From: zwork@starfighter.den.mmc.com (Michael Corvin)
Subject: Re: difference between Vx and Vy
Organization: Martin Marietta Astronautics Group
Distribution: usa
Date: Wed, 18 Nov 1992 22:00:01 GMT

chesky@squirrel.austin.ibm.com (Bill Chesky) writes:

>>
	Vx = best angle of climb speed; the speed that will give you the
	     greatest gain in altitude over a given horizontal distance
	Vy = best rate of climb speed; the speed that will give you the
	     greatest gain in altitude over a given period of time

To me this says: 1) your altitude will be greater if you use Vx over a
given distance than if you had used Vy.  2) your altitude will be greater
if you use Vy over a given period of time than if you had used Vx.  

... <discussion deleted> ...


3) 	     |                        
	     |                            
	     |                         * Vy altitude (plane is higher than
  altitude   |                     +     plane using Vx after a given amount
  	     |Vx altitude *    +	 of time but is lower than plane using
	     |         +   +             Vx at any given horizontal distance 
	     |      +  +                 from the orgin)
	     |   ++
	     |+
	     +--------------------------
	          horizontal distance

My friend claims that I am mistaken, though.  He says that the term "best 
rate of climb" is really a misnomer since you will be higher using Vx even
over the same *period of time*.  I don't totally understand his argument.
He says Vy is a "more efficient" rate of climb and that's why you wouldn't
use Vx all the time (or something like that).  So who is right?  Can anybody 
clear this up for me?

<<

Your understanding of Vx and Vy is entirely correct and well stated!  Your
friend owes you a beer (or equivalent atonement)... :-)

One only uses Vx in a short field take-off when trying to clear an obstacle.
In this case the objective is to gain as much altitude as possible in the 
shortest distance.  Your plot #3 shows this clearly.  It is correct in
showing that climbing at Vx will not get you as high as climbing at Vy
in the same amount of time.

Vy is more efficient in energy terms - assuming the engine is putting out
about the same (full throttle) power in each case you gain more altitude
in the same time - you've turned more of the work done by the engine (power
x time) into potential energy (mass x height x gravity). (With a fixed prop
you probably will be getting a bit _more_ power at Vy also because you'll have 
slightly higher rpm at the higher speed.)

Up here in Denver using Vy is critical in a 152 or even a 172 if you want
to get anywhere significantly AGL on a warm day...  Note that Vy changes
with altitude.

When performance isn't as marginal as that one typically climbs at a speed
somewhat higher than Vy to afford better engine cooling and a better view
over the nose.

(When grinding upwards at Vy in a 152 or the like, with the nose obstructing
your view forward, you can impress your instructor (or examiner) by doing
very gentle s-turns about your primary heading to clear the airspace ahead of
you.  Done smoothly, this reduces your rate of climb only a little.)

Hope this helps,

-----------------------------------------------------------------------------
Michael Corvin 	      PP-ASEL, PP-G		zwork@starfighter.den.mmc.com
just another spaced rocket scientist at    Martin Marietta Astronautics Group
-----------------------------------------------------------------------------
===============    My views, not Martin Marietta's   ========================
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