Thrust Lines. How Far Above is Okay??

bpearson

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The UK adopted Proffessor Houstons report and its now part of their gyro standards. Part of my questions is that I was wondering why the USA hasnt seen fit to adopt that report given the depth of the study or do you already have similair standards now[ within 2 inches of CLT]
Maybe the USA still believes people should be free to make their own mind up.

None of the current crop of factory machines allowed to fly in the UK conforms to the 2" rule!
 

Doug Riley

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Paul Plack, the concern I've heard in private discussions is this:

If you stuff the RTV far enough aft of CG, you may in effect be winding a spring quite tightly -- a spring that will lift the tail very abruptly if, say, your LTL engine quits. I believe you can construct a worst-case scenario out of (1) a light, high-RPM rotor with consequent low damping values; (2) a long, massive airframe with high moment of inertia (3) ineffective or missing H-stab and (4) extreme LTL. In this nighmare machine, the aft RTV will induce a forward tumble simply if the engine quits. The tumble will proceed thanks to the airframe's rotational momentum, unopposed by much rotor cyclic lag. The rotor's rapid compliance with the changing spindle angle assures that rotor thrust is quickly lost.

This, I think, is the speck of theoretical truth to the (otherwise bogus) notion that gyroplanes are prone to "bunting" apart from either PPO or drag-over. I have not done numbers that would verify that this "perfect storm" is even possible, but it is not obviously IMpossible. You may need nothing more than a truly horribly-designed aircraft to make it happen.

In fact, I believe that stock Bensen gyros were at the edge of meeting the criteria for a "self-bunting" gyro, even apart from PPO.
 

RotoPlane

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I think it is important for everyone to read and understand Doug's post #15 and then come back and read post #22 again.

Thankfully many of us now understand the importance of NCLT for a gyro. In my opinion, I'm not sure that some of the same people fully appreciate the importance of the CG verses the RTV position. A properly rigged stock Bensen gyro (with no HS) that has NCLT, can be considered relatively safe as long as the RTV remains near the CG, and this means the airspeed is below say 50 mph. Go faster and it must have an effective HS to be truly safe. I have seen several of this type of gyro flying fast and the nose is quite low….scares me. This is because the RTV will move further aft of the CG as the cyclic stick is pushed forward to gain more forward speed, and this allows the RTV to have more leverage to rotate the tail up. It wouldn't take much to trip this "spring". This condition is unsafe without an effective HS to stop the rotation.
 

PW_Plack

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Doug, understood, although I'd expect high MOI to at least buy an experienced pilot a little time. A side-by-side LTL machine with it's lower MOI would worry me more if it had an inadequate stab. (Or inadequate attachment strength of an adequate stab.)
 

Doug Riley

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Er, actually not quite, Ed.

It's true that the rotor disk's AOA has to be flatter to avoid climbing as you speed up. In a CLT-no-HS gyro, however, this doesn't move the RTV aft of the CG, in steady flight.

The RTV can't stay aft of the CG for any length of time unless there's a counter-moment that holds it there. That usually means either LTL or a down-loaded stab. With neither (a Bensen has neither), the RTV may go aft of CG for a moment when you push the stick forward, but this will simply drop the nose. Dropping the nose moves the CG aft relative to the RTV until the two meet again. The gyro simply will fly along nose-low, with RTV and CG still aligned.

This reaction is buffered to some extent by increasing rotor blowback.

The typical pitch-instability accident in Bensens was, I suspect, a combination of PPO, torque-over and self-energizing "inertia bunt" such as I described earlier. It took a few cycles of PIO before the frame developed enough rotational momentum to overcome (the rather small) rotor damping provided by Bensen blades. More damping power was lost as RRPM fluctuated with the PIO.
 

RotoPlane

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Doug, you type a lot of er's to me, Heheh. I am going to need more thought on the CG moving aft when the nose drops….just doesn't make sense to me right now. It seems to me that the CG would stay put…but past experience has shown me that you are most likely correct….sigh ;).
 

Doug Riley

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Ed, I don't mean that the CG shifts on the airframe, of course. That happens in an airliner when someone walks back to the toilet, but on a gyro all the ballast is fixed.

The CG does move aft relative to the rotor, though, when the nose drops.

When the pilot moves the (joy)stick forward, the rotor disk's AOA flattens. This swings the RTV to a more vertical orientation in space. The frame remains level momentarily. As a result, the RTV's new orientation puts the RTV behind the CG.

However, this situation can't persist. The RTV pulling up on the tail (as it will when it's behind the CG) rotates the frame nose-down. It's this rotation that brings the RTV and CG back into line. From the frame of reference of the teeter bolt, the CG has "moved aft." It has to if equilibrium is to be restored.

If, OTOH, the gyro has either LTL or a down-loaded stab, the RTV can and will stay behind the CG during power-on equilibrium flight.
 

RotoPlane

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Yep….I just had that rotation alignment epiphany and was about to post it. Thank you Doug….one more piece of this puzzling gimbal puzzle in place. Rigid is good…….
 

Redbaron

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flying a high thrust line gyro with no stab is akin to pointing a loaded gun to your head! its not a good thing to do but won't kill you, its when you take away the rotor thrust " lowered or zero g" you pull the trigger! :rip:
no amount of training will help!
 

Chopper Reid

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Doug, am I correct in saying that heavy rotors will help keep the rotor disk flatter when the nose is pushed down? or am I on the wrong thought pattern?
 

Doug Riley

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Brian, I'd put it the other way. Heavy rotors help keep the nose up when the rotor disk AOA is reduced.

Heavy blades can eliminate HTL, simply by their dead weight. The CG moves up because of all that mass at the top of the mast -- maybe enough to hit the prop thrustline and create CLT. For example, adding McCutchen blades to a Gyrobee should make a CLT gyro out of it.

HTL with no HS requires that the rotor thrustline (RTV) stay ahead of the CG, to hold the nose up. When you flatten out the disk AOA, the RTV becomes more nearly vertical. That requires that the frame drop its nose enough so that the CG swings aft of (the new location of) the RTV. In a gyro rigged to Bensen hang specs, this can result in some hair-raisingly nose-low flight stances -- all because the CG MUST end up aft the RTV for the craft not flip forward from prop thrust.

My 1986 vintage, low-rider Air Command could do well over 80 mph on only 40 hp. Your feet and pedals were so low, however, that the sensation was like sitting in a wheelbarrow that was being dumped. LTL gyros and gyros with adequate H-stabs don't do this big nose-drop when you speed them up.

They also don't become "twitchy" on the controls, which that old lowrider also did.
 

Gyro_Kai

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Hello,

there are some conflicting effects, which play out in a way I didn't expect. It is a fact, that the Magni flies faster, than an aerodynamically similar MT03. Assuming that the Magni requires a more downloaded Hstab to avoid the higher thrust line, thus wasting some energy, there is a second effect: The prop spins in cleaner air up there, probably being more efficient. The overall result seems to be positive.

Also the Magni 24 performs better than the CTL Xenon. While the Xenon prop hides almost completely behind the cabin, the Magni prop is more or less behind the mast.

Kai.

Kai.
 

Resasi

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Performs better? The Xenon can under certain conditions carry three people which is pretty good performance.

Are we talking about speed perhaps?

I would also be interested in comparative wind tunnel smoke flow testing. One might find that the airflow round the cabin does in fact conform to fuselage shape as it passes round the rear to the prop and provide a good feed of air into it.
 

Gyro_Kai

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I am talking about the only xenon model available here, with 912S 100hp. It compares to a 914 in the Magni, if you don't go over the notch of the turbo.

Kai.
 

Chopper Reid

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Brian, I'd put it the other way. Heavy rotors help keep the nose up when the rotor disk AOA is reduced.

Heavy blades can eliminate HTL, simply by their dead weight. The CG moves up because of all that mass at the top of the mast -- maybe enough to hit the prop thrustline and create CLT. For example, adding McCutchen blades to a Gyrobee should make a CLT gyro out of it.

HTL with no HS requires that the rotor thrustline (RTV) stay ahead of the CG, to hold the nose up. When you flatten out the disk AOA, the RTV becomes more nearly vertical. That requires that the frame drop its nose enough so that the CG swings aft of (the new location of) the RTV. In a gyro rigged to Bensen hang specs, this can result in some hair-raisingly nose-low flight stances -- all because the CG MUST end up aft the RTV for the craft not flip forward from prop thrust.

My 1986 vintage, low-rider Air Command could do well over 80 mph on only 40 hp. Your feet and pedals were so low, however, that the sensation was like sitting in a wheelbarrow that was being dumped. LTL gyros and gyros with adequate H-stabs don't do this big nose-drop when you speed them up.

They also don't become "twitchy" on the controls, which that old lowrider also did.

Thanks Doug for taking the time to answer my questions. You make a lot of sense there to me and it also verifies other stuff that I have been told.
I use a Fibergalss rotors, quite heavy, I havent got the weights but they are noticeably heavier than extruded alloys and I reckon I have noticed a change to my gyro [HTL] when I have gone from the fiberglass blades to the alloy blades. We in Australia are doing hang and tilt tests for a survey and I will try both sets of rotors to actually see what chnages to the thrust line take place.
I guess that it follows that having a car starter motor for a pre rotator will also help move the thrust line somewhat?

If I read it right in Proffessor Houstons study, he questioned the effictivness of stabs under 50 MPH which makes for interesting thoughts ??
 

Resasi

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I have noticed a change to my gyro [HTL] when I have gone from the fiberglass blades to the alloy blades.

The CG relationship to thrust line changed with the change to lighter blades. I would expect the handling to have changed.
 

Jazzenjohn

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If your doing a number of hang tests and have access to different blades, I think it would be enlightening to see how much difference between the lightest blades (dragon wings or Bensens) compared to the heaviest extruded or glass blades. I would expect the VCG to change several inches on a light weight gyro, maybe more.
 

Heron

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Doug
One of those squetches showing the positioning would be great.
and someone could make them move to show the mods and changes.
thanks
Heron
 

Doug Riley

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Here you go, Heron. Cruise flight (say, 50 mph) on the left and fast flight (say 75 mph) on the right.

Gravity always pulls straight down.

Engine thrust is show net of frame drag, and frame drag is presumed centered on the CG. It takes less thrust to propel the rotor along at 75 than at 50,. Frame drag is much greater at 75, though, so the total engine thrust needed (which I haven't shown) is greater.
 

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Jens

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The short explanation..

The short explanation..

..........
 

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