I'm Curious

[Dean_Dolph]

Every time I draw sketches of enclosed pusher gyros arranged for better aerodynamics, I wind up with the engine farther aft, and the pilot farther foreward, to allow a more gradual streamlining at the rear. I understand how this could make the machine react more slowly to control inputs, but is there any actual danger in spreading out mass farther from the CG?I extracted this post from another forum where no one has posted an answer to Paul's question.

I'm also curious as to the effects of moving weight away from the center of rotor thrust which may be different than the CG Paul mentions. But I'm not sure because I got up too early this a.m. to be posting anything. I wake up about 10:00 a.m. despite what time I get up!

[Vance]

Hello Dean, In several of my Helicopter design books they talk about keeping the movable load near the center of gravity, so changes in the movable load have less effect on weight and balance. Movable load includes the pilot, pasangers, fuel and bagage. On a Robinson 44 when you fly with two fat guys in the front seat it defitnatly flies nose low. This can make landing kind of a rocking experance. Gyroplanes are also sensitive to weight and balance. If you were to move the weight further from the center with one of the larger movable weghts (pilot/pasanger) in my opinion it would limit how fat or skinny they could be. Thank you, Vance

[Victor Duarte]

Vance, Dean,
that subject also came to me.
If i think by myself : the more the "significant" weights are far from the CG, the less, changing one, will affect the CG, or am i wrong ?
Isn't the recommendation quoted by Vance justified by the yaw control ?
The more the weight is concentrated close to the CG, the more it will be easy to rotate (less inertia) , the less is power needed on the anti-torque.
Thank you for enlighting me.

[Vance]

Hello Victor,

You are changing terms here. My books are speaking of movable loads. If you move the heavy items away from the center of gravity that gives you a larger polar moment. A large polar moment slows responce, like adjusting the pendelum down in a clock.

What my books adress is thinking of it like a teeter totter. As the kids get further from the center, you have to move them farther to to compensate for weight differences. We can't move the pilot very far in an Autogiro, so having him further from the center of gravity moves the center of gravity further when we change his weight. We have a range of cg that is acceptle to fly, if we fall outside that range we shouldn't fly. Having the movable load further from the cg makes it more sensitive to changes in the load. That is why most Autogiros have their fuel tank close to the center of gravity, so the change in the weight of the fuel during flight doesn't affect the CG any more than necasary.

In my opinion, all else being equal, a high polar moment will slow the autogiros response to yaw control imputs. In my experance all else is never equal.

Thank you, Vance

[GeneWeber]

Hi Dean & Paul,

You’re dealing with the moment of inertia.

From Wikipedia, the free encyclopedia.
Moment of inertia quantifies the resistance of a physical object to angular acceleration. Moment of inertia is to rotational motion as mass is to linear motion.

In general, an object's moment of inertia depends on its shape and the distribution of mass within that shape: the greater the concentration of material away from the object's centroid, the larger the moment of inertia.

This is demonstrated by some formulas for moment of inertia. For a solid disk rotating about its symmetry axis I = 1/2MR^2. For a hoop rotating about its symmetry axis I = MR^2. So the hoop with its mass away from the centriod has twice the moment of inertia of the solid disk.

Don’t know if that really provides the practical answer you’re looking for. :D

Since the moment of inertia is a summation of the point masses times the square of their distance from the axis of rotation, you can calculate the effects of moving estimated mass values out specific distances. In this way you could estimate the total effect you would have by modifying the position of large masses such as the engine and pilot.

Regards,

Gene

[Victor Duarte]

Vance,
sorry, i confused the terms.
your imputs make sense.
That would be interesting to hear from tandem pilots vs 2 aside seats.
Thanks

[darrellwittke]

Basically the movement of cg away from rotor thrust line does not matter AS long as you have adequate tail feathers to compensate.

A very good (in depth) discussion was on an old thread I started called 8 simple rules of good gyro design. Might want to review it if you're really really curious. darrellwittke

[Doug Riley]

This has been discussed before on the old Forum. The moment of inertia (MOI) is a measure of the resistance of the mass to increasing or decreasing its rate of rotation -- IOW, it's the "flywheel effect." The term "angular acceleration" is techno-speak for a change in rotational rate.

A tandem gyro has a greater MOI or "flywheel effect" in the pitch and yaw axes than a side-by-side of the same mass. The body of the tandem will therefore have a greater tendency to resist pitching to a new orientation after a control input. Note that this doesn't directly affect the speed with which the ROTOR moves to the new orientation. (The rotor reacts to control inputs at a speed based only on the rotor's own mass, RPM and airfoil). As a result, the pilot of the tandem will PERCEIVE more "lag" in the pitch control response.

In any machine, a significant lag between operator input and desired result tends to lead to operator-induced oscillations -- the operator gets impatient with the slow response and wrongly adds more input, causing overshoot, then a "panic" control input in the opposite direction, and the familiar PIO scenario.

Fortunately, adequate horizontal tail surfaces can help "hurry up" the pitch response time of a frame with a high MOI. The surfaces add a moment tending to rotate the frame toward its new flight path more quickly. At the same time, the damping provided by tail surfaces reduces the tendency to overshoot and oscillate.

Moving the pilot or fuel out away from the CG does make the craft more sensitive to changes in pilot weight. In fact, if the moveable weight is placed right ON the CG (as with the rear seats of some tandem gyros), then changes to that weight have no effect at all on CG location.

These comments apply to the pitch axis. In yaw, the response really IS slower, rather than being a mere perception. You need more powerful vertical tail surfaces for a given machine mass if it's a tandem than if it's a SXS. "More powerful" in a tail surface usually means having either more force or a longer lever arm. The tandem is apt to have the latter anyway, as a natural by-product of its layout.

The SXS will have a higher MOI -- more lag and flywheel effect -- than the tandem in the roll axis.

[Victor Duarte]

Darrell, i think that would be good for everyone and particularly for me, please do so.
thanks

Doug > thanks again, your comment is VERY important to me, i think that in my building, the engine and pilot may not be as close as in a classic gyro, so your comment about PIO and lag is a warning.
Thanks

[Brian Jackson]

I can add nothing here except to draw a parallel betwen this issue and the design of newly engineered Golf Putters; the mass of the club head is split into two volumes and placed at the extremities of the head, much like the 2 iron balls that swing out from the governor on a steam engine. The more outward from CoG the mass, the more energy it takes to rotate it. Thus the new putters tend to stay where you aim them without twisting as much as the traditional design. It's why figure skaters can spin faster by pulling their arms inward. In the case of the new golf putters, like gyros, spreading the mass out from the CoG has a "stabilizing" effect, but the wrong kind when maneuvering the craft.

At least that's how I understand it. As always, there's more to it.

Respectfully,
Brian Jackson

[Dean_Dolph]

ThanX for the response, guys!

Now next question that looks to be related. I know this was discussed on the old Forum, better known as Rotorcraft.Com, at one time. But since these types of questions will probably come up from real newbies sooner or later then this old newbie might as well speed up the process by being the one asking them and let the knowledgeable populate the 'new' Forum with the info.

There was a discussion that centered around the effects of moving the vertical Cg up toward the rotor. If I remember correctly Chuck. B. related that Carl Schneider kept moving his up until he almost had a rotor haircut. And each time he moved it up it improved the machine. What I don't remember is in what way the machine was improved. This discussion may have been strictly about CLT.

So, what are the pros and cons of moving the vertical Cg as high as possible? That is other than Aussie Paul's objection to needing a ladder to get in such a machine!

[Victor Duarte]

Dean, as a newnewbie, i would ask the same question, and the answer will be almost important to me. I designed the mast height according to height clearance (goodbye free haircuts), i made it also at least 1.5 times the distance pilot-engine (CoGs).
I readed in my helicopter design book that the most higher the cog is the better, they state that it is better because it counters the adv blade extra-lift (sorry i din't know the term).
They say that a too much low CoG make, at the extrem, an aircraft unstable.
I understand it but how is it in real life ?
Thank you

[Udi]

...The SXS will have a higher MOI -- more lag and flywheel effect -- than the tandem in the roll axis.
Why is that, Doug?

Udi

[banaari]

The side-by-side moves the mass of the two occupants LATERALLY away from the roll axis of the aircraft (relative to the tandem configuration).
Assuming the total overall mass remains the same, moving it outboard will increase the rotational inertia... around the roll axis.

[mceagle]

There will be plenty that will speak in favour of raising the vertical C of G up towards the rotor, so I will try to suggest some possible "not so desireable" side effects.
Lets assume that you leave the rotors the same height that they are otherwise you only have a dog chasing its tail. These observations and trials are only of a general nature and are not perculiar any particular makes or models.

The already mentioned inconvenience of embarking and disembarking. (This IS a real problem, especially for those without prerotator and electric start)
The inconvenience of having much longer and consequently more tender undercarriage (especially the nose wheel) - any undercarriage failure means a "total".
A more complex and consequently heavier airframe.
An airframe that is harder to build, especially the undercarriage.
In some cases it is desireable to raise large items of mass, although it often means a much larger mass movement than that which was originally hoped for, because raising the lower items of mass also raises the C of G, making it harder to keep variable items like fuel and pilot/s on the C of G.
I quicker pendulum frequency (PIO - normally only a problem with low hour pilots).
As the C of G gets closer to the head, it restricts the size of the propeller that you can run. (propeller diameter = thrust)
As the C of G gets close to and above the thrust line the gyro will start to run out of forward stick movement at higher speeds.
As the C of G gets close to and above the thrust line the gyro will behave in a more and more adverse manner with sudden engine failure.
A higher C of G can make the gyro less "G force" stable in tight turns.

[Doug Riley]

Udi: In a side-by-side configuration, the masses of the two occupants are displaced to one side or the other of the roll axis, like the weights on each end of a barbell. In the tandem, both occupants sit more or less centered on the roll axis. It's the same sort of difference as when you look at the pitch or yaw axis, except that the MOI rankings as between tandem and SXS are reversed.

(I imagine that the MOI difference between the two layouts is usually not as great in roll as in yaw or pitch. You can cram two people closer together side-by-side than you can front-to-back, unless your front-to-back seating is motorcycle-style.)

[Aussie_Paul]

This CLT thing is getting a little out of hand, and we are seeing come knee jerk reactions.

I guess that I started the Pitch Stability discussion in Oz with my Raf situation. There was basically no discussion, and Barrys Oz forum was doing nothing, just lying dormant!!!!! The forum is quite active now. :) Love me or hate me I got people discussing and using Barrys forum.!! :D
The effective stab was a great improvement. The Raf 10" offset and a quite a large effective airfoil stab is still not in the same street as Hybrid!!! :eek:

Hybrid with almost CLT, between 1" and 2" thrust line high depending on fuel load differeing pilot weights etc, is a wonderfull machine. I can adjust the stabs to fine tune the stability that the almost CLT gives. I can have the nose rise or lower as full power is added just by adjusting the stab AoA. That is what I call "doing it correctly" without all the knee jerk reactions and emotions.

The airframe is so stable now that I can really pick the difference in the stability of the various rotor blades that I have had the pleasure of testing. This was quite a surprise to me to find such a variation in the pitch stability of the various rotor blades availible. Until you get the airframe stable, you do not know how much of the instability is airframe or rotors.

Tim, I have proven that you can achieve the desired result without having those "real" problems that you described.

Aussie Paul. :)

[Dean_Dolph]

Okay, guys we are getting a little bit off track! This isn't a discussion on the merits of CLT.

Tim, Carl Schneider probably has the fastest/highest performing machine in the U.S. I say that without proof other than observations by several of us. His machine is unique in several ways and is WAY off the ground! He is a very experienced pilot so I'm not sure if there is any issues with control that a new pilot would have a problem with. The machine appears to be at least NCLT if not CLT. It would be helpful if Carl posted here. Just for the record his machine is powered by a three cylinder Hirth!

What I'm asking from our more knowledgeable people is what effects on control/performance can be expected by moving the vertical Cg WAY up!

[PW_Plack]

Tim,

At the risk of hijacking a thread that I inspired in the first place, I have to call you on three of your statements, if only to understand what you're talking about...

"As the C of G gets closer to the head, it restricts the size of the propeller that you can run. (propeller diameter = thrust)..."

No, only moving the thrustline closer to the head does that. Moving it closer to the keel does the same thing! If you just move the pilot, battery or other weight up a few inches, how does that limit prop size?

"As the C of G gets close to and above the thrust line the gyro will start to run out of forward stick movement at higher speeds..."

Perhaps, if you assume someone does a hack job on a formerly HTL machine. Re-doing the plates and rehanging would solve this one, right? If you have a machine which wants to fly nose-down at speed, you lose reserve stick travel back, don't you?

"As the C of G gets close to and above the thrust line the gyro will behave in a more and more adverse manner with sudden engine failure..."

You lose me on this one. A machine with HTL will want to suddenly nose up in an engine failure, which is unstable. A CTL machine will exhibit no nose up or down tendency at all when the engine fails, except as influenced by the incidence of the horizontal stab. A machine with a thrust line below the CoG will tend to pitch nose-down in an engine failure and hold airspeed, which might be startlingg, but at least is stable.

Your statements seem to imply a machine which has had the engine and prop moved up on the mast, without any dilligence to the other associated mods needed. In the case of the Air Commands, for example, the engine could have been mounted right-side up, the redrive arranged to put the prop below the crank, and the CoG would have been raised with none of the negative effects you mention above.

Am I right, or just misunderstanding you?

[Udi]

Udi: In a side-by-side configuration, the masses of the two occupants are displaced to one side or the other of the roll axis, like the weights on each end of a barbell. In the tandem, both occupants sit more or less centered on the roll axis. It's the same sort of difference as when you look at the pitch or yaw axis, except that the MOI rankings as between tandem and SXS are reversed.

(I imagine that the MOI difference between the two layouts is usually not as great in roll as in yaw or pitch. You can cram two people closer together side-by-side than you can front-to-back, unless your front-to-back seating is motorcycle-style.)
Gotcha. I fell into the old trap of seeing the gyro as a pendulum. If you take the teeter bolt as the axis of the roll then the seating arrangement would not make much difference in terms of MOI. But I guess the real axis of roll is the CG, which is more or less in line with the people's CG.

Thanks.

Udi

[Udi]

I agree with some of Tim's points, and I have one more (negative) to add. Raising the CG closer to the rotor would require more stick correction for engine torque. I felt that very well with my old single place Air Command. I had to move the cyclic maybe a third of its range to counter a full-throttle torque (I am light weight and I was sitting high). This is less of a problem with a longer mast/lower CG. Obviously, using a tall tail and a centered stab helps too.

In my opinion, a very-low thrust line/high CG gyro can almost be as dangerous as a high thrust line gyro. It is true that, as long as the engine is providing thrust the gyro would fly super-stable. But what happens when the engine quits? Remember, a low-thrust engine is trying to flip the gyro backwards (tnub = bunt^-1 ;) ). The pilot has to move the RTV backwards to counteract this nose-up moment by pushing the stick forward. When the engine suddenly quits (imagine a high-power low airspeed scenario), the nose up moment goes away instantly and, before the pilot can react, the gyro would pitch nose down quickly – maybe even bunt. The severity of this reaction will obviously depend on the size of the offset and the power of the engine.

I can see one advantage to having a CG very close to the rotor and that is nimbleness. The gyro would be more responsive in pitch and roll.

Udi

[Doug Riley]

I agree with Tim's last point about the pitch effects of CG above thrustline and with Udi's and others' comments about the problems with this arrangement.

I understand that Carl Schneider's machines have the CG several inches above the thrustline, and that Carl advocates that setup. Carl bases his position on experiments he did some years ago with a Bensen-style frame on which the seat could be easily moved up and down the mast. The Dominators have the same arrangement, to a lesser degree.

Greg Gremminger's way of discussing this is really more comprehensive and useful than any ironclad rule about placing the CG X inches above or below the thrustline. Greg's analysis starts with the fact that, in steady flight, the moments affecting the gyro are always in equilibrium. That is, one way or another, the pitching moment created by your high or low thrustline gets cancelled by some other moment affecting your gyro. The key is to figure out (1) WHAT is creating that other moment (2) what can upset the balance and (30 what the consequences are of upsetting the balance.

We all know by now that a HTL machine with no HS or inadequate HS uses the rotor to make the counter-balancing force. That answers question #1 for the HTL machine. The answer to question #2 is that changes in the G-load on the rotor can upset the balance. The answer to question #3 is that the nose will drop in response to the high thrustline once the craft is unbalanced -- and that this worsens the low-G situation that started the sequence. IOW, the gyro is pitch-unstable and subject to PPO. All old hat by now.

In a machine with CG much above thrust line (LTL), you'll still probably use the rotor to balance out the pitching effect caused by prop thrust. In this case, though, the rotor thrust is pulling UP on the tail in order to counter the tail-down moment created by the prop. As a result, the answer to Question #3 is that, in a low-G situation, the engine thrust becomes unbalanced in a way that causes the nose to rise. Unlike the HTL case, this rotation of the frame tends to increase the G load on the rotor and prevent PPO.

So far so good. Do we really want a large pitch-up moment and rotation of the nose with every low-G event, though? A dramatic, uncommanded mid-air flare is likely to freak the pilot out, possibly tempting him to shove the stick forward, re-entering low G. Even if the pilot holds the stick still, the abrupt re-establishment of G load and a nose-down moment from the rotor will cause the craft to pitch back down again -- IOW, it'll do a little porpoising half-cycle all by itself, if the thrust undersling is big enough and the reaction times of the rotor and frame have sufficient lag.

These effects can be muted by a suitable HS. As with aspirin, however, just because a little thrust undersling is good does not mean that a lot is better. We should try to keep the moments on the frame close to balance in all situations, allowing a MEASURED amount of imbalance when that imbalance will result in a stable response -- but not an excessive imbalance that will cause wild pitching reactions.

It would be interesting to hear whether Carl has explored this corner of his gyros' stability envelope. There is some suspicion that the fatal crash of Steve Adler's very LTL gyro 3 years ago or so was caused by excessive pitching of the type I've described.

[rehler]

Chuck Beaty's article in the recent Rotorcraft magazine clearly illustrates the value of the thrust line going through the CG. Check out the article, as it is very good.

[quadrirotor]

Is it as useful as this one?
http://www.aircraftdesigns.com/gyro3.html