Aerodynamics question...

brew3departure

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Hello all,

I received a question today from a commercial student that I wasn't able to answer completely or to my satisfaction. So, roll up your sleeves and take your best shot:

We all know that in a normal, single engine airplane, Vx is normally a little lower than Vy at sea level, and as we increase altitude, Vx increases and Vy decreases - the point at which they meet is the absolute service ceiling (or absolute ceiling) of the aircraft.

The question is - why? Why does Vx increase and Vy decrease as we increase altitude? I tore my house apart looking for my fluid/aerodynamics books last night, couldn't find them (in boxes somewhere).

Sigh. You don't teach or use something for a year and a half, and it gets a little fuzzy. Thanks much for any/all responses.

-brew3
 

tarp

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Tough one to explain without getting too mathematical, but here goes.

Vx is the best angle of climb and is a function of excess THP or Thrust horsepower for a given airplane. As you know, in a normally aspirated, prop driven airplane, the amount of power is reduced for any given increase in altitude due to the decrease in ambient pressure and therefore manifold pressure. I.e., as you climb the air gets thinner and the engine has a hard time breathing and the prop has a hard time moving the thinner air. So at low altitudes the airplane is relatively efficient. To maintain the best angle of climb (that is the amount of distance up for the distance forward) you will have to lower the nose and increase the speed at altitude or the airplane will lose the ability to gain the maximum amount of altitude. Think about an acrobatic plane demonstrating a hammerhead. By pointing the nose straight up, there is a point where the airplane runs out of steam and simply cannot climb anymore. If that same pilot just lowered the nose and powered out, he still has climb performance.

Vy is the best rate of climb and is a function of time. The airplane will gain the maximum amount of altitude in a given time. At sea level, our given plane is pretty efficient again. And depending on the aerodynamic form, there is probably a speed greater than max angle that will give a better rate of climb. However, as the airplane climbs and becomes less efficient, you will have to raise the nose to attain this fastest “rate”. Hence the airplane is going slower.

There comes a point when the angle of climb speed and the rate of climb speed meet and this is the absolute ceiling of the airplane. Please also note that Excess THP is largely dependent on the weight of the airplane. Ceilings published in the POH are rated on Max Gross Weight. If the airplane is lighter or you perform a test on a better than standard atmosphere day, you will have more excess thrust and thus the ceiling could be higher.
 

avbug

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Note that this isn't the case for turbocharged, turbonormalized, or supercharged engined airplanes. These airplanes may certainly have the capability of climbing higher, but are often capped out by limitations in certification, at the absolute ceiling of the airplane. This is often the case for airplanes which can remain boosted to great heights. In this case Vx and Vy may will remain separate numbers at the maximum altitude to which boost may be maintained on the engine, or to which sea level boost may be maintained for turbonormalized operations.
 

brew3departure

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Thanks to you both!

Tarp and avbug,

Thanks for your replies! Your information, combined with what I finally managed to dig up in my aerodynamics texts have enlightened me the second time around and my student.

Take care,
-brew3
 
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