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Grumman American Cougar

Build Blog by Mark Elen

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Mark Elen01/06/2020 22:20:42
527 forum posts
1022 photos

Thanks Gents.
I’m going to rename those ‘easy touch’ sanding bars to ‘easy to remove your touch’😂

Cheers

Mark

Mark Elen05/06/2020 18:19:05
527 forum posts
1022 photos

I have got a little more done today.

I hinged up the rudder and got the top piece glued into place:

f9b61437-8071-44bc-be75-525334ba8343.jpeg

It still needs sanding down to shape.

Then I got out the wing plan and traced out the horizontal stab profile and set to work drawing out a built up version. I'm not sure if my designing skills are up to scratch, but here it is:

a734f35b-7ed9-4069-9ee9-07a42492d1d8.jpeg

I'm going to have a go at it and see how it turns out.

Cheers

Mark

Danny Fenton05/06/2020 18:27:36
avatar
9635 forum posts
4451 photos

Looks good to me, well done.

What are the dihedral braces made from?

Cheers

Danny

Mark Elen05/06/2020 18:30:17
527 forum posts
1022 photos

Hi Danny,

Thanks for your kind comments. The braces are shown on the plan as hardwood, I have 1/4" bass and spruce in stock, but am toying with the idea of using 1/4" ply, what would you recommend?

Cheers

Mark

Danny Fenton05/06/2020 18:37:14
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9635 forum posts
4451 photos

Remind me of the tailplane span?

Mark Elen05/06/2020 18:39:37
527 forum posts
1022 photos

Hi Danny,

Its 28 inches.

Cheers

Mark

Danny Fenton05/06/2020 18:42:08
avatar
9635 forum posts
4451 photos

Okay 1/8 real birch ply would do it, but the 1/4 of any sort should be more than enough. What thickness spruce have you? That would be best weight/strength

Cheers

Danny

Mark Elen05/06/2020 18:48:04
527 forum posts
1022 photos

Hi Danny,

I have 1/8 and 1/4 Bass, 1/8 Spruce and both 1/8 and 1/4 Birch ply

Cheers

Mark

John Minchell05/06/2020 18:51:13
22 forum posts
1 photos

Danny / Mark

The plan says "hardwood not ply".

Mine is two pieces of 1/8" bass wood cut from 1/8 sheet and laminated with epoxy before gluing into the 3/8" balsa sheet tailplane halves.

John M

Mark Elen05/06/2020 19:00:04
527 forum posts
1022 photos

Hi John,

Thanks for your comments.

The 2 pieces laminated together sounds like a plan.

Cheers

Mark

Danny Fenton05/06/2020 19:10:21
avatar
9635 forum posts
4451 photos

Mark, do what you think best, your instinct seems pretty good, trust it.

I would not trust that plan. The whole design appears overweght.

Also watch out when it comes to epoxy strength and weight.

I did an interesting test on a Fury strut. One brass strut end attached to 1/16 ply with 30 minute epoxy, one with medium CA. I was very surprised how it was almost impossible to get the CA one apart, and it in fact split the ply. The epoxy was solid, but I was able to pop it apart.

Epoxy is old school......Hysol now you are talking.....

Cheers

Danny

John Minchell05/06/2020 19:58:53
22 forum posts
1 photos

Hysol 9466 is still epoxy, just a more modern recipe.

John M

Mark Elen05/06/2020 23:53:15
527 forum posts
1022 photos

OK,

So, my instinct all along has been that this will turn out nose heavy, so I have decided to build the tail plane as per plan. If it turns out heavy, so be it. It won’t fly much anyway....I always say, 90% of my enjoyment of the hobby is building, rather than flying.

If it’s anything like my last effort, it will end up in bits (not of my doing) on its first outing.
I will laminate up the tailplane dihedral brace, as per Johns comments.

I have got the majority of it made tonight, but no photos.

Cheers

Mark

John Minchell06/06/2020 00:24:55
22 forum posts
1 photos

Mark if you are a bit worried at the strength of the tail v join you could put a glassfibre bandage reinforcement (like joining trainer foam wings) and keep it narrow inside the fuselage.

John M

Nigel R06/06/2020 08:48:44
avatar
3938 forum posts
687 photos

Mark I would stick to your guns on the built up tail. It will resist warping better than a solid lump anyway.

I'd only suggest losing the rear spar and moving the brace back to the rear 1/2 x 1/4. Otherwise it looks perfect.

My choice for the braces would be lose the braces entirely and join the halves with some thin glass. It can be blended in quite easily. But either way will work

Nigel R06/06/2020 08:50:56
avatar
3938 forum posts
687 photos

This design definitely needs light wood selection Danny.

A lot of old school designs are like that though. Modellers must have been surrounded by forests of cheap contest grade wood.

Mark Elen06/06/2020 11:08:50
527 forum posts
1022 photos

Hi Gents,

Thanks for your comments. I used to be indecisive, now I'm just not suresmiley

I have now swung back around to the built up idea. I have had a re-jig of the design, and have decided to keep the rear spar, but it is now spaced 1/4" forward of the rear 3/8 block that will hold the hinges. I have decided to laminate 2 x 1/8 bass joiners and they will fit between those two.

The front spar will have a 1/8 bass joiner each side. It will be interesting to get this on the scales, along side the sheet tail that is nearly complete.

6c4dd4b5-8448-4809-9c4e-3e187a738b1f.jpeg

b0f4d4ae-bcb0-4736-9a78-1074504b1f99.jpeg

96cae009-69b8-45c9-afd0-3153a6c6dd8e.jpeg

I have used aliphatic for all of this.

Cheers

Mark

Mark Elen06/06/2020 18:12:21
527 forum posts
1022 photos

I have got on with that today, a couple of hours work and its nearly done:

f0712872-7243-4fc0-9c21-fda315e3d687.jpeg

6b8d3d0c-b34c-434d-80d0-26d4b73a7196.jpeg

d3af53cf-1fb1-4df8-9f2d-5e81414b19e1.jpeg

The 2 solid block stabilizers on the scales are 75 grams - without any joiners, that 1st half with the joiners, but no top sheeting came in at 28 grams, so it looks like I'm going to save a little, I only hope my nagging 'nose heavy' intuition isn't correct.

I'm now reconsidering having a go at the fibreglass nacelles. If only I could make a decision.....laugh

Cheers

Mark

john stones 106/06/2020 18:23:02
avatar
11546 forum posts
1517 photos

Have some faith in yourself, get in there with the fibreglass.

Mark Elen10/06/2020 22:53:33
527 forum posts
1022 photos

Hi John,

Thanks for your comments.
I haven’t got a lot more done in the last few days, I have put this to one side, to see if I can ‘build a wing in a week’ for the Peggy Sue 2, to replace the one that got damaged on its maiden day.

One thing I have definitely decided though, is that I’m not going down the fibreglass route for the nacelles. At 76 inch span, 815 square inches and with a target weight of 9 and a half pounds, by my reckoning, that gives a wing loading of 26.85 oz per square foot.

Reading some other twin blogs, and seeing a couple with a shorter span but coming in well over this figure, has given me a bit of reassurance. Although, I have just ordered some ‘contest grade’ 3/8” and 1/2” to make the nacelles with. Talk about ‘Equador Gold’... it needs to be, the price of it.

Cheers

Mark

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