Re: LG wheel hydraulic drive motors

From:         ehahn@fairlite.mitre.org (Ed Hahn)
Organization: The MITRE Corporation, McLean, Va.
Date:         30 Oct 94 21:29:40 
References:   1
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In article <airliners.1994.1646@ohare.Chicago.COM> news@aol.net writes:

   I saw a paper that had been given at an SAE  A6 meeting  on  the use of
   hydraulic motors  to  apply torque though gearboxes to landing gear wheels
   and thus move the airplane on the ground.  This would allow maneuvering
   the airplane on the ground with only its APU operating to drive a
   hydraulic pump.  <deletia> ...  The motors would have to freewheel
   or be mechanically disconnected during takeoff and landing. 

<additional deletia>
							       Thanks
								Bill Simpson
------
>From an economic standpoint, I'll talk about what the system would
have to overcome:

1) Utility:  it seems that this kind of system would only work for
moving the aircraft forwards, due to the problem of a) seeing what's
behind the aircraft, and b) stopping the aircraft while moving
backwards.  Typically, the only time aircraft move backwards on their
own power is during a powerback, where there are people looking out
behind the aircraft, and the engines running to stop the reverse
motion if it is necessary (hitting the brakes generally would cause
the aircraft to rotate back on its tail).  If only the APU is running,
there wouldn't be a way to stop the reverse motion.   

Therefore, assuming it can only move the aircraft forward, there must
be an incentive to use this kind of system rather than the engines or
a tug. Which leads to:  

2) Maintenance: because of the moment that the wheel motors will put
on the landing gear structure, the maintenance schedule of the landing
gear may be impacted, especially as the moment would be typically in
the opposite direction of the moment applied by wheel brakes.  This in
itself might not cost so much.

On the other hand, the maintenance of these motors would contribute to
added cost to maintain the airframe, compared with the cheaper
maintenance cost for tugs and other towing vehicles.

Furthermore, unless an entirely independent hydraulic system is
included in the aircraft, the motors will be hooked into the main
hydraulic system, which means the motors must be designed to minimize
impact on the flight control system in case of a leak, etc.

To size the added maintenance cost, flight delay/cancellation roughly
impacts airline operations anywhere from about $2500 for a short delay
to $75,000 for a cancellation, depending on aircraft size.   

3) Weight:  each extra pound an airliner must carry for an entire year
roughly costs $1000/year.  Is the utility gained by the addition of a
wheel motor system going to offset this additional cost? 

4) Safety:  if the above mentioned freewheeling or torque disconnect
failed on a landing, how would this affect the aircraft?

Anyways, these are my thoughts on the wheel motor system.  Any
inaccuracies in the concepts or numbers are mine only.

ed

////////   Ed Hahn | ehahn@mitre.org | (703) 883-5988   \\\\\\\\
The above comment reflects the opinions of the author, and does not
constitute endorsement or implied warranty by the MITRE Corporation.
Really, I wouldn't kid you about a thing like this.