Vx, Vy changes with altitude

11 Replies - 690 Views

Vx, Vy changes with altitude

Posted by Joel Hamm on Apr 2, 2018 4:44 pm

e-FIRC's got me confused. The Mountain Flying module links to the ASI course, which stresses that IAS for climb speeds remain the same despite risinfg density altitude (while TAS increases), and that high altitude departures should be flown at the same IAS as at sea level.  Sounds reasonable, because IAS is the direct measure of pressure against the wing, and the same pressure produces the same quantity of lift wherever you are.

The  second lesson  in the module is "Teaching Mountain Flying", which says, in one of the quiz questions, that as altitude increases Vx increases while Vy decreases.  Can't for the life of me figure out, or find ou why.  Querry to AOPA's Pilot Answers dept just got me a reference to the AIM (or whatever) which described  the  convergence of the 2 speeds at the absolute altitude, again, with no explanation why.

can anyone provide clarification? 
  • Bookmark

Re: Vx, Vy changes with altitude

Posted by Michael Jesch on Apr 16, 2018 6:53 pm

This graph, which was given to me a while back by Rod Machado, is one of several I use to illustrate this phenomenon in my seminar titled "Hello! My Name Is Airspeed!" I'm giving the talk again this coming Wednesday evening at Camarillo, CA (KCMA) for the Ventura County 99s. The red arcs are rates of climb achieved at various indicated airspeeds. The top of the arc is Best Rate of Climb, or Vy. The blue lines coming from the origin and tangent to each arc indicate where Best Angle of Climb, or Vx, is achieved. Note that, at higher altitudes, the top of the arc moves left, while the tangent point moves right. Vx is a bit of a compromise between rate of climb and distance travelled, but that's exactly what we want when we are trying to outclimb an obstacle right after takeoff. I never really understood this concept myself until I wrote this seminar, and Rod graciously offered the use of his graphs, and I thought they illustrate it perfectly. He gave me permission to publish them here today, too. Thanks a million, Rod!

I'll also note that a similar thing occurs on the glide. It's the difference between minimum sink airspeed and best glide speed.