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Old Jun 16, 2007, 09:13 AM
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Al,

interesting report. What they say about the location of the thrust line is certainly true for teetering head gyros.

Fortunately, with DC head gyros you seem to have a bit more leeway. All my DC gyros have the motor's thrustline a bit above the vertical c.g. I'm using this configuration to counter the additional drag of the rotor at higher speeds, comparable to the down thrust of the motor on a normal plane. Without this my gyros would definitely be more difficult to fly.

Jochen
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Old Jun 16, 2007, 04:05 PM
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New Jersey, USA
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I enjoyed the accident report, But I find it hard to understand how the rotor blade could strike the rudder when witnesses stated the gyro appeared to be flying normal and then "BANG". Even though ossalations were observed during the flight, none were reported just before the "BANG" Dispite what the report says, I'm thinking a broken prop hit the rotor.
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Old Jul 20, 2007, 11:13 AM
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Quote:
Originally Posted by David A Ramsey
I enjoyed the accident report, But I find it hard to understand how the rotor blade could strike the rudder when witnesses stated the gyro appeared to be flying normal and then "BANG". Even though ossalations were observed during the flight, none were reported just before the "BANG" Dispite what the report says, I'm thinking a broken prop hit the rotor.
Draw a vertical line on a piece of paper. At the top put an arrow facing right and call it rotor drag. Further down draw another arrow facing left and call it thrust. then put a blob near the bottom and call it CentreOfGravity. In normal flight you have the rotor system causing drag and trying to pull the Autogyro over backwards. You also have an engine developing thrust and trying to push the autogyro over forwards. Both of these forces work around the pivot point which is the CofG. The two forces (drag and thrust) roughly balance each other, with any imbalance being held in check by the CofG moving fore or aft about the vertical axis. Now imagine a scenario where the pilot gets a PIO going (Pilot induced oscillation). If it develops enough there will come a point where the autogyro enters 0 or negative G. When this happens the rotors will be providing zero lift and consequently zero or only a tiny amount of drag. If the engine is developing thrust it will push the autogyro over forwards which is known as a PPO (Power Push Over). The centre of gravity position vs the thrust position will determine the rate of push over (typically less than one second). The further they are apart the faster the push over. The disc formed by rotor blades won't keep up with the push over and so the tail will in effect be lifted up into the path of the rotor blades hence the rudder chop (It is more correct to say that the disc is pulled down by the mast into the rudder). Modern gyros try to overcome this inherent design problem by raising the pilot and lowering the engine to try and put the thrust line as close as possible to the CofG hence on more modern designs you see tall spindly undercarriages and booms with cranks in them to clear the prop. Early tractor (engine and propellor at the front) autogyros already had the thrust line in line with the CofG and didn't suffer these sort of problems, but the Bensen design and all similar pusher configurations that followed especially those where the engine was in fact raised to provide more room for bigger props when the owners wanted more power have suffered with this problem.
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Old Jul 20, 2007, 01:28 PM
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Great discription and very clear. I can easily see that as the reason. The bit about, appearing to be flying normal, made me wonder if might not be the prop. Of course the witnesses may not have known what flying normal really was. The provided information seems to peclude the prop as a reason, but could it be?
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Old Jul 20, 2007, 01:55 PM
Ozzie GyroNut
Hervey Bay, Australia
Joined Mar 2007
138 Posts
Hi All,

I spent years during the 80's, building Bensen Gyroplanes, original B8 gyrogliders, plans-built B8M's, B80 kit's, Ken Brock clones, the whole tootie. My arms still ache from swinging the prop on that noisy drone engine!! In short, they are flying deathtraps!! Their flying "envelope" is so tricky, it's easy to get yourself into trouble. Yes, there were "masters" flying them, but most of them died in them as well. The whole CG, Horizontal tail, pump action stick issue is old news. Only when the long legged high seated models came out, it became better. But still, after a few flights, I stayed on the ground, and am around to write this mail.

Hey, I loved the machines, but they were as scary as hell, if you were clever enough to have a brain, understand the aerodynamics of what was going on, and realise how rudementary the original Igor Bensen design was. Maybe he should have stayed with the towed overhead stick flying boat glider design, that was more stable. It was towed along, not "trying to push chain shackles along".

My 2 cents worth...

Francois
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Old Jul 20, 2007, 06:31 PM
Very slow flyer indeed
BragMan's Avatar
Bedford, UK
Joined Aug 2006
41 Posts
Bensen and boats

Yes, Francois,

Bensen did indeed start out with towed gyro gliders and my experience of these is that they fly exceptionally well.....
....all except the gyro-boat!

Well, Bensen did use the UK-built Hafner-designed Rotachute design as the errr... 'basis' for his 'designs' , having been tasked with evaluating it at work. So it should work, since the brilliant Hafner's brief from the Military was to design it to be towed behind a bomber at over 100 knots. The mind boggles......

To this end it had considerable delta-3 and was allegedly pretty stable in its final evolution. Fortunately it was never pressed into service, since the frozen fingers of the poor Rotachute pilots would hardly been of much use by the time they reached their destination. As for the Rotabuggy.....
Poor Hafner, he got all the rotten jobs, and yet his contribution to rotor head design was immense.

Rotachute:
http://www.aeroflight.co.uk/waf/uk/a.../rotachute.htm

Rotabuggy:
http://www.pilotfriend.com/photo_albums/potty/5.htm

What I recall of Bensen's description is that the gyro-boat was (in Bensen's view) a stable machine until one day it was towed too fast, and pitched forward, plunging in a seemingly unrecoverable dive towards the water with maximum 'up' cyclic applied. Well, it did pull out, and that was probably due the tow-rope helping to pull the nose level, I would guess. Gliders have their advantages.

Gyro boat: http://www.hmfriends.org.uk/bensenb8b.htm

With all that boat ahead of the CG and Rotor Thrust Vector, it can't help but become divergent in pitch with increasing speed. At what speed it goes out of control only Bensen could say for sure. But it will go out of control if you go fast enough.

Yes, Francois, I feel it would have been better if Bensen had stayed with gliders, but maybe not the gyro-boats! His superb piloting skills may have convinced him that his machines were safe for the general public, but in my view he bears a grave responsibility for many incidents over the years.

When Bensen nailed various engines in his glider, he did take some basic precautions, and had a fairly low thrust line if the engine nose was tilted down as shown on the later plans. Actual relative position of the thrust line was usually within the spec suggested by Glasgow University (which matched the original Cierva one fairly closely). One can't be much more specific, since pilot weight, seat cushion, and fuel load can play a large part in the final CG of a machine that is so light. Ones that I have hung (or seen hung) and had the CG evaluated have been within those limits, that's all I can say. Others may differ.

Now modified Bensens often have larger props fitted, and need a taller mast as a result. Misguided builders often think that a lower CG will be more stable, and as an engineer working for an engine/prop supplier we were asked for engines mounted under the reduction gear 'to lower the CG'. Oh dear. We can't blame Bensen for everything.

Of course, the small rotor of a Bensen can easily have a following rate (Hi Mickey!) that is less than optimum. Materials enter into this, of course, and the early wooden blades feel pretty heavy - don't know their mass or radius of gyration, but Bensen did add massive leading-edge weights at around 70% radius.

The best design (in my view) that was (only loosely) based on a Bensen was the Wallis 'Agile' series. http://www.kenwallisautogyro.com/

Ken Wallis built a Wallis-Bensen with a Mac drone engine in the early days, and added a proper stick to it, since that's the conventional approach. He rapidly discovered that it was divergent in pitch, so he independently invented the offset-gimbal head, shared the idea freely with Bensen, received no acknowledgement, and saw it promoted as a Bensen innovation some time later. Fine.

Within a couple of years one Wallis Agile at a Norfolk flying club was flown by well over 100 assorted (mostly fixed wing) pilots of varying skills who had four things in common, they had no previous gyro experience, they had around a half-hour of briefing and taxiing, and they then went solo and they loved it.

This is clearly a very different class of machine from the Bensen, and indeed many of the autogyros available today.

The young man who died at Shipdham need not have done, if only....
He and I were to have met a few weeks later and had exchanged a few emails and 'phone calls. We were to discuss some of the topics in this very thread. At the time I was editor of 'Gyro Flight' magazine, and readers would often email or call for a chat. I was looking forward to meeting him, and I suppose I still am.

Despite the many marginal gyro designs around there is a growing movement towards better machines backed up with both science and the art of careful aircraft design.

May pilots enjoy their gyros in safety, and models are a good way to do that!

All the best, Ben
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Old Jul 21, 2007, 06:46 AM
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New Jersey, USA
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Good read. Thanks Ben.
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Old Jul 21, 2007, 10:31 AM
Keep it simple
alfoot's Avatar
Salisbury,England
Joined Jan 2005
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I'm glad that the report has prompted some good comments and explanations. You can rest assured that every possible credible scenario is explored during such investigations, with the aim of making things safer for all of us.

Al
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Old Jul 21, 2007, 11:27 AM
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Quote:
Originally Posted by alfoot
I'm glad that the report has prompted some good comments and explanations. You can rest assured that every possible credible scenario is explored during such investigations, with the aim of making things safer for all of us. Al
There are 2 Quotes from the report that made me wonder about a prop/blade failure. In the first, the pilot is feeling so good he waves to the witness. In other words, the pilot is so comfortable about his current flight condition he has time to wave to a person on a bicycle. In the second quote, the prop had been damaged in a prior incidence, but only one blade was replaced. Were the remaining 2 blades and hub tested? Then I wondered which of the 3 prop blades were broken during the incident. Was the replaced blade still attached to the hub?

I'm resting assured Al. I'm not a currently active pilot, but a firm believer in pilot error.


"One other witness, who was cycling in the local area,
stopped to look at the aircraft to the east of the runway,
as it flew apparently straight and level in a northerly
direction. The gyroplane passed close to him and its
pilot waved to him. There was a constant noise from the
engine until this witness heard a “clunk” and the engine
noise stopped.
He watched the aircraft tip nose down and
fall to the ground with the rotors stopped; his impression
was that the rotors were hanging vertically down each
side of the aircraft. This witness was approximately
500 metres away from the crash location."

"During the investigation two people reported that the
accident pilot had attempted some wheel balancing on
his aircraft without supervision at sometime during the
latter half of 2002. During this attempt the aircraft had
suffered a ‘blade flap’ incident on the ground2 resulting
in a rollover and damage to the propeller and rotor.
These accounts are supported by the fact that the pilot
purchased new rotor blades and a new propeller blade
in October 2002.
The new rotor blades were of the
same type but, at 23 ft diameter, one foot larger than
the authorised rotor diameter specified in the machine’s
Permit to Fly. However, there is no evidence to suggest
that any aircraft damage from that accident led to the
pilot’s subsequent fatal accident."
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Old Oct 14, 2007, 06:41 PM
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Wrocław, Poland
Joined Jun 2007
696 Posts
Hi, I am very impressed by this thread.
I see that around post #24 you mentioned about delta hinging.

I have D=1m girocopter, fantasy-semi-scale, with radial engine.
I am not the builder, the machine never flown but appears to have nice proportions.
After reading your posts I have paid attention to learn as much physics as I can and modify the rotor head to get as much reliability as possible before maiden flight.

What I have is 3-blades, very elastic flapping hinge layout, 500grams, blades not twisted.
After reading your posts I have introduced lead lag hinges as in regular Trex 450.

I would be grateful if you colud tell us more about the 2 following topics:

-How do I chose offset angle in delta offset head?

-I want to avoid surprises during diving or faster flight.
This is why I want to use offset gimbal
(even if I have two HS-225MG tilting the rotor, I don't want to bend control rods).
Does using offset gimbal makes sense in flying model?
http://www.autogyro.com/technic/offsetg.htm
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Old Oct 19, 2007, 10:32 AM
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Quote:
Originally Posted by kbosak

-How do I chose offset angle in delta offset head?

-I want to avoid surprises during diving or faster flight.
This is why I want to use offset gimbal
(even if I have two HS-225MG tilting the rotor, I don't want to bend control rods).
Does using offset gimbal makes sense in flying model?
http://www.autogyro.com/technic/offsetg.htm
I assume you mean delta three angle? If so you will have to experiment, but 10 - 15° is a safe start. On models the effect of delta 3 may not be so pronounced.

Offset gimbal is definitely a good idea. Most successful models seem to use it. You can determine the right amount experimentally by removing the pushrods and holding the model in stiff wind and adjusting the offset until the rotor trims by itself. Alternately you can assume a ~15° tiltback angle and use this to project where the gimbal joint should be offset. This will depend on the vertical distance from the center of the hub to the gimbal. This will give you a good first estimate.
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Old Dec 11, 2007, 09:36 AM
edi
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Elstertrebnitz, Germany
Joined Aug 2004
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Coning again

Hi all!

Since I am a scale nut, I'd like my autogyros to be scale, too. Right now I am building a Cierva C.6 and one concern is that the mast angle on the original looks like about 0 degress. When I tilt it back it will not only be less scale, but also seriously reduce the clearance of the rotor.

Since aerodynamics most probably did apply back in 1926 and the Cierva C.6 was pretty successful there must be more things here to consider. One thing is airfoil efficiency, and I'll certainly have to live with that so I might just *need* a higher mast angle. Or maybe I don't? The other is flapping. The original had its blades rigged so they probably flapped up high to achieve a decent angle of incidence, but also led to coning which is bad as I understood from your explanations. Now I have found the following link on youtube:

http://www.youtube.com/watch?v=Gf7p1...eature=related

which states that "Coning Effect of the rotors on a flapping hinges [sic!] provides STABILITY!"

And finally: Where would you assume the CG to be for an autogyro with stub wings? My intuition tells me that if it's in the region that would be sound if the stub wings were the only wing and if the hang angle is such that the rotor disc will be tilted slightly forward I should be about ok. But then again, my intuition sometimes fails me ...
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Old Dec 11, 2007, 11:56 AM
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The tilt-back angle in models is caused by the poor efficiency of model sized airfoils (the L/D). This also creates a need for a different blade rigging angle as opposed to the full sized aircraft.
You may have to "cheat" on the tilt back angle and the blade incidence if you want to get a good flying model.
Alternately you could just fly nose high and re-rig the wing an tail incidence accordingly.
I think the C.6 has tiny wings, so I'd go with just the hang angle estimate as a starting point.

Coning is an un-desireable byproduct, nothing more, nothing less. If you want to believe every blanket statement made on youtube I've got some options on some under-developed Florida property we need to discuss. Make sure you send your Visa card number in the email so I know who to charge for the investor kit.
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Old Dec 11, 2007, 02:19 PM
edi
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Thanks for confirming what I thought and thanks for the input.

(I include my Visa card number in all my emails just in case somebody wants to charge me for something I didn't know I urgently need. )
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Old Dec 11, 2007, 04:40 PM
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imsofaman's Avatar
Nowhere
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Quote:
Originally Posted by mnowell129
Coning is an un-desirable byproduct, nothing more, nothing less. If you want to believe every blanket statement made on youtube I've got some options on some under-developed Florida property we need to discuss. Make sure you send your Visa card number in the email so I know who to charge for the investor kit.
I will defend that staement...I never posted that movie here on RC Groups and therfore the next sarcastic jab is in reference to the ones I just recieved.....in case the forum police have a heavy trigger finger....not that they have ever had that probelm before.

Hmmm....I guess those top selling model autogyro mfg.s who all use delta hinge heads or flexing rotor plates must be informed of their engineering blunders. From the big Kellet kit from Autogyroros of Arizona right down to the Rotorshape.....they must all be reprimanded, chastised and taught that their designs are no good! How on earth did they design ANYTHING with your approval first Mickey???? Think about Hal Debolt and the countless others who all agree with the coning is actually good. I will be the first to go ahead and email all of the current mfg.s stating that any kind of coning is just not desirable! I will send a link to this sticky post. How dare them!!!!!

I know of only ONE single rotor kit MFG that does NOT have flapping hinges or some type of flex rotor plate. Hmmmmm wonder if he is pro flexing rotor plate or against it....

Look at all the gyros designed and built in the twenties and thirties with flapping hinges! OMG! I don’t mean toys...I mean the real things! They must have been very lucky....they never read any of these stickies! How did they manage? I guess Jaun De La Cierva and the engineers of the day were all just lucky and really did not understand what they were doing. I am sure that it's it! NOT!

On the serious side.....This sticky has awesome info....just some of it is very bias in my opinion. One man does NOT rule all info concerning autogyros..though he is delusional enough to think so!

So...Edi......."get your credit card out" and buy a book written by Juan De La Cierva "Wings Of Tomorrow The Story Of The Autogiro" if you REALLY want to know the truth. It is very interesting.....makes you wonder how Cierva ever got it done without this sticky.....

Got to go....I have a lot of emails to send out...let's see. A of Arizona.......uh.....
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