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Seagull P40 - weak bulkhead / poor design


Ron Gray
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I bought my P40 off another flier who hadn’t quite finished the build and the engine hadn’t even been run up in the airframe. This model is sold by Seagull to be powered by a 30 - 35cc gasser and this one was fitted with a DLE35RA. I finished the build but during this was not too impressed by some of the factory construction but pressed on. Due to the lack of space, a Pitts type silencer was fitted to the engine and despite me fitting pepper pot sound reducers the bark was a bit on the loud side. Cutting a long story short I recently bought a Laser 240v (beautiful engine) and decided to swap out the gasser. Having done all the associated work, I took the P40 up to the field today for its maiden.

Despite this being its first run, the 240v started after about 10secs on the electric wrist and after warming up on mid throttle I ran it up to max revs for a short while. At this end of the rev band it was fading a bit so I throttled back then slowly back up. At this point there was a rather loud bang and the spinner shot across the tarmac. The bang was, I think down to the 4mm spinner bolt shearing. Anyway I noticed that the engine was not at the same angle and upon removing the cowl, this is what I found:

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What really struck me (apart from the wayward spinner) was the extremely under engineered bulkhead fixing which fixes to the fus at either side of the bulkhead the contact area being about 2 x 30mm x 6mm. The top and bottom of the bulkhead was fixed to 1/16th balsa sheet, so nonstrength there. There were triangular strengthening fillets but one split in 2 and the other just pulled off. As I had my doubts about other areas on this ‘plane I had checked the bulkhead but a more thorough check wasn’t possible as you can’t really get to the back of it.

The bottom line line, for me is now what to do with the ‘plane. Sure I could stick it back together but it would still be a weak point and to put strengthening framework in place would probably necessitate removal of most of the nose.

So I’m going to look for another home for the 240v and in the meantime, if you have a Seagull model designed for a 30 - 35cc gas engine, check the bulkhead fixing!

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I haven't owned that model but, from other stories I've read, I'm not too surprised by the design/build quality of an ARTF.

But what does intrigue me though, is what on earth would cause the spinner bolt to shear? Presumably the prop stayed attached, but was it perhaps a bit loose so that a backfire would have caused it to slip? Doesn't explain the spinner bolt, even if that is the case.

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I’ve a few other ARTFs but IMO this one is the poorest in terms of factory design / build quality. The Great Planes and Hangar 9 ones I have are far superior.

Prop stayed intact, prop bolt was not loose, in fact it had Locktite applied, and no backfire from the engine. My guess is that the bulkhead parted company and the engine dropped causing the spinner backplate to come in contact with the cowl. The backplate, being plastic (!!!) deformed popping the spinner off which then was off centre causing the prop bolt to bend and snap.

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looking closer at the photos Ron the construction seems reasonable in terms of the design. Those tabs on the side frames are clearly intended to lock into the firewall but I wonder if they were a little light on the glue.

I agree with your sequence of events regarding the failure. A spinner cone flailing around would put huge loads on the retaining screw and could easily break it

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This certainly highlights the wisdom of making sure people are behind the propeller when an engine is being started and/or revved-up.

Some ARTFs leave a lot to be desired with the quality of their materials and construction (and the quality of the glue they use). They are notoriously bad in the engine mounting areas, and also U/C mountings. These highly stressed areas have been known to be wonderfully under-engineered with badly fitting bits of thin plywood (daylight visible at the joints) which is then hidden with even thinner balsa. On some of the worst examples, it is possible the twist the engine "box" by hand. Some extra scrutiny is required.

Basically, an engine will not stay in place if it is mounted to flimsy materials which have been plopped together with a couple of blobs of hot glue. . We need to be in front of the "game" and mend things before they break.

The engine bays also need a decent coating of fuel proofer.

B.C.

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Posted by Brian Cooper on 17/02/2018 09:02:12:

Basically, an engine will not stay in place if it is mounted to flimsy materials which have been plopped together with a couple of blobs of hot glue. . We need to be in front of the "game" and mend things before they break.

The engine bays also need a decent coating of fuel proofer.

B.C.

As Brian says, you spend a couple of hours on any ARTF looking for weaknesses and building up with doublers and whatever is necessary while it is conveniently empty of gear

We should not have to do this, as the model should arrive fit for purpose

But realistically, they are never " ready to fly "

And there is always something that needs changing or adjustment

To be honest, It is quite enjoyable getting an ARTF up to spec

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It looks as though it's supposed to be keyed to the crutch at the top either side too. It all looks to have come apart cleanly, can you not epoxy it all back together from the front? It sounds to have failed because of a lack of glue rather than a lack of strength. I find that you can nearly always get some more epoxy on from the rear by using long brushes bent as needed inserted through the openings left in formers for tank access. I don't know this P40, is it not set up this way?

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I agree with what has been said about checking for weakness and strengthening where necessary. I’ve done this on the U/C mountings on my GP ‘planes and engine boxes (50cc SU26) but the problem with this one from Seagull was that you cannot get to the back of the bulkhead as the space to get there was limited by the size of opening in the next former (only enough room to get a fuel tank through). On the face of it I saw that it had triangular reinforcement behind it so thought that it was “OK, plus there was no sign of movement when I twisted the engine.

For once Jon, I disagree with you! The tabs you can see do fit into the bulkhead but the ply they are part of does not extend further back down the fus. The actual mating surface between the bulkhead and these ply pieces are about 30mm long each side, there is no structural support along the top or bottom of the bulkhead, in fact the bottom half of the bulkhead is made from 4mm ply. The tabbed pieces of ply are themselves stuck to 2mm lite ply which is where all of the strength in the fus isvat this point. I should have listened to my own thoughts when I saw that the platform to mount the battery and the fuel tank on was made out of balsa not ply, warning bells, warnings bells.

Still, we live and learn from our mistakes but now I need to look for a suitable home for my shiny new 240v, any ideas (as long as it’s not a Seagull produced one!!!)?

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Wow! Thank heavens the model didn't make it to a test flight, a lot of lovely engine to dig out of a hole and an uncontrollable model, sans engine as well. That firewall looks terrible to me, very little glue and contact area intended for such big motors. I'd have much more in the way of gusseting and extra glueing area, so as to integrate and direct mechanical stresses into the fuselage more effectively. Much more 'meat' needed in that area for sure, and going back some way down the fuz. Extra weight is hardly likely to be an issue.
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I've got a Seagul Rv8 with a split level firewall which was falling to bits the same as this and for this reason it's put me off buying any more Artf. I have a Hangar 9 Piper Pawnee and the fibre wing tubes were glued in with wood glue which didn't even bond at all. Traditional building for me as soon as I get a better workshop.

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not that long ago I remember a large ARTF Sopwith camel was lost due to structural failure, and upon examination the strut mounting blocks had weevils in them that had eaten all the wood leaving only the epoxy......and captive nut.the model was flying around on borrowed time.....crepe materials used in the manufacture..

ken Anderson..ne...1..weevil dept.

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I'm surprised to hear those tabbed side frames do not extend further back. most modern artf's seems to build a stroke box structure that takes all the loads and then decorate it with spitfire/mustang/whatever shape formers. I assumed this was the same setup.

If it was all glued back together properly I think it would be ok. Looking at the triangular stock still on the fuselage sides they really didn't waste any money on glue

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This sort of thing is limited to ARTF's is it?

Has no one has ever built a model themselves and only found out afterwards that it has weaknesses?

The P40 certainly shouldn't have come apart as it did and it's an issue worth raising but this thread is at risk of knocking all ARTF's generally and that just isn't a balanced discussion.

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@Percy - back in the day I used to build all of my own ‘planes from plans, control line, free flight and early days of single channel r/c so building in strength where needed was something I learnt when in my teens! Coming back to the sport a couple years ago I came across ARTFs and, as you say, they do vary greatly in quality. However this, in my experience, has largely been down to glue, or the lack of it! Basic strength in the engine bay area has largely been OK, that is adequate material being in place but this Seagull model falls well short and that is what this thread is all about.

Having had a closer look at the construction I can see a way of providing more strength and this involves cutting the bulkhead in half where its thickness reduces. I will then add horizontal braces to form a crutch box behind the bulkhead which it can be fixed back to. Then I plan to add some horizontal braces back to the first former. This photo shows how the bulkhead is reduced in thickness.

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Edited By Ron Gray on 17/02/2018 18:36:30

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Looking at that Ron the real issue is the lack of glue. It seems only the balsa skin was really holding it on as there seems to be no damage or glue residue at all to the ply side frames or any of the other ply longerons. I would remove the existing triangle stock, epoxy the firewall back in properly and then add new triangles afterwards. If you are still concerned get some glass cloth and slap that in around all the inside joints to help it along

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Certainly was lacking glue Jon but relying on only 2 vertical joints with no lateral bracing for a ‘plane designed to take a 35cc gas engine seems like a bit of under engineering. I have now rectified this and am putting the finishing touches to the restrengthened bulkhead, pictures tomorrow.

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I have had a fare few Seagull Models and not had a problem like this. Though I have seen more than one of there spinners disintegrate. I do tend to go around any glue joint that I can reach. May be a Friday afternoon one!

Just for info I bought a Hangar 9 Carbon Cub that came in it’s shipping box inside packing box was a label that said Manufactured by yep you guessed Seagull Models.

Phil.

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All closed up now Jon, hoping to run the engine up again in the garden tomorrow before taking it back up to the field.

@Phil - the spinner is a bit scary, for one so large not to be made out of alloy or at least have an alloy backplate. I may end up turning an alloy backplate on the lathe.

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