B.O.T. Polyhedrial Modification

[ForeverFlying]

I am thinking of modifying my Bird of Time wing to produce better handling. It doesn't have a lot of "dihedral", and I have noticed the newer designs have an extra "break" in the polyhedral. I was thinking of making another break on each side, a little over midway out on the far panel, kind of like the Bubble Dancer, or AVA. Will this have the desired effect?

Thanks!:)

[Ollie]

"Better" is relative. It will increase the yaw to roll response. That may be what you are after. It will reduce the tip stall margin in turns. You can accept this or compensate with a little washout in the mid pane. However the wash out will reduce the glide angle a little in at high speeds (penetration). It is a matter of trade off and priority.

[markdrela]

Originally posted by ForeverFlying
I was thinking of making another break on each side, a little over midway out on the far panel, kind of like the Bubble Dancer, or AVA. Will this have the desired effect?


I wouldn't to that. Besides killing the classic BOT lines, the new outer poly break may cause tip stall problems on the pointy tip planform.

The best way to add more dihedral to the BOT is at the existing breaks. Either at the center break, or the outer breaks, or both.

[BMatthews]

Perhaps a different solution that results in the same effect. I built an electric fuselage for a set of glider wings and stabs and made the tail moment a bit longer on the electric version. The longer tail handled SO much better than the short tailed version that I could not believe the difference. And all thanks to an extra 1 1/2 inches. I also made the fin and rudder about 10% SMALLER to compensate for the extra moment arm and keep the vertical tail volume coefficient close to the same. The model in question is an RO-8 and the original had a VERY large find and rudder. Hence the reduction for the longer version. The longer tailed version snaps to commands much more crisply and is more neutral in the turns. The short version tended to level and leave in thermals if the speed dropped too far but the longer tailed version has no sign of this trait. It just holds turns at any speed that will maintain flight like it's on rails.

An extra 1 1/2 or 2 inches in the tail and an inch in the nose (BoT's can tend to be hard to balance at the best of times from what I've read, mine included) may provide the extra crispness in turn entry and gooviness in the turns that you are looking for without any dihedral changes. Certainly when you compare the old school very short fuselage of the BoT to some of todays RES models the shortness of the fuselage is very noticable.

PS: My BoT does tend to tip stall if I really start horsing it around at lower speeds and that is despite the generously sanded in tip washout where it transitions to a symetrical airfoil over the last 6 inches of the panel. So Mark is right in that you probably don't want to mess with the upturned tip idea. Not to mention it would look funny unless you could do it by bending it up over the last foot in a smooth curve.

[ForeverFlying]

Thanks B. I had no idea that added length would have that effect. The groviness in turns that you describe is just what I am looking for. It tends to require control input to maintain a thermal turn. Certainly lengthening the fuse is the easiest fix!

[BMatthews]

I should have added that YMMV of course. After all this is all based on only one sample. But the results in my case were SO positive that I would have no concerns about lengthening the moment arms as I suggested. My BoT has similar handling charactaristics to the short tailed RO8 so I suspect it can be made better with a stretch mod. I should add that my BoT is heavier than most thanks to a couple of major repairs in it's past and the use of fiberglass on the fuselage, fully sheeted wing center panels and flaps on the center panels. AUW on mine is about 65 oz so it's no lightweight. The extra weight may be prompting the poor slow speed turning charactaristics. OTOH if you have a 45'ish BoT that shows signs of this same desire to level and leave thermals when circling at low speeds then a stretch job will probably help.

Iin any event it can't hurt.

[winchdoc]

Dave Thornburg said to keep the wingtips LIGHT! He said each wing panel should balance between root ant tip at a certain inboard point. The less wingtip inertia to deal with, the better it would handle. I am always amazed that the BOT was also an early F3B ship!

WinchDoc

[schrederman]

I built one that came out at 44 oz. and was winchproof. I really loved the way it handled. I never had problems with it falling out, even low and stood on the wingtip with the stick full back. The Eppler 205 airfoil seems to do better at the lighter loading. I was very careful to make my tip panels as light as possible. They were 3.7 oz. each but still bulletproof. I sold it in a weak moment to Walter Higgins.....

Jack Womack