Page 10 - Demo

9This explanation developed from discussing propeller sizing with several long term IC flyers where it became obvious that the principles Brian PerriamComparingI/C and Electric PropellerSelectionof propeller selection are very different for IC(internal Combustion) to E (Electric) model engines.Over time I have converted 5 models from IC to E. Initially I would try and match the E motor to the existing IC prop but as I have gained experience I have realized that greater efficiency and hence battery life can be achieved by making sure you have the finest propeller pitch that matches the model speed and minimum diameter to achieve a realistic climb. On my Taube I started with a 11/6 propeller and ended up with a 9/4 which has given me far better flying performance and 4min more flying time. The different principles became very obvious when I changed my Vintage duration 1/2a model from a .049 ICto electric. The Cox 049 had a large, course pitch prop to slow RPM and extend fuel life as long as possible. The Electric needed a much smaller propeller and finer pitch to minimize engine load and extend the limited battery power, as long as possible. To help understand why this is, one needs to look at the operating principles of both systems and consider how much of the energy source (petrol volume or battery capacity) is available to the propeller at any given moment. For IC this increases as RPM increases but for E, most of the energy is available at all RPM%u2019s. An example of this was a model with poor UC position and tendency to nose over. On take off the motor drew approx 20A but a prop strike could blow the 30A fuse instantly. Even taxying strikes would blow the fuse.Note, The following is a very simplified overview and not intended to explain all details etc. Values quoted are for example only and not calculated.ServoServoServoTorqueTorqueTorqueRPM 5,000RPM 10,000RPM 8,500AirAirAirButterflyGapButterflyGapButterflyGap Med Butterfly Gap * Med Fuel/Airflow * Med Bang ** Med RPM Med Torque * Full Butterfly Gap * Large Fuel/Airflow * Big Bang ** High RPM High Torque * Full Butterfly Gap * Large Fuel/Airflow * Big Bang ** Reduced RPM High Torque * More Heat Generated * in EngineAn I/C engine at Med. throttle setting has increased fuel flow allowing the engine to produce more power which should increase propeller speed before the power and propeller drag are again equal.An I/C engine at high throttle setting has further increased the fuel flow allowing the engine to produce more power, which should further increase propeller speed before the power and propeller drag are again equal.An I/C engine at high throttle setting with increased propeller size will increase the propeller drag which will reduce the propeller speed until the drag again equals the power available from the engine, which is relative to the fuel flow setting. If overloaded to much the engine speed will drop too much and it will overheat and fail.FuelFuelFuelServoTorqueRPM 2,000AirButterflyGap Small Butterfly Gap * Small Fuel/Airflow * Small Bang ** Low RPM Low Torque *An I/C engine at low throttle setting has restricted fuel flow which limits the (energy) power the engine can produce. This power is absorbed by the propeller which will only reach a low speed before the available power equals the drag forces on the propeller.FuelSummary.In an IC engine. Opening the throttle position introduces more fuel to the engine which allows it to produce more power and increase the propeller speed / thrust. Bigger propeller%u2019s will have more drag, but as only a set amount of power is available, the propeller will slow until the drag reduces sufficiently to balance the available power.