Getting a grip on gyroscopic precession?

[Learjet]

"Gyroscopic precession" : these two words are almost guaranteed to get just about any gyro instructor sweating... so it's not really a topic that gets much in-depth coverage in those theory of flight lectures.

Wiki pretty much sums it up as follows:

Gyroscopic precession also plays a large role in the flight controls on helicopters. Since the driving force behind helicopters is the rotor disk (which rotates), gyroscopic precession comes into play. If the rotor disk is to be tilted forward (to gain forward velocity), its rotation requires that the downward net force on the blade be applied roughly 90 degrees (depending on blade configuration) before, or when the blade is to one side of the pilot and rotating forward.To ensure the pilot's inputs are correct, the aircraft has corrective linkages which vary the blade pitch in advance of the blade's position relative to the swashplate. Although the swashplate moves in the intuitively correct direction, the blade pitch links are arranged to transmit the pitch in advance of the blade's position.

Which in plain language I understand to mean; if you want to tilt the disc down the control input is made 90' degress earlier - and this input occurs thanks to the offset position of the Y-shaped rotor-head fork in gyros?

So, if this is correct... how the hell do you fly one of these?


If I'm understanding the theory of gyro precession correctly, and in the absence of offset input linkages .i.e direct input - you'd need to make 90 degree offset inputs to the direction you actually want to go ???? E.g forward stick to roll left, aft stick roll right

Or am I smoking something?

[saraf]

Hey Dave , don't think about it just fly it!!!!!!!!! lol

[Learjet]

Eben, that's the problem! Fly gyros enough and you begin start wondering why and how they fly. Then you start scratching into topics like gyroscopic precession and you begin to wonder how on earth we actually manage to control the damn thing. I've also found a convincing argument to deter those who claim gyros are weight shift... The cyclic leverage factor on a R22 helicopter is 14feet. Dis nou n moerse koevoet!

[Wagtail]

Think about it this way. Even with direct overhead control, if you tilt the head forward the angle of attack of the advancing blade ( on left hand side for our "western" rotors ) is reduced and this will cause the disk to "fly" lower 90 degrees later, that is when the blade is in the front. The nett result is that the disk actually tilts forward because the angle of attack input was given 90 degrees "before"...sort of neat solution given to us by mother nature.

[Gyronaut]

"Gyroscopic precession" : these two words are almost guaranteed to get just about any gyro instructor sweating...

I object m'lord

I have a bicycle rim in class that I use to demonstrate gyroscopic precession and it becomes very clear very quickly if you spin the rim in the horizontal plane and then give it inputs. They DO happen 90 degrees later.

I agree 100% with Wagtail.

The offset torque-tube on the Magni makes no difference, just gives it more leverage and thus smaller inputs on the controls. Other machines, like the MT03 don't have that and work just as well.

Len (sans sweat)

[Learjet]

Hi Len & Wagtail - thanks for your comments - we're all agreed on the 90' input Can we put AOA to one side (for the timebeing) and focus simply on the gyrsoscopic precession input - reason being this would have the same nett result on a spinning bicycle wheel or a child's gyrsoscope toy i.e where the AOA is irrelevant.

Now if the off-set torque-tube is not playing a role in the 90' control input (which we all agree occurs) :
(i) why bother to off-set it? Or are we saying that the change in AOA (and thus lift) is such that it takes precedance over the forces of gyrocopic precession which would want to tilt the disc to one side - not forward? (As per Wagtail's post above)
(ii) why do heli's require an off-set pitch-horn to achieve the same?

From Helicopter Flight Info's chapter on gyro precession. http://helicopterflight.net/gyroscopic%20P.htm :

What many don't quite understand is how the control rigging in the helicopter compensates for gyroscopic precession. In helicopters, the controls are rigged is such a way that when forward cyclic is applied, the helicopter moves forward, likewise for aft, etc. To accomplish this, the pitch horn is offset 90º to the rotor blade. The controls still tilt the swashplate in the same direction as the control input is made, but due to the pitch horn placement, the input to the blade occurs 90º earlier in the plane of rotation.

gyroscopic precession. This principle is behind those experiments in science museums where you hold a spinning bicycle wheel and it feels so wierd if you try to tilt it. The principle states that any force applied to a rotating object will manifest itself 90° "later" in the direction of rotation. Say you're holding a bicycle wheel that's spinning clockwise so the wheel is on its side, parallel to the ground. If you try to tilt it forwards - away from you - it will tend to tilt to your right: 90° further in the direction of rotation.This complicates rotary flight, because the helicopter design must take account of it when control motions are applied. If we have a main rotor spinning anti-clockwise, and want to tilt it forwards, we must attempt to tilt it to the right. That way, the tilt will actually occur in the direction we want. All control inputs must be 90° "behind" the desired direction.

Archie Woods article http://everything2.com/title/Helicopter+pitch+controlon helicopter pitch control:

[weedy]

The following is from the Rotor Craft Handbook produced by the USAs FAA.

"GYROSCOPIC PRECESSION
The spinning main rotor of a helicopter acts like a gyroscope.
As such, it has the properties of gyroscopic
action, one of which is precession. Gyroscopic precession
is the resultant action or deflection of a spinning
object when a force is applied to this object. This action
occurs approximately 90° in the direction of rotation
from the point where the force is applied. [Figure 3-8]
Let us look at a two-bladed rotor system to see how
gyroscopic precession affects the movement of the tippath
plane. Moving the cyclic pitch control increases
the angle of attack of one rotor blade with the result
that a greater lifting force is applied at that point in the
plane of rotation. This same control movement simultaneously
decreases the angle of attack of the other
blade the same amount, thus decreasing the lifting force
applied at that point in the plane of rotation. The blade
with the increased angle of attack tends to flap up; the
blade with the decreased angle of attack tends to flap
down. Because the rotor disk acts like a gyro, the
blades reach maximum deflection at a point approximately
90° later in the plane of rotation. As shown in
figure 3-9, the retreating blade angle of attack is
increased and the advancing blade angle of attack is
decreased resulting in a tipping forward of the tip-path
plane, since maximum deflection takes place 90° later
when the blades are at the rear and front, respectively."


With gyros we must manually tilt the rotor head to change the tip path of the rotors thus affecting to path of the gyro.

Regarding offset I think its mostly for convenience sake, as I have seen rotor heads that have a rear and side attachment for the controls.

[Gyronaut]

Dave, a great topic indeed. We cannot, unfortunately, leave AOA out of the discussion if we wish to understand it fully.

My attempt at describing the way I understand it a little more ...

Remember that a gyro's rotor is not driven and we are not changing the pitch of the blade at any stage, as in a helicopter and we need not 'wait' for the pitch change to have effect. Granted, we are applying forces at the root of the disc which happen 'later'. Gyroscopic precession in a dynamically driven rotor blade is taken care of by the rotor aerodynamics. The direction the rotor "disk" is tilted is exactly as mechanical intuition would suggest. Move your Gyrocopters joystick/cyclic around while stationary and look at the head ring gear - it moves in the direction of the control input. If the rotor wing rolls left, say, there will be gyroscopic precession trying to tilt the rotor disk rearward (i.e. front up, rear down). But, this actually means the rear blades see increased AOA, while the front sees reduced (or even negative) AOA. (or is it the other way round? confusing myself here now... lol) The practical upshot is that the blades (disc) stay in the same plane as the control input thankfully and therefore it all works for us.

am I making any sense?

[Learjet]

Thanks for the responses and info guys. Len, you make complete sense, but I think there's still some confusion (though perhaps only in my mind!) about the various forces at play in a flying wing / spinning rotor i.e gyrsoscopic precession and the seperate(?) issue of AOA and resultant increase / decrease in lift effecting the flying charateristics of the airfoil. At risk of repeating myself, gyroscopic precession can occur in a vacuum or on a flat spinning disc, bicycle -wheel or gyrsoscope toy where there is no flying wing to speak of and absolutely no role of AoA in the precession forces that occur. This is the part I'm interested in. (if the resultant AoA and lift counteracts / balances the gyro precession force then great... but we still need to tilt the disc in the direction we want it to go. And the laws of gyrsoscopic precession are quite clear. It's going to (initially) "turn" 90' degrees offset to the direction which we are "steering" it.

So to clarify this in my own mind - and if we use Len's bicycle-wheel example (which has no wing or AoA) but still displays gyrsoscopic precession, I'm unclear as to why we keep referring to changing AoA to increase / decrease lift and resultant direction of flight? This all happens after, and is a secondary effect and ensuing result of our initial direct input to the disc. I'm interested in the direct input itself, and the gyrsocopic precession response. And why a helicopter needs adapted linkage configuration and off-set pitch-horn to compensate for the input offset and 90' desired response - and yet some are saying here that a gyro doesn't?

Imagine a gyro had a flat, clockwise spinning disc instead of rotors. Or even Len's spinning bicycle wheel sitting on the top of the mast. (for the sake of discounting all the AoA and advancing & retreating blade stuff lets just say the gyro is stationary and there is no wind blowing.) If you tried to tilt the disc or wheel forward by direct input it will want to tilt 90' to the right. If you wanted the disc to tilt forward, you would have to try and tilt it to the left. That is gyrsoscopic precession. Nothing to do with AoA, nothing to do with lift, nothing to do with rotor airfoil shape. Pure gyroscopic force at play. Helicopters correct for this control input through the linkage configuration and off-set pitch-horn. I guess the part I'm still trying to understand is why that apparently doesn't seem equally applicable to gyros like the direct column input Bensen above...? i.e .with that direct input configuration you'd in theory need to make 90 degree offset inputs to the direction you actually want to go ???? Anyone here flown one who can answer?

[Gyronaut]

You do make a very valid point Dave, but I think your statement

"AoA and lift counteracts / balances the gyro precession"

answers most of it.

Not being an engineer I cant explain it succinctly but I also believe/sense that the precession is less pronounced in gradual movement. If you YANK the stick backwards from the horizontal plane while on the ground at 200+ RPM, chances are you will end up on your side. Move it back gradually and let the airodynamics take effect, it doesnt happen. No?

[Wargames]

It blows my mind that you smart blokes can discuss procession of gyros without even a mention of its rigidity. (Menu->Edit->find->Rigid->No results)

Here is the catch, rigidity determines the procession. Ie. If a gyro is very rigid, the procession would be close to nothing, and vice versa.

Now I am no gyro nut, and hence do not know the properties your gyros would have wrt this. Can someone please give me an idea of how rigid your gyros gyroscopic forces is.

Very interesting topic.

[Gyronaut]

THAT may well be part of the answer Dirk !!

I suspect it may be the other way round actually. My bicycle wheel rim is VERY rigid and has strong gyroscopic precession while an un-rigid system would probably be less prone.

The teetering rotor makes a gyro rotor system un-rigid and allows the blades to 'hunt' up and down to find their happy medium between the varying forces acting on the advancing and retreating (relative to the wind) blade. Similarly I suspect the gyroscopic precession forces are possibly absorbed/damped largely by the teetering system.

Ag ek weetie meer nie... al wat ek weet is die ding vlieg en hy vlieg LEKKER!

[Wargames]

THAT may well be part of the answer Dirk !!

The teetering rotor makes it un-rigid and allows the blades to 'hunt' up and down to find their happy medium between the varying forces acting on the advancing and retreating (relative to the wind) blade. Similarly I suspect the gyroscopic precession forces are possibly absorbed/damped largely by the teetering system.

Ag ek weetie meer nie... al wat ek weet is die ding vlieg en hy vlieg LEKKER!

That may well be it. Because the blades has a lot of flex in them, they dampen most of the forces acted out on it, to make it a very rigid gyro, with very little precession. And that len, is why you can enjoy a beer while flying. Although I have Never seen you do it.

[Gyronaut]

And that len, is why you can enjoy a beer while flying. Although I have Never seen you do it.

If this stuff keeps smokkeling with my head ... watch me !

[Learjet]

Eurika... I finally get it. It took a good chat on the phone to Magnifan while sitting in my office chair with my arms spreadeagle changing the AoA of my fingertips while swiveling in circles! My staff think I'm nuts but I got it. Wagtail & Len - you guys were spot on! All we do when we wiggle the stick is change the AoA... the actual precession input is the rotors flying response to our AoA change off-set by 90'. Pull stick back - AoA changes 90' to the pilot - input is 90" which results in rotor tilting disc back. AAAhh shweet!