Hi all. Here’s a little update on my project, with picture.
As you can see, there have been a few changes. The wing span is larger, and the wings are now tapered and swept slightly. The latter is to get the wing’s aerodynamic center better positioned compared to the CG (which is farther aft than my early rough calculations showed), and the former is largely to keep the weight of the wing down and improve the rate of climb. The increased span is largely to preserve the range to my 500sm requirement. (The wing tip shape is just for drawing convenience - they'd be more 'raked' in the real thing.) That's a Cessna 152 and a 6' guy ghosted in the background for scale, by the way.
The other big change is the power plant. I started out with an optimistic hope to keep the power required down to a 55hp Hirth 3202. Once I got to more detailed calculations, I discovered that wasn’t enough for an adequate climb rate, so I bumped up to the 65hp Hirth 3203. As I tied in even more accurate formulas into my sizing routine, I kept watching the weight climb and the climb rate drop. Then I ran the numbers for a hot day here in Southern California, especially at Big Bear and Mammoth airports, which are within my design range. Rate of climb sucked, to be honest, even with the 65hp motor. So I’ve baselined the 100hp Hirth F-30S as my new motor. That drove the weight up a little more (through increased fuel consumption and a just-plain-heavier motor), but now I can meet all my range, speed, and runway length requirements, and still have about 800fpm hot-and-high at the maximum takeoff weight. Sizing has stabilized at about what you see here.
The ‘funny’ thing is that I could’ve had a more realistic weight and power estimate early on, if I’d only ‘listened.’ When I started the project, I ran my requirements through Dan Raymer’s ‘rubber engine’ statistical first-order sizing routine and came up with about 1200lbs. maximum takeoff weight and about 100hp power required. That seemed too high to me, so I wrote it off to the fact that his statistical curves were based upon ‘conventional’ airplanes and not a flying wing like mine.
My current MTOW is 1291lbs. and the Hirth F-30S puts out 105hp.
****.
Right now I’m working on getting a tighter drag coefficient estimate for the entire airplane (using Raymer’s component buildup methods) and further refining my empty weight estimates. I’m hoping to recover a little bit of the weight gain through that process. Also, if the stability and trim work (next job) shows that I can reduce the span of the elevons, my maximum lift coefficient with elevons ‘up’ gets larger and my wing can get smaller (and therefore lighter). I’m looking at my canopy design and the engine mount/rear spar attach structure. The canopy is currently a flat-wrap piece attached to a frame, so I don’t need any expensive canopy tooling. Easy and cheap to build, but it’s a little heavy. As for the aft fuselage structure, I’m tinkering with making it a ‘regular’ part of the structure and not a welded space-frame as you see roughed-out here. The engine will have to have a short drive shaft because it’s moved forward slightly and I’ve moved the prop aft to get it farther clear of the wing trailing edge. That shaft will need to be supported at the prop end by a bearing, and it might be lighter/simpler to just extend the fuselage pod structure aft to accommodate that than attach a separate structure. I just don’t know yet. The engine/driveshaft combo ‘ought’ to be in a separate, rigid structure to preserve their relationship to each other, and the whole thing mounted by rubber mounts to the airframe.
As always, your thoughts and comments are more than welcome.
As you can see, there have been a few changes. The wing span is larger, and the wings are now tapered and swept slightly. The latter is to get the wing’s aerodynamic center better positioned compared to the CG (which is farther aft than my early rough calculations showed), and the former is largely to keep the weight of the wing down and improve the rate of climb. The increased span is largely to preserve the range to my 500sm requirement. (The wing tip shape is just for drawing convenience - they'd be more 'raked' in the real thing.) That's a Cessna 152 and a 6' guy ghosted in the background for scale, by the way.
The other big change is the power plant. I started out with an optimistic hope to keep the power required down to a 55hp Hirth 3202. Once I got to more detailed calculations, I discovered that wasn’t enough for an adequate climb rate, so I bumped up to the 65hp Hirth 3203. As I tied in even more accurate formulas into my sizing routine, I kept watching the weight climb and the climb rate drop. Then I ran the numbers for a hot day here in Southern California, especially at Big Bear and Mammoth airports, which are within my design range. Rate of climb sucked, to be honest, even with the 65hp motor. So I’ve baselined the 100hp Hirth F-30S as my new motor. That drove the weight up a little more (through increased fuel consumption and a just-plain-heavier motor), but now I can meet all my range, speed, and runway length requirements, and still have about 800fpm hot-and-high at the maximum takeoff weight. Sizing has stabilized at about what you see here.
The ‘funny’ thing is that I could’ve had a more realistic weight and power estimate early on, if I’d only ‘listened.’ When I started the project, I ran my requirements through Dan Raymer’s ‘rubber engine’ statistical first-order sizing routine and came up with about 1200lbs. maximum takeoff weight and about 100hp power required. That seemed too high to me, so I wrote it off to the fact that his statistical curves were based upon ‘conventional’ airplanes and not a flying wing like mine.
My current MTOW is 1291lbs. and the Hirth F-30S puts out 105hp.
****.
Right now I’m working on getting a tighter drag coefficient estimate for the entire airplane (using Raymer’s component buildup methods) and further refining my empty weight estimates. I’m hoping to recover a little bit of the weight gain through that process. Also, if the stability and trim work (next job) shows that I can reduce the span of the elevons, my maximum lift coefficient with elevons ‘up’ gets larger and my wing can get smaller (and therefore lighter). I’m looking at my canopy design and the engine mount/rear spar attach structure. The canopy is currently a flat-wrap piece attached to a frame, so I don’t need any expensive canopy tooling. Easy and cheap to build, but it’s a little heavy. As for the aft fuselage structure, I’m tinkering with making it a ‘regular’ part of the structure and not a welded space-frame as you see roughed-out here. The engine will have to have a short drive shaft because it’s moved forward slightly and I’ve moved the prop aft to get it farther clear of the wing trailing edge. That shaft will need to be supported at the prop end by a bearing, and it might be lighter/simpler to just extend the fuselage pod structure aft to accommodate that than attach a separate structure. I just don’t know yet. The engine/driveshaft combo ‘ought’ to be in a separate, rigid structure to preserve their relationship to each other, and the whole thing mounted by rubber mounts to the airframe.
As always, your thoughts and comments are more than welcome.
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