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: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: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: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: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.