Re: Physics Question

From:         Art Intemann <>
Organization: Earthlink Network, Inc.
Date:         11 Aug 96 00:03:57 
References:   1
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Ramesh Pillutla wrote:

> here's the scenario:
> How does a fully-loaded 747 maintain it's speed, altitude and engine power
> settings over a long flight like LAX-SYDNEY.

It's generally done by one of three autopilots (on the 747-400), which
are programmed by the pilots.

> Fuel is constantly
> being burned off which implies that the weight of the aircraft is constantly
> changing. If the weight is constantly changing, isn't the center of
> gravity constantly changing.

Yes, but within a design (small range), which the autopilot pitch trim
adjusts for.

> Alos, is fuel burned equally from both
> the port/starboard fuel tanks in the wings to avoid an imbalance ?


> The law of conservation of momentum states that for a body moving with a
> certain velocity, changing the mass will result in change in velocity
> to conserve momentum. The intuitive conclusion is that the auto-pilot
> actually adjusts the engine power settings to account for the mass
> of expended fuel.

Actually, the autothrottle system adjusts throttle position to
maintain a preset airspeed or mach number.

I'm not expert,
just a driver           Art