Sagita Displays Hot Air Powered Helicopter

rcastro0 writes "Gizmag reports on the Sherpa, an interesting helicopter design at this year's Paris Air Show. As the article explains 'Rather than driving the rotors directly, the Sherpa's engine instead powers a compressor with an air intake at the rear of the helicopter.' There's no tail rotor. This approach is supposed to be more efficient, more reliable and more affordable than the traditional. A one-fifth scale model was shown to fly. Sagita, the 2008 startup behind the project, has yet to build a full scale prototype. They plan to sell a Sherpa two-seater for around US$ 200k in 3 years."

Comments

Test it in Washington

[reboot246]

Just put one of these helicopters over Washington D.C., and you'd have an unlimited supply of hot air.

The thing would be able to run forever!

Re:Test it in Washington

[NettiWelho]

And get yourself arrested by the feds by siphoning illegally off the national strategic hot air reserve.

Re:What are they up to?

[russotto]

I'm wondering if they have separated the compressor and turbine stages in a conventional jet engine in an effort to get a fuel economy or weight improvement.

I cheated and read the article, and it appears that is what they've done.

However, they haven't actually built it; they have a 1/5 scale model, but it's using an electric motor (so it's not clear what it proves), and furthermore the laws of aerodynamics are not scale-invariant.

It's not clear why they're trumpeting the lack of a tail rotor as a new thing; this isn't a NOTAR design (single main rotor with no tail rotor); it's a coaxial helicopter, which is nothing new at all.

Something isn't adding up...

[wagnerrp]

According to the article, the air is compressed, then heated by the engine's cooling system, then mixed with exhaust gasses, and the resultant flow into the turbine is only 100C? Something doesn't seem right here. That temperature difference implies such a low boost that it won't even operate effectively as a supercharger, much less provide anything like enough power to those turbines. There's a reason gasoline engines with worthwhile turbochargers and superchargers all have intercoolers, and that's because compressing air makes it really damn hot.

Re:Something isn't adding up...

[dywolf]

This is actually not new technology.
In fact, most turboshaft helicopters already use this sort of system, eg: Hueys and Cobras (my background)

What you have here is a non-physical or indirect link between the engine and rotor transmission.
In some helicopters the link is direct or physical. IE, the transmission gearbox is directly attached to the engines output shaft, just like a car with a direct connection. This means the transmission must input must therefore spin at the same RPM as the engines output.

What they have done is the helicopter equivalent of the torque converter in your car. The rotor transmiion and gearbox is NOT directly connected to the engine's output shaft.

So how this works is you have a turbine engine. A jet engine basically. It produces a jet exhaust. What Hueys and Cobras, and Im sure some other types as well, what they do is rather than put the xmission gearbox directly on the engines output shaft is they isntead have a seperate unconnected turbine in the exhaust stream. Thus, the hot, compressed and fast flowing exhaust turns this second turbine. And it's this 2nd turbine that is connected to the xmission gearbox and thus to the rotor. This is essentially like the torque converter now common in cars.

By not having a direct link you do reduce mechanical complexity, as well as increase reliability. The 2nd turbine, the one driven the by the exhaust, can be keyed differently, thuis naturally resulting in a lower RPM than the engines axial shaft, and without using a gearbox in between to step down the RPM. Thats where the reduced complexity and higher reliability come from.

In the case of Hueys/cobras we're talking about stepping down from a jet turbine (say, 40k RPM) to a Rotor speed of say ~120 RPM. That kind of gearboxing is going to have high streess, high wear and tear, be extremely hot, and need careful babying. So instead we put this power turbine in the exhaust stream and design it so it naturally spins at a slower speed for a given flow rate, achieving the same effect as a gearbox. Now this is obviously still going to be a higher RPM than the final Rotor output, so theres still gearboxing and further reduction (say ~2k to 120 rpm). But the big jump from the engines axial RPM is accomplished via exhaust air flow.

In the helo community the engine is thus commonly known as the "Gas producer" (Ng, rpm of the gas producer). So what they've done here is replace the jet engine as the gas producer with an air compressor, to achieve the same goal: air to drive a Power Turbine that actualy moves the rotor.

Also note, for space sake the two turbine shafts, engine and power, are usually coaxial, hence the name: turboshaft.
Compared to jet engine with a propellor directly attached via gearing, which is the turboprop.
(lot of words, but now you know a whole lot more about modern helo engines)

Re:What are they up to?

[m0n0RAIL]

The tail provides stability during forward motion. A helicopter without one would need constant corrections to maintain its orientation during forward flight.

Re:this word power seems to have been lost

[wagnerrp]

You've got that backwards. Work is power done over a period of time.

Re:What are they up to?

[wagnerrp]

Without the "turbo" part either. "Turbo" implies a gas turbine engine. Turbojets are nothing more than a gas turbine. Turbofans have a gas turbine at their core. A turbocharger is a gas turbine wrapped around an internal combustion engine. This is a piston powered helicopter with a fluid dynamic transmission. It's merely different from the typical fluid dynamic transmission in that it is an open loop. Anyone in the automotive world will tell you a fluid dynamic transmission is lossy and inefficient. The only reason to plumb hot gasses around is because your engine design means you already have those hot gasses handily available. Oh, and they decided to supercharge it for some reason...

As for hot, fast gasses, they claim to be powering the rotor turbines with 100C compressed air, that's only that temperature after being heated up by the engine coolant and engine exhaust. At that temperature, there's not even enough energy in the flow to make that supercharger a net gain.

This would be a very interesting design if they actually used a gas turbine engine, rather than a piston engine (or realistically, an electric motor in their scale demo), as they would actually have enough exhaust flow to make it work. It would basically be the same thing GE did with their unducted fan, mating the contrarotating fan disks directly to a pair of contrarotating turbine disks, eliminating any deswirler or gearbox. The trouble is that these rotors have a strict top speed. Their tips cannot exceed the speed of sound, as doing so would cause all kinds of noise and vibrational issues on the helicopter. The scale difference between the rotor and the turbine means that means you're going to be running at a painfully low RPM, which is exactly why no one has done this commercially yet. It's really tough to get a turbine running that slowly, efficiently, and with significant power output.

Re:What are they up to?

[stockard]

Coaxial helicopters still have a tail due to the controls on the empennage. Helicopters are a bit odd in that the pilot is basically flying the rotor disk, and the fuselage is kinda just "along for the ride". So, if you wanted to rotate or adjust the pitch of the fuselage, you'd need some sort of controls on the fuselage to do so. (Some adjustment can be made with the rotors, but the standard tail controls are a bit simpler.) So, while coaxial helicopters are more inherently stable and don't need a tail rotor, they'll still have a tail. (See the Ka-50, X2 and Ka-27 among others, as examples.)

The reason you want controls at the aft end of the the tail is because for things like the elevator, you want as big a moment arm as possible to reduce the force required to adjust the pitch of the aircraft. Similarly, the vertical stabilizers are there to help reduce sideslip at higher speed, since a helicopter can fly in any direction, regardless of the orientation of the fuselage. (Generally for lighter helicopters, the vertical stabilizers are fixed, though the larger ones can have a movable rudder.)

Additionally, having a tail will help you if you need to do an autorotation, as it will help prevent the rotors from impacting the ground at the rear when you flare it right before landing. (Here are some examples: http://www.youtube.com/watch?v=T-5ARzqF3R4)

Disclaimer: While I'm a helicopter engineer, I don't work on the controls, so this may be a bit of a simplistic explanation. :)

Re:this word power seems to have been lost

[Anonymous Coward]

I research and teach physics, and even I think you're being kind of stupid here. Yes, that is a stab at the physics definition of power, but it should be pretty obvious to anyone with some basic understanding of English that there is a non-jargon meaning to the word, and usually it is quite clear from context which is meant. Many words have more than one meaning, get used to it, unless equivocation is a hobby of yours.

The very definition of vapor ware

[bdwoolman]

FTFA "Though a full-scale working prototype is yet to be built, Sagita claims to have proven the concept (albeit with an electric motor) with a one-fifth scale model. You can see the video of it in flight below.">

And it actually uses vapor. But not the good kind.

As for me? I am developing a teleportation device. I don't have a working prototype, but I have a proof of concept using my automobile. Any VCs out there can reach me on my FTL communicator. Also in development.

Re:this word power seems to have been lost

[wagnerrp]

Or rather... power = work / time....

Re:What are they up to?

[modecx]

As far as I can see, their innovation can be accurately summed up thus:

They've decoupled the power turbine (the one which provides power to the rotors via a gears and such) from the engine, and mix cooler air into the exhaust stream. They've done away with the transmission and drive train by forcing this much cooler exhaust stream through a power turbine directly coupled to the main rotors, and they have also eliminated all of the complexity of a tail rotor and associated drive train by going with co-axial counter-roating rotors to provide necessary anti-torque.

It's the laws of thermodynamics I'm most concerned about, since most thermodynamic cycles suffer when cold fluids are introduced. Maybe eliminating the losses in mechanical transmission and weight offset that? Who knows, but if it works like they claim, that's pretty cool.