Smooth Magni Rotors
Part 2
By Greg Gremminger
How does Magni keep the rotor coning angle this shallow, and therefore allow shorter teeter heights? I'd like to say it is a trade secret, but almost anyone can tell you it is done by using shorter diameter rotors. The rotor-induced vibration issues get exponentially worse with rotor blade diameters over about 25 ft. The Magni rotor on the 2-seat gyroplanes are 28 ft diameter - this compares with normally 30 or 32 ft diameter rotors on most other 2-seat gyroplanes. So, just the shorter rotor diameter makes the vibration issue a lot less challenging - in consideration of the other factors described above.
Why don't all gyroplanes use shorter rotors - now here is the BIG secret! Also, not so much of a secret, just ask any rotor blade manufacturer. Shorter rotors spin faster - they also spin faster to carry the heavier weights of 2-seat gyroplanes. The concern with most rotor blades is that when they get so fast, the tips have air velocities that are a significant fraction of sonic speed - compressibility starts becoming a factor on the center of lift on the tip portions of the blades. The closer the tip of the rotor blade gets to sonic speed, the center of lift on the blade tip starts moving aft on the blade. Ideally, the center of lift on the blade should align chordwise with the feathering axis of the blade - this is to balance stick forces. If the center of lift on the blade starts moving aft, most blades will twist somewhat, torsionally from tip to root. If/when this happens at the tip of the blade, the rotor naturally speeds up to handle the same gyroplane weight load but with less blade angle of attack. The blade spins even faster, with the tips seeing even more compressibility effects, and the whole cycle continues again. In the extreme this can result in "blade runaway", where the blades get destructively faster and faster, the blades twist flatter and flatter and the cycle repeats to destruction. Actually, what the pilot first notices is strong forward forces on the stick, as the center of lift on the blades moves further and further aft on the blade tips. To avoid this possible situation, because most rotor blades can twist torsionally from tip to hub, most gyroplane manufacturers keep the rotor diameters exceptionally long to keep blade tip speeds low enough to avoid compressibility speeds where blade twisting might become significant.
Magni shorter rotors are allowed to spin faster because the construction of each fiberglass blade is designed to resist torsional twisting. If the blades cannot twist from tip to root, there is little danger of blade runaway and higher rotor RPMs can be allowed. Higher rotor RPM means shallower coning height and shorter teeter heights with all the paybacks noted above. The Magni rotors are also very heavy and very stiff, reducing the coning angle further. It is interesting to note that a fully loaded Magni under high g loading (steep bank turns or sharp pull-ups) can achieve rotor RPMs of nearly 500 RPM - with no perception of excess forward stick pressures above what would be expected by the higher g load - the rotor shows no indication that the center of lift on the blades is moving excessively aft on the rotor blades.
A blade parameter called "tracking" can play an important part in rotor-induced vibrations. For rotor blades that are exactly aerodynamically balanced (aerodynamic center coincident with dynamic mass center) the blade tips should track along the same path in the air. If the blades are not precisely aerodynamically balanced, the visible tracking of the blade tips may not be ideal - some offset may actually present minimum vibration. The vibration from poor tracking or aerodynamic misbalance usually results more in vertical "cabin hop" than stick shake. Most rotors allow tracking adjustments to compensate for aerodynamic out-of-balance. Magni rotors have no tracking adjustment, relying on the proven quality precision of the blade fabrication. When the aerodynamic shape of each blade is precisely controlled as it is, any tracking error really means a slight dynamic mass balance error - center of mass is slightly offset from the spindle axis and the coned rotor is angled a bit to the heavy side. With the aerodynamic and geometric centers assured. Any slight out-of-track can be adjusted by slight balance weights at the tips of the blades. In fact, this is how the final dynamic balance is achieved, adjusting weights at the tips to attain perfect track. Now, I'm almost running out of things that play a part in the extreme smoothness of the Magni rotor system - but there is a little more. No rotor system is perfect, and 2-blade teetering rotor system have at least one vibration issue that cannot be avoided completely. Rigid 2-blade rotor systems will incur a 2-per shake, fore and aft, due to the differences in rearward drag on the advancing blade relative to the retreating blade. More involved rotor systems such as on helicopters employ a lead-lag hinge on each blade to eliminate this cause of vibration. To keep our gyroplane rotors simple this mechanical complication is not normally employed on gyroplane teetering rotor systems.
To minimize this drag-related 2-per vibration, Magni rotors employ very clean and low drag airfoil and surface perfection - no rivets, no seams, precise airfoil contours throughout the whole span, etc. Just keeping the drag low minimizes the drag resultant shake.
But, there will always be some minimal amount of rotor-induced shake. Magni uses one more trick to prevent the pilot and controls from seeing this remaining vibration as stick shake. Some amount of friction is applied in the roll and pitch pivots in the rotor head. This friction significantly prevents any rotor shake from transmitting through the controls to the stick, it prevents the rotor head from moving on its roll and pitch pivots with the vibration. By restricting movement of the rotor head itself, any rotor vibration is transmitted into the mast and airframe instead of through the control system. For the high airframe moment of inertia and very stiff mast and frame, the resultant body shake is hardly noticeable. What this is really doing somewhat, is to not allow the rotorhead spindle to completely move as the rotor centers remains stationary. Because of the high inertia of the airframe, some of the rotor out- of-balance or shake is forced into the rotor disk and not all of it into the airframe body. At any rate, when this slight friction in the roll and pitch pivots is applied, increased body or airframe shake is not noticeable, but the "stick shake" transmitted through the controls is very noticeably reduced.
Ok, the more perceptive among you might argue that it is not a good thing to have friction in the controls! Without starting into the whole issue of pitch stability in gyroplanes, I'll agree that control friction is a thing to be avoided on unstable type gyroplane. If a gyroplane's airframe is unstable, that means it moves or pitches in the wrong direction in response to a vertical gust or g load transient. Because historically, most gyroplanes were negatively or neutrally stable in this regard, it has been very important to avoid friction in the controls or even a tight grip on the controls in those gyroplanes. Such friction or tight grip transmits the airframe wrong-pitch movements into the rotor, exacerbating the unstable situation into possible PIO or PPO. The Magni airframe, however, is very aerodynamically stable. So, its airframe moves in the correct pitch direction relative to a gust or g load transient. This means that it is actually a GOOD thing to couple this airframe pitch movement into the rotor - it further enhances stability. So, we apply some friction into the roll and pitch pivots of a Magni gyroplane to accomplish two things - reduced stick shake and further enhanced pitch stability. Of course the amount of friction applied in these pivots is kept below the level which might induce pilot over-control to overcome the friction. Magni gyroplanes also employ a high degree of control force feedback - for pilot over-control prevention - so the friction applied in the rotor head pivots is not really noticeable in the control forces.
Another factor in the Magni gyroplanes that minimize stick shake is simply that there is no play in the control linkage to the rotor head. All joints are precision bearings. In many gyroplanes, some joints are simple bushing joints, which accumulate some play. When you let go of a cyclic stick with a lot of play in the controls, the stick naturally shakes a minimum amount in this "play". This would not be actual, forceful stick shake, but it appears to have a lot of shake when you let go of the stick. Magni eliminates control play for a different reason - any play in the controls is a potential source for over-control by the pilot. If there is "play" or slop in the controls, the pilot must physically move the stick through this "play" - with no effect on the rotor - and then suddenly adjust his stick movement when the slop suddenly is "bottomed" out and suddenly further stick movement is actually affecting the rotor. Stick "play" should be minimized to minimize this potential for pilot over-control.
Now I'm finished, but I suspect there are other things that Magni has employed in these rotors that I am not even aware of. A prominent original designer of fiberglass helicopter rotor blades developed the fiberglass rotor blade technology that goes into the Magni rotor blades. He and Vittorio Magni conspired decades ago to employ this technology for autogyros. Vittorio Magni advanced the technology for his autogyros - solving other rotor shake problems along the way. With this much expertise and experience and time and money spent on these rotor systems, I'm quite sure I don't know the entire story. After all, we do need to keep some competitive edge!
One last thing - no you can't buy a Magni rotor for your gyroplane. First, it wouldn't match your rotor head design, and secondly, the rotor dynamics, inertia, and other features would not be proper for most other gyroplanes. As explained above, there is much more that goes into a smooth rotor than just the rotor and rotor head. And, Magni nor I could either compensate for or assume responsibility that all those other factors are properly addressed on your gyroplane. It is certainly not recommended that you attempt to apply any of the design principles above unless you thoroughly conduct the required testing in close consultation with that rotor manufacturer. Many development and test hours went into this Magni design. While some issues seem straight forward, many important considerations are not immediately apparent, such as the issues involved with maximum rotor RPM, airframe stability and rigidity, etc. Kids, do not attempt this at home!
Greg Gremminger - Magni USA, L.L.C.
