Aerodynamics question

flyingcheesehead

flyingcheesehead

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So, I know that as density altitude gets higher, Vy goes down and Vx goes up and they meet at the absolute ceiling.

I'm curious as to what the final Vx=Vy number is, and whether this is a linear relationship. Would the final number be exactly halfway between the sea-level Vx and Vy?

Also, what is the absolute ceiling of a 1971 C182? Service ceiling is 18,000, but I don't know the absolute.

I'm wondering all of this mainly so I can ensure max performance when I'm out west later this summer... I'd like to know what Vx and Vy really are when I'm up high.

Thanks,
 
flyingcheesehead said:
So, I know that as density altitude gets higher, Vy goes down and Vx goes up and they meet at the absolute ceiling.

I'm curious as to what the final Vx=Vy number is, and whether this is a linear relationship. Would the final number be exactly halfway between the sea-level Vx and Vy?

Also, what is the absolute ceiling of a 1971 C182? Service ceiling is 18,000, but I don't know the absolute.

I'm wondering all of this mainly so I can ensure max performance when I'm out west later this summer... I'd like to know what Vx and Vy really are when I'm up high.

Thanks,
Click to expand...

I know that neither Vy nor Vx varies linearly with altitude and that the point where they meet isn't necessarily halfway between the two sea level values. Also, Vx varies with wind speed, Vy doesn't but I'm assuming you were only considering a no wind situation.
 
Never heard any rules of thumb, and never been able to understand the forumulas ;)

You're dealing with two separate conditions in Vx and Vy, though, so it's unlikely that they'll meet halfway. Vx is an "excess thrust" figure, while Vy is "excess power". IIRC (and that's a bit of a stretch in this case), Vx is a "squared" curve and Vy is a "cubed" curve.

Generally, though, you'll get a good approximation if you go whatever max altitude you think you'll be dealing with, determine a number for Vy there by minimizing the time to climb a given altitude, and use linear interpolation from there to sea level (adjusted for MGW, of course). You can determine Vx up there, too...you minimize the product of (airspeed x time) for a given altitude change.

Takes some pretty precise flying to determine them, but so does actually using the correct speeds.:D

Fly safe!

David
 
lancefisher said:
Also, Vx varies with wind speed,
Click to expand...

I guess I need to go back and review the definition of Vx. I never thought of it as being affected by wind speed. I guess it would be the lowest speed at which I can still climb at all without the wind moving me backwards. I always thought of it as the speed that would achieve the steepest climb through the air mass I was flying in.
 
Graueradler said:
I always thought of it as the speed that would achieve the steepest climb through the air mass I was flying in.
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I believe you are correct. Wind speed only comes in to the equation if you need the optimum airspeed to clear an obstacle. If wind speed = Vx, you will go straight up... no problemo!

-Skip
 
Vx is indeed a still-air speed, and while wind velocity will affect your climb angle with reference to the ground, that effect isn't large enough to make a different airspeed much more effective. Also, having played with this some, my experience is that Vx and Vy meet about one-third the way from Vx to Vy (e.g., SL Vy=90, Vx=70; Vx/Vy meet about 77 at absolute ceiling), but that's not scientifically based and may not work with planes other than those few I've examined.
 
I can't find a reference but wouldn't Vx and Vy meet at L/D max, right around best glide?

Joe
 
The only speed I know that varies with wind speed is ground speed. Vx & Vy are a function of weight, wing efficiency and power available. I don't think any of those care which way the wind blows. It would be a very complicated ASI that could adjust for windspeed if it did.

Graueradler said:
I guess I need to go back and review the definition of Vx. I never thought of it as being affected by wind speed. I guess it would be the lowest speed at which I can still climb at all without the wind moving me backwards. I always thought of it as the speed that would achieve the steepest climb through the air mass I was flying in.
Click to expand...
 
Areeda said:
I understand but when you get to the absolute ceiling and can't climb, I thought there was only one speed you could use to maintain altitude.
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...with full available power, yes, but at that point the power-off speed for least sink rate will not be the same -- faster, I should think.
 
Ron Levy said:
...with full available power, yes, but at that point the power-off speed for least sink rate will not be the same -- faster, I should think.
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True, but "best glide" means most forward motion for each unit of lost altitude while "least sink" means least altitude lost per unit of time. Those speeds are not equal.

What I remember is that Vx is where L/D meet which is also "best glide". I read this in one of Barry Schiff's Practical Pilot columns - complete with graphs, etc. I'll have to see if I can find it tonight.

John
 
Ron Levy said:
Yes, it does, too, but the L/D portions of the equations are to the 3/2 power in computing Vx/Vy but not for best glide.
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OK.. right, since determining L/D has already compensated for weight.

Seems quite the Aerodynamics computation that would be nigh impossible in flight.
 
jsstevens said:
True, but "best glide" means most forward motion for each unit of lost altitude while "least sink" means least altitude lost per unit of time. Those speeds are not equal.
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My bad. Either way, though, adding or reducing power changes the speed for best climb/descent gradient.
What I remember is that Vx is where L/D meet which is also "best glide".
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Not sure I follow that statement, but best glide is found at the AOA where L/D is maximized and no other factors adhere, while Vx is a function of (L/D)^(3/2) among other things including power.
 
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Ron Levy said:
My bad. Either way, though, adding or reducing power changes the speed for best climb/descent gradient.
Not sure I follow that statement, but best glide is found at the AOA where L/D is maximized and no other factors adhere, while Vx is a function of (L/D)^(3/2) among other things including power.
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I poorly worded it, as well as being wrong - otherwise it was perfect :D. Best glide is where parasitic drag and induced drag meet, which is minimum drag. I was mis-remembering the graph as plotting lift/drag and that's not correct - it was plotting the parasitic drag curve and the induced drag curve. At the absolute ceiling, it's all the same place: Vx, Vy and best glide (and probably minimum sink, although I'd have to think about that one).

John
 
jsstevens said:
At the absolute ceiling, it's all the same place: Vx, Vy and best glide...
Click to expand...
Even at the absolute ceiling, why would power-off best glide speed (which occurs at the AOA for L/Dmax with no power) be the same as full-power Vx/Vy which (IIRC) occurs at the AOA for max (L/D)^(3/2) at full power? I don't see it. Remember that best glide speed does not change with altitude, but Vx and Vy do. In my Tiger, Vx and Vy start at 70/90 and meet at the top at around 76, but Vbg is 73 at all altitudes.
 
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Ron Levy said:
Even at the absolute ceiling, why would power-off best glide speed (which occurs at the AOA for L/Dmax with no power) be the same as full-power Vx/Vy which (IIRC) occurs at the AOA for max (L/D)^(3/2) at full power? I don't see it. Remember that best glide speed does not change with altitude, but Vx and Vy do. In my Tiger, Vx and Vy start at 70/90 and meet at the top at around 76, but Vbg is 73 at all altitudes.
Click to expand...

But isn't Vx also at L/Dmax? That is my best recollection. If not it may have to do with downward vector of the propwash providing some lift.

John
 
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jsstevens said:
But isn't Vx also at L/Dmax? That is my best recollection. If not it may have to do with downward vector of the propwash providing some lift.

John
Click to expand...

Aerodynamics for Naval Aviators says "L/D Max is highest angle of climb for jet powered aircraft," but does not explain why this is not also true for propeller powered airplanes.

It may be as you say...
 
dmccormack said:
Aerodynamics for Naval Aviators says "L/D Max is highest angle of climb for jet powered aircraft," but does not explain why this is not also true for propeller powered airplanes.
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Probably because in a jet thrust is constant and power varies with airspeed, in a CS propeller driven plane the opposite is true (HP is constant, thrust varies).
As to the early comment RE wind and Vx, wind doesn't "affect" the published Vx, but it definitely does affect the actual Vx i.e. the airspeed that delivers the maximum climb angle. That said, Ron is correct that the effect of a typical wind is small enough to ignore and in any case if you use the published Vx you will achieve at least the corresponding published climb angle (distance to clear the mythical 50' obstacle) as long as you avoid tailwinds.
 
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