Sopwith Pup C of G

[David Monk]

I am currently finishing off my 1/4 scale DB Sport and Scale Sopwith Pup and am uncertain where to set the C of G.

According to the plan it should be at 122mm from the top wing leading edge, which is just less than 25 % of the horizontal distance from the top wing leading edge to the bottom wing trailing edge.

However, I calculated the C&G using 'Method 2' described by David Burton in the May 2012 RCME ( connecting the points which are 25% of the wing chord on the top and bottom wing and projecting upward vertically to the top wing from a point on the line in proportion to the wing areas ...... OK, I never was good at explaining things - have a look at the article!). The answer came out to 155mm back from the top wing leading edge, ie 33mm further back than shown on the plan.

The model is the same as that built by Pete Low and described in RCME. He had to add 2.3 kg of lead to the nose of his to get it to balance (Nov 2012 RCME)- he doesn't state what C of G he used but I assume it was as on the plan.

As I really don't want to cart around that much lead (the total all up weight is only supposed to be 6.8kg!) I would like to set the C of G at the 155mm point, but am a bit nervous about doing it unless someone tells me soothingly that it will be alright!

I would be grateful for any relevant experience with this model?

Thanks, David

[jeff2wings]

Dear ol' D.B. liked to set his models at a very safe c.o.g to make sure that people who made his kits and plans had a good chance of succsses on that maiden flight .However it should fly safely with the c.o.g at 150-55mm in fact some have suggested up to 40mm back from that on the plan !! It would be prudent to use some expo on the elavator though, I use around 40% to start with on my models.

[David Monk]

Thanks for that Jeff. I'll go ahead and set it at about 150mm and see how I get on.

[jeff2wings]

BTW the c.o.g recommendation comes from a Mr G.Whitehead who apparently knows a thing or two about these sort of things ! On a personal experience note (of sorts  a clubmate brought a s/h DB pup at a show last year with a old mk1 Laser 120vt,then sold to another clubmate unflown , who then re-engined it with a OS 91 fs and a extra 6oz of lead (that's all I had in my flight box) the cog a good bit back from the plan recommended position, first flight very twitchy elevator ,second with expo set and reduced elevator travel very plesant to handle.As for Mr Low and 2.3 kg I thought lead paint had been banned ! !

This is a old airframe that has been around for a couple of decades ,first seen at a bring n' buy down Winchester/Alton area

[David Monk]

Ah well, thats even more reassuring. An OS 91 plus 6 ounces doesn't sound like much weight at all. Funnily enough I have put a very old (pre 1990?) Laser 150 in mine - it was reliable in the last model it was in so I think it should be OK. Lovely pictures - biplane magic!