Phil Winks Posted October 12, 2015 Share Posted October 12, 2015 One of my projects for the winter is to convert my Seagull E-Pioneer to a twin allowing me to fit some FPV gear into her with the camera where the old single motor lives. my quandary is between the following 2 prototype designs for the wing motor pods, As you can see it's a choice between conventional nacelles and a pod slung under the wing. the 1st is self explanatory, the advantages I see with the 2nd is the thrust line matches the original and the pod is, I believe, less likely to affect the lift characteristics of the wing. All opinions and advice welcome Quote Link to comment Share on other sites More sharing options...
Colin Leighfield Posted October 12, 2015 Share Posted October 12, 2015 Personally Phil I'd go for the underslung pods, as you say they'll emulate the existing thrust-line and perhaps be a little more efficient as far as the wing is concerned. If you go the other way I don't think there would be any big issues though. Sounds like an interesting project. Quote Link to comment Share on other sites More sharing options...
Phil Winks Posted October 12, 2015 Author Share Posted October 12, 2015 Thanks Colin it certainly will be interesting if only to find out if the conversion improves her, Even though the motors are smaller, they still swing the same diameter prop though they do have a finer pitch, though not by much and the power output combined is a little better but the predicted thrust is about 25% up so she could be a nifty mover if required, on the pod design thanks for confirming my thoughts it's all help full stuff Quote Link to comment Share on other sites More sharing options...
Tim Hooper Posted October 12, 2015 Share Posted October 12, 2015 Phil, Is either design going to be tied in to the wing's internal structure? From the drawings they both look to be attached to the outer skin alone. If you plump for the first design then you may need a little bit of upthrust, due to the high thrustline. tim Quote Link to comment Share on other sites More sharing options...
Phil Winks Posted October 12, 2015 Author Share Posted October 12, 2015 Hi Tim, both will be fixed to one of the ribs with a diagonal brace running from the fix/spar intersection back to the rear wing bolt with an extra cross brace in the fuselage to help spread the load along the wing root and prevent the wings trying to bend forward, on the thrust line you have confirmed what I though about the 1st option and the reason I came up with a lower thrustline alternative, thanks. On the question of adding weight, because I know someone will make this point, I've already tried her with a 6oz lead payload and apart from needing a slightly higher landing/take of speed she behaved very well, so hope fully I can make all the mods without going over that limit, at present it looks like it'll be about a 4oz increase including the FPV gear and extra motor/esc and both pods. Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted October 13, 2015 Share Posted October 13, 2015 Does not the under slung pod bring the props tips that much closer to the ground? The normal prop has the nose wheel quite close behind and central to the plane has to achieve a significant angle before a prop strike. Would the under slung props have the same degree of protection? Quote Link to comment Share on other sites More sharing options...
Allan Bennett Posted October 13, 2015 Share Posted October 13, 2015 I can see the advantage of underslung, but it's my gut feeling that the conventional nacelles would be better simply because of their robustness, and slightly less weight because of the lack of pylon. I'm sure that the difference in thrust line could be compensated for by motor down- or upthrust -- whichever is appropriate. The engine mounting plate could be mounted directly onto the leading-edge spar (with a hole drilled for the protruding motor shaft), with not much extra support structure needed. Edited By Allan Bennett on 13/10/2015 08:30:37 Quote Link to comment Share on other sites More sharing options...
Phil Winks Posted October 13, 2015 Author Share Posted October 13, 2015 Simon the under slung/pylon mount lowers the motor centre to the original thrust line, ie same shaft height as original and as the new motors spin the same diameter but smaller pitch props, the prop to ground clearance is the same.And with the main wheels positioned as they are even a tip over onto the wing tip leaves the prop still clear, only just but clear all the same. Allen I see your point there but remember the E-Pioneer isn't designed for wing mounted motors so where ever I mount them much the same beefing up will be needed and yes the pylon will add a tad more weight but to be fair it's negligible and after my experience with a mobious mounted on the top of the LE breaking the airflow over just under 2", I suspect that although the wing lifts well it doesn't take much to upset it's efficiency and I do want it at it's best possible, on the point about mounting the motor plate directly to the LE I've done a crude mock up and the motors do need to be forward to achieve the correct COG without the need for to much church roof being added Edited By Phil Winks on 13/10/2015 18:02:43 Quote Link to comment Share on other sites More sharing options...
Colin Leighfield Posted October 13, 2015 Share Posted October 13, 2015 You know what you're doing Phil. This is going to work. You can't beat the idea of keeping the upper surface of the wing clean. I know that you can't translate everything from full size aerodynamics down to model size, but one of the reasons that the Lancaster wing was much more efficient than the Halifax was said to be that Chadwick designed it with the engine nacelles low-slung to minimise airflow disturbance over the top. I'm sure that he was right. Quote Link to comment Share on other sites More sharing options...
Chris Bott - Moderator Posted October 14, 2015 Share Posted October 14, 2015 What I found when converting a model to a twin - using leading edge mounted motors - was that rather than upsetting the flow across the wing, the prop wash dramatically increased it. This was borne out by two things. I tried differential throttle, which worked really well for taxiing, but in the air the extra lift from the side with the faster motor was dramatic. I also found that cutting the throttle to transition to glide appeared to dramatically cut lift, so throttle had to be treated gently. Of course this effect may have been caused by a change from prop wash smoothing the flow, to a state where the nacelles were upsetting the flow? I doubt that the under slung option would have these traits. Quote Link to comment Share on other sites More sharing options...
Phil Winks Posted October 14, 2015 Author Share Posted October 14, 2015 Interesting Chris on the issue of increased lift the props on the under-slung variant will be centered just 2" below the wing center and the tip 3 1/2" above it so there will indeed be a goodly area of prop affected air flow above the wing also it occurs to me that with a contra rotating design the upwards traveling portion of the prop vortex will be hitting the underside of the wing out board of the pylon and possibly could have a marked affect on A, lift and B, reducing tip stall, not that B is an issue on this bird in fact quite the reverse the stall is so stable she all but drops straight down in a stall situation Quote Link to comment Share on other sites More sharing options...
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