Re: LG wheel hydraulic drive motors

From: (Matthew J O Scott)
Organization: Yale University
Date:         30 Oct 94 21:29:40 
References:   1
Followups:    1
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Bill Simpson <> wrote:
: 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.  Have high torque low speed wheel hub or direct drive
: motors been considered?  These are widely used for off-the-road vehicles,
: etc.  The motors would have to freewheel or be mechanically disconnected
: during takeoff and landing.  Very high torque low speed motors would be
: required to move a 747 size airplane.   The motors would have to be built
: to aerospace standards of light weight and high strength.

I don't see how or why this idea would fly. (Excuse me)
1.  The weight penalty would be exceesive for a feature that has no clear 
and immediate benefit.  (Aircraft have done fine without directly driven 
wheels for the entire history of aviation.  The Wright Brothers of 
course, had a ground drive system of sorts, but we understand why it was 
necessary for them)
2.  Airlines are interested in fuel savings.  Yes, a hydraulic torque pump 
running on APU juice would consume less fuel than all four fires on a jumbo, 
but as it is, almost every airline only runs essential engines 
while taxiing after landing (Jumbos shut down the two outboard 
ones, commuter twin-turboprops shut down the port engine next to the 
door) after landing.  Most big jets get pushbacks from their gates 
(unless they are able to taxi straight forward from the gate) that 
save fuel.  Frankfurt Airport at one point in the late eighties (someone 
help me out, tell me if they still do it) practiced a policy of towing 
all the widebodies right out to the holding point to save fuel (and cut 
down incredibly on jet fuel - hundreds of tons a year).  
I have a hard time seeing how even lightweight, high strength 
direct-drive motors designed for ground handling would take off (excuse 
me again).  A simple system to spin up landing gear wheels just prior to 
touchdown might reduce wear and tear on tires, but it is hard to see 
designers embrace the weight penalty involved across the board.
Matthew Scott                   "I'm an Englishman in New York."  Sting
Yale University                 "I'm a Canuck in Connecticut, eh!" M.S.