ponderings.....

[julian.i]

hi
i have a question thats been bugging me,
i have heared people say counter-rotating blades on a heli are more efficient, is this true, as far as i understand the larger the prop the more efficient so the tail rotor hellis should win but then i am no expert, i know also that the counter rotating blades are normally a cambered airfoil wheres the tail rotor heli blades are normally symetrical, without this being a factor which is potentially more efficient. :confused:
thanks
julian

[Brandano]

It's always a tradeoff. That said, the earliest practical helicopters used counterrotating or intermeshing rotors. Using counterrotating rotors allows you to use a smaller diameter rotor, since you have more blade area. However it can't be half the diameter of a single rotor since the combined system isn't as efficient (the lower rotor is in the wake of the upper one) and you have twice as many rotor tips to deal with, with consequent drag losses. Control linkages are more complex, but you save weight and complexity since you don't have to deal with a tail rotor transmission.. and this is just the tip of the iceberg. The considerations and variables are so many that whole books could be written on what system is better suited for what task.

[biskit]

People often consider tandem rotor or co-ax rotors to be more efficient because all power goes into producing lift, while with a conventional tail rotor a significant portion of power is wasted producing lateral thrust instead of lift.

It all depends on how you define efficiency - there are many ways.

[Erich Preitler]

The real gain of koax-concepts is not efficiency, but stability. You can build a koax without any gyros, and if designed right it will fly stable - which is not possible with 1 rotor.

Erich

[vintage1]

The mere fact that in the real world, no coaxial helicopter exists, shows you that they are almost certainly not efficient at all.

Twin rotor designs like the Chinook, do exist.

[Brandano]

Vintage, I am not trying to undermine you, but.. http://en.wikipedia.org/wiki/Kamov
The naval designs have been flying for ages, and don't seem any less reliable than other designs. If kamov keeps on making coaxial designs it must have its own good reason...
And on the front of intermeshing rotors: http://en.wikipedia.org/wiki/Kaman_Aircraft
Dunno, must be something with manufacturers whose name starts with a K...

[vintage1]

Ah..forgot Russian stuff. But look how far apart the rotors are on them.

Which suggests that close coupled coaxials don't work well.

BTW I said nothing about reliability, only efficiency.

The Kaman is more of a Chinook with the rotors even closer, turned sideways ;)

The thing about that is the rotor blades never pass over each other. It's that which I believe is the main source of inefficiency.

[MCarlton]

As far as I know, or understand, the "older" Kaman helicopters had the coaxial layout so that the helicopter could have a shorter overall length, plus a higher deck clearance for operation from Frigates and the like. Also, if you look, many of them have a rear opening "clamshell" cargo bay, which would be an awkward s*d to arrange with a traditional tail rotor layout.

It also appears that co-axial rotors can offer some manoeverability advantages, as seen on the latest Ka5x series, I suspect this comes from the fact that rotor torque is automatically cancelled, and perhaps gyroscopic stability is reduced (eliminated?) think about holding a spinning bike wheel in your hands and trying to move it, the rotation works against you. Now, if you cancel the rotation by having an equal amount of rotation in the opposite direction, does it thus become easier to move?

Just a thought

[Brandano]

Nope, it doesn't become any easier to move. However, it doesn't tend to tilt at 90 degrees from the initial input anymore, but just tends to oppose it, wasting the energy as a bending moment on the rotor shaft.

[Cory]

Ah..forgot Russian stuff. But look how far apart the rotors are on them.

Which suggests that close coupled coaxials don't work well.

BTW I said nothing about reliability, only efficiency.

The Kaman is more of a Chinook with the rotors even closer, turned sideways ;)

The thing about that is the rotor blades never pass over each other. It's that which I believe is the main source of inefficiency.

My understanding is that the main reason for the vertical distance between the two sets of rotors is to prevent them from striking each other. Notice on the KA-50 that the space between the two rotors is similar to the space between the tail boom and the lower rotor. Blades flap up and down more than many people consider due to gusts and control inputs. Helicopters have experienced tail boom strikes from the main rotor. This will ruin your day. :eek: Equally, rotor to rotor strikes are not good either.


As we've discussed here in other threads, all design is a compromise. I do not know how fuel efficient the Kamovs are, but they are efficient in taking up less space on a ship, ability to land in small clearings, maneuverability at least in the yaw axis, lack of loss of tail rotor effectiveness in quartering tailwinds, and have a higher top speed than most other helicopters. The Ka-50 is quite maneuverable in all axis. Ka-50 on Wikipedia (http://en.wikipedia.org/wiki/Kamov_Ka-50)

Edit: That article is an example of the failings of Wikipedia. It states that low advancing rotor tip speed allows a helicopter to fly faster. That is false. Low advancing tip speed results in even lower retreating blade airspeed. This requires an increased angle of attack to maintain lift on the retreating side of the rotor disk. Too much AOA = retreating blade stall. Other than that, however, that article seems to pretty accurately reflect what I had read about the Ka-50 in Aviation Week a few years back.

I'm not saying that I think they are the best design out there, but since all design is compromise, they are very good at what they are designed to do.

[Brandano]

The retreating blade stall on a coax rotor isn't as important as in a conventional helicopter, as long as the advancing blade is producing enough lift to maintain the helicopter airborne the fact that the retreating blade is stalled doesn't affect too much the flight of the helicopter. On a conventional machine this would cause a roll in the direction of the retreating blade, and loss of control at high speed. This was also investigated by Sykorsky in its s69 prototype: http://en.wikipedia.org/wiki/Sikorsky_S-69 . Note also the use of rigid rotors instead of articulated rotors to allow for more narrowly spaced disks.
[edit:] oh, and apparently the X2 (http://en.wikipedia.org/wiki/Sikorsky_X2) has made its first flight:
http://www.flightglobal.com/articles/2008/08/28/315257/sikorskys-x2-speedster-completes-first-flight.html

[Cory]

Right, I agree that the retreating blade stall doesn't have the same effect on a coax as on a conventional rotor system. I just thought that the way the wording was in the Wikipedia article didn't accurately describe the effect.

The rigid rotor system does allow for less space between the rotor disks, but it still isn't flex free. That's why you don't see the two rotor disks as closely spaced as the counter-rotating props on some airplanes.

[biber]

Brandano, I think a stalling retreating Blade will make the rotor disc nose up, rather than induce roll.

biber

[Brandano]

Nope, because the stall happens on the sides of the helicopter, and not behind the helicopter. It's also fully balanced by the advancing blade on the other disk. Effectively, the retreating blade is unloaded so that no stall as such happens, it just doesn't produce any lift.

[biber]

But generally less lift on a part of rotor means the part 90 degrees later will move down because of precession.
I don't see where precession does not apply in coaxial rotors.

biber

[biskit]

"Precession" is not always 90 degrees, it varies considerably based on the design of the rotor system. Aircraft exist with angles from as low as 40deg up to 180deg. The 1-sentence explanation is that a rotor system is not a perfect gyroscope because it flexes(gyros are rigid bodies) and it has huge aerodynamic forces on it. In the engineering world it isn't even called "precession", it is called the phase angle. Additionally, the phase angle changes based on loading, aerodynamics(airspeed, atmospheric conditions, etc), and other things. An interesting side note, the US Army Commanche(?) had fly-by-wire controls and the phase angle was continuously adjusted in flight by computers to make the pilot inputs true(a roll input produced only roll, etc) within the entire flight envelope.

As far as retreating tip stall, it doesn't happen at a single point on a rotor disc, a large area is stalled and all (single rotor)helicopters will roll to the stall side and pitch up, how much roll and pitch depends on conditions, the aircraft, and a hundred other things.