aileron throws, dual rates and end points

[Eifion Herbert]

Hello,

Wonder if anyone can throw some light on what (to me anyway) is puzzling.

I'm starting to practice aerobatics with my foam Wot 4, and noticed that rolls to the right are nice and snappy whereas rolls to the left are quite sluggish like the plane is strugling to get round. Checking the throws visually at the field, haven't looked at them on the bench, it was quite obvious that the ailerons move significantly more with the stick all the way to the right than all the way to the left. It also does require quite a bit of left trim to bring the ailerons visually centralised and the plane tracking straight, but that shouldn't affect the amount of throw avialable, right? it just sets the centre point.

So anyway, I go to the endpoint adjustment bit of mt DX6i, and increase the left aileron throw to 125% (as much as it will go) this helps, but I still have to decrease the RH aileron throw to about 85% to get them moving the same amount left and right. Then I have to incrase up the rates to get the same amount of deflection as I had before. It seems better now, but still not quite there, it still rolls to the right more easily - or so it feels to me, this could be psychological with me bing more comfortable rolling one way than another.

So my questions are, if you have Dual Rates and End Points set, which setting wins if they're in conflict? Like if I have DR set to 80% and have EPA set to 60% how far will the servo arm move when I move the stick all the way over? 80% of total, or 60% of total? or 80% of 60%?

Should I be doing all this electronic jiggery pokery anyway or is it a sign that something mechanically with the linkages isn't quite right and I should sort that out rather than compensating with the transmitter?

I notice that the tailplane on the Wot4 isn't quite level compared to the wing, could this make it roll one way easier than the other?

Thanks in advance,

Eifion

[Pete B]

So many things to think about but I'll raise a couple of points which may be worth considering.

When you say 'the tailplane on the Wot4 isn't quite level compared to the wing', am I right in suspecting that, looking forward from the rear of the a/c, the left end of the tailplane is lower than the right end? This would introduce a clockwise turning effect and could be the reason why you need some left trim and it tends to roll to the right more easily.

Secondly, your remark about mechanical linkages could be another reason.

Are both servos the same make and type? Are you using identical servo horns?

Are both aileron horns mounted with the clevis holes equally over the hinge line on both?

Were the servos centred on the Tx before fitting the horns and are they, as closely as possible given the spline settings, at 90 deg to the servo body at the neutral point? Quite often they are a few degrees out of absolute plumb and this can result in unequal throws. I find that it is sometimes necessary to try a four-arm horn in various position on the spline until I find the nearest that gives me 90 deg.

Sorry about the grilling but it helps if we know where you're at with the installation!

Pete

ps just thought of something else - are the wings laterally balanced? If one is significantly heavier than the other, this could also contribute to the condition.

Edited By Pete B on 29/05/2012 22:31:53

[Eifion Herbert]

Thanks for the quick reply. They tailplane is, as you suspect lower on the left hand side than the right, when viewed from behind the model. While I knew this wasn't right, I didn't think it would make that much difference - and it didn't when all I was doing was pootling round the sky and glad to get it down in one piece. Now I'm doing more aerobatics I guess it does matter - especially if I want them to be accurate and consistent enough to go for my B one day.

Not sure I understand how a slightly off level tailplane introduces a turning force though. Must go off to read up some more aerodynamics.

I *assume* the servoes are the same make and centred before fitting the horns, I certainly expect they would be but don't know for sure if this is the case with my particular model as it was an RTF with the servos, horns and linkages all preinstalled. The servos have a plastic cover that is glued in place so it's not really easy to get at them to check, and even if I took the covers off the way they're installed I still don't think I could get at the horns.

If the horns were not put in dead 90 degrees on the servos I thought that would make the down going aileron deflect more than the up going, or vice versa. This is not what I'm seeing, the up and down traveling ailerons move the same amount (within the accuracy limits I can measure) but they both move with greater deflection when giving right aileron input than left.

Would playing with the servo reverser on the TX while measuring the throws tell me anything?

I think I'll need to make a thin shim of foam and glue it in to set the tailplane level, then start thinking about the servos and linkages. I don't want to start cutting into the wing if I don't need to.

Thanks agian.

 

Eifion

Editted to add:

Ha, now I think a bit more the servo horns being off could cause what I'm seeing, if one is off one way and the other is off by the same amount in the other direction. If that's the case I suppose I have to choose either start cutting to get at the servos or continue fillding with the Tx till it behaves as I want. I could always take out the Y-lead and have the servos on a channel each, then I could set the end points more independantly, and try out flaperons. But there's something unsatisfying at using the Tx features to fix something that's lacking in the model's setup. Feels like cheating somehow.

I have no idea if the wings are laterally balanced, I've never checked. I assumed they would be to within Ripmax's manufacturing tolerances, and they would be sufficient for the model. Goes to show what happens when you assume!

Edited By Eifion Herbert on 29/05/2012 23:06:24

[Pete B]

Blimey, I'm on dodgy ground here - I could get eaten alive by the aerodynamicists for this (particularly that BEB chappie - he's very good, you know - and British! ) but my understanding is that the flying attitude of an aircraft with a flat-plate tailplane (not sure about an aero-section tailplane) is dictated by the tail surface datum, ie, the tailplane will always be at 0 deg incidence to the direction of flight.

Phew, a bit of a mouthful, that......but I am quite happy to be corrected

As a result, skewed tail surfaces will tend to have a profound effect on the line of flight and make trimming for stable flight difficult.

The chances are the servos are identical but, given the QC uncertainty, there's no guarantee that the servos have been set up to give symmetrical movement. It's a bit of a pain when they are glued in, though - makes a chore out of a quick check of the geometry.

If you've got more throw on the right aileron, try moving the clevis on the left aileron horn one hole closer to the flying surface. This will give a slightly greater throw on the aileron which may be enough to even them out.

Having a channel for each wing does make life so much easier when setting up, too. With Y-leads you lose a lot of versatility, such as differential, flaperons and so on.

Assumptions - ha! Given the criticisms of build quality, suitability of parts, etc, often aired about ARTF's on this and other forums, it's much safer to work from the principle that there is no such thing as an ARTF model! Treat it as a kit which you don't (usually!) need to glue together and check it through as if you've built it yourself!

Pete

Edited By Pete B on 29/05/2012 23:46:29

[Bob Cotsford]

As for the original question, I'm pretty sure that EPA will be the primary setting with DR acting on the range defined by the EPA but limited by the software (in Futaba at least) to a maximum of 140% between them.

Foamy 4s are built down to a price, assembled in a factory by people who are just doing a job and probably on piecework into the bargain so I tend to accept that mine will never be a precision aerobat. Come to that, trim seems to vary with the weather and where the model's been stored simply because foam is a non-rigid material.

Ideally you would want to take off the covers on the aileron servos and check that the output arms are set the same on both, and that the servos have equal movement both in each direction and equal to each other. I don't think the tailplane will affect rolling to any great degree but it will cause the model to skew out of loops. What will affect rolling (apart from the servo settings) are: warps in the wings, sidethrust not ideal, banana fuselage and misaligned rudder.

ps, AFAIK the tailplane will attempt to achieve a 0 degree angle of attack, but this AofA will be affected by the dynamic stability of the model (ie nose or tail heavy, area of tailplane and tail moment) and downwash off the wing.

Edited By Bob Cotsford on 30/05/2012 10:50:06

[Eifion Herbert]

Well sounds like I've got a fair bit of checking to do, I know the foamy 4 wil never by a precision aerobat, and to be fair the main source of imprecision is at the moment still the man at the controls, but hopefully I can get that left roll rate a bit less mushy. The fuze isn't twisted or banana-shaped and the fin is vertical and square to the wing, so I thnk it's basically sound, just needs a little tweak.

Having said that I'm enjoying flying it so much it's making me think about getting a "real" wot 4 one day, made of wood and stuff, and maybe even one of them noisy smelly things up front.

[Evan Pimm]

Simple answers to your questions, yes, you have a mechanical problem with aileron, assuming the servo arm is 'centred' and the throw has no mixing, and is set at 100% each way.

The tailplane being not square to the wing will make little difference, unless you have set it up that way for a bit of glide circle trim...

Evan.

[Eifion Herbert]

Well I've taken the aileron covers off, and set all my trims at zero and all the endpoints and rates at 100 and indeed the servo horns are not at 90 degrees to the servos, but canted slightly forward.

I did try to unscrew the servos to sort this out, but ended up rounding off one of the screws so that servo is in there for life. I can always use subtrims to set the horns at 90 degrees to the servo, but was wondering whether they were canted forward a little by design, as this would reduce movement on the downward going aileron, thus combating adverse yaw, so decided to leave them be for the time being.

The pushrods though I did adjust so that the ailerons were visibly neutral to the wing with the radio's trim set to neutral. In the air she just needed a click or two of left to track straight, and the roll rate is now virtualy equal for both directions. Certainly close enough for a foamy wot-4.

The bad thing is that I now can't blame the plane when I completely mess up the consecutive rolls that I'm trying to practice for the B cert....

[Bob Cotsford]

It sounds as though the servos are set up to give differential ailerons, that will not affect left v right roll rates. I know mine does quite barrely rolls.

[Steve Hargreaves - Moderator]

As Bob sez it sounds like the ailerons are set up to have some differential....this is a good thing so I would leave well alone. Lots of trim will use up servo travel in the "trimmed" direction so worth setting any offset (if large) by adjusting the pushrods so the trim can go back to neutral.....