Attention engineers.

[birdy]

I posted this question on the Oz forum and thought I'd run the idea by you blokes.



This is probably a dumb question[cum'n from me,more than likely],but has anyone ever heard of or tryed a teetering prop??

I was do'n loops today in some not so good weather ,and It occured to me that when I try to wheel around tight ,most of the effort goes into fight'n the prop's gyroscopic toque.It's alot easer to reef the machine around with the motor at idle coz there's less gyroscopic power in the prop to work against.

I reckon it'd make a slippery machine evan more slippery,and alot lighter in the mouth.

Or am I think'n with my a.. again?

I'd be willing to try such a setup if there's someone who'd make it.
:D :D

[Chuck Irby]

I think your thinkin' is correct, Bird Man!

[Brian Jackson]

Do you mean with the prop pivoting a bit on the hub like the rotors do? I'm not a certified engineer, but it would seem to me you would still have gyroscopic effect, just delayed slightly until the teeter stops of the prop hub bottomed out. When they do, I would assume that the effect would be violent shock to the hub with every 1/2 rev of the prop (2-bladed rigid) as the prop hub slams violently from one teeter stop to the other, since the prop and hub are no longer on the same plane of rotation, having reached their teetering limits.

But I could be way off... I'm trying to visualize the system in my mind. Also, wouldn't such a system play havoc with your thrust line since the teetering mechanism would be acting like a U-joint? Just wondering.

Respectfully,
Brian Jackson

[C. Beaty]

In theory, Birdy, 2-blade props ought to have teeter bearings and 3-bladers ought to have flap hinges just like helicopter tail rotors.

The judder you get from flying sideways is mostly gyroscopic vibrations from the prop.

But easier said than done. In those direct drive applications where the prop also serves as the flywheel, piston pounding would make life next to impossible for the teeter bearings. Redrives with separate flywheel and soft coupling wouldn't be too tough to do.

[quadrirotor]

not dumb at all!!!
to be tried!

[quadrirotor]

may be this concept is more suitable...

http://www.unicopter.com/

[birdy]

Thanx Chuck and Andre,
Chuck,would the drive flange on a 912 be too rough,maybe a rubber bush to take the power shocks out ??

Andre,I never thought of coning angles/underslings.
Correct me if I'm wrong,but isn't the undersling engineered into rotor blades to account for the blades flex??A more ridged wide chord ,two blade[Ivo magnum]prop wouldn't flex much.The teeter bolt and blades could be in the same plane,no??

Brian,I'm no engineer ether,just a frustrated gyro pilot with too much time to think.

[C. Beaty]

I don't know, Birdy. I just looked at a 912/914 parts catalog and the redrive doesn't appear to use a soft coupling. There is something called an overload clutch, a torque limiter I suppose, to protect the crankshaft from sudden stoppages.

Most of the smaller Rotax engines now use a Lovejoy or look alike rubber compression coupling to cushion the gears. It wouldn't be difficult to design a teetering propeller for any of these engines but the 912/914 would require some serious thought.

[gyromike]

I saw a setup like this on an ultralight fixed-wing (Kitfox Lite type) at Oshkosh in 1997. The builder claimed that there would be no gyroscopic loads on the crankshaft, and the teeter joint was skewed at a 45º angle like a helicopter tail rotor.

He continued extolling the virtues of this design, until I asked him how many units were actually flying. Everything started getting fuzzy then. :confused:

I'll dig around in my old pictures. I may have snapped a shot of it.

[quadrirotor]

may be there is something there?

[nsheryka]

I still don’t understand what this would accomplish, there would most certainly still be a torque force on the machine, can you explain this in greater detail?

[C. Beaty]

A propeller is a gyroscope. The torque to change its plane of rotation is equal to the product of its angular momentum and precession rate. This results in a 2/rev shake with 2-blade props and a steady force displaced 90º in the case of 3-blade props.

With flap or teeter hinges, a cyclic pitch is applied to the prop and aerodynamic forces do the work. No different than applying cyclic pitch to the rotor of a Gyrobee.

[birdy]

Any more details on that thing Andre???
I suppose a 3 blade setup would be smoother,and that one looks simple enough.If it could incorperate some sort of shock absorbing bush it could fit a 912.[the lack of a flywheel on the rotax 4 bangers is a pain :mad: ]

C. Beaty mate,dose this mean we agree on somthing. :D :D

[birdy]

BTW, could someone conferm/correct this.

"Correct me if I'm wrong,but isn't the undersling engineered into rotor blades to account for the blades flex??"
Yes C.B.,they have overload clutches in the redrives,but it wouldn't take out any chatter from the power strokes.

[C. Beaty]

The purpose of undersling is to locate the teeter bolt on the CG of the coned rotor.

Take a stick of welding rod and bend it into a shallow V to mimic a coned rotor. Lay it on top of another stick of welding rod and position until it balances. The teeter bolt needs to lie along the line of the straight rod and be directly above the crotch of the V.

[birdy]

In other words,if they didn't cone/flex,you wouldn't need the offset/undersling,no?

[C. Beaty]

Yes, Birdy, if the rotor was either impossibly stiff or impossibly heavy and didn’t cone, the teeter bolt would be at hub center.

The ratio of centrifugal force to load supported is typically 20:1. A slope of 1 foot in 20 is about 3º.

A Robinson helicopter hub has individual coning hinges; a central flap hinge with coning hinges a few inches outboard on the hub.

[Doug Riley]

I wonder how much improvement could be had simply by using a flexible 2-blade prop -- with less weight, less cost, and fewer things to break. With a 2-blade prop turning 2800 RPM and the gyro yawing at 180 deg./sec.*, it looks like the gyro turns 2 deg. for each half-turn of the prop. That's within the amount that a teeter hinge could handle without too much danger of exceeding the flap stops, but it's also little enough so that a bendy prop might at least smooth out the pounding... if the whole system didn't resonate!

A flexible prop wouldn't help with 3 or more blades, since they produce a steady gyroscopic reaction, not a 2/rev pounding.

* Birdy, is that a quick enough turn for you?

[C. Beaty]

The only really floppy prop I’ve seen on a gyro fluttered. Tough to make one limber flapwise while at the same time with adequate torsional stiffness. External noseweights?

[AZSpyder]

I would have to agree the precession would still be there. Either delayed and continued after the turn with a flexible prop or with a surge if the stops were contacted with a teeter. The teeter would likely be spending most if the time with some small amount of thrust offset. It may help with P factor. Now a cyclic control on the prop connected to the pedals could be interesting!

[birdy]

Good to see some constructive input from some good thinkers.

Doug.
While it's an interesting thought ,it is basically the same as a ridgid prop coz it's not changing it's flight plain through airodynamics,but force.[still fighting it's gyroscopic tendencies]
Think'n with me eyes shut,I reckon it'd probably be airodynamicaly worse coz you wouldn't have the ridgid control/force over the indervidual blades.
Them's interesting numbers though,if you could turn the machine 180 degrees in one second without the prop hitt;n the stops,would the rotors be able to keep up without hitt;n the stops??[asuming you'v alot of back stick.]
Still,I don't think I'd like to be fly'n round with blades that "floppy".


* Birdy, is that a quick enough turn for you?

Doug,a cow can turn tail in less than half a second at full gallop,if I could in one second,I wouldn't git left behind so much.[still too slow]

Spyder,mate,that's gitt'n complicated.

BTW,wouldn't the prop thrust always be streight through the driving shaft at all times in a teetering systm???Uneven airflow fed to a ridgid prop will offset the thrust from the shaft,so wouldn't the teetering prop be more efficiant,driving always streight through the shaft.[wich is where you want it]

[Doug Riley]

Birdy, with a 2-blade prop, springy prop blades might shave off the peaks of the 2/rev propeller pounding. Yes, you'd still be forcing the prop around rather than using cyclic pitch to get it to "fly" itself around, so the total change in velocity you have apply to the prop during the time it takes to reverse course would be the same -- just potentially smoother.

There'd be no difference between limber and not when using 3 or more blades.

In a 90-deg. bank (whatever the stick position) at 400 RRPM, you'd be flirting with precession stall if you could do a 180 in a single second. For each half-turn of the rotor, the body of the gyro would have turned 13.5 degrees. As the body turns, it takes the spindle with it. The spindle cranks 13.5 degrees of cyclic up-pitch into the advancing blade. That's enough to stall most airfoils if applied all at once, even if the foil starts out with a zero angle of attack (which it likely won't). The cyclic pitch change isn't really applied instantaneously; rather, it comes on smoothly during the blade's sweep from the fore-aft position to the "abeam" position. The blade has some chance to get out of the way and so dilute the pitch change, but that input rate is still scary.

There's some evidence that a very fast, flat yawing turn (so-called "pedal turn") in a gyro can cause a cyclic or precession stall if the stick is held back as you suggest.

It would be great if rotors could be rated by the maximum cyclic pitch change rate that they can tolerate without cyclic stall. Testing for these limits could be pretty spectacular. A rotor head mounted on top of an armored vehicle might be the appropriate test rig!

[birdy]

Hmmm........ kinda sounds like a bit of a snag ay.
So wot will actualy happen if the spindle outruns the rotor,besides kick'n the stick out of your hand.???Stick shake would be a minor detail compared to blade stall right??

I reckon if the blades could handle the 13.5 degree pitch demand without stall'n,the feed back through the stick[from the bar hit'n the stops] would be a unignorable warning that your push'n your luck.Even so,with 9 degrees of teeter,[airspeed would be slow do'n this,so you would still have nealy all avalable teeter room]you have plenty of time to rudder out the shake before the head movement limit was exceaded,and your head got ripped off.

Please be gental,remember I'm a SCG.

[Doug Riley]

Birdy: SCG, my foot.

The idea that a too-fast cyclic input (caused by radical maneuvering) can cause a cyclic or precession stall is mostly theory. As far as I know, it hasn't been investigated systematically (I think it should be, as I've said before). It's known that helo tail rotors can do this, but that's not quite the same thing.

When the performance requirements for Light Sport gyroplanes were being tossed about, we initially included some language about precession stalls. The FAA objected, claiming it was all theory not mentioned in the literature. The "theory," though fits the many witness reports of what happens during a PPO (the most radical maneuver of them all). The nose drops so fast that the retreating blade stalls (can't keep up with the spindle) and makes a loud "crack" as well as chopping the tail off.*

People who've gone right to edge of PPO have reported violent stick hammering as you describe. Chuck Beaty has a good story about this, from one of the wild men of the '70's gyro scene in Florida.

If a radical turn can do the same thing, then the rotor is going to seesaw uncontrollably in one direction or another. How realistic the scenario actually is I don't know. I have a vague sense that, in a coordinated turn, there's not much danger unless the body of the craft is very unstable and fails to "weathervane" in the direction the craft is going. That's because the rotor itself is pulling you through the turn. How can the spindle "outrun the rotor" when it's the rotor that's leading the body through the turn? If the body DOESN'T weathervane, but instead rotates rapidly in some non-weathervane direction, then maybe the spindle can get out of sync with the rotor. That's what happens in a PPO (engine thrust whips the body sharply nose-down). It's also what appears to happen in a violent pedal turn (rudder whips the body about the yaw axis).

The kind of turns you describe in mustering sound like sideways half-loops. That is, you lead with the rotor. I would think (but don't bet your life on what I think!) that, with the rotor in the lead, outrunning the spindle isn't a danger as long as your airframe does not rotate the wrong way or otherwise create inappropriate, abrupt spindle movements.

* a precession or cyclic stall is especially likely in a PPO, for a couple reasons. First, the body's rotation is nose-down. This cranks extra pitch into the RETREATING blade. The retreating blade is already operating at a higher AOA to start with and is partly stalled in any case. It's extra-vulnerable to adding too much AOA all at once. Second, the overturning force on the body is huge in many cases -- it's a torque equal to the thrustline misalignment multipied by the prop thrust at the time. This can be hundreds of foot-pounds.. perhaps 500-600 ft.-lb. in the case of a stock RAF at full bore. No amount of hammering of the teeter stops is going to stop such a body rotation once it gets going.

[birdy]

I rekon you mite be right Doug.
The only way the spindle could outrun the rotors would be by hard rudder with the stick back.In most hard turns you roll first,then pull back to turn.There's buggerall rudder involved in these turns.And as you say,the machine is led by the rotors.
Probably the only way the rudder could cause the spindle to outrun the rotors would be at low speed,back stick,on the power curve.But I'v kicked hard rudder in this very situation plenty of times,often going past 180 degrees,and never felt any unusual feed back.
I'll quiz me blade maker about the pitch /stall limits.

* Once you'v lost your airodynamic cyclic control[ppo/blade stall] you better be quick to kiss your .ss,coz that's all you could do ay.

[birdy]

I'v been ponder'n this stick kick/blade stall thing today.[had an easy day]
In the post above I said,
"But I'v kicked hard rudder in this very situation plenty of times,often going past 180 degrees,and never felt any unusual feed back."

No unusual feed back,..........but now I'v been think'n bout it,the feed back I git when do'n this only occures ........when I do this.
Doug,when I'm hang'n off the power curve and creep'n along,forward,everything is smooth.If I now apply rudder[moreso right than left] I get feed back.It's a sidways wobble in the stick[stick is on the back stop]and the harder I rudder,the stronger the "wobble".Wot's caus'n this??
I used to think it was just the blades hitt'n the full power propwash with the stick back but now I realise it don't happen when I'm go'n streight OR take'n off,both times the stick is right back and the 912 is on full tap.
Could it be hitt'n the teeter stops or am I pull'n me maggot??
Maybe it's just throw'n the coning angle/undersling out coz the load would be high and the RRPM would be low..........but then why don't it do it when I'm go'n streight????
I'll leave it to you Doug,mate.
I'll be chase'n some more cows in a couple o days and I'll pay special attention to wot's go'n on.[The wobble seems to resonate through the hole machine.]

[birdy]

Doug,I payed a bit more attention today and it's definatly a side wobble in the stick.
Next time I'll git it on video and I'll have a better understanding of wots go'n on then.
BTW Doug,wots wrong with ya foot mate?? :D

[birdy]

Just windern if anyone has come up with a plan, or seen a teetering prop yet?
I'm still keen on tryn one.
Was think lately along the lines of a 2 blade [ ivo mag] with flap hinges.


Also, the one down side i can think of with such a setup is, wen the engine is at idle rpms and you stick or rudder hard, its go'n to need alot of flapping room to avoid hitn the stops at this speed.
No worrys at full rpms coz its getn plenty of cycles per second to react.

That 'folding' prop looks promising, at least itd have plenty of room before it hit the stops:).

[karlbamforth]

Birdy,

I know this may seem a strange question, but "Why" ?

Teetering is used to overcome advancing/retreating blade differences in lift.
Can't really see how this would apply to a prop as the airflow is approaching both blades at the same angle/speed.

Unless you are thinking of vertical descents, when the problem is fairly insignificant compared to the complexities of a teetering prop.

Maybe you just want to try it. If so that is as good a reason as any form me.

Added later
Ooooooppppps just read the earlier posts.

[quadrirotor]

For you birdy (hinged prop): look at the rear rotor of an helo! i am looking for a good photo...May be you could use a two IVOPROP magnum blades and make a hub which could accomodate a 45 degrees Delta3 effect...

[Doug Riley]

Birdy: the old Averell one-bladed prop had a Delta-3 hinge of sorts; the idea was to create a constant-speed effect without all the gadgetry.

I assume you want to reduce the gyroscopic reactions that result from quick moves. A light, limber prop does almost the same thing without the hardware and failure points. A 2-bladed prop doesn't produce a steady gyro reaction; it just pounds the crank at 2-rev when you yank it around. 3 or more blades give you a true gyroscope, for better and for worse.

[PW_Plack]

Guys, would a teetering prop's coning angle remain the same over most of its operating RPM range? In other words, would changes in centrifugal force and thrust always be equal and offsetting?

[Al_Hammer]

Guys, would a teetering prop's coning angle remain the same over most of its operating RPM range? In other words, would changes in centrifugal force and thrust always be equal and offsetting?

Not if you go by the following rules of thumb, Paul:

Centrifugal force varies as the the square of the rpm ratio(new rpm/old rpm). Going from 2000 rpm to 2500 (a ratio of 1.25 )would increase CF by 1.56 times. which is 1.25 to the 2nd power)

To find the new power delivered by the prop, take the rpm ratio to the 3rd power. (so I've been told ).

The same increase in rpm as above would give an increase in power of 1.95 times.(1.25 to the 3rd power)
The new thrust will depend on the new airspeed, since power = Thrust X velocity, but will change by the same amount as the power initially, when airspeed has not had a chance to change.

So, it appears that increasing the throttle will drive up the thrust faster than the centrifugal force. Certainly this is not going to be a self cancelling situation and the coning angle(which depends on the ratio of thrust to CF) will change with throttle setting. The degree of change will depend on whether your working with static thrust(at zero airspeed) , or if you're at cruise speed where thrust is less a contributor to total power.


In case it hasn't been mentioned, two counter-rotating props will result in zero total torque at the shaft. The individual 2 bladed props will still be hammering the shaft, but out of phase.

[birdy]

the old Averell one-bladed prop had a Delta-3 hinge of sorts; the idea was to create a constant-speed effect without all the gadgetry.
Could you explain this a bit more please Doug, i got no idea wot a delta hinge is.

A light, limber prop does almost the same thing without the hardware and failure points.
Maybe, but i'm not sure it could put up with the flogn it'll get here.
Is there one avalable that would adaquately replace the ivo mag i'm usen now?


[I]would a teetering prop's coning angle remain the same over most of its operating RPM range?
I'm not sure if the cone angle would affect the prop the same as a rotor Paul.
The main reason an 'out of cone' rotor makes the machine/stick shake is coz of the airflow direction over the DISC. In a virtical decent, the airflow over the disc is constant 360* round the disc, so theres nothn to make the TV move to cause shaking. Apply forward speed and everythn changes.
A prop would nearly be in a constant virtical decent mode.
[ no dout ill be corrected ere if im off the mark;)]

In case it hasn't been mentioned,
It has been mentioned before Al, and would counter torque roll but not gyroscopic effect. The GE wouldnt be felt through the machine like it is now, but its still there, only its confined to the prop shafts. And see'n as there'd be alot of GE confined to such a small shaft during a full rpm high rate turn, the shaft would need to be very strong. You wouldnt know your stressn it till it breaks, coz you couldnt feel it through the machine. As it is now, the GE itself is stopn you from pushn it too hard.
Besides this, its complex, and SCGs dont like complexity.;)
And on top of that, no ones tryed a teetering prop.:)

[SamL]

What about a twin counter rotating prop ?

[M. Pearce]

birdy,

Yep, I tried it once on a small airboat.

It works!

A helicopter tail rotor is basically a teeterin prop but with you guessed it, in flight ajustment!

Forward and Reverse!

I just sold my Huey tail rotor on ebay that I had used for the project.

[birdy]

mmmmmm, sounds tempting:).