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Schleicher ASH31 4m

Let us all watch your new project progress.
MartinE
Posts: 122
Joined: 28 Oct 2021, 14:36
Location: Essex

Re: Schleicher ASH31 4m

Post by MartinE »

Once I've layed the top spar cap in, I'll then bond in a shear web as I did with the elevator, then bond the bottom spar cap to it.
Jolly Roger
Posts: 514
Joined: 30 May 2015, 20:35
Location: Sutton Bank, North Yorkshire

Re: Schleicher ASH31 4m

Post by Jolly Roger »

Hi Martin,

I wondered if you'd considered using pultruded carbon strip rather than laying up your own using tows? Something like this...

https://www.hyperflight.co.uk/products. ... rbon-strip

or here..

https://carbonfibreprofiles.com/index.p ... cts_id=629

The benefit would be that the strips would be stronger and more uniform than hand-laid tows, due to a higher fibre/resin content and most importantly that the fibres are much straighter and more aligned.

I think the main drawback of hand laid tows is that the vast majority of them are twisted or wavy (even if only slightly) with the result that they do not evenly share the tensile stress. When the spar is loaded, the tiny minority of straight strands find themselves having to accommodate a stress that was designed to be taken by all the fibres in the spar - so they fail and pass the stress onto the next wavier strands...which fail etc and then whole spar fails. A pultruded spar on the other hand contains very accurately aligned fibres due to the pultrusion process, so that when it's loaded they are much more likely to share the stress across the whole crossection of the spar.

Added to this, hand-laid tows will probably include imperfections like air bubbles and specks of foam etc which will act as stress concentrators and further points of failure.

I hope this doesn't sound negative Martin, I'm just offering it as a suggestion.... :)

Rog
MartinE
Posts: 122
Joined: 28 Oct 2021, 14:36
Location: Essex

Re: Schleicher ASH31 4m

Post by MartinE »

Hello Rog,

Your suggestion is very welcome. I have & do use those pull-truded strips, particularly on shorter lengths, like tails.

My reasoning for using tows on wings is the ability to achieve tapered spar width and depth. Granted the layup and bond won't be as good, however, I tend to over engineer the amount of tow.
I will also cure under vacuum which should reduce the risk of voids, etc.

So far I've never had a spar cap failure (kiss of death :D ), de-lamination/buckling of the skin seams to occur first (& never in flight), although I don't fly my models in the alps!
MartinE
Posts: 122
Joined: 28 Oct 2021, 14:36
Location: Essex

Re: Schleicher ASH31 4m

Post by MartinE »

Here is the first inner wing section after spar cap has been bonded in (under vacuum). For this stage I used perforated film and kitchen roll, to absorb excess resin (& distribute vacuum).

I've added a 48gsm leading edge strip & an 80gsm bias D box.
The spar cap top is below the wing profile so I will now fill and shape, before adding the final carbon skin.

Note the added foam end blocks to stop the tows wrapping over at the ends.
Attachments
20220407_113308.jpg
MartinE
Posts: 122
Joined: 28 Oct 2021, 14:36
Location: Essex

Re: Schleicher ASH31 4m

Post by MartinE »

Left hand inner wing section now in the bag, to cure the upper spar cap and d-box.

Weighted down using the profile negative and my old gym weights (long since used to improve muscle tone :lol: )
Attachments
Perforated file & kitchen paper
Perforated file & kitchen paper
20220412_202341.jpg
MartinE
Posts: 122
Joined: 28 Oct 2021, 14:36
Location: Essex

Re: Schleicher ASH31 4m

Post by MartinE »

Elevator at the back with filler skim, to fill pin holes
MartinE
Posts: 122
Joined: 28 Oct 2021, 14:36
Location: Essex

Re: Schleicher ASH31 4m

Post by MartinE »

The mid section wing joiner are assembled and bonded (with spots of 5min epoxy).

10mm carbon rod, joiner tube previously formed over it then cut into 4.

5mm carbon location pins into brass tubes
Attachments
20220412_200257.jpg
20220412_201859.jpg
Kenneth Paine
Posts: 12
Joined: 14 Nov 2021, 09:50
Location: Cheltenham

Re: Schleicher ASH31 4m

Post by Kenneth Paine »

MartinE wrote: 12 Apr 2022, 20:32 Left hand inner wing section now in the bag, to cure the upper spar cap and d-box.
Good progress Martin. Just perforated film and kitchen towel is the way I want to go in future; no more Mylar nonsense.
MartinE wrote: 12 Apr 2022, 20:32Weighted down using the profile negative and my old gym weights (long since used to improve muscle tone :lol: )
When you get to applying the full skins I suggest you weigh down the trailing edges with a straight bar to prevent them curling. I make an MDF board for - and the shape of - each of my cores to put between them and the weights to spread the load evenly.
MartinE
Posts: 122
Joined: 28 Oct 2021, 14:36
Location: Essex

Re: Schleicher ASH31 4m

Post by MartinE »

For the outer skin, I think I'll try a test bagging with perf-film (on a sample profile), not Mylar. I'm a little worried about that trailing edge.

1 piece of MDF makes sense Kenneth - i have tended to use some flat pieces of oak, of various sizes, to spread the load, but just for bagging the outer skin.
MartinE
Posts: 122
Joined: 28 Oct 2021, 14:36
Location: Essex

Re: Schleicher ASH31 4m

Post by MartinE »

Top side of the elevator has been filled and sanded, then 3m putty applied & sanded again (it's good stuff, fills small imperfections really well - BE WARNED, it melts foam).

A light coat of primer was sprayed on, still small areas needing attention.

Countersunk fixing hole also added.
Attachments
20220415_100426.jpg
20220415_102439.jpg
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