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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(lán)天飛行翻譯 作者：航空 點擊：次

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It usually arises from the action of the venturi in the throat, just before the butterfly valve, which regulates the amount of fuel into the engine. You will remember the venturi's purpose is to accelerate airflow by restricting the size of the passageway, which has the effect of reducing the pressure and pulling the fuel in. Unfortunately, this process also reduces the temperature, as does the fuel vapourisation, hence the problem (the lower temperature means greater relative humidity, and closeness to the dewpoint, and the vapourisation takes its latent heat from the surroundings, making the situation worse). In fact, the vapourisation (and cooling) can carry on most of the way to the cylinders, causing the problem to persist, especially with the butterfly semi-closed, which produces another restriction and more of the same. Any water vapour under those conditions will sublimate, or turn directly to ice. Note also that warm air will produce more ice because it can hold more moisture.
With smaller engines, use full settings—that is, either on or off, with no in-betweens - the greatest risk is at reduced power. Out of Ground Effect hover performance charts for helicopters usually assume the carb air is cold (the R22 requires heat below 18" MP). In fact, when heat is applied, an engine will typically lose around 9% of its rated power.

Rough running may increase as melted ice goes through the engine. Also, be careful you don't get an overboost or too much RPM when you reselect cold. Of course, aeroplanes have some advantage if the engine stops from carb icing, as the propeller keeps the engine turning, giving you a chance to do something about it.
In a helicopter, due to the freewheel that allows autorotation, the practice of only selecting hot air when you actually get carb ice may not be such a good idea – usually, a gauge is used with a yellow arc on it, showing the danger range.
Use carb heat as necessary to keep out if it. The other peculiarity with regard to helicopters is that they tend to use power as required on takeoff, whereas aeroplanes use full throttle. This makes them more vulnerable, as the butterfly opening is smaller, and is particularly apparent on the first takeoff of the day, when the engine and induction system are still cold. If it is filtered, your carb heat may be used to preheat the induction system during the engine warm-up.

Oil Cans
Whatever you use to open them, don't bang it down on the lid, but gently prise it open, so you don't get slivers of metal in the oil which may disagree with your engine.

Schermuly Flares
Flares should be stored for a maximum of 4 yrs between 0-25o C, and only be installed for 1 year or 1000 flying hours. Check that the centre contact in the mounting moves freely and is clean, and the locking and securing ring threads for damage. When not carrying flares, the plastic cover plate should be fitted and the securing ring locked with the locking ring.
Test the firing circuit with a suitable 50W 24V bulb and holder in a cut-down fired flare case. Before installation of any flares a "no volts" check of the firing circuit (using a millivoltmeter) should be made.

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