Living with partial blade stall of rotors and propellers does not only reduce efficiency of generating thrust. In the case of tiltrotors it can ruin aircraft stability.

Two GIFs below show schematic interpretation of the formation of blade stall:

Scenario No.1.

Fixed pitch propeller. Changing axial speed of a working propeller causes a serial transformation of the triangle of basic speed vectors of the airflow around blades. As a consequence, at different points of the blade length AOA will deviate from design value. Growing AOA errors appear and zones of stall (partly those of windmilling) along propeller radius get created.

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Legend for the images: Optimal AOA is considered relatively small with a value of around 4 degrees. Only significant deviations, i.e. “AOA errors” are shown in the charts.

Scenario N.2.

Variable pitch propeller. At 75% radius blades are always aligned with the resulting wind. This technology allows great reduction of both AOA errors and stall zones, but still they can’t be eliminated fully.

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Legend for the images: Slightly twisted surface in violet, fitted with an arrow, indicates vector field of the resulting wind.

Notice the twisting vector field of the resulting wind (surface in violet, fitted with an arrow). Using stiff blades it is impossible to align them together without errors in AOA.

Propellers used on tiltrotor aircraft are supposed to function in regimes of both hover and cruise. Variable blade pitch technology is usually present though on all sophisticated tiltrotors, this expectation still creates some contradicting requirements.

High speed cruising needs strongly twisted blades (high blade angles) in contrast to hover, which requires very light twist, i.e. low blade angles. Today, as a rule, stiff blades are available only. The value of the constant blade twist is defined by engineering compromise. Especially, when no RPM control is available (non-electric drive) the need for high cruising speed prevails: blades with relative high twist get chosen. This feature causes the rotor to operate with partial blade stall in hover, by design. That is always.

Zones of blade stall in hover make “holes” in the rotor disk. Thrust (lifting force) becomes reduced and asymmetric. In slightly adverse circumstances tendency to get into Vortex Ring State (VRS) increases.

The solution proposed by us offers concept of a variable-twist rotor/propeller blade capable to be fully aligned with the resulting wind at any axial speed. Actuator complexity does not exceed that of the present day variable pitch propellers.

Become the first company to offer variable blade twist capability to their propellers. A dramatic increase of propulsive efficiency can follow.

A description with design details sufficient for starting the prototyping project is provided in the free eBook downloadable on the front page of this website (stallfreepropellers.com).

Higher Propeller Efficiency for Sustainability

Subsonic flight is far from having been fully explored

Birds and butterflies have way more efficient propulsion systems than the ones we humans use

For sustainability – it is time we catch up!

By boosting efficiency, you cut fuel costs without penalties in flight performance.

Upgrade your fans and/or propellers with Stallfree technology to increase efficiency of propulsion system by 30-50%. Or more.

And get an even better flight envelope for your aircraft.

Stallfree technology turns fans and propellers into a VIRTUAL CLASSIC PROPELLER with adjustable design speed.

The Stallfree propeller concept is a quantum-leap development of the variable pitch technology. Speed range improvement is 200-300%. The props operate near to their maximum efficiency from zero (hover) up to a speed of about 0,8 Mach. No sharp fall of efficiency beyond this value – smooth decline spans well beyond 1 Mach.

(Optimum speed selected/regulated automatically or manually. )

Classic propellers

… have beautiful shape.

They also have beautiful airflow pattern around their blade-sections.

Good airflow means excellent thrust and efficiency

Yes… – at their DESIGN SPEED and RPM. Only.

With SFP you have a classic propeller for ANY SPEED! (Optimum speed selected/regulated automatically or manually, from zero up to about .8 Mach.)

It is scientific fact that in the subsonic speed range – this is the range where most (>90%) of the global air transport is operating – efficiency of propellers exceeds that of the jet propulsion by 2x, 3x, even four times.

(See some trusted technical references below.)

Why do airline operators keep choosing the more costly solutions, when fuel consumption per passenger capita has ever been their most scrutinized and critical business parameters.
The reason is, to put it simply, the lack of an appropriate gearbox.

(Contra-rotating propeller arrangement comes close but has several drawbacks. See more on page „Contra-Rotating Props Beaten”. )

Common practice is to combine propeller and jet propulsion systems together, to receive hybrid engines that provide efficient takeoff and low speed climb, thanks to their propeller component. Then, at higher speeds (above 0,4 Mach) the jet part of the combo steadily improves and keeps propelling the aircraft up to about 0,8 Mach.

Above that value (of about 0,8 Mach) however trouble starts, as the propeller’s blades can’t help developing very high drag.
The resulting trust reduces to near zero, and the aircraft won’t accelerate any further.

The above gearbox-problem is addressed by the present invention. It enables the aircraft to fly the whole subsonic speed range using clean propeller propulsion, with the propellers operating all the way near to their peak efficiency.

There is a very simple mathematical proof to support the operational viability of the proposed structure of the blades and the propeller. Improvement of propeller efficiency has been shown by proving mathematically the factors causing deterioration of characteristics are reduced to zero.

The potential of fuel saving – and of course that of its positive environmental impact – is huge; considering the recent volume of global air traffic, it is immense.

Subsonic Flight Champions