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Seagull P40 - weak bulkhead / poor design


Ron Gray
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Sorry to see that Ron . Looks like you will need to make an access hatch and repair from that. For strengthening the front end on ARTFS first of all I take any pre installed star nuts out then I line the front fuselage and rear of the firewall with fibreglass tape (similar to wing joining tape) if it’s poorly designed or needs stiffening I add a few wood beams too. Using a long tube to extend the brush I can get into tight spaces. I also use chopped waste fibreglass mixed with epoxy as a super strong filler around the firewall. I then re drill and install the star nuts after all the reinforcement. I had to do that as well as adding extra ply on the firewall of my Seagull P47(And the Wots Wot xl) . Once done it was super strong and no added lead was required. I think you were unlucky not noticing the weakness but to be honest I have never yet found an ARTF that I felt was strong enough without reinforcement. At the very least I paint the inside with epoxy....edit oops missed your second page 😉

Edited By Timothy Harris 1 on 19/02/2018 14:40:15

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Tim, I'm as thorough as you and can get quite 'anal' when it comes to strengthening. I don't go as far as you removing grab nuts but my models are usually bomb proof.
Great Planes (USA) do Milled Fibreglass which when mixed with quality epoxy is superb for any repairs. It's deadly stuff and care must be taken not to breathe in the fine powder. Thanks for the tip on extending brushes.
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No problem Ash very happy to help. I also buy the big bulk boxes of brushes😊 . For the brush extension I use old split bamboo or carbon fibre tubes with tape on the end. I have often shown the guys at our club the “boaters technique” for making great fibreglass filler. Just fairly finely chop up any odd scraps of matting ( thin covering matting us also fine ) into a small pot with a blob of epoxy in it and stir. We used to fix our engine mounts into hulls that way and they never came out despite taking massive beating from the prop shaft and high rpm .

Edited By Timothy Harris 1 on 19/02/2018 16:01:55

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Saying all the above I had a firewall break and pop out recently (1st time) on my Toledo Special with a dead stick landing in the rough. It was a pain to repair and was made of liteply (Hangar9) , but if it hadn't broke there the damage would have been further along the fragile spiderweb fuselage.
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@Tim - I took your advice when building my Wots Wot XL and added reinforcing beams from the bulkhead back to the first former but I could get access to do that. On the P40 I should really have cut out the bottom of the fuse front to give me said access and then I would have been able to add strength. Anyway, I've added strutting and bracing as you can see from my photo plus the bulkhead has been glued back with copious amounts of epoxy so it should now hang together. I'm planning on flying her on Wednesday so will be able to see then but as a precaution I'm going to run it without the spinner and backplate (a nylon backplate is too flimsy IMO).

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Hi Ron it does look like the access was very limited as to say. Anyway good thing is it didn’t crash and you have repaired it well . On my p47 I added 2 half inch balsa beams on the inside to stiffen it that I glassed in as the front. Once I have the LA7 done I’m going to buy a Seagull Hurricane. I certainly will be reinforcing that too 😊 ready for nose overs😉. Regarding spinners I do prefer metal ones especially on warbirds, I think the larger ones also have heat sink properties too and aid cooling slightly. ( one for Jon😉.

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A friend of mine made an ali backplate for the spinner on his escale seafire as the plastic one was like soft cheese.

Im not sure what the cooling properties are like, im sure it helps but i would imagine that is a small effect.

Of greater importance is the weight as its about as far forward as you can get and it helps with c/g

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With artf models i have taken to thinning epoxy resin and spraying it around the inside of the firewall/tank bay as a method of fuel proofing. It also has the secondary effect of soaking into all the joints and firming them up.

Looking at the photos i think it would have been strong enough as it was had they glued it together properly, but as they didnt then there was no chance

I dread to think what a dle55 would have done if someone decided to fit one

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Epoxy filler is very neat and structurally strong once u get used to applying it . I repaired the end of a broken wing strut on my Wots Wot XL Biplane with it. Where it needs to be flush with a surface it’s easy. You can either repair the part on polythene or use insulating tape and repair a hole from the other side removing tape once it’s set hard. I tend not to thin my epoxy as I use west Systems big cans and that’s fairly thin already and soaks into wood well . Too much thinner and you will get weak rubbery glue. For small amounts of filler mixes I just use regular deluxe epoxy and cut a bit of leftover Matt into a small “ take away “ pot wit a small blob of epoxy in it . You don’t need much matting to make filler and a tiny bit will do . I apply with a coffee stirrer😊

Edited By Timothy Harris 1 on 20/02/2018 13:18:11

Edited By Timothy Harris 1 on 20/02/2018 13:23:27

Edited By Timothy Harris 1 on 20/02/2018 13:24:03

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I checked my stock of ali bar / ingots last night and the largest I have is 125mm diameter before turning. Unfortunately the spinner is 128mm dia so I need to get an off-cut from my local supplier which means that I will be definitely be running it tomorrow without the spinner in place! At least that's one less thing to go wrong and the engine will get some cooling air blowing over the pots.

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Posted by ASH. on 20/02/2018 14:44:50:
I am very particular in having my props perfectly balanced. How does one go about balancing a spinner?

Fit to model, start engine, does it shake? if yes throw it away, if no then its a winner!

I have to confess i have not balanced a prop in probably 15 years. Modern props are usually so close that it make no difference at all.

Spinners i am suspicious of i do try and balance on a vertical spike that locates in the centre of the cone. The problem is once you start trying to fix weights on the inside its just a mess and now i only buy the spinners from just engines as the 3 have had so far have given no balance problems at all so i just run with them as they came out of the box

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Posted by Jon - Laser Engines on 20/02/2018 15:41:42:
Posted by ASH. on 20/02/2018 14:44:50:
I am very particular in having my props perfectly balanced. How does one go about balancing a spinner?

Fit to model, start engine, does it shake? if yes throw it away, if no then its a winner!

I have to confess i have not balanced a prop in probably 15 years. Modern props are usually so close that it make no difference at all.

Spinners i am suspicious of i do try and balance on a vertical spike that locates in the centre of the cone. The problem is once you start trying to fix weights on the inside its just a mess and now i only buy the spinners from just engines as the 3 have had so far have given no balance problems at all so i just run with them as they came out of the box

That’s interesting coming from an engine man. I must admit I only ever need a drop of paint to balance my props.

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Posted by ASH. on 20/02/2018 14:44:50:
I am very particular in having my props perfectly balanced. How does one go about balancing a spinner?

 

THIS might be interesting reading. I too , balance and check every prop I get. Even big Xoar props need a dab of balancing glue sometimes.

Edited By cymaz on 21/02/2018 06:46:45

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Posted by Rich too on 21/02/2018 06:22:13:

That’s interesting coming from an engine man. I must admit I only ever need a drop of paint to balance my props.

If its so close that a drop of paint will do it then you need not worry. Single cylinder engines are impossible to balance anyway which is part of the reason they vibrate in the first place. The weight of a layer of paint will not make any significant difference. If i find a prop that vibrates badly and then is miles out on the balancer i just throw it away. Given that i have not noticed excessive vibration from any prop i have bought in many years i dont tend to worry about it any more.

That said, if you fit something heavy like a spinner and its out of balance you will immediately notice vibration which is beyond what you normally see from the engine. This is where knowing your engine really helps. In this case balancing of the spinner is needed as the last thing you want is to get to a stage where you reach resonance with the airframe.

The method shown in the article cymaz linked is the best way but i am yet to find a balancer that has a long enough shaft and large enough sliders to fit a 5 inch spinner. If i could find one i would use it, but alas no joy

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I only use APC props and remove the sharp edge and balance using a file. Putting an Ali spinner on out of the box negates this I feel. I'll take a look at link cymaz, thanks. And also the Just Engine spinners.
True Turn from the US are supposed to be the best but too pricey for me.
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tru turn are really nice but when the spinner costs half as much as the engine you know its getting silly!

i always de flash my plastic props but most props i use these days are wooden.

When it comes to spinners i always run the engine with its intended prop, then add the spinner. Any change in the vibration level is then immediately apparent and i take action if i need to. Since i switched to the JE spinners i have not had any trouble

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I found some 3mm aluminium plate so was able to machine a backplate after all. Still needs a bit of finishing but does the job.

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@Jon - as per the article Cymaz linked to, make a new base plate that the prop balancer end pieces can fit on then shafts from silver steel, 4mm, 5mm whatever. Cone locators for front of spinner and backplate with silicone and spring retainers - job done!

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