Expert: Paul Soderman - 4/30/2005

Question
I did a Google Search and came up with this site.  Thank you for this opportunity.  I'm an airline captain working on my Certified Flight Instructor rating, more for the fun and challenge.  I'd like to ask why, specifically, airspeed increases with altitude for the best angle of climb, and the airspeed decreases with higher altitude for best rate of climb? I suppose graphs can provide some of the answer, but I'm looking for a more intuitive, verbal answer that I could explain to a new student (or to myself!)  I know that thrust and power decrease with altitude, but why does one speed increase, and the other decrease?   
Thanks, Paul, for your time and effort!
Terry Taylor

Answer
Hi Terry

You mention two types of airspeeds without clarification.  If I understand correctly, you are asking:  Why does aircraft airspeed increase during best angle of climb and then decrease after some point ?

OK, let me assume that we are talking about jets.  Jets have pretty much constant thrust available as a function of airspeed.   During climb at the optimum climb angle, the aircraft will accelerate and reach maximum L/D, at which point the aircraft will have optimum excess thrust (thrust available minus drag).  But power available is still increasing,  and maximum climb rate will occur at a somewhat higher altitude.  But as airspeed increases, drag increases as velocity squared (and decreases with density reduction), excess thrust decreases, power required increases, and acceleration decreases.  At some point, the power available is just that required for max cruise speed in level flight.  Acceleration is zero and climb is no longer possible.

The story is slightly different for propellers.  You might visit: http://142.26.194.131/aerodynamics1/Performance/Page10.html to see a  comparison of jets versus propellers.  If I missed your point, please contact me at psoderman@direcway.com, and we can discuss this further.  I can also run this by my flying buddies at NASA who have much better hands on experience than me.

Paul