Helicopters as windmills?

September 12, 2005 11:03 AM

Would a helicopter make a good makeshift windmill?

This came up in discussion about a post-apocalyptic scenario where stranded military personnel have to live off the land. I had this idea of a helicopter-using military unit converting their helicopters to windmills. (Imagine a row of military helicopters standing on their tails on the side of an airbase runway.) The basic idea is to just lift the helicopter on it's tail and either extract the energy at the tail rotor or just replace the engine with a generator. Obviously this would not be the most efficient wind-generator available, but just how unefficient would it be? Would the rotor blades be any good as windmill blades?

posted by lazy-ville to science & nature (17 comments total)

You'd probably want to take the rotors off the choppers to do this. They'd be a lot lighter, for one. Also, helicopter blades swivel, so you'd need to get them in the right orientation.
posted by delmoi at 11:25 AM on September 12, 2005

You've seen how long it takes a helicopter's blades to get up to speed, right? The blades start very slowly because there's a lot of intertia to overcome. More inertia, I'm afraid, than a stiff wind could overcome.
posted by pmbuko at 11:35 AM on September 12, 2005

Well, I supposed they'd turn if you set the pitch right, but having the fuselage as a big blobby lump immediately behind the rotor would do bad things to the airflow. You'd probably also want to mount the rotor good and high, as wind speed varies massively by height.

I suspect the rotors, being designed for high-speed motored use, wouldn't be that efficient. They're also nowhere near heavy-duty enough; aerospace equipment often needs hours of maintenance for hours of operation, while wind turbines get a few hours of maintenance every year -- and you can't wheel a wind turbine into a hangar when the weather turns nasty.

You might also have interesting resonance problems if the rotors had four blades. Two-bladed designs might need to be motored up to speed.

I don't know enough about helicopters to comment. Someone who would know is Jay Carter; he used to build wind turbines, though not with much commercial success.
posted by scruss at 11:39 AM on September 12, 2005

Windmills need to be way further off the ground than helicopter height to reap any good wind benefits. This is why you see them on ridgelines. You'd be better off using your helicopter fuel barrels to make smaller windmills [like so] and just put them on top of small towers. You'd really need to completely dismantle the helicopter blades to do anything even halfway decent along these lines. Also, this will only work if you're someplace with decent wind, so there are a lot of variables that determine if this is a good idea or not, my vote would be, generally, "not."
posted by jessamyn at 12:31 PM on September 12, 2005

Yes, you could. But not while on the rotor, and certainly not through the rotor, mast, engine, compression and out through the tail boom rotor.

Strip the blades off the rotor entirely and build a Darrieus Rotor wind turbine. If the blades are flexible enough, bend them into an arc and mount, if not so flexible, cut into strips and join a segmented arc, or do a flat-bladed Darrieus.

Helicopter blades would probably pretty ideal for a "found object" Darrieus compared to just about anything other than purpose-built Darrieus rotor blades. They're light, they're an airfoil, they can handle high RPMs. And A Darrieus thrives on high RPMs and a really stout wind.

Plus they're unidirectional. There's little to no lag or change in output as wind shifts around in direction, whereas traditional horizontal-axis turbines must turn to face the wind.

See here, here, here, and here.

The last image is a combo/hybrid Darrieus-Savonius.

Savonius rotors are rather easy to build out of fuel drums, bent wood, and all kinds of cylinder shapes.

Tag this as "Things I learned as a kid from hippies and "The Whole Earth Catalog".
posted by loquacious at 12:38 PM on September 12, 2005

Yes, loquacious, but Darrieus turbines suffer from fatigue problems like you wouldn't believe. Their output is not constant throughout their rotation. Wind direction seldom changes that fast, so the horizontal axis design can still keep up. I think the wind direction changing thing is perceived to be more of a problem by people who don't design windfarms for a living ;-)

Oh, and thanks for giving me the horror of the day with the hybrid Darrieus-Savonius picture. That would be like yoking a racehorse to an ox; completely different tip speed ratios and Cps!
posted by scruss at 1:01 PM on September 12, 2005

True, I'm not a windfarm or wind turbine designer, but the question involves survival/post-apocalypse scenarios, not best condition scenarios.

*grumbles* Damn non-generalist engineer nerds. ;)

Granted, getting a Darrieus bolted/welded together and balanced out of cut up (probably honeycombed-core) helo blades in survival conditions with simple tools would be a task in and of itself.

Coud that Darrieus-Savonius hybrid be intended for use across a wide range of windspeeds? It looks like it's very well (and expensively) built. (I don't read German. Text for that pic about 1/3rd of the way down.)
posted by loquacious at 1:47 PM on September 12, 2005

I know nothing about the specifics of helicopter transmissions, but I suspect you're best bet would be to remove the rotor from the helicopter entirely, find some other way to support it as a windmill and connect it through some improvised drive to the generator. Probably lots of other parts in the helicopter that could be scavenged for that.

Your biggest problem, however, would be the blade design. Helicopter blades have relatively small surface area, to keep weight down, but rotate at high speed to give the required lift. Windmill blades have larger relative surface area, but rotate at slower speed.

I suspect you'd need a huge wind to accelerate them, but maybe a not so big wind once they were running, if you had them on a good smooth bearing. Maybe some kind of initial power assist, just to get them up to speed? I'm speculating.

On preview: Good point, the Darrieus might be the way to go, but aren't helicopter blades made of some kind of composite, nowadays? Might crack if you bend it into a catenary?
posted by normy at 1:49 PM on September 12, 2005

loquacious, if you select the English link at the top of that page, all is revealed. Seems this design used the drag-rotors to start the turbine; I'd forgotten that Darrieus turbines didn't self-start. It still gives me the fear, though.
posted by scruss at 2:04 PM on September 12, 2005

Scruss: Thanks. I didn't even see that one.

Normy: From what I've seen in videos and in person of helicopters in flight the blades seem pretty flexible - but they probably only appear that way, and only flex that much when under extreme loads.

So simply bending them to form a catenary or hyperboloid under a load of tension or compression is probably pretty unlikely, especially considering the lateral loads a Darrieus exeriences at high RPMs, which it needs to create enough energy and/or/or for it to be able to be geared down for torque to be useful for energy output/work.

Without a whole machine shop of tools, welding a segmented arc out of slices of the blades would probably be pretty unlikely as well, even if the rotor was metal, metal skin and honeycomb, or other metal. It's probably going to be an exotic aluminum. If it's pure non-metallic composite, well, welding is right out anyway.

The best solution I can think of in this particular survival scenario would be to make a Darrieus like this one. (H-Rotor? Scruss, what's that kind called?)

Applying my non-specific generalist geek skills of MacGuvering intuition, I'm assuming that that kind of flat-segmented Darrieus is just utilizing the flat blades as though they were the near-vertical tips of an arced Darrieus rotor.

You could probably use a number of parts from your dismantled helicopter as parts for the Darrieus rotor. You could probably get bearings and make a turntable or spindle and you could probably get some gearing from various places in the helicopter.

Though, if you were crashed in a remote place or on any sort of metaphorical island - rather than, say, an air base, constructing this thing well enough so it didn't fly apart and kill someone would be a serious task, above and beyond the realm of duct tape and a Leatherman.

But people have been able to juryrig some really impressive stuff when the need arose, so I wouldn't be too awed or surprised to see it happen. Especially if I was there and I wanted to go home - say, in a situation that required some juice to power a radio transceiver.

If it was my only option, I don't care if I had to mill and hew the blades by hand with rough bits of rock or coral as grinding bits and tying/lashing it together with plant matter. I would balance it with my own spit if that's what it took.

But then, in this scenario of "I want to go home!" I'd probably work on building a stationary bike or other cranking mechanism to turn an alternator or generator for my juice. It'd be easier to do.
posted by loquacious at 2:51 PM on September 12, 2005

Yes, in the end, I suspect your biggest problem would be the tools to do it, regardless of any clever engineering improvisation you might be able to dream up. Do military helicopters carry much in the way of a toolkit? I'm guessing they might have some basics, but nothing like what you'd need to dissemble the transmission.
posted by normy at 3:07 PM on September 12, 2005

Wind can spin a helicopter rotor with at least some power generating capacity. We know this because helicopters can autorotate, using the wind alone to maintain the blades RPM and holding the weight of the helicopter against the force of gravity. Based on the "What happens when the engine stops" section here, 20mph of wind (1800ft/min down) can keep the rotor in question spinning at ~500rpm... (presumably the forward speed of 55 knots is to maintain enough airflow over static lifting surfaces for stable flight, it doesn't have anything to do with braking the fall).

The effectiveness as a wind generator will likely depend somewhat (or a lot) on the model of rotor used. Helicopters intended for operation from unprepared fields don't induce as much velocity in the wake as helicopters designed to operate from concrete slabs (basically, you can't blow as hard in an unprepared area because you will cause debris to fly around and damage things). I think the figure of merit for this is disc loading, measured in lbs/ft^2. A CH-53E has a disc loading of 14, while the Bell 206B has a loading below 4. I suppose that choosing a rotor with the appropriate disc loading would greatly improve the usefulness of the design, but I'm not at all sure what the optimal disc loading should be for a windmill.

Of course helicopter rotors, like windmills, already have a built in mechanism for varying blade pitch. This should be very useful for optimizing the power generation. However, helicopter blades are hinged. I believe the hinges have built in angle limits to keep the blades from touching the ground while parked, but it might be necessary to fuse the flap hinge anyway, or to limit the angle even more, or something...

I agree that you would certainly want to take the rotor and shaft off the helicopter, maybe even the transmission, but it is not at all clear that you would want to dismantle the rotor itself. Figuring out the best post apocalypse wind generating system would require a lot of testing (lots of time for that post apocalypse :P), but I think a helicopter rotor is a reasonable starting place, assuming you don't have any modern windmills around.
posted by Chuckles at 3:53 PM on September 12, 2005

That should read "helicopter blades are often hinged", there are lots of rigid rotor helicopters. However, the blades in such designs are more flexible, and they might actually be a poorer choice for a windmill... It is easy enough to fuse or limit a hinge, but not at all easy to make the actual blade stiffer.
posted by Chuckles at 4:00 PM on September 12, 2005

Plus they're unidirectional. There's little to no lag or change in output as wind shifts around in direction, whereas traditional horizontal-axis turbines must turn to face the wind.

Is there any reason they would have to be HAWT? WOuld it not make more sense to take advantage of the helicopter blades as designed and use them VAWT style, (perhaps having taken them off the helicopter already)?
posted by biffa at 2:21 AM on September 13, 2005

loquacious, they're called H-rotors, or Musgrove VAWTs. I used to work for a company that built a few of them.

I think the survivors (if there were any) would be better off building things like Hugh Piggott does.
posted by scruss at 1:01 PM on September 14, 2005

but just how unefficient would it be?

physics aside (I know a bunch of pilots who would laugh histerically now, no offense) I have to think of the last time I rented a dinky jet ranger. $550/hour wet plus my instructor. uhm... highly uneconomical. I imagine a ch53 (three turbines instead of one) to run you about $2500/hour.

also, a windmill is powered by wind whereas rotorblades create wind out of previously stale air. opposite concept.
posted by krautland at 9:31 PM on August 28, 2006

also: turbines compress air and that causes heat - and you need at least one to power the rotor (the exception being a really small R22 or R44 or the likes but there would be other issues).

you will never be cooled by a helicopter unless you take the doors out and race down the beach at 85 m/ph. if you do that in shorts, you will freeze even in 90F.

turbines are HOT.
posted by krautland at 9:36 PM on August 28, 2006