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Scale rescale

Modifying a scale plan

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kevin b10/01/2017 20:20:56
1447 forum posts
150 photos

I wonder if anyone can offer advice please.

I have been looking for a nice scale biplane to put my OS 40FS in. Something a bit "different", but vintage.

I have found a nice plan ate 41ins span (1/9th scale), but it is for a 45-60 four stroke.

I wondered if I reduced the scale down to 37.5ins (1/10th scale) and build light, would that be about right for my engine ? Scaling down plans is not a problem to me.

I did consider trying to build the original very light, but of course there is the drag factor as well.

It's curious that when I look at the old free flight plans the engines are all under half the size of the modern RC equivalents, but I suppose they weren't intended to perform aerobatics (intentionally) !

Denis Watkins10/01/2017 20:32:51
1874 forum posts
94 photos

You are on the right track Kevin, I fly 3, .40 4 strokes and they all comfortably fly 4.5 lbs, and I estimate 0.6hp to be about 450 watts. All 3 models are high wingers, but one is 50inch span. A biplane as you know doubles on wing drag, but you are in the ballpark and should consider weight also. I ran one motor in on a Boomerang, it was not sprightly, but it served as a platform, and that is 62inch at 5lb, or more. I would build the 41inch light as you say, as below that wingspan, flying can become a little twichy.

Edited By Denis Watkins on 10/01/2017 20:35:07

kc11/01/2017 19:49:31
5197 forum posts
159 photos

I would expect an OS 40FS to be fine for a 41inch span biplane or up to about 48 inch span, depending on the performance required. Only a bipe like the Pitts or Ultimate that need excessive power for vertical performance would need larger.

Early 4 strokes were not as powerful as later models so that might account for the 45 to 60 being specified, however it might also be because the model had a very short nose and used the weight of the larger engine for that reason.

Martin Harris11/01/2017 19:57:16
6995 forum posts
171 photos

My Puppeteer (60" biplane weighing 8 lbs according to Flair) flew perfectly well on an OS 40 FS with no meaningful compression - so your one should be a doddle (even unscaled) unless you're looking at Pitts performance!

kevin b11/01/2017 20:20:45
1447 forum posts
150 photos

Thanks for the information guys.

I think I will go with the plan as is. If I build light and it is ok, fine. If not I could always put a bigger engine in it. I have a spare Irvine 60 2 stroke !!!

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