That game of darts...

[saraf]

Ebie, I think what PtKay is (correctly) pointing out is the mistaken assumption that the gyro disc attitude is changed by means of weight shift (i.e "pulling or pushing up, down,left or right etc.as per a trike wing - albeit on a gyro via the control linkages). This is not the case.
It is gyroscopic precession (not weight) which changes the disc attitude - i.e. the pilot input (via the control linkages) merely changes the rotor blade Angle of Incidence and gyroscopic precession (and thus the disc attitude change) occurs 90' degrees later.

wiki explains it quite nicely:
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 that blade gets to the 12 o'clock position. This means the pitch of each blade will decrease as they pass through 3 o'clock, assuming the rotor blades are turning CCW as viewed from above looking down at the helicopter. The same applies if a banked turn to the left or right is desired; the pitch change will occur when the blades are at 6 and 12 o'clock, as appropriate. Whatever position the rotor disc needs to placed at, each blade must change its pitch to effect that change 90 degrees prior to reaching the position that would be necessary for a non-rotating disc.

I am not stepping on anybodies toes but maybe we need to fully understand what ours rotors are doing and what forces are applied to them.....

Sometimes Wikipedia can make mistakes too.......

Summation:

A 'floppy' teetering rotor will not experience any gyroscopic precession. This is because it is a 2-blade rotor and there is nothing at 90-degrees ahead of the blades to gyroscopically 'precess'.

An absolutely rigid rotor will act like a gyroscope and therefore experience precession. The amount of gyroscopic precession should be very small though, because;

The rotor has relatively little mass,
It must overcome the inertia of the helicopter as well as the rotor, and
No rotor yet has achieved this high a level of rigidity

Regards Eben Jnr

[saraf]

Gyroscopic precession has *virtually* nothing to do with a helicopter's rotor.

Gyroscopic precession is little more than a simplistic means of describing the basic end result at the rotor, after the application of a cyclic input. (i.e. 90-degrees later ~ basically). It is no good for explaining the 'how'.

The simplest argument against the use of 'gyroscopic precession' is to look at its algorithm. Mass and rotational velocity are the predominant variables. The relative mass and rpm in a gyroscope are humoungeous. The relative mass and rpm in a rotor disk are negligible.

The helicopter's rotor disk 'flies' to position. The eventual demise of the sinusoidal swashplate will result in rotor blades that can fly to position in 12.3-degrees or 123-degrees.



The results of gyroscopic precession and aerodynamic precession are the same for a basic teetering rotor, but the result of gyroscopic precession is easier to see. It therefore is a quick way to explain precession to mechanics and pilots who have to work with it but do not necessarily have to know its 'innards'. If a pilot or mechanic want to fully understand precession then to continue to explain it in terms of gyroscope precession only becomes a problem.

I am not saying "that gyroscopic precession has nothing to do with helicopter rotors". It plays a role, but its direct roll is a very small one and its indirect role is exactly that ~ 'indirect'.

A gyroscope (gyroscopic precession) functions because of high mass and high rpm. Aerodynamic plays no significant role.

A rotor (aerodynamic precession) functions because of aerodynamic forces. Mass and high rpm play no significant role

Direct role:

I believe the magnitude of gyroscopic precession's direct role in aircraft is dependent upon the rigidity, mass and speed of the device. A propeller will have a reasonable amount of gyroscopic precession, whereas a teetering rotor will have very little.

[Gyronaut]

Ok, Great. At least we're all in agreement then.

He did say

...merely changes the rotor blade Angle of Incidence and gyroscopic precession (and thus the disc attitude change) occurs ...

Exploring and trying to best understand rotor dynamics can't possibly do anyone any harm when flying Gyro's.

[saraf]

So let me be the thorn in the bush again, but i cannot agree that gyroscopic pres is the reason. The AOI or AOA DEF does but GSP has very little to do with it. You still need an outside force to make another force move........

[Gyronaut]

Ebie, I certainly don't believe Gyroscopic precession is the reason but I do believe it is present and has an influence on the behavior of the rotor.
Inertia is a force and is present? Or am I confused?
Len

[PTKay]

I am not stepping on anybodies toes but maybe we need to fully understand what ours rotors are doing and what forces are applied to them.....

Sometimes Wikipedia can make mistakes too.......

....

In an independent research Wikipedia has been proven more precise than Encyclopedia Britanica.
Multiple aerodynamics textbooks are also clear about the matter.

Like one of the most experienced gyroplane pilots from Australia,
David Bird, "birdy" on The Rotary Forum states in his signature:

Ignorance is bless, but only till you realise you were.

[crazydoc]

So just to get my head around it.
If gps plays a minimum role, due to / because of /not entirely because of, a teetering system then its basically a weightshift . which starts to make more sense to me. i think??
Traditionaly one first shifts your weight then the plane follows. Here you change the AOA via the stick , the blade flies itself say downwards which puts the fuselage closer to the blade ie weight is closer to one side and whole plane goes in that direction.?

[crazydoc]

i mean gsp

[saraf]

Hi crazydoc,

Yes, You change the rotor disc AOA to your forward motion as well as shifting the weight by introducing an outside force to another force via your controle stick. You can not change the pitch of the blades like in a helicopter.

Regards

[Learjet]

Hi crazydoc,

Yes, You change the rotor disc AOA to your forward motion as well as shifting the weight by introducing an outside force to another force via your controle stick. You can not change the pitch of the blades like in a helicopter.

Regards

Please do me one little favour...

Place your rotor at the 9'oclock and 3 o'clock position i.e cross-wise to the gyro.
ask someone to pull back on the cyclic while you stand in line with the advancing rotor and observe what happens to the AOI - or the "pitch" of the blade.

Please come back and tell me what you saw.

[Gyronaut]

I have done that Dave.
Aggressive AOI change.
Why does this control input not roll the Gyro left then? It actually results in a pitching moment.
Methinks this is where GSP comes into play - to assist/damp the aerodynamic force input which peaks 90deg later.

[crazydoc]

Hi,
I have also done that and confirms that it is as gyronaut says ie the pitch changes but what eben has being saying is (i think )that the teeter negates the gsp and that the blade flies the whole disc into a pitch down attitude and since gsp is negated and since the fuselage is now closer to the disc that the weight moves foreward making the plane go down(weightshift) BUt now i start thinking: Eben, for the same reason, the weightshift should also be negated by the teater ,because it can just shift back to where it was before, through the teeter. So, is it not just a matter of the rotor blade that flies downwards from 90 deg abeam to the front that makes the rotor plane go downwards and everything else just follows. (downwards)..Sorry .i might talk lot of nonsense but im trying to understand.

[Gyronaut]

I think you've got it.
The rotor-disc flies dependent on the AOA/AOI/GSP relative to the airflow.
I have said elsewhere that the force exerted by the control rods on the head is FAR less than the weight of the machine.
This supports the 'flying disc' theory.

[Learjet]

You can not change the pitch of the blades...etc.

Place your rotor at the 9'oclock and 3 o'clock position i.e cross-wise to the gyro.
ask someone to pull back on the cyclic while you stand in line with the advancing rotor and observe what happens to the AOI - or the "pitch" of the blade.

I have done that Dave. Aggressive AOI change.

I have also done that... the pitch changes.

So we are all in agreement now that blade "pitch" change can in fact occur and very visibly so. And it does so every time we make a cyclic control input.

The argument that gyros are "weight shift" (like a trike) is usually put forward because gyro's have a "fixed rotor" (and no swash plate and collective etc like a helicopter).
BUT - in a gyro it's the rotor-head gimble which enables the change of the advancing blade "pitch" AoI when making a control input to the rotary wing.

(This "pitch" change occurs 90' degrees ahead of the resultant change in overall disc AOA, which is suggestive of GSP, however whether this is due to GSP or simply a case of the blade flying to that new position due to the increased AOI (or a combination of both), depends on one's aerodynamic or scientific point of view, and that's something even the mathematicians & propeller heads haven't been able to quite satisfactorily resolve yet!)

What is not in dispute is that a weight-shift control (WSC) aircraft is dependent on the pilot's ability to change the Centre of Gravity (CG) with respect to the wing. This is accomplished by the pilot physically changing where the aircraft frame is in relation to the wing during flight.

In our above demonstration - when the rotor-blade AoI changed - did the gyro's CG change? No
Did the gyro's fuselage position change in relation to the rotor? No - not by one inch!

I have said elsewhere that the force exerted by the control rods on the head is FAR less than the weight of the machine.
This supports the 'flying disc' theory.

I guess that's why Fairey Rotodyne (or any other large gyro / heli) pilots had no problem flying the massive gyro with it's four x 90' foot stainless-steel rotors, covering nearly 600 square metres of rotor disc area with a 15 ton gyro hanging underneath!

Weight-shift?? Aikona!

[saraf]

With all due respect....a well known aeronautical engineer, 25year rotorcraft president of the EAA America, well known gyro manufacturer.............all agreed with me. I am no expert and do not hold there qualifications.....so it is difficult to not believe them......is it not???????

I will pick there brains a little more and get back to us non experts.........

Regards