From: firstname.lastname@example.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> email@example.com 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 | firstname.lastname@example.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.