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Grumpy Tiger Cub Increased Size


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Having just come across this design thought I would have a go at building an IC powered twin.

 

Thought at first I would be able to get a short kit from Sarik or other suppliers but none seem available now. Therefore if I am going to cut my own ribs possible best if I scale up the design to approx 60 inch to give me more options on the motors to use.

 

To save me further work therefore looking for a CAD drawing of the original plan, or indeed a scaled up drawing, can anyone assist?

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Its rather a long story, but I own the cutting files (not the drawings as they are Peters) for the standard GTC which SLEC can produce. Chris at SLEC has rescaled other vplans for me in the past so I don't think it would be an issue.

Message me if you are interested as it might save you time and money in the long run.

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Well in the absence of any easy way to generate cutting files for the ribs I made a start today on importing a copy of the original pdf plan (thanks Peter) into Inkscape.

 

Increasing both plans by 10% gives me just shy of 60 inch wing span with little noticeable error on the spar dimensions which will remain as 1/4inch square but probably selected  from a hard wood rather than balsa.

 

Fuselage length is just over 36inch now which is a shame but since my laser cutter can only cut just over 14 inches the sides will have to be cut using a paper template but that is no big issue.

 

Continue to trawl good old ebay for suitable engines so would be interested in what others have used for a 60" version. Suspect a pair of 0.15 would still be ok as not looking for a hot ship to fly.

 

Have read several old threads on this model build but for the life of me I don't see an easy way to build a semi symmetrical wing profile that tapers so I am open to further advice on suitable methods without reverting to some form of jig.

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  • 2 weeks later...

Scaled up plans arrived yesterday, so it's just a matter of now checking the chord dimensions of each rib before generating a cutting file.

Have purchased one OS 20 Max for the build but not 100% happy with it at the moment as it took a long time to get it running so may have to do further work on it to ensure some degree of reliable running. 

Two more engines should arrive this week so sure I can get two good engines out of the three,

Still lost as to the method of building the wing given the taper and symmetrical ribs so will have to play with a dry assembly before adding glue.

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Tapered wings, you'll need a jig of some sorts, can't do it without, but it doesn't have to be complex. I use the most basic of jigs possible - one strip of 3/16" med/soft sheet (or similar) near the TE. The strip will be slightly tapered, thicker at root end and thinner at tip.

 

Draw centre lines on each rib, this helps with alignment. The spar is pinned to the board. Jig strip goes under the TE. Make doubly sure the LE of the ribs lines up with the LE strips (any errors in rib thickness around the spar get magnified) and the TE of the ribs lines up with the TE strips. Make sure the root and tip are parallel to the board. Adjust the jig strip until the above is right. Mark where you need to pin it on the plan - you'll need to repeat the position later (probably). Then hit everything with thin cyano, and it's all done.

 

When adding any sheeting, you'll need the jig strip in the same place again. Just hold the wing panel in place while you're doing it (pins are a bit difficult to extract if there is sheeting in the way). Contact adhesive makes this step easy and quick - you can just gently press the sheeting into place while holding the wing down on the board and jig strip. If you're doing a D box and the rear is open you could probably pin the whole thing down whilst doing the sheeting which avoids needing to hold it in place.

 

Other methods are possible. The above works for me. The key is double checking everything whilst you're getting the jig in the right place, once that's done it is plain sailing.

 

As to engine size, I did some quick sums on a 60" GTC - you're looking at about 650sq in of wing, and I would guess around 6 pound weight - i.e. something larger than the average Acrowot which usually gets a good single 45 or 55 two stroke. Two 20 FPs would be way undercooking it, I think, which isn't ideal on a twin. I'd look for a good pair of 25s (at least) or 30s (preferably).

 

Edited by Nigel R
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Have read the Tim's thread but couldn't get quite understand the build process he described. If the lower spar & TE are pined down to the build board then all the ribs would have to be inclined at a slight angle to ensure they are perpendicular when the wing is removed from the board given the wing tapers in depth. 

If the ribs are not correctly set then each engine pod sides are going to be twisted away from the vertical.

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Andy, you are totally overthinking it. IIRC

  1. Put plan on building board
  2. Place plastic sheet to protect drawing - optional
  3. Pin lower sheet and spar to board
  4. Place and glue ribs, don't forget the sheer webs and servo cable tube
  5. Sheet top wing
  6. Fit wing tips and center dihedral brace
  7. Build other wing, glue together, build fuz and nacelles
  8. Add ancillaries and cover
  9. Set throws and C of G 
  10. Go fly and enjoy it ?

There are at least a couple of build blogs on here of which one was an over size with extras added in

 

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image.thumb.png.f292ec97c9b2e872e35b83ecead8035e.png

 

image.thumb.png.8540a613a63a2d9be7cd3648028089be.png

 

 

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5 hours ago, Nigel R said:

As to engine size, I did some quick sums on a 60" GTC - you're looking at about 650sq in of wing, and I would guess around 6 pound weight - i.e. something larger than the average Acrowot which usually gets a good single 45 or 55 two stroke. Two 20 FPs would be way undercooking it, I think, which isn't ideal on a twin. I'd look for a good pair of 25s (at least) or 30s (preferably).

 

May have dropped a bit of a clanger regarding engine size. Certainly not looking for a hot ship and many said the 53 inch version flew like a home lost angel on a pair of 15's.

So do you think I should stick with the 20's or go looking for two 30's.

Was not going to beef up the structure much. Perhaps 1/8 for the fuselage sides and sticking with 1/16 skin for the wing. Spars will be 1/4 spruce. 

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Depends how light you build I guess. Twin in my avatar is 6-3/4lb and is 700sq in, I used two 40FPs and I would not say it is overpowered... but it is certainly not underpowered. Couple of 25FX or similar would seem about right to my mind for 60" GTC.

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Well making some progress now with £90 of balsa ordered today from SLEC. Rib cutting file is now ready for export to the cutter.  Just a matter of pressing go when I get the delivery of wood as would like to double check the fit of the 1/4 inch spruce spar on a sample rib or two.

 

 

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Well the balsa arrived today so started to do some rib cutting. First two ribs went OK but then problems started with one file only cutting 50% of the path. Tried again on another rib but the problem persisted.

Currently at a loss as to the reason why but will take a lòk at the grbl settings tomorrow.

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Well after a day of smelling the house out with balsa wood smoke whilst cutting out the ribs I made a start on the assembly today.

 

Only one rid missed in error and one rib 2mm short so not too bad a start.

 

Assembly is following the method of 'just build it flat' although I did prop up each rib by 1/16 to allow the underside sheeting to cover the rear 1/4" TE spar.  Will probably do a similar thing with the LE so that the 1/16 sheet goes over the front 1/8 LE and then the second 1/8th strip buts up against both top and bottom sheeting.

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Good progress made over the last few days. Wing is made along with the nacelle sides. As can be seen have departed from the plan with a fully sheeted wing and to avoid scarfing in the nacelles each side goes full depth into the wing which in theory should be a lot easier.

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Bit too windy for flying Nigel so made good use of my time! More done today with the balsa and ply nacelle sides laminated together, plus made a start on the rear stabiliser which will be an open framework rather than solid balsa.

 

Missed out on a SC32 engine on ebay at the weekend, so think a pair of OS 25's will have to be purchased as there are quite a few up for sale at reasonable prices.

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Gone as far as I can go on the wing without some more 1/16 balsa, so made a start on the fuselage today. Basic bulkhead formers at the wing location cut out and a 1/8 balsa doubler added for each fuselage side rather than using ply as I cant see the need for additional strength across the wing chord.

 

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  • 3 weeks later...

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