Home Engineering Small Unmanned Fixed-Wing Aircraft Design. A Practical Approach
Range is given by the Breguet equation (for piston engined aircraft): Range = (3600/9.81) X (ц/SFC) X (Lift/Drag) X ln(MTOW/(MTOW - Mfuel)) where SFC is in kg/kWh and the resulting range is in km. ц is the propulsive efficiency at the cruise speed, typically around 0.6 but clearly varies with propeller choice and also speed. Note that here Mfuel does not include the fuel reserve.
Structural Loading Calculations
To size spars and booms, and hence estimate their weights, some form of maximum g to design against will be needed. This will come from gust or maneuver loads or some arbitrary choice such as 4g. Then simple beam theory can be used alongside design decisions on the likely form and materials to be selected for these items (typical materials are carbon-fiber tubes, solid or box-section plywood beams, or aluminum rods or tubes). Note that the FAA regulations (FARS 14, CFR, part 25) state that the maximum maneuver load factor is normally to be 2.5 but if the airplane weighs less than 50 000 lbs, the load factor is to be given by n = 2.1 + 24 000/(W + 10 000), though n need not be greater than 3.8; while UK JAR 25 says: “the limit load maneuvering load factor n for any speed up to Vn may not be less than 2.1 + 24 000/(W + 10 000) except that n may not be less than 2.5 and need not be greater than 3.8, where W is the design MTOW.” Here the unit of weight is lbs. JAR-VLA (the equivalent standard for very light aircraft) simply says the limits should be between 1.5 and 3.8.
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