Wednesday, December 29, 2010

Propeller design rethought

Most manufacturers design propellers in the same way and they read the old books and reports and the prop gets no more than 80-85% efficiency at best. It is written in books that propeller efficiency will be about that at best and it is left often open how low it can be at worst.

Here is an interesting article about a guy that made a prop that was 90 percent efficient by not abiding the "old truths" but thinking out of the box:

Having a strong taper certainly makes sense since the propeller tip travels very much faster than the root through the air. Also the old saying that single blade prop is most efficient does not make sense if you think it in detail: the air that enters in the next blade is not the same air that went through the previous blade because of the forward movement of the aircraft. This could be extrapolated in a such way, that the faster the aircraft travels, the more blades the propeller can have without sacrificing the propeller efficiency. This should not actually require very high mathematics, but I am quite sure that it could be estimated with simple calculations where the downwash of the previous blade goes in relation to the next blade on the speed range intended for the aircraft being designed.

High altitude propeller will require some additional thinking for the tip chord because the Reynolds number will become low if the chord is this short. The TAS is much higher at high altitude, therefore the air travels faster through the prop, that would mean that the prop could have more blades. The high altitude propeller does not require full efficiency at low altitude because to be able to operate at high altitude, there needs to be a lots of excess thrust available regardless.


Thad Beier said...


This is an interesting article, and he has proven his theories in the most stringent way possible.

I wonder if the same ideas would apply to helicopter rotor design as well. Almost all helicopter rotor planforms are rectangular, if you could get another 10% efficiency by tapering the blades as Lipps specifies it would be amazing.

Unknown said...

I am quite sure it would affect to helicopter rotor as well. In fact, the helicopter rotor is a wing that just rotates. The root is moving slower than the tip of course, which could actually call for strong taper. Also instead of using a super critical airfoil near tips might be a good idea because the blades are moving so fast, the usual choice of some turbulent profile may be really inferior choice. It could be also inferior in the propeller but one of the things which are just done because that's how they have been always done. Of course there are limits to that, namely the Reynolds number. If the reynolds number ends up too low <400k then the laminar airfoil may have higher drag than the turbulent rival.

The 10% efficiency feels very small difference as a number but in horse power it can mean significant amount and as excess power it can be even more significant - it is the excess power that makes the planes to climb.

I would estimate that the 200 hp equivalent on standard prop with changing the prop to 10% more efficient e.g. in our Diamond would make the already good (on GA scale) climb rate to even better. Given that this would be excess power with no additional weight penalty, it could mean significantly better climb rate in fact. I haven't done any math to calculate the prop efficiency in the Diamond on different flight conditions so I can't compare, but this is just a guess.

ALso I have been thinking, that the efficiency loss of multiple blades apparently is not significant unless the forward movement is close to zero (zero airspeed). If the plane is moving forward, the prop becomes like a screw and more than two blades suddenly becomes like a good idea. The faster the plane goes (critical parameter here is the climb, the faster the climb speed), the more blades the prop can have without efficiency loss.

Unknown said...

Also 10% improvement in fuel efficiency would be nice. E.g. we could go with the Diamond from 6.8 gallons per hour to 6.1 gallons per hour. That at least feels quite a bit lower even if the difference is not that high in gallons. Of course the difference becomes more significant if we count liters instead or Cola bottles. Almost 2 cola bottles volume of gasoline per hour could be saved.