Propeller Thoughts

The following came from reading Chapter 14 of Model Aircraft Aerodynamics by Martin Simons

The purpose of the propeller is to convert engine power into thrust used to drive the speed plane.  In general, speed planes use small diameter high pitch propellers.  These are thought to be low thrust as compared to large diameter low pitch props.  In reality, speed props develop as much if not more thrust as an aerobatic plane.  Before going further, lets look at what is happening with the prop.

As the airplane moves, the air in front of the prop disk area (called the inflow) is drawn into the propeller and is accelerated so that the speed of the air after the prop (called the outflow) is greater than that in front.  This is intuitive as we have all felt the prop wash on our models.  The acceleration of the inflow causes a low pressure in front of the prop disk and the out flow causes a high pressure behind the disk.  The difference in pressure between the inflow and the outflow is the thrust generated by the prop.  The greater the volume of air that the prop moves, the greater the cumulative pressure difference.  Thus an increase in prop diameter increases the prop disk area and correspondingly increases the volume of air.

Props, like wings, generate greater pressure differential when the angle of attack in increased.  In props, this is the pitch.  Like a wing, too much pitch can stall the prop.  A curious thing happens though when the model plane starts to fly.  The inflow begins to pick up speed as the model picks up speed.  The prop gets more efficient, that is, the drag on the prop is reduced as the plane picks up speed.  We know this as the unloading of the prop.  At some point as the inflow air speed increases, the low pressure in front of the prop turns the inflow to the direction of the prop rotation.  The result of this is to decrease the effective angle of attack (pitch) of the prop.  If the initial pitch of the prop is too low, the increased air speed can reduce the pitch to zero or to a negative value where the prop becomes a brake.  To compensate, speed planes run very high pitches.

The difficulty with high pitch props is that as the pitch increases, so does the drag.  As prop diameter increases, so does the drag.  Consequently, for a motor to run an increased pitch prop at the same rpm, the diameter must be reduced or the power output of the motor increased.

All the above tell us is that low pitch props provide good acceleration until the airspeed causes the effective angle of attack to be reduced.  High pitch props provide poor acceleration because the high pitch prop is stalled until the air flow gains enough speed to reduce the pitch to an operating level.

So how to use this information on a speed plane?  If the event allows the flier several laps before the timed run begins, use a high pitch prop.  The first few laps as the airplane takes off can be used to build up the airspeed and get the prop working before timing commences.

If the event is timed from the first lap, a compromise is required between the acceleration of a low pitch prop and the higher speed of a high pitch prop.

Another important point is that the props modelers generally use are constant pitch props.  The combination of pitch, diameter, and blade shape make a prop efficient at a specific air speed.  If the model or engine is modified so that the air speed will be higher, a new prop must be found.
 
 

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