Reverse Pumping as a method for launching Rotary Airborne Wind Energy Systems

Dear all,

I’ve been experimenting with the concept of “Reverse Pumping” as a method for launching Rotary AWES.

The idea consists of adding additional energy to the rotor while reeling in and converting it into potential energy (i.e. height) when reeling out. The reverse of what a pumping RAWES does during power generating cycles - hence the name. We have built a rotor to test the concept. For simplicity and space reasons the rotor uses a horizontal guide line and can therefore only move along one axis such that we do not have to worry about things like cyclic pitch control for this experiment. Also the horizontal orientation takes gravity out of the equation. We therefore don´t have to worry about the rotor mass. If we manage to maintain the rotor speed through various pumping cycles we could measure the tether force and see if it would be enough to actually lift the rotor. Sadly we have not come to that point.

So far the rotor drag exceeds the energy added by pumping and the rotor loses energy quickly. Maybe the concept can be implemented with the right combination of rotor dimensions, mass, airfoil, RPMs, reel in speed, pitch angles etc - but the current design seems so far off these parameters that I have decided not to continue with the experiment until somebody suggests significant design improvements - preferably by running a successful simulation and finding a set a parameters that should work.

Nobody was hurt in the making of this video. Big thanks to for providing the helmet :slight_smile:

What do you think about the concept?




Hi. Really interesting experiment.

I was thinking if you started with the blades acting as a windmill, and with a wind of, say 8 m/s. Then you reel in and the rate of rotation should increase even without any change in pitch angle. Because the blades are acting like a corkscrew until they stall. Rotary speed is forced by airspeed + reel in speed. Once airspeed is achieved, you should be able to let go and the blades should just propel outward.

Actually the simpler case of doing the thing described above would be just to act as a windmill, dont care about pumping, and then reel out slowly.

So I guess you are looking for something that can work in zero wind.

So if we assume you can give the blades an initial spin, then tilt the blades just enough to keep the airborne part flying (creating additional force will result in more blade drag but no motion). Then reel in quite fast to increase the energy. This is the tricky part. Because if you want the windmill to go from being a propeller to a generator, it will require quite a lot of airflow for that to spin at any reasonably fast speed. I would expect you need to reel in at least 5-8 m/s. That does not seem feasible in your test setup.

The alternative seems to be being building blades that are so lightweight that they will be flying even at minimal rotary speed and wind speeds. But those are probably not useful once you have engaged the whole tether and are getting ready to produce electricity.

Also reeling in so fast would shorten the tether very fast, making net reel out difficult.

There is only one effect that maybe could save you which is that when the prop is running, the airflow in the swept area is not zero. It will be something related to the pitch of the props and the rotary speed. unfortunately, this airflow will be in the “opposite” direction of where you want the wind to flow, making transition to generation even more difficult.

I think this technique makes more sense for a windmill placed in the wind, then by adding a reel in/reel out periodic movement on top of your slower reel out motion, the blades should spin a little faster and thus be able to launch in less wind. But the wind still must be more than zero for it to be possible.

I think your conceptualization in the video is slightly off. I don’t think you go up higher and higher hills, the tether length should look somewhat like f\left(x\right)=\sin\left(3x\right)+x, with the first leg after you launch longer to account for the extra rotation you are able to give the rotor on launch.

Then you only reel in to get up to autorotation speed quicker and, once you reach that, you let the rotor rise again. Your question is how quickly do you reach autorotation speed given the reel in speed and the initial remaining rotation speed. You’ll also want to know what the autorotation speed is you want to reach.

If I understand what you are doing in your tests, you seem to be doing the opposite of what I think you should be doing. You want instead to give the rotor a small initial rotation and then quickly reel the tether in to see if you can get it up to autorotation speed. A first test perhaps could be to let the rotor fall under its own weight and see what happens, then gradually raise the tether tension and see what happens then. My intuition tells me the tether tension should probably look like what you’d get from a whipping motion, the more drawn out the more inertia the rotor has.

If you can then quickly change the pitch of the blades, you can then measure the tether tension as the rotor would be rising, or you could measure tether tension separately while you vary rotor rotation speed and pitch angle by some other means.

Yes… but once you get faster rotation the energy in a reel in pump will be a lot more. So there is probably some minimal rotary speed that makes sense

Okay, how about a rotation speed where the rotor still generates more than enough lift to lift the system and keep it at the correct elevation angle with the blades at a minimal angle of attack, and then a pump that would raise that rotation speed and tether tension to something significantly higher. That would perhaps ensure that the rotor doesn’t just fall out of the sky when you start a pump cycle.

To begin testing this, I don’t think you need to be able to adjust the angle of attack, if you use the two stage testing method I describe above, if you only want to prove the concept.

The important thing seems to be you can only pump when acting as a generator. In zero wind the blades must act as a propeller driven by moment of inertia… So there must be a transition there…

Ideally you should not need to change the AoA dynamically to achieve pumping. In that case maybe the design is too complicated.

I guess I don’t really understand this. Let’s use the analogy of a weighted and unweighted glider: pumping would be temporarily weighing the glider down while trying to maximize the glide ratio to maximize speed while minimizing altitude loss, and the other mode would be releasing the weight and with the help of inertia from the gathered speed pitch up (quickly) and try to gather more altitude than what you started with.

For this to work, it would be best if the acceleration would be close to infinite to limit altitude loss, and that the glider had lots of mass to have lots of inertia, and would have very little drag. But then if you had lots of mass, acceleration would not be close to infinite, and the wing would need to be strong to withstand the extra weight and acceleration, so there has to a balance.

I think this analogy could give you more of a clue if this could work out or not by calculating the acceleration and so on of a variably weighted and unweighted glider and see if you can make that work.

One idea could be to add (water) ballast that somehow gets shed after the rotation slows down to give the first rotational/linear launch more energy, or at first to add ballast during testing to give you more time to react during slow down. For testing maybe instead of aluminium or carbon fiber tubes use steel rods or weights. I’d probably want to see how high of an altitude I was able to reach with the ballasted and unballasted glider/rotor and only be interested if it was some significant fraction of the desired altitude for both. Maybe achieving that would be a first step, and then adding this pumping capability would be another step that would maybe help in low wind.