Vx / Vy relationship as alt goes up

[leedwpilot]

Does any CFI knows why Vx increases and Vy decreases as altitude goes up and where I can find a reference.

thanks

 

[midlifeflyer]

This is the explanation I use. The mathemeticians will no doubt cringe.

Vy is based on how much power you have in excess of the amount of power you have to maintain straight and level flight. Think about flying straight and level at 70 kts in a 172. Very little power is needed. Increase power, maintaining the 70 kts, and you climb. More power, higher rate of climb. Vy is the airspeed at which the difference between power available and power required for straight and level flight is the greatest. As you increase in altitude, there is less power available, so the airspeed at which the difference is the greatest goes down.

On the other hand, Vx is based on available thrust. Vx is the airspeed in which the greatest amount of thrust is available. An airplane with virtually unlimited thrust could fly straight up and the distance it can climb within a set horizontal distance be "infinite" (very loosely speaking). Think of our cartoon view of a rocket ship - straight up to outer space with no horizontal movement at all - forward airspeed is zero. (Of course, we don't fly airplanes that go straight up, but it helps visualize the concept.) As altitude increases, the amount of thrust also decreases - less power available to produce the thrust, more propeller "slippage" as air density gets thinner, etc. So, the airplane can't go straight up any more. In order to climb, we now have to lower the nose. Which in turn means an increase in airspeed.

 

[TEXAN AVIATOR]

This is the explanation I use. The mathemeticians will no doubt cringe.

Vy is based on how much power you have in excess of the amount of power you have to maintain straight and level flight. Think about flying straight and level at 70 kts in a 172. Very little power is needed. Increase power, maintaining the 70 kts, and you climb. More power, higher rate of climb. Vy is the airspeed at which the difference between power available and power required for straight and level flight is the greatest. As you increase in altitude, there is less power available, so the airspeed at which the difference is the greatest goes down.

On the other hand, Vx is based on available thrust. Vx is the airspeed in which the greatest amount of thrust is available. An airplane with virtually unlimited thrust could fly straight up and the distance it can climb within a set horizontal distance be "infinite" (very loosely speaking). Think of our cartoon view of a rocket ship - straight up to outer space with no horizontal movement at all - forward airspeed is zero. (Of course, we don't fly airplanes that go straight up, but it helps visualize the concept.) As altitude increases, the amount of thrust also decreases - less power available to produce the thrust, more propeller "slippage" as air density gets thinner, etc. So, the airplane can't go straight up any more. In order to climb, we now have to lower the nose. Which in turn means an increase in airspeed.

I’m going to have to say excellent explanation!

 

[Lead Sled]

Nice explanation, too bad it's misleading. The premise is wrong. When it comes to things like Vx, Vy, and Vs they are actually based upon specific angles of attack and not specific airspeeds.
These are not an isolated examples. Many of the airplane's critical performance numbers are really angle of attack numbers:
The stall occurs at a particular angle of attack; The smallest power-off descent rate occurs at a particular angle of attack; The best power-off glide ratio occurs at a particular angle of attack; The recommended "approach speed'' is really an angle of attack recommendation; The best rate of climb occurs at a particular angle of attack. The best angle of climb occurs at a particular angle of attack.

The problem is one of instrumentation (or lack of it). The proper answer to the question is that there is a specific angle of attack required to achieve Vx and a specific angle of attack to achieve Vy. Most light, general aviation aircraft do not have angle of attack indicators and therefore we are forced to estimate our AoA with indicated airspeeds. It is because of the lack of altitude compensation on our pitot-static systems (and a few other various and sundry reasons) that we are forced to vary our indicated airspeeds.

Lead Sled

 

[midlifeflyer]

Nice explanation, too bad it's misleading. The premise is wrong.

Are you saying "the maximum rate of climb would occur where there exists the greatest difference between power available and power required," is an incorrect premise?

or that

==============================
The effect of altitude on climb performance is illustrated by the composite graphs of Figure 2.22. Generally, an increase in altitude will increase the power required and decrease the power available. Hence, the climb performance of an airplane is expected to be greatly affected by altitude. The composite chart of climb performance depicts the variation with altitude of the speeds for maximum rate of climb, maximum angle of climb, and maximum and minimum level flight airspeeds. As altitude is increased, these various speeds finally converge at the absolute ceiling of the airplane. At the absolute ceiling, there is no excess of power or thrust and only one speed will allow steady level flight. The variation of rate of climb and maximum level flight speed with altitude for the typical propeller powered airplane give evidence of the effect of supercharging. Distinct aberrations in these curves take place at the supercharger critical altitudes and blower shift points. The curve of time to climb is the result of summing up the increments of time spent climbing through increments of altitude. Note that approach to the absolute ceiling produces tremendous increase in the time curve.
==============================

is incorrect?

 

[Lead Sled]

Are you saying "the maximum rate of climb would occur where there exists the greatest difference between power available and power required," is an incorrect premise?

or that

==============================
The effect of altitude on climb performance is illustrated by the composite graphs of Figure 2.22. Generally, an increase in altitude will increase the power required and decrease the power available. Hence, the climb performance of an airplane is expected to be greatly affected by altitude. The composite chart of climb performance depicts the variation with altitude of the speeds for maximum rate of climb, maximum angle of climb, and maximum and minimum level flight airspeeds. As altitude is increased, these various speeds finally converge at the absolute ceiling of the airplane. At the absolute ceiling, there is no excess of power or thrust and only one speed will allow steady level flight. The variation of rate of climb and maximum level flight speed with altitude for the typical propeller powered airplane give evidence of the effect of supercharging. Distinct aberrations in these curves take place at the supercharger critical altitudes and blower shift points. The curve of time to climb is the result of summing up the increments of time spent climbing through increments of altitude. Note that approach to the absolute ceiling produces tremendous increase in the time curve.
==============================

is incorrect?

Of course those are accurate statements. But they don't go far enough. They only address the power side of the equation. I don't care how much "surplus" power you have, you're not going to climb without some amount of positive AoA. And if you want to climb at the maximum possible rate or angle then you MUST optimize the AoA. Believe it or not, it's possible to fly a flight using the AoA indicator alone and not even referencing the ASI at all. The fact that most light aircraft don't have AoA indicators doesn't change the aerodynamics of it. The specific angles of attack required to achieve best rate and best angle of climb are constant and don't vary with altitude. The indicated airspeeds required to achieve those angles do.

Lead Sled

 

[midlifeflyer]

I don't care how much "surplus" power you have, you're not going to climb without some amount of positive AoA. And if you want to climb at the maximum possible rate or angle then you MUST optimize the AoA.

I don't disagree at all. It's pretty obvious that with all the power in the world, if you point the nose down so that you have a negative AoA, you will descend, not climb. And that if you don't optimize the AoA, you won't have the best climb. But by the same token, give me optimum AoA with no power and you won't climb an inch.

But that doesn't make looking at the at the effects all this has on airspeed "misleading" when we're dealing with an airplane in which the =only= "AoA indicator", as poor a replacement as it is, is the ASI.


So take my post and replace "you have to lower the nose" with "increase the angle of attack"