Matching Propeller to Model Mission

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.

Measuring Prop Thrust

As a beginning Electric flier, I became very interested in both propeller thrust, and the propeller equations suggested for predicting performance. My modeling is done in Idaho Falls, Idaho, at an elevation of more than 4,700 feet-predicting thrust of Electric flight systems is important. I wanted accurate measurements of actual thrust with a particular system, and I wanted to know what thrust would be available if I changed the propeller or rpm. As a first step, I designed and built a portable thrust-test stand. With a portable test stand, I'm able to measure propeller thrust anywhere. (This comes in handy, as you will see.) The second step was testing the suggested thrust equations. To explain my own propeller thrust measurements, I tried applying equations that I found in modeling literature. None verified. Some of the suggested equations predicted results that were almost 30% different from the static thrust I measured with my equipment.

Prop Pitch Gauge: A Useful Tool?

What is your prop's pitch-really? Who needs a pitch gauge? Indeed, what is a pitch gauge? Would I use a pitch gauge if I were an umpire in a game of slow-pitch softball? Or is this gauge something that a music teacher uses to tell if your song is on pitch? The pitch gauge of interest to aeromodelers is not a tool for softball or music, but rather a tool used to measure the pitch of a propeller. Propeller pitch is the forward travel (in inches) of a propeller during one revolution, if the propeller acted like a section of a bolt being screwed into a nut (i.e. without any slippage). With uniform pitch, the pitch should be the same at all locations along the propeller blade.

Optimizing Prop Performance

I thought I had reached the zenith of performance with my souped-up Goldberg Electra sailplane. I had designed a new T-tail for the airplane, which kept me from punching holes in the horizontal stabilizer and breaking the fuselage all the time when I landed in tall grass. I had improved the thrust output of the kit stock motor 70 percent by converting the model from direct-drive with an eight-inch prop to a gear drive with a 12-inch folding propeller. The folding prop I had installed was readily available from the local hobby store-a Master Airscrew model. This propeller was durable, relatively inexpensive, and greatly increased the thrust available with the stock motor and a seven-cell battery (from 13 ounces on direct drive to 22 ounces with the AstroFlight 035 gear box).

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