3 or more blades worth the effort??

[Jim]

In reply #30 under “Theory of Flight” > Two-per-rev- shake, Chuck Beaty gives a significant answer:
“Two/rev rotor shake is probably the single most frustrating and intractable problem afflicting gyros with seesaw rotors.”

“The large gyros, 1,000 lb. and up, present a far more difficult problem than the lightweights. The big ones suffer from poor rotor efficiency and would benefit immensely from lower blade loading. But increasing blade chord much beyond the present 9” or so would compound rotor shake problems. Rotors in forward flight are subject not only to periodic aerodynamic forces but the blades must undergo cyclic pitch variations. Increasing the moment of inertia of the rotor about the feathering axis severely compounds the problem. Two/rev stick shake from feathering axis cyclic motion becomes a steady force with 3 or more blades.

Three blade rotors anyone?”

That brings me to a question that has been in my mind for a long time: Are three and four blades much better?
I know that respondents will be quick to show the simplicity in construction and convenience in storing 2-bladed rotors. And yet....As recently as April 21 Ken Rehler wrote: “So far the world record for non stop flight in a gyro is less than 700 miles. And that has been hard to beat.” Our machines are extremely inefficient. Any discussion to better the performance is therefore legitimate.

Lester W. Garber, Ph.D. and Bill Friedlander, Ph.D did wind tunnel tests on RC gyros. The basics of this Wind Tunnel Test project has been published in the August 2001 Issue of RCModeler. Part if this article can be read in: http://www.autogyro.com/technic/tun_test.htm Has someone the full text? I would love to see it!

They conducted their tests on 44" models – thus about 1/6 scale of what we would use. A little small, but still interesting.

Their first conclusion is: Rotors containing 4 blades perform significantly better than those with 2 or 3 blades. 3 blades are much better than 2, with 4 being slightly better than 3.... You may read the rest of their findings on their site.

Now: before we venture into the tedious work of building 3 or 4 bladed hubs, with all the problems associated with lead-lag hinges, and less simplicity will someone tell us if their conclusion is really valid? Especially:
"3 blades are much better than 2..."

Thanks :D

Jim

[Spaced]

Whith all the discussion relating to controlability and PIO Im surprised this topic hasnt come up before.
One of the big advantages to having more blades is tha ability to have a flapping hinge offset. This gives far greater control especially under low rotor thrust conditions.
With reguard to PIO, to my understanding atleast, the problem is largely bought on by the pilot reacting to the airframes moments, as opposed to flying the rotor. With even a small amount of hinge offset, this problem would be almost eliminated as the airframe would align with the tip path plane in a much shorter period.
Another area in which safety would increase is in the area of low G cuasing a bunt. With a teetering rotor, as the rotor thrust reduces so to does the effectiveness of the rotor to produce a pitching or rolling moment. This means in a low G situation, there is a point of no return, where the prop thrust is now the dominant force, pushing the gyro over.
With a 3 or greater bladed rotor, due to hinge offset, even with greatly reduced rotor thrust, the pilot is still able to maintain effective control over the aircraft.
One area which will suffer is that of induced drag. That is for the same blade area, and same diameter, greater airspeed would be required, although this can be offset by increasing chord, you will pay some penalty in profile drag. However you theoretically will be able to fly faster, because you can reduce the rotor diameter. This allows you to have a faster spinnning rotor, whist maintaining the same tip speed, which has the effect of delaying retreating blade stall.
Its the same old conundrum, if it helps speed it hurts slow, and vise versa.
However one of the main problems I guess will be in the increased complexity of the head, not nessecarily due to the addition of lead lag and flapping hinges, but with a cyclic control system.
The other way to go would be to add more blades, bu keep the gimballing head. While you would more than likely need the flapping and lead/lag hinges, it would simplify the head a little. Personally I dislike this system because it doesnt take advantage of having a hinge offset.
I guess it all comes down to what you want, if you want a faster smoother ride, go for more blades. If you want to fly slow and dont mind a bit of 2per, and like simplicity, stick with the teetering system.
For my design, Ill be going with more than 2 blades, but speed is a consideration for me, and I have a plan for slow speed.

[Chad]

http://www.magnigyro.com/USA/magniday2000.htm

Three pics here and a brief write up on the Magni w/ 4 blades

[6432R]

Does anyone know where toget plans for a 3-blade head? The complexity of construction does not seem that bad> thanks, Mel Bony. mbony@Prinetime.net

[skyguynca]

The 4 blade head can be done with just a lead/lag hinge. I did a thread about that earlier with drawings of a simple hinge and using two teeter towers one above the other for simplicity. I should work fine. I will be doing a two place with 4 blades this winter.

[gyroguy]

Somewhere in my readling, I remember a reference that a gyro can be subject to ground resonance with three or more rotor blades. Can't find it, though.

Has anyone else heard of this, and if so, will you please post some details?

[skyguynca]

ground resonance is a problem but normally experience more by 3 bladed systems. The degree arc from a 3 blade system is usually 120 degrees. The ground resonance is linked to either a low tire, low strut and a abrupt jar to the airframe, then two blades close in on each other to where the angle is now 60 degrees on one set and 150 between the other two causing a great unbalance in the rotor system which gets worse and will tear the machine apart in just a few seconds, with even number systems it is alot harder to cause because the blades on a lead/lag hinge as they advance or retard due to the shock it happens equally with the other pair of blades also or if two blades advance from the standard 90 degrees it just creates two blades at say 70 degrees and two blades at 110 sort of balances itself back out even though they are not flying at standard settings and then they return to normal flight. I am not sure but is this clear enough? Anyway I have seen a 3 bladed rotor system enter resonance and tear apart the airframe in about 20 seconds.......4 bladed I have not heard of it.

[darrellwittke]

I remember the oleo struts (being long travel and not enough damping) being one of the reasons for the Air and Space 18A gyro being so susceptible to ground resonance. Any lengthy suspension to the landing gear of the craft you witnessed tear itself apart Skyguynca? (I suspect not since I suspect a helicopter.) Thanks in advance.

[brett s]

There's a couple of videos of some ground resonance testing using a CH-47 at the following site, near the bottom of the page:
http://www.chinook-helicopter.com/video/video.html

In this case it was intentionally induced, pretty scary stuff. I can't remember ever seeing a case of this in a Chinook happening accidentally, it's not very susceptible but as the videos show not impossible.

[skyguynca]

Yes, it was in a Chinook CH-47. During a landing on a hillside to pick up troops. Acft had a bad strut that was spongy, not hard and shock absorbing. Pilot hit that strut first, bounced and load transfered to the front gear also on the upslope of the hillside. Pilot could not lower thrust anymore to settle aircraft due to slope. Applied power to hold position as the resonance started, kept position instead of either lowering power and settling acft or applying power back into the air imeadiately. The blades first impacted the tunnel cover on top the aircraft then proceeded to dismantle the acft. Was quite a show, luckily no one was seriously injured. After the long investigation it was determined that the rear rotor system entered ground resonance and the system went out of balance one blade struck the tunnel cover the following blade struck the side of the aircraft then they started hitting the blades in the front rotor system since the combining shaft had then failed and with pieces flying the imbalance became severe and the rest is just a bunch of parts.

[skyguynca]

OH, by the way the unit listed in the page with the ground resonance videos, Bco 159th was my old unit. I have even flown in that aircraft. I remember when we sent it to CCAD for overhaul.....you can see in the video they had to buck the controls around pretty good to get the resonance to start. If you keep looking at the rear rotor system from the rear view you can see when the resonance starts. You will see the rotor blades, two of them getting really close together and the third by itself causing the imbalance that tears the rear transmission pylon from the airframe. Cool video. Thanks for the link.

[brett s]

I crewed Chinooks in the 1980's before I started flying, C & D models (stationed in Korea & Ft. Eustis VA) - I miss those things :)

[skyguynca]

I started out as a 67N in 79 then went on to 67T, 67U then into subshops 68G and B before I started flying 47's, great ships. Spent time with 300th avn, Bco 159, Black Cat in Korea, and tsk fc 160 and then a really long stay at Gieb in Germany.

[brett s]

Hmmm, we probably know some people in common - was in the Black Cat's from 84-87.

[skyguynca]

I was abit later, 94-95 went there from Haiti and from there back to Geibelstadt. I am sure we probably know alot of the same people though. One guy I kept running into all the time another 47 guy, Glenn Moore been in the game a while.

[Al_Hammer]

If you are looking to improve efficiency, I suspect that number of blades won't have as much effect as going to a full time partial power drive on the rotor.
Supplying up to 15HP to the rotor allows it to fly at a reduced angle of attack, thus reducing drag considerably. Any more than about 15Hp and torque is a problem.
See **** DeGraw's DeBird gyro for an example.

(guess I can't type the word D*** without being censored, that's funny) :D

[jolly467]

2 vs 4 Blade - would love to see that on a new tractor gyro....My helo experience (UH1 to HH60G) would bear out that the 4 blade is the way to go...MUCH smoother in the 'Hawk compared to the Huey.....'Course it is much more complex, but look at all those Pitcairns back in the day - they were by and large 4 blade rotors.....

[Brian Jackson]

If you are looking to improve efficiency, I suspect that number of blades won't have as much effect as going to a full time partial power drive on the rotor.
Supplying up to 15HP to the rotor allows it to fly at a reduced angle of attack, thus reducing drag considerably. Any more than about 15Hp and torque is a problem.
See **** DeGraw's DeBird gyro for an example.

(guess I can't type the word D*** without being censored, that's funny) :D
Testing: **** **** **** ****

Damn, you're right. s**t f**k... and my father-in-law's name is ****. This s**ks. :D

[Victor Duarte]

2 vs 4 Blade - would love to see that on a new tractor gyro....My helo experience (UH1 to HH60G) would bear out that the 4 blade is the way to go...MUCH smoother in the 'Hawk compared to the Huey.....'Course it is much more complex, but look at all those Pitcairns back in the day - they were by and large 4 blade rotors.....
Hi Jolly, i hope i will see it on a tractor I want to build next year...
thanks

[skyguynca]

the 4 blade was my discussion on another thread where we were sorting out the lead/lag hinge so that we could still use the teetering head stacked to keep it simple. I am still using that on my 2 place I am building over the winter. I am also planning on using partial power on it. Having a second small light engine to provide it with a shaft drive with sprag clutch to the head. I have most of the drawings done and should start construction in December...right now I am still painting my 172 so my hangar is full.

[Al_Hammer]

Isn't the lead lag hinge going to act like a solid connection when it is under load?
The blades can lead and lag just fine if they are "sped up" or "slowed down" by coriolis forces, but in the case of the 4 bladed rotor, the hub of one rotor is being twisted from the center by the movement of the other rotor. I'm not explaining it very well, but, basically any hinge is not going to swing freely when loaded with 7000 lbs or whatever the CF force is. It might make more sense to simply allow the teeter bolt some freedom of movement by using a soft mount for the bushings.

[skyguynca]

the hinge should move and the movement only has to be a small amount. others have tried the 4 blades with stacked teeter heads and developed cracks in the trailing edge of the blade near the blade attachment point. This hinge should eliviate that problem. Really not much different than the attachment hinge on 3 three bladed head like used on the CH-47. A pin thru a hole in the grip. True it uses a bearing but the bushings should provide enough bearing surface and movement. This will allow the blades to speed up and slow down without transfering the twisting load to the hub and trying to twist the two teeter heads should be taken care of. I don't know if I would want to try and make a teeter bolt in a slot that moves, seems like that would set up some serious vibrations.

[Al_Hammer]

There's an important difference between a double teetering 4 blade system and a 3 bladed or 4 bladed fully articulated system.
The blades in the double teetering rotor are not really speeding up and slowing down as they do in a 3 bladed system, just as they don't in a 2 bladed underslung teetering rotor.
The source of the cracking may be(my speculation) that the hub is forced to slow down and speed up due to the cardan joint/hookes joint effect as we discussed on another thread. You are then trying to turn the hub against the force of centrifugal force of the blades trying to hold it stiff. It's like trying to turn your wrist when you are spinning a weigh on a string. The greater the CF, the harder it is to turn your wrist, even if you have a lead lag hinge , its still stiff. C. Beaty made some comments on the 2 per rev thread that backs up my position on this.
I don't think you will induce vibration by having a soft teeter mount because its not a driven system.(well you did say partial power, so that's not totally correct)I'm not criticising the design, just trying to establish if that hinge will do what you want.

[skyguynca]

me too..........I am looking for a fix. Bensen did it originally but I can not find any more data from his work, he mainly dropped it because it was supposed to be a twin engine also but the single engine ops was not what he expected. Magni is flying one now also but they are very tight with any data at all concerning their stuff.

[Al_Hammer]

Right, there isn't much info out there. . . Good luck in your quest.

[gyromike]

Al/Brian,

I deleted Mr. DeGraws first name from the censorship list.
Soyy, I didn't think of that when I made the list! :o

[skyguynca]

Thanks Al, I am determined to do it. Not just for me but if it works the drawings will be free to everyone.

[Victor Duarte]

David , as some one said to me : GET BUSY ;) !
good luck ! and keep posting.
cheers

[Jim]

In Norm’s old forum I asked why we need lead-lag hinges for more-than-two bladed systems and not for our present 2-bladed systems. I knew that lead-lag hinges can be very dangerous. See http://www.fwcvhpa.org/fw/ground.htm or http://safecopter.arc.nasa.gov/Pages/Columns/RayProuty/pdf_files/Ground_Resonance.pdf

At that time I wrongfully thought that the Coriolis effect have something to do with it. (Coriolis have to do with the rotation of the earth. That causes water to spin in your bath outlet and winds to blow in rotation from high to low pressure. Cierva used this myth to fool his competition! — Chuck Beaty wrote: The Coriolis hoax, ....Was perpetrated by Cierva to bamboozle his competitors.

To have a Coriolis torque, you must have radial movement of mass in a rotating system. The commonest example is to stand at the center of a carousel and walk toward the rim. Your body, as it gains kinetic energy, exerts a torque opposing the rotation of the carousel.)

Al Hammer was quick to point our the fallacy of my thinking: “It is not Coriolis, but rather hookes joint effect that causes a cyclic speed variation in the rotorhead” (Sorry Al you goofed earlier in this thread by saying “The blades can lead and lag just fine if they are "sped up" or "slowed down" by coriolis forces,”) ;)

To understand what is going on, the following is important:
1. “The blades move in a single plane(the blue plane in the diagram attached), but the plane is tilted with respect to the rotorhead and ring gear.” (Al Hammer, emphasis mine)
The blades neither speed up/slow down in RPM (angular velocity) with respect to their own axis of rotation, nor do they climb and descend like a circular roller coaster. Both such behaviours would require the application of large forces at various points in their orbit. (Doug Riley)
These two gentlemen, rightfully observed that:
a) The blades do not accelerate or slow down.
b) They do not climb or descend out of the single plane in which they turn.
Al gave this diagrammatically in attachment 1.

2. Blades are coned. If all the blades are coned in the same angle with respect to the spindle, no problem with leading and lagging would have existed. Chuck Beaty have shown that, in order to compensate for the different wind velocities that the advancing and retreating blades experience, but in order to still carry equal load, the plane in which they move goes op on the one side and down on the other. This reduces or increases the angle of attack. (Type “Flap, flap, flap” into your search engine to find Chuck’s article.)

One blade will therefore be at a larger angle in respect to the axis of the rotorhead than the other. – something like my over-inflated diagram in attachment 2. While A and B are exactly the same length, the radii of a and b in regards to the axis differ. If both are turning at the same speed (as Doug Riley rightfully said above) then a, having a shorter radius, wants to turn quicker around the axes. A wants to lead and B is lagging. 180 degrees later B will be leading and A lagging!
Paul Plack describes this: "When the figure skater raises her arms over her head and reduces the radius of her body, her spin RPM increases, despite the fact that the attachment points for her arms are overslung above her body's CG. While less esthetically pleasing for the judges, she could put her arms straight down at her sides, and the same thing would happen...”3. To compensate for this leading and lagging, someone (at Bell) invented undersling. Al Hammer again gives a clear explanation: “If you look at the attached diagram it can be seen that no matter how the coned rotor tilts , a point on the rotor will always be at a constant radius from the teeter bolt or center of rotation if the undersling is correct.” His diagram is attached as the third attachment.

Again Al Hammer rightfully observed: "If the hub bar were not underslung (teeter bolt in the hub bar) we would certainly need lead lag hinges."

4. Lead-lag hinges have been suggested in this forum. For the problems associated with them, I really think that they are a very bad solution for a not-so-bad problem.


Ask me to write a long intro for a short question!!
Multi - bladed systems often come up in our conversations. They may enhance our machines. (See my first posting)

Now: to you who are smarter than I: Have anyone tried a system with undersling such as in the last attachment? – NOT on paper, but in real life!

Jim

[Al_Hammer]

Jim,

its always good to see my name in italics, I think
I used the term "coriolis" above, mainly to avoid a lengthier disccussion (and to bamboozle my competitors..)
The blades do move relative to the lead lag hinge and that's the main thing. In a 2 bladed, underslung rotor, they do not lead and lag.
The logical step that some people would take is to use lead lag hinges to eliminate the stresses in the double rotor(4 bladed).
Unfortunately, they won't work for that, I don't think, as I tried to point out earlier.

I have no idea if anyone has used a system like in the drawing.
The L shape in the blade seems like it would be problematic.

[skyguynca]

There is another way, after doing lots of thinking I dropped the lead lag hinges, I will have 4 blades, two teetering assy but with the upper able to move 15 degrees fwd and aft in rotoration to the main assy for the lower teeter assy, I did a wooden mockup which seems to work well. I will start making the aluminum test head soon.

[Rotor Rooter]

Al Hammer said; " I have no idea if anyone has used a system like in the drawing."

Nope. The Offset Teetering Rotor has never been built. The only 'build' was a mockup for the intermeshing SynchroLite (pictures in following web pages).

Theoretically, it has a number of advantages, but it has two disadvantages;
- A 2-blade rotor will result in an unacceptable 2P vibration. The rotor must have 3 or more blades.
- It has a large number of components. For gyrocopters, this is a big disadvantage, since the rotor must also include conventional helicopter pitch bearings and flight controls etc.

For the interested ~ Pictures of Three blade - Offset Teetering Rotor; ~ http://www.synchrolite.com/Dragonfly.html
~ Hub ~ http://www.synchrolite.com/1181.html
~ Flight-control - Spider ~ http://www.synchrolite.com/Dragonfly_Spider.html
~ Flight-control - Swashplate ~ http://www.synchrolite.com/Dragonfly_Swashplate.html
~ Yoke Assembly ~ http://www.synchrolite.com/1190.html

__________________

skyguynca;

Al Hammer has raised a valid concern. You may find the Kaman intermeshing rotorheads and the means of handling lead-lag of interest. As with your rotor, the Kaman rotor(s) consist of two teetering rotorheads that are mechanically interlocked at 90-degrees to each other. A couple of sketches may be seen near the top of this web page. http://www.synchrolite.com/0433.html



Dave Jackson