is a two blades rotor with offset hinges possible?

[quadrirotor]

Long time ago, i was told that a rotor with two blades with offset hinges is not possible. As you know, offset hinges could be a solution to all the stability problems of the gyro. So my question is: is it possible or how can it be possible to have offset hinges? Futhermore, Dick Degraw used them for his Syncopter...

[Vance]

As you know, offset hinges could be a solution to all the stability problems of the gyro. ...

I would be gratefull for help in understanding how offset hinges could be a solution to all the stability problems of the gyro. Thank you, Vance

[quadrirotor]

Because with offset hinges, you can generate a moment to stabilize the gyro! this moment can't be generated by a rocking head! hinges blades are used with three bladed (and more) rotors, i wonder if it's possible to have a two bladed rotor with hinges blades, as most of whom are interested in gyro, want two bladed rotor because they are more easy to store!

[banaari]

Hmmm... *Conventionally* in a gyro, that angular difference between spindle and disc-plane is the fundamental control mechanism by which the rotor disc is steered; i.e. the disc tracks the spindle.

If (and only if) the force applied to the spindle by the disc is enough to usefully stabilise the airframe then you'd have to employ a different means of steering the blades; probably a fully-blown cyclic system... complex.

[MichaelBurton]

The Hawk 4 from GBA uses a two blade fully articulated rotor system. This does not solve all of the stability problems but it does reduce the lag time between the input and the gyro reaction.

[quadrirotor]

Michael; The GBA has not a offset hinged blades rotor!
Banaari; "Things must be simple, not more" Einstein.

[quadrirotor]

I am looking for this with only two blades, is it possible? (Hughes 269)

[KenSandyEggo]

Notice the similarities? Same designer for the J-2.

[quadrirotor]

I found this one too but nothing about its rotor. (Fairey, jet-gyrodyne, 1953) and the french Djinn.

[C. Beaty]

Here are a couple of pictures extracted from fuzzy, grainy, faded out 8mm movie film.

The pictures were taken at Bensen Days, 1974.

The 3-blade rotor had neither flap nor drag hinges; the hub was a triangular aluminum plate with blade feathering bearings attached to each apex and itself attached to the fixed rotorhead via Chevrolet engine mounts.

If the hub can self-align with the rotor tip plane axis, there is no absolute need for drag hinges.

I built the thing mostly to get under Martin Hollmann’s skin who had said such a scheme couldn’t possibly work.

Martin had gathered up an audience and was explaining how my rotor was going to fling itself asunder just as I came motoring sedately past.

[spaceman spiff]

"Martin had gathered up an audience and was explaining how my rotor was going to fling itself asunder just as I came motoring sedately past."

Love it when that happens. :D

[quadrirotor]

OK, Chuck Beaty, but can we have only two hinged blades?

[C. Beaty]

André, you’re barking up the wrong tree.

Increasing control power by coupling the airframe to the gyroscopic forces of the rotor makes a gyro controllable during periods of reduced or zero rotor thrust but does not make it more stable; quite the contrary.

Bell thoroughly investigated using torsion bars instead of normal teeter bearings to increase control power. They did substantially increase the control power at the cost of increased vibration. I have a copy of the Bell paper around here somewhere.

[quadrirotor]

What about the system of the Syncopter of Dick Degraw?

[C. Beaty]

I don’t know why Dick DeG. used offset flap hinges on his syncopter and I doubt if he does either. In any case, the offset isn’t enough to increase control power significantly.

A syncopter is inherently stable in pitch because with skewed rotor shafts, a component of rotor torque appears as a nose up force, requiring forward stick to keep it level. That forces the CG to always be forward of the rotor thrust vector. That’s because outboard blade motion is rearward; with reversed rotation, a syncopter becomes unstable.

A pure coaxial helicopter has no residual rotor torque.

RC model helicopters can be flown upside in spite of teetering rotors because there are rubber snubbers that can be tightened enough to make it nearly a rigid rotor. But nobody has to ride in one of these things so there’s no problem as long as rotor vibration doesn’t shake the radio apart.

[skyguynca]

Chuck, I would luv to hear more about your head, looks great!!!! How did it fly? vibrations? Tell us more!

[Georgi]

Skyguynca,Chuck's head is just fine.That is why he can answer all our questions and more...And now a weird question to Chuck. Chuck,has anybody ever tried to coax add a short enough gyro rotor (free or partially powered) to a helicopter? (Of course,rotors planes should be well separated: they are crossing each other.)
Georgi.

[C. Beaty]

Here’s another 3 blade rotor on a Rotax 447 powered highrider taken at Bensen Days, 1990.

This was intended to be a jump takeoff machine but it was soon evident that without a feathering prop, the 447 didn’t have enough extra power to get the rotor to jump speed.

Wood spar blades with 10 lb. lead slugs in each tip.

The rotorhead was similar to the one shown previously from 1974 except the hub was filament wound fiberglass. It was connected to the fixed rotorhead by 6 tapered shock absorber style bushings; 3 pairs of 2 each.

Properly adjusted 3-blade rotors by nature are glass smooth. The problem is proper adjustment. Without drag hinges and fixed in-plane; getting the blades set to exactly 120º apart is a chore.

These pictures were originally shot of Sony Beta tape and transferred to VHS, so the sharpness is little better than the 8-mm film.

The reason I have this stuff handy is that I’m trying to get a ton of old tapes and film burnt on DVD. I even have some grainy film of the Japanese Army playing with gyros during WW II.

[C. Beaty]

Georgi, Bensen had a coax helicopter using 2 different rotor diameters.

Each rotor was driven from opposite ends of a differential so there was no torque exerted against the airframe.

The reason for the torqueless drive was to enable the use of a tilt head system for cyclic control. Otherwise, if torque must be reacted out through gimbel pivots, a component will end up snatching the stick out of your hand.

[Georgi]

Thanks,Chuck.I was just thinking for a second about situation when helicopter tail rotor or main transmission fails and a gyro rotor saves the day.
Georgi.

[C. Beaty]

That would have worked for Bensen’s coax helicopter rotor, Georgi.

If I remember correctly, the larger rotor was set for autorotative pitch and collective was applied to the smaller rotor.

[skyguynca]

Chuck, I would really like to talk to you about the head, I am working toward something like what you are describing. I am interested in you design and its operation. Please contact me off list, thanks

[skyguynca]

I never said anything was wrong with it Georgi, I was just asking for more info.
Skyguynca,Chuck's head is just fine.That is why he can answer all our questions and more...And now a weird question to Chuck. Chuck,has anybody ever tried to coax add a short enough gyro rotor (free or partially powered) to a helicopter? (Of course,rotors planes should be well separated: they are crossing each other.)
Georgi.

[quadrirotor]

What about this one?

[C. Beaty]

I’m not sure what I’m looking at, André.

It appears to be a syncopter transmission with very close rotor spacing. I can make out drag hinges but don’t have a clue as to the location of flap hinges.

In any case, with rotors that close together, offset flap hinges are necessary to keep the rotors from fouling one another.

I’m still not sure just what it is you’re driving at. Of course offset flap hinges can be made to work on a 2-blade rotor if you’re willing to tolerate the accompanying shake.

I have in my hand a paper written by Walter Sonneborn and Jing Yen of Bell Helicopter, titled: “Hub Moment Springs on Two-bladed Teetering Rotors.” It was published in the Journal of the American Helicopter Society as #B-74-WE-19-3000.

Their conclusions were:

(1) “Two bladed rotors with hub restraint are suitable for zero-g flight.

(2) Hub restraint which added some 27% to the one-g control power of an OH-58A helicopter with a Bell Model 640 rotor caused a negligible increase in 2/rev vibration during hover and level flight.

(3) The 2/rev oscillatory moment component due to hub restraint in a two-bladed rotor can be balanced about a point below the rotor hub by additional rotor underslinging. The amount of this underslinging depends on the location of the natural frequency of the first cantilevered inplane blade mode.”

By whatever means the rotor is tied to the airframe, the end result is the same. I gather some of the resulting shake can be balanced by increasing the undersling in the case of a teetering rotor with moment springs.

All schemes of increasing control power with 2-blade rotors require the use of fixed rotorheads and some kind of feathering cyclic control.

The basic problem with many gyros is instability rather than the lack of control power during reduced g maneuvers. A stock RAF-2000 would still be unstable and difficult to fly even if the airframe was more closely coupled to the rotor.

Stable gyros don’t go about tumbling out of the air so stability is an issue that ought to be addressed before we worry about more esoteric things.

[C. Beaty]

Skyguy, the attachment shows a cutaway of a Bell-47 rotorblade.

That is the general scheme I followed with my wood spar blades except for the use of a modern airfoil.

The steel balance weight plays no structural role but its own CF is carried by the root fittings.

The balsa afterbody is sawn from balsa planks with the grain running top to bottom.

Yellow birch has adequate strength for the spar, the handbooks giving a tensile strength of 16,600 psi for short, straight grain samples. The challenge is developing adequate strength in the spar to root fitting junction.

This is dangerous stuff to be playing with.

[skyguynca]

Thanks for the picture and the advise. I am not taking it lightly at all believe me. I am just collecting a ton of info really. I will not build a set just love reading and learning about the development of the blades over the years. Very very interesting stuff.

[Georgi]

Hi,Skyguynca.I just tried to make a light joke:I talked about Chuck's anatomical head and not a rotor head he designed for his gyro (my russian sense of humor). Hence, my apology to you.
Georgi.

[rtfm]

Here’s another 3 blade rotor on a Rotax 447 powered highrider taken at Bensen Days, 1990.

Properly adjusted 3-blade rotors by nature are glass smooth. The problem is proper adjustment. Without drag hinges and fixed in-plane; getting the blades set to exactly 120º apart is a chore.
Hi,
Correct me if I'm wrong (as I probably am...) but if 3-bladed rotors are so smoothe, why are all gyros not using them? Surely setting them at 120º can't be that hard? And wouldn't it be worth the effort? No rotor shake, smaller diameter blades?

I'd be very interested in more info?

Regards,
Duncan

[rtfm]

Skyguy, the attachment shows a cutaway of a Bell-47 rotorblade.

That is the general scheme I followed with my wood spar blades except for the use of a modern airfoil.
Hi,
It strikes me that a material like Graphlite carbon fibre rods might make an extremely light and rigid rotor blade. Has anyone built rotor blades using Graphlite?

Regards,
Duncan
BTW, which airfoil did you use for your blades?
Duncan

[rtfm]

I am looking for this with only two blades, is it possible? (Hughes 269)
Hi,
Why stick to 2 blades? If (as C. Beatty suggests) 3-bladed rotors are "glass smoothe" why not use three?

Duncan

[rtfm]

The 3-blade rotor had neither flap nor drag hinges; the hub was a triangular aluminum plate with blade feathering bearings attached to each apex and itself attached to the fixed rotorhead via Chevrolet engine mounts.

If the hub can self-align with the rotor tip plane axis, there is no absolute need for drag hinges.

I built the thing mostly to get under Martin Hollmann’s skin who had said such a scheme couldn’t possibly work.

Martin had gathered up an audience and was explaining how my rotor was going to fling itself asunder just as I came motoring sedately past.
Hi,
What was wrong with the system? Obviously there was something less than ideal with it, or you'd be flying something similar today. This is new territory for me, so please don't take my enquiries as anything other than curiosity. It just seems to me that if a glass-smoothe 3-bladed rotor system could be used, there would be more lift, less shake - what's not to like?

Regards,
Duncan

[C. Beaty]

What’s wrong with a 3-blade system?

(1) Complexity

(2) Space hog

(3) The unknown: This was the fly in the ointment. Without drag hinges, in-plane resonances are possible that are not apparent to the operator. I wasn’t prepared to fly one of these things much higher than I was willing to fall without performing a strain gauge survey. I had obtained strain gauges but never got around to finishing the job.

By lift you mean lift to drag ratio. If a gyro will leave the ground, it has enough lift. But many gyros that will leave the ground have rotors so draggy that performance is marginal.

A 3-blade rotor doesn’t necessarily have less drag than a 2-blade rotor. It’s mostly a function of total blade area, the primary factor controlling rotor tip speed and therefore profile drag.

Properly designed 2-blade rotors are acceptably smooth.

The Dreadnought class gyros, which need more blade area to get rotor tip speed down to sane values, could benefit from 3-blade rotors. But I’m not personally interested in Dreadnoughts. To my way of thinking, a gyro that tips the scales at more than 250 lb. is overweight.

[skyguynca]

C. Beaty, do you have any drawings or such for the rotorhead? Or could you describe it more. I am fascinated with using chevy motor mounts on the ridgid mount to the mast to fasten to a trangulated plate with just feathering bearings. So the plate with the feathering bearings would not only rotate alittle fwd and aft in plane but also allow flapping? The pictures just made me more interested in seeing it.

[C. Beaty]

Here are a couple of photos of the hub. It consisted of 4 each 3/32” aluminum sheets structurally bonded together.

The Chevy engine mounts are antiques used on stovebolt sixes. Just rubber donuts.

The same rotor system was later used on a small helicopter that flew fairly well.

As originally laid out for a gyro, I had given no thought about transmitting 400 ft-lb of torque through the engine mounts so machined up Teflon plugs to carry the torque as a stopgap.

Rotorblades don’t flap or drag except in the imagination of the beholder. The blade tips run in a perfectly logical circular orbit.

The illusion of flapping results from the rotor tips rotating about a different axis than the rotorhead. In that case, flap and drag hinges are merely universal joints accommodating the different axes.

When the hub is free to self align with the plane of the rotor, the need for individual universal joints vanishes. The rubber donuts become a universal joint that permits the axis of the rotor plane to differ from that of the rotorhead.

[rtfm]

Hi,
OK, an obvious question, but one for which I don't readily have an answer, so perhaps someone more knowledgeable can help me out here?

Why do rotor blades have such narrow chords? Most seem about 7" or 8" wide. Why not make then 15" for example?

Regards,
Duncan

[rtfm]

Hi,
Another question: If space is a consideration in the choice of a three-bladed rotor, why not mount the blades on bushes or bearings, so that they are free to fold when stationary? As soon as they spin up they are going to align themselves as a result of the centrifugal force anyway...

Duncan

[rtfm]

Hi,
A third question:
Since the airspeed over a rotor blade differs depending on its distance from the hub, why are rotor blades not designed with different airfoils depending on their distance from the hub? EG A high-lift airfoil close in where the airspeed is lower, progressing to a airfoil optimised for higher speed further out towards the tip. One might also think of wider cords closer in, narrowing as distance from the hub increases. Props are made that way, why not rotor blades?

Regards,
Duncan

[KenSandyEggo]

Because.......X3. :confused:

[skyguynca]

Thanks C. Beaty, that is awesome!

[C. Beaty]

Duncan of Devonport, you sure have a bag of questions.

(1) Rotorblade chord: Rotorblade area, i.e., blade loading sets the tip speed. The tip speed is approximately 66*(blade loading)^0.5. A blade loading 35lb/ft² results in a tip speed of ~390 fps. A practical upper speed limit is about 35% of rotor peripheral speed at which point, the cyclic stick will be on or near the forward stop. So, a blade loading of 35 lb/ft² gives a rotor limited top speed of 93 mph.

If you must go faster, you need less blade area.

In the case of 2-blade rotors, the moment of inertia about the feathering axis causes a 2/rev stick shake as a result of cyclic pitching.

It is a steady force resisting control input in the case of a 3-blade rotor.

(2) Blade folding: A couple of pre WWII Autogiros did have blade folding so it’s doable if someone wanted to go to the trouble. Naval helicopters nearly always employ blade folding so they’ll fit on the lifts.

(3) Airfoils: The military, for whom money grows on trees, do in fact use rotorblade airfoils optimized for prevailing conditions along the blade radius. Feasible only for plastic blades and then only when cost is no object.

[rtfm]

Duncan of Devonport, you sure have a bag of questions. Yes, sorry, mate - I just find this fascinating, and can't stop... Thank you for taking the trouble to answer.

(1) Rotorblade chord: Rotorblade area, i.e., blade loading sets the tip speed. The tip speed is approximately 66*(blade loading)^0.5. A blade loading 35lb/ft² results in a tip speed of ~390 fps.
Could you explain the formula above? Where does the 66 come from? And what is the "^0.5" mean? The square root? If so, square rt of 66*35 is approx 48.

I assume that I am wrong in this, so will pass over it and move on to your conclusion. A blade loading 35lb/ft² results in a tip speed of ~390 fps. Is this dependant on blade length? I guess it is, because this would affect blade area (and disk loading). Am I correct, then, in assuming that a four-bladed rotor system would turn half as fast?

A practical upper speed limit is about 35% of rotor peripheral speed at which point, the cyclic stick will be on or near the forward stop. So, a blade loading of 35 lb/ft² gives a rotor limited top speed of 93 mph.
If this is the case, and if my assumption above is correct, slowing the rotor down would decrease the craft's upper speed limit? But this can't be right, because the Carter Copter slow their rotors in an effort to fly more quickly. I'm confused...

And what if the gyro had wings which progressively begin to provide lift as speed increases. Rotor speed is dependent on the load it has to bear. So as the weight decreases, the rotors slow down. Am I correct in this? If so, this too would decrease the max speed of the aircraft. Surely there is something else, because the CC seems to prove otherwise. Max flying speed can't be limited by rotor tip speed only...


If you must go faster, you need less blade area. So this isn't necessarily true?

Thank you again for your help. This is a most intriguing thread.

Regards,
Duncan

[skyguynca]

I just love these discussions, feeds my fire for knowledge.

[C. Beaty]

66*(blade loading)^0.5 means 66 time the square root of blade loading. The square root of 35 is 5.916 and that multiplied by 66 equals 390.5 fps.

The 66 is a consolidation of the constants used to calculate lift; air density, lift coefficient, etc.

Going to a 4-blade rotor from a 2-blade rotor with identical blades would reduce rotor rpm by the square root of 0.5 which is 0.707. A four-blade rotor would turn 71% as fast.

The Carter Copter at speeds above 100 mph or whatever is flying on the wing rather than on the rotor. There, the rotor is just in the way and the more slowly it turns; the less power it eats.

For normal rotorcraft, those without wings or gas filled balloons, top speed is limited to some fraction of rotor tip speed; specifically, retreating blade stall. Some of the old Autogiros were flown as fast as 50% of rotor tip speed but 35% is a more reasonable limit.

[skyguynca]

I have some pictures you sent of the is head assembled and arrows showing what it does, very nice and thanks. I also have a picture of it flying on a helicopter too, very impressive. Was the head removed from service for any specific reason or just "because"? The reason I ask is that if you were to use modern lycoming mounts like those used on a 182 instead of the chevy mounts I think they would be a bit stronger with a longer life possibly. The head looks great and is very simple, something we all need. Its versatility for both gyro and helicopter use would be outstanding and it definitely is a giant step in the right direction away from the complex ideas that are out there.

[C. Beaty]

Here’s a photo of the very first liftoff of the helicopter using the aforementioned rotorhead, about 1976.

It was powered by a 440-cc Canadian Kohler snowmobile engine.

I purchased the engine as an OEM product direct from Kohler and it came without exhaust system, carburetors or a handbook.

The engine eventually threw a rod through the side of the crankcase and I gave up helicopters.

It was really quite an advanced engine but probably wasn’t intended to be run with crankshaft vertical. The big end of the rod, riding on the crank cheek overheated and the rollers looked like pieces of bread dough that had been rolled between the palms of your hands.

Outboard engines have serrated faces on connecting rod big ends to wedge oil. And they don't turn as fast.

[rtfm]

I have some pictures you sent of the is head assembled and arrows showing what it does, very nice and thanks. I also have a picture of it flying on a helicopter too, very impressive.
Hi,
Care to share? I'd be very interested in taking a look.
The head looks great and is very simple, something we all need. Its versatility for both gyro and helicopter use would be outstanding and it definitely is a giant step in the right direction away from the complex ideas that are out there.
Do you think it might be an honest contender in the quest for a 3-bladed system on gyros? I have to confess, that even though I've read and re-read the posts about this head, and studied the pictures of the disassembled head carefully, I can't follow the logic of how it works. In particular, I'm trying to get my head round Chuck's When the hub is free to self align with the plane of the rotor, the need for individual universal joints vanishes.
Would you care to play the teacher and help me out? If I figure this right, the head is a simple, rigid triangular plate, with three blades attached to it by rubber donuts. The blades don't flap. Why/how does this work? I thought blades HAD to flap in order to equalise the lift produced by advancing and retreating blades...

Regards,
Duncan

[rtfm]

66*(blade loading)^0.5 means 66 time the square root of blade loading. The square root of 35 is 5.916 and that multiplied by 66 equals 390.5 fps.

The 66 is a consolidation of the constants used to calculate lift; air density, lift coefficient, etc.
Thanks, Chuck. I knew there was an easy answer.

For normal rotorcraft, those without wings or gas filled balloons,
Do I notice just a hint of irony in this? ;)

Regards,
Duncan

[C. Beaty]

Here is a filament wound version of the rotorhead above.

The stem projecting through the hub center, on a ball bushing, is tilted for cyclic and raised or lowered for collective.

The spider atop the rotorhead is the equivalent of a swashplate.

Cyclic feathering and cyclic flapping are one and the same. In forward flight, the rotor “flaps” when viewed along the rotorhead axis but this is virtual flapping that exists only in the mind of the observer.

Viewed along the axis of the rotor disc, there is no flapping but there is a cyclic pitch variation exactly equal to the apparent flapping.

But don’t feel bad if this is confusing. Martin Hollmann, an aeronautical engineer and self-proclaimed rotorcraft expert, couldn’t understand it either; at least not 30 years ago.

Don’t think for a minute this stuff is simple. A couple of tons of centrifugal force trying to fling the blades into orbit, the need to be able to oscillate the blades in pitch at a torque of no more than a few inch-pounds, aerodynamic drag variation as the thing rotates trying to excite interblade resonances and a host of other things don’t make life simple. The fact that Bensen’s scheme is about the only rotor system seen on homebuilt gyros is no accident.

[skyguynca]

that is the other picture I have. Chuck B. how come you stopped with this head?

[C. Beaty]

I still have this head and the gyro that came with it.

I do this stuff solely for my own entertainment and move on to something else when I lose interest. I wouldn't dare sell or even give these things away. Somebody would splatter and then the lawyers would descend.

[skyguynca]

That is great, THANKS CHUCK B.!!!!!!!! I would luv to see the head and gyro in person, or would luv a ton of pics emailed to me or posted here. This is the kind of stuff I am interested in........One offs that work but were never pursued. Alot of these are out there (one off's I mean) that are simple and would work. Even if the rubber motormount dampeners and such need to be replaced every 50 or 60 flight hrs, that is fine, it is a better head and we know we could all use it. I would luv to see the simple yet "Better Head" out there for us, the gyro community. Chuck you have alot of practical experience and knowledge, these pictures just fire the imagination and the movtivation to do more.

Thanks for sharing Chuck!!!!!!!!!!!!!!!

[skyguynca]

Chuck, what blades did you use on the gyro installation of this head? Did you use different blades when you flew it on the helicopter? How many flights did you do, gyro? helicopter? Was there any particular reason why you stopped experimenting with it?

Thanks

[C. Beaty]

The first 3-blade rotor, flown during Bensen Days ’74, only made 4 or 5 flights before I pulled the rotorhead and reinstalled the Bensen style rotor. I made 3 flights, Ken Brock made 1 flight and a friend, Gary Yanson, made 1 flight.

The helicopter of ’76 was first flown in my parking lot for 5 minutes or so and then had the blades removed and was taken to Bensen Days ’76 at Dunnellon, FL. There it made a half dozen flights before the connecting rod went through the side of the crankcase.

Both versions used runout Hughes ex-military OH-6 rotorblades. These blades have an NACA 0015 airfoil of 7” chord, not the greatest for either gyros or helicopters.

The later gyro with wood spar blades used a Boeing VR-7 airfoil of 6” chord. I still have those blades. It was flown at altitudes of a few feet for a couple of years; total time of perhaps an hour or two.

I’m not nearly as brave as some people and don’t fly at potentially lethal height without knowing the stress level of critical parts.

Three blade hingeless rotors can have in-plane resonances, similar to a tuning fork, that are not apparent to the pilot. Without a strain gauge survey, it’s Russian Roulette. No matter how much calculation has been done or by whom.

The nice thing about see saw rotors is that inplane resonances announce themselves. There is always a residual force that shakes the rotorhead. A straight, unsupported rod has no resonance that places a node (a point of zero translational motion) at its center.

[skyguynca]

Wow, thanks for the info Chuck