Except for glider-only enthusiasts, all of us model flyers have to depend on propellers to provide the motive power to get -- and keep -- our model planes airborne. (Ducted fans are merely small-diameter, multi-bladed props.) Yet few of us truly understand propellers: exactly how they work, and why.
The Brits call propellers "airscrews". This name suggests that props thread themselves along through the atmosphere in the same way as a bolt threads into a tapped hole. But that concept is FALSE. A propeller's a rotating airfoil, not a screw. It pulls itself through the air exactly like a wing produces lift: by deflecting the air molecules it strikes. A prop pushes these behind it, and obtains forward propulsive force as a reaction.
True, the "pitch" of a propeller IS specified as if the prop was intended to screw into something solid. For example, a "9-6" prop has its blade surfaces tilted so they'd move forward six inches if the prop was rotated one revolution through, say, a block of clay.
(A propeller's "pitch angle" decreases from hub to tips, because the tips travel farther circumferentially. The lesser-moving inner portions of the blades have to be inclined more steeply, so they'd "advance forward" the same distance per revolution as the fast-travelling tips do.)
The real reason a propeller's blade surfaces are "twisted" is to provide the angle of attack any airfoil requires to generate "lift". However, there's a tremendous difference between the way a WING airfoil produces its lift and the way a prop develops thrust. In flight, an airplane's wing moves forward into an essentially motionless atmosphere.
BUT A PROPELLER PULLS THE AIR IT WORKS WITHIN TOWARDS ITSELF.
If a model airplane wing's angle of attack exceeds 9 or 10 degrees, it "stalls". Then the airplane stops flying and becomes a falling object. But a propeller can work just fine with its blades set at 45 degrees or more! Such a prop's steeply-angled surfaces definitely ARE "stalled", when they begin revolving. However, the airfoil of a wing doesn't stall because its lifting ability suddenly vanishes at high angles. What actually happens is that first its "drag" shoots way up. That excessive drag slows the aircraft greatly; lift drops in consequence; THEN the airplane quits flying.
When a "high-pitch" propeller begins spinning, its blade airfoil is also "stalled". But engine (or rubber) power keeps the prop rotating anyway, despite the excessive drag the blades are developing. The "lift" that the prop airfoil produces pushes its working atmosphere backwards (the "slipstream"); then to take its place more air gets drawn in from the front.
The blade's attack angle is lessened by this incoming airflow, because that's already moving in the desired direction when the blade strikes it. Thus drag goes DOWN and thrust goes UP as incoming and outgoing slipstream velocities both increase. Within a second or so, the propeller establishes its optimum "working environment". The incoming airflow automatically adjusts the angle at which it meets the blades, until maximum effectiveness occurs for that particular prop diameter, blade shape & pitch, and rpm. And this happens regardless of whether the propeller is moving forward or not...
(That's why, when flying a rubber-, CO2-, or electric-powered model, you should always allow the prop to attain its optimal airflow before releasing the airplane. The "torque surge" of a fully-wound rubber motor is more an illusion than a reality. That sudden left-turning effect comes mostly from the prop blades' excessive drag during the brief period between the beginning of rotation and attainment of "optimum airflow".)
The most important variables affecting the thrust a propeller can develop are its diameter and rpm. Thrust increases when either of those go up -- in proportion to the square of the rpm, and the FOURTH POWER of the diameter. If you should speed up an engine-driven prop from 10,000 to 14,142 rpm, its thrust output would double. And if you spin a ten-inch prop at the same rpm as a geometrically-similar 5-incher, the thrust developed will be SIXTEEN TIMES as great.
(I take advantage of these relationships in choosing props for my 1/2A R/C models. Most .049 flyers employ 6-inch props. I gain almost 40% more useful thrust with a 7-3 prop instead of a 6-4. The rpm goes down, true. But the extra diameter far more than makes up for that...)
Naturally, the power of the motor (or rubber) to turn its propeller enters strongly into all this. A big prop absorbs more horsepower than a small one; one with high pitch needs more power than a low-pitched type. Thrust doesn't come as a gift! However, the "motivating devices" we install in most of our model airplanes today can put out FAR more power than the air- craft need to fly with. This allows us a wide choice of usable propellers -- particularly with geared-down electric motors!
....Lately the noise engine-powered airplanes make has become a "public relations problem". Besides using mufflers of various types, many model flyers have experimented with changing propellers in attempts to diminish the overall sound output. Lowering prop tip velocity (with a smaller diameter at the same rpm, or a larger one spinning slower) is one useful approach. Another is reducing engine speed and thus the frequency and intensity of its exhaust noise.
These sound-reduction methods can be combined, by either of two opposite techniques. One method's to increase the propeller pitch and decrease diameter. But I use exactly the opposite approach for my own engine-powered models: it's far more efficient.
Here's why. The working efficiency of any reactive propulsion system (propeller, jet, or rocket) is found by comparing the velocity of the vehicle to the velocity of its "backflow" relative to the vehicle. If these were the same (an impossible condition, of course), the propulsion would be 100% efficient, and there'd be no slipstream behind the aircraft. The air it moved through would stay as motionless as the highway behind a moving car. However, the closer we can approach this ideal with a propeller-driven aircraft, the higher its efficiency. Less energy gets wasted in engine heat, air turbulence -- and in noise generation.
The thrust we need to keep our airplanes flying comes from the reaction that results from accelerating a mass of air rearward. If this mass is low, like that behind a small-diameter propeller, it's got to be accelerated quite a lot to provide appreciable thrust. This produces a fast-moving slipstream, and efficiency suffers. On the other hand, the air mass that a big prop acts upon needs much less acceleration to achieve its thrust output. (That's why a single-bladed propeller outperforms multi-bladers: for the same shaft power it can be bigger in diameter.)
Another thing I like about large-size, low-pitch props is that they minimize variation in their models' airspeeds. At takeoff and at low model velocity they pull hard -- just when high thrust is needed. But as airspeed builds up, their thrust diminishes in proportion. The faster the model flies, the smaller the effective attack angle of the prop blades becomes. Below about 3 degrees, thrust output drops sharply. If the blade angle of attack should go all the way down to zero degrees, as in a high-velocity dive, the prop may even act as an airbrake.
One more important factor in propeller performance is blade STIFFNESS. If a prop has flexible blades, they'll bend and twist under the forces generated by torque, flywheel action, and aerodynamic effects. There's little likelihood that such bending and twisting will improve efficiency! For one thing, when torsional forces on the blades of a flexible propeller cause pitch to increase, the effect becomes larger as it moves away from the hub. This is exactly what you DON'T want to happen. As you can easily see by examining one, the pitch angle of a propeller DECREASES from hub to tip. Reversing this relationship by allowing the prop to deform under power -- as with certain "scimitar blade" designs -- cannot improve efficiency.
True, SOME of the blade's area might possibly twist into a better angle of attack than it had. But the rest of the propeller would then be forced to operate at a WORSE angle. ALL of a prop's blade area produces drag that absorbs power from the "motivating source". For maximum efficiency, as much as possible of the blade area must develop the most thrust it can. That normally requires true helical pitch.
(I've performed experiments with identically-shaped model props made from materials with different flexibilities. In every test, the stiffer the blade, the higher the thrust output! Stiffness is the main reason today's reinforced plastic "gas engine" propellers work so much better than plain nylon ones, and why carved-balsa rubber-model props [covered with tissue for added stiffness] will outperform the flexible plastic types.)
Most engine-powered sport models and scale airplanes perform best with a prop diameter roughly twice the pitch -- e.g. 7-3, 9-5, and 12-6 -- with maximum engine speed in the 10,000 to 12,000 rpm range. This also applies to direct-drive electric motors. But for all other model power types: rubber, compressed air, CO2, and geared electrics -- high pitch, large blade area props provide better performance.
Here's a handy "rule of thumb" concerning propellers. The maximum speed a propeller-driven aircraft can attain is roughly equal to the prop pitch in inches, times its rpm in thousands.
Example: An 8-4 prop at 10,000 rpm can't pull its model faster than 40 mph. (4 inch pitch times 10K rpm = 40 mph maximum.) The airplane can fly a lot SLOWER -- particularly if it's a high- drag design. But even in a dive, it won't go faster than this "rule of thumb" limit.
