The first practical single-blade propeller, also known as the Everel Propeller, begins in Lancaster, Pennsylvania, in the mid-1930s. While most engineers of the time were debating and comparing the performance advantages of two-blade, three-blade and four-blade props against each other, Baltimore-based inventor Walter Everts calculated that the single-blade propeller not only had less drag than its two-blade cousin, but that it moved more cleanly through the slipstream.
Of course this is basically hogwash, as the propeller blades on a moving aircraft, no matter how many blades, are always cutting through ‘clean’ air as the aircraft moves forward.
Everts took his eccentric innovation one step further, creating not just a counterweighted single-blade propeller, but one with a blade that freely pivots on an ingenious hub, allowing it to automatically change pitch in flight.
As the center of pressure on the blade changes, the propeller’s pitch changes with it. At full power on takeoff, the blade pivots to fine pitch. At reduced power in cruise, it pivots to coarse pitch. In other words, the single blade moves fore or aft on the hub, and swivels, allowing the prop to self-adjust to the most efficient angle.
During pre-flight, you can test this by moving the tip of the blade forward and back several inches with your hand. Not what you would want on a standard propeller of today.
Everts and partner Frank Ellington combined their names to create the ‘Everel Propeller Corporation’. They hired the Sensenich brothers in nearby Lititz, Pennsylvania, to carve the blade, and got Jesse Jones, a well-known local aviator and former barnstormer to flight test and market the propeller.
In 1939 the Everel prop was tested on a Taylorcraft in a race and actually won.
However, shortly after the introduction of the prop, more powerful engines were developed which rendered the efficiency gains of the single blade moot. Additionally, considering that the balance of the prop was very fickle in changing weather, the already mechanical complex prop just wasn’t worth the effort, so the design never caught on.
However the single-blade propeller idea didn’t die.
Designers like Bolkow took the idea and used it in helicopters. No kidding.
They thought along the same lines as Everts, believing that the blade passes through ‘clean’ air and is thus more efficient compared to the multi-blade configurations.
Some self- launching gliders like the Alisport Silent 2 Targa fold their propellers inside their fuselage using a single-blade propeller.
Today a few other manufacturers use single-blade designs to launch their gliders.
Some RC models also use this technology, creating unique designs.
ADVANTAGES/DISADVANTAGES OF SINGLE-BLADE PROPELLERS
- A major advantage is that the single-blade propeller is compact.
- The counterweight for reducing vibrations is heavy. This may be acceptable for small and lightweight propellers used in gliders and models. A major problem in helicopter blades and larger aircraft blades using larger horsepower engines creates significantly heavier counterweights.
- The lever arm is huge and this increases drag, which defeats the objective.
- The blade may be balanced at the pitch axis to reduce vibrations, but induced torque will cause significant vibrations as torque increases.
- The blade has aerodynamic forces acting on it, while the balance weight does not. This causes imbalance and vibration.
- If the weight is fixed, the rotor will tend to rotate the engine and could rip it off. The only way to prevent this is to balance the blade, both aerodynamically and centrifugally, which is easier said than done.
- The drag of the blade is different from that of the mass arm, with torque being created parallel to the axis of rotation.
- In helicopters, the rotor blades usually ‘cone’ in flight; this means that the lift vector is slightly tilted towards the center, not concentric with the rotor plane.
In the end, a propeller with more blades, or a bigger propeller has a better efficiency because it can use a larger mass of air for thrust and lift creation.
The number of blades determine the solidity ratio of a propeller. If you need to limit tip speed and propeller diameter, the propeller will have a high solidity ratio by using many, short blades.
If the propeller has a low advance ratio, the wake of one blade might interfere with the next, so packing more blades will increase the risk of interference.
Efficiency generally runs inverse to the number of blades.