TODAYS Performance exam

[Send Clowns]

Spitfire, TAS of Vx and Vy always increases with height, because Vx increases and Vy reduces slowly. The TAS of a given CAS increases more quickly than either effect, so is dominant.

Hence the answer is "both increase".

[spitfire747]

SC

Thanks buddy, got that one wrong then

what about the second question about induced drag..
what the opinion the correct answer is for that one

Cheers
Spitty

[Dick Whittingham]

Cutting right back to the key point on the second question, and ignoring the side issue of stall definitions, the answer is C.

Cdi is proportional to Cl squared, but induced drag itself is a function of Cdi and V squared. In level flight -note the qualification - you can reach Cl max for your configuration at various speeds, depending on what Cl max your configuration gives you. As you fly slower V is coming down, and as lift is constant Cl required for level flight is going up proportional to the square of the speed reduction. Cdi is thus going up as Cl to the power 4. So with actual induced drag being Cdi times V squared, induced drag is going up as the square of the speed reduction. Got that?

This means that, however you adjust Cl with flap etc, you will still need a big Cl for slow flight, and this means a massively bigger Cdi and a bigger induced drag. This is a key practical point in handling rotate and lift-off. Do not rotate to extreme attitudes and do not lift off early or induced drag will bite you.

Dick W

[spitfire747]

Thanks Dick

I will leave it and call the chapter closed.. unless of course I need to re-open it once the white envelope arrives from the campaign against... which i do hope i never need to..

Thanks for all you input... roll on the weekend

[Send Clowns]

Dick is right that C is correct. However, according to the fine people in the Belgrano, it was not one of the answers on offer! This is an RTFA, i.e. read the answers. The answer is therefore stall speed.

On the plus side, this is a useful, technical disussion of PofF for our students sitting next month

[MorningGlory]

Well I'm awfully confused now, maybe the atpl's were too long ago, because I remember one of my instructors telling me that in prop as altitude increases vx increases and vy decreases, and in a jet as alt' increases, vx remains constant and vy also decreases.

[Send Clowns]

MG - V speeds are all CAS. This was asking about the effect of altitude on the equivalent TAS. Since The TAS equivalent to a given CAS is increasing with altitude much faster than any change in Vx or Vy, this is the dominant effect. Therefore the TASs always increase with altitude.

[MorningGlory]

Yep gotcha, after sitting here scratchin' me head trying to figure this one out all over again for the last 20mins, I now remember and yes you are quite correct