Expert: Paul Soderman - 6/1/2007

Question
Hello Paul,

I would like to come back, if I may, to the question asked by Terry Taylor about best angle and best rate climb speeds. I am actually preparing a JAR PPL in Belgium and this specific issue is still a mystery to me

Looking at performance charts it is clear that best rate of climb speed will decrease with altitude and that, at the same time, best angle of climb speed will increase with altitude.

At a certain altitude best angle speed will be equal to best rate speed and, as I understood it, the aircraft will be flying at its absolute ceiling.

But it is not clear to me why one speed is decreasing with altitude while the other is increasing with altitude.  

Answer
Patrick
Sorry for this late resonse, but here goes.

Consider a propeller aircraft climbing at the best rate of climb with a speed Vx.  That is to say, it is climbing as steep as possible to minimize the horizontal distance needed to get to altitude.  Now, if the pilot wants to climb faster and is no longer trying to minimize horizontal distance, he puts the nose down and accelerates to Vy because the gravity component is now weaker in the flight direction.  Vy is always greater than Vx until the absolute ceiling is reached and both velocities converge.  In either case, as the aircraft climbs, the power available decreases as air density decreases, but the aircraft accelerates because the drag decreases and the climb angle decreases.

So, I think it is easy to see that Vx increases with altitude.  The tricky part is that Vy also increases with altitude because Vy is always at the peak of the power curve, and speed for peak power increases slowly with altitude even though the amplitude of peak power goes down.  However, Vy is increasing at a slower rate than Vx, so Vx catches up with Vy at the absolute ceiling.  Note that I am speaking of true airspeeds, indicated airspeed complicates this a little because indicated airspeed may appear to decrease with altitude even though true airspeed is not.

To help understand this, it really helps to see a graph of Vx, Vy and power available as a function of altitude.  You can see that at:  http://selair.selkirk.bc.ca/aerodynamics1/Performance/Page10.html  The difference between jets and propeller aircraft is also interesting.  Hope this helps.

Paul