Calculate VX and Vy

JohnAJohnson

JohnAJohnson

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Given Vs and Vso, is there any way mathematically to calculate Vx and Vy?

Also, how does adding Vortex Generators effect Best Glide, and can it also be calculated off Vs or Vso?
 
You'd be better off calculating the ratio of ants and termites to grains of sands on the beach then messing with that calculation... Just google "calculate Vx and Vy" and you'll get your answer.
 
There are ways to GUESS AT Vx and Vy given Vs0, but they are rules of thumb and should be taken with truckloads of salt. A real airplane designer might not have gotten the memo....

Like, cruise speed is usually around twice the clean stall speed. But if you depend on that for anything, you're going to get in trouble....
 
Kevin H. said:
Just google "calculate Vx and Vy" and you'll get your answer.
Click to expand...

I think you'll get answers to a different question with a Google like that. Perhaps something like

Vx = V cos(theta)
Vy = V sin(theta)

Google can be a useful tool, but you have to be careful what you ask for.
 
JohnAJohnson said:
Given Vs and Vso, is there any way mathematically to calculate Vx and Vy?

Also, how does adding Vortex Generators effect Best Glide, and can it also be calculated off Vs or Vso?
Click to expand...

There's no aerodynamic basis for a consistent relationship between Vs and Vx or Vy. Both Vx and Vy are affected by thrust HP because when you are climbing a portion of thrust (albeit a small one in many light airplanes) actually supports the airplane. Another factor is the "draginess" (parasite drag) of the airframe, all else being equal I'd expect a slipperier airplane (think M20J vs Stearman) to have a greater spread between Vs and Vy. Wing planform and aspect ratio probably have different effects on Vs and Vx/Vy as well. And different airfoils with different lift/drag curves are likely to have different relationships between Vso and Vy or Vx regardless of the thrust issue. Finally, Vso brings another variable to the equation in that the difference between Vso and Vs is mostly a function of the effectiveness of the lift enhancement devices (flaps and slats).

VGs likely have a minimal effect on best glide airspeed but might lower it slightly. The same issues I mentioned above (except thrust) affect the relationship between Vs and the Vx/Vy.

FWIW, the "normal" way to "compute" Vx, Vy, and best glide speed (and others like max endurance speed) involves starting with a power required curve and a lift (or airspeed) vs drag curve. Those V speeds are represented by certain easily identifiable points on those curves.
 
Thanks all. I'm adding VGs next week and am trying to understand what impact they will have on Vx and Va. I'm fairly sure Vy shouldn't change but since the stall speed will be about 5 knots lower, I know I can climb out steeper (but don't know how it will actually impact Vx altitude and distance), and should probably lower Va by 5 knots too. Thoughts?
 
JohnAJohnson said:
Thanks all. I'm adding VGs next week and am trying to understand what impact they will have on Vx and Va. I'm fairly sure Vy shouldn't change but since the stall speed will be about 5 knots lower, I know I can climb out steeper (but don't know how it will actually impact Vx altitude and distance), and should probably lower Va by 5 knots too. Thoughts?
Click to expand...
Without VGs you can already climb slower than Vx, I really doubt that the VGs will have a noticeable effect on Vx. The effect on Va should be directly proportional to the real (CAS) change in Vs (flaps up). I'd be surprised if your actual calibrated Vs drops 5 Kt but it's possible. More common is a large drop in indicated airspeed due to increased errors in the pitot-static system and that won't affect Va. And strangely, AFaIK, the FAA has never required an increase in published Va as part of a VG STC.
 
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