Coning of the blades

[baobab72]

Hi guys can someone explain me why when you move the cyclic aft the blades come up?

Thanks

Baobab

[Peter-RB]

Baobab,

The rotor blades are attached to the hub(which sits on top of the power delivery shaft from the gearbox/engine) the hub is sort of sat on a gimble, that is attached to the collective that is the pilots control stick( cyclic in Heli.. joystick in A/C) when you move the cyclic Left or right or forward or to the rear(or indeed any angle you command) if the rotor( thats the collective name for the group of blades attached to the hub) is spinning the flow of air being collected by the rotor blades is deflected or directed to the direction of the gimbled rotor hub which in turn is controlled by the pilot holding the stick.

In otherwords if you are in the air or on the ground, with power available to the rotor the Heli would move into the commanded position of the stick in the pilots hand.

Peter R-B
Lancashire
UK:ok:

[baobab72]

Hi peter
Thanks for the prompt replay, hoever i still dont see why the blades cone up when you move the cyclic aft and not when you move it forward since as you said when you move the cyclic basically you are redirecting some total lift in horizontal thrust, propelling the heli in that direction.

Thanks

Baobab

[baobab72]

Hello
My question was geared to the recovery procedure that my instructor told me for the SFAR sigmoff about the low rotor rpm, involving lower the collective to decrease the pitch angle and thus the aerodynamic load on the blades allowing them to spin faster and rolling the throttle and moving the cyclic aft to take advantage of the fact that when you move the cyclic aft, the blades cone uo and as they do so and their center of mass moves closer to the axis of rotation due to the coriolis effect they tend to spin up, restoring the RPM's.
My understanding of the system is that when you move the cyclic back you tilt the whole rotor disk backward to re-direct some total thrust along the horizontal plane which in turn propels the aircraft in that direction, but i still don t see whi the blads cone up! Cosmif it is true that one blade moves closer to the mass the other one should move further away, canceling the cone effect out! But evidently i am mistaken!

Just to clarify all this was related to the r22.

Thanks

Baobab

[Arm out the window]

Flare effect.

With some forward airspeed, moving the cyclic rearwards tilts the disc that way. The relative airflow direction is changed, now approaching the disc more from underneath, so to speak, which opposes the induced flow.

Lift increases, so the blades cone up.

Due to conservation of momentum (the old ice skater analogy), RPM increases.

[GipsyMagpie]

Bravo...what he said. One part of flare effect in CFS(H) speak

[Rivet]

If your are in forward flight and you apply aft cyclic you will increase the G-force on the rotor system, similar to an ice skater raising their arms which will increase speed to the rotors rpm.

Hope this helps :)

[Ascend Charlie]

How about this explanation then:

The coning angle is the resultant of two forces - the lifting force, or total rotor lift, which acts at 90 degrees to the length of the blade (not at 90 degrees to the tip path plane - it is tilted a bit towards the middle of the disc), making the coning angle move up; and

The centrifugal force generated by the blade spinning around, acting through the centre of gravity of the blade, and trying to pull the blade outwards in the same plane as the plane of rotation, making the coning angle flatten out.

Add these 2 vectors together, and the resultant acts along the length of the blade, pointing upwards from the tip path plane/plane of rotation.

So, if you make one force bigger than the other, one vector will be predominant. Increase the RRPM, the centrifugal force gets bigger, and the blades will tend to flatten out. Increase the lift (more pitch/power) and the blades will cone upwards.

Heavy helicopters have big coning angles, little light ones have flatter coning angles.

Now we get to your question: apply aft cyclic, the blades cone up.

When in forward flight, and you pull back on the cyclic, you are rapidly changing the angle of the relative airflow - it now comes in more from below the blade disc, so it opposes induced flow, increases the angle of attack, the lift goes up, and the blades cone up.

But now another effect comes in - conservation of angular momentum. The energy of the blades spinning is now in a smaller total disc area, so the blades will spin faster. This increases the centrifugal force, and tries to flatten out the coning angle. Therefore, initially when you apply aft cyclic, the RRPM will increase as the aircraft pitches upwards, and it should stabilise with the new balance between rotor lift (which is now tilted more towards the mast instead of upwards, and is less effective) and increased RRPM, trying to flatten the blades out again.

It depends on how long you leave the cyclic input in place. If you started straight and level, you will very soon release the back stick when you see the attitude out the front, and your instructor will be turning pale as he recalls the effects of poking the nose over to fix the attitude.

However, if you are in a turn and increase the back stick, the effect will remain for as long as the pitching moment is there and you keep on turning. That is why the RRPM go up in a turn, especially when in autorotation, and you need to control the revs with more collective.

[tartare]

Have often tried to imagine dynamics of the rotor system as follows:
The collective introduces force (via pitch) into the blades `collectively' all at once.

The cyclic is then used to point that force in the direction you want to go, further increasing then decreasing the pitch of each blade as it `cycles' around each rotation.

Push cyclic forwards, back of disc begins to bite, tilting disc forward. Pull cyclic backwards, vice versa.

I think I'm understanding it correctly...:confused:

When you introduce principles like gyroscopic precession, translational lift and lead & lag, its incredible anyone ever worked out how to make the damn things fly.

[riff_raff]

I may be wrong, but isn't the only type of rotor where the disc actually "tilts" a gimbal rotor? With other rotors, the cyclic only varies the direction of the rotor's net thrust vector, right? While the collective varies the total thrust (or lift) from all the blades along the vector controlled by the cyclic.

I also don't believe that the cone angle of a rotor's blades technically changes. The blades may experience more bending deflection from root to tip as the amount of lift each one produces increases, but the cone angle at the blade's hub attachment will remain essentially unchanged. As cyclic is applied, the lift produced by any given blade varies during the course of a single rotor rotation. And it's the asymmetric lift condition resulting from this that produces the cyclic directional control. The cyclic inputs also result in each blade experiencing a varying amount of bending (flapping) as it goes thru a single rotation.

As for the situation where the aircraft must auto-rotate, I think we must consider the difference in a rotor that is being driven by an engine, and a rotor that is relying on the free-falling/forward motion of the aircraft to produce sufficient airflow over the rotor blades to spin them fast enough to create enough lift to slow the aircraft's descent rate to allow a safe landing. Basically, the direction of airflow thru the rotor during powered flight is somewhat opposite of that during auto-rotation.

[Ascend Charlie]

I may be wrong,

That's correct.

I also don't believe that the cone angle of a rotor's blades technically changes.

Whether you believe it or not, it happens.

Stand beside a helicopter when it is at 100%RRPM and flat pitch, on the ground. See how there is almost no coning, the blade tips are spinning at almost the same level as the hub.

Then watch as the pilot applies collective to lift to the hover - the bigger and heavier the machine, the better demonstration. See how the tips are now spinning around much higher than the hub. The blades are coning up. It might be through the use of the flapping hinges, or through the rubber bearings, or (in the BO 105 and BK 117) the blades themselves bending up.

Believe it now?

[Arm out the window]

Or watch at the bottom of an auto where RPM is low and a lot of collective pitch is being applied - the coning angle is large and easily visible.

As far as the disc tilting, they all do! Just move the cyclic on any type, be it a teetering head or otherwise, and watch the tip path change. Flexing in the blades and attachment hardware, or movement around a hinge, it doesn't really matter - the net result is a tilt of the disc as a whole (which as we all know is a fictitious concept anyway, it's just a number of individual blades doing their thing as they whirl around at ridiculous speeds.)

[[email protected]]

Riff raff - one of the reasons that debates about blade motion get so heated here is that people tend to argue from one standpoint based on a design they are familiar with - some are used to gimbal (Bell) style 2 bladed teetering designs, others fly the R22 combined teeter and coning hinges in the head - then you get onto fully articulated heads and semi-rigid rotors.

And don't forget Hooke's joint effect when you are looking at coning and apparent tilting of the disc;)

[tartare]

Thanks - have learned something - had suspected the disc appeared to tilt, but did not do so in reality.

[riff_raff]

crab- thanks for the reply.

I guess I have a different notion of what the term "coning" implies. I was considering what "cone angle" means with regards to the current hingeless/bearingless/rigid rotor configurations. These rotors all have a fixed, defined cone angle designed into the hub/blade attachment, and this cone angle does not change during operation. The only change in the out-of-plane displacement of these blades is that resulting from bending in the blade structure itself. These types of rotors use a few degrees of cone angle to orient the operating longitudinal load vector of the blade normal to the hub attachment structure, thus reducing dynamic stress levels.

Obviously the situation is different with older rotor designs that use a flap hinge, as you noted. So the correct answer seems to depend upon what type of rotor is being considered.

[Ascend Charlie]

Riffy, you are talking about pre-coning, where the manufacturer sets the coning angle at the expected angle for minimal stresses under normal conditions.

Therefore, at flat pitch on the ground, there is a downwards stress on the head (blades not coning up at all) and at the end of an auto or under heavy load, there is an upwards stress - these heads can't please everybody all the time. Fully articulated are able to sort themselves out.