View Full Version : C30 inverted
03-16-2011, 11:21 PM
I have often heard the story of a C30 gyro on floats suffering "drag over" due to the floats. The aircraft recovered and landed. If it was a "drag over" then I guess that would be like an outside loop.
I recently came across an account of an inadvertant half loop, once again when righted the gyro landed safely albeit with damage. On this occassion it was attributed to blade flutter.
How many times did C30 style gyro's get inverted but landed safely ?
Would a gyro with modern blades survive this ?
Here is a cut/paste from the article. Mr H A Marsh was a very well respected test pilot involved in rotorcraft development for many years.
For many years folk speculated as to whether an Autogiro could get on its back in the air—and if it did, whether the rotor would stop and fold up. Mr H A Marsh answered these queries in 1935, when he was putting a seaplane version of the C.30 through its paces at the Marine Experimental Establishment at Felixstowe.
Starting at just over 5,000ft -and diving with engine off at about 95 m.p.h., the rotor blades developed a most violent flutter, and the machine half looped on to its back
The fact that Marsh had no parachute with him was of little moment, because he could not have got safely through the rotor blades whirling beneath him. Sitting tight or rather suspended by the harness—he waited for what might happen.
However, after losing some 2,000ft in the inverted position, the C.30 righted itself, and Marsh was able to park it back on the water all in one piece, despite the rotor blades having been severely damaged by ballast having fallen from the front seat whilst in the inverted position. The flutter,
too, had done a lot of damage; the plywood surfaces of the blades were all misshapen and broken.
Shortly afterwards, whilst flying another C.30 prototype at the R.A.E., Marsh had an almost similar, though not quite so hectic, experience.
The machine had blades of the same design and construction, and that was the cause of the trouble
Without any scientific background in rotor design, and on nothing more than a hunch, I would guess that if this had been a "teeter type" rotor head it would have completely failed.
03-17-2011, 02:08 AM
Why, I'm not sure a happy ending would have been imagined in advance, even with three-blade articulated rotor. The reason for happiness is perhaps only that the rotation torque remains positive when the wind blows over disk. R.p.m keeping everything remains possible. (just my opinion)
03-17-2011, 06:18 AM
Fortunately, airfoils can generate lift in the appropriate direction even when flown upside down (that is, with the camber inverted). The more camber, the less well the airfoil works upside down.
Our underslung teetering rotors will not do well upside down, however. Any rotor will expereince reversed coning if it generates lift in the "other direction." Reversed coning, combined with reversed undersling, will produce a violent 2/rev vibration that is likely to break things.
The offset gimbal head doesn't work upside down.
It should be possible to build a gyro that can be flown in sustained inverted flight. It ought to have aerodynamic controls (ailerons and an elevator), collective pitch, a fully-articulated blade suspension and blades with little camber. Obviously it must have enough rotor-to-frame clearance to allow reverse coning without any collisions.
The necessary compromises make it hardly worthwhile to accomplish something as relatively useless as flying upside down.
03-17-2011, 07:10 AM
What Doug said but note that there is a difference between an aircraft preforming an outside loop where the lifting surfaces are producing "negative lift" to push the aircraft to the inside of the loop and an aircraft that is tumbling about it's center of gravity.
03-17-2011, 07:19 AM
Cierva C-30s in original form had rotorblade airfoils with a significant negative pitching moment. As airspeed increased, the advancing blade, having greater airspeed, would twist nosedown more so than the retreating blade. A form of built in swashplate that compensated for dissymmetry of lift and suppressed cyclic flapping. At some high airspeed, the stick position would actually reverse and move rearward as airspeed increased.
One C-30 was lost when the pilot got into a high speed dive from which he was unable to recover.
The floats might have saved the pilot’s life by dragging the machine inverted and limiting dive speed. A rotor isn’t too fussy about which way the wind flows through the disc, especially when the blades aren’t highly cambered.
Offset flapping hinges can provide some positive control under zero “G” conditions with a fixed spindle rotorhead via the centrifugal pitching couple but in the case of tilt head cyclic control, no moment can be applied to the airframe except through the cyclic stick and the pilot’s arms.
The standard buntover that has plagued amateur HTL designs has a mousetrap characteristic that rotates the airframe so fast that the blades can’t follow and stall. Seesaw or flap hinge is irrelevant except, as Doug says, the reversal of undersling.
03-17-2011, 11:36 AM
The three bladed rotors invariably had flapping hinges that allowed for reverse coning of the rotor and the centrifugal forces will keep the blades happily from folding up while flying upside down as they do in the normal flight attitude. The inverted lift on the blades would try to iron out (and reverse) the built in coning angle of a teetering rotor which would most likely break the blades. One crucial point is why the rotor still flew at an angle of attack where it still would autorotate. My guess (deep down from the engineering tummy) is that the long tail of a tractor gyro together with a generous H-stab made this possible.
03-17-2011, 12:18 PM
The pilot had switched off the engine when he started losing control so the machine most likely was sinking vertically or nearly so.
03-17-2011, 09:27 PM
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03-17-2011, 10:52 PM
Does anyone have a reference for the "drag over" incident ?
Where did it occur, UK, USA ?
I am trying to work out if there were 2 different incidents with different causes or just one with 2 different claimed causes. This is why I highlighted the differences in the reports. I see a dragover caused by the floats would be a nose down pitching motion, where as the test pilot reports a half loop which would imply a nose up pitching moment. Maybe it is the same incident and something about the loop explanation was lost in translation.
Marsh does briefly mention a second incident.
03-18-2011, 12:01 AM
With the float incident the pilot recovered the C30 and survived.
The other incident was blade airfoil related as Chuck said in his post and pilot and machine were lost.
03-18-2011, 12:23 AM
That makes at least 3 then and possibly 4.
Marsh mentions 2 both of which he survived, plus the one fatal that Chuck mentions makes 3.
By 2, I meant was there one drag over caused by floats and this one which appears to be a pull up when reading the article. Or are they both the same incident ?
03-18-2011, 07:28 AM
The C-30 on floats flipping inverted was discussed in the book: “Cierva Autogiros” by Peter W. Brooks.
It was not called “dragover” in the book and was attributed to rotorblades having a negative pitching moment coefficient.
“… It seems from these incidents that the C-30 had a tendency to pitch nose-down uncontrollably when dived above a certain speed (apparently about 115 mph). This was, in fact, due to twisting of the highly cambered blades of Gö 606 aerofoil section, as confirmed by the NACA in the United States about three years later, during tests on a Kellett YG-1B, which had blades of the same section.”
03-18-2011, 07:46 AM
I'm having trouble picturing how a rotor with a tendency to "tuck" could put a C-30 on its back. If the rotor simply dipped to a low- or no- lift disk AOA, the C-30 with its big HS would dive faster and faster, thanks to gravity. A lawn dart. We've had a couple incidents where similar chains of events may have caused crashes in modern gyros.
If the rotor tucked all the way to a negative "G" AOA, then the reversed rotor thrust would push through a spot BEHIND the aircraft's CG. This would push the tail down and the nose up.
Seemingly, a force generated by something other than the rotor flipped the C-30. The floats are the logical suspect. This could be checked using an anatomically correct scale model of the float-equipped C-30, minus the rotor and having accurate CG placement. Just toss it and see if it flips.
03-18-2011, 08:34 AM
I expect the nosedown pitching of the rotor and the floats worked together, Doug.
There is only one mention in Brooks of a C-30 flipping onto its back and that was one on floats.
There are several mentions of the C-30 having a tendency to abruptly pitch nosedown and enter a highspeed dive if the throttle is not promptly closed.
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