RADIO CONTROL AEROBATICS
Rick Allison 26405 SE 160th St., Issaquah WA 98027
Introduction
Last month I briefly touched on Pattern prop selection and promised more on propeller theory. Much of this material will be familiar to veterans, and much of it is basic — but the ground bears covering again. New readers need the fundamentals, and many experienced builders still have gaps in their propeller knowledge.
A propeller is a device that converts rotational power (torque) into thrust. A propeller blade is a rotating wing. It helps to think of a propeller as a system composed of several variable features chosen by the designer (and ultimately you, the customer): blade length (disc swept area), blade planform (shape), blade cross-section (airfoil), blade pitch (set-angle), and number of blades. To keep things simple, this discussion will consider two-blade systems.
Each feature brings advantages and disadvantages; together they determine the prop’s efficiency for its intended purpose. All aviation deals with compromise, and propellers are an outstanding example.
How a propeller works
A prop pulls air from in front and accelerates it rearward. The accelerated airmass produces thrust that reacts against the blades and propels the aircraft forward.
Key points:
- Prop diameter (disc area) determines how much air is available for the blades to work on each revolution.
- Pitch and rotational speed determine how much that airmass is accelerated as it moves through the disc.
- Because the airmass is accelerated, a rotating prop creates a low-pressure area in front of the blades that draws air into the disc, and a high-pressure area behind the blades. This flow is the slipstream.
- Acceleration of the air through the disc produces thrust; roughly half the acceleration effect occurs ahead of the prop and half behind it.
- Slipstream velocity relates to blade pitch and rpm.
Diameter and efficiency
To produce equivalent thrust, a small-diameter prop must turn at higher rpm and accelerate a smaller amount of air to a higher velocity than a larger-diameter prop. You have two choices:
- Move a large amount of air slowly, or
- Move a smaller amount of air quickly.
At the relatively low speeds of model aircraft, larger-diameter propellers are more aerodynamically efficient. For aerobatic models, the bottom line is to extract the greatest amount of thrust from a fixed amount of torque by increasing diameter and decreasing rpm rather than the reverse.
Pitch (blade angle) and load
Pitch is the theoretical distance (in inches per revolution) a prop would move in a slip-free medium. In practice, props always slip a bit — propeller slippage is analogous to losses in an automatic transmission. For our purposes, be aware it exists.
Pitch is also a slippery subject because measurement is nonstandardized and because most model props vary local pitch from root to tip in order to maintain an approximately constant angle of attack along the blade (helical-pitch or true-pitch concept). Manufacturers vary in how closely they adhere to that concept. Prop airfoil sections also vary from undercambered to flat-bottomed to symmetrical.
Pitch and diameter together determine engine load. Increasing pitch increases load because it increases the drag the prop must overcome. Props create induced drag as they create lift; that induced drag resists rotation (torque).
Blade lift, drag, and effective area
Not all lift produced by a prop blade is thrust. Blade lift acts perpendicular to the relative airflow across the blade — not perpendicular to the prop disc. Because blades are inclined to the direction of flight, a component of their lift acts to resist rotation (added drag). The outer one-third or so of a prop blade produces most of the useful thrust because:
- Rotational speed is greater at the tip,
- Blade angle is less extreme toward the tip (so more of the lift becomes thrust),
- Inclined lift-vector component and blade drag are lower at the tip.
Small reductions in diameter can significantly reduce thrust; trimming as little as 1/4 inch from a good prop can noticeably degrade performance.
Torsional stiffness and practical limits
Adding diameter increases load and introduces torsional stiffness concerns. Props that lack torsional rigidity can:
- Flex and depitch (go flat) under load,
- Flutter and buzz at the tips (increasing noise),
- In extreme cases, break in flight.
To achieve the same torsional rigidity, a longer prop must be thicker and heavier, especially near the hub. Increased mass raises inertia, degrading acceleration and increasing gyroscopic precession. Ground clearance is another limiting practical consideration. The solution is compromise: use enough diameter to move air, but not so much that structural, clearance, or handling problems arise.
Blade and tip shape
Props produce tip vortices where high-pressure air flows to the low-pressure side. These vortices represent drag and wasted power. The most efficient blade produces the smallest tip vortex. Because tip speed is highest, a blade that is progressively narrower and thinner toward the tip is best. Tip shapes:
- Rounded tips are more efficient than squared ones.
- Angled or raked tips can be a bit more efficient than round tips.
- Tip shape correlates closely with noise: less vortex production usually equals less noise.
- Lower tip speed reduces noise, but increasing diameter at a given rpm increases tip speed — another limiting factor for diameter.
High-aspect-ratio blades (long and narrow) are more efficient than low-aspect-ratio blades (short and broad). Increasing blade width increases drag faster than it increases thrust. Wide blades can increase aerodynamic braking on down lines and may be a bit stiffer due to added material.
Airfoil sections
Most commercially successful model prop airfoils are fairly ordinary. RC props operate over a wide speed range with considerable cross-flow, so exotic sections are generally of little practical use.
Pattern flying: practical recommendations
My preference for Pattern work is long, rigid, light, and relatively narrow props. If you must prioritize features, choose in this order: long, stiff, narrow, light.
Specific guidance:
- Use sufficient diameter to move air efficiently. A bare minimum of 15 inches for a .20 is recommended.
- A Y.S. .40 can turn 15.5–16 inches easily; modern large two-strokes handle up to 17 inches comfortably.
- Increase pitch until engine rpm is load-limited to at or just below the torque peak. This keeps tip speed reasonable and reduces noise. For most common Pattern engines, 11–13 pitch is appropriate.
- Rule of thumb: the narrower the blade, the higher the pitch you can use.
- Popular, good choices include APC props: 15 x 12 and 15 x 13N (for .120s), 15.5 x 13N and 16 x 13N (for .140s). The APC 17 x 12N is a good choice for some two-stroke .120/.140 engines.
- Avoid larger-diameter, lower-pitch, wide-blade setups like 16 x 8–10 or 17 x 10 for Pattern work — they tend to be noisier, less efficient, and heavier by several ounces.
- Avoid multiblade (three- or four-blade) props for general efficiency reasons. Additional blades operate in more disturbed air and are inherently less efficient, especially at model-scale rpms.
None of this replaces your own testing: each model/engine/pilot/environment combination is different. Use theory to start in the right direction, then experiment.
Summary — points to remember
- Use enough diameter to move air efficiently.
- Use enough pitch to load the engine properly (near the torque peak).
- Keep tip speed down to reduce noise.
- Prefer long, stiff, narrow, and light props (in that order of priority).
- Narrow blades allow higher usable pitch.
- Avoid unnecessarily large, wide, low-pitch props and avoid extra blades unless you have a specific reason.
- Test and experiment for your specific setup.
Prop-comparison tests are less common now, but equipped with this background you'll be better able to interpret them when you find one.
Transcribed from original scans by AI. Minor OCR errors may remain.
