- Jul 17, 2004
- Hamburg, New Jersey USA
- GyroBee Variant - Under Construction
Pendulaire is too hard to say. I assume they are called trikes due to having three wheels? Apollo just replaced the trike wing with a rotor and called a gyro.Why exactly are they called "trikes"?
"Pendulaire" actually sounds more apt to me.
I still have questions about combining (especially) an AOA-sensing, variable-RPM rotor with a centered flap hinge, though. It's one of a couple reasons that I feel that all the small gyros on the market are flying with scanty hard-engineering data in hand. In this respect, among others, none of them is the equivalent of a type-certified aircraft. We have both "known unknowns" and "unknown unknowns."
Which is not to say that I won't/don't fly them. I enjoy the experience enormously. But we ought to be honest with ourselves.
Hi Abid,AOA sensing variable-RPM rotor with a centered flap hinge?
So AOA sensing on which section of the blade exactly and how would a variable-RPM be done in autorotation exactly except for a pitch changes in flight for blade with a full mechanism to do it and centered flapping hinge like an offset flap hinge or what exactly do you mean. Offset flapping hinge should reduce 2 per rev a bit but not sure exactly what you mean?
Doug,Here's a page from Gessow and Myers covering the offset-flapping-hinge idea. Note especially the comment that this rotor suspension setup provides control power in situations of low rotor thrust -- another way of saying "in low-G situations."
IMHO, the hobby-gyro "industry" has not explored this sort of meaty technical issue, instead unquestioningly copying Bensen.
By "AOA sensing," I'm referring to disk AOA, not blade-element AOA. All gyro rotors are disk-AOA sensing devices.
A heli rotor is designed to be essentially constant-RRPM. A gyro rotor, in contrast, is a variable-RRPM device.
The heli rotor is designed to respond to changing G-load needs at constant RRPM, by means of increases or decreases in collective pitch and throttle (normally at the same time). The engine and rotor turn at constant RRPM, and throttle changes affect only manifold pressure.
The gyro's rotor, OTOH, responds to changes in its disk AOA. We commonly say that the rotor speeds up in high-G situations but, in fact, that's a backwards way of talking about it. What occurs is that disk AOA increases (because the pilot pulls the cyclic aft or an updraft hits from below). The rotor experiences an immediate increase in the blades' AOA* and therefore an increase in thrust -- but the rotor also speeds up. There's the variable RRPM effect.
This increase in rotor RPM at higher disk AOA is well and good in "G exceeding 1.0" scenarios (such as landing flares). But it's problematic at the low-G end of things. The rotor responds to low disk AOA by losing thrust and by slowing down. We call this a "low G" situation but, again, the rotor doesn't know about G's, it knows about disk AOA. It responds as designed, by losing RRPM.
Central flap hinges (whether teeter or other style) mean that only the rotor's thrust is applied to the gyro's frame. Your control and stability forces are akin to pulling from above on a rope attached to the teeter bolt. Less thrust and RRPM mean less control and stability. Slack the rope altogether, and you have zero control/stabilizing force.
If OTOH, you move the flap hinges outboard from the spindle, the centrifugal force of the blades also feeds back to the airframe, providing some control and stabilization force even at zero thrust. This is sometimes called the "T-bar effect."
The Cierva direct-cyclic gyros that retained the tilt-spindle cyclic system had slightly outboard flap hinges (perhaps intentionally taking advantage of this auxiliary supply of control authority), but they suffered as a result from high stick forces. You have to muscle the spindle around against the T-bar effect, once you move the flap hinges out from the spindle axis. A swashplate gets rid of this control-force trap... I believe Groen went through the process of re-discovering this dilemma; Jim Mayfield may want to comment.
My point is that the combination of centered flap hinge and autorotation has not been subjected to controlled experiments. We don't know exactly where the edge of the cliff is, or whether we can/should try to design our way back from it.
* Notice that blade AOA is not the same thing as blade PITCH. We can have no collective control, and still have variable blade AOA (that is, the angle at which the air actually hits a blade element can vary). In particular, increasing disk AOA also increases blade AOA and vice versa. IOW, our variable disk AOA, though achieved with cyclic only, has the same aerodynamic effect on rotor thrust as a collective pitch control.
My understanding is that the original "taildraggers" had NO back wheel, just a skid that actually did drag on the ground. They actually had only two wheels. I believe those two wheels would still be considered "conventional" landing gear.With the nosewheel up front we call airplanes "tricycle" , but at the other end we call it "taildragger " while the FAA calls it "conventional gear" despite the wheel count being the same.
To be fair @fara on a trike I have never worried too much about low g. I have with a passenger, left the seat completely when I entered a strong downdraft, least I think it was a downdraft, I know we fell like a stone, my mate made a great impression of a mouse with his squeal..So you are basically worried about low G situations.
Same as trikes, PPC, PPG and even many helicopters.
To be fair @fara on a trike I have never worried too much about low g. I have with a passenger, left the seat completely when I entered a strong downdraft, least I think it was a downdraft, I know we fell like a stone, my mate made a great impression of a mouse with his squeal..
P.s. larry's videos re trike flying are exceptional and helped me understand my pendulaire much better and the fact that the pendulaire can easily cope with meteorlogical conditions that would makes most men squeal.
p.p.s. I don't think my "crown jewels" are bigger than anyone elses, but, with age, they hang lower
Brian: Re your Post #22:
What keeps our blades from folding up in response to their own lift is not the rigidity of the (Bensen-style) hub bar, but rather the outward pull of centrifugal force. Cent. force* pulls out at right angles to the rotor's rotational axis, while lift pulls up; the combination of the two settles the blade into a coning angle of a few degrees only. It doesn't matter if the blade is free to rise as much as it wants; unless RPM is lost catastrophically, it won't rise more than a few degrees because cent. force won't let it. There have been rotors with centered, but independent, flap hinges designed as you described. Chuck Beaty built one that looked like a giant door hinge, and some old helicopters had blade-root yokes somewhat like the one you're picturing.
If the flap hinge is not at 90 deg. to the blade's spanwise axis, then the hinge is said to have a "Delta three" offset. The A&S 18A uses some of that. It provides the 18A with an automatic reduction in collective pitch as the blades use up their stored rotational energy and slow down after a jump. They drop from helicopter pitch to gyro pitch.
Jim Mayfield is describing a different way of achieving the same effect as a Delta three offset, while still employing a rigid teeter bar and hinge: in either case, pitch-cone coupling is the result.
Fara, yes, I am concerned about low-G situations. And, yes, trikes and teetering-rotor helos and PPCs have issues with low G, too. (BTW, trikes may have been so named to distinguish them from the original foot-launched powered hang gliders. Just a guess.) I don't know of any trikes or PPC's that have achieved STC, though. I don't know if one could pass.
But the situation with gyros and zero G (= zero disk AOA) is, in any event, unique. Our RRPM is variable (but not instantly), and is related to disk AOA. As you point out, loss of RRPM is a one-way street beyond a certain point. Most, if not all, gyro low-G crashes have clearly resulted from airframe instability (HTL, engine torque and/or low center of drag), which is preventable by design, but low RRPM is certainly another route to disaster.
My point is not that Bensen-style gyros are clearly and unavoidably uber-dangerous. Still, in discussing gyro safety, we shouldn't ignore the fact that these issues have not been explored systematically to see just where the safe edge IS, or how much better some other setup might be. This lack of knowledge is itself a risk factor: a "known unknown." Compare a C-172. It's pretty unlikely that any unexplored coffin corners are left in one of those.
* No, it's not really a force; it's an inertial effect, but the difference doesn't matter here.