Center Line Thrust Question

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Bruce Beasley

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Hello everybody. I am a new member. About thirty years ago I had a Bensen gyrocopter that I had begun to fly. Becoming a new father and the best pilot in our club being killed in his Benson led me to give it up. I am dreaming of flying a gyro again and I am confused about the Center Line Thrust claims of American Autogyro for the Sparrow Hawk versus the RAF gyros. Can anyone help me figure out if this is something important?
 
Welcome, Bruce.

Center Line Thrust is something that wasn't understood in the gyro world of 30 yrs ago. It had been known about by the early autogyro pioneers, and in fact, there were very few if any fatal gyro accidents in the early tractor design autogyros in which CLT was incorporated.

When I first got into gyros about 7 yrs ago and got on the gyro forums as you are doing, it became obvious to me that CLT was very important for stability. I bought a CLT gyro and never had any worries about unstable behavior leading to pilot induced oscillation (PIO) or power pushover (PPO.)

A HTL gyro can be mastered by a pilot with experience, but under conditions that are beyond pilot control, such as a zero g event, due to a gust or manuevering, all control is lost. A teetering rotor such as used in mostly all gyros, is not able to prevent a "bunt over" in zero g, if the propeller thrust is pushing the craft above its center of gravity.
Even in normal, 1 g conditions, a HTL gyro is susceptible to pitching instability because any disturbance of the rotor from a gust tends to lead to divergent behavior, rather than a return to the previous attitude.
 
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Hi Bruce,

Yes, it's important. You should get a good understanding of the issue before making a purchasing decision.

Try using the search feature on the forum; there are LOTS of posts about CLT. ;)
 
Hi Bruce - welcome to the forum.

What was the cause of the accident in which your friend died? Today we know that gyroplanes that have a large horizontal stabilizer and in which the engine thrust line is passing close to the gyro center of gravity (CLT) are more stable, and thus safer, than gyros without these features. Also, a gyro in which the engine thrust line is passing thru the CG cannot and will not PPO (power push over).

Stabs and CLT are not just hype - there are very compelling scientific reasons for having both. Test pilots have shown that these features make gyros more stable and easier to fly, and the accident statistics show very clearly the results of these additions.

Good luck with getting back to flying!

Udi
 
Oh no..... You are opening a heated topic here Bruce! ;)

Bruce, IMHO here is my answer.

Any of todays " typical " sport gyroplanes - and these include everything from Bone Stock RAF2000's, to Early Bone Stock Aircommands, to Bandits, Bensens, Ken Brock Machines, Dominators, Sparrowhawks, Sportcopters, Falcons, Sportsters, Twinstars, etc..... etc.... - Can be flown with a reasonable safety margin no matter if they have a horizontal stab and no matter if the thrustline is centered with the Vertical CG or not.

What does it take to fly any gyroplane?

Training.

With proper training, you can fly any gyro. The fact that a gyro has a thrustline offset, or no stab..... or has all the modern advancements in design such as CLT and a good effective stab, A gyro is still a gyro and they will all basically fly the same.

With the proper training, you can learn to fly a machine with a high thrustline such as the RAF, and even without the addition of a horizontal stab. People have done it for years, and people will likely continue to do so for the forseeable future.

Now there is a catch!

No matter how much training you recieve, a gyro with a high thrustline can, and still wants to do the dreaded power pushover. It is only your training to respond to the machine in certain situations that prevents the power pushover from happening. So in a nutshell, it will always be up to the pilot to insure that the gyro remains in normal flight, and it is really the pilot that makes these ships stay in the sky. Again, this is possible with the right kind of training and the proper amount of training.

The difference a CLT - centerline trust - arrangement makes on a gyroplane is it eliminates the possiblity of the power pushover. When you understand what a power pushover is.... it is clearly obvious why the CLT gyro can not do it.

A power pushover - also called a buntover - happens when the rotordisk is unloaded and the engines power setting is still at a mid to high setting. There is many situations where the rotor can unload... Such as at the top of a fast steep climb, in a strong downdraft, in stormy or gusty wind conditions, AND most popular is once pilot induced osicilations are allowed to get to the point of something simular to the fast climb and pushover at the top of the climb.

When the rotordisk is unloaded, the gyro experiences more or less a G factor of zero. You feel like your floating, such as at the top of a hill when riding a roller coaster. Since the gyros weight is normally suspended - for a lack of a better term - from the rotor, the rotor is normally under at least 1 positive G. As the gyro flys around the rotor is holding the gyro under the rotor, and it is making lift, just like the wings on a airplane. This lift factor also called the rotor thrust vector, is what sort of magically allows any gyro to fly. Even gyros with no horizontal stabs and with the thrustline placed above, below or on the vertical center of gravity. In other words, the rotors thrustline vector can allow what is more or less a out of balance gyro fly, and even fly reasonably well.

When you unload the rotor, experience Zero G, or whatever you want to call it.... You TAKE AWAY the rotor thrustline vector. When you take the RTLV away you basically take away any stability factor the rotor blades add to that gyro. When this happens the gyro is nothing more than a go kart in the sky that is basically out of control until you reload the rotor and get the RTLV back into effect.

The bad thing about a High Thrustline gyro, such as a RAF2000, is that during the time the blades are unloaded and the RTLV is gone the gyro is now going to fly, react and respond to forces as they happen based around the gyros VERTICAL CENTER OF GRAVITY. So if you find through hangtests and caculations that the VCG of your RAF2000 is located 12 inches below the location of the center of the prop - which is where the thrustline is - then the propellers thrust is pushing the the gyro higher than the CG, creating a forward tumbling force that can and will become a Power pushover- Bunt over.

It is the same as grabbing a basketball and pushing on the top third of the ball... it will roll forward quite easily. Push on the middle of the ball and it will just want to slide fowards, not roll at all.

The Centerline thrust machines are designed to place the Vertical center of gravity inline with - or in some cases like my Dominator, slightly above - the Thrustline.

In doing this, the CLT gyro, if flown into a condition that results in the rotors unloading, it does not have any force trying to tumble it foward into a bunt, even if the power if left at maximum power. It is this that makes the CLT gyro not able to Bunt Over and this is the reason the Sparrowhawk exists. To offer what the RAF offers but to do it in a design that eliminates the possibility of Bunt overs.

Don't forget that once a Buntover begins, there is absolutely no recovery and nothing you can do to save yourself. It has also been caculated that on a high powered machine such as the RAF2000 that it only takes a fraction of a second to Buntover, once the rotors are unloaded if the power setting is at a high setting when the rotors become unloaded. So there is no recovery, and little time to react on the onset of a unloading.

Again like I said in the beginning of the post, if your properly trained, you can normally detect and prevent the rotor from ever becoming unloaded to begin with, and you could very possibly fly a HTL gyro like a RAF2000 till you are too old to get in the seat and reach the starter button. BUT..... then again, we are all prone to screw up at some point or another. It is possible to get behind in flying the gyro and end up unloaded and in a Buntover. It has happened to several experienced gyropilots, several high time airline pilots flying gyros, and many many Newbies to gyros.

It is alot like handling a loaded and cocked handgun with no safety switch. If your trained and careful you can handle that gun forever. But the one time your careless, that gun can and will go off. A HTL gyro is EXACTLY the same. For this reason, I and many many others believe the best choice in gyroplanes FOR EVERY PILOT, NEW TO FLYING GYROS OR NOT, is a CLT machine. It is safer than a HLT machine without dispute.

A horizontal stab is also a not needed item on a gyroplane. All a Horizontal stab does is to dampen the gyro airframe in pitch. It makes PIO less possible and helps smooth out the ride in gusts and thermals. It does the same thing as a Shock aborber does for your screen - storm doors. It just dampens things.

So far most Horizontal stabs are not powerful enough on their own to counter a Buntover at slow speeds. At very highspeeds, a properly designed and installed stab can make enough force on it's own to counter the force of a HTL in the event that the rotor becomes unloaded. But again, at the slower speeds a stab is just a shock absorber in pitch and makes getting into a Zero G situation more difficult, by making the gyro fly more stable.

Even a CLT gyro benifits from a Horizontal stab. It dampens the gyro in pitch and makes for a smoother ride.

So these are just some facts and thoughts to maybe help you with your decision. Again, any gyro can be flown with enough proper training. A HTL gyro can buntover, a CLT gyro can not. Any gyro with a Horizontal stab will fly better and have a more stable ride than one without.

You will read alot of comments on stability on this forum. Some people I think are confused on what stability is. To some folks stability is just a gyro that flys reasonably well and you can trim it out and fly hands off for periods of time. To others a stable gyro is one that can be flown for long periods of time with the control stick locked in position, preventing it from moving to counter pitch excursions. Me myself, I consider a gyro to be stable only if it can not or is extremely highly unlikely to be possible of going into a mode of flight of which there is no recovery - A Bunt over on a gyro, or say for example a spin in a airplane with the CG past aft limits....
 
Of course its not necessary for an aircraft to be stable, but why in the heck would you choose one that isn't?
The horizontal stab, by the way, does more than slow down or damp a power pushover. It makes the gyro far less twitchy in its pitch response, as was recently discussed in another thread.
A gyro inherently has the quickness of a fighter jet in its pitch and roll response, due to the way the rotor controls the ship by vectored thrust. This makes any gyro susceptible to overcontrolling by the new pilot.

A stab changes the way a gyro responds to the stick. With a stab, when the stick is pushed forward or pulled back, the gyro will pitch at a steady rate, like an airplane does. Without the stab, the rate increases as long as the stick is held off center, until it eventually reaches a steady rate at some very high value.
That is why gyro instructors often teach about the "second jab"- for every stick input, there must be a second input which nullifies the first before it gets too far. With a stab, and CLT , you don't need to do that.
The stab can also be set to provide a down force on the tail, which will partailly or fully overcome the tendency for the gyro to pitch forward with propeller thrust(HTL), making it more pitch stable.
 
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To Gyroron, That was the best post on the subject yet! I learned more reading that than I have reading all the other posts in the last couple of weeks on the subject. Thanks again Dave
 
Thanks Dave, I am not the best person here to explain these things, but I think that post should give most folks a better idea of what is going on with gyros.
 
Al_Hammer said:
Of course its not necessary for an aircraft to be stable, but why in the heck would you choose one that isn't?

Al, I don't know why someone, or anyone would want to fly a gyro with no stab and or a HTL either. But my point is that some of us have, some do, and some will...... and that means it is possible.

I guess I was mainly trying to point out the fact that in what we consider a truely stable gyro, the pilot is not ever the sole thing that makes the machine stable. The machine is stable by itself. A unstable machine is able to be successfully flown because the pilot is well trained and he or she becomes the factor that gives that machine stability. Most of us will agree that we would feel more comfortable knowing the machine we are flying will not depend on us to prevent something that is non recoverable such as a Bunt Over.
 
Ron, maybe you can coin a new term- Pilot induced stability: PIS. :)
 
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Thanks Al, and your right, it is PIS......

I have asked some gyro pilots a question in the past and this kind of shows my point. The question is....... Besides Takeoff and landing, would you allow a Pilot - someone who already has experience flying airplanes only - fly your gyro with no training?

Most of the people flying true stable machines would say yes, because on a machine like a Sparrowhawk or a Dominator the machine won't fly a whole lot different than a regular ole airplane. There is nothing spooky that could happen.

I myself have allowed a friend with ZERO gyro training or flight experience to fly my Dominator on three occasions. He had no problem.

Some machines do require special skills to be learned to keep the gyro flying. All pilots of bone stock RAFs have learned this skill. All of Steve McGowans students that learned to fly in his Parsons learned this skill. And most people flying HTL gyros, even with nice effective stabs have learned this skill and use it. the ones who didn't learn that skill and flew that style machine ended up dead, period... end of story!

You can learn that skill, become good at flying that type of machine, and put on a good stab and try to minimize the thrust offset as much as possible - such as avoiding adding heavy items below the thrustline, moving existing items up higher on the machine, etc..... - And fly with a reasonable margin of safety. With the key Word being Reasonable! Stan Foster and Harry and many others are proof that with some modifications they can fly these HTL machines and rack up a bunch of safe hours flown.

The fact will still exist though, that Stan or Harrys or any of the other machines still count on the pilot to be a factor in the stability of their gyros. They find that acceptable. Some others don't...

I believe the smartest choice, When you have a choice, is to stick with the machine that is stable on it's own. One that won't bunt, one that practically flys itself, one that doesn't require special skills to keep it in the air.

I edited this post to add that even though I let a friend fly my gyro with no gyro training, he is a very talented pilot and we discussed how gyros fly and the whole process from taxi to takeoff to landing on several occasions and he had a complete understanding of what is what. I am sure he would have got signed off by a instructor in a record low number of hours. In retrospect it was dumb on my part to have allowed him to fly my machine with no training, no matter how talented he is. I used him as a example to show that a stable gyro could be flown easily and without much special skill. But I do want to make it clear that you should get training before flying, no matter how stable the gyro is. Very important!!! you do need training!!!
 
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I myself have allowed a friend with ZERO gyro training or flight experience to fly my Dominator on three occasions. He had no problem.

Congratulations, Ron. I wouldn't do that, for a variety of reasons, most of them selfish.
 
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Ron, Good posts. Your writing style is improving.

Now if you can get a couple of articles together for the PRA mag.. ;)
 
Ron: Most excellent post. I agree with it completely. I like the PIS part of it. Like I have always said....the pilot is the worst link in the chain. I will take a relaxed pilot anyday in a less than perfect machine that a ham fisted one in a perfect one. :D


Stan
 
Ron,
Here is to your point..... I have talked about it and Al you posted the happenings but here is the result. I just wish I had pictures of this painted and the engine cowling on it....

Picture 1 is this original confuration, Picture 2 is after the chain saw :D, Pictures 3 & 4 are in the new cofiguration and painted with the cowling off.


Fly Safe,
Chris
 

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gyrochuck said:
Ron, Good posts. Your writing style is improving.

Now if you can get a couple of articles together for the PRA mag.. ;)

Chuck, thanks. I got my lastest and greatest new issue of the magazine today and looked through it. Took about 3 minutes.... :( I really would like to put something together for the magazine, but.... I don't think I got the credibility to really be putting articles out in the magazine, and also my posts here are usually quick thoughts put into words on the fly and these are just what comes from the heart. I think if I set out to make a article for the magazine I would have writers block and not really write anything worth printing.

On the other hand, if anyone likes something I wrote copy and paste it to your computer, Fix all my misspelled words and bad grammer and then submit it to the magazine on my behalf. But I honestly don't think I am in the leagues of the people who are normally in the magazine.
 
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articles

articles

Hey Ron,

Rember what Henery Ford said," If you think you can, you can. If you think you can't, you can't. Either way, your right.

I think you should give it a shot.

Best Regards
 
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