Expert: Paul Soderman - 3/29/2001

Question
Dear Mr. Soderman,
I am currently a student at Southern Illinois University studying Aviation Flight and Mechanical Engineering. I recently been trying to figure out the science behind best rate and best angle of climb for and aircraft.
What I do know is that best rate of climb deals with the amount of excess power available, and best angle of climb deals with  excess thrust available.
My 'theory' thus far has been that as altitude increases the power available to a normally aspirated engine decreases, so best rate of climb decreases. However, my thought sort of fell apart when I looked to see if a turbocharged aircraft would maintain the same best rate of climb until it reached it's critical altitude. Although it's change was very slight there was a change (about 4 knots over 20,000 ft then it proceeded as expected).
For best angle of climb, I have thought that as altitude increases the true airspeed of the propellor would increase, thus increasing thrust available. This of course would only hold true if power available remained constant or decreased at a slower rate.
I hope you can help me to see the light at the end of this tunnel.
Thank you for your time.
Sincerely,
Aaron Barber

Answer
Hi Aaron

I am not quite sure what your question is unless you merely want some comments on propeller aircraft climb.  In that case, your theory is pretty good as long as you consider efficiency.  The first paragraph is true.  

Your second paragraph is also correct as long as you consider that a turbocharger efficiency falls off somewhat with altitude.  The turbine cannot totally overcome the loss in density.

In the third paragraph, I am not sure what you mean by propeller true airspeed.  A turbocharged aircraft would have a fixed speed, variable pitch propeller.  So, the local angle of attack on the blade changes with altitude, but the airspeed remains more or less constant.  However, the propeller efficiency will decrease.  And power available will decrease somewhat as discussed above.

Good luck.

Paul