Delta3 for gyro rotor

quadrirotor

André MARTIN
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Why the earlier autogiros had, almost all, a rotor which had Delta3 effect whereas today, almost none have this feature???
 

LGoodhind

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Contemporary gyroplanes pretty much all use a two-blade teetering rotor without drag hinges for simplicity. Delta3 is not a feature, it's a fix for a problem that sport gyro rotors don't have.
 

quadrirotor

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Contemporary gyroplanes pretty much all use a teetering rotor without drag hinges for simplicity; it's not a feature, it's a fix.
Not the good answer!... (cf. the tail rotor of an helico)
 

LGoodhind

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I certainly don't understand your response ... "not the good answer"? As far as I know that is the answer.

The early machines needed it because they were heavy and had multiblade rotors- they couldn't control the rotor if it didn't have pitch coupling. A two bladed teetering rotor on a light machine doesn't have the same problems so they don't use the same solutions. Do a search on delta3 .. was already discussed back in September.
 

brett s

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Tail rotors don't get cyclic pitch inputs, the object is to keep their plane of rotation from ever changing at all. Hence lots of delta 3...
 

C. Beaty

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The early gyroplanes did not employ delta-3 coupling; in fact, it was carefully avoided. Take a look at Cierva’s patent drawing for the C-30 rotorhead below.

The A&S 18-A uses what looks like a 45º delta-3 angle to make collective pitch reduction automatic following a jump but they pay a heavy penalty for so doing. I once tried to discuss this with Don Farrington and his sidekick, John Potter, but drew a blank look. Neither had ever heard of delta-3 coupling.

Delta-3 coupling has the effect of imposing an aerodynamic spring between rotor and mast. That raises the rotor natural frequency above the rotational speed so the normal 90º phase shift between cyclic pitch and rotor tilt no longer exists. That is partly accommodated by swash plate phasing but it only works somewhat over a limited airspeed range and roll/pitch coupling is a vexatious problem.

Delta-3 coupling also reduces the damping supplied by the rotor and degrades stability.

So tell me, André, why would you be recommending delta-3 coupling?
 

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Not Yet

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Pitcairn’s later rotor heads had a delta 3 of about 30 degrees.
I was of the impression that delta 3 was to lesson the aft and left tilt of the rotor disk verses that normal to the spindle.
 

C. Beaty

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Here are scans of both the Kellett KD-1 and Pitcairn PA-22 rotorheads. Both have non-skewed flap hinges, i.e., no flap-pitch coupling.

The Pitcairn PA-36 looks like it might have had flap-pitch coupling but the photograph (from: AUTOGIROS by George Townsend) isn’t clear. In any case, only one was built.
 

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Not Yet

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Here is a picture of the wood model in the pioneer airport at Oshkosh WI.
You can easily see the 30 degree delta 3.


Sorry for the bad quality of the picture, the glass display case gave me trouble.
 

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C. Beaty

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I imagine the delta-3 coupling in the PA-36 was for automatic depitching of the rotor following a jump, same as for the A&S 18-A.

The PA-36 had a coarse pitch screw retaining the blades which were held against the zero incidence stops by hydraulic cylinders. When the hydraulic pressure was released, centrifugal force would sling the blades into jump pitch but jump pitch might have been too much for forward flight.

As the blades slowed down and coned up, the skewed flap hinges automatically reduced pitch. My best guess.

Almost every conceivable hinge arrangement was experimented with during development of jump takeoff. One system involved skewed drag hinges that pulled pitch out of the rotor under the influence of driving torque.

The first really satisfactory scheme was developed by Kellett; collective pitch via a torsion/tension pack just like the Bell-47.
 

XXavier

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Beaty writes, among other things, that...

(...)

Almost every conceivable hinge arrangement was experimented with during development of jump takeoff. One system involved skewed drag hinges that pulled pitch out of the rotor under the influence of driving torque.
Yes, that was Cierva's 'autodynamic' rotor head. A famous -and successful- demonstration took place at Hounslow Heath in July 1936. Two gyros made 'jump takeoffs' there, I believe (off the head) in the same day: a modified C-30 (with a two-bladed rotor) and a smaller 'Weir'.
The skew angle (off the head again...) was 25º. The system was simple and automatic, but severe vibration was encountered. Cierva died in a KLM crash a few months after that demonstration, in December 9, 1936, and the 'autodynamic' head was -apparently- abandoned.

Rgds

Javier
 
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quadrirotor

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C.Beaty:So tell me, André, why would you be recommending delta-3 coupling?

I would like to evaluate the possibility to use a coaxial set up as a gyro, and so the use of delta3 could help to keep the two rotors as a close 90° angle of the mast as possible...obviously to avoid the contact of the two rotors, which is the most feared situation for anyone interested in coaxial set up.
 

C. Beaty

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Perhaps, André, but delta-3 coupling must be explored carefully.

Because the periodic cyclic input is lower in frequency than the resonant frequency of a delta-3 rotor, rotor tilt, initially, will be less than 90º from input; perhaps 70º, depending on delta-3 angle.

Also, as delta-3 angle approaches or exceeds 45º, the rotor can break into a nutational oscillation at the heterodyne frequency. Like a wobbling top.

Stuff gets complicated.
 

quadrirotor

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:violin: :violin:

OK Chuck, must be tried in a wind tunnel or at least on a RC model, say 1/4...then must be tested very carefully, because the RC doesn't give the interaction between aerodynamics and rehological behaviour of the materials...without forggetting the truckmush behaviour... :help: :drama: :violin: :violin: :violin:
 
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XXavier

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Hello all...

Concerning the jump take-off rotors, Beaty writes, among other things, that...


I imagine the delta-3 coupling in the PA-36 was for automatic depitching of the rotor following a jump, same as for the A&S 18-A.

The PA-36 had a coarse pitch screw retaining the blades which were held against the zero incidence stops by hydraulic cylinders. When the hydraulic pressure was released, centrifugal force would sling the blades into jump pitch but jump pitch might have been too much for forward flight.

As the blades slowed down and coned up, the skewed flap hinges automatically reduced pitch. My best guess.

(...)
Just to try to make a small extra contribution to the subject, let me quote Brooks (page 212) on the 'jump rotor' of the PA-22 (9th version):

(...) The cantilever three-blade jump take-off rotor was mounted on a faired rotor pylon. Its hub incorporated a hydraulic system, holding the blades in zero pitch during rotor spin-up to jump take-off speed. On release of the rotor starter clutch hydraulic pressure was suddendly released, and centrifugal force drove the blades outward on steep-pitch multithreaded shanks in each blade root. This had the effect of rotating the blades through 4 3/4 degrees to the flight pitch setting so that, under the impulse of the kinetic energy ¡n the over-speed rotor, the aircraft would jump smartly into the air. The system worked extremely well (...) The same design was later used successfully on the PA-36 and PA-39 (...)

Regards

Javier

P.S. I'm really impressed with your reflection on nutation and heterodyne oscillations... I'll copy it verbatim in my pocket agenda, so I may also 'explain' any trouble that I might have with my rotor... My prestige at the airfield will improve real fast... :) :) :)
 
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Not Yet

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Here is a picture of the same rotor head on the gyro that was parked next to the display.
The second picture is of the gas cap. The paint job might suggest a military gyro.

Oh by the way, both the R22 and the R44 use an 18 degree delta 3 on their 2 bladed gimbaled rotors.
 

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quadrirotor

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Thanks Mark, i had it, very interesting indeed...There are some solutions, which could be used in a fail-safe concept: The touringcopter need not the propulsor to fly...but for a monorotor helico, when you lose the tail rotor, most of the time...you are done!
 

Rotor Rooter

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Boring stuff ~ For those that have run out of reading material.

Boring stuff ~ For those that have run out of reading material.

Couplings:

The axis of delta-3 hinges are located in the plane of the mast. They are essentially a flap(teetering)-pitch coupling, although one version has a lead/lag component. Pitch-Flap Coupling

The picture of the PA-36 appears to have hydraulic cylinders to change the pitch of the blades. The flapping hinge is outboard of the pitch bearing. Therefore as the pitch is increased the flapping hinge acquires a lead/lag component. In other words it appears to be a lead-flap coupling. In addition, changing the pitch varies the ratio of the coupling. This lead-flap coupling may be intended to compensate for Coriolis. However, I believe that the lag and the Coriolis will be out of phase with each other by 45º. Lead-Flap Coupling, for Intermeshing Rotors & Pitch-Lead Coupling


Delta-3, Offset hinge, and Phase angle:

The delta-3 hinge and the offset hinge provide somewhat different functions. The thing that they have in common is that they both use the phase angle for control, and this is because the phase angle is the only thing that is adjustable on a swashplate control system.

An offset hinge changes the rate at which a blade responds to an input. The greater the offset hinge -> the faster the blade's response rate -> the smaller the phase angle (phase lag).

Delta-3 does the opposite, for a main rotor. It slows down the rate of response. However, it does it over a number of revolutions in increments that are smaller than 90º per revolution. For the bored that want more information on this subject see the first web page link above and the link to [Delta-3 and Phase Angle] at the bottom of this page.


Perhaps Chuck expressed it best when he said " Stuff gets complicated."


Dave
 
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Al_Hammer

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Dave,
from your site:
The pitch-flap coupling introduces an aerodynamic spring that increases the effective natural frequency of the flap motion.
That means that the blade will move into position sooner in the cycle, following a cyclic input. Therefore, the rotor begins its tilt a fraction of a rev sooner. I doubt this would be very noticeable.
You state elsewhere that delta-3 makes for a SLOWER response. I think by this you mean slower for the rotor tip path plane to come into complete alignment with the control plane. This may be true, but the rotor actually flaps into position faster in the initial cycle and thus the rotor tilts into position earlier. Granted, its tilt will be phase shifted by a few degrees, but it will be mostly tilted in the desired direction on that first cycle. The remaining cross coupled component will wash out in a couple of revolutions as the tip path plane aligns with the swashplate and the overall effect is of a crisper response. Anyway, that's my take on it.

Here you talk about the stick tracking:
It is assumed that the right-hand graph represents a teetering rotor with delta-3, such as the Robinson. In this situation, the phase lag has biased the teetering to left. Therefor at moderate speeds there is a need for a small amount of right lateral cyclic, whereas at fast forward speeds there is a need for a small amount of left lateral cyclic.
I'd like to quibble slightly with your interpretation of how delta-3 adds "bias". The graph is probably not for a R22, but the point is, Delta-3 in the R22/44 counteracts a right tilting tendency of the rotor from coning. It provides a left tilting bias which gets greater as the flapping angle gets greater, i.e, as forward speed increases. At low speed, the delta-3 is not doing much of anything to left bias the rotor.

Delta-3 only acts when there is a *changing* stick input and/or during flapping.
At these times the tip path plane is not lined up with the control plane. Note that this occurs continually during flapping and a rearward flapping blade will tend to constantly tilt left slightly with delta-3(up flapped blade flaps into position early, which means its high point is in the forward right quadrant.)

At one particular airspeed, right tilt from coning is exactly countered by left tilting tendency.
===============

The excerpt below, taken from a patent, alludes to a potential problem with delta-3 on multi-blade rotors, which is flap-lag instability, a growing problem in this permissive society.

"Large positive values of delta-3, however, will cause the flapping frequency to increase and approach the rotor inplane mode frequency. This can lead to a rotor flap/lag instability at high speed. Likewise large negative values of delta-3 will improve the rotor flap/lag stability by preventing coalescence of these two rotor modes." -United States Patent 6616095
 
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