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204 Canadian Professional Pilot Studies The mixture control has a secondary function, which is to cut fuel from the engine on the ground when you want to stop it (you don't just switch the magnetos off). The Idle Cut Off (ICO) in the carburettor is joined to the mixture lever with a Bowden cable. When the lever is operated at the end of a flight, the engine is starved of fuel, and stops. Carburettor Icing This is actually one aspect of induction system icing. The other two are fuel icing, arising from water suspended in fuel, and impact ice, which builds up on the airframe around the various intakes that serve the engine. Even on a warm day, if it's humid, carburettor icing is a danger, especially with small throttle openings where there's less area for the ice to block off in the first place (as when descending, etc.). Also, the temperature drop (between the OAT and that in the venturi) can be anywhere between 20- 30°C, so icing (in an R22, anyway) can happen even when the OAT is as high as 21°C (70°F), or more. Tests have produced icing at descent power at temperatures above 30°C, with a relative humidity below 30%, in clear air. Because it is more volatile, and likely to contain more water, you can expect more fuel and carb icing with MOGAS than AVGAS. 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 for every application— that is, carb heat 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 carb 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 Airframes, Engines & Systems 205 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. With a fixed pitch propeller, the first indications will be a slight loss in RPM, followed by rough running, then more loss of RPM until the engine stops. With a constant speed propeller (and in a helicopter) keep an eye on the manifold pressure gauge and the EGT gauge, if you have one, which should show a decrease. Fuel Injection Most of the above problems with the carburettor are avoided with fuel www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：Canadian Professional Pilot Studies2
