Vx or Vy and why?

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TimTaylor
Posts: 1491
Joined: Tue Feb 21, 2017 7:17 pm

Re: Vx or Vy and why?

Postby TimTaylor » Tue Mar 05, 2019 2:03 pm

I think the issue has more to do with an engine failure one minute or less after lift-off. At Vy, if your plane flys at 72 mph and climbs at 1000 fpm, in 60 seconds you will be 1000 agl and 1.2 nm from lift-off spot. If that same plane at Vx flys at 67 mph and climbs at 800 fpm, in 60 seconds you will be 800 agl and 1.1 nm from lift-off spot. With a headwind, the distances would be shorter, of course.

Maybe the bigger worry would be losing an engine in 30 seconds at 500 agl and .6 miles from lift-off at Vy versus 400 agl and .56 miles from lift-off at Vx.

If we were concerned about losing an engine at 1500 agl, we would always make a downwind departure and circle the airport until we reached whatever we felt was a safe altitude. This is something I do if the plane just came out of maintenance. I doubt many people are going to fly at Vx until 1500 agl, IDK.

I use Vx to get over the 50 feet trees and power lines. That takes about 5 to 10 seconds at which time I start to lower the nose to Vy. Once clear of obstacles, I'm more concerned with best rate of climb, better engine cooling, better vision over the nose, and getting out of the way of other aircraft using the runway. Also, the airplane handles much better at Vy than Vx, especially in gusty wind conditions. YMMV.
Commercial Pilot Airplane Single & Multiengine Land; Instrument Airplane
Sport Endorsement Airplane Single Engine Sea
Flight Instructor Airplane Single & Multiengine
Ground Instructor Advanced Instrument
BS Engineering NC State
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TimTaylor
Posts: 1491
Joined: Tue Feb 21, 2017 7:17 pm

Re: Vx or Vy and why?

Postby TimTaylor » Tue Mar 05, 2019 4:38 pm

Commercial Pilot Airplane Single & Multiengine Land; Instrument Airplane
Sport Endorsement Airplane Single Engine Sea
Flight Instructor Airplane Single & Multiengine
Ground Instructor Advanced Instrument
BS Engineering NC State
MBA Wisconsin

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dstclair
Posts: 1085
Joined: Thu Mar 06, 2008 11:23 am
Location: Allen, TX

Re: Vx or Vy and why?

Postby dstclair » Tue Mar 05, 2019 5:22 pm

Nice article -- thanks for posting.
dave


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