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Old 16th Apr 2013, 11:19
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Join Date: Aug 2004
Location: Aberdeen
Age: 63
Posts: 2,021
In modern aircraft, of course there tend to be no throttles / levers, just switches that select off, idle, fly for the FADEC software.

In more traditional types the levers typically connect to the fuel control unit (FCU) on the engine. There is typically a shutoff position, that stops all fuel getting to the engine, an idle position that takes the engine to its minimum operating N1 (rotors probably less than flight speed on the ground) and is typically used for starting, and a flight position where, from the lever's point of view, maximum fuel flow is authorised, but of course the FCU then limits the fuel flow so as to maintain the rotor rpm at the correct value.

Imagine the lever as a tap at the inlet of the FCU - when its shut off, no fuel. When its at idle, a little fuel is allowed through for idle speed, and when its in the flight position the tap becomes fully open. So it may be possible to set the lever in an intermediate position between idle and flight - in the case of an FCU malfunction where the FCU is trying to give too much fuel, the pilot can set the power by partially closing this "tap" but then of course there is no rotor rpm governing.

Some types also allow fuel flow to be increased beyond the FCU demand, for the case where an FCU malfunction is not allowing sufficient fuel in. This may be a seperate lever, or may involve pushing the lever further forward than the flight position. Think of this as a tap bypassing the FCU and just letting fuel straight into the engine.

Beep switches are typically used to overcome the static droop of an analogue governing system, and to match engines in a twin engine setup. Static droop is the phenomenon whereby as the load is increased (raising the collective) the governor can't quite maintain the desired rotor rpm and it droops a bit. So for example at minimum collective pitch on the ground the rotor rpm would be more than when hovering at high pitch. The beep switches modify the governed datum a little, so that the pilot can set the rotor rpm to the optimum value at a given load (collective position). For twin engine aircraft, the beep switches can be used independantly to ensure that both engines are working equally hard and sharing the load.

Of course well designed helicopters, even of 1980s vintage, don't require beep switches!

Last edited by HeliComparator; 16th Apr 2013 at 11:21.
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