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bultaco
24th Mar 2006, 09:57
Hi all,

Maybe someone can help with this question.

You are climbing from MSL at a constant IAS of 200kts to 40,000'. I think your power requirement is increasing and power available is decreasing. The angle of climb and rate of climb is decreasing. I heard that at 40,000' the density is 1/4 the MSL value and your TAS is twice MSL i.e. 400kts for this question. Now the lift equation should stay in balance cause the density is 1/4 but the TAS is double. So:

1)Does the AOA remain the same throughout the climb
2)Do you need to reduce the pitch of the aircraft

Bultaco

24th Mar 2006, 12:45
1)Does the AOA remain the same throughout the climb

Almost certainly no.

Mach number is varying quite significantly during that climb, chances are the AoA for a given CL is different at start and end.

Crowsnest
24th Mar 2006, 12:54
1. No
2. Yes

You're right, if the figures you have assumed are correct: the amount of lift for the given scenario would be the same at altitude as at msl.

However, this given amount of lift would have to be density-compensated (as you correctly state) by an increase in thrust (Increasing T req, decreasing T av) and decrease in Alpha accomplished by a lower pitch attitude.

The next problem is whether climbing at the constant IAS compromises the stalling speed of the aircraft with decreasing density (it will do eventually), so the transition to constant Mach No. has to occur.

If anyone has other ideas/opinions/corrections I'd be glad to hear.

Charlienov.

bultaco
24th Mar 2006, 14:32
Thanks Crowsnest,

In the PPSC notes I have on performance it seems to state that the AOA should remain constant once you are climbing at a constant IAS. I have printed the paragraph below but now I believe that it's not correct.

"Climbing at a constant IAS will cause the TAS and Mach number to increase. Although a constant angle of attack is maintained, the climb gradient and rate of climb will both decrease because of the decreasing excess of power available over power required. After changing to a constant Mach number, both the IAS and TAS will gradually decrease. Because of this the angle of attack must be increased. Consequently, despite the decreasing power available the climb gradient increases. Therefore, in a normal atmosphere, at the crossover altitude the climb gradient changes from one that is decreasing to one that is increasing."

Keith.Williams.
24th Mar 2006, 15:21
For JAR ATPL(A) purposes you need to think of the way reducing air density affects the thrust.

If we ignore any changes in mass and any compressibility effects, then at constant CAS we will need constant angle of attack to give constant lift. Constant angle of attack at constant CAS will give constant(ish) drag.

But as altitude increases, the thrust available will gradually decrease. So to maintain constant CAS we must reduce the drag to match the reducing thrust. To do this we must decrease the angle of attack.

All of the above is typical of the simplfications that are used in JAR ATPL(A) exams.

bultaco
24th Mar 2006, 15:50
Thanks Keith :)