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Seagull Super Chipmunk ARTF

Build comments and review

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Keith Miles 219/08/2019 20:31:17
170 forum posts
6 photos

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 Doctor20/08/2019 09:51:54
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2272 forum posts
25 photos

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 sad

Good luck hope you can sort it .

Keith Miles 220/08/2019 12:08:03
170 forum posts
6 photos

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 220/08/2019 23:34:42
170 forum posts
6 photos

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 221/08/2019 01:15:19
170 forum posts
6 photos

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.

sad

Keith Miles 216/09/2019 14:54:58
170 forum posts
6 photos

And now there are two!

cimg1476.jpg

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 216/09/2019 15:07:53
170 forum posts
6 photos

Just one more for the road!

A pretty pair?

In the foreground, that is!

cimg1473.jpg

Bob Cotsford16/09/2019 15:38:49
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7931 forum posts
436 photos

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

Keith Miles 216/09/2019 17:26:45
170 forum posts
6 photos
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 Cotsford16/09/2019 20:35:01
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7931 forum posts
436 photos

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!

Keith Miles 217/09/2019 10:19:02
170 forum posts
6 photos

Bob,

Yes, with retracts, raking the u/c forward ain’t really an option is it?

Not really viable on this one either unless one dispenses with or somehow modifies fairings and spats as per Jim’s model and as you will see in the picture. In his case, he may have been influenced by the previous crash damage!

That said, at full power, the tail on mine only comes up slightly on the prep bench but I do tend to use high rate and judicious “up” elevator when taxiing, anyway!

On which point, and as I previously failed to mention, the “book” control throws, particularly aileron, seem excessive. Not uncommon, in my experience and I have always tended to set up “book” figures, especially for aileron and elevator, as “full rate” with 20-30% expo at all rates and have found that low rate (60%) is more often than not perfectly adequate for “scale-like” flying, as it also seems to be in this case.

Who needs a sport scale model with, for example, the roll rate of a spin drier?

Keith Miles 220/09/2019 15:33:00
170 forum posts
6 photos

OH, DEAR, Part 2!

What did I say about greasers? My second landing wasn’t one!

Yep, you certainly do need to fly this one to touchdown with a little bit of power maintained and to be very careful with the round out, especially on a no-wind day!

Chop the power a tad early and raise the nose just a little too much and too soon and that left wing suddenly drops like a brick!

Luckily, damage seems, initially, to be confined to the undercarriage. Wing tips are unscathed as the model was almost down so no cartwheeling! Did have to pull out a large tuft of grass from the front of the cowl though! Didn’t even break the prop!

The fairings are flimsy, anyway and difficult to secure properly, so might scrap them and either make some from balsa (as my mate Jim did) or not bother. If spats are okay, might leave them on or might not!

Surprised that the wheels rotated. I’m guessing that the leg assembly must have rotated on the piano wire despite seemingly being a very tight fit in the top of the leg. Only one leg appears to have bent slightly.

So, time to get the tools out.....

150aad7f-6d46-4447-bc22-f68a9d3d5b22.jpeg

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