Helicopter with Main Rotor Below the Cabin

skeptic2

Overlooking the difficulty in landing such a craft, would a helicopter with the main rotor below the center of mass, differ significantly in stability from normal one with the main rotor above the center of mass?

mgb_phys

Yes - technically speaking the stability would be a right bugger.

Emicro

Think of it like this, it's like standing on a pencil that is stood on it's end.

berkeman

At least it makes an ejection seat option easier (you don't need explosive bolts in the rotor blade assembly).

rcgldr

It would be less stable, but radio control helicopters flown upside down don't seem significantly less stable because the angular momentum of the main rotor is the dominant factor. The center of mass is set to be very close to the axis line of the main rotor, which reduces any torque reaction related to up thrust or down thrust from the main rotor.

boit

Yep. I remember seeing a documentary on the tv. There was this early model a guy was flying while standing on the craft. It was a public demonstration in a stadium. He was flying so close to the ground it reminded me of a lawnmower. I understand the americans settled for Zarkorzy's? roter over cabin design.

Phrak

Why would it be unstable? There are low wing aircraft...

sophiecentaur

boit: Sikorsky was the helicopter guy.

Once airborne, the blades on a full size helicopter curve upwards quite a lot so the centre of effort could be arranged to be above the centre of mass whilst in flight.

BUT one snag with a rotor below the fuselage would, surely, be finding somewhere to put the wheels!

skeptic2

I am wondering if a pendulum effect, in which the center of mass is below the center thrust, helps to stabilize a craft when hovering. At higher velocities the location of the center of pressure becomes more important and at higher accelerations the pendulum effect becomes insignificant. Is this the dominant stabilizing force for a hovering craft and if not, what is?

sophiecentaur

Your "pendulum" effect may help a bit bit they are, inherently, not very stable wrt any axis.
I just think they're v. difficult to fly and hovering is the worst.
If the machine is not designed with the cm on the rotor axis, with the fuselage horizontal, then it will tilt until the cm is below the rotor spindle and will need to be compensated for by the blade angle controls if you want to be horizontal.
But it's normal to get the trim of all aircraft (and boats) about right before you take start off, isn't it?

mgb_phys

It's very similar to the stability problem of a boat - where you need the center of boyancy above the center of mass.

Imagine the weight at the centre of mass of the cabin pulling down and the lift at the centre of the rotor pushing up.
If the rotor is on top it's like hanging the weight from a rope, if the cabin tilts to the side then it's own weight will pull it back down into line - while the rotors will pull back up and straight.
If the rotor is on the bottom it's trying to increase the tilt, as is the weight - and it will flip over.

It works for model helicopter because the cabin is much lighter (compared to scale) and the rotors are much heavier

sophiecentaur

Yes - true but the blades curve upwards* when providing lift (they are flexible) and the cm will be lower, wrt the centre of effort than you might imagine.
*they may curve up much more than they droop down when 'resting' - which is an appreciable amount in pictures of some helicopters I've seen.

BTW, I can't imagine anyone is being really serious about this idea!

skeptic2

Of course not.

I was trying to look at the pendulum fallacy from a different perspective. In another thread "Rocket Idea: Bell Shaped Pendulum" posts #1, #3, #4 & #5, the claim is made that positioning the mass below the thruster would not be effective in stabilizing a lunar lander due to the pendulum fallacy. I understand that the pendulum effect (my term) would not be effective when a rocket is traveling through the atmosphere at velocity or at high acceleration, but in the case a hovering lunar lander I do not understand why it would not add stability. I am not suggesting that if the lunar lander is wildly out of control, the pendulum effect is all that would be needed to stabilize it.

RonL

I think you will find this idea has not only been looked at in a very serious way, but has in fact been quite successful. Look at the "Hiller flying platform" and many other video's on u-tube.
Your comment about the seriousness of an idea, sounds much the same as the wall of resistance that, Sir Frank Whittle, ran into. there is always a chance old ideas might once again become new.

sophiecentaur

I imagine a "serious" proposal would include a mention of where the wheels go. Please enlighten me. I have tried drawing one and there seems to be a topological problem.

RonL

Two links that give some idea of what has been done. It seems that discovery channel may have had u tube remove some videos, as all I could find are poor quality.
The Blakebrough's winged shelf at least gives two views of less quality test flights. If I can find the better discovery clip, I will try to post it.

It seems to me that a larger platform and rotor blades could be a possible design that would give a more stable machine and using lipo batteries attached to the ring, an electric power setup could keep it pretty quite.

http://www.hiller.org/flying-platform.shtml

http://www.youtube.com/watch?v=tR1QI4wUbPc

sophiecentaur

The machines shown in the links aren't so much helicopters as hovercraft plus a bit. That actual flying demo seems to require ground effect (it only gets a metre or so high).
To lift a serious payload out of ground effect and to hover, you need larger rotor diameter than that, surely, and then my "where to put the wheels" question would still need to be answered.
With the cm above the rotor, you must also be in a seriously unstable situation - as was the 'flying bedstead' and the Lunar lander. There's no 'pendulum effect' from those arrangements ( no restoring force when perturbed). I think the same problem must exist even with the James Bond 'jet pack'.
With modern control systems this needn't be too much of a problem if there happen to be other advantages. All rockets need stabilising until they are moving through the air fairly fast and that is achieved reasonably well, these days.
But this thread has developed two issues - the stability and the feasibility of such a system. They are getting a bit mixed up, I think.