Choose the right prop for your engine

Model Airplane News, Jun 2001 by Lennon, Andy

MODEL AIRPLANE NEWS HOW TO

Understanding torque, brake horsepower and thrust

For best flight performance of sport or scale model airplanes, the propeller diameter and pitch should match the characteristics of both the airplane and its engine. The fine engine reviews by Mike Billinton, Dave Gierke and the late Peter Chinn published in Model Airplane News provide complete details on engine performance. The MDS engine in Figure 1 is an example; it appeared in "The Right Combination" (August 2000 issue). The table "MDS .46 rpm on standard propellers" is self-explanatory, but it provides only half the performance picture; the Bhp curve (upper) and the torque curve (lower) give the other half.

In my experience, few modelers understand the significance of these two curves, and even fewer use either in propeller selection. There is also some confusion about which curve to use-torque or Bhp. I hope what follows will explain both curves and which to use for prop selection.

* Torque. Torque is the elemental force that rotates the prop. In Figure 1, the MDS .46 RC engine's maximum torque is 75 oz.in. at 9,798rpm with a standard silencer, as measured on a dynamometer. The torque load imposed on the engine by various diameters, pitches and makes of propellers is marked on the torque curve. The 12x6 Graupner prop spins at 9,780rpm and demands the maximum torque. Larger props would overload the engine and cause a reduction in both torque and Bhp-as indicated by the steep downward slope of the torque curve to the left-and risk overheating the engine.

To the right, smaller prop sizes demand progressively less torque from the engine, permitting increased rpm. A 9x4 Zinger prop turns just under 18,000rpm; this corresponds to just over 60 oz.-in. of torque.

* Brake horsepower (Bhp). This calculated figure reflects force over time. Dave Gierke's Bhp formula is:

[Note: this is an approximation. The exact figure to divide by is actually 1,008,384; it has been rounded off for simplicity.]

As torque demand diminishes with the size of the prop, the engine can spin more rpm. As long as the rpm increase at a greater rate than the torque declines, the engine's Bhp (as calculated above) will continue to climb. At some point, this figure will peak-specifically, when the rate of torque decline begins to exceed the rate of rpm increase. The engine in Figure 1 peaks at 1.1 Bhp and 18,000rpm, which is the speed at which it spins the 9x4 Zinger prop.

Thrust. An engine and propeller generate thrust by blasting a column of air backward to propel the airplane forward. It is a logical conclusion that the thrust thus generated is proportional to the volume of air per minute being blasted back. The greater that volume, the greater the thrust and vice versa. The volume per minute is easily estimated by multiplying the area of the prop disc (in square inches) by the static rpm and again by the nominal pitch. Disc area can be calculated with the following formula:

This air-volume-per-minute figure is conservative. In-flight rpm are higher than static rpm, and on some propellers, the true (or geometric) pitch is higher than the nominal pitch (see Figures 2 and 3).

* Comparison of torque vs. Bhp. From the many engines reviewed in Model Airplane News, I selected four that ranged from .46 to 1.5ci. In addition to the MDS unit in Figure 1, I chose the SuperTigre G90 (Figure 4, from the December 1996 issue), the Webra Speed 1.20 (Figure 5, from the October 1994 issue) and the Irvine 1.50 (Figure 6, from the January 1996 issue).

For each engine, the air volume per minute was calculated in two ways; for prop diameter/pitch at or close to the rpm where peak torque occurs, and for prop diameter/pitch at or close to the rpm where peak Bhp occurs. The results are shown in Table 1.

Obviously, "propping" the engine near its peak torque range produces the greater volume of air per minute, and, therefore, the greater thrust.

* The model. In Table 1, the prop sizes listed opposite "torque" match the engine's performance characteristics, but more than likely, they will not match the model's performance characteristics (see "The Right Combination"). Models with lower wing loadings require props of larger diameter and lower pitch to match their lower flying speeds. Models with higher wing loadings fly faster and need smaller-diameter, higher-- pitch props. For best results, however, both props should load the engine into its peak torque range.

This is where Dave Gierke's "propeller load factor" (PLF) formula (July 1999 Model Airplane News) is very useful. Note that his formula should be limited to families of props from specific manufacturers.

Its usefulness is best described by examining, as a practical example, the SuperTigre G90 engine with the ST Quiet Muffler (Figure 5). The APC 14x14 prop most closely matches the peak torque because it spins at 7,060rpm. By placing a straightedge on the points corresponding to pitch and rpm on the rpm/speed/pitch nomograph (Figure 7), the level flight speed is shown to be 115mph. For a model that has a moderate wing loading of 20 ounces per square foot and a level-flight speed of 80mph at the same 7,060rpm, the nomograph indicates a prop pitch of 10 inches and, obviously, a larger diameter.