Eagle1
7th Feb 2002, 17:31
A question from a PPL who is not satisfied with what a read in my PPL textbooks.
My question concerns the, probably well-known story about increasing TAS with increasing altitude. Now, I know that, with increasing altitude, the air density gets lower, so the IAS falls behind of the TAS. In other words, you'd have to fly faster (higher V) to compensate for this. To stick to the formula: you have to increase Velocity to compensate for the lower Rho (the symbol should be somewhere on the computer, but don't ask me where), and if you do that right, the result will be the same IAS, but with a higher TAS. Ram-air pressure has to stay the same.. .I was always perfectly happy with this answer, untill I realised that inside the altimeter there is also the static pressure. Now, it seems to me that, with increasing altitude, the static pressure will go down as well, probably with the same rate as the ram-air pressure (doesn't air density play a role also in the static pressure?). So, I thought, basically the pressure differential between both sides of the diaphragm will not change - the air comes from the same source, doesn't it?. .To put a long story short: with increasing altitude Rho will go down, but doesn't that go for the ram-air pressure as well as for the static pressure? So, how come that you fly a higher TAS with the same IAS?. .I hope this question is clear (I'm Dutch, not English), and that there's anyone who is able to shed some light on this.. .Thanks in advance, hope to hear from all of you.
My question concerns the, probably well-known story about increasing TAS with increasing altitude. Now, I know that, with increasing altitude, the air density gets lower, so the IAS falls behind of the TAS. In other words, you'd have to fly faster (higher V) to compensate for this. To stick to the formula: you have to increase Velocity to compensate for the lower Rho (the symbol should be somewhere on the computer, but don't ask me where), and if you do that right, the result will be the same IAS, but with a higher TAS. Ram-air pressure has to stay the same.. .I was always perfectly happy with this answer, untill I realised that inside the altimeter there is also the static pressure. Now, it seems to me that, with increasing altitude, the static pressure will go down as well, probably with the same rate as the ram-air pressure (doesn't air density play a role also in the static pressure?). So, I thought, basically the pressure differential between both sides of the diaphragm will not change - the air comes from the same source, doesn't it?. .To put a long story short: with increasing altitude Rho will go down, but doesn't that go for the ram-air pressure as well as for the static pressure? So, how come that you fly a higher TAS with the same IAS?. .I hope this question is clear (I'm Dutch, not English), and that there's anyone who is able to shed some light on this.. .Thanks in advance, hope to hear from all of you.