A blade for a propeller is made to generate thrust. The curved side of the airfoil should face in the direction you want the resulting force to pull.
A blade for a wind turbine should spin with an approaching wind and generate a lift force mostly downwind and a little bit to generate a moment to rotate the shaft. The curved side of the airfoil is facing away from where the wind comes from.
Propellers come in CW and CCW types, but no matter how you orient them, a propeller could not become an efficient turbine blade. If you place the curved side of the airfoil away from the wind, the wind will approach the airfoil of the blade from the thin (trailing edge) side.
If a propeller is used as a generator blade, you are essentially using the negative angle of attack part of the airfoil. There will be a little lift, but it will not perform well.
In particular if you decide to use an inefficient turbine blade, you will generate relatively more drag and the blades will not achieve the high tip speed ratio you could otherwise expect. If you place that turbine on a flying vehicle, the drag losses may become severe and power output modest.
Actually I believe for a turbine on a flying vehicle, many blades rotating slower (low tsr) is better than a high tsr because if you have high tsr a large force is generated downwind. For a fixed installation this is not a big deal because resisting downwind pull is energy free. For a moving vessel this is not the case.
Just investigate the way the lift works on the blades and it should become clear
I measured (with a multi-meter) 9 volts at 13.7 m/s then 3 amperes at 14.4 m/s, by using a Master Airscrew GF 9x4 on a motor Mabuchi RS-380PH used as generator, at about 7100 rpm, so 85 m/s tip speed and TSR of 6 for an apparent wind of 14 m/s. I extrapolated the 7100 rpm value from the voltage (3.8 volts) obtained with a drill running at 3000 rpm, which is of course very approximate. The apparent wind speed by car was measured with an anemometer.
So about an average of 27 W at 14 m/s, for a swept area of 0.041 m² (diameter of the propeller 0.23 m). At Betz limit the power would be 39.8 W. So 27 W is more than 67% Betz limit, and was obtained with the loaded generator: not too bad for a propeller as wind turbine. I have had slightly or much worse results using other combinations. Of course in all cases the propeller was turned upside down.
In flight on the FlygenKite things are very different because of variations of power according to the place of the kite in the wind window, leading to various significant losses by cosine ³.
You can get away with running small, thin-profile, injection-molded, toy propellers backwards.
A propeller of normal thickness is terrible when running backwards as a turbine rotor.
I once did a serious, documented, experimental project to develop a wind turbine rotor that could work in either direction for a non-aiming turbine in a bi-directional wind resource. Talked an architectural firm into paying for it. Tried a few versions. Turned out not to work very well - unacceptable performance.
Right, every first impression in wind energy is “clear” for people who don’t know about wind energy. That’s how so many crazy projects get funded. Yes, a wind turbine blade is backwards and inside-out from a propeller blade. At least that’s my Windergarten explanation.
Next: A high-solidity, low TSR wind turbine rotor can never achieve high efficiency. I’m not going to tell you why, because that is just something I’ve noticed nobody in AWE has had the slightest clue about for 14 years and counting, and I hate to ruin such a perfect record.
Indeed my propellers were tiny. Larger propellers would have been dangerous for tests in addition to be inefficient: I was driving the car with one hand, and I held the turbine with the other hand, while I look at the multi-meter.
I think kiteKraft, Windlift and Makani have correctly conceived their turbines aloft for crosswind flight.
I agree to this, but my thinking is that high efficiency could be a non-goal for an onboard airflow turbine because with a slightly less efficient blade, you may get a much better power to «drag» ratio, which maybe os more important
Yes this is indeed interesting, as Makani were using blades both for turbine and propulsion. And from my experience VTOL is very demanding, so you probably could not get away with having the blades running backwards. On the other hand using the blades as a turbine was the main function of the kite. A very difficult proposition indeed.
I have no idea how they did this. It may have been a great engineering feat. Or it may have been an unsolved problem. Or not a problem at all…
I can on the other hand say that we did testing at Kitemill using props as turbine blades, and found the performance to be absymal compared to proper blades. This was in the 20-30 cm diameter range blades. Some props work slightly better than others, but in the end you really dont want or need to go there
If not for any other reason, people in wind power will not take you seriously if you do it
Small and light blades for wind turbines were not available during the time of experiments. For almost nothing I was able to buy propellers as blades for wind turbines and small motors as generators: this worked well, with a high TSR of 6, high efficiency at the apparent wind speed of tests.
I was also able to test different variants (low amperage by using other generators, and lower kite speed) to obtain lightning.
The answer to the question if a propeller not designed for a wind turbine can be effective under certain conditions is definitely yes: I measured it. If you dispute my tests you can carry them out with the equipment I have mentioned and linked, then report the results.
I’d say you did a good job for your experiment, Pierre.
The reason the small injection-molded blades can work OK is the profile is so thin there is not much difference between a leading and trailing edge.
Also maybe depends on the induction factor used. But if you are closing in to the Betz limit, that proves you are close to optimum. I would have liked to see the experiment repeated though, because I find the results better than I would expect. But I think there is noone with enough interest to do this at the moment
Tallak: I was under the vague impression they probably used somewhat symmetrical airfoils(?) and possibly even active pitch control at the hubs(?). Glancing at their videos, I’m not so sure though. Looks like the blades have camber(?). They may just use extra power at takeoff to overcome the inefficiency of nonoptimal camber. They did use high-solidity rotors such as you mentioned, but I think the reason was noise control, limiting RPM, since otherwise their blades might have run at high Mach numbers.
Anyway, the proper terminology for the reverse force is thrust, or in this case, reverse thrust, and it could be called reverse thrust loading on the aircraft, rather than “drag”. In wind energy, the “push” on the rotor is called “thrust” or “thrust force”. Same with aviation for the push of the rotor.
It is easy to see a range of numbers when doing such truck testing. A quick glance at the meters can show more favorable numbers than one might expect. If you are looking for high numbers, the high numbers are what you will remember, ignoring times with lower output. Best to use long runs and go in two directions in calm air and take the average, since even a 2 MPH wind can skew your results a lot. Even a crosswind will add power. Also, you can beat the Betz coefficient briefly when the rotor first spins up, before a bubble of dead air has built up ahead of the rotor. How is this possible? The Betz coefficient assumes steady-state, and startup is not steady-state.
Yes Tallak, sorry for not addressing your original point.
I do not know the answer to that one off the top of my head, although I think thrust force vs output, with regard to rotor solidity and design TSR is a known, rather than an unknown, in the art. It would be something to get ones head around when designing a “drag” er um I mean compound two-stage lift device producing reverse rotor thrust. As I pointed out regarding Makani’s plane, it’s easy to see the high solidity of the 5-blade rotors, but my take is that choice was made to reduce the Mach number of a turbine designed for a much faster relative (induced) windspeed than a normal turbine. The high solidity may also be desirable for using the blades for propulsiion, since wind turbine blades are non-optimal for that use. Anyway, you can get away with some variance from technically optimal parameters and still get acceptable performance as long as you are “in the ballpark”.
Did you find blades for wind turbines in the 20-30 cm diameter range (“proper blades”)? As for me, I didn’t find any, nor did I find any small generators, only small motors and propellers for model planes.
Indeed the curve of a propeller blade is inverted with respect to that of a wind turbine blade. As a result, my Master Airscrew 9x4 (23 cm diameter) performed slightly better using the trailing edge as the leading edge than the other way round with an inappropriate curvature. But the profile is almost flat due to the “thin-profile”, in such a way that it is not easy to distinguish the trailing edge from the leading edge in term of thickness…
Also the speed I tested these propellers was higher (inducing in a higher Reynolds number) than the wind speed for wind turbines, capping at 11-12 m/s, while propellers for model planes are used at far higher speeds. So for this reason, and also other reasons that are mentioned on this thread, the suitable blade profile for crosswind fly-gen (high speed) could be likely a bit different, likely that of Makani’s turbines.