Servo interactions

[scallybert]

Yes the endpoints scale the travel, but when you adjust them, you use them to adjust just one control function (i.e. collective pitch),

They adjust just one servo - which is exactly what you want to do.

On an ECCPM set-up, all 3 servos will be participating in collective and (both axes of) cyclic pitch. This isn't the point, though. You're adjusting the zero-point and travel of the individual servos. Which is exactly what you want to do.

but the same 3 servos are used to also control the aileron and elevator, and now you have skewed those controls by limiting the travel of one or two of the servos.

No - as I explained before.

I don't know about 450 size Hitec servos, but expensive JR and Futaba full size digital servos are pretty consistent and accurate - that's what you pay all that dosh for.

But all you're really arguing about is whether the servo trimming is done: in the factory; with a servo programmer; or on the TX. It has the same overall effect (set the correct centre & equalise the throws) - except the factory can't centre the servo for your application, and it seems unfortunate to reject a servo when a little TX trimming (sub-trim & end-point) will sort it out.

You may not need to set sub-trim and end-points, but there's nothing wrong in doing so.

[LaurenceGough]

Things change inverted and in any different position, with wind etc, if all you're doing is just hovering and nothing else, not even forward or sideways flight then go ahead and use whatever trim you want.

Level swash at all extremes and mid point, have the linkages as close as possible, use as little subtrim as possible and go fly. Simple, that's it.

Each to their own though.

[bolders]

I agree with Scallybert on this one. When I am using subtrim and end points I do it with the head removed and using a swash tool. I am effectively making adjustments to try and maintain a level swash throughout its movement so inaccuracies in head links etc have nothing to do with it in this instance. It really is making adjustments to compensate for differences in servos so for a given output from the TX all servos move the required amount whether it be from collective or cyclic commands. I would certainly expect higher quality servos to be more accurately matched and so require less adjustment but that is beside the point.

If after this process there is interaction in the head once it has been fitted then I can see some merit in adjusting head links and using mixes to dial out interactions but I believe that you need to get the swash moving with minimal interaction before proceeding with this step. However there is not an easier way that I am aware of to remove swash interaction than using subtrim to get arms at 90 (assuming you cant get there with trying different servo arms) and using end-points to match servo movement.

As a little aside just thought I would mention that it is not necessarily a 90 servo arm that is required but for the servo arm to be perpendicular to the servo to swash link.

[Nick B]

Massive difference on my 700 now ive adjusted the servo wheels to minimise the sub trim values to below 3 points and end points set top and bottom it goes straight up and down at full -/+

Its back to flying like a 'normal align' again

[cjcj1949]

It's not a lot, but defo just a tiny bit.

The point about swash levellers is that they are a point of reference, somewhere to start from which you can be confident is always the same on every heli you build and setup.

I use one to setup my initial zero degree position and then work from there by testing the paddles and flight testing.

At the end of the day, it's how it flies that really matters, not what the swash leveller or pitch gauge says.

That's interesting. It won't be doing what you said as it will produce a rotation in the roll axis. I know someone who understands the maths involved, I expect it's something to do with gyros and probably very difficult to understand intuitively, if at all. Interesting that swashlevellers are only a starting point. Must remember that if I go flybarless. My current problems are rather more basic regarding inverted flight and contact with the ground. It will be a while before I complain about the finer points of setup. I've still got to use subtrim instead of normal trim.

[bolders]

That's interesting. It won't be doing what you said as it will produce a rotation in the roll axis. I know someone who understands the maths involved, I expect it's something to do with gyros and probably very difficult to understand intuitively, if at all. Interesting that swashlevellers are only a starting point. Must remember that if I go flybarless. My current problems are rather more basic regarding inverted flight and contact with the ground. It will be a while before I complain about the finer points of setup. I've still got to use subtrim instead of normal trim.

This is spot on and is why the quote below cannot be accurate.

Most helis require a little bit of right swash tilt to compensate for the sideways force imposed by the tail rotor

You cannot compensate for a sidewards drift by applying a constant roll input which is what tilting the swash is effectively doing.

To counter the sideways drift and maintain a steady hover the heli will need to have a slight lean to resolve all forces to zero. However a tilted swash will apply a continuous roll when what is actually required is a roll input to induce the lean and then re-center the controls.

The rate of roll in degrees, around the helis axis, is the integral of the tilt angle of the swash. The integral of a constant (i.e. in this instance a constant angle of tilt on the swash) is a linearly increasing function - i.e. the amount of roll in degrees will keep increasing. Also the greater the angle of the swash the steeper the gradient and thus the faster the roll rate will be.

The integral of zero (i.e. a level swash) is also zero and thus the heli will not roll

[cjcj1949]

Thanks for your support but we are talking about a second order effect.
My comment doesn't disagree with a spot of swash tilt. I was only disagreeing with the reason for the swash tilt. The heli contains three rotating masses with signifcant Moment of Inertia. The main rotor, the tail rotor and the motor. I am prepared to believe that a small roll rotation might be required for stability. That does not mean I disagree with your comment that the swash must be level. It's just beyond my maths level. A flybarless system could be set up with any initial conditions. There may be quite a wide or a narrow tolerance. That's up to the system designer. That's not to say that there isn't an optimum setup for any system. Hopefully all systems are and will be designed to a common standard, but I doubt it.
We will also be affected by Coriolis forces as we are rotating. The direction of flight will make a difference. Hopefully nobody is good enough to notice this.

[discoferret]

I didnt realise this would turn out to be a major discussion. Very intresting.

[scallybert]

To counter the sideways drift and maintain a steady hover the heli will need to have a slight lean to resolve all forces to zero.

Yep. And the rotor disk will similarly be inclined - ie remain perpendicular to the mainshaft.

However a tilted swash will apply a continuous roll [...]

I don't think this is necessarilly true.

The rotor disk (and flybar disk) will (in general) align themselves with the swash. ie so that there is no cyclic pitch relative to the disk.

Now, I believe it's possible for the heli to be in a non-rolling equilibrium with a tilted swash.

An example is when the pilot has used trim to compensate for the centre of mass not being on the mainshaft. eg if it's tail heavy, the heli flies tail down (COG hangs under rotorhead); so cyclic stick is pushed forward; rotor disk tilts forward wrt heli - but is actually level wrt ground.

I think it's vaguely similar in fast forward flight. The heli has to be orientated so the forces cancel - but the rotor disk may need to be at a different angle to generate forward thrust (eg canted forward more).

[I think roll rates are quite interesting with a flybar. ie it's governed by the flybar paddle pitch - but that in itself is governed by more than the swash angle. (flybar disk isn't perpendicular to the mainshaft, it's pitch relative to the flybar disk that we're interested in, etc.) ]

I suspect a lot of this is why FBL controllers that act like 3 heading hold gyros don't fly like people expect - or want...

[cjcj1949]

After agreeing with the advice on subtrims versus trims, I've just set the trims to zero and used subtrim. The aileron subtrim adjusts the two side servos and the elevator one does all three. Front two in one direction and back one in the other. Perhaps different txs use subtrim differently, or there is some option I have set differentl. My tx is a DX7 and it's a trex 450 120 degree swash.

[the doc]

Thats weird man - I have a DX8 and it definitely does NOT do that on subtrims.

You have a setting messed up somewhere.

[cjcj1949]

Does anyone with a DX7 have a comment??

[Steve]

DSX9 here - sub trim moves the servo, stick trim moves the relevant combination of servos - i.e. the control channel. A bit of right aileron stick trim is like permanently holding the stick over a bit, so it has to affect both the aileron servos.

[cjcj1949]

I've got a Multiplex Cockpit Tx with telemetry as I'm not too happy with DSM2 when there are others flying on DSM2. I've used it on my glider with a vario but I've not got round to setting up the heli. Perhaps only better txs handle subtrim properly. I'm flying the heli further away now and I may be getting to the limit on the AR6100. Not much point in finding out when I've got a Multiplex full range rx just sitting in my glider.

[scallybert]

Does anyone with a DX7 have a comment??

I'm sure that's not how it works - but I'd have to go and dig out a DX7 to check.

You have got the TX doing the ECCPM ? (ie the TX is putting out servo data; rather than collective, elevator & aileron ?)