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Seagull Super Chipmunk ARTF

Build comments and review

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Keith Miles 217/08/2019 23:11:54
170 forum posts
6 photos

Update, update. 110 or 88?

Model balances level, as it is, at the 110 mmm position.

BUT, with fingers at 88mm position, 12oz of nose weight (a lump of old lead gas pipe) had to be taped to the cowl, at the engine position to achieve balance!

AND the model is also already 8oz above design weight, although I suspect that design weight of 7.9lbs is optimistic.

The three high torque servos (my mod) for rudder and elevator only account for a less than 3oz difference above fitting two standard ones and, also, offsetting that, are about a couple of ounces of added ply and fuel proofing at the front end.

So, as others have experienced, CG and balance seems to be a bit of an issue with this model, especially in view of conflicting supplied CG information from the manufacturer!

So, I would be interested to know, if at all possible, of those models already flying successfully, where your ACTUAL balance points are (lipos fitted or fuel tanks empty, of course!).

Need to have a little think about mine, at this point, but I definitely won’t be adding 12oz!

Denis Watkins18/08/2019 08:27:21
3814 forum posts
54 photos

In this smaller scale size class, Keith; warbirds included, and even as we move up to 10lb, 15lb, 20lb yipes!!!

We have no choice with C of G, where weight has to added, the model then needs that weight, you know that

We all don't want to add weight as speed goes up.

I maiden most models at my patch, and if the C of G comes out On or just in front of the wing tube, then I go for it.

In this size class, I can compare the Super Chipmunk to a Corsair we fly, a Kyosho 50 Hurricane, and the Seagull P51.

All need flying with care, should be kept moving, no loitering, and have planned flights without ad lib stick inputs that allow them to bite you.

When you know the model better, then explore the envelope.I

This Super Chipmunk with the OS70fs Balances at 90mm, upright or inverted.

There looks to be 4oz noseweight across the front of the engine mount

We cast weights to bolt under motors

Don't stick weight in a cowl, as an heavy arrival will break the cowl

Keith Miles 218/08/2019 09:02:17
170 forum posts
6 photos

Denis,

Thanks for your CG info. Will check again later but not happy adding 12oz even if there was room to get it in. It would put the model way over design weight and that ain’t good either.

What was interesting was that the thread originator first balanced at 110 mm then just added 60g. He never came back with his new CG position, hence my question.

I note that your models, apparently, have only relatively small weight added as well.

Whether my servo mod has made mine especially tail heavy remains to be seen. May have to change that to avoid adding too much balancing weight. No other reason for it that I can see, other than a flawed design.

And I have never had to add more than 3 or 4 ounces for balance on any model thus far, the worst case being my Wots Wot.

Denis Watkins18/08/2019 10:28:44
3814 forum posts
54 photos

To add Keith, after having a look

Our flight battery is alongside the tank on the front bulkhead

And the OS motor is way up front on the engine mount

Keith Miles 218/08/2019 15:30:34
170 forum posts
6 photos

Hi, Denis, thanks for info.

No, I would never add weight to a thin cowl, attached by self-tappers, especially 12oz!

As for engine position, moving mine forward would be unlikely to shift the balance significantly (see previous photo). Moving the lipo flight battery forward would make it difficult to remove for charging and, similarly, I doubt that it would make a significant difference either in terms of achieving 88-90mm CG (see photo below).

in case you have not seen one, I also include a photo of my deceased World Models version. You will immediately notice significant visual differences. There are quite a few more, including the one-piece wing! That said, overall dimensions are pretty identical but the WM version is a recommended pound lighter (that comparatively heavy Seagull u/c having much to do with it, I reckon!). Stated CG of the WM model is 105mm and it balanced at that with no adjustment and flew fine with just a touch of aileron trim.

Noting that David Rayner added only 60g after balancing at 110mm, and noting your recent comments, I’m minded to do similarly and then just test fly it, or get one of our far superior pilots to do it! I’m reasonably confident that it won’t be uncontrollable. Touch wood!

dadb81fc-c7ff-4fa8-b767-ccd41d9eebc0.jpeg

9c1ea643-9665-4f2d-966f-99ca3738b21a.jpeg

And, yes, that is a Corsair in the background! Bought second hand. Flown once. Had to repair one of the retracts. Subsequently noted side slop at one of the hinge positions, so rudder may have to come off. One day, I’ll get around to figuring how best to go about fixing it. It’s a closed loop system and access is a little tricky!

Keith Miles 218/08/2019 17:08:12
170 forum posts
6 photos

Denis, going back to one of your comments, I just checked the model and note that the CG recommendation “A” of 110mm is, in fact, the front of the wing tube.

Life is much simpler with a parallel chord, ain’t it where main spar is generally a good starting point?

One other thing, on reading the WM Chipmunk manual, it states that, for electric power, the CG should be moved forward 10%. Seagull, however, don’t mention this. Fuel weight is constant, of course, for batteries! So, for electric, that would move the WM Chippie CG from 105mm to 94.5mm and the Seagull one from 110mm to 99mm.

Might help to explain the problems that my aforementioned colleague(s) had with his (their) electric powered Seagull version. In that case even the pilot was eventually moved forwards!

So, I still suspect a slight design flaw with this model compared to others, certainly ones I’ve had thus far!

Denis Watkins18/08/2019 17:57:45
3814 forum posts
54 photos

In fairness to the designers, trying to entertain both power sources, requires a more recent skill.

The weight of knowledge on this model appears to be I/C

But with I/C we have always flown heavy with fuel at take off.

As you say, our electric flights are constant weight distribution, throughout the flight.

PS, is that a PT19 lurking in the background?

Keith Miles 218/08/2019 18:33:34
170 forum posts
6 photos

Denis, sorry to be a pest (ongoing!), or anyone else, perhaps (all quiet so far!), would be interested in knowing final flying weights achieved.

Just taped a 4oz slice of lead to the cowl at the engine position. Model balances halfway between the two given CG values (i.e. at 100mm, give or take a fraction). Taping a 2oz kitchen weight (the weight of a servo) at the elevator servos position makes only a very slight change in balance.

The model currently weighs 8.5lbs and I would prefer to keep it as far below 9 as possible to avoid too much percentage increase in apparent design wing loading.

So, my plan now is to see what weight I can attach to the sides of the engine mount, using some lead sheet, making allowance for possible removal (he said, optimistically!) after a flight test. Similarly, I will also have the option of perhaps removing an elevator servo and, albeit with a little reluctance, reverting to single servo operation of elevators.

On we go........

Maybe I should also weigh the yet to be added decals?

smiley

Keith Miles 218/08/2019 20:09:09
170 forum posts
6 photos

Denis, posted before seeing your most recent reply.

It is indeed a PT19, another Seagull ARTF. Flies really well but have never managed to get the OS FS52 running as well as it should. Tried all sorts, even different fuelling set-ups. In fact, have tried everything you can think of. I have come to the conclusion that the tank position might be too high because it has always had a tendency to leave quite a bit of fuel residue inside the cowl and around the tubes. I have even seen the carb inlet filling up with fuel, on occasions, with the model just sitting on the ground, engine off. Need to see if I can do a bit of reconstruction but another job I’ve yet to get around to!

Bet you wish you hadn’t asked now!

smiley

Denis Watkins18/08/2019 20:20:58
3814 forum posts
54 photos

Keith, you will have to trust me on this, as an OS FS52, my most prolific ownership size

This motor will draw fuel from an inch too high tank, to an inch too low, so get your tank near enough and job done.

It will only draw fuel like this with pressure

So no leaving pressure off this one

They, OS, like most 4 strokes, like correct tappet setting and a complete valve spring

A broken valve spring can hide itself quite well, so look closely and lift and prod the springs with a toothpick

A broken spring will display inconsistent to non running

Engine Doctor19/08/2019 09:45:26
avatar
2272 forum posts
25 photos

Those metal pushrods that seagull use on the rudder elevator add weight to the rear. Properly fitted snakes will lose some of that unwanted weight. Worth a try ? Think about it 1 Oz of weight at the tail is approx 3 to 4 times further from CG than the bulkhead so will need 3 to 4 Oz's to counterbalance. Does the chippy have a heavy tail wheel set up? Same applies. Seagull kits have improved immensely over the years but they still inside some heavy junk fittings to keep price down . Look at any way lose weight at the rear and you will achieve the CG without a ton of nose weight.

Good luck.

Keith Miles 219/08/2019 11:07:08
170 forum posts
6 photos

Denis, I will check the valve springs, as you suggest, however.....

Two of these were bought, unused, from a former club member who was selling up. My brother-in-law had the other one and put it into a plan built PT19. His has always been okay. Actually, rang him last night to see if he could check his tank position. He runs his without tank pressure as, apparently, he does with almost all of his engines, without problems. I have previously checked tubes, main needle, tappets and tried three different fuelling systems. Mine was originally pressurised and, actually, it seemed to run better with the pressure line removed, in all cases, so in the end we left it off! Whilst I can get a consistent run, it is very sensitive to main needle setting and If I set slightly rich of peak on the bench it becomes very sensitive to any raising or lowering of the nose. Both engines are mounted inverted. Oh, and I’ve tried different plugs and the same fuel (Optiifuel 5%) is used for all of my models.

Evidence of siphoning and excess fuel residue inside the cowl is what finally left me to consider that the tank might be too high, notwithstanding your recent comments which are all duly noted.

it’s a puzzle!

Keith Miles 219/08/2019 11:43:42
170 forum posts
6 photos

Engine Doctor,

I totally follow and understand your comments but the parts to which you refer would seem to be nothing unusual, in my view, and modifying control runs on an ARTF would, in general, not be very easy to do unless you are referring to merely replacing the inners which in this case, might introduce excess flexing but I will certainly take a look. Tailwheel is pretty standard. Of course, I think that we would agree that such action should not be necessary if the design is right!

The only unusually heavy item on this model is the undercarriage but that, of course, sits close to the CG.so largely merely adds to overall weight.

My previous WM model, as said, was lighter, despite having similar components except for having two fewer servos and a much lighter wire undercarriage. And I had no problem with balance. Oh, and it was cheaper!

Keith Miles 219/08/2019 12:08:24
170 forum posts
6 photos

P.S. I also still think that the target weight stated, of 7.9lbs, is optimistic bearing in mind that I have only added 70g of servo weight and a few grams of additional ply and some foam padding!

That said, it’s probably true to say that such optimism is not unusual and merely varies by degrees!

Keith Miles 219/08/2019 15:44:10
170 forum posts
6 photos

As suggested, had a look at some SLEC snake inner (yellow/ribbed) which I already have and, likewise, threaded rod, if necessary for clevises.

6 inches of protruding snake inner would flex too much, alternatively, finishing with 6 inch lengths of metal pushrod (or bicycle spoke), whilst reducing overall model weight slightly, would seem not to reduce tail weight enough to give a significant improvement in balancing, all the weight being taken from the middle of the control runs rather than at the extremes.

I’m just off to check out some carbon rod and might experiment with that, in the workshop, of course!

Keith Miles 219/08/2019 15:54:18
170 forum posts
6 photos

P.S. One metal pushrod, 24 inches long weighs just under half an ounce.

So, wouldn’t think that even three would greatly affect balance/tail weight but I could be wrong!

ASH.19/08/2019 16:13:25
avatar
301 forum posts

Keith, on the PT19 you could try looping a longer carb line over the engine mount and then down and up to the inlet nipple, (securing with tie-wraps). It's worked for me. I do it on all installs. It's a fallacy to believe the carb line has to be short and direct as possible.

You could also reduce tail weight on the Chipmunk by replacing the 2mm long heavy metal push rods with 2M carbon tubes with couplers and ball joints..

Edited By ASH. on 19/08/2019 16:14:41

Keith Miles 219/08/2019 18:12:20
170 forum posts
6 photos

ASH,

RE: PT19/OS FS52. Already tried that. Referring to my previous post, just been round to my brother-in-laws house who has precisely that arrangement on his plan-built PT19. His tank is also set in the same position as mine. When the Chipmunk is finished, I intend to take the engine and tank out and check everything yet again. I might even run it on a bench and see what effect raising and lowering the tank has before I put it back on the model. Brother-in-law is very knowledgeable and experienced and he has been as puzzled by it as I have. It’s always been a problem despite a successful and trouble free bench mounted run-in.

RE: Chipmunk. After much thought, having just looked at some 2mm carbon rod (see previous post), which aforementioned brother-in-law has, we have both concluded, after much discussion, that whilst total weight would certainly be reduced it would be too much trouble at this stage for a possible limited advantage. And neither of us could say, with complete confidence, that a glued joint between carbon rod and a clevis or rod and clevis adaptor would be entirely trustworthy on a model of this size.

Keith Miles 219/08/2019 18:45:02
170 forum posts
6 photos

ASH,

Just remembered that I had these in my “might come in handy” collection. They came off a model I bought second hand some years ago which crashed on its first flight!

d671825f-8ff9-4eea-800e-9ca4532fa68d.jpeg

Is this the sort of thing you mean, i.e. epoxied on adaptors for carbon rod?

Looked like a possible idea, initially however the rod is not 2mm, in this case, nor the threads on the fittings, which seem to be 3mm.

Would also need to avoid using too many metal fittings or that might defeat the object!

I’m thinking snake inner combined with something like the above at either end or 2mm carbon rod throughout with suitable adaptors, if available?

Keith Miles 219/08/2019 19:33:45
170 forum posts
6 photos

Further to last post. Cannot use snake inner as the exit gap is too narrow and can’t get in to widen it as the exit fairings are already glued in place!

So, it’s 2mm carbon rod and fittings to suit or leave model as it is.

Currently looking for the bits.

Thus far can only find “sets” including the 2mm carbon fibre pushrod.

The search continues!

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