Rotor Damping

[C. Beaty]

Whenever the name Magni is mentioned, the Faithful bow their heads in reverence and chant the Master’s Mantra; “The road to Everlasting Stability is paved with Harmony.”

Along comes the Irreverent Mr. Bird who says; “The bloomin’ thing drives like an overloaded dump truck with the power steerin’ busted.”

The original US importer of the VPM predecessor machines, Bill Parsons, continuously bitched about stick pressure. That’s been 20 years ago.

Other, more discreet individuals say; “It flies like it’s on rails.”

There are some that credit the horizontal stabilizer for the excellent safety record of the Magni line and dismiss out of hand the University of Glasgow windtunnel testing that showed the horizontal stabilizer, buried in the fuselage wake as it is, to be less effective than it should have been.

I think what might have been underestimated in the Glasgow studies was the effect of damping provided by the rotor.

Rotor damping is produced by the rotor lagging behind a control input, whether its spindle is tilted by the pilot manipulating the cyclic control or by a disturbance tilting the airframe. A rotor with flapping hinges (teeter hinge) exchanges its gyroscopic rigidity for damping. A rotor that lags behind airframe motion produces a force that opposes such motion and provides a restoring moment.

Cyclic stick force is a good relative measure of rotor lag and therefore damping provided by the rotor. The more the rotor disc axis lags behind the spindle axis for a given tilt rate, the greater is the stick force and damping. A rule of thumb is heavy rotor = heavy stick. Check FIG 1.

If there are no tricks involved, the lag of the rotor disc vs. spindle tilt rate is a relatively simple quantity to calculate. But I suspect the Magni rotor has some tricks that I’ve yet to uncover. Otherwise, there is no plausible explanation for a Magni having higher stick force and slower response than similar gyros with the same rotor weights.

Other gyros with similar rotor inertia and AUWs have significantly lower stick force and greater agility. I believe that an RAF-2000 fitted with a Magni rotor would behave nearly identically to a Magni and that a Magni with an RAF-2000 rotor would behave nearly identically to an RAF-2000, after due allowance for differences in horizontal stabilizer configuration.

Some of the tricks applied in the past have used auxiliary gyroscopic devices to artificially increase the apparent inertia of the rotor and its damping.

Perhaps the best known is the Bell flybar rotor. The flybar is pivoted to the mast and driven by it but maintains a fixed position in inertial space. The rotor, with respect to cyclic pitch, is tied to the flybar and also maintains its position in inertial space in the case of rolling or pitching motion of the airframe. The pilot’s input is via a differential linkage that applies input in series, between flybar and rotor.

The flybar is slowly precessed into alignment with the airframe by a pair of friction dampers but which do not respond to short term disturbances. See FIG 3.

Another artificial inertia device is the Hiller Rotomatic control system and stabilizer. With this system, the pilot “flies” the servo rotor, which in turn, controls the main rotor. It is a system related to the Bell flybar rotor but with a very different control mechanism. The response to pilot input is quite sluggish and slow. The paddle blades of the control rotor can be ballasted to have almost any control lag imaginable. There is a well known publicity photo of a Hiller hovering with sandbags strapped in the seat and with the “pilot” standing alongside; probably wouldn’t do anything but hover with the necessary amount of lead installed in the paddle blades.

The Hiller servo rotor is the device that first enabled ordinary humans to fly RC helicopter models; without such a device, model helicopters are far too twitchy to be flown by mere mortals. Now, of course, the coming of inexpensive piezo rate gyros has rendered such devices unnecessary.

The sketch of FIG 2 illustrates how a Hiller servo rotor might be applied to a tilt spindle gyro. The near side hub plate was omitted to show details. Whatever stick force and rotor response wanted could be easily obtained by proportioning mass to area ratio of the paddle blades.

There are advantages to artificial inertia devices. A low inertia autorotating rotor has some desirable features: namely its rotational speed can more quickly respond to load, suppressing cyclic flapping and the resulting excess velocity stability of rotorcraft.

An increase of load, say an upward gust, increases the angle of attack equally on both retreating blade and advancing blade but since the advancing blade has greater airspeed, its incremental lift increase is greater, causing the rotor to tilt noseup, an unstable response. If a rotor was inertialess, its rotational speed would increase instantaneously, reducing airspeed differential between the two blades and tending to suppress cyclic flapping. Excess velocity stability (rotor blowback) is a primary contributor to angle of attack instability along with propeller thrust line offset.

Airpeed stability is a different kettle of fish. It is related to angle of attack stability.

I think in the final analysis, Juan de la Cierva had it right; get rid of the gimmicks, arrange the propeller thrust line to pass through the CG and provide ample horizontal stabilizer power with surfaces centered in the propeller blast.

The first man to successfully hitch an ox to a cart must have had at least an intuitive understanding of the relationship of thrust line and CG.

[troed@aon.at]

I still do not get what this should tell us ?!

The M 24 Orion has surprisingly low stick forces compared to the M16/22. Actually the M24 feels on the stick like a heavier MT03 (and this model has VERY light stick-forces).

So there MUST be somethin´else contributing to this than only rotor-weight.

All European gyros have a similar design, esp. concerning H-Stab, based on the gyro-concept of Juka Tervamakki and Vittorio Magni. All fly stable, never heard of a non-pilot-error fatal incident.

The incidents I know caused by pilot error would have happened the same with LTL/CTL-gyros.

Please get me the POINT You are aiming at to make me FULLY understand about the issue of this discussion.

Angelo

[Doug Riley]

Chuck: I agree that rotor damping probably has a good deal to do with pitch stability. A wingless gyro's rather weak roll stability demonstrates to us that rotor damping isn't especially powerful in general.

The only PIO I've ever managed to get a gyro into is in the roll axis. The first time I got my gyroglider off the ground, I got into a wild side-to-side rolling PIO that terminated in a snow pile on the edge of the runway.

The Bensen gyro was a "perfect storm" of poor rotor damping. The blades were fast (400 RPM) and light. That may account for the Bensen's proneness to porpoising and pitchover, even though its HTL wasn't too bad by today's standards. Bensens were smoking in at almost one a month when I first got involved in this hobby.

Fast forward to the 80's. McCutchen baldes were dramatically heavier, and slower, than Bensens. The first time I flew these blades and saw the shadow of the rotor languidly coasting about overhead, it was positively weird. Slow and heavy equals more damping. McC blades probably helped blunt the effect of the increasingly high thrustlines brought on by redrives.

[bpearson]

Originally Posted by troed@aon.at

The M 24 Orion has surprisingly low stick forces compared to the M16/22. Actually the M24 feels on the stick like a heavier MT03 (and this model has VERY light stick-forces).

Glad I wasn't imaginning it Angelo. Maybe the set up of the 24 allows for more stick to head movement (no rear seat for the back stick to hit).

The MT has much longer control sticks as the seats sit higher. Maybe that is all there is to it ! I've seen Magnis thrown around Chuck just like the Italian on your machine in the video but maybe you need bigger arm muscles.

[C. Beaty]

Just trying to flesh out opinions with some facts, Brian,

The only quantitative data we have are the 2-axis hang tests done in Australia, showing the prop thrust line/CG offset to be about 12 inches and the Glasgow data showing stagnation pressure at the horizontal stabilizer to be 50% of free stream.

If anyone wishes to dispute these numbers, he ought to come up with his own set, not dismiss it all out of hand.

I suppose everyone knows how to locate the CG from a double axis hang or weight test.

The stagnation pressure can easily be measured with an airspeed indicator and a proper pitot/static probe mounted at various locations along the stabilizer leading edge.

"When you can measure what you are speaking about, and express it in numbers, you know something about it, but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind: it may be the beginning of knowledge, but you have scarcely. . . advanced to the state of science." –Lord Kelvin

[C. Beaty]

Doug, although lateral PIO has happened, it’s pretty rare. The lower roll axis MOI and the crisper response probably account for that.

I’ve also experienced lateral PIO that was caused by too much backlash in a rotor system that had no component of thrust fed back into the stick.

It was with a rotor where the roll/pitch pivots were coplanar with the teeter bolt.

I think three of us tried to fly it, Ernie, David Seace and myself. No one could fly it until I solved the backlash problem.

The self centering action of a standard offset gimbal rotorhead mitigates control slop.

[Doug Riley]

Chuck, that probably means that I'm laterally challenged (a politically correct way of saying "retarded").

My Bensen gyroglider had a spindle head (no centering force and very weak rotor damping). The first time I had it off the ground, I got it into a wild lateral oscillation. It and and I were saved from damage by my colliding with a soft snowbank instead of hard tarmac.

The next time was nearly 30 years later, on the initial flight of my Gyrobee. It lifted off at about 18 mph and I got it wallowing around pretty nicely, though nothing like the old B-8 glider. The 'Bee has a large, slow, medium-weight Rotordyne rotor and a gimbal head... but (like the glider) it has an overhead stick with a very slow leverage ratio. The total throw in each axis is about two feet, giving a rate of over 1" per degree. It takes a measurable amount of time to physically deflect the stick far enough for anything to happen. This is a source of lag/overshoot.

Anyway, I got used to both of them after awhile.

[Lee Scatt]

Chuck, Thanks for posting such info. It really helps to understand what I have read in other threads.

Your statement concerning inertia produced velocity stability as a negative, due to angle of attack instability, is interesting.

Do you think it would be possible to fabricate a set of rotors resembling the Hiller paddles? Basically a 12 foot dia. hub bar with 5 foot airfoil sections. Filament wound tubing to resist torquing and bending. Seems it should have a lower mass.

I can't imagine the inboard 5 or 6 feet of rotor having enough airspeed to have any appreciative lift.

[bpearson]

Originally Posted by C. Beaty

Just trying to flesh out opinions with some facts, Brian,

I think the debate will go on Chuck until someone puts a raf rotor on a Magni or vis versa. The numbers are too much for me to follow.

I'm sure the V CofG thrustline offset was found to be around 4" by Glasgow or somebody so I suppose that figure needs to found out with some accuracy first.

I remember walking into my first A level physics lesson to be told everything I'd learnt at O level was wildly inacurate and fairly useless. I dropped out of school a few weeks later and got a job !

[Learjet]

Whenever the name Magni is mentioned, the Faithful bow their heads in reverence and chant the Master’s Mantra; “The road to Everlasting Stability is paved with Harmony.”

he he at least the Magni Faithful aren't having to bow their heads because the too heavy rotors are busy chopping off the too low tail-plane situated too far below the too high thrust line... funny that! :-)

Arrow engined VPM M16 G-BWGI modified with air data system, inertial measurement unit, recorder, power supplies etc

Mass (zero fuel, 78 kg pilot in rear seat) 426 kg
Normal distance from propeller geometric thrust line to c.g. 1.75" (c.g. below)

Mass (full fuel, 78 kg pilot in rear seat) 459 kg
Normal distance from propeller geometric thrust line to c.g. 3.24" (c.g. below)

Results for Rotax 912 modified VPM M16 G-BUZL showed a slightly higher c.g. If I can find photos of G-BWGI it can be seen that the c.g. is likely to have been lower than normal due to the batteries from a Cessna 406 being place where the front seat used to be.

I also did a VPM M16 that was to be used for a round-the-world trip by a British Army pilot. This had c.g. nearly 6" below. This aircraft had been modified for the trip, so was non-standard, but it had other features that I hadn't seen before on 'GI or 'ZL. The Royal Air Force weighing team repeated the exercise a week later and obtained the same result I believe.

Will get the details for 'ZL and the round-the-world machine tomorrow and post here.

[Udi]

I know the Magni blades are supposed to be torsionally stiff but -- wouldn't an over balanced airfoil rotor be highly damped as well? Is it possible the Magni rotors contain a piece of lead or some other heavy metal at the leading edge, far out there - close to the tips? This may be one of the "company secrets".

Dr. Angelo and Mr. Pearson - I am not sure a light stick on the M24 is good news. If Chuck's hypothesis regarding the Magni stability is correct than the M24 may not posses the most important trait - high rotor damping.

Doug - I am not sure if I read your statement correctly, but a high-RPM light rotor is better damped than a low RPM light rotor. Whether a low-RPM heavy rotor is better damped than a high-RPM light rotor depends on the proportions. MOI is linear with weight but the square of the arm (of blade CG).

[bpearson]

Originally Posted by Chippydriver

Arrow engined VPM M16 G-BWGI

Mass (zero fuel, 78 kg pilot in rear seat) 426 kg
Normal distance from propeller geometric thrust line to c.g. 1.75" (c.g. below)

Mass (full fuel, 78 kg pilot in rear seat) 459 kg
Normal distance from propeller geometric thrust line to c.g. 3.24" (c.g. below)

Wildly differing numbers to the Australian test and IF right then explains a lot.

[C. Beaty]

Doug: I have always disliked a slow stick and suspect it could cause control difficulty.

Lee: On the NACA site, there is complete wind tunnel test data on rotors with various amounts of the inboard portions of the blade replaced with a streamlined spar. It doesn’t look good with too much cut away. I don’t have the URL stored on this computer but have it on another if you can’t find it.

Udi: Overbalanced blades was what I’ve been thinking about when saying there might be a secret vis-a-vis rotor damping. Greg G. has assured me there is no overbalance.

However, overbalanced (nose heavy) blades will provide an effective increase of rotor damping. I suspect that’s Wing Commander Wallis’ secret.

[Udi]

There are two ways to know whether the Magni blades are overbalanced - one is to get the manufacturer plans and the other is to dissect a blade postmortem. If only the outer section of the blade is overbalanced, it would be difficult to detect while the blade is intact. If you have a section of the blade and you don't know what part of the blade it came from, you won't be able to rule out overbalancing in other sections.

What other "secret" could there be?