trying to make on scratch build a rubber powered 12inch wingspan model, i think the propeller should be about 1/3 of wingspan , please correct me if i am wrong,
not able to decide what should be the rubber motor size. once i know that, will try to get it from some online order.=pravin
(so far what i have tried is to cut strips of rubber tubing used for catapults, not giving  very good results)

Yesa  third WS for prop is a  good start You can start with 1/16 rubber should be plenty. The length should be 30 % of the ready to fly plane. If it does not climb you have 2 choices : Try a second loop  of 1/16 (my prfrence) or a 1/8 loop single loop. Good luck, let us know how it turns out.

What type of rubber tubing you cut ? Cycle rubber will not certainly work. Rubber toundicate pipe will not be properly cut by stripper but rubber is like TAN Sports.

 I have checked some TAN Rubber (Given by Saikat da) and Nylon rubber too. TAN Sports rubbers are not available in small. If you are buying, You can sell some of them to here so that we can purchase. You may try some rubber bands (Nylon Rubber Bands, some off-white transparent color) gives medium result, but you have to add lubricant s that it wont dry, otherwise, it will soon broken. However this nylon bands are very cheap (Rs. 38/kg) I purchased. I have some, and you can try it.
Micro calculation is tough and so tough the duration. Just try 3mm Balsa with 16" wingspan and single loop 1/8th motor. Use 2/3rd Fuse length as motor length.
You can use either 1.5mm balsa as wing  or can make with the same 3mm balsa structure and covoring.
If you have balsa supply I can send you some of proven plane like Veron and others. Even with cut shapes from Balsa Strip. Just PM me your e-mail ID (Preferably G-Mail).

And please post some of your model pics.
A few files attached herewith.
But SORRY as I have no download link in archive from where I downloaded previously, and so cant' refer right now.

Thanks

free flight and kalyanprodhan, thanks for reply,
what i was looking for was, --i am interested in 12inch to 16inch wingspan, so if i have to order rubber for this, what size should i need to order--
 minimum quantity , i think is around 1/2lbs, so if kalyan sir, can guide me for the apprproprate size then i can put a order for the same.
=pravin

Hi,

Please check the attached files. there are more. I need ur mail ID for sending as some more size may be copyrighted and so can not post publicly till I become sure.

thanks

Hi Pravin,

It is good that you are experimenting with your own design. See if you can get this book, well worth for future refrence too :

http://www.ebay.com/itm/Don-Ross-book-building-and-flying-rubber-powered-model-airplanes-/330745079178

Rubber i very vital for flight. So if you wan to get it from FAI model supply. Geta 1/4 pound box of 1/8 and 3/32 and should covera large spectrum of models. Will post my video of a an old rubber power plane. Good luck.