Equal Lift across the Disk?

[Chiplight]

Blade tracking and balancing is said to involve getting equal lift on all blades. Sometimes blade track needs to be off in order to achieve perfectly matched lift.

My contention is that equal lift is NOT a requirement for track and balance, (speaking only theoretically.)

It is true that a cyclic change in lift will result in tilting the rotor; one blade goes up, and the other goes down, in a 2-bladed system.
But, if one blade has more pitch then the other, not just for PART of a cycle, but ALL the time, then the blade reaches a new coning angle and stays in one plane, despite what the lift of the other blade is.

A one-bladed helicopter is surely the ultimate in disymmetry of lift-one blade has 100% of the lift, the other blade, which is just a weight, has none. Yet, the one blade helicopter does not wobble horibbly, assuming the mass is balanced, and one bladed props are not vibration monsters, as far as I know.

A vector diagram of lift and centrifugal force shows that the CF forces of blade and counter-weight cancel, leaving only the vertical component of lift pointing straight up. There is no wobbling lift vector!

Likewise, in a 2 bladed rotor, what if one pitch link is made longer than the other by having unequal threads showing. How much difference does it take before the vibration becomes extreme? One thread? 1/2 thread? 10 threads?
My guess is it doesn't matter much, so long as the blades are dynamically balanced i.e. If the coning angles are different you need to adjust the location of the center of mass accordingly, such that a horizontal line can be drawn connecting the cg's of the blades.

Am I missing something, or is the equal lift idea a myth.? :}

[Gaseous]

Interesting.

When there is no pitch on, I can see that the single blade and weight will balance but as pitch is added, will the position of the thrust force will move out along the blade? There is more lift on the end of the blade than the root even when there is washout on the blade.

If it does this, it will act as as a cantilever on the mast causing severe vibration. The lift vector will not be central over the mast but will revolve around it.

The effective thrust vector must be centred over mast for no vibration. Tell me if I am talking rubbish.

On an Enstrom, 1/2 turn error on a pitch link is unflyable.

It matters a lot that tracking it is right. Small errors in dynamic balance are less noticable in flight.

Edited to remove really badly phrased bit.

[Chiplight]

Gaseous, thanks for your comments.


I believe that lift on the blade does not act as a cantilever unless the helicopter is still in the hangar and you are lifting it by hand.
When the rotor spins, Centrifugal force resists the tendency of the blade to bend up. You can look at it as a torque on the blade opposing the torque that lift is producing. The two torques are in balance at the coning angle.

When lift increases, the balance is shifted and the blade will cone up more because the ratio of lift to centrifugal force is changed. The lift is still acting up on the root of the blade at all times, assuming that CF forces are exactly equal on both blades.

You could replace the blade with a rope pulling on the mast at the coning angle. A rope cannot act as a cantilever.
If you balance out the horizontal pull of the rope, only vertical pull is left and the mast will move straight up. That's why a one-bladed rotor is possible.

The trouble is , that when the blade cones, the center of mass swings closer to the mast, changing the CF. So, if pitch links are made unequal, the difference in pitch will create a difference in coning angles, which will make the cg of one blade closer to center and higher as well, so you get 1 per rev and 2 per vibration. It is not because of unequal lift, per se.

It does make me question my theory when you say that 1/2 turn on a pitch link is unflyable.
All I can think is that this shifts the center of mass inward or outward enough to throw off the dynamic balance severely.
Adding mass to one blade should theoretically eliminate the vibration, despite the unequal lift between blades.

I'm putting on my flak jacket.

[Gaseous]

I'm only really familiar with the Enstrom articulated head where the flapping(coning) hinge is about 3 inches out from the mast. It is the couple formed by the distance between these hinges that gives the articulated head its control power. Thats what I was trying to get at in my last post(badly).

Agreed, if you put the coning hinge over the mast that is not an issue.

What goes on during tracking and can mis-tracking be balanced out?

Imagine an Enstrom in the hover. The thrust on all three hinges must be the same magnitude and direction (vertical) for smooth flight. Given that the blades will flap to equality, if one blade has more pitch, it will flap up and the disk will tilt. Unlike cyclic input, that blade will have more pitch for 360 degrees so the disk will wobble, which you see with your tracking light.

The thrust vector will be at right angles to the plane of the disk which will always be higher at blade with the increased pitch, hence the thrust vector will 'wobble' around the vertical at a fixed but small angle, giving the vibration which is felt as up and down bounce. This is not correctable by adding mass to the blade as that would act in the wrong direction.

What you need is the thrust vector dead stable for smooth flight and for that the disk must run true. I think it is stability of thrust vector rather than c of g/differential coning that is relevant to smooth tracking.

You shouldnt have difficulty tracking a one bladed rotor!! I don't know if the C of G changes due to coning would be an issue. I suspect not.

Its a bit of a pain to carry a large counterweight around though. Probably best make it shaped a bit like a blade.:D

[Chiplight]

Thanks , Gaseous. :D I think I see it now. If the flapping hinges are offset, such as on an Enstrom. then any difference in lift will create a force couple on the head which rotates at 1-per-rev.

In a teetering system, the lift of each blade is transmitted throught the center point of the head and there's no force couple.


-Chiplight

[Vfrpilotpb]

Gentlemen,

Consider this, no doubt somebody will cover me with Horse S**t for asking this but here goes,

in the light of the questions you are posing we have three different types of head, the Teetering ( two Blade) the semi articulated(three and more blades) and the fully articulated( 3 or more blades, any of these blades seem to be very complex pieces of equipment, both to use and to keep in tip top balance conditions, they seem to need very strong and complex hinges and pins to allow them to Flap and to cone, also to twist(feather) when power is needed to lift. We see many different lengths, profiles and cords but the one thing they all have to do is to transmit power to the air in order for the Helicopter to lift off the ground and fly.

Why can't this be done with the same sort of blade and propeller system that is employed by a normal configured aircraft, ie, take the propeller and fit it on top of the Helicopter, it could still have a swash plate for direction, it would still be able to be feathered(twisted) for lift and would still be able to thrust for forward flight, the blades would be much more rigid and would cause less drooping of the blade tips.

Granted this would be a heavier proposition but when you look at the materials available now this could be engineered to be just perfect, this would allow for blades to be set on the engineers bench, being rigid they would always be in track, the feathering could be accomodated by a small hydraulic fitting going down the centre of the transmission shaft linking the collective to the hub for any requirement of blade twist alteration.

I realise what I am saying would make in peoples minds a rather strange looking Helicopter but this would have many advantages that normal blades do not.

Having written all this I will now go in doors and put on my armour,
remember, Cos its different, dont mean it wouldn't work!!

PeterR-B
Vfr :8

[Gaseous]

Peter,
You just want to say there is a propeller on the roof dont you?!

I think the reason is that props don't generally go sideways through the air so don't have the abilty to flap to equalise lift on the advancing and retreating sides.
Solve that one and you are on to something.

I think your solution might look like what we already have.

Phil.

Why are there no 2 blade fully articulated heads - or are there?
That would solve the mast bumping problem and still let you store your helicopter in a skip.

[212man]

"Solve that one and you are on to something."

Yes, it's called a helicopter rotor system!

Ignoring the other factors for a moment, I'm not sure why you desire this propellor like rigidity? If you had a rotor blade that was perfectly horizontal while not rotating, it would be massively over-engineered compared to the strength it actually needs to function when rotating, this in turn would lead to the whole head and gearbox structure being over-engineered to cope, and for what purpose?

A propellor is too short and light to derive the required stiffness from rotation; a rrotor blade is not.

[Chiplight]

Peter,
I think it's been tried. :D
http://www.rotaryforum.com/forum/attachment.php?attachmentid=5977

Its great as a leaf blower, but I've heard that it autorotates at 4000 ft/min.

[Dave_Jackson]

Gaseous,Why are there no 2 blade fully articulated heads ... There would be a 2/rev vibration when the tip-path plane was not aligned with the mast plane.


212man, I'm not sure why you desire this propeller like rigidity? ......... If a compound helicopter was to take its rotor and wings, and combine them into a pair of rotors with "propeller like rigidity", we would be looking at the next generation of rotorcraft. :D


A brief look at history.

Cierva's early gyrocopter had a rigid rotor. When he tried to takeoff, the dissymmetry of lift rolled the craft onto its side. He then went to his English financial backers and attempted to get more money to add a second rotor. He knew that the two latterly located rigid rotors would give the craft symmetry of lift. His backers went ballistic. They started flapping their arms at the prospect of putting more money into his crazy idea. Cierva's English was very poor. He therefor though that they wanted flapping hinges on the existing single rotor.

Well the rest is history. For the next eighty years, Rotorland has been flapping its gums and spinning its wheels trying to make the single rotor a success. http://www.unicopter.com/LaughRolling.gif


Dave

[Gaseous]

Thanks Dave,
You beat me to it. It dawned on me as I was driving back from work and I was going to post that it was a stupid question. Fewer than 3 blades clearly won't work.

[212man]

Dave, I agree that there may be some installations where it is desirable, such as the advancing blade concept a/c or tilt rotor, but the question seemed to relate to a convential design (propellor on the roof).

The other draw back, as you say Cierva found, would be that the passengers would get sick from the constant 360 degree rolls in forward flight.

[Graviman]

"But, if one blade has more pitch then the other, not just for PART of a cycle, but ALL the time, then the blade reaches a new coning angle and stays in one plane, despite what the lift of the other blade is."

Hmmm, nobody here has mentioned the fact that rotor hub shaft stresses are likely to exceed material fatigue stress from the continuous undemanded cyclic input. If i was the guy that released that drawing i would specify the highest tensile steel for the rotor hub, keep stresses well below half UTS and still loose sleep over it! Better to specify an out of balance tolerance...


"A brief look at history."

I laughed very hard when i read this...

Mart

[HeliEng]

I like those before me, will light blue touch paper and retire, preparing to be shot down in flames!!!

Where to start???


The biggest problem with the prop on the roof theory (Sorry Peter!!) would be autorotative performance. To get the required airflow through the blades during auto would require a VERY VERY fast rate of descent, and if you look at a propellor in operation the blades DO actually flex. So after working out all the problems we'd be back where we started from with a lighter, higher surface area rotor system.

As for tracking and balancing the basic idea is to bring the centre of lift over the mast. By having a single blade generating more lift throughout a rotation you are effectively hanging the aircraft from a point offset from the centre of the mast.
With an even number of blades (ie: 4 or 6) a "w" type pattern is acceptable (ie: one blade up one blade down etc in the track picture) because you are still maintaining a central lift point.

A tracking adjustment, pitch links or tabs, will not only affect the vertical balance, but the lateral balance too by effectively moving the c of g of the blade either in or out from the mast. And conversely a mass adjustment will also affect the vertical balance by moving the blades flight path and also it's interaction with the other blades in the pack.

Phew!!! How's that?? :8 :E Preparing for :ouch: