Forum Notes Page Four

**This is Forum Notes.... Page Four**

From Ralph Kalb, July 1997

JB: Referencing your "engineering" students questions as to why "no flybar" is needed on R/C gyros, one has to look at fixed wing aircraft for a clue..............

On a large fixed wing aircraft, the control surfaces are HUGE, the airspeed is high, and the forces that the pilot can exert on the rudder pedals is limited by his feeble physical muscles.    In order to give him maximum control (assuming no hydraulics/electrical hookup), the designers came up with the idea of letting his muscles deflect a small "servo" rudder tab (for instance), and the force generated by the "servo" tab would, in turn aerodynamically, cause the larger control surface to move in accordance with the pilots wishes.    On smaller planes (Cubs, etc) the forces were neglegable, and "servo" tabs were not needed, since the force required to move the controls was reasonably low.

With the advent of the helicopter, it was found that the forces of a large rapidly spinning heavy rotor could overcome the muscle force of the pilot.   In addition, the problems of directly controlling the rotor by tilting the engine or using a kind of universal joint to achieve fore/aft and roll control sort of ruled out direct control.

Sikorsky, in his early efforts went so far as to use multiple engines/props at the tail to control pitch, yaw and roll, to keep the rotor shaft fixed.   This obviously heavy and complex solution was soon abandoned in favor of the swashplate, which used either aerodynamic shaped "paddles" or the gyroscopic forces on weights to act as the "servo tabs" used for large fixed wing aircraft.

The "modern" autogyro, other than those like the Air & Space 18A which uses collective pitch for "jump takeoff capability" do not need flybars, although they can and have been used, usually on the larger R/C gyros.    Since the gyro is an anachronism in our world of helis, it is concievable that many R/C gyro designers merely copied what worked on helis, without realizing that an alternative, lighter, less complex solution existed long before the advent of the helicopter.

Certainly, either method of controlling a rotor can be used.   However, the KISS (Keep It Simple, Stupid) philosophy is one to keep in mind when selecting a gyro control design!

Ralph Kalb

SINGLE BOLT BLADE GRIPS:
I wrestled with this for some time trying to decide ahead of time about the necessity of "lead-lag". In general, the literature generally says for models, it isn't necessary. However, in balancing model heli rotors it becomes clear that any deviation in lead/lag changes the apparent balance. Many times I've "chased my tail" trying to balance a rotor with one blade cocked off slightly. So, for convenience in balancing, mostly, I use a single bolt on the blade grip. Find those little locking nuts with the plastic inside work great for all nuts and bolts on a gyro, helicopter, etc. and have never had a blade come off. Tightening the bolts snug (so the blades can't flop around on their own), but loose enough so you can still move the lead/lag by using your hands, works well for me. Need some pressure to move them out of phase. Then, when the rotor spins up, this allows each to seek it's own "place". I usually then tighten them down a little so I won't innadvertantly move one especially if balancing the rotor is desired. Of course the added benefit of all this, is rare blade damage on a minor rotor strike. Like last weekend, with those tip-heavy blades on the BEECH whacking the desert flora, nary a scratch!

(* Edit note: You can have a second hole, and use either a small nylon bolt or just a balsa stick inserted to act as a shear bolt.   This works very well!    ( Jim Baxter),BR>

PS: The equation for calculating the pitch change on a Delta 3 gyro head described in Bill Youngs R/C Gyro Design manual is in error (Appendix d). Should be Sin Cone angle = Tan Pitch divided by Tan Delta pitch. (Ralph Kalb)

ALPHA-1 FOR JUMP T/O's
I don't know if you've been reading the past notes, but I suggested to "all" that the Whopper would be a natural for use of the Alpha-1 hinge for jump takeoffs, since the Whopper has a servo controlled disconnect to the pre=rotator (my Wallis doesn't- it depends on a centrifugal clutch). The Alpha-1 allows the blade pitch to reduce to a zero lift pitch (-1.5 degrees on a Clark Y) as long as the hub is being driven and the blades are lagging due to drag. When the pre-rotator is disconnected, the blades swing out slightly due to centrifugal force (ie their CG's and the hub center all line up!), the pitch increases to "normal" for gyro flight, and UP she goes. Actually, the blades swing slightly ahead of normal and then settle down, but that little extra UP pitch really gives the "jump" lift.    (Ralph Kalb)

ROTOR FLUTTER:
Yeah, guess your right, the 25% chord point should be at the chord being measured. Great if you have straight tapers, wha' hoppen if they are not? Dunno!
I guess you just have to think the tip is a narrow airfoil and the hub as a large airfoil and pick the 25% points on both and connect with a straight line. CG and hub mounting hole should lie on the line! Reason: Hold an empty box of paper matches out the window of the car by the stapled end. Due to the curvature of the cardboard, the "lift" will move the cover either up or down (depending on how youhold it).
As the cover moves past the zero lift point, it will continue to swing due to its mass until it is now curved in the opposite direction, where the lift vector will reverse and drive it back to its starting point, ad nauseum!
The rapid action is known as "flutter" and has been photographed by Kellet, Pitcairn, et al using a movie camera on the hub. Needless to say, flutter gets serious real fast!Best to keep the CG on or ahead of the zero moment line for any blade that is even thinking of twisting due to insuffiecient torsional stability! (Ralph Kalb)

DISK LOADING:

If you take the overall weight of the FA and divide it by the original area of the rotors (total area), then you have the disc loading in #/ft^2. If the disc loading of the modified design is lower than the original, you should be OK. The drag of the rotors can basically be imagined as having two pizzas in place of the rotors and inclined away from the wind. That frontal area is one big Mother, and will tend to rotate the gyro backwards in sort of a "on the ground loop!". With respect to the blades, the higher solidity (total blade area divided by total disc area will give you lower RPM's, both in flying and in per-rotation. Remember, those blades have to have a very good finish, so if you can shrink tube them do so!! (Ralph Kalb)

By the time I got here, the note you posted on undercambered vs Clark Y angles of incidence were gone, but here goes anyway. The undercambered has, I believe, a larger negative "zero lift" angle of attack, perhaps -3 degrees. The Clark Y has a zero lift angle of attack at about -1.5 degrees, and a symmetrical (NACA 0012) has a zero lift angle.

If the angle of incidence setting at the hub were zero, the tip ANGLE OF ATTACK would be somewhere around 5-8 degrees, dependingon how fast the rotor was turning, and how fast it was moving forward, and haw far the rotor mast was tilted back. The zero lift angle moves it to a slightly less angle of attack, so start with a zero angle of incidence, and then use the tripod to "fine tune" the angle of incidence for maximum RPM.

The KT-1&2 had no Bensen head, and were fixed hub machines. The rotor shaft was adjustable on the ground. Flight controls (per Nick's letters) were rudder, throttle, and elevators. His philosophy was with the rudders in the position they are, a right turn would torque the fuse (and therefore the rotor) right, giving a banked turn. With the rudders "up" as on most aircraft, the opposite is true. On the use of a wing vs no wing, I fully agree that a wing as a roll damping device is necessary on a Pitcairn type craft. I also believe it is very significant that as soon as the "direct control" head was evolved, all the need for a wing "disappeared"!! The difference of course is that with the "direct control" head, the fuse was hanging onto a free flying stable rotor like a Christmas ornament on the tree! (Ralph Kalb)

WEIGHT/DISK LOADING:
I was asked about disc loading (Weight/Disc Area) and what I thought was the "right" value. One thing I was hoping to do on this BB was to determine various machines weights and rotor diameters to get a ball park value to answer such a question! Although it may seem a cop-out, go for the lowest disc loading you can get away with, ie lowest weight and largest rotor you can safely swing! Decreasing the disc loading is probably one of the most important goals in designing a gyro, since as the disc loading decreases, the minimum AS decreases the maximum AS increases, and the power available for climb (PAFC) really increases, all of which improve performance. If the disc loading is too high, the gyro won't even get off the ground, although it will be a high speed taxi vehicle!

3 blades vs 2? Well, all other things being equal, the 3+ blader carries less of a load (ie total weight/# blades), so the coneing angle is smaller, which may affect the stability (sort of like dihederal in a fixed winger). In addition, the three (r more) blade rotor has a higher solidity (total blade area to disc area, and an increase in solidity decreases minimum AS, increases max AS and increases PAFC, so thats a benefit. However, the extra blade(s) cause a reduction in RPM. If we are talking about vertical descent (ala Maple Seed), who cares? However, with forward airspeed, the time for the blade to go from the rudder to the nose is important as to the total amount of flapping we can handle mechanically, and the Tip Speed Ratio (TSR= AS/Tip Speed), since that is a figure of merit that indicates how badly the blade is stalled! Again, the lower the TSR, the better!

Chord/Diameter ratio: Again, the problems of solidity are addressed as we increase the chord with a constant diameter, since we are increasing the blade area. Additionally, a large aspect ratio (radius/chord) has a lower drag than a small aspect ratio, so again long, narrow blades seem the best bet! (Ralph Kalb)

Disk loading seems to work out at about 3 to 6 on single rotor gyros, with the load increasing some on dual rotor models.
In my experience, Low aspect (paddle style?) seem to work reasonably OK on small models of the dual rotor style, however high aspect (long narrow) as stated above, seem to be the best, by far, especially on the single rotor models.
(Jim Baxter)

Chord/Diameter ratio: Again, the problems of solidity are addressed as we increase the chord with a constant diameter, since we are increasing the blade area. Additionally, a large aspect ratio (radius/chord) has a lower drag than a small aspect ratio, so again long, narrow blades seem the best bet! (high aspect ratio) (Emilio Cabezas.. compuserve..1996)

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