PeterP

I am trying to get my IMPACT dialled-in to my liking but I cannot seem to get it to roll as well as I like.

The rolls just don't seem to be as axial as I expected so I am taking a closer look at my linkage setup. I have tried different amounts of differential (yes, I have negative - more down than up) but it hasn't helped.


This is my first top hinged model with servos mounted on their side. I am not satisfied with my geometry with this configuration because of the length of arms involved. Can anyone give me some advice on the best approach to setup the geometry correctly. The angles look all wrong which I think is part of the problem. Ailerons are probably moving at different rates to each other throughout the entire travel range.


Thanks


Peter

tommy s

My Feedback: (55)

I have an OTOP which has the same aileron set up as the Impact and I didn't like the servo
arrangement either. For one thing it's a pain in the a** to mount and work on the servos and
using the supplied control arms you don't use all the resolution of the servo. With the long arms
you have to dial it down so much it kind of defeats the purpose of using a premium digital servo.
I decided to just surface mount the aileron servos and use standard Hanger 9 aluminum servo arms
as I have used on all my other airplanes. It sure is a lot easier to get to the servos and make any
adjustments and it only takes a few minutes to modify the mounting.

tommy s

Eric.Henderson

Peter,
I found that it only took one (1) degree more on the ailerons to get it close. I did fid that the rudder needed to be more sensitive around neutral. We added some negative rudder expo, (about - 5% in JR speak, + 5% in Futaba speak). Then rolls could be smoothedoiut all the way through the roll.

To take the rudder and elevator out of the equation, here is a test that you might try. Do some consecutive 1/2 roll-pause-1/2 roll exercises on long up and down lines to see if you get heading changes with aileron only.

Regards,

Eric.

riot3d

Peter,

I have the exact problem with mine on upline, no matter how much diff I dialed in, it still rolls very funky. However, the down line is pretty good. I am going to go back to the normal way next, more up than down to see whether it makes a difference.

Also, the snaps are not to my likings either.

Adrian

stek79

Very interesting issue guys! Adrian, I've seen the same weird thing happening on a top hinged plane... differentials seemed perfect on downlines, but on uplines every now and then I find heading changes during rolls...

Any expert opinion? Troy?

jamesjoneill

Differential usually involves setting more up than down, so it could be that you are going the wrong way?? [&:]

James

stek79

James,
with top-hinged ailerons a negative differential is needed actually - more down than up!

f3a05

with top-hinged ailerons a negative differential is needed actually - more down than up!
I know that that’s what is usually stated, but I think it’s worth keeping an open mind until having flown the individual model.
I’ve just flown my new Abbra a couple of times, and obviously it’s early days, but on doing my usual differential test( climb at 30--45 degrees, multiple aileron rolls), there is an obvious yaw in the opposite direction to the roll. To my way of thinking, this implies that the down-going aileron is dragging more than the upgoing one, and so it looks as though the Abbra will need more up ail than down, in spite of its top hinges.

stek79

Hello,
are you sure that the down going aileron is dragging more?

I would say that you have to put more down throw instead... anyhow, I'm not an expert.

oisf3a

Here in Norway, at least three Impact owners have had the same rolling problems as described by others in this thread.
I've investigated this in numerous flights and my preliminar conclusion is that the need for negative differential is a function of how much aileron that is actually used. For example, if i set up my Impact to roll perfect in vertical half- and quarter rolls (relative fast point rolls), the programmed amount of differential makes slower, horizontal rolls very difficult.

I beleive this has to do with nonlinear effects of the sharp lip that opens up on the underside of the aileron.
If this is the case, making the programmed differential a function of stick input would theoretically help, but my radio (Graupner MC24, and I believe other radios as well) has no convenient way to do this even it is possible.

I have now sealed the gap with tape and hope this will reduce the problem. In addition to this I have moved the servos from within the wing to the outside the enable perfect linkage geometry.
However, winter caught me before the mods could be tested.
The first tests will now be in april when the snow is gone.

Oistein

riot3d

It worked on my Angel's Shadow (also top hinged), with more down than up.

Then, again, each design is different. I'll go back to more up than down and try it again.

Adrian

PeterP

Adrian,

I am not sure what is happening in your instance but if it rolls well in downlines but a little funky in uplines then it must either be a thrust issue or you may be using a little rudder to throttle mix to correct for vertical upline tracking. Lets face it the only difference between a model going up and coming down is power. Every other variable cancels out.

Regards,

Peter

Dean Pappas

Hello Oistein,
Hello All,

I have only a little data, from watching airplanes that whistle with aileron deflection, but I think we can gather a better picture, here.
These planes generally feel like they need different amounts of aileron differential depending on whether they are loaded in positive G, negative G, or in a vertical, right?
The amount of aileron throw that is being used matters too, right?
I have also heard some say that there are differences in vertical up and down lines, right?
I suspect that there may be a pitch trim difference between the up and down lines that really makes this the difference between positively loaded, and truly unloaded.

Can you correlate the change in behaviour to the presence, or lack, of an audible whistle?
That sound energy has to come from somewhere, and it must come from the creation of drag.
It is, as you say, a non-linear effect.
An internal wiper seal may solve the issue.

best regards,
Dean Pappas

oisf3a

Hello Dean and others

Your analysis of the problem is perfectly in line with mine, but you probably described it with better words.

The aileron hinge arrangement on the Impact is definitely not optimal for a pattern model where linear and symmetrical flight dynamics is among the most important properties.

When pulling out from a dive (increased AoA), a slight whistle can be heard from my Impact. The same whistle can be heard for every single aileron input in a 8-point roll. As you say, the whistle energy comes from increased drag.
And the drag increase is a function of both aileron deflection and wing load factor (AoA). Complicating things further, in a roll, the effective AoA for the two wing panels are not the same. The wing panel with the up going ailerons sees a higher AoA due to the angular rotation. In a perfectly vertical roll, one panel sees positive and the other sees negative effective angle of attack.

Well.. anyway, I believe a good gap seal will cure most of this problem.

Anyone got a good method for doing this (gap sealing) ?

Regards
Oistein

PeterP

Hi Oistein,

I posted a solution for gap sealing in another thread in RCuniverse. This solution was effective in removing the whistle in all elements of the flight envolope

Click below

http://www.rcuniverse.com/forum/m_36...648420#3648420

See post number 13

Regards,

Peter

PeterP

Hi All,

I thought I would ressurrect this post to see what people have discovered in regards to aileron differential with this design. I have been putting a fair amount of time on this model lately and it just gets better. However I still dont think the differnetial is quite right. I can live with it but I know it can be better. I have every other part of the flight envolope sorted out. My model flies with no rudd-elev mix (On high-rate rudder)after I adjusted the angle the fuselage flies by adjusting both wing and tailplane incidence. The fuselage flies with a slightly more nose down attitude which also fixed the other problem I had trying to dial in more down thrust.

My model would now have over a 100 flights and the tailplane is still attached. (I did a fair amount of work during construction to ensure mine did not suffer the same fate as others on this list). I think I will build another but I will mount the aileron servos conventionally to ensre the linkage geometry is correct.

How are all the other IMPACT's going out there? It would be great to share some more experiences.


Peter

Malcolm H

Mine has done around 130 electric flights with no structural issues. I did fit the rear bulkhead, tail support plate and a wooden ladder crutch. It has wing and tail adjusters but I bonded in the rear wing incidence tube just in front of the adjusters to support the fuz in this important area.

It is certainly the best flying model I have owned, far superior to the Angel's Shadow which it replaced. I am building a second one now. As far as the rolling issue, mine is best with conventional differential - more up than down but I have still to seal the bottom of the ailerons to eliminate the shriek on snapping.

I don't have any problems with getting the servos in and out as I made a long extended hex driver and use hex headed sheet metal screws to hold the servos in. As far as the geometry is concerned having the two horns operating in the one plane (geometric plane that is!) must be better than in two. If you don't believe me and you have access to CAD plot the input and output angles for a two plane linkage. As my model is electric I don't need ball raced horns so I use the supplied fibre ones in the surface and th largest Dubro heavy duty one on the servo (JR 8401). With the clevis in the outer hole on the servo arm and the second outer on the aileron horn I still dial in 120-130% travel for the snaps with much less for normal flying. This is no different to the setups I use in conventional servo mounted models. With these hole positions the pushrod gives some conventional differential right off and I add some more on my 10X

Malcolm - satisfied Impact owner

Malcolm H

Just another point having re-read Peter's post. Mine did roll extremely odd with the incidences set as recommended in the manual. The model also looked like it was dragging its tail around too. I adjusted my incidences using wing and tail adjusters which cured both problems.

Malcolm

riot3d

Peter and Malcolm,

Did you guys set it to zero-zero, or even slight negative wing incidence?

Adrian

tggilkey

I remember JAS making the observation about flying tail low and making a stab adjustment to "get the tail up." I assume the adjustments mentioned here are lowering the TE of the stab (for positive incidence) and possibly doing the same thing to the wing. Would that be correct? How much? I'm about finish up mine and am glad to see some more Impact discussion.

Thanks,
Tom

riot3d

I started mine with zero-zero. Now, very slight negative

Adrian

PeterP

Hi Adrian,

I am not exactly sure were everthing is as far as incidences are concerned because I have fine tuned it by flying. I can tell you that I lifted the rear of the fuselage a further 17mm from were I originally had it and then reset my incidences accordingly. I started with .25 degrees positive on the wing and zero on the tailplane but after I flew it I was carrying some down elevator trim. At that point I dropped the rear of the tailplane by approximately 2mm. The model has no trim what so ever. Changing the position of the fuselage removed all the rudd-elevator mixing so I believe what is stated in the manual is wrong. These changes in effect added more down thrust to the model in relation to the wing. This model goes up straight. Sorry I forgot to mention that I do have 2% down elevator mix on low throttle and 3% right rudder mix on full throttle but I will play with that further. My model is flying with a DZ160. Total weight is 4.7kg.

I am also happy with mine and will probably build another with the things I have learnt from this one. I am convinced that flying a model with no or very little rudder/elevator mix makes manouvers like the horizontal 8 with integrated half rolls in P shedule and rolling circle and rolling S in F so much easier because as you use more rudder the model keeps on track without scrubbing of speed and changing pitch. A good friend of mine who is an extremely talented pilot was gob smacked when he flew this model doing those manouvers. After my previous model required something like 17% of rudder elevator mix to remove pitching on rudder application I can certainly tell the benifits. A neutral model will allow you to make these manouvers more compact.

PeterP

As a giude to what I have done as far as incidences these are some of my measurements.

with fuselage sitting in my jig the bottom of the fuse (lowest part just behind chin cowl is 2 mm above the surface of the bench. The tailpost is 47mm above the surface of the bench. At that point incidence was set to .25degreees positive on wing and zero on the tailplane. Then from here I dropped the rear of the tailplane by 2mm. I dont know what the relationship is now between wing and tailplane but I would guess between 0.1-0.2 degrees. The angle the fuselage presents to the horizon is extremely important and I cant stress that enough. You need to make sure that is correct as well instead of just looking at relativity between wing and tailplane. These setting are pretty close to the mark IMHO. By lifting the rear of thte fuse I in effect added more down thrust which it required.

Any other experiences out there?

Cheers,

Peter

Malcolm H

My wing and tail started off pretty much on the moulded marks on the fuz. Like Peter I hold no store with these or accurate bench alignment - I fly it and adjust to suit. I have never understood JAS comments about adjusting the angle at which the fuz flies by adjusting the tail. The wing incidence relative to the fuz centreline is the only thing that can affect this. The wing flies at what ever incidence it needs to generate the requed lift and the fuz then goes with it. All the tail is doing is trimming the wing to the required incidence. Adjusting the tail incidence and trimming the elevator produce broadly the same effect alhough one may vary with airspeed more than the other. My procedure is to fly the model and trim it for straight and level. Then I observe the fuz angle and if I am happy I then do the downthrust checks. If this requires adjustment I alter it by changing the wing incidence which has the same effect. I then recheck for fuz angle and if this is now wrong I physically alter the downthrust. Once I am happy with fuz angle and downthrust then and only then do I adjust the tail to remove any elevator trim the model may be carrying. Over lots of models from .60 size 2 strokes up to my electric 2m I have found this procedure works.

Malcolm

tggilkey

PeterP (and Malcolm, too!)

Thanks for the good information. I should have mine in the air in a week or so. Peter -- you mentioned using a DZ160 -- that's what I have to go in mine. What prop(s) are you using or have tried?