tom775257
Topic Author
Posts: 141
Joined: Fri Dec 22, 2000 1:51 pm

PPL Flight Performance + Planning

Hi all,
I did this practise UK (JAA) PPL question, I disagree with what is supposed to be the correct answer, see what you think:

‘The true airspeed for the best rate of climb when climbing from sea level to the service ceiling:
a) Tends to increase
b) Tends to decrease
c) Stays the same”
I said a)…because
Lift = Coefficient of lift x ½ x rho x V squared x S
OK, so V= TAS = proportional to IAS depending on Rho (pressure).
So with increasing altitude, rho decreases so you need to increase TAS (V) to compensate for the loss of lift from rho decreasing. Vy is an IAS not a TAS, correct? So if you fly Vy IAS, with increasing altitude TAS will tend to increase.

Book Answer = b) Tends to decrease. Explanation: “With increasing altitude, density decreases, so engine power available together with the lift generated for a given airspeed will decay. The reduction of lift can be compensated for by increasing angle of attack which will increase drag causing speed to decay”

OK so if you let speed decay you are no longer at Vy, and not actually flying best rate of climb.
Hmmm, any thoughts?
Cheers.

Ralgha
Posts: 1589
Joined: Tue Nov 09, 1999 6:20 pm

RE: PPL Flight Performance + Planning

Vy decreases as you climb.

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fspilot747
Posts: 3455
Joined: Mon Nov 01, 1999 2:58 am

RE: PPL Flight Performance + Planning

I got (b) when I first looked at the question, there's nothing wrong with the book.

You aren't going to climb at Vy the whole way to, say, 7,500 feet anyways. The higher you go, the lower you're going to have your rate of climb be. The density of the air is thinner as you go higher, therefor you have less air molecules for your RAM air pressure.

tom775257
Topic Author
Posts: 141
Joined: Fri Dec 22, 2000 1:51 pm

RE: PPL Flight Performance + Planning

Thanks for the replies.

Ok, so are we talking Vy IAS or TAS. Fair enough you might fly a lower IAS, but what about the TAS - even though you are say flying 5-10kts slower IAS your TAS might have increased. Surely you want to be getting more lift through increasing V than through AoA increasing coefficient of lift...obviously if you are decreasing Vy IAS while climbing, and relying on greater AoA to give the lift then the induced drag is going to be much greater. While speed up IAS and lower AoA and greatly lower induced drag (see AoA drag curve...most efficient AoA for best lift/drag generally around 4 deg +ve...surely this is what you want for best rate of climb??)
(1/2 rho V squared being IAS)

Play 'spot the big error in Toms understanding' ...please!
Hmmm

[Edited 2003-06-27 19:42:17]

tom775257
Topic Author
Posts: 141
Joined: Fri Dec 22, 2000 1:51 pm

RE: PPL Flight Performance + Planning

Oh yeah I would just add that I don't think I am better than the book...just trying to get you guys to help me understand why I think it is wrong and thus where my knowledge is incorrect!
Cheers

Ralgha
Posts: 1589
Joined: Tue Nov 09, 1999 6:20 pm

RE: PPL Flight Performance + Planning

FSPilot: incorrect, you will (should) climb at Vy for the whole climb, but Vy decreases as you climb, so your IAS will decrease. This doesn't hold true for all airplanes/pilots as cruise climb is popular, and faster, than Vy.

Tom: Whether TAS increases or decreases as you climb at Vy depends on how fast Vy decreases as you climb. For a Cessna 172RG (only POH I have handy), your true airspeed will increase ever so slightly as you climb. Of course this assumes standard atmospheric conditions. Vy at 0 feet is 84 KIAS = 84 KTAS. Vy at (for example) 4000 feet is 81 KIAS = 86 KTAS.

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POSITIVE RATE
Posts: 2121
Joined: Sun Sep 16, 2001 11:31 am

RE: PPL Flight Performance + Planning

I would have answered A also. For a light aircraft although your Vy IAS may decrease as you climb, this will be offset by the increase in TAS as you climb. But then again the question is asking about service celing which is around 18,000feet or something isn't it for a typical 172?

Ralgha
Posts: 1589
Joined: Tue Nov 09, 1999 6:20 pm

RE: PPL Flight Performance + Planning

The top of the climb chart for the 172RG is 12000, at which point Vy = 75 KIAS = 90 KTAS, so it's still climbing at that point. It will probably continue to do so up to the service ceiling of 16800.
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