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Andy Blackburn18/01/2014 22:41:34
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I thought it might be useful to add a few additional Jet Provost building notes, collated from various questions that people have asked.

Fuselage
There is a lot of wood in the fuselage for what is quite a small model, there will be lot of carving, whittling and sanding to shape; this is the JP I'm building to replace the one sold (unwisely) to Scott Edwards:

jp_16_mar_2013.jpg

quite a lot of wood has to come off to get the shape right; don’t rush it, but be bold. After much bold razor-planing and the production of many shavings...

balsa-shavings.jpg

..and after a rough sanding we will eventually end up with something looking a bit like this:

carved-fuselage.jpg

There's still quite a bit of wood there, even though a fair proportion has been removed.

Bearing all this in mind, I strongly suggest using soft and light wood for the fuselage (around 6 lb cu ft); if you're going to steal the wife's digital kitchen scales (much more exciting than buying your own) to weigh wood in the model shop, here are some wood weights (36" length) that you might like to aim for:

3/16" x 4" : 44 grams
1/8" x 4" : 28 grams
3/8" x 4" : 85 grams
1/4" x 4" : 57 grams
1/2" x 4" : 114 grams
1" triangle : 25-28 grams
3/4" triangle: 14-16 grams

If you don't want to go to the trouble of weighing it yourself and haven't got a stock of wood already, or if someone (probably me) has got to the local model shop before you and swiped all the good stuff, you could do worse than talk to the Balsa Cabin; they charge more for the very light wood ( less than about 6.5 lb cu ft, or what they call "contest grade" ), but - IMHO, anyway - it's worth it.

Wings
It's best if the wing skins are made from medium-hard (12 lb / cu ft) balsa sheet because there are no spars to speak of and the wing skins carry the bulk of the bending stress;

The dihedral brace fits in a slot in the top and bottom wing surfaces, so that the brace is properly bonded to both top and bottom wing skins and the shear webs; it wasn't drawn on the plan because it was made oversize, projecting right the way through the top and bottom surface and glued with PVA, and the excess ply sanded off afterwards.

The washout wedge is made from some scrap sheet ( circa 1/8" ) and extends from R1 (zero thickness) to R10 (0.8mm thick); cutting something that small sounds suicidal but it's actually quite straight forward with a) a bit of double-sided tape on the bottom of the straight-edge to stop it sliding around, and b) lots of shallow cuts.

The ailerons are shaped in situ like this:

aileron-small.jpg

The aileron blanks are spot-glued in place and carved/sanded to shape to match the wing surface, which is protected with masking tape; the hard balsa trailing edges do help avoid dings/dents, and also seem to make the aileron more rigid.

Here are some wood weights (36" length) that equate to wood of around 12 lb / cu ft):

Wing skins ( 1/16" x 4" ) : ~ 28 grams
3/32" sq : just pick the hardest, or strip from 43 gram 3/32" x 4"
L.E. : cut from ~ 125 grams 1/4" x 4"
T.E. : cut from 85 grams 3/16" x 4"
Ailerons : these can be made from the soft 1/2" sheet bought for the fuselage

Target Component Weights
These target weights might be useful when deciding whether to replace some of the wood in the wood pack;

  • Wing : circa 4 - 5 oz, but it's difficult to make it much heavier and it's not critical because it's not a large proportion of the total weight.
  • Tail surfaces : circa 1 oz
  • Tip tanks : aim for around 1 oz for the pair.
  • Intakes : as light as you can get (circa 1 oz)
  • Fuselage : about 8.5 oz; if yours is going to be a heavy model, most of the extra weight will be in the fuselage. And I suspect that most of that will be in the triangle stock that's behind the c.g.

 

Edited By Andy Blackburn on 18/01/2014 22:50:40

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