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Old 02-01-2017, 05:57 PM
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Brian Jackson Brian Jackson is offline
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Default The early days of PPOs and design mods

Greetings All.

A question has been turning around in my head for many years but I don't recall ever asking it. So here goes:

I seem to recall general agreement that Bensens were pretty stable as designed. Granted there are many models, but for the most part "Near-CLT" was a desirable attribute long before the masses who flew them included CLT in their vernacular. Brock and others naturally introduced variants, each espousing various changes justified by something that I'm sure was well thought out at the time.

I know there's not a "Designers Union", but what puzzles me is somehow in that Golden Era we overlooked a fundamental flaw and kept selling kits to the masses. Folks bunted and the sport was forever tarnished. To this day we're paying the price, evidenced in the poor number of us in GA.

Maybe what I'm asking is what standard are gyro manufacturers held to? (with respect to science and current understanding.)

I can think of many designs that our current knowledge would outlaw. Not meaning to offend anyone but the Bandit Low-rider? Seriously? Why is this even being offered? I just don't see the militant fanaticism going the other way to save lives.

OK, long post. Sorry. But really would like to know from the old timers. Thanks in advance for furthering an education.

Brian Jackson
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Old 02-01-2017, 07:02 PM
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Default A fuzzy answer to a fuzzy question Brian.

In the USA here is no standard that gyroplane manufactures are held to because the manufacturer is the builder of an experimental amateur built gyroplane and there are very few gyroplanes that aren’t experimental.

The British have section T that has some extensive detailed standards and allow factory built gyroplanes.

The same goes for most countries other than the USA.

I feel you may be oversimplifying why people died in gyroplanes when they were more popular.

There was very little dual instruction and many people did not have the patience to follow the Bensen Syllabus.

In my opinion a horizontal stabilizer had more value for stability than near centerline thrust particularly in relation to pilot induced oscillations.

In my opinion based on my observations a Magni is one of the safest gyroplanes out there and yet all of the currently produced Magni gyroplanes have a considerable thrust line offset.

A gyroplane is a system and there is more than one way to produce a stable gyroplane.

I feel there is value in reading the NTSB reports carefully.

The Bandit appears to me to have a very large horizontal stabilizer and as far as I know there were no fatal accidents caused by gyroplane stability in a Bandit despite considerable thrust line offset.


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Originally Posted by Brian Jackson View Post
Greetings All.

A question has been turning around in my head for many years but I don't recall ever asking it. So here goes:

I seem to recall general agreement that Bensens were pretty stable as designed. Granted there are many models, but for the most part "Near-CLT" was a desirable attribute long before the masses who flew them included CLT in their vernacular. Brock and others naturally introduced variants, each espousing various changes justified by something that I'm sure was well thought out at the time.

I know there's not a "Designers Union", but what puzzles me is somehow in that Golden Era we overlooked a fundamental flaw and kept selling kits to the masses. Folks bunted and the sport was forever tarnished. To this day we're paying the price, evidenced in the poor number of us in GA.

Maybe what I'm asking is what standard are gyro manufacturers held to? (with respect to science and current understanding.)

I can think of many designs that our current knowledge would outlaw. Not meaning to offend anyone but the Bandit Low-rider? Seriously? Why is this even being offered? I just don't see the militant fanaticism going the other way to save lives.

OK, long post. Sorry. But really would like to know from the old timers. Thanks in advance for furthering an education.

Brian Jackson
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Old 02-01-2017, 07:16 PM
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Bensens were never stable, they were near clt , on my first one I only needed to move the seat up a bit over an inch to achieve clt, a good sized horizontal stabilizer made it stable with the offset gimbal head so I had honest stability, a Bensen wouldn't ppo but it was easy for a new pilot to get into a pio situation, this could be stopped before it got out of hand with no problem , many had this happen and reduced power and pulled back on the stick and recovered, when rotax came along with the large prop many simply put a taller mast on and bolted the new engine on without knowing that the machine now had a deadly flaw due to the higher thrust line and the offset gimbal head to mask it, once you exceeded the limit things happened so quickly that it was impossible to recover even if the stick was moved quickly as long as the power was left on, it takes a rotor a bit of time to react to a control I put and it will never catch up in this situation, as the airframe rotates forward around its c of g the tail gets into the path of the rotor that's still on its original path.
Norm

Last edited by phantom; 02-01-2017 at 07:18 PM. Reason: Wrong word put in by the lovely Siri.
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Old 02-02-2017, 06:12 AM
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I agree with Phantom. A few years ago using aircraft scales I ran the CG program using a Bensen with a super Mac engine (dual carbs and tuned pipes). It flew with inverted Hughes blades. The pilot was around 220 lbs. The vertical cg point was within an inch of the engine thrust line.
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Old 02-02-2017, 07:47 AM
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Thinking back about that first Bensen back in the 70s if I had not been in contact with ken Wallis I would never have known that the vertical Center of Gravity was important and jerry Barnett insisted that I not fly without lots of horizontal stabilizer so there is a good chance that without their advice I wouldn't be here today.
Norm
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Old 02-02-2017, 10:02 AM
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Quote:
Originally Posted by phantom View Post
Thinking back about that first Bensen back in the 70s if I had not been in contact with ken Wallis I would never have known that the vertical Center of Gravity was important and jerry Barnett insisted that I not fly without lots of horizontal stabilizer so there is a good chance that without their advice I wouldn't be here today.
Norm
Norm, what did you use for a horizontal stabilizer. I ask because I just bought a 2'x4' sheet of .090 6061 T6 as an HS for my KB3. What dimensions do I need to end up with and should it be mounted under the prop to protect it from stone chips or just as far back and as large as possible? Any pics? Thanks
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Last edited by Stan V; 02-02-2017 at 02:21 PM.
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Old 02-02-2017, 02:11 PM
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I have always used the basic rule of what ever you have for a vertical fin, make two more like it and mount them at least as far back as it is, you can mount them on the keel but it's best to have them up in the prop blast, I have made them out of wood, tube and fabric, foam and glass or metal, the metal is .020 aluminum.
Norm
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Old 02-04-2017, 10:55 PM
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Brian Jackson Brian Jackson is offline
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Thank you all for the great replies. You have given me much to think about. As I said in the original post, I don't mean to offend... the Bandit was simply the first plane I thought of that seems (on the surface at least) to go against modern safety practices. Wasn't trying to single any design out.

H-stabs, CLT and a plethora of other design criteria that may not be instantly discernible save lives, and the safety statistics validate good design. All things evolve if given enough time, and gyros seem to be inclusive in that. Those features that improve survival rate live on to reproduce into features of new designs.

I can't speak here with any authority, merely as an astute student. So I hope folks here can appreciate the questions.
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Old 02-05-2017, 08:33 AM
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Ken Wallis always told me that horizontal stabilizers make a gyro dangerous in very strong turbulence, I have found this not to be true but I don't like offset gimbal heads in rough air, I don't think that they are dangerous to anyone use to them but they do cause things to happen that you don't have to deal with when flying a fixed spindle head.they sure do make things simpler so it's a compromise like many things that we live with.
Norm
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Old 02-13-2017, 12:14 PM
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The more serious problem than pure PPO in the Bensen was the relative ease with which it could be put into PIO, a/k/a porpoising.

The lack of an effective H-stab was probably most of the problem, but other design details almost certainly made it worse: The Bensen rotor was very light, and spun at a relatively high RRPM. Both of these details make for a "sporty" feel, as the rotor responds very quickly to cyclic inputs.

Effortless cyclic response means low rotor damping, however. That, in turn, means that the rotor does not provide even the modest stability that a heavier, slower rotor would supply.

With enough rotor damping, a rotor will lag so far behind its spindle that it will hammer the teeter stops, and send strong pulses back through the control system, when given a hard, fast control input.
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Old 02-13-2017, 01:35 PM
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Quote:
when rotax came along with the large prop many simply put a taller mast on and bolted the new engine on without knowing that the machine now had a deadly flaw
Thank you, Norman, for this very enlightening description. I dream of getting to the point where I can reproduce these problems with my numerical simulation program. I know positively that it is possible to achieve this with today's technology so if I don't make it it'll be entirely due to the fact that I'm to plain dumb.... (but I also feel that currently the number egg heads in this forum supporting the effort has dropped to zilch.... I'm looking at you, Raghu and Jean Fourcade...;-)
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