Author Topic: Brodak Thunderbird build senior moment (Read 5465 times)

RDJeff · « **on:** February 29, 2012, 09:36:41 AM »

D'oh! I started building the wing the other night, and figured out after I had most of the ribs glued in that I built the spar all wrong. When I spliced the spar together, I built it flat on the bottom, when it should have had equal taper top and bottom. I think I can still make it work, but it might look a bit funny...

FLOYD CARTER · « **Reply #1 on:** February 29, 2012, 10:14:35 AM »

Brodak uses a "jig" of balsa wedged under the wing as it is assembled on your flat surface. If you don't use it, the wing comes out crooked! All this can be avoided by using 1/4" rods passing through the airfoil center line. Then, it doesn't mater if the airfoil is tapered or constant.

Floyd

Balsa Butcher · « **Reply #2 on:** February 29, 2012, 11:20:27 AM »

That's too bad. The assembly booklet explains the procedure. If followed it will yield a straight wing. If you have not gone any further than the wing I suggest ordering another set of ribs from Brodak and re-doing it rather than going through all the work to finish the project and find out that it does not fly well. I don't think there is one builder on this forum who has not had to build a wing twice because of one thing or the other. Welcome to the club!

RDJeff · « **Reply #3 on:** February 29, 2012, 12:16:05 PM »

Hmmm, maybe I didn't explain it right. I used a wing jig with the 5/16" rods passing through all the ribs, and the wing is nice and straight. The problem is that as a result of the way I spliced the spar together, the wing thickness tapers from root to tip more on the top than on the bottom. In other words, the wing bottom is nearly flat, while the top surface has most of the taper.

I don't see this addressed anywhere in the instruction book, and just wasn't thinking it through when I put the spar together. The instructions only describe cutting out the spar, but mine was laser cut already.

Balsa Butcher · « **Reply #4 on:** February 29, 2012, 01:03:47 PM »

I made a similar mistake on a Hemstraught PT-19 except it was the bottom that was tapered and the top was flat. The plane flies fine and the flat top/tapered bottom may have helped as the the plane is a low-wing design. The tapered top/flat bottom may give the wing a slightly drooped appearance but if straight, should be OK. If you used a CA glue, liberal doses of debonder and some careful knifework may allow a takeapart and a re-do.

Clint Ormosen · « **Reply #5 on:** February 29, 2012, 06:51:03 PM »

Yeah, I'd say your going to have to redo it. Although it might fly fine, it's going to have the appearance of "droopy" wings. If you'd have put more taper in the bottom rather than the top, at least it would have looked like dihedral wings.

Jim Fruit · « **Reply #6 on:** February 29, 2012, 06:53:23 PM »

I just had my senior moment for February. Thank God the month is over. Maybe some of you might have experienced this before but I stumbled into it blindly.

What I learned was to not use cyanoacrylate while wearing bandaids on your fingers. I have glued my fingers to my work before, but never while wearing a bandaid. I was gluing the ribs into the rudders for my Beech C45J. They are quite small, so I was doing this while holding the assembly with one hand and gluing with the other. The cyano, being clear, ran undetected through the rudder and into the pad of the bandaid on my other hand. You know how cyano reacts, it does not fire right away. This did not also, but when it did it felt like each thread of the pad was firing separately - sort of like a hot glow plug filament against your finger. It sort of happened in slow motion. First one thread would fire and then I would think "glad that's over", then the next one would fire. Ouch! I'll bet it was funny to see a 240# idiot grabbing at his fingertip. The bandaid would not come off but in little bitty pieces.

I am OK. I wish I could say that I am a little smarter now, but that would probably be stretching it.

Jim Fruit

Paul Wood · « **Reply #7 on:** February 29, 2012, 08:33:15 PM »

Can you just turn the wing over? I'm guessing it has unequal span panels, right? Even if it does, with a little minor surgery to the wing panels, you could still make it work.

Paul

RDJeff · « **Reply #8 on:** March 01, 2012, 09:39:35 AM »

I did some careful measuring on it last night, and it turns out it's not as bad as I first thought. With the wing laying on the bench, I measured the center of the leading edge, and found that it is only 1/8" higher at the center than at the tips. It can be seen looking straight at the leading edge, but you might not notice it if you didn't know about it in advance. I decided to flip the wing over and just call it mild dihedral.

Balsa Butcher · « **Reply #9 on:** March 01, 2012, 10:24:34 AM »

Good choice. The dihedral won't hurt and won't be noticeable. T-birds rule!

EddyR · « **Reply #10 on:** March 01, 2012, 10:39:48 AM »

Many of the older designs were built that way except the top was flat and the bottom had the taper in it. Some of DeBolt's planes were that way. You could cut the center section open and raise up one end and it will come out correct. In a jet style stunt plane it looks good the way you have built it.
Ed

Bill Little · « **Reply #11 on:** March 01, 2012, 10:42:48 AM »

Quote from: RDJeff on March 01, 2012, 09:39:35 AM

I did some careful measuring on it last night, and it turns out it's not as bad as I first thought. With the wing laying on the bench, I measured the center of the leading edge, and found that it is only 1/8" higher at the center than at the tips. It can be seen looking straight at the leading edge, but you might not notice it if you didn't know about it in advance. I decided to flip the wing over and just call it mild dihedral.

Hi Jeff,

When we built a Brodak T-Bird, I believe the wings were not equal span, but I could be wrong. If they are unequal in length and you simply flip it over you will have some really weird trim problems to deal with.

BIG Bear
RNMM/Amm

RDJeff · « **Reply #12 on:** March 01, 2012, 12:08:48 PM »

Thanks Bill, I'll have to double check that. There are notes on the plan saying the original had some amount of offset, maybe 1 1/2", but that 3/4" worked better. I don't remember the wing being asymmetrical in that respect though, so maybe the offest was done by just offetting the whole wing. I'll have to look closely at the plan.

I want to thank everyone for their input, it has been helpful! This is my first "complicated" C/L build, never having seen a wing like this before, and I messed up on the very first step!

Randy Powell · « **Reply #13 on:** March 01, 2012, 03:34:44 PM »

As a side note, why is it that CA will ALWAYS stick to your finger better and more quickly than anything else you can name?

Clint Ormosen · « **Reply #14 on:** March 01, 2012, 06:14:08 PM »

Quote from: Randy Powell on March 01, 2012, 03:34:44 PM

As a side note, why is it that CA will ALWAYS stick to your finger better and more quickly than anything else you can name?

Dunno. Was it because it was developed for use in the medical industry? I know they use it now to close cuts.

john e. holliday · « **Reply #15 on:** March 02, 2012, 09:03:30 AM »

That is the story. Was developed for the body tissues that a suture would not hold. I remember using the original Hot Stuff on Bill Wright of Wright/Dunkin F2C fame. He cut his finger real bad. We washed it out with iodine and then as he held the cut closed a drop of Hot Stuff was put on. Never heard a grown man scream so much. Would you beleive it was still holding after the contest.

RDJeff · « **Reply #16 on:** March 02, 2012, 09:24:11 AM »

I think it's all going to work out fine. The plan shows the wing center line matching up with the fuselage side, that's where the wing offset comes from, so it won't matter which way the wing goes in, at least until the bellcrank gets installed.

Thank again for all the help!

Balsa Butcher · « **Reply #17 on:** March 02, 2012, 09:49:31 AM »

An early memory I have of George Aldrich is at an early VSC he cut the heck out of the back of his hand in the prop of his Nobler, yes that Nobler. Blood everywhere. I was a current EMT at the time so grabbed my kit and cleaned it off. Before he would allow me to wrap the wound in gauze he had me hold the edges together and covered it with hot stuff. No screams for him, edges of laceration held. I then dressed the injury. He he took his place back in the line-up and flew an official flight. Tough guy that George.
OH, so as not to hi-jack the thread, if you haven't attached them already, be be very careful with the alignment of those huge T-Bird wingtips. That's where a lot of T-Bird wings go bad.

RDJeff · « **Reply #18 on:** March 02, 2012, 11:40:05 AM »

I really am not happy with those wingtips, and am thinking of other ways to build them. They seem pretty flimsy to me, and look like they will be tough to attach properly. Any good ideas out there?

Balsa Butcher · « **Reply #19 on:** March 02, 2012, 01:20:12 PM »

I haven't made the Brodak T-Bird but did make a Brodak Smoothie. I will assume they are engineered the same. The part that forms the rear curve of the wingtip overlaps and attaches to the inside TE of the wing center section. This is as good a method as any and when finished will be strong.

RDJeff · « **Reply #20 on:** March 02, 2012, 04:31:30 PM »

Maybe I'll add some "riblets" to strengthen it up a bit.

RDJeff · « **Reply #21 on:** March 05, 2012, 02:30:30 PM »

Well, I managed to attach the wingtips without using any riblets. There are balsa blocks to brace both the top and bottom at the LE, and I added a "crutch" to better attach the tips to the TE.

I figured out the booklet is for an earlier version of the kit, as there are several references to building parts that came laser cut with my kit, such as the wing spar.

I'm muddling through it. The wing is all done except for the flaps and linkages, and the fuselage is mostly done now. Fiddling with all the engine cowl stuff is taking some time...