| PROPELLERS |
| Controlling the RPM Selection of the correct propeller is important for two reasons. Firstly, the faster the engine runs, the more exhaust, engine and airframe noise is produced. Secondly, high propeller speeds produce more noise. Although it is difficult to lay down strict criteria for calculating ideal propeller sizes, as a rule of thumb, the following target RPMs need to be approached if noise emissions are not to exceed 82dBA at 7 metres: 0.40 cu.in 2 strokes: 10000-11000 RPM 0.60 cu.in 2 strokes: 9000-10000 RPM 0.90 cu.in 2 strokes: 7000-8000 RPM Some commentators have suggested that the propeller tip speed should be kept below 350 mph to achieve the same effect. It is true that if the propeller tip speed becomes too high, the tips will emit a very significant amount of noise. This is not usually a problem on model aircraft unless large propellers running at very high RPMs are used. Tip speeds approaching the speed of sound are required for this effect to become significant, and a 12 inch propeller will not approach this speed even when running at 15000 RPM. Pitch vs Diameter Whichever rule is followed, the correct way to reduce RPM is to run coarse pitch propellers rather than large diameter propellers. This prevents RPM from rising dramatically in a dive or at high speed (this would negate any benefit of reducing RPM on the ground, leading to increased noise output). Incidentally, a coarse pitch propeller will often result in higher achievable airspeeds - think of it as an aerodynamic gearbox; coarse pitch equals a higher gear ratio. Some useful prop sizes are given below: 0.40 cu.in. 2 strokes: 11x6, 11x7, 11x8, 10x9, 10x10 0.50 cu.in. 2 strokes: 11x9, 11x10 0.60 cu.in. 2 strokes: 11x11, 12x10, 12x11 0.90 cu.in. 2 strokes: 14x11, 15x10 Similar principles can be applied to 4 stroke engines. Multibladed propellers Multibladed propellers have a place, especially for very large, fast running engines. Where unacceptably high tip speeds would occur with the diameter of a two bladed prop necessary to absorb the power, an extra blade can be used to absorb the power at a more modest tip speed. For most engines however, a multibladed propeller offers no advantages over a coarse pitch two bladed prop. Torque The other point to note is that the torque peak for most model aircraft engines lies quite near the above recommended RPM ranges. This means that a comparatively large propeller can be turned, and benefits result by virtue of moving a larger mass of air over the wings and control surfaces. Thus although the power output of the engine at these lower RPMs is lower than the BHP peak, more of the available power is converted into thrust. |
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| Propellers and Thrust: - Link to some of our results - Link to work by Roger Bellingham |
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