Engine’s Performance

Please note that the propeller’s power is coming from torque not engine’s RPM, but these are related to each other. As it is true with the cars, since everyone knows when a car is "becoming" powerful around 3000 RPM and above, and it is quite weak at 1000 RPM and below. So, it is good if you know your engine's torque-diagram or the optimal RPM range at least. High torque at low RPM requires bigger engine size, while the small engines are rotating faster to produce enough power. In parallel with the engine power the perfect load is very important! The load on the aircraft engine is the optimally sized propeller. If the engine is overloaded with a bigger propeller then it cannot rotate as fast as it needs, or when the engine is not loaded properly with a too small prop then it is over-rotating and will produce heat and noise instead of thrust. Regarding these facts the optimal propeller configuration is very important. This calculator is using HP (HorsePower) to describe engine's performance. If you don't know your engine's power, you have to make the following experiment: Fit a suitable prop (sized for that engine - read engine's manual), have it run on full throttle and measure the RPM with an RPM meter. Then use the Static Thrust Calculator to enter the data about the propeller you used with the measured RPM and then you will get your engine's performance in HP. Please note that these calculations are very complex, so the results may not 100% accurate but they are giving a very good base to start with.

Note! When editing the horsepower field, take care about the computer's regional settings: all Windows versions should use decimal point (not comma) between integer and fraction!

Here is a little table showing some average engine's performances and RPM limits. The Minimum RPM means the torque is good from that point. The Maximum RPM limit means there is no more torque from that point (just more noise and possible engine damage)

Please note that these are examples only and there are many different engines!

Calculated Data

If you enter reliable (true) data the calculator will show one or more possible propeller configurations which are good for the engine. It is based on a table which contains most of the RC propeller sizes. Please note that calculator is using standard shape, two-blade propeller configurations at a normal sea-level pressure and normal room temperature. (Note that the calculator's recommendations are working within the Planet Earth's Air only... :-) The used table is only designed for RC props, please keep this information in mind: The smallest prop in it is: 4.1 x 4.1 and the biggest is: 34 x 12. If you check the results, you will see that the biggest possible prop is listed on the top, and the list goes down till the smallest possible prop is found within the limits you provided. The first choice is often the best one, which gives the more static thrust and enough fly speed by maintaining the optimal engine's RPM. Remember: the engine will over-rotate if it is under-loaded and will under-rotate when it is over-loaded!

If you examine the results you may notice some rules between the required power, the RPM and the propeller's pitch. If you do so, then you will understand why most of the real airplanes have variable pitch propeller, often called: constant RPM propeller. If the plane goes faster and faster, the engine still produces same RPM and power, therefore the propeller should give higher pitch to be able to accelerate the aircraft. The higher pitch the higher speed, but it takes the power of course. The propeller blades are wings, therefore they have optimal "flying speed" as well. The optimal perimeter speeds are calculated as follows: The lover perimeter speed limit is 100 m/s. Under this value the propeller rotates too slow (in comparison with it's optimal rotation speed). The upper perimeter speed limit is 300 m/s. Over this value the propeller is over-rotated and the blades are closing to break the sound barrier. This is dangerous and should be avoided at all time! Using these rules, and using the limits with the engine performance you provided, the calculator will display some good configurations.

In order to get the best result you have to keep the lower and higher RPM limits as narrow as you can, since most of the torque-diagrams are showing similar narrow curves. If you do so, usually the first three results will be most effective for your engine. Mostly the first choice will give the more static-thrust with the relative slowest flying speed. This is the best configuration if the maximum power is the goal, but it may reduce the throttle-reaction speed, especially with bigger propeller sizes when the propeller's weight and inertia also counts. The second and third results usually give less static-thrust but they give more speed and increased throttle-reaction time. The other choices (if there is any) below may look crazy at a first sight, as this calculator is not fool-proof and it cannot count on every small details which are very important in the real world. If you are skeptic just make sure you have entered real (life-like) data and then try the first propeller recommendation with your model aircraft. Measure it's RPM and thrust. You will see, it is very close to the optimal! These calculations were working for us, so we do believe that they will be good for others as well. If you are a "complaining-kind-of-guy" for whom nothing is good enough, try to think about the huge job we did with this calculator absolutely free... But if you think the calculator is helpful and your RC model is now better than before, we are more than happy!