Gyroscopic feedback/ Chuck B.

[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.]

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 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!