hi kenny
just some thaughts on the centre section that may or may not suit the reason you have split the wing into 3.

if the centre section span was round 8 foot...you could get it into a tralor....perhaps even a longer span centre section would fit into a tralor if the fuselge and centre section were on a 45 deg angle or even rotated to 90 deg.

there is the posibility the centre section could stay permanently with the fuselage.this may allow you to use perminant carbon attachments and emit metal fittings and bolts and also the holes that the bolts need/week points.

perhaps the reason sailplane manufacturere split a wing into 2 sections comes down to packed size??

in ither case i thaught you would have stayed with the root airfoil for the centre section and stayed with a thiker spar.the idea to split the wing into 3 was for strenth through the root section and to hopefully pick up a weight saving?yer?with a view to carrying a hevier pilot.

russ.




On Tue, Mar 27, 2012 at 12:02 PM, Kenny <This email address is being protected from spambots. You need JavaScript enabled to view it.> wrote:


So, I've been looking a little bit at the wing profile in the context of making a 3-piece wing. I also assumed the split is made at rib 5, WBL 84 (wing butt line) and a 24 inch wide cockpit is assumed, the overall wing length keeping the same overall wing span. That would make the center wing 16 feet in length, with the outboard wing panels 14 ft each. One thing not obvious to the non-analyst is that the overall wing is a 4-point bending configuration; so, between the wing attachments at the cockpit there is no shear, only bending. In the case of the 3-piece wing however, there is both bending and shear at these joints. Maybe not a huge deal, but it has to be accounted for. This results in a slightly shorter wing than the original (252 in. in the revised condition, whereas the original is 256.5 in). There's your clipped wing Russ!

I calculated the original area as:

A = 256.5(60+22)/2 = 10516 sq inches per wing or 146 sq ft. total

I then back-calculated the constant section required to get the same area.

10516 = 84x + 168(x+22)/2

X = 51.6 in

However, at that cord-length the spar height has also been reduced some 16%, which increases the spar cap loads as well as the shear-flow in the spar web. So, what I'm thinking is that this might not be a bad time to bump up just a little bit the area to recover a bit of the increased pilot and chute weight. And, I don't think there should be SO much weight penalty as the spar height is a bit higher and the addition to the D-tube minimal (which is where most of the weight will come from). If the center section constant-cord is set to 56 in. with the tip kept the same 22 in. and the wing length at 252 in. due to the slightly larger cockpit (if I'm reading that correctly), then the total wing area is bumped up 7% (keep in mind the gross weigh has gone up a bit more than 20% so we still don't recover all of it, but a bit of it, and I think for pretty much no weight penalty. There is more weight penalty for making the outboard joints).

I calculate the original weight at:

155 Lb AC weight + 150 Lb pilot + 10 Lb throw-chute = 315 Lb

Revised weight is:

155 Lb AC weight + 200 Lb Pilot + 25 Lb ballistic-chute = 380 Lb

I set the pilot weight to 200 Lb for this portion of the exercise assuming that it could go a bit more than that and it will be stressed a bit more than that, but I like the idea of keeping the wing loading as low as practically possible. Also, what I'm thinking now is to reduce the actual load factors to 4g limit, 6 g ultimate with a 240 Lb pilot. Since I'm using fairly low allowables for the carbon rod, this should be sufficient for a safe aircraft without getting nuts.

Any thoughts, suggestions?

Kenny