How does a headwind affect Vy?
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.
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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.
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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.
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.
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.
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.
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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.
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.
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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.
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.
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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.
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.
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.
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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.
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.
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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 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.
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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.
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.
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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.
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.
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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.
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.
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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?
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!
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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?
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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.
Actually 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 (let´s 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...
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 path
In practical terms, if Vx matters, you have a mjor obstacle ahead, otherwise you will anyway fly Vy. If you have an obstacle ahead (let´s 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...
