The downwind turn myth

[MauleSkinner]

So....how does the airplane know where the "wind" is coming from?

GPS provides ground speed and track, AHRS provides heading, and Air Data computes TAS. The integrated computer uses this information to calculate wind.

Or, you could compute all this stuff on your whiz wheel and tell the airplane yourself.

Or, you could ignore this post, because it isn’t relevant.

 

[FastEddieB]

The Mythbuster answer...

Perfect. But I'm surprised they said their fans were revolting!

 

[Checkout_my_Six]

No...no...those are for the pilot, not the airplane.

GPS provides ground speed and track, AHRS provides heading, and Air Data computes TAS. The integrated computer uses this information to calculate wind.

Or, you could compute all this stuff on your whiz wheel and tell the airplane yourself.

Or, you could ignore this post, because it isn’t relevant.

 

[FastEddieB]

Yes.

No.

The Mythbusters video sums it up nicely.

 

[Capt. Geoffrey Thorpe]

No.

Somebody was asleep at the switch...

 

[Salty]

This brings up something I initially found counterintuitive…

Would those two planes hitting head on have the same force as one of them hitting a stationary object at 80 kts? Pretty sure the answer in no.

I like to think of it by imagining a brick wall in between them that they both hit head on. Same amount of damage as if they’d hit each other. 40 knots worth each.

 

[IK04]

The wind is adding no relative energy to objects in the air, only to the earth itself or objects fixed to the earth.

Exactly. That is my point. I am only concerned with the energy of the object relative to the Earth. Yeah, I know we are all moving at 600+ something MPH, but that is not considered in this example of acceleration.

If I were to frame this in free space, where there is no air or wind, would that make it more obvious?

Exactly. But we are not talking about colliding with anything attached to the earth.

How is a stationary object in the air different from an object on the surface of the Earth?

The moving object has more stored energy that the stationary object. Forget about the wind, or the objects being airplanes. Consider how we measure the muzzle energy of firearm projectiles- Foot-Pounds of energy. If a shotgun slug moving at 800 Feet Per Second impacts an identical slug that is stationary, the resulting transfer of energy will cause an acceleration in the direction of the moving slug.

They would each encounter enough force to reduce their speed by 40 knots.

Yep. If the initial speed of one of the objects is zero, there would be acceleration in the direction of the moving object.

 

[Hang 4]

How is a stationary object in the air different from an object on the surface of the Earth?

It isn't, but so is the wind in your example.

The moving object has more stored energy that the stationary object.

But your "stationary" object is moving as well and has stored energy. In the air, they have identical energy, regardless of the wind.

Yep. If the initial speed of one of the objects is zero, there would be acceleration in the direction of the moving object.

There would be acceleration in the direction of the moving object even if it wasn't stationary. There would also be acceleration if it flew into a granite cliff. The cliff would "move" backwards as the energy was dissipated, but so what?
I'm really struggling to understand the point.

 

[Tantalum]

there are highschool physics teachers all over this country weeping right now

 

[Stewartb]

Have any of you launched into a strong headwind, say 40 kts, in your small planes? In a climbing turn, what was your perception in the first 90° compared to the second 90°?

 

[MauleSkinner]

Have any of you launched into a strong headwind, say 40 kts, in your small planes? In a climbing turn, what was your perception in the first 90° compared to the second 90°?

I think everyone here who agrees that the downwind turn is a myth from an aerodynamic perspective also understands the perception issue.

 

[Checkout_my_Six]

there are highschool physics teachers all over this country weeping right now

The answer my friend.....is blowing in the wind. lol

 

[Stewartb]

I don’t think that’s correct. Please answer the question. Was the rate of turn in a climbing turn equal?

 

[MauleSkinner]

I don’t think that’s correct. Please answer the question. Was the rate of turn in a climbing turn equal?

Rate of turn would be the the same unless the pilot changed control inputs.

Have any of you launched into a strong headwind, say 40 kts, in your small planes? In a climbing turn, what was your perception in the first 90° compared to the second 90°?

But to directly answer your question, yes, I have. My perception of first 90 degrees was going sideways, and my perception of the second 90 degrees was going really fast. But since I maintained coordinated controls at the desired attitude, my airspeed remained constant.

 

[Stewartb]

In a climbing turn? We have very different descriptions.

 

[MauleSkinner]

In a climbing turn? We have very different descriptions.

Yes…a climbing turn. What is your description?

 

[EdFred]

I guess S-turns across a road cause a lot of planes to fall out of the sky if you fly a tail dragger in Alaska.

 

[Jim K]

Have any of you launched into a strong headwind, say 40 kts, in your small planes? In a climbing turn, what was your perception in the first 90° compared to the second 90°?

You don't have these problems in low wing airplanes because you can't see the ground

 

[Stewartb]

The first 90° is easy. The controls are very responsive. The airplane doesn’t want to turn from cross to down. It seems to fight it. Certainly some of that is perception. My flight experience is in very narrow confines. Turbulence plays a role, too Gusts are easier manage with a headwind component.

I understand the physics argument. I don’t understand the strong opinions that downwind turns are no different than other turns. I think a lot of pilots stay home when the wind blows.

 

[Stewartb]

I guess S-turns across a road cause a lot of planes to fall out of the sky if you fly a tail dragger in Alaska.

No, but turns in a confined space like a mountain pass do.

 

[EdFred]

The first 90° is easy. The controls are very responsive. The airplane doesn’t want to turn from cross to down. It seems to fight it. Certainly some of that is perception. My flight experience is in very narrow confines. Turbulence plays a role, too Gusts are easier manage with a headwind component.

I understand the physics argument. I don’t understand the strong opinions that downwind turns are no different than other turns. I think a lot of pilots stay home when the wind blows.

They are no different. And if you think they are you DON'T understand the physics argument. Don't look outside the plane and you won't know whether you are turning upwind, downwind, or crosswind if the wind is steady state.

Do you also cook magic grits on your stove?

No, but turns in a confined space like a mountain pass do.

Nope. Your ground track may be affected, but that has nothing to do with what we are talking about.

 

[FastEddieB]

The first 90° is easy. The controls are very responsive. The airplane doesn’t want to turn from cross to down. It seems to fight it. Certainly some of that is perception.

ALL of that is perception. An airplane in free flight behaves identically regardless of wind direction. It’s doing it’s normal thing - it just happens to be doing it in a moving airmass. It will not trim differently, nor cool differently, nor have any change in “response”.

 

[Stewartb]

I hope you guys get to go fly in the wind and experience it for yourselves. It’s blowing about 40 at my house now. Surrounded by mountains so no lack of turbulence. Just watched a King Air land. I don’t think he’ll want to do that again today.

Merry Christmas. I get to next week with my dept head Aerospace Engineer buddy. I’ll make sure to discuss this topic with a real rocket scientist.

 

[IK04]

But your "stationary" object is moving as well and has stored energy.

Nope. It is not moving relative to the Earth. Only the wind is moving.

 

[EdFred]

I hope you guys get to go fly in the wind and experience it for yourselves. It’s blowing about 40 at my house now. Surrounded by mountains so no lack of turbulence. Just watched a King Air land. I don’t think he’ll want to do that again today.

Merry Christmas. I get to next week with my dept head Aerospace Engineer buddy. I’ll make sure to discuss this topic with a real rocket scientist.

Turbulence has nothing to do with what we are talking about. We were talking steady state winds. And I'm sure you will frame the question to your buddy objectively.

 

[IK04]

I hope you guys get to go fly in the wind and experience it for yourselves. It’s blowing about 40 at my house now. Surrounded by mountains so no lack of turbulence. Just watched a King Air land. I don’t think he’ll want to do that again today.

That reminds me of the time I dropped off a US Senator at Hooper Bay, AK when the wind was 50+ and 90 degrees to the runway.

I landed on the ramp and was able to hold position long enough for him to jump out. That was a real eye opener! (totally safe, of course!)

 

[Checkout_my_Six]

so....what if you're doin this on a treadmill?

 

[MauleSkinner]

so....what if you're doin this on a treadmill?

It appears we are…this thread keeps going round and round and round.

 

[Dan Thomas]

I hope you guys get to go fly in the wind and experience it for yourselves.

Did a lot of it as an instructor. Never had any difficulty turning out of the wind. The IFR students under the hood couldn't tell me where the wind was coming from either. No airspeed changes, no ball showing skid or slip, no funny stuff from the airplane. It just did what it was told. The ground track sure looked goofy though.

One trick was to get the IFR student to look out and see where we were. Then under the hood to do a continous 30 degree banked turn for four or five minutes, then tell him to look again. Always surprised at how far downwind we had drifted, with no sensation of doing it.

30 or 40 knots at 2000 feet worked well for this.

 

[EdFred]

Did a lot of it as an instructor. Never had any difficulty turning out of the wind. The IFR students under the hood couldn't tell me where the wind was coming from either. No airspeed changes, no ball showing skid or slip, no funny stuff from the airplane. It just did what it was told. The ground track sure looked goofy though.

One trick was to get the IFR student to look out and see where we were. Then under the hood to do a continous 30 degree banked turn for four or five minutes, then tell him to look again. Always surprised at how far downwind we had drifted, with no sensation of doing it.

30 or 40 knots at 2000 feet worked well for this.

But were they in Alaska, where apparently the laws of physics are different?

 

[Stewartb]

Being a dick doesn’t advance your point. Not that you’ve contributed much. I get the physics. I can’t remember a windy day where the air mass was in steady state. The bigger basis for my viewpoint is I fly by ground reference. Instruments are there for the FAA’s pleasure or for scuzzy days, but on those days the ground reference is impact is enhanced by reducing the visual. I don’t need to try to convince anyone of anything, but I really do invite you to read Contact Flying. Chapter 7 is all about downwind turns. Not a physics lesson, but a pilot controlled aircraft maneuver. A maneuver that kills pilots, and lots of them probably took physics in school.

 

[MauleSkinner]

but I really do invite you to read Contact Flying. Chapter 7

Being a dick doesn’t advance his point, either.

 

[EdFred]

I fly by ground reference. Instruments are there for the FAA’s pleasure or for scuzzy days,
....
a pilot controlled aircraft maneuver. A maneuver that kills pilots, and lots of them probably took physics in school.

I think we found the problem and why you are having trouble. You're trying to make your turns by ground reference and ignoring instruments.

 

[Tarheelpilot]

That reminds me of the time I dropped off a US Senator at Hooper Bay, AK when the wind was 50+ and 90 degrees to the runway.

I landed on the ramp and was able to hold position long enough for him to jump out. That was a real eye opener! (totally safe, of course!)

If it was totally safe Salty must have been there to protect everyone from your propeller.