Sheathed Rotors

Please check out new trailer for Avatar. In the movie the helicopters use sheathed rotors. Which are rotors inside narrow duct fans. I don't know how realistic that is. But wouldn't that system prevent RBS and supersonic advancing blade. As the rotor tips are sheathed from incomming air. If so how is lift produced during forward flight?

You can still stall the retreating blade - all you need is sufficient forward airspeed. Also, you can still have parts of the advancing blade go transonic - again, all you need is sufficient forward airspeed. And lift is produced in the same manner as ordinary helicopters, pretty much.

I don't doubt that a thing like those could be made to fly, but I am slightly skeptical about the advantages.

The "sheath" would cause enough drag and turbulence on its own to remove any possible benefits, I think, and probably create more problems than it fixes (and I don't think it would really fix those problems anyway).

I first misread the title as "stealthed rotors". So here is the stupid question, could such a setup be used to create stealthier helicopter that are less prone to doppler radar detection (not that this this would be needed much today).

Don't think so, since the radar looks down at the rotor disk.

I guess this is true. Even though the horizontal component of the radar vector is usualy bigger than the vertical, the sheath would have to be very high to mask the opposing end of the rotor disk.

The greatest disadvantage of such an aircraft is its width, making it harder or even impossible to vertically land/take off into/from small confined areas. Traditional rotor is much better in this case. Another thing is that helicopters with two lateral rotors are harder to control than with rotors mounted centrally.

If you necessarily have to build such a machine with covered rotors, I think it would be a better idea to have small multibladed propellers turning with high speed, more like a tiltable ducted fan, than classic rotors. Something like GDI Orca: http://media.moddb.com/images/mods/1/10/9011/Orca1.1.png

As for the movie, I have to admit, that they look just fantastic and that was the point od "designing" them ;) Compare to UDL Cheyenne dropship from Cameron's earlier movie, "Aliens". Totally un-aerodynamic, all lift generated by vectored thrust nozzles (thus inefficient), with this strange, enormous tail. http://sftdb.com/images/aa19860718dropship.jpg Still, it looked cool and I always wanted to take her for a spin :)

Sheathed Rotors

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"Sheathed Rotors"????, "Ducted Fans" would be more accurate, I think,

just words I know, but in technical discussions, correct terminology avoids confusion

Ducted Fans end up less efficiant than conventional Rotors in helicopters, simply because its impractical to make them of large diameter without a lot of weight and drag off the duct and as we all know, rotors and propellors are more efficiant when they are relatively large and slow turning

"Sheathed Rotors"????, "Ducted Fans" would be more accurate, I think,

just words I know, but in technical discussions, correct terminology avoids confusion

For the aircraft in Avatar, it would be more accurate to refer to them as "sheathed" rather than "ducted" since the "duct" is so small that it doesn't really qualify to be called a duct.

GDI Orca looks like a blend of A-10 and Osprey engines mounted on a helo fuselage.

For the aircraft in Avatar, it would be more accurate to refer to them as "sheathed" rather than "ducted" since the "duct" is so small that it doesn't really qualify to be called a duct.

Incorrect, its a duct (yes ive seen the film)

The term "sheath" in relation to rotary wing aircraft has only ever been used to describe a protective cover of some sort that is attached to individual blades, entirely different thing to a duct

The term "sheath" in relation to rotary wing aircraft has only ever been used to describe a protective cover of some sort that is attached to individual blades, entirely different thing to a duct

We just call those "blade covers". ;)

We just call those "blade covers". ;)

Nope, you call blade covers..blade covers...a sheath is something else... .......something like this..........http://www.navysbir.com/06_s/37.htm

;);)

Why do they use the word "sheath" on that page, since they are talking about something that clearly only covers the leading edge, not the entire blade.

What's really interesting is that in Avatar (just saw it) the helicopter-thingys have contra rotating, co-axial rotor systems within their ducts (admittedly thin rotor packages). The word 'why' springs to mind. ? Anyone with insight other than it looks cool? ;)

The most obvious reason for that is, that they cancel each other's gyroscopic effect, which causes additional force applied ninety degrees further along the edge of a rotating body, when you try to tilt that body. As seen in movie, pilot (surely with the help of hydraulic or electric actuators) tilts the whole "sheath" along with rotors, instead of shaping a rotor in a such way, that it wants to tilt by itself - as is done on real helicopters with the help of aerodynamics.

This cancelling effect would be then used to lessen forces acting on the joint between fuselage and sheath, when the sheath is tilted or when the whole aircraft maneuvers in space. For example it would be hard for the actuators to hold the commanded sheath's pitch while the whole a/c was being banked, if they've used single rotors only.

Why would that matter? Helicopters manage well with only single rotor - in fact that's the 'classic' way of building them. It's because the helicopters use free rotors (you have to apply only small force to turn the blades and the airflow does the hard work), while Avatar's are just contra-rotating propellers solid-mounted into a tiltable ring (meaning you need to put some serious force to tilt them) - anyone familiar with Mustang or Corsair know, how bad the propeller torque induced yaw can be.

I've watched Avatar's rotors closely and I didn't notice any behaviour suggesting that they have cyclic control. Also if they were, it would negate the whole sheath concept and would also cause a real danger of a collision between the blades and the boom (on which end the prop hubs were mounted and which also supported the whole sheath structure) when the rotor plane is tilted by cyclic.

Also they are too close to each other, to be free flapping rotors. All of us know, what happens with classic contra-rotating rotors in certain flight conditions ;) I've read somewhere, that it was Kamov's discovery, that you need to put a spacing between rotors being at least 10% of their diameter and that's certainly not the case with Avatar's ones.

While on the movie they sounded just like Blackhawk, I doubt it would be the case for real ones. Tu-95 is known for it's being enormously loud and Kamov's helos as well as Cessna 337 have very distinctive sound footprint. And as you know, noise generated by an aircraft is an important factor both in civilian and military applications (noise pollution and abatement procedures, detection probability).

I've mentioned earlier about some difficulties in controlling such aircraft, especially with regard to longtitudinal instability (in pitch), but also yaw stability would be negatively affected, compared to the classic helo. On the other hand, even now we have fly by wire, which can help in that matter, but that surely would make the design way more expensive, which is a serious factor in RL to be considered.

And why the sheaths are built like dihedral wings? To help stabilise aircraft in a hover. Similar concept is used on Harrier - while in hover, the jetstreams from the engine don't go vertically, but slightly outwards. Think of it, as of standing in a boat on rough seas. It's easier to maintain balance with legs spaced further, than kept close to each other (the pyramid principle).

I'm also higly sceptic about autorotating in such aircraft. While the combined rotor area could be sufficient enough, I don't think there is enough rotating mass there. Remember - hard-tilt instead of free rotor, so you want them to be as light as possible, which is further made worse beause individual blades are so short (=even less inertia driven angular momentum to keep them turning).

As was said earlier, from aerodynamic point of view, the sheath would cause more trouble then help and that's correct. Ducts are used on propellers and fans to reduce blade-tip vortices, which improves prop efficiency and lessens force needed to turn it. Think of it them as of rotary winglets, but they only work in a laminar airflow, which is more or less paralell to the axis of rotation. In Avatar a/c the airflow is more perpendicular to it and you end up with a nice air brakes. Theoretically the sheath could improve hovering out of ground effect, but I doubt that this single property would justify using it. Helicopters for the most of time either hover low or cruise high with the help of other helo specific effects like translational lift.

Also tail surface looks too small to be efficient (however I could be wrong here), but certainly the 'butterfly' (a.k.a. V-tail) tail assembly is not the best for a helo or any VTOL for that matter. But it looks cool and unusual and that's the reason behind it.

What I liked in this design (Aerospatiale SA-2 Samson and AT-99 Scorpion, to be specific), was that the engines were mounted into the fuselage itself, rather than like on Mi-12 or Osprey. It greatly improves balance and static stability (beacuse more mass is closer to the a/c center of gravity), as well as makes easier to propell both sides with one engine, in case of an engine failure (shorter shafts and simpler main gearbox). It makes also easier to install armour protection and IR suppressors over the engines, fuel lines also would be less exposed.

I could examine it further, but that's all that is on my mind now. I must say that I totally love the Avatar's design from the point of 'coolness' and indeed it would be possible to build one with current technology, but think about it: there is a deep reason why contemporary helicopters are built like they are and not in a different way.

Is anyone going to make "Na'vi war paint" skin for Ka-50 please? :music_whistling:

IIRC in some interview I saw the whole tech side of Avatar has been researched quite a bit, and all the vehicles and concepts are based on reality. Whether you believe that or not is up to you :)

I've read that interview also. As I've written above:

and indeed it would be possible to build one with current technology

Yes, it is based on a reality, or to be more specific on the ideas researched from the RL, like Osprey, Bell X-22, Mi-12 (not to mention, that the whole fuselage looks like a crossover between Gazelle or EC and Huey). Just remember, that Osprey isn't widely used around the world (even though it's prototype was flown some time ago) and there aren't other constructions of that type, as well as X-22 and Mi-12 never made any significant career.

Yet another explanation why it uses contra-rotating props. More thrust from the same area. The Avatar's a/c in question are rather wide and it would be reasonable to take efforts to minimise that, in order to improve maneuverability and capability to operate in small areas.