by Joe Wagner
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
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
(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
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
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.
The above information is provided as a guide. Since WE have no way of determining
the ability of the individual using and understanding this information, we assume absolutely NO RESPONSIBILITY for any damage
to person or property from the use of this information.
BY: Model Engine Corporation of America / MECOA/K&B