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# How does a headwind affect Vy?

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# How does a headwind affect Vy?

18th Jul 2015, 15:32

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How does a headwind affect Vy?

Hi Guys,

With the few I've spoken to, there seems to be conflicting ideas or maybe more confusion as to how a headwind will affect Vy. Can anyone shed some better info on this? A friend got it in an interview.
19th Jul 2015, 10:15

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VY represents the speed for best rate of climb speed and that would be entirely unaffected by wind. Similarly VX, as defined, would be unaffected. The only climb parameter that I can think of that would be affected by wind would be the climb gradient calculated for obstacle clearance but even in that case the climb speed would not change with changing head or tail winds.
19th Jul 2015, 12:57

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I think Alex is right. It's rather counter-intuitive, as speed for best glide is definately affected by wind.
Surely the interview question would be about Vx, if the guys knew what they were about, as the Vy question is obvious, unless we are talking about a kite...
20th Jul 2015, 17:43

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Climb and descent

In simple terms, climb rate is calculated as follows:

Rate = (Thrust - Drag) x TAS / Weight

Thus to get the best rate you need the highest possible excess thrust (T - D), a high TAS and low weight. When you do the maths on the drag polar for a jet, Vy occurs when you get the best ratio of TAS to Drag. This will give you the highest possible rate of climb relative to the air mass. If the air mass is moving sideways (wind) it makes no different to the rate of climb.

Vx is different. It gives you the best climb gradient (i.e. Ratio of rate of climb to groundspeed) and, because this considers rate of climb AND distance covered horizontally, wind does matter. This is one of the reasons why takeoff with a tailwind can be a bad idea.

For gliding, the minimum drag speed gives the lowest rate of descent. This is also the best endurance speed for a jet, and gives you the best glide range in still air. When there is a head or tailwind then the effective glide angle with respect to the ground will change.

Hope this helps.

WF
20th Jul 2015, 17:48

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Vy is best rate and Vx is best angle.
Vy is altitude gained over time and Vx is altitude gained over distance.

This is where it gets confusing;
A headwind will affect the climb angle relative to the earth.
Extreme example is climbing into a headwind which is equal to your speed.

However, a climb into a headwind is always performance increasing and a climb with a tailwind is always performance decreasing.
It's not that the airplane performs better or worse because an engine is not affected but the net results are better or worse.

Consider air distance and ground distance and some easy numbers:
• Speed 120 kts or 2 miles/minute
• Rate of climb 1000 fpm
• No wind

After 1 minute the plane is at 1000' AGL and 2NM downrange
2NM Air distance and 2NM ground distance
• Speed 120 kts or 2 miles/minute
• Rate of climb 1000fpm

After 1 minute the plane is at 1000' and 1.5NM downrange so the climb angle is acutally steeper (better) at the same Vy
The airplane has still flown 2NM through the air but only covered 1.5NM over the ground.

So a headwind changes the angle of the climb which means we could fly faster into a headwind and make the same climbgradient as in a no wind situation but this is not accomplished by changing Vy or Vx

Make sense
20th Jul 2015, 20:35

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Can you give me a reference for your definition of VX please WF?
21st Jul 2015, 16:38

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WeekendFlyer:
For gliding, the minimum drag speed gives the lowest rate of descent.
Actually, minimum drag speed gives the best glide angle in still air. The speed for lowest rate of descent is less than the minimum drag speed and is used for circling in thermals, to maximize the rate of climb achieved for a given updraft velocity.
21st Jul 2015, 22:14

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Alex,`google` V SPEEDS, lots of references,;
B2N2 is correct; Vx is best angle,and any headwind will increase the angle,by reducing Ground Speed(think helos)
Vy is best ROC,however,the reduced G/S with a headwind will improve/increase the climb gradient,but not the rate(normally)..
and vice-versa for a tailwind.....
22nd Jul 2015, 04:22

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Have flown lots of different light aircraft types. Going strictly from memory, for a given altitude, I have never seen a performance chart or speed table that corrects Vx for wind. Only altitude. I assume therefore that density altitude makes the difference.

Would like to see actual technical written stuff that confirms that wind does make a difference.
22nd Jul 2015, 06:49

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speed for best glide is definately affected by wind.
how?
unless you mean "best glide distance".
But if you mean best glide speed, then it is what it is, no matter the wind.
22nd Jul 2015, 08:50

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Vx

Yes, I understand that headwind and tailwind affect climb angle relative to the ground, I know this as the 'flight path angle'. My understanding is that VX occurs where there is the greatest excess thrust over drag, approximately VMD for a jet but usually below VMD for a prop, and that this speed, defined as an EAS, IAS or CAS, does not change with wind. Of course the groundspeed will change with wind, but the target speed is not a groundspeed.
22nd Jul 2015, 09:56

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Quote:
speed for best glide is definately affected by wind.
how?
unless you mean "best glide distance".
But if you mean best glide speed, then it is what it is, no matter the wind.
Getting a bit off topic, here. However, best glide, as mentioned here means best glide distance, as you say. I'm not really sure what you mean by "best glide speed" as opposed to "speed for best glide (distance)".
Speed for best glide endurance (min sink) is one thing, and it is an absolute value. But it is fairly meaningless unless you are scratching around low looking for thermals (!).
Speed for best glide distance will vary with wind. Ask Mr McCready.
22nd Jul 2015, 12:22

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Vy is not affected

HW or TW does affect Best Climb Angle. However that is outside the control of the pilot. The best we can do is to fly the Vx speed. You are still flying the Best AOC speed for those particular conditions.

So in summary Both Vx & Vy are unaffected
22nd Jul 2015, 15:20

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I don't think that speed for best glide distance will vary with wind.
I think that distance glided will vary with wind.

If I am gliding at best glide speed in zero wind, then I will get the best possible distance in zero wind. If I glide slower than that, or faster than that, I will travel less distance.

If I am gliding into a 20kt headwind, gliding at best glide speed will get the best possible distance into the wind. If I glide slower than that, or faster than that, I will travel less distance.

And the same for a 20kt tailwind.

Best glide speed is just that, the speed at which you glide the furthest. It does not matter whether you're facing a headwind, tailwind, or zero wind. However, you will travel less/more distance.
22nd Jul 2015, 16:21

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You will get the best possible distance relative to the airmass you're flying in, but not the best possible distance relative to ground. Suppose your glide takes 6 minutes, then your 20 kt headwind will reduce your distance relative to ground by 2 NMi. If you fly faster, you will cover less distance relative to the airmass, but more distance relative to ground, because you cover that distance in less time.

Carried to extreme, when the windspeed is equal to your best glide speed, you cover zero distance relative to ground at that speed. You would need to increase your speed to gain some distance over ground against the wind.

Last edited by Gysbreght; 22nd Jul 2015 at 16:50.
22nd Jul 2015, 21:24

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He's right. It's all here:

https://en.wikipedia.org/wiki/Polar_curve_(aerodynamics)
23rd Jul 2015, 00:16

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Originally Posted by Gysbreght
You will get the best possible distance relative to the airmass you're flying in, but not the best possible distance relative to ground. Suppose your glide takes 6 minutes, then your 20 kt headwind will reduce your distance relative to ground by 2 NMi. If you fly faster, you will cover less distance relative to the airmass, but more distance relative to ground, because you cover that distance in less time.

Carried to extreme, when the windspeed is equal to your best glide speed, you cover zero distance relative to ground at that speed. You would need to increase your speed to gain some distance over ground against the wind.
I would think that if the windspeed equalled your best glide speed then if you go faster you will go backwards because to achieve the faster speed you need a steeper glide angle.

I read the wiki on the polar curve but must admit I don't understand it. If someone could explain why my intuition is wrong that would be good.

Edit: I've read it again and understand what it is saying but it still doesn't feel right, I need a different explanation to make it click.
23rd Jul 2015, 02:35

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Oh

My

God

If the participants on this thread are in fact licence or certificate holders, dog help us.. Obviously some are simmers pretending to know what they are talking about and others are gliders who have the need to tell us about their supreme knowledge.

Insomniac is correct. Vx and Vy are both independent of wind ( a steady wind that is), but not independent of windbags. (And regardless of IAS, EAS, CAS TAS wank wank wank)

The question is only interesting when wind shear is considered as the dynamics begin to have transient influence. Insomniac may know what I mean but sadly it seems very few others.

If you have a pilot licence you MUST know the answer to the initial question. If you are applying for a job that is dependant upon this info and you need to rely on pprune for the answer, you will get the job you deserve. You will never work for me, that's for sure.

HD
23rd Jul 2015, 09:18

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23rd Jul 2015, 09:49

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Excellent, Hoppy.
That's the perfect graphical demonstration.

What he said...