The right propeller for an airplane is as vital as its engine. Use of extreme propeller size and pitch usually has unwanted results. A prop is defined by two factors: First its diameter and second its pitch.
Diameter is the circle that is being defined by its tips, and as pitch is considered the distance to which it will advance itself if it completes a full revolution in ideal conditions, not counting the distance reduce which is caused in the air by opposite forces.
A propeller performance depends also on the following factors:
(If made of plastic, then pitch reduce can be observed in high rpms)
Due to its factors a propeller do matches to an airplane/engine or not. There is a common belief that a propeller only goes with the type and the capacity of the engine due to engines manufacturers’ right advice for specific propeller sizes. But the same prop is not suitable for the same engine on two different models. For a trainer model a longer diameter and relatively smaller pitch prop, gives better results.
On the other side on a pylon model a small pitch will have bad performance even with the same (theoretically) engine with the trainer.
As a modeling principle the pitch of a propeller goes with speed and diameter with torque. The produced power by the engine along with its torque, determine the spectrum of the propeller type that we will use and the type, the weight and the design of the model determines the specific propeller. It is not possible to specify using mathematics which propeller suits precisely a model. Experimentation is the indicated way to obtain the best performance.
Basic principles of use
As for every component of our model certain rules govern the use of the propeller. A stroke while the propeller is spinning can be proved fatal for your personal integrity. It’s worth mention that the tips of a 10 inch prop attached to a humble .40 engine, spins with a speed more than 450 Km/hour. Along with the above rule, avoid standing in virtual disk of a spinning propeller.
Always balance the props. Using a special scale so you can verify or adjust the balance of your props. If you have to reduce weight don’t lessen its diameter smoothing with sandpaper its tips. An out of balance propeller except keen to break up, is a serious vibrations generator. If it is made of wood, apply a coat of clear varnish.
If it is made of plastic, immerse it in boiling water.
In both cases you achieve to provide it with moist so that:
1. It will not break up in use.
2. It will not lose its balance due to moisture that will absorb from the air.
If one of its tips it’s partially damaged, then smooth it with sandpaper. Do the same to the other tip and keep in mind that the propeller has changed (even slightly) its diameter.
In the following table the indicative size of a propeller must increase by one unit in diameter or pitch in altitudes more than 1500 meters. Theoretically one unit larger in diameter equals to larger pitch by one unit. That said, a 10×6 propeller will produce the same results with a 11×5. If the engine shows symptoms of loosing torque, then pitch decrease and diameter increase is advised.
It’s emphasized that there are only suggested sizes (diameter & pitch). It depends on the user to try another propeller size, if the engine seems to be overloaded or the model doesn’t achieve the requisite flight speed.
|TOTAL AIRPLANE΄S WEIGHT|
45 – 90 cm
0.15 up to 0.20
0.20 up to 0.30
8×4, 8×5, 8×6
9×4, 9×5, 9×6
90 – 150 cm
1350 -1500 gr
0.20 up to 0.35
0.35 up to 0.45
9×4, 9×5, 9×6
10×4, 10×5, 10×6
11×4, 11×5, 11×6
150 – 190 cm
1500 – 2800 gr
0.45 up to 0.61
10×6, 10×7, 10×6
11×5, 11×6, 11×7
12×4, 12×5, 12×6