Gyroscopic feedback/ Chuck B.

[birdy]

From another thread bout gyros falling over after touch down, I mentioned the following as a 'maybe'.
The instant and compleat unloading of the disc on touchdown has it loaded with inertia ,but with very little airodynamic responce to any inputs compared to when it's in the 'flying state'.
If the disc left in this position [full back stick], the worst that will happen is the disc will cause the machine to creep backwards if no power is applied to stop it.
In a strong wind from any direction tho, the disc must be leveled to prevent the wind from getting under it and rolling the machine, BUT, with heavy blades it should be done slowly but surely, because there will be some gyroscopic effect one the machine fed back from the blades coz there isn't enough airdynamic responce from the blades in the 'no wind' situation to counter it.
[Slight correction; the feed back is felt through the stick and not the machine.]
Then Paul B. said
...this was discussed a couple of years ago. The universal joint of the gimbal head stops gyroscopic precession from affecting the airframe.[ i tried to find it ,without success]
The gimble will isolate the 'resistance to directional change' from the machine, but it still has an effect on the stick ie; push the stick forward and it tries to go right side down.

Question Chuck, is this right or am i full o sh1t again?? :rolleyes:
[ if it ain't gyroscopic 'resistance to directional change' that i'm feeln, its sumthn very much like it.] :confused:

[C. Beaty]

It’s all quite simple, Birdy; heavy rotor, heavy stick; light rotor, light stick.

The seesaw rotor in conjunction with teeter hinge is a universal joint so gyroscopic force can not directly be fed back into the control system.

But the pitch and roll pivots of the rotorhead are below the teeter bolt. Move the stick faster than the rotor can follow and the line of rotor thrust lags behind the rotorhead axis, producing a moment opposing rotorhead tilt.

The heavier the blades, the more pronounced the effect. Rotor lag is a function of the ratio of rotor inertia to aerodynamic force.

[Doug Riley]

If you move the stick far and fast enough, with a rotor that's massive enough, the rotor may hit the teeter stops, too. That'll produce a 2-per metal-on-metal slap felt through the stick.

[mceagle]

The universal joint of the gimbal head stops gyroscopic precession from affecting the airframe:As a primary effect, yes. As a secondary effect, once the rotor responds to the input, it will drag the mast (airframe) with it.

[birdy]

I / you may not be understanding it???
With wot you blokes are talkn bout, the feed back felt in the stick would be as a shake or pulse. Wot i'm talk'n bout is a constant resistance, as if sumone was pull'n against your input.
Also, the resistance your discribing would driectly oppose your input, wot i'm feeln is at 90* to an input. IOW, when you push forward, it pushes back, but when i push forward it pushes right.

[C. Beaty]

When you outrun the rotor with the stick, maximum flapping velocity is 90º ahead of flapping displacement and teeter friction tends to tilt the stick crosswise.

Move the stick forward, for instance, the rotor lags behind (or flaps) and maximum flapping velocity occurs to your right. Teeter friction trys to lay the stick over to the left.

Make sure teetering is free. If you have teeter bearings that are metal on metal, the only solution is to leave the teeter bolt loose so that it can behave like a floating piston pin.

Teeter bearings ought to be needle roller bearings of at least DU type plastic lined bushings.

Even with needle bearings, you can have excess friction between the top hats and the towers. You need slight end play.

[Butch S.]

Birdy
I flew a set of heavy glass blades today for first time & beleive I felt what your talkin about
but will not enter debate as I know FA bout arguments put foward
have you tryed same day tests on ally verses glass blades on same gyro ?
Butch

[birdy]

No butch, the time i realy noticed the feed back was when i changed from jerry's to AK's, a big weight difference.
[I know sweet FA to butch, thats why i'm ask'n.]

Am understanding wot your say'n bout outrun'n the rotors Chuck, I do it all the time when i'm in the air,[ ask Kenny J., he woulda seen it on the vid i sent him.] but i'v never felt it when landing.
I think if it was teeter friction, the baldes wouldn't be silky smooth when flyn [ as they are] , i'v experianced flight with the teeter blot torqued too much and it was shake'n the stick and the stick/machine kept want'n to roll left.
IOW, the teeters on both machines are free.[ one's a needle and the other is a free bush.]
Besides, if it was teeter friction, it wouldn't be a smooth, constant resistance, but a shake.

[birdy]

Hmmm..................... seems they threw me into the too hard basket :o .[ just that f#$@*% SCG :mad: ]

Warning, to all new and not so new gyro pilots, if you do attempt to shove the stick forward at the moment of touchdown in a spot landing and you have heavier than average blades, the stick will try to go right as you push forward, and if not corrected, any wind geting under the high side of the disc could tip you over.

I'm not say'n i'm right[ havn't been convinced otherwise] or its definatly gyroscopic feed back, but it sure as hell feels alot like his full brother if it ain't him. ;)

[Al_Hammer]

The pilot might note that when he pushes an inch of forward cyclic, the tip path plane might not go purely forward, but might have some roll as well. This particular effect is due to many contributers, including gyroscopic precession, rotor flapping resonance frequency, rotor blade flapping inertia and flap aerodynamic damping.
-Nick Lappos

Birdy,

cross-coupling as alluded to in the above quote by a helicopter test pilot can be caused by rapid tilting of the rotor disk. It happens to some extent whenever the stick is moved, but isn't normally noticeable.

Just after touchdown in the gyro, the stick may be moved a relatively large amount in a short time, such as when levelling the rotor in a wind.

If the rotor is moved from full back to full forward, say, in 1 sec, then the forward blade will be moving down and the rear blade moving up. The induced velocity changes (aerodynamic input) results in a tilt of the rotor 90 degrees later, to the right.(assuming CCW rotation)
A rough calculation shows that the rotor would tilt maybe 5 degrees to the right during a full stick movement from back to forward position.

The rotorhead (and stick)will tend to follow the rotor, especially with heavier blades. (the rotor normally follows the rotorhead, but it does work both ways)
This will be felt in the stick as a force at 90 degrees to the intended stick input.

I don't know if this is what you're seeing, but it's one possibility.

[C. Beaty]

I know your game, Birdy; “I’m just a simple grower of simple cows and get no respect………Woe is me, woe, woe……..I might as well beat myself with a bullwhip….”

Think you’ll suck me into a never-ending argument about the peculiar behavior of rotors, do you?

Well, just this one time I’ll bite, not wanting you to take a bullwhip to yourself on my account….

The sketch shows velocity about the teeter pivot when you outrun the rotor with the stick. A rotor built like a barbell set would remain fixed in space no matter which way the rotorhead was tilted except for friction about the teeter pivots.

You’re right inasmuch as teeter friction, if linear, would produce a 2/rev shake. But sliding friction of things starting and stopping isn’t linear. The coefficient of friction of things at rest is higher than sliding friction; the coefficient of sliding friction generally decreases with increasing velocity. As you’ve probably observed, more effort is required start something sliding than is required to keep it going. Starting friction is formally called “breakaway” friction.

Teeter motion starts and stops twice per revolution and the non-linear character of the sliding friction of plain bearings presents a nearly constant force on the stick. Teeter friction as depicted in the sketch would tend to lay the stick over to the left.

I’ve been through all this first hand; seems like a hundred years ago but suppose it was more like 30 or 40 years ago.

My first attempt at copying a Bensen gimbal rotorhead (the first one sold in Florida) failed miserably. The teeter bushings of the original Bensen head were black and the liner material was easily skinned up with a screwdriver; I didn’t know what it was but now realize it was some sort of plastic liner impregnated with graphite.

I thought I’d be clever and used bronze bushings with hardened steel top hats.

Torqueing the teeter bolt down so as to lock the top hats to the hub always resulted in instant seizure no matter what kind of lubricant. That always gave a hard left stick whether in the air or on the ground.

Leaving the teeter bolt loose enough to float like a VW piston pin solved the problem but violated my sense of propriety. Bearings ought to bear where you want them to.

I then tried Delrin bushings with hard steel top hats. Same problem but not as severe. The steel of the top hats would plate onto the Delrin and the whole mess would seize.

Plain bearings with a unidirectional, oscillating load will always seize. The lubricant is squeezed out, oxides are rubbed off which sets the stage for friction welding.

The only solution to teeter bearings is needle roller bearings; cheap and will last forever if not packed with sand.

[C. Beaty]

Al, the cross component Nick Lappos refers to is very evident on the A&S 18A but that’s caused by the large amount of Δ-3 coupling used on that machine, not a rate effect.

All rotors exhibit some rate effect which produces a cross component but it’s not normally detectable with mark-1 eyeballs.

[Al_Hammer]

You're probably right, Chuck, but I was just trying to relate this to what is seen in helicopters. This is an excerpt from a letter written by Ray Prouty:



"...If you are on the ground and apply maximum cyclic pitch to the rotor when it is over the nose, the tip path plane will tilt up 90 degrees later on the left side (and down on the right) following the laws that apply to a gyroscope.

If you are in the air, the same thing happens as you call for an acceleration of a right roll by applying maximum pitch over the nose. (If the rotor has offset flapping hinges, the angle is not 90 degrees, but something less--maybe 85 degrees). However, once you achieve some rate of right roll, the situation changes and the tip path plane will not be responding at 90 degrees, but at some smaller angle depending on the design of the blades. This angle can be as low as 60 degrees depending on the design of the blades and results in rate crosscoupling.

[birdy]

I know your game, Birdy Not play'n any games Chuck, I just wana know :( .

Think you’ll suck me into a never-ending argument about the peculiar behavior of rotors, do you?
No, its just that i'm still not convinced.[ I like to have a flawless understanding of these things] :D
I'm not convinced coz i think you'v missed a detail.
You said that a barbell hooked up in the same manner as the rotor would remain fixed in space no matter which way the rotorhead was tilted . Thats logical.
Question; would the barbell create any 'resistance to change feedback' to the stick?
BTW, wots linear mean? :confused: [ i could look it up in the dictionary but it may have a different mean'n ere in Oz.]
I understand wot your sayn bout friction not be'n constant from start to stop, but theres a 'start to stop' twice per rev, so any feed back would have to have a slight pulse, to the left.

Teeter friction as depicted in the sketch would tend to lay the stick over to the left.
I know, i'v mentioned that before in this thread,[ when i flew with an over torqued teeter blot.] But the feedback i'm refer'n to is to the right, so it couldn't be teeter friction.[ no, my blades spin the same way as yours. ;) ]

[C. Beaty]

Birdy, just for you, I’ve gone back to the original illustration of the barbell and added a white arrow to indicate feathering motion with flapping.

The blades are forced to undergo a periodic oscillation about the feathering axis; The white arrow always stays parallel to the teeter bolt. The acceleration of feathering axis mass is maximum (in the illustration) when the blades are at 180º-360º. That tries to lay the stick over to the right but is a 2/rev force and not very much unless the rotor is outlandishly heavy. Blade feather axis inertia is the only force that could produce a stick force to the right.

I expect Australian slang words, say for female anatomy, are quite a bit different from US slang words for the same body parts. But proper and technical words are identical in all versions of English. Linear means linear, whether in Ireland or Australia. Hint: Think of line as opposed to curve.

[C. Beaty]

Oops! I was wrong about feathering axis inertia laying the stick over to the right but I’ll patiently await Al Hammer’s correction. Acceleration is the second derivative of displacement.

Feathering axis inertia causes a fore/aft stick shake.

[Doug Riley]

Birdy, is it possible that your foward push has a sidways component? Our arms are made of straight links with pivots, so they naturally move in arcs, not straight lines. This gets really obvious if you try to draw a straight line without a ruler. For a short distance, you can manage a pretty straight line, but the longer the line, the more it looks like a series of arcs.

To avoid any sideways motion of the stick, your fist has to go straight forward -- but your shoulder is off to one side. You need just the right amount of shoulder rotation in two different axes, plus just the right wrist action.

We normally fly gyros with such small hand-forearm movements that precise control is easy. "Dumping" the rotor on landing is another story. It's a long, full-arm movement that won't be as precise, no matter how much of an ace pilot you are.

What's more, the leverage ratio of our arms changes when we extend them. We don't have as much power or precision of sideways movement with our arms extended. I found this out a few weeks ago when a student brought me a gyro to fly that had low a stick ratio. The gyro was properly balanced, but the head wouldn't tilt all the way forward before the stick hit the panel. The stick had to be pretty far forward to fly level. I found it more difficult to keep her from banking because my arm was contunually extended like that.

(To flog a dead horse, this is another argument for the Bensen overhead stick. For large movements, you usually hold it with both hands. The arc effects of one arm cancel those of the other. There's some hope that you can push the thing straight out when you need to.)

[birdy]

This computer near got thrown out the f^%$# window, the pice o sh1t. :mad: [ musta hit a wrong button and poof, the hole lot disappered :rolleyes: ]

Anyway, Doug, you got a point and i know wot your say'n[ i can't even draw a streight line WITH a ruler :p ] but i don't think its the problem coz it never happened with the lighter blades. The lighter blades also needed more trim spring pressure, which means there needed to be more effort when the arm was extended.

Bugger it Chuck, your loos'n me now. :( [ don't forget wot the S stands for in SCG.]
I can't argue bout this feather inertia axis thingy coz i ain't got a clue wot it is, or how it could apply any resistance to the stick. It wouldn't by any chance have any relation to wot a SCG would call pitch inertia??[ the feed back felt when apply'n huge cyclic pitch changes to a slow rev'n rotor.]
BTW,Question; would the barbell create any 'resistance to change feedback' to the stick?
Please.

Or have you already answered that in the post i got to study more? :o

[Butch S.]

Was wondering if rotation of earth makes water in plug hole turn ccw in northern hemesphere & cw in southern hemesphere & benson started this gyro thing in north why dosen't someone try cw rotating blades in southern hem they might be somwhat more efficient &&&&&&&
Then this [right hand force] may be reversed then you could have your next thousand hours to get rid of your tennis ellbow Biry
Butch

[thallett]

Are most "rollovers" to the RIGHT? If rollovers are (partially) due to gyroscopic effect, then stick FORWARD would produce RIGHT roll. Anyone know of a LEFT rollover?

[C. Beaty]

Birdy, I stuck my head someplace where the sun doesn’t shine.

Cyclic flapping and cyclic feathering are the same thing viewed from different axes.

In forward flight, the rotor disc appears to blow back relative to the rotorhead.

Viewed along the axis of the rotorhead, the blades “flap,” but there’s no cyclic pitch change.

Viewed along the axis of the rotor disc, the blades cyclically “feather” but don’t flap. The amount of “feathering” in this axis is exactly equal to the amount of “flapping” observed when viewed along the rotorhead axis.

The mass about the feathering axis is forced to oscillate and resists by producing a 2/rev fore/aft shake of the stick.

None of which has anything to do with laying the stick over to the right. You’ve got me beat on that one.

[Doug Riley]

Tinkerin' Tom: I've done one rollover in each direction. Two are quite sufficient, thank you very much.

The rollover to the right was clearly the result of a gusty left crosswind and my refusal to get the stick hard forward, because of a violent nosewheel shimmy. No gyroscopic whatchamacallit there, just ignorance about the nosewheel adjustment.

[birdy]

Not want' to flog the dead ores again, but,
Question; would the barbell create any 'resistance to change feedback' to the stick?

A simple yes or no will do, thanx.

[Doug Riley]

Some, yes.

[birdy]

Thanx Doug.:)
[some, yes is symple enough.] :p

Obviosly, if i was try'n to fight the gyroscopic force of a real gyroscope of the same size and mass as a rotor, i'd have no chance. But see'n as the rotorblades airodynamic effects are like power assist in the steering of your motor car, all i'm realy do'n is 'initiating' a change in the tip path, the airodynamic effect dose the hard bit.[ cyclicaly fight'n the gyroscopic forces]

IF, theres point at touchdown where the blades are neither autorotating nor propelling[helicopter'n], and a strong cyclic command is fed to the systm, then maybe the small amount of gyroscopic resistance can be felt coz theres a momentary lack of airodynamic assistance. [ and a small amount in a gyroscope this big is alot to a puny human arm with buggerall leaverage].

IOW, if the blades arn't fly'n, they can't have the airodynamic responce we need the counter the gyroscopic resistance...........no???????

[ i belive theres a few other situations where this happens, thankfully only momentarily.]

[Doug Riley]

Yes if the blades aren't flying, you can't have your power steering...

BUT, to a rotorblade, "flying" means simply "spinning." The fact that a rotor isn't delivering any lift to the frame doesn't mean that your usual cyclic aerodynamic control OVER THE ROTOR is gone. (It DOES mean that the rotor has little effect on the frame, but we don't care about that here; we've already landed).

The rotor right after landing is in very-low-G mode -- it's delivering little net lift to the frame.* HOWEVER, when you make a cyclic input, you increase one blade's AOA and decrease the opposite one by the same amount, same as usual. The NET lift over the rotor as a whole hasn't changed, but the individual blade that got up-pitched still experiences increased lift, while the de-pitched one loses lift. The disk will precess normally, in response to this imbalance.

I'm betting, Birdy, that what you're experiencing is some combination of the effects that Al and Chuck have described. I'll try to duplicate what you've described in the next day or two and see what it's like... if Dragon Wings on a needle-bearing teeter hinge can even do it.

*Because your forward airspeed is low and you're not sinking as you would in the air.

[MichaelBurton]

If we push the stick forward we increase the AOA on the retreating blade and reduce the AOA on the advancing blade. This will put an upward force 90 deg ahead and in the plane of rotation pushing the rear of the gyro up if the rotor hits the flap stop. The rollover could happen either direction based on the direction the nose wheel is pointed. As the gyro darts one direction and the end comes up the picture gets bad fast. There is also a gyroscopic action from the prop. Turning right will add to the back end coming up over the top.

Why do we want to push the stick forward quickly?

[birdy]

Now that we'v established that there is some gyroscopic presance that could be felt in the stick, we need to picture wots happen'n with the air that the disc is spin'n in at the point of touchdown.
Coz we have no AS and no weight on the rotor, it can't autorotate, but its loaded with inertia so its still spinin at fly'n rpm and in a short time will start to pull the machine backwards coz its tilted backwards. Doug, if you wait till the machine starts to creep back before you push the stick forward, you won't feel it anyway near as strong as you would if you push forward before it starts to reverse. [ as soon as the forward motion has stoped or even slightly before] then you will feel it.
This is why i recon the blades have little airodynamic effect at the point where they change from the 'autorotating state'[ flying] to the 'helicoptering state'[pulling the machine backwards], and if large cyclic inputs are applied here, you'll feel the gyroscpoic resistance.
The best explanation i can cum up with is , the air the blades are spining in at this point resembles the water a centriphical pump's impeller is in when the outlet is shut off. Its still spining but theres no friction. Kinda like cavitating??
Just listen to the noise they make when in this situation, its like no noise they make normaly.

Don't think i'v ever hit the stops in this situation Michael, i'd recon that that would be push'n a little too hard.
As the gyro darts one direction and the end comes up the picture gets bad fast. There is also a gyroscopic action from the prop. Turning right will add to the back end coming up over the top.
In the situation i'm refer'n to theres no forward ground speed and little forward AS, if any, and on idle power so theres buggerall feedback from the prop.

Why do we want to push the stick forward quickly?
Plenty o reasons in this line o work Michael.;)

[Aussie_Paul]

Why do we want to push the stick forward quickly?
Plenty o reasons in this line o work Michael.;)

Could you name a few reasons please Birdy?

Aussie Paul. :)

[mceagle]

Birdy, another thing that may be partially responsible for the effect that you are feeling is to do with rotor blow back angle. The second that the gyro stops "flying" and starts to sink, the rotor speed decays very quickly. As most landings are done into wind (no matter how gentle) the blow back angle becomes greater as the rotors slow, and can move the spin axis ahead of the pitch pivot (especially with higher teeter towers) The further forward the spin axis moves, the harder it becomes to move the stick forward.
This is particularly noticable by eager beginners on take off when their forward speed is too high for their rotor speed. They find it very hard (sometimes impossible) to move the stick forward to the correct take off attitude.

Another mitigating factor could be that when the rotors are lifting the total weight of the gyro, their offset lift force in the joystick is being cancelled out by the trim spring. When that lift depreciates, the trim spring force in the joystick becomes greater, tending to pull the stick harder rearwards. Even a small trim spring pressure would be very noticable after many hours of flying wity a trimmed stick.

[birdy]

is to do with rotor blow back angle
Wouldn't that be a rearward pressure on the stick Tim??
on take off when their forward speed is too high for their rotor speed
I get this alot when horse'n it off on short strips and its a rearward resistance.
the trim spring force in the joystick becomes greater, tending to pull the stick harder rearwards
Agreed, but the force i'm on bout is to the left, not backwards.

[Al_Hammer]

Tim,

I think you bring up a good point. Following up on what you said about blowback, I'm wondering if the rearward force you mention might possibly cause a sideways force right after touchdown.

The rotor has a blowback(flapping) angle at touchdown. As soon as the wheels touch, the load on the rotor goes away.
The blowback does not go away instantly due to inertia in the rotor. For a few moments, the rotor will be flapped back , trying to counteract disymmetry of lift, but there is no disymmetry of lift, so the rotor will tend to have reduced angle of attack on the advancing blade. This will pitch the blade forward and to the right due to rate cross coupling. The effect becomes much greater if the stick is moved forward during this time.
Just some speculation on my part...now I'll duck for cover.


Al

[birdy]

I duno, remember, this can happen in a no wind spot landing as well as with wind,[ only moreso in no wind] and if the rotors are pulln the machine backwards,[ in no wind] the air go'n through the disc is go'n forwards.[helicopterin] As for the frount half of the disc blow'n back, wouldn't the much greater ground affect on the rear of the disc counter it[ if it existed], coz its so close to the ground??

Another thing to ponder is when you stick forward as soon as you touch, the disc will pump air from the top down at the rear of the disc and from under to top at the fount, [which is why dust allways blows inot the RAF cab].IOW, its try'n to pump air in a direction that its already go'n,ie;forwards. [i know nuthn bout sail boats, but i recon if you tried to steer a bout to the right by pabbel'n on the left side while the sail is already push'n the boat forward, itd be less effective than it would be if it wasn't move'n in the water.] The blades when in left n right sides of the disc are do'n nuthn but chang'n pitch.

[Al_Hammer]

Birdy,

I don't believe ground effect or "helicoptering" of the rotor plays a big part in this.
The rotor is still producing lift after landing, as proved by the fact that you can roll backwards by pulling back on the stick. The rotor is not at zero g.
The rotor is pushing air forward if its tilted back, but it is also pushing air down in flight, so nothing has changed in terms of direction of flow. The driving force is no longer present, so the rotor will rapidly lose speed, but it continues to thrust air in the same direction as before.A gyro , like a helicopter, flies by thrusting air down.

The gyro will have forward speed just before landing , unless its a vertical descent. The rotor doesn't blow back upon landing- it blows back in flight.
As Tim says, this means that the rotor spin axis is not aligned with the hub axis and under these conditions the rotor has reduced AoA on one side compared to the other. After you land, the flapping may(i'm guessing here) be tending to pitch the rotor forward and to the right until the rotor re-aligns with the hub.

[birdy]

Your all loos'n me now.

The rotor is still producing lift after landing
Correct.
The rotor is not at zero g.
Correct.
The rotor is pushing air forward if its tilted back, but it is also pushing air down in flight,
Correct.
so nothing has changed in terms of direction of flow.
?????????????????????
I think theres gota be a difference. When in the air, [autorotating] the air is hitn the blade at an angle from underneath, when you land and take the weight off, the AOA deminishes near to or greater than 0*. Theres no thrust from the machine to counter the thrust of the disc, air is exit'n from the underside of the disc so its gota be cumn in from the top, no?
Coz the air passn around a flyn blade [ with the machine also flyn] has a sudden and dramatic change in position, the blade gets maximum airodynamic effect [ lift]. But when the air passn round a flyn blade on a landed machine isn't moving the air as much [ lower AOA and reversed flow*], the airodynamic effect isn't as great.
*When i say 'reversed flow' i'm say'n the body of air the rotor is spin'n in has started to move in the direction the blade is pushn it, ie; enter'n the disc from above .
When its flyn[ and the machine is flyn too] air enters from below, is suddenly pushed downwards, but not to the extent of a helicopter'n [ hover'n] blade. The air is only compressed for an instant, then resumes its natural position before the next blade hits it.[ it has to, or our teeter'n systm could never acomidate one blade with high AOA and the other blade with a lesser AOA, they'd never fly in track.]
Its only the rapid small downward pulses of a disc that creat a slight down wash, but most of the volume of air that actualy passes through the disc exits through the top between the blades.
The air [ when flyn] enters from under and exits from over the DISC, but with a downward flow compared th o the surrounding air mass.its only the NET EFFECT of rapid strong pulses pushn portions of air down that creats the slight down wash from a gyro disc.

The rotor doesn't blow back upon landing- it blows back in flight.
Correct.
After you land, the flapping ........
Theres no flappn in a no wind situation, why would they flap? they have equal AS and AOA.[ i'm talkn ONLY bout spot landings.]
may(i'm guessing here) be tending to pitch the rotor forward and to the right
I fail to see why any flappn [if there was any, which would only be caused by unequal ASs]could feed back through the stick in any direction unless the stops are hit or theres a massive pitch command. Either way, it wouldn't be a smooth, constant resistance.


I think the missunderstanding is cumn from the fact that i'm ONLY talk'n bout a ' no roll, no wind' landing, and everybody keeps thinkn of some degree of AS at the disc.
A landing WITH some AS will never have this feedback from the stick coz they are still autoing, and they still have airodynamic authority to counter [ if any] gyroscopic resistance.



Bloody ell, now I got a eadake.

[Doug Riley]

Birdy, I think I understand what Al's talking about. A flare is probably the most rapid change of straight-line velocity that you encounter in gyroing. In a full-flare landing, you go from forward airspeed to no airspeed over a short interval of time.

While you still have airspeed, the rotor will naturally be "blown back" -- that is, it won't be square to the spindle. You accomplish the flare by pulling back the stick while there IS still airspeed. The spindle tilts back when you pull. The rotor follows because you've put in a cyclic command. Once the rotor has finished responding to your back-pull, the rotor again will be tipped more aft than the spindle (because you still have forward airspeed). IOW, at the beginning of the flare, there's still active blow-back happening.

Once the extra RRPM stops you in your no-roll landing, the airspeed is gone. The rotor has mass, however, so for some interval of time after you stop, it's still tilted farther aft than the spindle (call this "leftover blowback"). The leftover blowback has, as a consequence of the teeter hinge, a by-product in the form of a cyclic de-pitch on the advancing blade and a cyclic up-pitch on the retreating blade.

While you had airspeed, this cyclic pitch differential was just what you needed. Once you stop and it becomes a mere "leftover," it will cause the disk to precess forward, toward a plane square to the spindle.

For the disk to do this, the front blade must descend relative to its old orbit. This increases its angle of attack. The blade will try to rise (relative to its old orbit), which, with the precession lag, makes the whole disk tip a bit to the right. Once the leftover blowback is gone, this effect should disappear, too.

I have no idea whether this reaction is strong enough or lasts long enough to be noticeable, but it makes sense that it would exist. It's a matter of the rotor's adjusting itself to your sudden new airspeed (namely, 0). It will happen to some small degree any time you change airspeed, but, unless the change is sudden, it would be too small to notice.

[Al_Hammer]

That's exactly what I meant, Doug. Thanks.

Sorry you got a "eadache" Birdy. I hope it doesn't add to it if I say that
I believe the downwash from a gyro is equal to that of helicopter of similar weight. It may not be as obvious since the gyro doesn't spend much time hovering near the ground, but in a spot landing notice how much dust is kicked up.
Your point about air coming in from the top after landing is valid(called inflow). That will lower the effective angle of attack on the blades a bit and reduce thrust. The blades are spinning down at this point, so it just further diminishes the lift. It doesn't alter the "left over blowback" effect. As Doug says , this may not be a major effect, but one that could be significant, we just don't know without being able to take some measurements.

[Doug Riley]

Downwash:

The change in momentum of the air that results in downwash has to be enough continually to hold the aircraft up. Momentum is mass x velocity. This rule applies to any heavier-than-air craft that supports itself by squirting matter downward -- whether it's a rocket, Harrier jump-jet,flying platform, FW plane, gyro or helo. You could imagine an aircraft that shoots B-B shot out its bottom to hold itself up -- it would work the same way. Each of these devices uses a different mix of mass and velocity in its downwash(somewhat like high voltage/low current vs. low voltage/high current).

It turns out that large mass and low change in velocity consume less power than low mass/high velocity. The Space Shuttle may possibly be the least efficient flying vehicle ever devised by man, for this reason.

The downwash of a gyro rotor probably is slower than the downwash of a helo of the same weight because of the gyro's lower disk loading. That is, the gyro downwash affects a larger mass of air per unit of time, but speeds it up less, than the helo. You can bet that both of them comply with the basic rule that "impulse equals change in momentum," though.

[Al_Hammer]

Doug, I stand corrected :D The gyro does have a lower disk loading, so the downwash will be be proportionately less. Here is a quick and dirty formula for figuring downwash velocity based on weight and rotor area.

[Doug Riley]

Al, I know you know this stuff far better than we basket-weavers do. It's all in the definitions -- the gyro downwash is slower, but it has more mass. Is that "less" downwash, or the "same" downwash with a different mix of V and M?

LFINO!

[mceagle]

Al/Doug, when the gyro comes to a stand still with the stick full back, the dihedral effect of the coning angle would mean that the rearward rotor would be supplying more lift than the front one. This would tend towards a rolling force to the left, requiring right stick pressure to counter. Perhaps this could also play some small part.

[Al_Hammer]

Tim, could you please explain for this basket weaver what you mean. http://theolddub.com/hammer/clap.gif :D
what is the increased lift at the rear due to? ground effect?

In forward flight, the coning produces a tendency for the rotor to tilt to the right due to the increased AoA at the front and the consequent precession, but when the gyro is at zero airspeed I would expect this effect to go away.

[mceagle]

Al, the gyro is now stationary with the stick full back. Collectively the blades have a thrust vector, made up from the individual blades lift vector. In this case, due to the coning angle and the rearward tilt of the disc, the individual lift vector from the rearward (more horizontal) blade is greater than that from the forward blade, which has to be compensated for by the pilot (more noticable if he has to move the stick to a new position).

Please let me know if I am barking up the wrong tree here.

[Al_Hammer]

I see what you're getting at,Tim, but even though the rear blade is more horizontal , it is not spinning in a horizontal plane. Both blades are spinning in a plane that is tilted back and both fore and aft blades will have the same AoA if the airspeed is zero, ignoring blowback effects for the moment.

[birdy]

I recon my bigest problem is not be'n able to put thought into words.:mad:

Al, the edake was gon, but after readn this, its back.:D

I understand wot yous are say'n bout the blowback still momentarily present after AS has washed off, but as the rotor cums into line with its spindle, it uses cyclic airodynamic force to accomplish it, which would transmit no feedback, and even in the most sever cases it MIGHT feed back a PULSING resistance.

I have no idea whether this reaction is strong enough or lasts long enough to be noticeable
Doug, if this was the case and it could be felt, wouldn't it be felt at the same intencity with light and heavy blades? Your talkn bout an airodynamic resistance not an inertia one , no?

I believe the downwash from a gyro is equal to that of helicopter of similar weight. It may not be as obvious since the gyro doesn't spend much time hovering near the ground, but in a spot landing notice how much dust is kicked up.
If they ever made a heli of the same weight and blade loading as me ferel Al, itd kick up as much dust as the ferel dose in a high inertia hover landing.[which BTW, are the bestest things you can do ina gyro;) .]
Your point about air coming in from the top after landing is valid(called inflow). That will lower the effective angle of attack on the blades a bit and reduce thrust. The blades are spinning down at this point, so it just further diminishes the lift.
And with dimished lift theres also diminished airodynamic authority available to the blades to counter the gyroscopic resistance to directional change [ which Doug says there is SOME].....no?

Not sure you could feel this differential in lift if it was there Tim coz its gota go through the teeter hinge.

Gota go n nurse me head again.:o

[Al_Hammer]

Birdy, take another aspirin and see if you can stand any more of this...Http://theolddub.com/hammer/lol2.gif

I understand wot yous are say'n bout the blowback still momentarily present after AS has washed off, but as the rotor cums into line with its spindle, it uses cyclic airodynamic force to accomplish it, which would transmit no feedback, and even in the most sever cases it MIGHT feed back a PULSING resistance.

Since the line of thrust is above the pitch/roll pivots, there is some torque transmitted to the stick if the rotor axis is not ligned up with the spindle axis.

Doug, if this was the case and it could be felt, wouldn't it be felt at the same intencity with light and heavy blades? Your talkn bout an airodynamic resistance not an inertia one , no?


Heavier blades would result in the leftover blowback taking longer to bleed off and giving more time for the blowback effects to be felt.

And with dimished lift theres also diminished airodynamic authority available to the blades to counter the gyroscopic resistance to directional change [ which Doug says there is SOME].....no?


I don't see, why any gyroscopic forces would pass through the teeter hinge. However, some inertial resistance to movement will be felt in the stick due to inertia in the feathering axis and some resistance to movement will always be present as a result.

Normally , the rotor blades act as a servo mechanism; small cyclic pitch changes by the pilot are amplified aerodynamically and the blades fly to a new position. If the rotor is not making much thrust, for whatever reason, the result is less control power.
What I don't see is how this loss of power would increase force in the stick directly. The force you feel must be due to the tilt of the rotor from the blowback effects, or some other effect. The pitch and roll pivots are below the center of rotation as I said, so that explains why there is a force in the stick.
In flight, you would not normally experience this force, since the aerodynamic forces always tend to keep the rotor aligned with the spindle. It is only when the aerodynamic forces can no longer do this, that the rotor starts to tug on the stick. As you can see, I am partially in agreement with what you were saying in the first place.

[birdy]

I don't see, why any gyroscopic forces would pass through the teeter hinge
Doug said that SOME dose but you don't feel it coz of the high athority of the cyclic control, when you have sufficiant airodynamic effect.

What I don't see is how this loss of power would increase force in the stick directly.
Only when you apply a cyclic command. If you don't move the stick, you won't feel anything. Same as a gyroscope, you only notice resistance when you try to move it.

Al says; If the rotor is not making much thrust, for whatever reason, the result is less control power.
And Doug says; Some, yes. after i asked him;would the barbell create any 'resistance to change feedback' to the stick?

Now, if i add these two answers together, it'll read sumthn like this.

If the rotor is not making much thrust, for whatever reason, the result is less control power.......... to override the ......small..... amount of gyroscopic resistance presant in the still spinning rotor. The result is a force to the right felt when you push forward, and if not checked, COULD result in a rollover if the wind happens to be cumn from the left.

I can feel me head clearn up already.:D

[Doug Riley]

Birdy, to clarify: There's no GYROSCOPIC resistance passed back from the rotor through the teeter hinge. What I was talking about was the moment of inertia of the blade itself, considering the blade just as a long plank. The blade resists feathering motions by virtue of the fact that it has mass distributed around, and some distance from, its own center of gravity.

Imagine a rowboat oar made of solid cast iron. Sit in the boat and hold the oar handle. Try to feather this oar (i.e. rotate it around its long axis). It'll fight you more than a wooden one, right? Try feathering and un-feathering it really fast. Your wrist will get more of a workout the faster you feather the oar.

That's the kind of inertia-effect feedback I was referring to -- it's not gyroscopic, just plain old mass-moment-of inertia stuff that applies equally whether the blade is spinning or not. Wide-chord blades, higher-mass blades* and blades with external noseweights all will have more of this kind of resistance than your average blades. As you say, this kind of inertial reaction will produce a 2/rev, not a steady force in the cyclic stick. It WOULD produce a steady force in a collective stick, but that's another story...

* Assuming that the mass is distributed along the chord and not all concentrated right at the feathering axis.

The key to Al's theory is that the rotor takes some time to get itself out of "blowback position" and square itself to the spindle. During this time lag, the portion of the disk in front of the gyro is descending. As each blade passes through this forward position, it descends. In descending, it meets the air with its bottom surface. This is an increase in angle of attack. The blade reacts in the usual lagged manner to this increase in AOA -- it tries to fly UP, maxing out 90 degrees later. This means it rises on the left and the rotor pulls to the right (if your rotor turns CCW viewed from above).

(If aspirin doesn't work, try a Foster's.)

[Doug Riley]

An increase in the stalled region of the disk while you're still moving forward should cause a 2/rev. Once you've stopped, leftover blowback (which causes a cyclic up-pitch on each retreating blade) will still mean that the stall wil be most pronounced in the retreating sector -- 2/rev again.

If the blades had more than the usual amount of stall uniformly all the way around the disk, it would be after you stopped and after the blowback went away. If this created a large negative blade pitching moment all the way around, the two opposing blades would just twist against each other through the hub bar. They wouldn't do anything to the controls... unless you had collective. The collective would try to pull itself down, I'd expect.

[Chopper Reid]

I read your early posts Birdy and I too know not much about what you are saying BUT I have had similiar expereinces too and I reckon its just that we are moving the stick too quick for the rotors.
I'm using Glass 27 ft 6 in rotors and they are heavier than alloy 27's and I have noted that I do have to be a bit gentler in the stick handling department. Also have felt the rotors protest on going from hard left turn to a hard right say. Just the beginnings of the rotors hitting the stops.

I have found the thread very interesting.

[birdy]

F$%KN ELL, now i feel like i'm standn in front o Al's pissedoff cat.:mad: