直升機飛行員手冊直升機操作手冊 The Helicopter Pilot’s Handbook

時間：2011-04-05 11:37來源：藍天飛行翻譯 作者：航空 點擊：次

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While I'm not suggesting for a moment that you should, piston engines will accept their limits being slightly exceeded from time to time with no great harm being done. Having said that, the speed at which the average Lycoming engine disintegrates is about 3450 RPM, which doesn't leave you an awful lot of room when it runs normally (in a Bell 47, anyway) at 3300! Turbines, however, are less forgiving than pistons and give fewer warnings of trouble because of their closer tolerances. This is why regular power checks are carried out on them to keep an eye on their health. The other difference is that damage to a piston engine caused by mishandling tends to affect you, straight away, whereas that in a turbine tends to affect others down the line.
In a turbine-engined helicopter, power is indicated by the torquemeter.
Apart from sympathetic handling, the greatest factor in preserving engine life is temperature and its rate of change. Over and under leaning are detrimental to engine life, and sudden cooling is as bad as overheating—chopping the throttle at height causes the cylinder head to shrink and crack with the obvious results—the thermal shock and extra lead is worth about $100 in terms of lost engine life. In other words, don’t let the machine drive the engine, but rather cut power to the point where it’s doing a little work. This is because the reduced power lowers the pressure that keeps piston rings against the wall of the cylinder, so oil leaks past and glazes on the hot surfaces, degrading any sealing obtained by compression. The only way to get rid of the glaze is by honing, which means a top-end overhaul. For the same reasons, a new (or rebuilt) engine should be run in hard, not less than 65% power, but preferably 70-75%, according to Textron Lycoming, so the rings are forced to seat in properly. This means not flying above 8000 feet density altitude for non-turbocharged engines. Richer mixtures are important as well. Also, open the engine compartment after shutting down on a hot day, as many external components will have suddenly lost their cooling. With some turbine engines (like on the AStar), you have to keep a track of the number of times you fluctuate between a range of power settings because of the heat stress.
In the cruise, better fuel consumption may be obtained at slower speeds and lower power settings, at the cost of extended running time, so you might not really save that much. For example, leaning to 10° lean of peak Exhaust Gas Temperature (EGT), without exceeding the maximum, loses about 5 knots. Typically, EGT probes are fitted to one cylinder of the engine, which is not necessarily the one that reaches peak temperature first, even though it may end up as the hottest, so a margin of 25° rich of peak may still not be enough to stop another cylinder from getting too close to peak for comfort, or even lean.

One consideration with using low power when it's very cold is that the engine may not warm up properly and water that forms from combustion may not evaporate, so oil won't lubricate properly.

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