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• ##### Question: Since a plane's weight decreases (0+ / 0-)

continuously during flight, how is lift continuously decreased to maintain a constant altitude?

• ##### Good question(1+ / 0-)
Recommended by:
AnnArborDem

Assuming we're at a constant airspeed, we can maintain the same amount of lift with less angle of attack and use less thrust as a result.

This was very noticeable in the B-52. At very heavy weights we would fly slightly nose high. At lighter weights we would have a very noticeable nose down pitch.

If the pilot's good, see, I mean if he's reeeally sharp, he can barrel that baby in so low... oh you oughta see it sometime. It's a sight. A big plane like a '52... varrrooom! Its jet exhaust... frying chickens in the barnyard!

[ Parent ]

• ##### I assume, then, that the computer systems of(1+ / 0-)
Recommended by:
RiveroftheWest

modern jetliners continuously adjust the thrust and angle of attack to maintain a constant altitude.  Otherwise it seems that the aircraft would accelerate upward until the reduced air pressure reduced the lift to equal the plane's weight.

• ##### You are correct(1+ / 0-)
Recommended by:
RiveroftheWest

At cruise we're normally on autopilot and have the auto-throttles engaged.

We dial in what speed and altitude we want it to hold and it does the rest.

On long flights we will sometimes "step climb" up to a higher altitude after we burn some fuel off.

Going higher gets you better fuel economy, once you're light enough to get up there.

I plan on devoting a diary to the auto flight systems  sometime in the future.

If the pilot's good, see, I mean if he's reeeally sharp, he can barrel that baby in so low... oh you oughta see it sometime. It's a sight. A big plane like a '52... varrrooom! Its jet exhaust... frying chickens in the barnyard!

[ Parent ]

• ##### With Concorde it was relatively simple....(2+ / 0-)
Recommended by:
AnnArborDem, RiveroftheWest

you simply pumped fuel from the rear to the front to alter the trim :) On G-BOAF, I watched this happen!

[ Parent ]

• ##### They had to build it that way(2+ / 0-)
Recommended by:
AnnArborDem, RiveroftheWest

When you go supersonic the center of lift changes noticeably and the center of gravity has to change with it.

Not significant on a fighter-sized airplane but very much so on something like Concorde.

I've talked to B-1 guys and they said they didn't even really know where the fuel was at any given moment because the plane was automatically shifting it around.

If the pilot's good, see, I mean if he's reeeally sharp, he can barrel that baby in so low... oh you oughta see it sometime. It's a sight. A big plane like a '52... varrrooom! Its jet exhaust... frying chickens in the barnyard!

[ Parent ]

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