piper arrow v-speed
- Thread starter GADD45B
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USMC-SSGT
Well-Known Member
Because you are comparing two totally different numbers
VY is for the best rate of climb or to attain the greatest altitude in the shortest period of time (not distance) using maximum power
Best glide is basically the speed at which the airplane loses the least amount of altitude over a given distance with typically zero or close to zero power
although VY is often equated to a speed where there is the least amount of drag possible it is a different situation being that the engine is using full power where best glide you are using zero power and typically producing less airspeed
i probably shouldnt have spoken up though because when i saw the question i knew the answer, however i can not properly explain it. I think one of the CFI's here can take what i have and actually put it into something that makes sense
VY is for the best rate of climb or to attain the greatest altitude in the shortest period of time (not distance) using maximum power
Best glide is basically the speed at which the airplane loses the least amount of altitude over a given distance with typically zero or close to zero power
although VY is often equated to a speed where there is the least amount of drag possible it is a different situation being that the engine is using full power where best glide you are using zero power and typically producing less airspeed
i probably shouldnt have spoken up though because when i saw the question i knew the answer, however i can not properly explain it. I think one of the CFI's here can take what i have and actually put it into something that makes sense
fish314
Well-Known Member
I'm going to expand a little on what USMC-SGT wrote, because he's basically got it right on the noggin.
Here's the difference between Best GLIDE speed and Vy. Best glide speed is usually the same speed as Vx, not Vy. This is because best glide speed is located at the airspeed where the ratio of lift to drag is greatest. This point occurs where the drag is the lowest. This is the same speed at which you will get your best ANGLE of climb (but not your best RATE of climb), because you will have the greatest amount of thrust compared to drag. Again, all of this occurs because your drag is at it's lowest point.
Vy is determined by another factor, power. Power is defined as a force applied across a distance in a certain amount of time, or another way to put it is power is a force times a velocity. So even though we talk about Drag (which is a force) being at it's minimum, and the ratio of thrust to drag being maximized at L/Dmax (or Vx, or Best Glide speed, since they are all the same) when you mulitply by the velocity to get it in terms of POWER, the maximum power available occurs at a faster speed.
So in layman's terms, if you are trying to glide a far distance, or clear a tree, then you use Vx.... Only distances are being considered. But if you care about climbing in a particular amount of TIME, then you use Vy...the point of maximum excess POWER.
Here's the difference between Best GLIDE speed and Vy. Best glide speed is usually the same speed as Vx, not Vy. This is because best glide speed is located at the airspeed where the ratio of lift to drag is greatest. This point occurs where the drag is the lowest. This is the same speed at which you will get your best ANGLE of climb (but not your best RATE of climb), because you will have the greatest amount of thrust compared to drag. Again, all of this occurs because your drag is at it's lowest point.
Vy is determined by another factor, power. Power is defined as a force applied across a distance in a certain amount of time, or another way to put it is power is a force times a velocity. So even though we talk about Drag (which is a force) being at it's minimum, and the ratio of thrust to drag being maximized at L/Dmax (or Vx, or Best Glide speed, since they are all the same) when you mulitply by the velocity to get it in terms of POWER, the maximum power available occurs at a faster speed.
So in layman's terms, if you are trying to glide a far distance, or clear a tree, then you use Vx.... Only distances are being considered. But if you care about climbing in a particular amount of TIME, then you use Vy...the point of maximum excess POWER.
tgrayson
New Member
That's only true if the thrust available curve is flat, which is not the case for a propeller airplane. Vx really ought to be a bit below L/Dmax.
fish314
Well-Known Member
This is true. My explanation was exactly accurate for jet powered aircraft, because they have a more linear power vs. velocity distribution. For propeller aircraft, Vx is usually close to but not exactly the same as L/D max (usually it's slightly less) because as speed increases thrust available drops off some. I just didn't want to confuse the issue any more than I had to.
The big difference that I was trying to point to is that Vy is based on POWER (a which is a force, like lift or drag, multiplied by a velocity). This is because what is important is Power available versus power required (or Thrust times velocity versus drag times velocity).
Best Glide (L/D max) and Vx, on the other hand, are based on FORCES only (Thrust versus Drag).
The big difference that I was trying to point to is that Vy is based on POWER (a which is a force, like lift or drag, multiplied by a velocity). This is because what is important is Power available versus power required (or Thrust times velocity versus drag times velocity).
Best Glide (L/D max) and Vx, on the other hand, are based on FORCES only (Thrust versus Drag).
tgrayson
New Member
Agreed.
Too late.
My observation is that most people's eyes glaze over during discussions of thrust vs. power. I've only been able to explain it mathematically, which most people don't find satisfying.