Teetering limits.

[birdy]

Had a dissagreement with a bloke the other day bout the limitaitons on teetering and when you get there.
Usualy i'd just tell him to restudy the concept of teetering coz i recon my understanding is right, but this bloke is sumone i hold in the highest regard coz of his abilities. I know he's very switched on coz i'v had a bit to do with his handywork, and he couldn't do wot he dose without knowlage and understanding of the principals of gyros.
I must add tho that the dissagreement we had had nuthn to do with his feild of experties. All the same, his differing understading on this subject has sowen seeds of dout bout my own understanding.
My understanding of the reason for the teetering hinge, and how it works you must understand, cums from the logical[ and sumtimes flawed] thinking process of a SGC.
But heres my understanding.
The hinge's main task is to allow the equalising of lift of the two opposing blades when they have different ASs.
In a true virtical decent, both blades are seeing the same AS and AOA all round the disc, so theres no need for the hinge coz the lift is equal round the disc.
Add some forward movement to the machine/disc and the advancing blade see's higher AS [ blade's rotational speed + forward speed] and the retreating blade see's less AS[ rotational speed - forward speed]. This differential in each blades AS on opposing sides of the disc is going to cause a differential in lift, and try to roll the disc/machine
The teetering hinge allows the blade with the higher AS/lift to rise, which allows the other blade, which has less AS/lift to drop. This equalises each blades AOA/lift with no change in the driection of the sum of lift of the two blades, so lift remains parralell to the rotor bearing bolt. [ up IOW]

Now, the dissagreement was "when do you reach a point when you run out of teetering". IOW, when will the hub bar start to touch the stoppers[ teetering limit] in S/L flight, if theres inadiquate teetering room?
Thinkn logical, like a SCG dose, i recon its at the highest speed.
If theres no teetering action in a VD, and teetering increases with increased forward AS, then logicaly the limiters will be touched at the highest speed. Or the VNE of a machine with inadiquate teetering room, is when the bar starts to touch the stoppers.
Remember, im only refering to S/L flight.

Like i said, im a SCG, and i'v been known to be wrong before.

Note; i like to know these things coz i recon, till you understand fully the physics and general working of a machine, any machine, then you can't get the maximum performace out of it............. without going past its/ your limits.

[Al_Hammer]

I think you're right, Birdy, except for that bit about the unequal lift tending to roll the machine. A pitch up tendency results from the 90 degree phase angle between force and displacement and it only effects the rotor, which is free to precess. If there was no flapping hinge, then the whole aircraft would roll and possibly pitch, too, depending on the stiffness of the blades.
As airspeed increases, the rotor increases its flapping angle, tilting away from the relative wind, eventually reaching the stops, assuming you don't run out of power first. The speed at which this happens depends on the pitch of the blades. With a lot of pitch you will hit the stops sooner and further application of power will cause a climb. VNE may be based on other factors besides flap stop limits.
Retreating blade stall is not the same concern in a gyro as it is in a helicopter, from what I've read, and so it is possible to hit the stops without ever encountering it. As you said, in a vertical descent there is no disymmetry of lift to cause flapping.

[C. Beaty]

The unequal airspeeds of retreating vs. advancing blades of a boomerang cause it to loop instead perform a corkscrew spiral if flung horizontally.

A rigid rotor without cyclic control would pitch noseup and begin a loop in forward flight, just like the boomerang.

With inadequate teeter travel, you’re most likely to contact the flap stops at liftoff, before the rotor’s fully up to speed. That’s something I learned the hard way. The teeter motion increases with airspeed but with the rotor at full flying speed and with normal teeter travel, ±9º, something bad will happen before you run out of teeter travel from too much airspeed.

Because of the teeter (or flap) hinge, you can only rotate the blades about their feathering axes. Human muscle power is too puny to tilt the rotor disc against its own inertia.

[Al_Hammer]

A boomerang appears to be very similar in some ways to a teetering rotor.
It will fly in a circle and come back(although for hunting, you don't want your boomerang to come back.)

If the rate of precession W exactly corresponds to the angular velocity of circular motion, then the boomerang rotates at exactly the correct rate to stay tangential to the flight path as shown. This gives an equation relating V to W,
V = RW
http://plus.maths.org/issue7/features/boomerangs/

Interestingly, the size of the circle does not depend on the spin rate or the velocity of the throw and is a constant for any given boomerang.

[gyropilot]

With inadequate teeter travel, you’re most likely to contact the flap stops at liftoff, before the rotor’s fully up to speed. That’s something I learned the hard way.Chuck,

Please tell us more about what you were doing when you "learned the hard way."

Thanks,

John L.

[C. Beaty]

Al, boomeranglike devices can have many shapes.

One way is to take your tinsnips in hand and snip out a 3-blade rotor from sheet metal. Twist some pitch in each arm and toss it. If the spin axis is vertical, it will loop, not roll.

That’s the case of an isolated rotor. When you have mass hanging beneath and blades that aren’t perfectly rigid, all sorts of things are happening which complicate a simple example.

*****************************

John, my first gyro was built with a partner and followed the Bensen plans religiously.

After 50 hours or so and imagining myself to be an ace, I decided to build one with an overhead stick. Being an ace, I thought all I would have to do would be to get it up to 50 feet or so to get the feel of the overhead stick.

But things didn’t go exactly according to plan, fortunately.

I never got it to 50 feet; I’m not even sure the wheels left the ground before I swatted the blades.

I patched up the rotor (homegrown copies of Bensen metal blades) and installed a conventional stick.

With the joystick, there was interference between rotor hub and the torque bar at the flap extremes so I installed a pair of ¼” bolts on the striker plate with the heads acting as elevated flap stops. That reduced flap travel to perhaps 2/3 of the necessary amount.

On each liftoff, the rotor hub would contact the bolt heads so violently that it would rattle my eyeballs. After a few seconds, when the rotor reached normal flight rpm, the machine would fly normally. I don’t remember how long I left the machine with restricted flap travel but I never contacted the boltheads in high speed flight.

It was simple enough to install a spacer to raise the rotorhead enough so there was no longer any interference.

[Al_Hammer]

That’s the case of an isolated rotor. When you have mass hanging beneath and blades that aren’t perfectly rigid, all sorts of things are happening which complicate a simple example.


Chuck, It is often stated in books that Cierva's early autogyros would roll prior to the introduction of flap hinges. We know that a flapping rotor pitches up, but if there is no flapping hinge then as you say, its more complicated. As a blade rises on its way to being higher in front, it will roll the aircraft to which its attached before it gets to the final position. I wonder if Cierva's gyros really did roll, or if writers just assume they did because of a simplistic understanding.

[birdy]

Hmmm............... i still seem to have the same ol problem.
Either i can't explain meself or you can't read proper.

With inadequate teeter travel, you’re most likely to contact the flap stops at liftoff,
I know that Chuck, thats why i stated,Remember,im only refering to S/L flight.

So im right when you said,The teeter motion increases with airspeed???

The stiuation i gave did stipulate the VNE of a machine with inadiquate teetering room ....................

So, if theres inadiquate teetering room, the most likely time you'll experiance contact with the stoppers is at high speed, once your flying.

[C. Beaty]

You’re right in theory but wrong in practice, Birdy.

Yes, if you have insufficient teeter travel and if you can get the rotor to flying speed beforehand, then you’ll start hammering the flap stops at some high forward airspeed.

At 100 mph, flap travel is somewhere in the range of 4º, depending upon rotor and load.

But with flap travel restricted to 4º, you’d never get airborne unless you had the ability to prerotate to more than flight rpm.

[C. Beaty]

I wonder if Cierva's gyros really did roll, or if writers just assume they did because of a simplistic understanding.
Most books about Autogiros, Al, were written by journalists or historians, not rotorcraft engineers.

To be sure, a rotor without flap hinges or some sort of feathering cyclic pitch is uncontrollable. Which way they might have flopped is a matter of conjecture.

Cierva was entirely aware of this problem. His first attempt had a form of cyclic pitch control using a cam to provide cyclic control and thereby equalize lift. Another attempt before the invention of flap hinges used counter rotating rotors to equalize lift and cancel gyroscopic effect.

[Al_Hammer]

Interesting tidbits on Cierva, Chuck.
A rotor without flap hinges can be controlled as long as the blades can swivel about their feathering axis, which is exactly what happens in some of the RC model gyros. The teetering is locked out and Hiller paddles enable the rotor to tilt about the feathering axis on a simple axle or pin. The blades flap by virtue of their flexibility(and there is a shift away from 90 degrees of precession). Also, the hiller paddles apply feedback to help take out the effects of disymmetry of lift in forward flight.

[Ga6riel]

very interesting thread
just to add an observation
all the trike equiped gyros Ive seen on take-off, at the point where the gear leaves the deck, seem to roll anticlockwise ~ seen from astern
In that description, the port main leaves contact last

[birdy]

Ga6riel, the only thing that will make a gyro roll at lift off is prop torque or imbalance of a side by side 2 seater, and to a lesser extent, slightly crossed controles ina xwind. If the machine you saw rolled anti, then the prop woulda been spinning clockwise, if not the other two situations.
If the pilots got everythn under control and theres nuthn to counter prop torque, then every gyro will roll slightly opposite to the prop rotation.

Yes, if you have insufficient teeter travel and if you can get the rotor to flying speed beforehand, then you’ll start hammering the flap stops at some high forward airspeed.

Thanx Chuck, thats all i wanted to know.
The reason i asked is coz i'v got teeter stoppers that don't allow the max teetering angle, [for reasons i'll keep secret]. But its not so limited that the stoppers are touched at high speed, which is where i recon theyre most likely to touch,AFTER TO. Which is the reason for the origional question.

[mceagle]

Birdy, as long as you appreciate the fact that the teetering referred to at high speed is not sideways across the machine, but at the front and back. There is no dissymmetry of lift as is popularly thought, but rather a tendency towards dissymmetry which is evened out by the cyclic control, making the lift even on both sides.
I don't know what secret experiment you are trying but I hope this helps.

[Aussie_Paul]

Birdy, as long as you appreciate the fact that the teetering referred to at high speed is not sideways across the machine, but at the front and back. There is no dissymmetry of lift as is popularly thought, but rather a tendency towards dissymmetry which is evened out by the cyclic control, making the lift even on both sides.
I don't know what secret experiment you are trying but I hope this helps.

It does not matter whether it is side to side or front to back, when you run out of teeter at hight speed the hub bar will touch the stops.

I think it is that simple, Birdy.

Aussie Paul.:)

[birdy]

as long as you appreciate the fact that the teetering referred to at high speed is not sideways across the machine, but at the front and back.
Tim, the AS differential IS at either side of the machine, and its the differential in AS that the teetering mechanism is designed to counter.

There is no dissymmetry of lift as is popularly thought,
Thats right, because the hinge allows the blade with the greater AS/lift to rise, lessening its AOA/lift and the opposit blade dose the oppoiste, decend and increase its AOA/lift to counter the AS differential. If the systm was rigid, then there would be an unequal lift, at the sides.

dissymmetry which is evened out by the cyclic control, making the lift even on both sides.
This be'n done automaticaly through the teetering hinge, not the pilot with the stick.

I think it is that simple,
I thought so too PB.

[C. Beaty]

Neither flap hinges or blade flexibility are essential qualities, Al. The following is from the Russian web site:

In 1945, Glidden S. Doman formed Doman Helicopters Inc. in order to develop various helicopter engineering concepts. These included a new hub system which was essentially similar to the mechanism used on a variable pitch propeller. The rotor system also incorporated a gimbal mounting to provide the necessary tilting of the rotor disc. Doman's theories were tested initially by installing an experimental system on a USAF Sikorsky R-6. This helicopter was known as the Doman LZ-1A and it started flight tests in early 1950 with remarkably good results. It was followed by the larger LZ-2A Pelican.

http://avia.russian.ee/helicopters_eng/doman.html

[gyropilot]

To be sure, a rotor without flap hinges or some sort of feathering cyclic pitch is uncontrollable. Which way they might have flopped is a matter of conjecture.Sounds like a good subject for that show "Myth Busters" on the Discovery Channel. They could build a recreation of an early Cierva gyro without flap hinges, attempt to fly it, and record what happens from many camera angles. Would be quite interesting. :)

Maybe Birdy would volunteer to be the pilot?

John L.

[PW_Plack]

...and both people who kept watching till the end would be amazed! :)

[Al_Hammer]

Tim said:
as long as you appreciate the fact that the teetering referred to at high speed is not sideways across the machine, but at the front and back.
And Birdy replied:
Tim, the AS differential IS at either side of the machine, and its the differential in AS that the teetering mechanism is designed to counter.
[/QUOTE]

I think Tim is correct on this point , Birdy.
The airspeed difference at the broadside position is counteracted by the blades rising and falling at that position. You'll notice that the maximum effect occurs when the blades are rising at their maximum rate and this occurs at the broadside positions.
The blades reach maximum rising and falling velocity as they cross through the midpoint of their travel, just as a child's swing is moving fastest when its at the bottom of the arc(the midpoint.)

The blades momentarily stop rising and falling when they reach the point of maximum displacement, which is at the nose/tail orientation, just as they do when you move the stick forward or back.
Flapping results in a disc that is tilted up in front, not tilted sideways and so the flap stops would obviously be contacted at the front and rear positions when the limits were reached in high speed flight.

[Al_Hammer]

Chuck, the Doman rotorhead eliminates the problem of the rotor not being aligned with the mast, but there still must be some way of dealing with disymmetry of lift in forward flight, right?
I am a little confused as to how this is accomplished, but it appears that blade flexibility is a key requirement.

from a patent on a Doman-type rotorhead(talking about equivalent hinge offset):
Actually, when subjected to blade pitch moment and roll moment loads, flexible hub and flexible blades act as springs in series. It can be shown that the combined offset achieved by the use of flexible hub and conventional flexible blades together is less than would be achieved by using either conventional flexible blades with a rigid hub or flexible hub with a theoretically infinitely rigid blade.
http://www.freepatentsonline.com/4323332.html

[Doug Riley]

You could probably pull off this experiment (or demolition derby) by un-shimming and tightening an ordinary Gyro's teeter bolt. An ordinary gyro that you didn't like much, that is.

Blade airspeed dissymmetry begins the moment a blade leaves the six-o'clock position and builds as it swings forward to three o'clock. I would think that a no-flap-hinge rotor might even flip a taildragger gyro onto its nose.

[Rehan K.Janjua]

Interesting.

Thank you Doug and all.

Rehan

[Al_Hammer]

When it comes to rotational mechanics, it is sometimes necessary to throw intuition out the window. A good example is a rocket ship in orbit around the earth. What happens if you fire the rockets as you point straight down at the surface? Do you wind up closer to the earth? Nope, you get kicked into a higher orbit. Definitely counter-intuitive.
A rotor with a teetering hinge is resonant at 1 per rev and will exhibit the classic 90 degrees of precession, but if you rely on blade flexibility for flapping instead of a resistance-less hinge, then you change the resonant frequency and the rotor will roll as well as pitch in response to an aerodynamic input, because when you change the resonant freq, you alter the phase response.
In other words, an input takes effect maybe 70 degrees later in the cycle instead of 90.

[C. Beaty]

Chuck, the Doman rotorhead eliminates the problem of the rotor not being aligned with the mast, but there still must be some way of dealing with disymmetry of lift in forward flight, right?
Yes, Al. That way is called a swashplate.

The aerodynamic input to a rotor translating edgewise isn't pure first harmonic motion so flexibility reduces blade bending stress in response to higher harmonics. But flexibility is not an essential requirement.

[mceagle]

It does not matter whether it is side to side or front to back, when you run out of teeter at hight speed the hub bar will touch the stops.Paul, not only does it matter, it is also very critical. If the rotors hit the teeter stops hard enough on the left hand side then the gyro will bunt over forwards. If you hit the teeter stops at the rear then the gyro will roll to the left. The difference being that a roll to the left will maintain or increase positive "G" loading, and may be recoverable.
I believe Birdy is trying an experiment here and some of this information could be helpful.

[Al_Hammer]

Thanks, Chuck

[Aussie_Paul]

Paul, not only does it matter, it is also very critical. If the rotors hit the teeter stops hard enough on the left hand side then the gyro will bunt over forwards. If you hit the teeter stops at the rear then the gyro will roll to the left. The difference being that a roll to the left will maintain or increase positive "G" loading, and may be recoverable.
I believe Birdy is trying an experiment here and some of this information could be helpful.

Birdys original question is what I was replying to Tim. I agree with your explination, BUT Birdy asked a simple question and everybody has muddied the water with all the Technical stuff that goes way beyond Birdys simple question.

Aussie Paul.:)

[birdy]

Never thought id hear it PB, thanx.:D

Tim, Chuck, Al and Co., The origional question was,
"when do you reach a point when you run out of teetering". IOW, when will the hub bar start to touch the stoppers[ teetering limit] in S/L flight, if theres inadiquate teetering room?
I asked WHEN, not WHERE. Where isn't my concern, its when, and i was correct, its at top speed, if at all in S/L flight.

Chuck, you'll be pleased to know i did a shortish [ 200'] TO this morn'n with me new 30'ers, prerotated to 140rrpm, no wind assistance, full power from standing start, full fuel and the bar never touched the stoppers. So theres no chance itll touch in S/L ay.

Tim and Al,
I think Tim is correct on this point , Birdy.
Yes he is, but its not answering the question.
Teetering LIMITS are reached in the 12 and 6 oclock positions, but the greatest reaction and AS differential is at 3 n9 oclock, or there abouts.

As Doug said,Blade airspeed dissymmetry begins the moment a blade leaves the six-o'clock position and builds as it swings forward to three o'clock. 3 is at bout the point of greatest differential, which means the greatest teetering reaction, not the limit.

[Aussie_Paul]

Never thought id hear it PB, thanx.:D

Tim, Chuck, Al and Co., The origional question was,
"when do you reach a point when you run out of teetering". IOW, when will the hub bar start to touch the stoppers[ teetering limit] in S/L flight, if theres inadiquate teetering room?
I asked WHEN, not WHERE. Where isn't my concern, its when, and i was correct, its at top speed, if at all in S/L flight.

Chuck, you'll be pleased to know i did a shortish [ 200'] TO this morn'n with me new 30'ers, prerotated to 140rrpm, no wind assistance, full power from standing start, full fuel and the bar never touched the stoppers. So theres no chance itll touch in S/L ay.

Tim and Al,
I think Tim is correct on this point , Birdy.
Yes he is, but its not answering the question.
Teetering LIMITS are reached in the 12 and 6 oclock positions, but the greatest reaction and AS differential is at 3 n9 oclock, or there abouts.

As Doug said,Blade airspeed dissymmetry begins the moment a blade leaves the six-o'clock position and builds as it swings forward to three o'clock. 3 is at bout the point of greatest differential, which means the greatest teetering reaction, not the limit.

Don't get too carried away Birdy.:D I will kick your butt if needed.:eek: You were being given very complicated answers to a simple question!!!!:)

Aussie Paul.:)

[mceagle]

As Doug said,Blade airspeed dissymmetry begins the moment a blade leaves the six-o'clock position and builds as it swings forward to three o'clock. 3 is at bout the point of greatest differential, which means the greatest teetering reaction, not the limit.We will just have to agree to disagree with this one Birdy - We might have to disagree over a small rum.
Doug is correct, but he is talking about airspeed differential. not teetering differential. Teetering differential is at its greatest at 12 oc and 6 oc, in straight and level flight. The higher the forward speed, the closer the hub bar comes to the teeter stops so there is no definitive answer to your original question without knowing many more variables (and then you need a name something like Chuck or Al work it out.

[Aussie_Paul]

It still gets back to the fact that mast bumping, as the heli boys call it, or hub bar hitting the teeter, as Birdy and I call it, will happen at the fastest speed, not the slowest speed as his mate must have been trying to convince my SCG mate!!!

It does not matter if the stops are hitting while the blades are for and aft or cross wise, the result is still the same.

Aussie Paul. :)

[Al_Hammer]

Birdy said:Teetering LIMITS are reached in the 12 and 6 oclock positions, but the greatest reaction and AS differential is at 3 n 9 oclock, or there abouts.

Birdy, airspeed differential may be greatest at 3 and 9 0'clock, but not "reaction", if you mean point of greatest reaction to be the point where force and acceleration are greatest.

Flapping motion of the blades follows a sine curve and the following factoids apply:
The point of maximum displacement(highest/lowest teeter) is at 6/12 o'clock as noted.
The point of maximum teeter velocity is at 3 and 9'oclock.
The acceleration is zero at 3 and 9 o'clock. Mathematically, this is an inflection point, where the 2nd derivative changes sign, but the main point is that the acceleration is least when the blades are crossing the midpoint of their teeter travel.

So, you may ask, why isn't this a point of maximum acceleration giiven that airspeed difference is highest?

Its because flapping causes the blades to be at their max velocity of rising/falling at the exact point where the airspeed is greatest/least and the two effects cancel out to zero.

The blades are actually accelerating or decelerating the most at the point of highest teeter, (6 and 12 o'clock)when the blades are reversing their motion from up to down.

As Tim said, there is a tendency for lift differential, but there is no actual lift differential as long as flapping neutralizes it and therefore there is minimal reaction force at 3 and 9 o'clock.

On the other hand, if you make a stick input, there is a momentary reaction at the point of aerodynamic input which realigns the disc 90 degrees later and then the reaction once again drops to zero once the disc has moved.

[Ga6riel]

Ga6riel, the only thing that will make a gyro roll at lift off is prop torque or imbalance of a side by side 2 seater, and to a lesser extent, slightly crossed controles ina xwind. If the machine you saw rolled anti, then the prop woulda been spinning clockwise, if not the other two situations.
If the pilots got everythn under control and theres nuthn to counter prop torque, then every gyro will roll slightly opposite to the prop rotation.


many thanks Birdy, indeed that makes sense

[autorotate]

i dont think it would be safe practice to ever alow the head to hit the mast stops this could lead to mast bumping failures witch usaly results in death the disk plane shouldnt exeed the corasponding swashplate angle. thit somtimes happens in low gravity situations witch are to be avoided anyway

[Aussie_Paul]

many thanks Birdy, indeed that makes sense


If the machine is "horsed" into the air with low rrpm the retreating blade can partially stall for a moment and the machine will roll to the retreating blade. I believe that depending on teeter limits that this could happen without the blades teetering out.

Aussie Paul.:)