David Hayward Posted December 2, 2015 Share Posted December 2, 2015 As I have previously mentioned under a discussion about ‘grippers’ versus ‘turnbuckles’ for the rigging flying wires, I am in the early stages of building a DB Sport & Scale ¼-scale Sopwith Pup with an inverted Laser 155 4-stroke. As has been previously highlighted at http://www.ivyandmartin.demon.co.uk/pup_review.htm, as a flying model, it has an excellent reputation but, as a scale replica, it leaves a lot to be desired - 1) the nose is too long due to the long-chord aluminium cowl which has no doubt been done to minimise the amount of lead ballast the model may need to carry to achieve a satisfactory balance; and 2), the tailplane and fin/rudder are significantly oversized. Item 1) can be rectified by cutting the excess material off the back of the cowl (approx. 15-20mm say), mounting it immediately ahead of the front-section struts without the original front bulkhead, or at least with the centre of the latter cut right out. A new ply former can then be set inside the fuselage sufficiently far back to accommodate glass-reinforced engine bearers or, depending on the engine selected, to radial-mount the engine with the prop-driver just forward of the cowl. This approach also does away with the kit’s integral ply fuel tank boxing and integrated ply engine bearer with built-in 3 deg. downthrust. However, with a new ply bulkhead set back, the latter downthrust can be readily achieved by slightly angling the bulkhead during its fixing. Arguably, the downside of adopting the above cowl/bulkhead position modification is that more lead ballast will be required to achieve the require balance. However, in view of the long fuselage (moment arm) behind the wings, this latter problem can neatly be minimised by addressing Item 2), noting that a small decrease in tail weight = a much more significant reduction in upfront lead ballast which will otherwise be needed, particularly for a short-nosed model - i.e. by reducing somewhat the size of the tailplane and the fin/rudder (by approx. 8-9% and 11-13% respectively in order to match true scale proportions). My question is, will such a reduction in the tailplane/fin/rudder area be detrimental to the flying characteristics of this biplane model? The above-mentioned ‘ivyandmartin.demon’ link suggests ‘definitely not’ (and even talks about ‘further improvements’ involving modifying the shape of the wing ribs to introduce undercamber, which I’m not planning to do) but no supporting actual flying performance comments are mentioned to back up the suggested rear end modification. Also, this link refers to the original DB ¼-scale Pup design with two fixed wings rather than the latest design involving four-wing halves with working flying and landing wires. I don’t know whether any of the key dimensions of the original design have since changed - perhaps not? As a compromise, I could of course just leave the rear end of the model alone and just undertake the cowl modification described and then just accept whatever lead ballast penalty is required. But I would prefer to maximise the model’s scale appearance overall as long as I won’t be jeopardising its flying performance. Does anyone have actual flying experience with this model having undertaken the modifications described under item 1) and 2) above? All comments gratefully received. Sorry the above is a bit long but I wanted to get my concerns clearly across! David Edited By David Hayward on 02/12/2015 01:17:29 Quote Link to comment Share on other sites More sharing options...
Jon H Posted December 2, 2015 Share Posted December 2, 2015 Hi David I have not yet modified the DB pup but the mick reeves 1/4 scale and the other exact scale 1/4 pups available as plans all fly just fine so i would not be concerned by the reduction of tail area. Most likely the smaller area will make the model marginally less stable but given the generous tail area of the full size i very much doubt it will be any problem at all. As i think we discussed when you bought the engine i have both the DB and the MR pups to build and was planning to do these mods to the DB just to bring it slightly closer to scale. I am not looking to win the nats with it so as long as it looks like a pup at 10 paces i am happy with that Quote Link to comment Share on other sites More sharing options...
Don Fry Posted December 2, 2015 Share Posted December 2, 2015 Just a thought, but does not the area of the tail have a bearing on the centre of gravity, from memory, increasing the tail area brings the centre of gravity back. There are formula for working it out. Best done before take off. Quote Link to comment Share on other sites More sharing options...
Ken Lighten Posted December 2, 2015 Share Posted December 2, 2015 I've always felt that the tail plane is over engineered and could be lightened quite easily by using a more conventional approach to its structure, also, the fin and rudder could be lightened by using a laminated outline - my opinion only obviously! Quote Link to comment Share on other sites More sharing options...
Cabbage Man Posted December 2, 2015 Share Posted December 2, 2015 The design is from an era when under powering was common and every possible trick was needed to provide safe flight. This included changing the rib profile and enlarging tail sections. I flew the DB Pup and it was ultra stable to the point it was a dog to get it to do anything lively. When I flew a 1/4 scale Reeves version, the difference was immense. A proper aerobatic war machine. Id have no quibbles loosing as much weight as possible, thinning the rib profile and changing the tail profile. It will fly better. Quote Link to comment Share on other sites More sharing options...
David Hayward Posted December 3, 2015 Author Share Posted December 3, 2015 Many thanks for all your responses. These give me the confidence to go ahead with the tailplane end mods as well as the cowl mods I have described. I have reservations about going further though, i.e. thinning the wing rib profile in view of the extra work required, particularly if undercamber is included which is moving into virgin territory for me and would make covering the underside more tricky. Also, I won’t be looking to achieve extreme aerobatics with this model! Like Jon (Laser Engines) says, so long as the model looks like a Pup ‘at 10 paces’, I shall be happy! As for the effect on C of G position following cowl + tailplane end mods mentioned by Donald Fry, all the C of G calculators I have come across to date only seem to allow plugging in of wing dimensions, not those of the tailplane (and fuselage) as well - am I missing something? Post-mods, I was thinking of simply sticking with the original design’s (safe) C of G of 122mm back from the top wing LE initially, as indicated on the kit’s fuselage plan, but mindful that other threads on this subject on this Forum (Feb 2013) have talked of up to 150-155mm, or even 162mm back from the LE, albeit with expo built into the elevator control set-up; moving the C of G back of course also helps to minimise extra lead ballast at the front. Quote Link to comment Share on other sites More sharing options...
Don Fry Posted December 3, 2015 Share Posted December 3, 2015 David, I have dug out the relevant pages of Gordon Whitehead, Scale Aircraft, and sent them to you in a PM. As I remember, you will need more weight up front to get it to balance with the smaller tail area. Quote Link to comment Share on other sites More sharing options...
David Hayward Posted December 4, 2015 Author Share Posted December 4, 2015 Donald - Thank you for your PM and associated reference; I have replied to you directly. Quote Link to comment Share on other sites More sharing options...
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