two bladed rotor with no teeter hinge

[slowrotor]

I have two radio control models with two bladed heads but no teeter hinge. The head itself is solid and the blades are flexible. What would this type of rotor be called?

I think it is a two bladed rigid or hingeless rotor. The rotor is common for RC models and allows full aerobatics without problems related to zero gravity loss of control with teetering heads.

Could I use this rotor for a full scale ultralight helo?

thanks
slowrotor

[NickLappos]

Often, pprunes confuse the type of rotorhead with the number of blades, which is simply not a factor. You hit the nail on the head when you describe the blade hinge type, which is the biggest single determinate.

A rigidly attached blade, with no flapping or lead-lag hinge, is commonly called a "rigid" rotor, or a hingeless rotor.

A balade that is free to move in both axies is called an articulated head, where we count the position of the flap hinge relative to the radius as the percent of hinge offset. Typical values range from 2 to 6%.

A head that is hinged at the mast, so that it gimbles or teeters is said to be teetering. It can be a teetering head with 2, 3 or 4 blades. For example, the head on the V-22 tilt rotor is a teetering head, since all three blades attach to a gimble ring that teeters about the mast.

[slowrotor]

thanks Nick,
I am wondering if there may be some reason why the simple head used by models (without the teetering hinge) would not be the best design for a simple slow ultralight. Are there any full size two bladed hingeless helos?

Actually, the models incorporate a blade attach bolt that serves as a lead lag hinge so I was wrong to call them hingeless. The flapping hinge (or lack of) is the focus of my question however.

My logic is this:
There is a principle called "the equivalence of flapping and feathering".
It might be possible to eliminate the flapping, provided feathering can serve the same purpose of controlling the dissymmetry of lift in forward flight. An ultralight is limited in speed by law and lack of power. So if cyclic feathering could work alone without the need for flapping up to about 50kts, the idea might have some merit.

If anyone knows why models work without flapping hinges, please advise.
It could be the flexible blades simply flap in bending at the root, or is it done with cyclic feathering? Or both?

Sorry for all the questions, but I need to figure this out before my design can progress.
thanks
slowrotor

[Dave_Jackson]

slowrotor,

A question ~ and then some rambling (which has not been totally thought out)

What is the approximate phase angle on the flight controls of your two R/C helicopters?

Theoretically, if the rotors were 'absolutely' rigid the phase angles should be close to 0º. If the rotors were teetering ones the phase angles should be 90º.
________________

Assuming that you were able to get a 2-bladed craft to fly properly without flapping, teetering and lead/lag hinges, there would be a large 2/rev vibration when the cyclic stick was moved off center.


Here are two web pages on an idea for consideration Offset Teetering Rotor (http://www.unicopter.com/1127.html) , Offset Bi-teetering Hinge (http://www.unicopter.com/1128.html )

The reason for suggesting this idea is that a small 5-6% flapping hinge offset should give the rotor more control and safety than that of a teetering hinge rotor. While at the same time, the 2/rev vibration should be less then that of a comparable very rigid rotor. Basically, it is a compromise.

The same results might also be achieved with a light hub spring on a teetering rotor.

Dave

[The Rotordog]

Ride quality is not so important in an RC helicopter

[slowrotor]

Dave,
Your question about phase angles for RC models is hard to answer. They all seem to incorporate the Hiller flying paddle control system for stability. The Hiller setup is needed because the 1300rpm main rotor would react faster than the pilot could handle. The servos are on each side, so the phase angle could be 0, 90 or 180. I have no idea. I would not use the Hiller system on a large ultralight helo rotor that turns slow.
You say a two bladed absolutely rigid rotor would have a two per rev vibration even with cyclic feathering? What is the cause? By absolutely rigid, I mean stiff in bending like an airplane wing but still able to bend a little. The cyclic feathering would replace the usual flapping if the principle of equivalence works.

Cierva did not have cyclic feathering when he invented the flapping hinge. He had ailerons on wings for roll control I think originally.

[Dave_Jackson]

slowrotor,You say a two bladed absolutely rigid rotor would have a two per rev vibration even with cyclic feathering? The following comments are based upon the application of forward cyclic and the 0º azimuth being located at the rear of the disk.

A teetering rotor will have the maximum pitch at 270º azimuth and the maximum upward flap (teeter) 90º later at 0º azimuth. The tip path plane of the disk is tipped forward and wanting to pull the craft forward. The center of thrust is located at the center of the disk.

An 'absolutely' rigid rotor will have the maximum pitch at 0º azimuth and a phase lag of 0º. The tip path plane of the disk remains normal to the mast. The center of thrust is located aft of the center of the disk because there is more thrust at the aft of the disk.

On the absolutely rigid rotor, when the two blades are at 180º & 0º azimuth they will be applying a pitching moment to the craft. One-quarter revolution later the two blades are at 270º & 90º azimuth and they will not be applying a pitching moment to the craft. Hence, the 2/rev vibration about the lateral axis of the craft.

A teetering rotor, which incorporates a hub spring (http://www.unicopter.com/B329.html#Hub_Spring) for better control, will experience the same 2/rev for basically the same reason.

To eliminate this oscillating moment AND have better controllability, plus safety, you will have to go to three or more blades.

Dave

PS How about cartage inflatable floats to get the allowable empty weight up from 254 lb to 284 lbs. ;)

[slowrotor]

thanks Dave, that makes sense. But if it works on models it might work on a slow ultralight as well. Or there may be other fixes instead of more blades to get the center of thrust back at the disc center.

Yes, the floats allow more empty weight under US regulation part 103.
I was told that any one seater can be overweight and the authorities will not care. It is the two seat ultralight that kills passengers that federal agents are concerned with. Hence the new Sport Pilot rules that will control two seaters.