't suddenly crash into the trees when turning in that direction.

We could go on and on...but hopefully that's not necessary?



Don.



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Old 11-22-2010, 11:04 AM
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Default RE: Downwind turn Myth

highhorse

That may all well be true if you let the model drift with the wind, however, if you fly a loop directly in front of you (i.e. fixed in space relative to the ground and not the air) then the model will have more airspeed one way than the other at different parts of the loop. Otherwise, why do we bother taking off into wind?
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Old 11-22-2010, 11:15 AM
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Default RE: Downwind turn Myth

There is absolutely no AERODYNAMIC reason to take off or land into the wind. We do that for shortened ground rolls.

Once an a/c is in the air and not in contact with the ground, steady-state wind direction is completely, totally, 100% irrelevant aerodynamically.
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Old 11-22-2010, 11:18 AM
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Default RE: Downwind turn Myth

I would not recomend a downwind takeoff.......... the runway gets short really fast!

Rick
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Old 11-22-2010, 11:18 AM
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Default RE: Downwind turn Myth

Ok, i'll ask another question then.

When you fly a perfect loop in front of you (as if I do) why do the control inputs differ depending on where the wind is coming from?
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Old 11-22-2010, 11:19 AM
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Default RE: Downwind turn Myth

I think airspeed vs. ground speed is where most of the confusion stems from; airspeed is airspeed, while the ground speed will increase / decrease relative to direction as long as the airspeed stays constant.

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Old 11-22-2010, 11:23 AM
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Default RE: Downwind turn Myth

More info on the topic:

http://www.djaerotech.com/dj_askjd/d.../downwind.html

http://www.aeroexperiments.org/brainteasers.shtml
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Old 11-22-2010, 11:27 AM
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Default RE: Downwind turn Myth

You didn't answer my question
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Old 11-22-2010, 11:28 AM
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Default RE: Downwind turn Myth

ORIGINAL: highhorse

There is absolutely no AERODYNAMIC reason to take off into the wind. We do that for shortened ground rolls.

Once an a/c is in the air and not in contact with the ground, steady-state wind direction is completely, totally, 100% irrelevant aerodynamically.
a
And by what form of education can you instruct us as to a final theory. you don't know what your talking about. if 'im going 100 mph with a tailwind speed of 20 mph and i turn into that tailwind which now becomes a 20 mph headwind and all the time maintaining the said 100mph . your saying my airspeed doesn't increase? I'm talkng about airspeed over the wing , not ground speed. last time i knew 100 and 25 made 125. it makes no difference weather you enter a loop from the tailwind position . once you enter a headwind your airspeed "at least over the wing" increases . it's a common sense issue. the guy probably got to the top of the loop and stalled it, then fought it to the ground . or maybe something inside the plane messed up because he was inverted like his battery came loose ....

let me see you take off downwind with a scale turbine jet with a 20mph tailwind. wait ,let me get my video camera out first. i don't want to miss this one
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Old 11-22-2010, 11:33 AM
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Default RE: Downwind turn Myth

Warbird1, your airspeed remains constant at 100. It is your groundspeed that is varying in that instance (unless you are taling about flying a 100mph groundspeed).

George has it nailed. My argument is that a loop is flown mainly with respect to groundspeed (or your loop will drift downwind) hence the variation in airspeed.
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Old 11-22-2010, 11:34 AM
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Default RE: Downwind turn Myth

Highhorse is correct but only addresses an aircraft in a constant state. As a pilot of RC aircraft and full scale planes for many years I know that one can see obvious effects when you turn away from or into the wind. The factor not addressed is acceleration. An plane flying in a strong wind which quickly turns 'downwind" will not suddenly accelerate into the new airflow (wind). Depending on variables such as mass and drag the plane will take some time to accelerate to the new speed. During this time there can be a loss of speed and a resultant loss of lift. It is usually easily corrected with elevator application but the effect is real. The same thing can happen in reverse which is why many planes will climb as they turn into the wind.
Yes, a plane does not care what direction the wind is going, but it does care about changes in airspeed/lift. Nearly all of us have taken off into a strong wind and felt the plane try to sink out when we turn downwind. It's not imagined.........
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Old 11-22-2010, 11:34 AM
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Default RE: Downwind turn Myth

You gave the answer yourself! You're still flying 100mph, period.
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Old 11-22-2010, 11:35 AM
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Default RE: Downwind turn Myth

ORIGINAL: siclick33

Ok, i'll ask another question then.

When you fly a perfect loop in front of you (as if I do) why do the control inputs differ depending on where the wind is coming from?
As a former full-scale aerobatic competitor, this quite sensible Q is close to my heart.

The answer is that u are adjusting the path thru a moving body of air to make the loop LOOK round from your perspective. But what the airplane is actually DOING is not a round loop at all, it's flying an elliptical loop that has been "squished down". Note that the wind affects the VISUAL SHAPE of a loop from YOUR perspective as you remain fixed to the ground. The wind does not affect the "feel" or aerodynamics of the loop from the airplanes perspecive. It is YOUR PERSPECTIVE that is changed, not the aerodynamics :-)))

Draw a loop on a piece of paper. Then do it with your eyes closed till you can consistently make a fairlt round loop. THEN do the same thing as someone else slowly pulls the paper sideways.
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Old 11-22-2010, 11:36 AM
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Default RE: Downwind turn Myth

ORIGINAL: siclick33

Warbird1, your airspeed remains constant at 100. It is your groundspeed that is varying in that instance (unless you are taling about flying a 100mph groundspeed).

George has it nailed. My argument is that a loop is flown mainly with respect to groundspeed (or your loop will drift downwind) hence the variation in airspeed.
[sm=thumbs_up.gif]
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Old 11-22-2010, 11:38 AM
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Default RE: Downwind turn Myth

i'm talking form airspeed over the wing perspective. ground speed is irrelevant if you can maintain enough lift to stay in the air. that's why a trainer can fly backwards with enough wind. i'm arguing the point that headwind and tailwinds don't matter.
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Old 11-22-2010, 11:40 AM
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Default RE: Downwind turn Myth

i'm talking from airspeed over the wing perspective. ground speed is irrelevant if you can maintain enough lift to stay in the air. that's why a trainer can fly backwards with enough wind. i'm arguing the point that headwind and tailwinds don't matter.
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Old 11-22-2010, 11:43 AM
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Default RE: Downwind turn Myth

ORIGINAL: warbird_1

ORIGINAL: highhorse

There is absolutely no AERODYNAMIC reason to take off into the wind. We do that for shortened ground rolls.

Once an a/c is in the air and not in contact with the ground, steady-state wind direction is completely, totally, 100% irrelevant aerodynamically.
a
And by what form of education can you instruct us as to a final theory. you don't know what your talking about. if 'im going 100 mph with a tailwind speed of 20 mph and i turn into that tailwind which now becomes a 20 mph headwind and all the time maintaining the said 100mph . your saying my airspeed doesn't increase? I'm talkng about airspeed over the wing , not ground speed. last time i knew 100 and 25 made 125. it makes no difference weather you enter a loop from the tailwind position . once you enter a headwind your airspeed ''at least over the wing'' increases . it's a common sense issue. the guy probably got to the top of the loop and stalled it, then fought it to the ground . or maybe something inside the plane messed up because he was inverted like his battery came loose ....

let me see you take off downwind with a scale turbine jet with a 20mph tailwind. wait ,let me get my video camera out first. i don't want to miss this one
LMAO... ya, ok. U smart, me dumb. Yes, I'm saying ur airspeed DOES-NOT-INCREASE. I'm saying that because it's a fact. (also, dude, the Airbus I make my living in is quite capable of of taking off in winds more than twice that, given a long enough runway. Even then, the limitation becomes one of tire speed, and not aerodynamics)
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Old 11-22-2010, 11:46 AM
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Default RE: Downwind turn Myth

ORIGINAL: warbird_1

i'm talking from airspeed over the wing perspective. ground speed is irrelevant if you can maintain enough lift to stay in the air. that's why a trainer can fly backwards with enough wind. i'm arguing the point that headwind and tailwinds don't matter.
Trainers don't fly backwards, they just seem to fly backwards relative to the ground.
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Old 11-22-2010, 11:48 AM
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Default RE: Downwind turn Myth

quote:

ORIGINAL: siclick33

Ok, i'll ask another question then.

When you fly a perfect loop in front of you (as if I do) why do the control inputs differ depending on where the wind is coming from?
As a former full-scale aerobatic competitor, this quite sensible Q is close to my heart.

The answer is that u are adjusting the path thru a moving body of air to make the loop LOOK round from your perspective. But what the airplane is actually DOING is not a rounf loop at all, it's flying an elliptical loop that has been "squished down". Note that the wind affects the VISUAL SHAPE of a loop from YOUR perspective as you remain fixed to the ground. The wind does not affect the "feel" or aerodynamics of the loop from the airplanes perspecive. It is YOUR PERSPECTIVE that is changed, not the aerodynamics :-)))

Draw a loop on a piece of paper. Then do it with your eyes closed till you can consistently make a fairlt round loop. THEN do the same thing as someone else slowly pulls the paper sideways.
And taking this to an extreme to try to explain my thoughts.

Assume flying at 100 knots in a 100 knot tailwind and pulling up for a loop. The only way you will penetrate into wind over the top (to make a round loop) is to increase your airspeed. This is obviously an exagerated example but the same must happen to a lesser extent with normal windspeeds if your loop is to appear round.

I agree that the downwind turn myth is a myth, but am not sure that the same argument applies in the case of a loop flown fixed with respect to the ground.
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Old 11-22-2010, 11:51 AM
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Default RE: Downwind turn Myth

Highhorse,

I'm not going to feed into a lengthy debate,but your blanket statement is incorrect.Depending on how aggressively the course reversal is made,the relative wind,and resultant coefficient of lift can change.The basic principal of physics,that an object in motion,stays in motion,relative to the stored energy,which varies as a function of mass x velocity,is a player.

In simple terms,think of a 3D airplaneflying into a 20kt headwind,and doing a a 3D type push/pull over of 180 degrees.At completion it is now in a 20 kt tailwind.It used thrust/lift to overcome the physics principal described earlier,yet realized a 40 kt loss of airspeed,with a corresponding loss in coefficient of lift.The above is an extreme example,and with the smooth,high energy type flying we do with jets,this effect is usually negligable.

For what it's worth,I don't believe wind was a significant factor in the viperjet crash.It appears he had low energy over the top,and pulled too hard,with a resultant stall/snap.Unfortunately,some don't understand that to get the plane flying again,you have to unload the wing.An airplane can be stalled at any airspeed or attitude.At Delta,when we teach upset recovery,in the sims I have seen the jet remain stalled in a 40 degree nose down attitude,with power on.The answer is to level the wings,and push slightly,unloading the wing,and letting it start flying again.Then pull and recover gently to not over-g the airframe.

Erik
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Old 11-22-2010, 11:55 AM
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Default RE: Downwind turn Myth


ORIGINAL: siclick33

quote:

ORIGINAL: siclick33

Ok, i'll ask another question then.

When you fly a perfect loop in front of you (as if I do) why do the control inputs differ depending on where the wind is coming from?
As a former full-scale aerobatic competitor, this quite sensible Q is close to my heart.

The answer is that u are adjusting the path thru a moving body of air to make the loop LOOK round from your perspective. But what the airplane is actually DOING is not a rounf loop at all, it's flying an elliptical loop that has been ''squished down''. Note that the wind affects the VISUAL SHAPE of a loop from YOUR perspective as you remain fixed to the ground. The wind does not affect the ''feel'' or aerodynamics of the loop from the airplanes perspecive. It is YOUR PERSPECTIVE that is changed, not the aerodynamics :-)))

Draw a loop on a piece of paper. Then do it with your eyes closed till you can consistently make a fairlt round loop. THEN do the same thing as someone else slowly pulls the paper sideways.
And taking this to an extreme to try to explain my thoughts.

Assume flying at 100 knots in a 100 knot tailwind and pulling up for a loop. The only way you will penetrate into wind over the top (to make a round loop) is to increase your airspeed. This is obviously an exagerated example but the same must happen to a lesser extent with normal windspeeds if your loop is to appear round.

I agree that the downwind turn myth is a myth, but am not sure that the same argument applies in the case of a loop flown fixed with respect to the ground.
Good thought, but I hi-lited the part where u went wrong. Ur thinking along the right path, but there is another way to make the loop "appear" round which u forgot: What we actually do is ease off of the elevator and spend MORE TIME in that portion of the loop than in the bottom portion. It's impractical to suddenly accel 20 kts at the top and fly over the top in the same amount of time (and many a/c don't have the power), so we fly that portion at the lower speed, pull less, and take longer.
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Old 11-22-2010, 12:02 PM
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Default RE: Downwind turn Myth

Good thought, but I hi-lited the part where u went wrong. Ur thinking along the right path, but there is another way to make the loop "appear" round which u forgot: What we actually do is ease off of the elevator and spend MORE TIME in that portion of the loop than in the bottom portion. It's impractical to suddenly accel 20 kts at the top and fly over the top in the same amount of time (and many a/c don't have the power), so we fly that portion at the lower speed,pull less, and take longer.
Highlighting continued
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Old 11-22-2010, 12:08 PM
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Default RE: Downwind turn Myth

ORIGINAL: warbird_1
if 'im going 100 mph with a tailwind speed of 20 mph and i turn into that tailwind which now becomes a 20 mph headwind and all the time maintaining the said 100mph . your saying my airspeed doesn't increase?
Yes, that is what he is saying but let's clarify this further.....

If the airplane is flying at an AIRSPEED of 100mph that means it is moving at 100mph relative to the air it is flying through. You mention having a tailwind of 20mph. So if your AIRSPEED is 100mph, and the wind (speed of the air relative to the ground) is 20mph blowing from the direction of the tail, then the GROUND SPEED of the model is 100+20 = 120mph.

If you then perform a 180 degree turn, the model (which is still flying at an AIRSPEED of 100mph) is then flying into a 20mph headwind so the GROUND SPEED of the model is now 100-20 = 80mph.

ORIGINAL: warbird_1
I'm talkng about airspeed over the wing , not ground speed. last time i knew 100 and 25 made 125.
That's true, but since you mentioned a wind speed of 20mph (not 25) above, let's continue to use 20mph in the example.

ORIGINAL: warbird_1
it makes no difference weather you enter a loop from the tailwind position . once you enter a headwind your airspeed ''at least over the wing'' increases .
Nope, AIRSPEED remains constant regardless of whether you are flying into a headwind, with a downwind, or flying crosswind.

ORIGINAL: warbird_1
let me see you take off downwind with a scale turbine jet with a 20mph tailwind. wait ,let me get my video camera out first. i don't want to miss this one
Downwind takeoff certainly are possible provided that you have a long enough runway and high RPM tires. What you will see is a very long takeoff roll. Let's say the plane takes off when it's AIRSPEED is 50mph and we'll say that the wind is blowing at 20mph.....
- when the plane is sitting with its nose pointed into the wind (turbine off, brakes on) it has a GROUND SPEED of 0mph but it already has an AIRSPEED of 20mph.
- in order to take off INTO THE WIND, the plane only has to reach to a GROUND SPEED of 30mph (since 50-20 = 30mph) to achieve an AIRSPEED of 50mph
- but if you tried taking off DOWNWIND, the plane has to reach a GROUND SPEED of 70mph (50+20 = 70mph) to achieve an AIRSPEED of 50mph. Accelerating to 70mph GROUND SPEED will require a significantly longer runway than if you only had to reach a 30mph GROUND SPEED.

Regards,

Jim


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Old 11-22-2010, 12:30 PM
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Default RE: Downwind turn Myth

ORIGINAL: Erik R

Highhorse,

I'm not going to feed into a lengthy debate,but your blanket statement is incorrect.Depending on how aggressively the course reversal is made,the relative wind,and resultant coefficient of lift can change.The basic principal of physics,that an object in motion,stays in motion,relative to the stored energy,which varies as a function of mass x velocity,is a player.

In simple terms,think of a 3D airplaneflying into a 20kt headwind,and doing a a 3D type push/pull over of 180 degrees.At completion it is now in a 20 kt tailwind.It used thrust/lift to overcome the physics principal described earlier,yet realized a 40 kt loss of airspeed,with a corresponding loss in coefficient of lift.The above is an extreme example,and with the smooth,high energy type flying we do with jets,this effect is usually negligable.

For what it's worth,I don't believe wind was a significant factor in the viperjet crash.It appears he had low energy over the top,and pulled too hard,with a resultant stall/snap.Unfortunately,some don't understand that to get the plane flying again,you have to unload the wing.An airplane can be stalled at any airspeed or attitude.At Delta,when we teach upset recovery,in the sims I have seen the jet remain stalled in a 40 degree nose down attitude,with power on.The answer is to level the wings,and push slightly,unloading the wing,and letting it start flying again.Then pull and recover gently to not over-g the airframe.

Erik
The "thought experiment" u put forth regarding Newton's first and second is interesting, challenging, and pretty cool, but I'm gonna ask u to re-think it... Do an instantanious 180 heading change at 20 kts airpeed into zero wind, into a 20 kt wind, and into a 40 kt wind. The resultant indicated speed loss in each case is never more than the 20 kts.

And in "the field" I have lived this with my skinny pink butt strapped to the seat at G's varying from +12 to -8, speeds from 500 to -30, and altitudes from 0 to 39000, I have never observed a "downwind turn" effect in the slightest.
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Old 11-22-2010, 12:36 PM
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Default RE: Downwind turn Myth

It comes down to lift. We generate lift by forward motion. Forward motion is measured relative to airspeed not ground speed. If there is no lift generated as it referenceqs true airspeed then we are not flying but are stalled. A good example of this is carrier takeoffs and landings. Ask Doolittle and any of the pilots the difference 30 knots of headwond made when flying from a carrier. It was basicallylife and death. Sao lift is be.seated relative to the air movement over the wing. No air movement no lift no lift no flying
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