Main rotor coning and forward flight roll.

[Tor Arne H]

Hi.

 

I'm a helicopter technician in the norwegian airforce by profession and as a hobby I design and build radio control model helicopters and rotor mechanics. My latest model is a 1:7,5 scale Agusta A-109BA with a four blade, CCW rotation, articulated rotor. I am experiencing a phenomenon where the helicopter has a strong tendency to roll right in forward flight, i.e. roll towards the advancing side of the disc. Hovering is fine and stability is good while hovering. The roll tendency in forward flight is actually strong enough to overcome the cyclic range with loss of control as the result and I crashed yesterday on the first forward flight test. The damage was minimal because of deep snow, but I have to find a solution to the right roll tendency, as well as increase the cyclic range.

 

This helicopter has a low main rotor rpm (for a model of this size) of 1100 which means the coning is quite aggressive, about the same as on a full size Agusta 109 from what I have seen in pictures. After thinking and reading helicopter theory for hours yesterday I have come to the conclusion that I need to increase the negative delta-three angle on the MR feathering hinges because the coning of the rotor causes the front of the disc to experience an increase in AOA with increased velocity, and the rear of the disc a reduction in AOA (transverse flow). Right now I have a delta-three of around -4,5°. I can easilly increase the angle to -20° with some spacers. If this is not the cause of the problem then I am at a total loss as to what to do about it.

 

I found some text written by Frank Robinson where he explains how an increased delta-three will eliminate/reduce this tendency (under the section on Robinson R-22): http://www.unicopter.com/B185.html

 

Does increasing the delta-three sound like a possible solution? I am also thinking about increasing the mass of the main rotor blades by 30% to reduce the coning a little which will also increase stability.

 

I have noticed that the elevators on the full size 109 are angled at different incidences which wouls apply a left rolling force on the fuselage, possibly to help counter the right roll tendency. Thre does not seem to be any delta-three in the flapping hinge of the full size Agusta.

[Tor Arne H]

I modified the head by changing the delta to around -21° and increasing the mass of the blades by 25 g to reduce the coning. There is still visible coning but it's much reduced. Today I did some slow forward flight at speeds that would provoke the roll tendency before. The roll tendency seems to be completely absent now so the modification seems to have worked. I have yet to fly it in fast forward flight so I don't want to celebrate too much yet, but I'm gaining confidence in the helicopter again.

 

The modifications I did changed the phase of the rotor and I had to change it in the computer. It was around 80° before the mod and now it seems to be around 90°.

[Gomer Pylot]

You may be experiencing retreating blade stall at higher speeds. It's hard to say without seeing your model and knowing the speeds you're flying when it starts rolling, but that roll is a characteristic of retreating blade stall. Models don't really have the same flight characteristics of full-size helicopters, though. Inverted flight and many other maneuvers can only be done with models, which seem to do it easily, while real helicopters can't come close.

[Tor Arne H]

If it was retreating blade stall it would pitch up/roll left, won't it? The rotor is spinning CCW when viewed from above, as on the full size Agusta, so the left side is retreating, whereas the model rolled to the right.

 

The tendency I experienced increased with velocity and was present from low speed. After I changed the delta and reduced the coning by increasing the tension it felt very good. I need to test more though.

 

 

I also fly aerobatics with conventional model helis with a flybar and two blade. My Agusta has free flapping hinges and is very different from a normal model heli.

Edited May 3, 2008 by Tor Arne H

[kodoz]

How about a TR efficiency problem? In the transition to forward flight, you get increased TR efficiency. If your disc is lightly loaded, that could produce a right roll (TR thrust acting above the CG). I'm thinking something analogous to the right roll tendency experienced in low G situations. It's a stretch, since you should also get increased MR efficiency (and a left roll) from ETL.

[CharyouTree]

Transverse flow effect would cause a right roll.

 

(Throwing it out there)

Edited May 8, 2008 by CharyouTree