Small hobby engines with masses less than 1 kg are attractive for use in low-cost unmanned air vehicles, because they are mass-produced and inexpensive. However, very little information about their performance is available in the scientific literature. This paper describes the development of a dynamometer system suitable for measuring the power output and efficiency of these small engines and presents detailed performance measurements for a particular engine with a mass of 150 gm that could be suitable for powering a low-cost unmanned air vehicle. When the mixture setting is adjusted according to the manufacturer's instructions, the peak power of this engine is 112 W at 9450 rpm with a brake specific fuel consumption of 3:0 kg=kWh. The performance can be improved to 159 W at 12,000 rpm and brake specific fuel consumption of approximately 2:1 kg=kWh by controlling the mixture.Nomenclature F = force f a , f m = atmospheric factor and engine factor I = moment of inertia k lc = stiffness of the load cell L=D = lift/drag ratio of the vehicle m = mass of the additional weight added to the engine cradle _ m = mass flow rate N nat = engine speed associated with the natural frequency of the cradle-load cell system P = measured output power P r = corrected power under standard reference conditions p, p r = test pressure and standard reference pressure p sr , p s = test saturated water vapor pressure and standard reference saturated water vapor pressure Q r = heating value q = fuel mass per cycle per liter of air q c = corrected specific fuel delivery R = length of the moment arm r = distance between added mass and cradle axis of rotation r r = boost pressure ratio T r , T = test ambient air temperature and standard reference ambient air temperature c = atmospheric correction factor = overall damping coefficient for cradle bearings = measured engine torque max = maximum deflection of the load cell = efficiency e = angle of cradle associated with steady state operation = density of air, mixture, methanol, nitromethane, and castor oil r , = test relative humidity and standard reference relative humidity = volume fraction of methanol, nitromethane, and castor oil in the fuel f = fuel mass fraction at takeoff ! = engine speed ! n = natural frequency of the system Subscripts a = air CH 3 OH = methanol CH 3 NO 2 = nitromethane c = cradle e = engine f = fuel lc = load cell o = overall oil = castor oil p = propulsive th = thermal
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