question about the "rocking" motion

chuter

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I’ve having trouble visualizing something and I’d like to ask some help from anyone who can explain it.

We talk about the “rocking” motion of a HTL gyro without a HS; I’ve experienced this myself and it’s quite obvious.

We say that a gyro in flight moves about its CG.

Straight and level; if it’s moving about the CG, and rocking, wouldn’t that mean that the rotor is going through acceleration/deceleration in relation to the CG? (not rrpm changes, but fore/aft movement of the top of the mast)

Or is the CG accelerating/decelerating and the rotor movement through the air is constant?

Or is it just a higglety-pigglety collection of both?

It seems to me that the rocking motion sort of indicates some pedular action (sorry to bring this up again).

================

A while back I brought up the question of relating a powered parachute to our discussions of all flying things rotating about the CG, and trying to make sense of how the PPC does this.

I think it was Udi who said that perhaps the chute is so airspeed-stable that there is actually some pendular motion going on with a PPC.

Is it possible this could apply to our rotors too (to a smaller degree)?

Just trying to put all this together, any help appreciated,

Thanks,
 
The rocking chair mechanism of HTL gyros is tied in to rotor RPM.

HTL; propeller line of thrust above the CG of the machine requires, for equilibrium, that the rotor’s line of thrust must pass forward of the CG, producing behavior similar to that of any other tail heavy flying machine.

An upward gust tends to tilt the machine noseup, increasing the rotor angle of attack, increasing rotor thrust and thereby increasing the fuselage noseup force, etc.

But as rotor thrust increases, rotor RPM also increases which decreases cyclic flapping and tilts the rotor nosedown.

The period, which in the AAIB report was ~15 seconds, depends upon the amount of thrust line offset, the machine’s moment of inertia and the RPM characteristics of the rotor.

The heavier the rotor for a given chord, the slower will be the pitch bobble. But it will have greater amplitude with heavier rotors.
 
Thanks Chuck,

I’ve got a good grasp of your first two points, the third about decreasing flapping was a refresher for me.

Since I made the first post I was thinking; perhaps in perfectly calm air, after establishing perfect straight and level, constant airspeed flight, maybe there wouldn’t be any rocking?

So the gentle rocking is caused by even very minor changes in the air, possibly just air density/temp changes, or even minor power fluctuations in the engine……….I can see how this would be the airframe moving about the CG.

In this case the flight path would actually be oscillating up/down slightly……..?

Edit: In my visualizing I was assuming a perfectly level flight path and trying to figure out how that could happen.

Have I got it?
 
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Here’s a scan from Gessow & Meyers of a helicopter undergoing a similar pitch oscillation, no doubt exaggerated for the sake of illustration.

A helicopter doesn’t have the option of placing a propeller thrust line below the CG in order to force a nose heavy configuration. The only cure in that case is a horizontal stabilizer.

But a helicopter has the luxury of being able to locate the stab on a moment arm that extends past the rotor tips.
 

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I'm kind of confused about why HTL craft would be more prone to this rocking. I'm taking a statics course right now and based on what I've learned, once the autogyro is trimmed for straight and level flight the HTL shouldn't cause it to pitch over anymore. Based on the way I'm thinking about it, HTL should only be a problem when adding power. Adding the power creates a moment about the center of drag.
 
I'm kind of confused about why HTL craft would be more prone to this rocking

My guess is that any gyro without a stab would be this way.........?
 
sla10m says...

...once the autogyro is trimmed for straight and level flight the HTL shouldn't cause it to pitch over anymore. Based on the way I'm thinking about it, HTL should only be a problem when adding power. Adding the power creates a moment about the center of drag...

Louis, the various moments balance around the center of mass (aka center of gravity), not center of drag (aka center of pressure). The center of mass is always in the same spot, while center of pressure (for anything other than a sphere) will vary based on angle of attack.

You correctly note that a change in prop thrust will start an oscillation in a trimmed HTL machine, but the balance between the moments of prop thrust and rotor thrust can also be disturbed by variations in rotor thrust, which are frequent, and caused by variations in angle-of-attack as we pass through air currents.

If we always flew in perfectly still air masses, this wouldn't be an issue.
 
Most all tail heavy contraptions are unstable in pitch; a gust or other disturbance produces a pitch response in a direction that tends to magnify the disturbance rather diminish it. I suppose blimps and submarines would respond the same way if CG was aft of center of buoyancy.

A conventional helicopter with central flap hinges has an aft CG in forward flight because fuselage drag swings the CG aft of the rotor thrust line.

Statics is ½ of the equation, dynamics is the other half.
 
Chuck, looking at the diagramme from Gessow and Meyers, I have a question. Three values are used, alpha, V, and omega.

alpha is angle of attack -- I assume we are referring to the rotor disc, here.

V is velocity. Both of these are bog-standard aero engineering terms.

What's omega? Is that pitch (as it would appear to be from context)? And is this also a standard Ae. E. term, with which I happen to be unfamiliar?

Finally, I wonder if the list engineers would comment on this book, or some of the alternatives, for a little more "mechanical" depth than one gets in your basic statics and dynamics class:

http://www.amazon.com/Fundamentals-Kinematics-Dynamics-Machines-Mechanisms/dp/sitb-next/0849302579

But my real curiosity is about the "omega" in the diagram -- also, is there anywhere a standard table of "common greek symbols" as used in Ae. E. or ME?

90% of engineering, after all, is knowing where to look stuff up.

cheers

-=K=-
 
Upper case omega simply designates angular velocity, radians/second.

In the context of the illustration, Ω designates fuselage angular velocity.

Other authors sometimes use lower case omega (ω) to indicate angular velocity of driveshafts and the like. G & M uses Ω for all angular velocities.

G & M has a six-page glossary of symbols which I believe were NACA standards at the time. I can scan the glossary to a CD and mail to you if you like.

I’m on a dialup connection at the tail end of a rural telephone line so E-mail has limits.

Better still; purchase your own copy of G & M. You won’t regret it.

Edit: Oops! G & M uses lower case omega to designate angular velocity when discussing oscillatory behavior. I should have looked at the picture I posted.
 
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Statics and dynamics

The Greeks, influenced by mathematicians such as Archimedes, had a pretty good handle on statics 2200 years ago. The Greeks could have analyzed a roof truss without difficulty.

The beginnings of modern dynamics came from Polish astronomer Copernicus’ (that’s his Latinized name) publication of his theory of planetary motion in 1543.

Copernicus theorized the Earth rotated around the Sun while spinning on its axis.

That theory raised questions that sent scientists scurrying to find answers.

How can the Earth be hurtling through space at many thousands of miles per hour while we, on its surface, are totally unaware of motion?

What is the motive force?

Galileo solved part of the riddle by rolling balls down an inclined plane. He observed that after a ball rolled down an inclined plane it would regain its original height if allowed to roll onto an upwardly inclined plane and that angle of inclines was irrelevant. From this, he theorized that an object once set in motion, would maintain that motion forever unless acted upon by external force. Since the Earth and everything on it moves in unison, there is no relative motion, producing the illusion of stillness.

Newton introduced the concepts of force, mass, acceleration and momentum in a formalized way.

From Newton, we’ve derived the concepts of conservation of energy and of momentum.

Bernoulli’s theories in fluid mechanics are derived from conservation of energy and momentum concepts.
 
Getting back to Louis's question about the effect of HTL on the propensity for rocking. At the anecdote level, I can testify that my old no-stab HTL Air Command rocked gently. After a day of flying it, I'd still feel the motion when driving home or eating supper -- like the leftover motion you feel after a long day on a boat.

The reason has to with the layout of forces that leads to a statically unstable system. With HTL, the prop thrust pushes in a direction tending to force the nose down. The fuselage dangles at an angle that allows the rotor thrust line to pull up on the nose to balance things out. So far, you have classic static equilibrium (sums of moments and forces are zero). Equilibrium remains as long as there is no disturbance.

If rotor thrust varies for any random reason, the thrust pushes or pulls the nose in a direction that tends to alter the aircraft's angle of attack. In the case of a rotor thrust line ahead of CG (or CM), this change is in a direction that tends to magnify the disturbance.

The rotor itself is, however, stable with respect to angle of attack. Therefore, the system will eventually tend to recover from small disturbances.

Part of the recovery process comes about through changes in rotor RPM. The rotor, being massive, doesn't change RPM instantly. The lag in this change is likely responsible for most of the oscillation.

An airframe with positive static pitch stability "de-magnifies" disturbances. This leaves less disturbance for the rotor's own laggy responses to deal with. Hence stable gyros don't bob.

I haven't had a case of gyro "sea legs" since switching to stable gyros -- and I've flown some long days in them.
 
Your "bod'" must be more sensitive than mine, Doug.:peace:

The only recognizable after-effect of flying gyro's over the years, was and is, in my hearing. I always used ear plugs before I started using a radio with the ear muffs. Driving my car after a flight, I always have a feeling of "solitude", if you will.

I just happened to think back...I did have a very slight tingling sensation in my hands and feet, after flying my Bensens, in addition to the feeling of solitude.


cheers :)
 
Harry, I don't know how much of my own indistinct hearing is due to working/playing around gyros and other loud machinery, and how much is to be blamed on amplified music.

Either way, the "rocking" in the Air Command was gentle and had a long period -- maybe 15 seconds. It's what the professionals call "phugoid" motion. The sensation was similar to riding in a boat going a little slower than a following sea.

It didn't build up explosively and it was not the short-period motion that becomes porpoising when it gets out of hand. I was very well warned about THAT and have never ran across it in any gyro I've ever flown.

I don't recall experiencing the phugoid motion in my Bensen, but that was a long time ago... and the engine quit so often that I may have been distracted.
 
In the few hours I flew a gyro that bobbled, it felt like it was flying level, when I guess it was really oscillating up and down a bit; that was throwing me off, trying to figure out how it could stay level and still be rotating around the CG.

I can see how someone who was trained on a gyro that bobbled could get used to it and it would be second nature to them, how they could come to like the flying characteristics of it and not having a HS and think a HS is not necessary.

I for one am a bit lazy I guess, I like not having to work the stick to straighten out the bobble.
 
That is the reason they invented autopilots Mike. A powered parachute (PPC), weight shift control (WSC), helicopter and gyroplane all operate under a hinge point. Some are metal and some fabric (PPC). They all osillate below this point. Except for the helicopter, they all have thrust below the hinge point. The CG establishes the location of the angle of attack of the airfoil in relationship to the relative wind from the hinge in normal cruise flight. The pilot determines the angle of attack with the controls for other modes of flight.

Any disturbence from air movement such as horizontal or vertical gusts will affect the lifting surface and therefore make the hanging body oscilate. PPCs oscilate all of the time unless you are flying in perfectly still air. WSCs oscilate when someone on the ground farts. I have learned new meterological terms flying these aircraft. Texture of the air (vertifcal and horizontal movement); sporty (mild turbulence; e.g. someone sneezed or farted on the ground); and cracker (your don't want to be there).

Helicopters are inherently unstable unless you put a autopilot or some fancy computer device on them. Ask anyone who has tried to learn how to hover (right paulp). All gyroplanes are more stable then helicopters without autopilots because they have a big drag chute they hang from.

Sensitivity of the controls determine how easy the machine handles. The vertical and horizontal stabilizer help to point the machine in the right direction. The pilot flying the machine, with qualified instruction, is taught how to fly the machine within its envelope and to establish their own envelope. Operating out of either can cause significant porblems. Therefore we have accidents.

Thrust lines affect the body operating below the hinge point. The arm of the thrust line from the hinge point will determine the amount of affect of the thrust and the movement of the body. If you eliminate the drag attached to the hinge point (ergo.: no lifting surface) thrust line will determine the angle you hit the planet.

The rotor disc is a lifting body in helicopters and gyroplanes. The results are the same in both machines. And at the accident site you will notice very little rotational damage to the blades in both aircraft as a result of unloading the rotor disc. Ask any accident investigator. That is what the teach at the NTSB accident investigation course for FAA and NTSB investigators.
 
Marty,

Thanks for posting here.

I can see some differences in what you have posted and what I’ve come to understand from reading here and other places and I’d like to ask you and others to help this shade-tree engineer to understand.

Your description of gyro flight seems to be saying it is a weight-shift aircraft that operates using the pendulum model.

The other description I see most often here and other places indicates that gyros move about the CG, not swing under a hinge.

As far as I know (I never took physics, but I’ve searched a bit on the Internet) the evidence seems to say a gyro will move about its CG.

Can anyone provide this simple minded soul a model or simple proof that will let me see for sure?

I gotta say, right now I’m leaning toward the model that says the RTV rotates the craft around the CG.

Help!!!!

I can see some common ground. You seem to be saying a HS is a good thing, and that loss of rotor thrust will result in a forward tumble in a HTL gyro.
 
Me too

Me too

Yep count me in with those questions.

Also I have been to a lot of crash sites and both gyros and helis can show rotation damage to blades, it all depends on what happened.
 
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