PDA

View Full Version : How does a headwind affect Vy?

Runway Change
18th Jul 2015, 16:32
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.

Alex Whittingham
19th Jul 2015, 11:15
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.

16024
19th Jul 2015, 13:57
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...

WeekendFlyer
20th Jul 2015, 18:43
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

B2N2
20th Jul 2015, 18:48
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 :E

Alex Whittingham
20th Jul 2015, 21:35
Can you give me a reference for your definition of VX please WF?

Gysbreght
21st Jul 2015, 17:38
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.

sycamore
21st Jul 2015, 23:14
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.....

JammedStab
22nd Jul 2015, 05:22
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.

darkroomsource
22nd Jul 2015, 07:49
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.

Alex Whittingham
22nd Jul 2015, 09:50
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.

16024
22nd Jul 2015, 10:56
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.

InSoMnIaC
22nd Jul 2015, 13:22
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

darkroomsource
22nd Jul 2015, 16:20
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.

Gysbreght
22nd Jul 2015, 17:21

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.

16024
22nd Jul 2015, 22:24
He's right. It's all here:

https://en.wikipedia.org/wiki/Polar_curve_(aerodynamics)

AerocatS2A
23rd Jul 2015, 01:16
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.

HarleyD
23rd Jul 2015, 03:35
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

FE Hoppy
23rd Jul 2015, 10:18
http://avia.tion.ca/documentation/polar/polar7.gif

16024
23rd Jul 2015, 10:49
Excellent, Hoppy.
That's the perfect graphical demonstration.

What he said...

AerocatS2A
24th Jul 2015, 12:12
Thanks. I had seen that graphic but it still didn't click. I think I've got it now. Basically the penalty of a higher sink rate in a headwind is offset by the benefit of the faster speed. You are trading time in the air for penetration.

A Squared
25th Jul 2015, 08:59
Thanks. I had seen that graphic but it still didn't click. I think I've got it now. Basically the penalty of a higher sink rate in a headwind is offset by the benefit of the faster speed. You are trading time in the air for penetration.

TO help solidify it in your mind, try this thought experiment. It's essentially what Gysbreght said in a previous post, but in my experience it halos to throw some numbers on it and think through it little.

Let's say that you're flying an airplane with a angle of glide speed of 65 knots. You're flying into a 65 knot headwind at 6000 ft AGL. The wind is constant all the way to the ground. You have an engine failure. You immediately pitch to your glide speed of 65 knots. Your descent rate is 1500 ft/ minute. You will glide for 4 minutes. How far forward do you go?

If your airspeed is 65 kt, and the headwind is 65 kt, then your groundspeed will be zero, and obvious you will not move forward at all with a groundspeed of zero.

So what happens if you were to pitch over further to glide at 75 knots? Now your groundspeed is 10 knots because your airspeed exceeds your headwind. Now, obviously, if you are moving forward, you're going to travel further than if you were not moving forward, so it can be seen that increasing the speed at which you glide into a headwind will increase the the power off glide distance.

A Squared
25th Jul 2015, 10:00
A similar thought experiment would suggest that angle of climb, relative to the ground could be increased when flying into a headwind by climbing at an airspeed below Vx.

DISCLAIMER: I am not in any way suggesting that a pilot should actually do this in flight. It would be a very bad idea, would wouldn't likely produce any real gains. I'm just saying that technically, some small increase exists.

Take a hypothetical airplane with a Vx of 60 knots. Now consider departing into a constant 50 knot headwind. If you climb at Vx of 60 knots, you will have a forward airspeed of 10 knots, so your effective angle of climb, relative to the ground will be something less than 90 degrees. If you were to pitch up and slow to 50 knots (assuming that you didn't stall and drop out of the sky) Your forward groundspeed would now be zero, so (again, assuming that you have a positive rate of climb) your climb angle, relative to the ground, will be 90 degrees, Obviously, a 90 degree angle of climb is steeper than an angle of climb less than 90 degrees.

Now, I'll reiterate, I am not suggesting that operationally, this effect can be used. At wind-speeds which are not close to Vx, the gain in angle would be negligible, or even negative and more importantly, climbing at an airspeed less than Vx is a really bad idea. A true Vx, if Vx is actually published, results in a pretty nose high attitude. and in a lot of GA airplanes its is getting fairly close to stall speed. A sudden loss of power would require an immediate, aggressive pitch over maneuver to keep the airspeed from decreasing rapidly. If you're climbing at a speed les than Vx, you're that much close to stalling, you have less kinetic energy, and your pitchover is going to have to be even faster to avert disaster.

I have been told, that Cessna no longer publishes the actual Vx for their single engine aircraft, instead they have a "recommended obstacle clearance climb airspeed" or some such, which is actually a little faster than true Vx. Their reasons for that change were because of the steep pitch attitude at Vx and the hazards that creates in the event of an engine failure.

I don't know for certain this is true, but when I first heard this some years ago, I looked at a number of Cessna 172 pilots manuals from different years, and there was indeed an increase in the climb speed and a change in the phrasing thereof across model years where it wasn't apparent to me that there was a real change to the aircraft which would result in a real change to actual Vx. That seems to support the claim.

So, in case I haven't been clear: really bad idea to be mucking around climbing out at airspeeds less than Vx. DON"T DO IT.

AerocatS2A
26th Jul 2015, 01:00
TO help solidify it in your mind, try this thought experiment. It's essentially what Gysbreght said in a previous post, but in my experience it halos to throw some numbers on it and think through it little.

Let's say that you're flying an airplane with a angle of glide speed of 65 knots. You're flying into a 65 knot headwind at 6000 ft AGL. The wind is constant all the way to the ground. You have an engine failure. You immediately pitch to your glide speed of 65 knots. Your descent rate is 1500 ft/ minute. You will glide for 4 minutes. How far forward do you go?

If your airspeed is 65 kt, and the headwind is 65 kt, then your groundspeed will be zero, and obvious you will not move forward at all with a groundspeed of zero.

So what happens if you were to pitch over further to glide at 75 knots? Now your groundspeed is 10 knots because your airspeed exceeds your headwind.

That doesn't really help. I was thinking that to increase speed you would increase your descent angle so you'd actually just end up going backwards. What I wasn't getting was that the trade off between descent rate and airspeed is not linear.

darkroomsource
27th Jul 2015, 08:48
Let me try again.
The distance travelled when gliding does vary with respect to wind.
The best glide speed, which provides the maximum lift to drag ratio, does not change with respect to wind (air mass).

I get it, if there is a headwind equal to the best glide speed (which is NOT a normal occurrence) then you will make no distance whatsoever. And, if you push the nose over and gain some speed, you will gain some distance. However you will no longer be gliding at the best glide speed (maximum lift to drag ratio).

Let's put it in more realistic terms.
You have a 65 knot best glide speed (max lift to drag ratio).
You have a 15 knot headwind (far more reasonable than 65 knots at the level you're at where you need to seriously consider this)
Your airplane has a 1:6 glide ratio at best glide speed (about normal for most SEL aeroplanes).
You're at 3000 to 5000 feet AGL - again, reasonable for the exercise.
At this altitude, looking down at the ground, in a 45 degree "cone" angle, as I was taught. You will make any "field" (you can do the maths, I did).
But if you think that, due to the proof that at a 65 knot headwind, you will need to nose over to achieve 90 knots (or some other speed) in order to get to your "field", guess what, there are some areas within that 45 degree cone, that you will NOT make.
And, in most SEL aeroplanes, best glide speed is often full back trim, so you simply set full back trim, and then focus on trying to restart the engine, rather than trying to do the maths to figure out what speed you want to be doing to get the most forward distance (at a reduced time, by the way) and thus spend less time on trying to restart the engine.

Gysbreght
27th Jul 2015, 09:45
darkroomsource,

If your plane has a glide ratio of 1:6 at a speed of 65 kt, and you maintain that speed against a 15 kt headwind, your glide ratio relative to ground will be 1:4.6 . So don't worry, you will still make your 45 degree cone, despite your performance being less than optimal.

darkroomsource
27th Jul 2015, 09:52
darkroomsource,

If your plane has a glide ratio of 1:6 at a speed of 65 kt, and you maintain that speed against a 15 kt headwind, your glide ratio relative to ground will be 1:4.6 . So don't worry, you will still make your 45 degree cone, despite your performance being less than optimal.

Isnt that what I wrote?
But, if you change the glide speed, based on the discussions above, and on other places, then you will not be assured of making that location.

Again. Best glide speed, which is the best lift to drag ratio, is NOT affected by the wind speed. Best Glide Distance, is affected by the wind speed.

Gysbreght
27th Jul 2015, 10:22
darkroomsource,

The Best Glide Distance is achieved with a speed that depends on the wind. With the optimum glide speed adjusted for wind you will be able to reach more places, not less.

darkroomsource
27th Jul 2015, 10:39
The Best Glide Distance is achieved with a speed that depends on the wind. With the optimum glide speed adjusted for wind you will be able to reach more places, not less.

I am not arguing that.
I am arguing that the best glide speed does not change as a result of different winds.
The best glide distance, I have already stated, is affected by wind, and therefore you can get different results, some times, but not always, better, by changing your speed.

But it is dangerous to think that you should adjust your glide speed for a headwind. To think that you can "dive for the landing spot", will lead to trying that when there is no headwind.

The safest thing to do is establish the best glide speed (which will not necessarily give you the best glide distance, but will give you the best lift to drag ratio glide, and thus the longest - timewise - glide), and then focus on a place within the 45 degree cone, then try to restart the engine, never losing track of your chosen landing spot, but also, not changing the speed because you think you might make it if you go faster - that leads to fatal crashes.

Gysbreght
27th Jul 2015, 10:53
darkroomsource,

the best lift to drag ratio glide does not give you the longest - timewise - glide, so you have to choose whether you want to optimize distance or time.

darkroomsource
27th Jul 2015, 11:36
the best lift to drag ratio glide does not give you the longest - timewise - glide
it doesn't?

darkroomsource
27th Jul 2015, 11:39
But honestly, who here would espouse that the best thing to do when you lose the engine in a SEL is to try to calculate what glide speed you should use to reach a safe destination?

To be safe, would you not just set the best glide speed by using full back trim, as every SEL that I know will have the best glide speed by using full back trim?

And wouldn't you focus on the emergency rather than the glide speed?

And, absolutely, without question, the best glide speed, does not change with wind. The best glide distance might not be achieved by the best glide speed, but the Vspeed does not change due to wind.

Above The Clouds
27th Jul 2015, 15:59
Vx or Vy indicated speeds are not affected by wind, the angle or flight path in relation to the earths surface will change with wind.

The best lift drag speed gives optimum glide distance or the best rate of climb performance, gliders do use polar charts or glide computers on the vario/vsi to obtain the best wind penetration glide speed to cover the greatest distance in the shortest time with least loss of altitude or another way the optimum flight path.

The same principle as the best lift drag speed indicator on the PFD.

16024
27th Jul 2015, 17:36
To be safe, would you not just set the best glide speed by using full back trim, as every SEL that I know will have the best glide speed by using full back trim?

Well let's confirm the malfunction.
If you think you have picked up some carb icing, or have a solveable fuel delivery problem, then you will want to stay in the air as long as possible while you sort it out. The best speed will be minimum sink speed, which is slightly slower than best L/D, and not affected by wind.
If you have a con-rod sticking out of the cowl and a landing is inevitable, you might want to glide as far as possible. With a headwind, speed for best glide is faster than best L/D, which I think is what Darkroom is calling "best glide speed".
And please, nobody mention the once-taught technique of turning downwind to cover more ground!

A Squared
29th Jul 2015, 17:05
I was thinking that to increase speed you would increase your descent angle so you'd actually just end up going backwards.

Why would you start going backward as a result of going forward, faster?

Not being a smart Ass, serious question. How would increasing you airspeed to 75 knots into a 65 knot headwind cause you to go backward?

Volume
3rd Aug 2015, 15:21
First of all, indeed Vy is independent of wind, as it is defines via pure vertical speed, which is not influenced by horizontal wind.

Vx is defined via the tangent to the climb polar at maximum power. If you movethat polar horizontally (by adding a wind speed to the ground speed), the tangent touches at a different point, corresponding to a diffenent IAS.
In Headwind, Vx reduces, in tailwind it increases. So the change of Vx as GS is even larger than as IAS. Small changes (per definition of the tangent...) do not change much.

gliders do use polar charts or glide computers on the vario/vsi to obtain the best wind penetration glide speed to cover the greatest distance in the shortest time with least loss of altitude or another way the optimum flight pathActually glider pilots do both. In a competition, speed matters, so you optimize your glide speedwise, not distance wise. You prefer to spend times in updrafts (typically thermals) than in glide, so better arrive a little lower but significantly earlier in the next thermal, as long as it is strong enough to gain more altitude in the longer climb, than you have lost in the faster glide. You basically look at your speed polar from the viewpoint of your compettitor in the thermal ahead. He will see you descending faster, hence you must fly faster to see an optimum glide from his perspective. However, if you do no longer expect to reach your destination, there are no more points to earn for speed, just for distance, you better switch tactics and go for maximum distance.

In practical terms, if Vx matters, you have a mjor obstacle ahead, otherwise you will anyway fly Vy. If you have an obstacle ahead (letīs say a mountain), the horizontal wind will also have a vertical influence, hence your Vx is anyway invalid. Trying to climb over a mountain in strong headwind is no smart idea...