Kelly W
Posts: 863
Joined: 1/21/2002
From: Delta,
BC, CANADA
Status: offline
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No, there really wasn't anything distinctly new about the design. I think the key to the strategy for this year was the 10lb maximum empty weight, as apposed to last year's optimum weight with a max displacement rule.
Taking a step back... Last year, there was a max displacement of 0.92 cu in, using however many engines you wished to use, of any brand, modified or stock, etc... Some teams went with .91 2-strokes and optimized the prop, added gearboxes, etc... The really competitive teams used a pair of Jett 46's with tunes exhausts. The top 2 (Akron and Missouri-Rolla) both had variations of this combo. They were achieving in the range of 22lb of static thrust with custom modified Bolly props, large diameter and very low pitch.
Now on to this year...
Both of these teams used Jett engines again, Acron used the same engines as last year, while Rolla switched to a brand new set of BSE 60's since the weigh increase was negligible. (ironically, our team's Jett 76L was lighter that a 60, although the reliability may have been an issue from pushing a 46 size crank case so far...) Each of those teams then proceeded to optimize their last year's concept by reducing the weight down from ~14lb to ~10.
UBC's approach was a clean slate, using absolutely nothing from last years design as it really wasn't competitive. The first thing they nailed down was a strict weight budget. We all sat down together and ran numbers of what each control surface, fuselage component, etc should weigh, and they stuck to it as best they could. If a component came in over budget, it was scrapped or in some cases the budget was deemed to be inaccurate and therefore it was adjusted. This is basically the way things are done on the real thing so it proved to be an excellent learning experience for them. When considering the budget, it became evident that they could get 2 of these marvellous Jett BSE Fire 76L engines (@13oz each+pipe and hardware) and achieve 30+ lb of thrust! The downside is the amount of weight it consumed from the budget, although it was tempting to do a twin. The chosen alternative was go with the best 'power to weight' option available (the Jett 76L) and make the best (larger and most efficient) wing possible... In the grand scheme of things, wing loading is the number 1 factor, raw thrust is 2nd according to the 2005 rules... More engine would still produce a competitive aircraft, but takeoff and landing speeds would go way up by the calculations, due to the proportionally smaller wing.
The wing was constructed from a small carbon spar on top and bottom. The spars were spaced apart by a closed-cell PVC foam structure and a layer of fibreglass (vac bagged in place with epoxy) was utilised as a shear web on each side of the foam. the wing eventually came out to just under 4lb and we estimated its strength to be in the range of 200lb of payload centrally mounted. The entire design was done to accommodate a 4g gust or landing load. The wing was a multi-tapered design that had a tremendous chord in the centre section, thus allowing a very large moment of inertia in the spar. The guys also spent a great deal of time analysing airfoils and came up with a very effective lifting surface, although the downside was suppose to be a nasty tip stall tendency. I had to stay conscious of the tip stall threat and use spoilers at the tips to avoid stall at times.
The fuselage was constructed primarily of the same carbon used in the spar. It was built as a long truss, optimized in the cross member placement by a larger iterative spreadsheet that calculated the buckling case in each position. As the lengths changed, the truss member positions changed, thus changing the lever arm from the tail surface, again changing the buckling case... They then used a few analysis techniques to appropriately size the Kevlar webs across each truss segment and built it to comply with torsion case from max deflection of the rudder and elevator at the 'do not exceed' speed limit. The fuse weighed in at ~1 lb.
Overall, it was an exercise in optimizing and eliminating redundant material. Use of Kevlar, carbon fibre, THIN plywood, and a variety of appropriate industrial adhesives (from Loctite-Canada, a sponsor...) were some of the keys to the deign.
I'll post more pictures in the RCCanada thread, linked in the 1st post of this thread.
Kelly
< Message edited by Kelly W -- 5/5/2005 5:17:46 AM >
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Kelly Williams, GTBA #1969
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All Forums >> RC Airplanes >> M.A.A.C. >> SAE Heavy Lift 
SAE Heavy Lift - 
) just in case another team scored what we thought was a competitive flight. Based on the 48.8 lb flight characteristics and the numbers we ran at that density altitude, the 58lb would have been tough but doable. This kind of competition sure teaches a pilot a LOT about energy management in R/C models. With 58lb on board, that would have been a payload:aircraft weight ratio of just over 6:1. That's way up there in terms of the SAE contest history from what I've heard. the 48.8 lift was just over 5:1. 
