Seagull Super Chipmunk ARTF

[Keith Miles 2]

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 Watkins]

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 2]

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?

[Keith Miles 2]

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!

[Denis Watkins]

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 Doctor]

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 2]

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 2]

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 2]

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 2]

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 2]

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.]

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 2]

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 2]

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!

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 2]

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!

[Keith Miles 2]

LATEST THOUGHTS!

Having weighed a similar length of metal pushrod to those fitted, I estimate 1.5 oz total for the three.

Have also just weighed SIX metal quick links, as per THREE fitted at the tail end, and get 1/4 oz!

So, other than shortening the fuselage or removing bits of control surface, I doubt that I can significantly lighten the tail by fitting carbon pushrods.

So, rather than worry about achieving that suggested 90mm CG, I think I’ll go with about 4 or 5 ounces on the engine mount and see how she flies with the CG about half way between the two given figures.

Will report back, in due course.

Feel free to add further comments, if you wish, and many thanks for your input.

P.S. Would still be interested to know what final flying weights have been achieved.

Edited By Keith Miles 2 on 19/08/2019 20:32:53

[Engine Doctor]

As you say trying adding the balast and test flying it might be the only option. The the problem could be caused simply by the wood used in the construction ? Nothing can be done if that's the case ,simply not viable.

I had a Seagull Ultimate bipe some years ago and that was heavy and flew like a brick . I tried all sorts with that but it was the basic structure that was heavily built so short of a strip down and rebuild ( not worth the trouble) nothing could be done. I have seen others that flew very well so can only assume I had a heavy one

Good luck hope you can sort it .

[Keith Miles 2]

Engine Doctor, yep, I agree!

I would, perhaps, not have been so concerned had it not been for the apparent experiences of two club members with the one model, and the CG anomaly in the instructions as mentioned by David Rayner and, apparently, also noted by RCMW, although I have not read the actual review. Also, having lost my previous WM version to a mid-air collision, I suppose that might also be influencing my cautious approach, not that the aforementioned incident was the result of any aerodynamic issue!

Test flying is a nervous enough experience, without the added stress of not knowing, for sure, where the CG should be!

Nobody likes crashing their aeroplanes, especially on a first flight!

I will report back in due course!

[Keith Miles 2]

Well, back sooner than expected!

Our John (bruv-in-law) just lent me his balancer!

FIRSTLY! Looks like I might, indeed, be needing to remove an elevator servo and revert to the two-into-one set up. Good news is that I have a much better pushrod joiner that I saved from the WM version. Will still need that nose weight, though!

SECONDLY! I decided, while the model was in the balancer to take a closer look at wing/tailplane incidence and the wing chord profile. The former is pretty much zero/zero, HOWEVER.......!

My friend Jim (the earlier mentioned aircraft engineer) is absolutely right about the wing tips having a higher angle of attack than the wing root i.e. wash IN. Looks like about 3-4 degrees to me! As he rightly says, this would cause the wingtips to stall BEFORE the wing roots which, as he says, and I would agree, is contrary to convention!

This would certainly explain his (and maybe others?) experience of a vicious left wing drop at the stall (which he demonstrated to me!) and would probably also explain other owners’ reports of a tendency for this model to flick on applying a strong “up” elevator input!

By contrast, the stall characteristics of my WM Super Chipmunk (it’s one of the first things that I test) were entirely benign and easily controllable without any rudder input, even with flaps down to the set maximum of about 30 deg.

Design fault(s)?

Looks that way to me with conflicting CG figures quoted, an apparent heavy tail and positive wing tip incidence. Not a good combination, especially if total quoted design weight also has to be increased, and I now wonder how many more have been crashed with owners, thereafter, scratching their heads!

That said, as I look forward to an eventual flight test, forewarned is forearmed, as they say!

And finally (for now!), I noticed, from the outset, how this model seems to take up a noticeable nose-down appearance when the tailplane and wing root chords are horizontal. Optical illusion and true to scale or another possible design fault?

[Keith Miles 2]

OH, DEAR!

With hindsight, should have done some research before parting with money and while still in mourning!

Just been reading some reviews on RC Groups. With one or two exceptions, not positive, encompassing all of the same concerns and observations and confirming a number of crashes due, it would seem, as suspected, to the very design issues that I have been concerned about.

Not exactly inspiring!

And, at present, rather than looking forward to a test flight, it’s making me feel much more nervous than usual, much less confident than usual and not a little disappointed.

[Keith Miles 2]

And now there are two!

As promised, an update!

On the left, the fellow club member's (Jim) aforementioned, previously owned "crashed on its very first take-off", subsequently bought, repaired, modified and eventually successfully flown electric version with my yet to be flown, at the time that this picture was taken, OS81 four-stroke powered version on the right.

Bearing in mind the 88/110mm CG anomaly, Jim apparently struggled to eventually achieve a 90mm CG with a larger battery fully forward, likewise the pilot, plus some nose weight! Not sure what total weight he achieved but the model flies well. As per other previous reports, it does suddenly drop the port wing at the stall, wings level, but only when provoked i.e. at VERY low airspeed with significant "up" elevator applied. That said, it recovers pretty quickly when the nose drops without much stick input. He has yet to set up the flaps. Also, it only flicks out of a loop with excessive "up" elevator. Originally, the model, when he bought it, was considerably underpowered but he eventually sorted that out as well with extra cells and a much bigger (16in.) prop!

As for mine, finally flew it yesterday!,

In view of my various "worries", I got a better pilot to do the take-off and the initial trials before handing it over!

No take-off drama and he only had to add a few clicks of "up" elevator and, again, despite the negative reports and concerns, this model also seems to fly well despite being almost a pound above the optimistic, in my view, "book" weight of 7.9lbs and with a CG of 98mm, halfway between the 88mm and 110mm figures given by Seagull! Full flap produced no pitching tendency at moderate speed which also bodes well. Better still, after flying it myself for five minutes or so, I even greased the landing without flap!

Just after my previous posting, I did eventually opt to revert to a single standard elevator servo, as per design, removing the two Hi-Torque ones to reduce weight and improve the CG position. I also still had to add lead sheet, screwed and epoxied to either side of the engine mount (a total of 5oz.) to achieve that 98mm CG. Adding more to bring the CG further forward was briefly considered but it seemed both impractical and undesirable.

So, whilst there were (are) a number of design and constructional flaws with this model that needed to be addressed (and one that can't be i.e. wash IN at the wing tips!) I am considerably happier and more confident with it now than I thought I was going to be!

As for the engine, I've got it running beautifully on a 14x6 APC prop and I was relieved prior to the test flight to obtain 79dB (3dB below club limit) at a healthy 9700rpm!

Just need another nice day, now!

And plenty of greasers!

[Keith Miles 2]

Just one more for the road!

A pretty pair?

In the foreground, that is!

[Bob Cotsford]

They are a nice looking aeroplane, those pictures make me wish I'd still got my World Models one.

[Keith Miles 2]

Posted by Bob Cotsford on 16/09/2019 15:38:49:

They are a nice looking aeroplane, those pictures make me wish I'd still got my World Models one.

Like the one in my photo earlier in this thread, you mean?

That had one or two design flaws as well but, overall, I was impressed with the general quality and it certainly flew well until a mid-air write off on its third outing. Couldn’t get another, nor any Chipmunk at this size, except for the Seagull one. Didn’t like the latter as much as the first and the subsequent negative reports didn’t help!

It’s growing on me now!

[Bob Cotsford]

Yes Keith, mine was the retract version complete with the legs retracting the wrong way. All the photos of the original appear to have rotating rear retracting legs. Ah well, cheap and cheerfull. Landings were ugly as it always tripped up on our grass strip even after packing the rear mount point, I always intended fitting larger wheels but never got around to it. As you say, once in the air it was a real joy to fly, mine was on a 91 4 stroke which made an ideal pairing. At one time we had three of them in the club!