Vx, Vy changes with altitude

13 Replies

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? 
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Re: Vx, Vy changes with altitude

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

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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.

Re: Vx, Vy changes with altitude

Posted by Michael Smith on Jun 14, 2018 4:54 pm

 Last year I was flying back from Idaho falls heading back to Texas. I'm flying a Cherokee 180. My second stop around 4pm was Farmington NM.
I got fuel planed and filed my flight to Roswell NM about 380 nautical. Field elevation at Four Corners is 5500. At time of departure density altitude was about 7,000.
What you have to do is lean the engine before taxing to prevent fouling then at run up lean to max rpm. I took off  on runway 25 lifted off at Vx+10 (70 mph) accelerated to Vy 90. Normal take off, but I used 2500 ft of run way. At sea level I would have used a max of 800 ft. My POH says at 7000 ft density altitude and aircraft weight I should get 300 ft per minute climb which I did. But they don't tell you that you may need to add a notch of flaps.So I think what they are trying to tell you. Vx Vy are the same as indicated at any altitude, but you have to take into consideration distance to clear that 50ft obstacle. At high density altitude your ground speed increases and you eat up more run way before reaching Vx or Vy. Hench what most Instructors teach at high altitude airports with obstacles. "If you're not airborne by 1/2 the runway distance abort".
 

Re: Vx, Vy changes with altitude

Posted by Ronald Levy on Jun 16, 2018 6:51 pm

Michael Smith:
At sea level I would have used a max of 800 ft. My POH says at 7000 ft density altitude and aircraft weight I should get 300 ft per minute climb which I did. But they don't tell you that you may need to add a notch of flaps.

I think if you look in the POH for your Cherokee 180 it will tell you to use TWO notches of flap for a short-field takeoff.  Adding a notch to that would create so much drag you'd never climb even if you did eventually get airborne.  If you meant to use one notch instead of two, yes, they don't tell you that, and while I'm pretty sure one notch would work better than none, I don't know that one notch would work better than two.

So I think what they are trying to tell you. Vx Vy are the same as indicated at any altitude, but you have to take into consideration distance to clear that 50ft obstacle.

I don't think that's what they're telling you at all, since the indicated airspeed for Vy goes down with density altitude while the indicated airspeed for Vx goes up with density altitude.  That means at higher density altitude, you have to use a slightly higher indicated airspeed for rotation and Vx for best short-field/obstacle clearance performance compared to the sea level numbers.

At high density altitude your ground speed increases and you eat up more run way before reaching Vx or Vy.

Absolutely true, because acceleration is lower and Vx is higher.

Hench what most Instructors teach at high altitude airports with obstacles. "If you're not airborne by 1/2 the runway distance abort".

The general guidance I learned was 70% of rotation speed by 50% of the runway, but your more conservative guidance would certainly give you a greater safety margin.