Menu Sign In Contact FAQ
Banner
Welcome to our forums

New propeller - how can this work?

10 Posts

They claim a 1000ft reduction in takeoff distance, which is a huge improvement.

I thought that prop efficiency is if the order of 85% already, so how can one achieve what amount to (of the order of) 20-30% extra thrust?

Administrator
Shoreham EGKA, United Kingdom

It sounds plausible. They extend the prop by 3in, which accounts for the increase of effectiveness. The downside is a higher tip velocity, hence more noise. Using a swept wind blade they claim to reduce it and transfer more engergy into motion, because noise is engergy lost. Best prop efficency is around 80% with slow turning wide baldes. It is not only the prop, you need an EPIC system to gain 1150ft. New leading edges, stakes, ram air inlets......

EGBE - Coventry
EGBE - Coventry

They don't make any claims about cruise performance, it could very well be that there's an actual reduction in cruise speed.

In my flight testing of MT three blade props on a Twin Comanche, I measured a 200 FPM climb rate increase over the two blade Hartzells which had been replaced. It is necessary for the prop to be optimized to the intended speed of the aircraft, but there are certainly improvements to be realized.

As said, a diameter increase can increase thrust, and this was very common with floatplanes, which really needed the low speed thrust, but strict noise requirements have brought that to an end. If you have it, you can keep it, but apparently no new "larger" diameter props will be approved. I'm mired in this right now with a 182 amphibian project.

Home runway, in central Ontario, Canada

They don't make any claims about cruise performance, it could very well be that there's an actual reduction in cruise speed.

They actually do: up to 24kts faster

EGBE - Coventry

Prop efficiency in cruise is generally reasonably high, although capped at the 80-85% level due to things like Mach tip effects. Without the compressibility issues, efficiency should continue to increase towards 100% the faster the aircraft goes.

However, the efficiency at the beginning of the takeoff roll is 0. (Thrust but no motion means 0 power). Static thrust is overwhelmingly determined by diameter, so the efficiency for much of the takeoff roll could be substantially higher on these props. Also, the other modifications referenced seem to lower the takeoff speed by 6 knots, which will have a very big effect on the distance to 50 feet.

EGTF

Don't forget if you increase the diameter of the propellors the area of "blown" lift over the wings in the slipstream will also increase significantly.

London area

I think that "blown lift" would be a minor to indistinguishable factor. The power to weight ration of GA aircraft is such that to have any blown lift effect, you would have to operate well behind the power curve. In this region of operation, you are nowhere near any ideal "performance", other than perhaps slow flight. Any aircraft with enough power to make this effect noticeable, will probably overheat during such operations, before the effect can be noticed.

Increased diameter of propellers on GA aircraft is in the order of a few inches, on a 70 to 85 inch diameter propeller. The percentage change is small. There are blown lift type aircraft, but non successful in civil operations. A major reason being that blown lift on single engine aircraft will have large torque effects to be overcome, which are very hard to overcome with the flight controls. If a multi engine blown lift aircraft, the failure of an engine during that phase of flight will probably result in an unrecoverable aircraft. search Custer Channel Wing. Those multi engine aircraft which have overcome this, have done so by linking all engines and propellers with driveshafts, so any engine failure does not stop the associated propeller. Very complex, heavy and expensive. More successful has been the augmentor wing (Shin Miewa), but still heavy and expensive.

Home runway, in central Ontario, Canada

Thanks for the Custer Channel Wing - very interesting.

The increase of diameter in this case is of the order of 4%, which I would have thought to have some effect. I am obviously not an aerodynamicist, and my multi-engine theory discussing blown lift was mainly to do with the engine failure case as you describe, but I had thought it would not have been insignificant.

Is it the case then that blown lift is only particularly important in the OEI regime and in the difference between power on/power off stall speeds?

London area
10 Posts
Sign in to add your message

Back to Top