A Question about coning on helicopter rotor blades.

[stu78]

First off, there are lots of ‘head stiffeners’ available for micro helis, to stiffen the head and reduce coning, thus improve FF characteristics i.e. the heli has less tendency to pitch up during FF.

So is it true to say that helicopter blade-coning acts very similarly to fixed-wing dihedral?

If this is true then a helicopter with a lot of built in coning (dihedral) should hover on the spot with very little input from the pilot or at least be very stable, right?

OK, I hope everything above is true, otherwise this part is a bit meaningless :)

If I had/built a multi rotor helicopter (imagine 3 helis all connected at a central point by their tails 120 degrees apart) assuming I could control the thing at all, would angling all three rotors in towards the centre increase the stability in the same way as blade coning does?


I think it will, but I’d like some other opinions before I even put this thing on the drawing board.

I have attached a simple diagram to help explain, hope it is clear!

Stuart

[TMorita]

I think the two multirotor systems are about the same stability.

If I understand this correctly, you lose the stability of a coned rotor head once you start tilting it. Once it's tilted, then the coning is a disadvantage since it decreases the size of your effective rotor diameter.

Also, it may create blade flutter problems in ground effect due to the blade alternately supporting more and less weight as it rotates.

Toshi

[Discharger]

Quote "So is it true to say that helicopter blade-coning acts very similarly to fixed-wing dihedral?
If this is true then a helicopter with a lot of built in coning (dihedral) should hover on the spot with very little input from the pilot or at least be very stable, right?"

Good question and sounds like a reasonable assumption to me!

Quote "If I understand this correctly, you lose the stability of a coned rotor head once you start tilting it. Once it's tilted, then the coning is a disadvantage since it decreases the size of your effective rotor diameter."

If it is the same as dihedral then it should return to a stable condition? If the coning angle remains constant then the effective disc area should be the same?

This is a job for Ollie and B Matthews!!

[stu78]

I think I need to explain a little more about how I came to my coning=dihedral conclusion.

The diagram should explain it all, but basically for my multi rotor heli it is not important whether the individual rotors have any coning, it is the overall built in coning/dihedral that I am interested in.

The diagram is simplified, but I think it is valid, and if it is then my theory about stability and coning should be correct.

As always, more comment welcome...

If I understand this correctly, you lose the stability of a coned rotor head once you start tilting it.

Why? I think that is when it actually does something different from a totally flat disc. :confused: see the diagram.

Once it's tilted, then the coning is a disadvantage since it decreases the size of your effective rotor diameter.

Coning will ALWAYS decrease the effective size of a rotor disc, irrespective of angle to the ground.

Also, it may create blade flutter problems in ground effect due to the blade alternately supporting more and less weight as it rotates.

The blades will produce the same amount of lift (therefore support the same amount of weight) regardless of their angle to the ground, just some of it may not be vertical lift! so i don't think this will be a problem, at least not for this reason.

Stuart