XXXX

[jm-urbani]

xxxxx

 

[XXavier]

No, the two-blade cierva 'autodynamic' rotor wasn't of the teetering type. The blades were individually hinged in flapping and lead-lag; the blades pitching down when lagging and up when leading, thus making the jump take off possible...

Lots of information on the 'autodynamic rotor', here:
https://rotorcraft.arc.nasa.gov/Publications/files/Harrison_TP-2015-218714.pdf

 

[C. Beaty]

Drag hinges were inclined in such a way as to produce pitch change vs hinge motion. Under the influence of rotor drive torque, drag hinges would swing back until reaching a stop; at which point, rotor blade pitch would be at zero lift. When drive torque was removed, blades would swing forward to normal position and blade pitch would be returned to normal flying pitch.
With a 3 blade rotor and normal drag dampers, the forward swing was so slow that too much rotor rpm was lost and jump wasn’t satisfactory. Only a 2-blade rotor could be flown without drag dampers but Cierva was unable to solve the 2/rev vibration problem while in flight. The vibration solution had to await Arthur Young and the Bell Helicopter.

 

[wolfy]

Would there have been any pictures posted here?

wolfy

 

[Arco]

Like a fix pitch autogiro. See almost vertical landings done by C 30 and others Cierva models. No changes

 

[C. Beaty]

Low disc loading (~ 1.5 lb/ft²) and low blade loading (~ 35 lb/ft²) = low tip speed and high rotor lift/drag ratio.
Long stroke landing gear provides good absorption of landing energy.

 

[XXavier]

How does helis manage to perform auto-rotation zero "roll" landings without wheels and without suspension ?

They have collective pitch control. The helicopter is descending with an important component of forward speed. Just before landing, the pilot pitches up the craft in order to load the rotor, converting that forward speed component into extra rotor revs, and then he cushions the landing by increasing the collective pitch, putting that extra rotor energy to work...

 

[XXavier]

Hi Javier,
does the pilot control the collective pitch manually or is there a system that keeps the rotor rpm constant acting on the pitch to achieve this constant rrpm ?

I don't know, but I suppose it's a manual control.

 

[wolfy]

It is up to the pilot to maintain correct rrpm while in auto rotation. Get it very wrong and there won't be second go.

wolfy

 

[XXavier]

For a given helicopter descending in auto-rotation, there's an interval between minimum and maximum pitch, so that, within that interval, the auto-rotation régime is preserved. Of course, there's an optimal pitch value, corresponding to the minimum sink speed, but the auto-rotation régime will be preserved for other, non-optimal pitch values, always within that interval...

 

[C. Beaty]

Because of flap hinges, coning angle changes with load. No load, no rotor cone. Double the load and the coning angle doubles (approximately).

 

[WaspAir]

touchy !

Not really.
In many helicopters, you can just shove the collective down and leave it there until the final pull, and rpm will stay in a reasonable range with the typical flight loads you induce through the maneuvers you use to get to your aim point.
In a super-low-inertia system like the Robinson R-22, you have to play with the collective as you load and unload the rotor to stay in the desired rpm range. With low inertia, rpm can both decay and rise rapidly, making for a higher workload, and with much less margin for error (there is a low rpm limit beyond which you can't recover). With a more massive system, the response is much more leisurely in both directions (higher and lower rpm) and it is far less "touchy".
My Bell 47 flies autos nicely with the collective kept on the floor until shortly before touchdown.

 

[WaspAir]

The R-22 has coning hinges.

My Bell 47 blades will cone up more under more load.

I've never been convinced that "cone" is really the best word because it suggests flat sides that meet at a point. The lift distribution can make it look more like slight curving upward, in a gentle arc, so it can be more like a really shallow bowl than a cone.

 

[WaspAir]

I use throttle and collective to hold constant rpm manually in the Bell 47 (it has no governor). Turbine helicopters and the Robinsons have governors that vary fuel flow/throttle. My rotor rpm is about 310, which is appropriate for the long blades to keep the tip speed optimized (37 foot rotor diameter).

 

[WaspAir]

I don't have a g-meter in it like I do in my sailplane, but judging by feel, I don't think I've ever gotten beyond the 2 to 3 g range. Like most rotorcraft, it's a matter of mushing through, not a structural problem.

 

[WaspAir]

What response do you expect from less than 1g load, and what lower limit?

 

[WaspAir]

So the idea is to function a little bit like the philosophy of a Watt governor (not the structure), with centripetal reaction to stabilize rpm at 1 g or above?

Centrifugal governor - Wikipedia

en.wikipedia.org

 

[C. Beaty]

The PA-36 had helicopter style cyclic and collective pitch control.

 

[XXavier]

what about the pitcairn PA-36 ? it seemed to jump perfect

Pitcairn didn't use the 'autodynamic' system, but an independently-developed hydraulic pitch control. I enclose two relevant text fragments scanned from Brooks' 'Cierva Autogiros', pages 212 & 224:

 

[XXavier]

More on the Pitcairn system, this time from Harris' book, vol. 1