My all-time favorite aircraft to fly is a .40-powered semiscale P-51D Mustang. However, landing this model used to be another story entirely; it was too fast on final to make a three-point landing. Every good landing had to be a “wheels” landing with touchdown on the mains. Landed in this manner, the model often flipped over and struck the fin ignominiously on the pavement as it skidded to a stop.

            My trips out on the runway to retrieve my upside-down aircraft were a source of shame. I almost decided to go back to my high-wing trainer and forget the Mustang. Then, during one particular flight, the most amazing thing happened: the engine flamed out. I had to shoot a dead-stick landing. It was the best approach and touchdown I had ever flown with the model; I even three-pointed the landing.

            With the 10 x 6 propeller I was using and with the engine at idle, the model’s airspeed was too high for an easy approach and landing. To reduce the airspeed on final, I switched to a 10 x 5 propeller. What a difference! The P-51 was still a pleasure to fly, and landings no longer ended with a flip and skid on the fin. Sometimes, when not limited by the pilot’s skill, the landings were even graceful. Bring out the observers! I was ready to show them a thing or two.

Whether you are flying a hot warbird or a slow-flying Piper Cub, the propeller you select makes a great difference in how a model performs. With the right propeller for the model’s mission, each flight is a delight.

            “So, what is the model’s mission?” you may ask. I judge the model’s mission to be adequate performance in each of three phases of flight: takeoff and maneuvering, cruise level flight, and landing.

            The takeoff-and-maneuvering phase tends to require a larger-diameter but shallower-pitch propeller for maximum thrust. Wing lift increases in step with the airspeed squared. To generate sufficient lift to maintain level flight with an aircraft having a high wing loading, we must fly at a higher cruise airspeed. So for cruise we may need a more steeply pitched propeller to get the higher speed.

            If we increase the propeller pitch, we may also have to decrease the propeller diameter to maintain the engine rpm in the best operating range.

            If we don’t have flaps on the aircraft, we may be back to needing a lower-pitched propeller for the landing phase.

            Since most of us don’t have a variable-pitch propeller on our models, we must select a propeller that best matches at least the minimum requirement for each phase of flight. Therefore, selecting the propeller to match the mission requirements will be a compromise. To do this job properly, we need guidelines for making informed judgements.

            I’ll show you three tools you can use to make objective judgements on adjustments to match the propeller to your model aircraft’s mission: graphs for calculating stall and minimum cruise speeds, graphs for calculating pitch speed, and equations and a graph for calculating the static thrust produced by a propeller.

            To use these tools, you will need a tachometer. You will also need a way to calculate or measure static thrust. I’ll show you how to do that. I’ll make the judgements based on two rpm measurements: one at full throttle and one at idle.

            Does that sound simple? It is. I wish I had these tools when I was trying to solve the problem with the P-51. I would have been much more confident in the outcome of the propeller change and its effect on the Mustang’s flying performance.

Stall and Cruise Airspeeds for Models: An estimated stall speed can be calculated using the equation in the sidebar. The calculation is even easier if one only has to look up a number on a graph, so in Figure 1 I’ve plotted graphs of model stall speed as a function of wing loading for four elevations: sea level, 2,000 feet, 4,000 feet, and 6,000 feet.