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?