Inflow

If you put your hand in front of a spinning propeller you will feel a draught although not as strong a draught as it is in back of the propeller. The approximate rule is that about one half of the velocity is gained in front of the propeller. This velocity, called inflow, requires the pitch of the propeller to be increased to stay at the correct angle of attack. As a general rule for control line model airplanes at maximum level flight speed, the inflow velocity can be estimated as a percentage of the forward speed of the aircraft. Under these conditions the inflow can range from 1% to perhaps as high as 6% of aircraft velocity. At very low speeds the inflow velocity tends to a constant value proportional to the propeller thrust.

The estimates here are based on a simple, idealized model of inflow. The more exact procedures are used in industrial practice, however for the lightly loaded propellers considered here the approximation seems adequate. There is some "swirl" to inflow. It has been considered in the estimates discussed here and it increases the effect of inflow by values on the order of 10% for conditions considered here.

Large inflow velocities are associated with high propeller "disk loading". This is the thrust of the propeller divided by the product of propeller area and dynamic pressure. A high disk loading is like "blowing holes in the air". It results in low efficiency even if the propeller blades have zero drag. This is like induced drag on a wing. From this standpoint the largest propeller diameter is best. Of course with the propeller fixed to the engine shaft as it is on the great majority of model airplane engines, the diameter is limited. This, however, is one reason all large propeller driven aircraft use gearing to reduce the shaft speed. Another reason diameter is limited is to avoid high Mach number. Both of these features are prominent in helicopters, rotor geared down a great deal and diameter just at the limits imposed by compressability.

For an example, the F2C (FAI Team Race) model described in Göran Olsson's Excel spread sheet has a thrust loading of 0.167. The efficiency of the propeller just due to the effect of inflow is 0.957. The inflow increases the velocity at the prop disk by 2.5%. To compensate for inflow the blade pitch must be increased approximately 2.5%. This is one reason the "propeller slip" used so often in model aircraft publications has to be used with caution. In each case some of the "slip" computed in this way is really just do to the inflow.

F2A, (FAI control line speed) has a significantly higher thrust loading than F2C, The drag of the aircraft and lines is similar to an F2C model, the propeller has the same or smaller diameter but the thrust is 2.0 to 2.5 times higher. Because of this the induced efficiency will be 0.9 or less.

In conclusion, propeller efficiency is decreased by the effects of inflow. Blade angle must be somewhat larger to account for the effect of inflow, but generally in the order of a few percent. Mach number, however, can play a much large role in fixing the correct blade angle. the next section will discuss efficiency in a general way and then Mach number will be treated.