Oxford vs. JAA

[BEK]

As an JAA ATPL student, I have the following question on Performance. According to Oxford/Jeppesen theoretical knowledge manual on performance the speeds for best rate of climb, minimum drag, minimum power should be in the following order(prop):. .Vmp - Vmd - Vy. - so here's the thing.. According to JAA speed for best endurance(Vmp)is approximately equal to Vy... . .This is all theoretical. In real life(PA-28-161) speeds are listed as: Vx 63kts, Vmd(Vglide) 73kts, Vy 79 kts.

Why this inconsistency? <img src="confused.gif" border="0">

N.b. Had a great graph for illustration, but don't know how to upload it to this site.. <img src="frown.gif" border="0">

BEK

[Zeke]

The Vx you cite would be with flaps, not clean.

Have a look at this link for a Vx Vy discussion

<a href="http://www.pprune.org/cgibin/ultimatebb.cgi?ubb=get_topic&f=57&t=002835&p=" target="_blank">click me</a>

[Keith.Williams.]

BEK,

I have not seen the Oxford manuals to which you refer, but I can assure you that the sequence you quote is incorrect. For a prop VX is just less than VMP and VY is just greater. VMD is greater than all three. Best prop endurance is at VMP and best range at VMD. For a jet VX and best endurance are at VMD and VY and best range are slightly greater.

You should also be aware that the Perf exams often ask questions about how these speeds vary with things like mass and temperature, but they are shying away from the effects of increasing altitude (as discussed in other strings recently).

If you send me your e-mail address I will post you a whole aircraft L :) polar diagram depicting these and many other (16 in all) interesting (for exam purposes) speeds.

If you look in the pilotwarehouse.co.uk web site under "JAR Question Banks" you will find an excellent book on the subject.