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Brodak F-82 Twin Mustang

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Brodak F-82 Twin Mustang

Post  Ken Cook on Sun 1 Jul 2018 - 17:45

I got a good start on this at the beginning of the week and I hoped to complete it this weekend. Well, it was too hot and there was just many problems with this kit. All the newer kits are laser cut and I had this one for probably more than 10 years now. From poor wood selection to ribs that wouldn't fit, this took the wind out of my sails pretty quickly. I built one of these a few years ago (Without issue) which met it's demise one afternoon coming out of the wingover. I wasn't going to fix it and it went into the trashcan.

             If one is going to build this plane, I highly recommend redoing the center ribs and sheeting the entire center section using 1/32" top and bottom. It looks nicer, much stronger and the wings won't flex. I really wanted to make my own fuselages and I wish I did as the wood was very punky. I also would've liked to make my own firewalls and cheek blocks but seeing I got into this and ran into many snafus I elected to use the stock nylon mounts. They're a bit heavy so I sanded a radius on the corners and took out some material on the sides. This took 1/4 oz. off of the combined weight of the mounts. The plane builds nose heavy so stealing weight without adding weight is the goal. I also routed in some maple hardwood to keep the mounts from crushing the wood. It also offers me a place to screw my tanks onto when I put them on.

           Power currently is going to be 2 Silver Bees and then I'm switching to product engines. I got 2 coats of dope on today and looking forward to some tomorrow. The stock landing gear is poorly designed as the wire is too small and the diagonal leg used to support the down leg is the wrong angle and will collapse. If needed, I will switch using 1/16' wire as they designed this with gear that a rubber powered model would use. The maple blocks I installed will be very beneficial for this. I covered the wing prior to installing the fuselages. I learned this the last time and this made covering much easier.

            hardwood to prevent crushing

Almost ready for finish paint. Fillets completed

No rudder offset     elevator has kit supplied cloth hinges which were doped on

     These mounts are heavy, plywood firewalls and cheek blocks would've made this a much better looking model. I suppose maybe next time. I sanded out the sides and sanded a radius on all hard corners to reduce a little weight. I'm hoping when I go with the Sure Starts, that will   shift the weight to the tail without adding weight. Last build had nearly a 1/2 oz. on the tail.
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Re: Brodak F-82 Twin Mustang

Post  GallopingGhostler on Sun 1 Jul 2018 - 20:39

Ken Cook wrote:I got a good start on this at the beginning of the week and I hoped to complete it this weekend.

Your build looks really good so far, Ken. Love the black film covering on the wings.

If one is going to build this plane, I highly recommend redoing the center ribs and sheeting the entire center section using 1/32" top and bottom. It looks nicer, much stronger and the wings won't flex. I really wanted to make my own fuselages and I wish I did as the wood was very punky.

Is this one where to lighten things, you'd build the fuselage as a hollow profile using sticks in the middle and sheeted over to look like profile?

I've got Walt Musciano's hollow log F-82 plan from his book I got, reflects Scientific style kit building except cowlings are folded sheet aluminum. Would be a fast flier as there is not much wing area for 2 engines, plus the less than optimal modified Clark-Y built up wing.
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Re: Brodak F-82 Twin Mustang

Post  Ken Cook on Sun 1 Jul 2018 - 21:05

George you have mentioned the hollow logs at times. I'm a hollow logger myself. The F-82 is on my to do list. Your spot on in terms of wing area. Walt is friends of several of my club members and he came to our field in 2004 I believe. Walt said that many of his designs were redesigned to maximize wood, (2 18" wings from a 36" sheet) and some other modifications to fit things into the box. With two engines, your really taxiing the wing and the suggestion on the plan is one. However, I'm thinking a Sure Start would be lighter not to mention it would pull the cylinder back vs a Bee. Keep the tanks internally over the CG and it should do fairly well when the power is on. This is true of many of those designs, a integral tanked Bee out there is going to offer power but kill maneuverability. I have a Atwood Shriek that might be destined for a log.

This Brodak Twin has 1/4" solid fuselages. They're light, as a matter of fact too light. I'm going to have to silkspan these for some strength. One of the guys the other day had one and he broke the nose off of it flipping it. I found humor in it but he didn't. C grain should be used or a medium density balsa at minimum. I taper the rear of the fuse to the same thickness as the rudders so it's pretty fragile in the rear. I could of left it and this would've helped with the CG but I like the look. I was equally concerned with the rudders as they were extremely flexible and while doping they started cupping. When dry, I applied a very diluted mix of thinner and dope to the opposing side and it straightened up. It's quite hot here and doping in this heat causes fast drying which shrinks. it did allow me to get 2 decent coats on it.

The wood was so soft that I had to swipe the paper opposed to going at it because it would just tear right into it. The dope stiffened up everything and it looks good now in two coats aside from the blushing. Brodak dope and high humidity don't play well. I'm progressing away from using dope. It stinks, it's highly flammable and I'm sure it's real good for you. The final finish on this plane is not going to be dope but Systems 3 water based polyurethane which looks stunning.

There's no spar in this wing. The leading edge is the backbone but the entire assembly is made up of 3 butt glued sections with a ply joiner across the mid section. The design calls for one over each joint. I figured it would be fine as designed and I should've used my better judgement. I should of sourced a new leading edge mid section of the hardest balsa I had or run a continuous ply jointer. It will be fine, but I know the wing is going to be subject to flexing . It appeared to me that somewhere this design possibly had internal wing gear that was going to be used due to cutouts in the ribs for ply platforms that were not used or called for in the plan. Or it was just a oversight mistake.
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Re: Brodak F-82 Twin Mustang

Post  akjgardner on Mon 2 Jul 2018 - 18:48

Looking good Ken, I've been wanting to build one and convert it to RC.
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Re: Brodak F-82 Twin Mustang

Post  roddie on Tue 3 Jul 2018 - 8:38

Really NICE Ken!! I'm curious about the bellcrank arrangement. Is it similar to the way Sterling recommends with the P-38?



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Re: Brodak F-82 Twin Mustang

Post  Ken Cook on Tue 3 Jul 2018 - 13:52

It's pretty much the same. However, they designed it without enough room. The fuselages don't utilize a root cutout and slip over the wing. They put a rib on each side of the fuse and allow the fuse to insert the full depth of the rib. Neat idea, it's a lot of extra wood and work and if not done properly is a big headache. If those ribs aren't square to the leading edge ( Which they were not) you need some wiggle room to square both fuselages. With all this material inside the wing, you really need to overcut most of it to allow for leadout clearance and swing.
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Re: Brodak F-82 Twin Mustang

Post  roddie on Tue 3 Jul 2018 - 21:23

Ken Cook wrote:         It's pretty much the same. However, they designed it without enough room. The fuselages don't utilize a root cutout and slip over the wing. They put a rib on each side of the fuse and allow the fuse to insert the full depth of the rib. Neat idea, it's a lot of extra wood and work and if not done properly is a big headache. If those ribs aren't square to the leading edge ( Which they were not) you need some wiggle room to square both fuselages. With all this material inside the wing, you really need to overcut most of it to allow for leadout clearance and swing.

Thank you for explaining that lay-out Ken. As you know; my scratch-built 1/2-A U-control P-38 utilizes a 1/8" sheet-wing construction. I decided to set-up the control-linkage "outboard" of the model's center-line.. and route the lead-out cables through tubes in the central-pod and inboard-boom. I hope that the arrangement will work-out.

Your model looks to be very close to the same scale as mine.. and we're both planning for Cox .049 reed-valve "twin-engine" power. I'm psyched that you're building this model.. because it's giving me some incentive to get mine in the air.
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Re: Brodak F-82 Twin Mustang

Post  Ken Cook on Wed 4 Jul 2018 - 8:27

The nice thing about this plane is the wing as it's pretty generous. The wingspan is 32 1/2". That's pretty decent size and with two Black Widows honkin away this plane did very well. It also has a fairly long tail moment that allows for tight turning. It's really cool in the overhead maneuvers and stays out there on the lines which is pretty impressive. I think the newer laser cut version would even be better especially with beam mounted engines. 2 Medallions would be very optimal to use and they would fly this plane beautifully. Even a pair of Babe Bees would fly this plane nice. If your ever looking for a home for anemic engines, put them both on a twin and you would be surprised how much power they offer combined.
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Re: Brodak F-82 Twin Mustang

Post  getback on Wed 4 Jul 2018 - 9:47

Airplane looking Sharp Ken , I've got a printed wood kit that i am not looking forward to building don't recall with one right now but not a twin . I hope the wood is good though! You're right about one thing to hot to fly and good building weather - the humidity , with the twins being built and flown lately gets me thinking but have some 1/2 built planes on the table now that need attention and thinking RC is where i want to go next anyway good luck with it and i will be checking in on the progress . 4th of July Flag
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Re: Brodak F-82 Twin Mustang

Post  Ken Cook on Wed 4 Jul 2018 - 17:40

A neat tip when you have print wood, photo copy the sheets on the printer and you can spray some "77" on the paper and attach it onto new wood. This allows intricate parts and shapes to be easily replicated. If you really thin some dope down and brush on one coat before cutting it also prevents crushing when cutting using a X-acto.
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Re: Brodak F-82 Twin Mustang

Post  GallopingGhostler on Wed 4 Jul 2018 - 17:52

Ken Cook wrote:A neat tip when you have print wood, photo copy the sheets on the printer and you can spray some "77" on the paper and attach it onto new wood. This allows intricate parts and shapes to be easily replicated. If you really thin some dope down and brush on one coat before cutting it also prevents crushing when cutting using a X-acto.

Excellent suggestion Ken on both points, and advantageous too over printing the wood. One, when they cut through the paper has clean wood when they remove the paper. One thing I found somewhat annoying about kit wood is seeing all the labeling through translucent doped tissue covering and part outlines. Some, you could sand the ink off, but then weakened the wood by making it thinner.

Some time ago, may be 30 years ago, there were a few cottage industry kit makers of peanut and walnut scale rubber powered stick build aircraft, that would sell kits with the printed paper temporarily pasted over the wood sheets. This was during the hay day of Peck Polymers. They usually had the more obscure WW1 aircraft and some WW2 aircraft that other kit manufacturers never bothered with. Since, I've lost touch with who these manufacturers were.

To fix die crushing or cut crushed wood, I'd put a drop of water then hit it with a Monokote iron. The steam generated would swell the wood fibers and help restore the crushed edge. Of course, some kit parts were unsalvageable because of such poor quality wood, would use the bad parts as templates on new wood.
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Re: Brodak F-82 Twin Mustang

Post  Ken Cook on Wed 4 Jul 2018 - 18:20

I like rubber power and I built a Pacific Ace last year. This was a Golden State model which was printwood. I know what you mean by seeing the ink through the covering. It was quite difficult to rid the model of the inking.It wasn't horrible, but in the nose area you could see it. Currently, I have a Scientific kit which I want to do next which is from the 50's. This pre-dates Musciano models and this kit is printwood. It's a B speed "Atomic". Not only is it printwood, but it also utilizes planked surfacing for the cowling and turtle deck.
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Re: Brodak F-82 Twin Mustang

Post  Ken Cook on Sun 15 Jul 2018 - 12:45

I finalized the twin today up to the point of bolting on the engines. I put it on the scale which ended up as 10.89 ounces. Unfortunately, when I did a quick cg check, the plane almost flipped forward out of my hands. I wasn't very happy with this. I built this model way back and it needed 35 cents on the tail to balance. This thing was WAAYYY over the top needing 1 ounce. I just don't get it. I cut weight in every possible manner to the point of even stealing 3/8" off of the fuse length. Seeing the problems I had early on just getting it together took the wind out of my sails. I could've mocked all of this up and did some adjusting. I just thought I pretty much nailed this one in terms of the last build.

One thing I did recognize is that the Silver Bees are a couple of grams heavier than the Black Widows I used before. Between the two engines equals almost 3/8 of a ounce. Not that that would fix the problem, It certainly would help. I knew better than to taper the fuselages, I just couldn't resist though and I tapered them in both directions to the rear. This is a common practice on Sterling models however, your taking away material where you need it the most. It still looks like I'm putting a big chunk of tail weight in it though.
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Re: Brodak F-82 Twin Mustang

Post  944_Jim on Sun 15 Jul 2018 - 21:42

Ken,
How much you figure the CG will slide back if you use postage stamp engines and external tanks? My Scientific profile P-40 was pretty nose heavy with the Golden Bee. Parking the AP .061 on it with an external tank behind the firewall moved CG back a bit and made for a looser flying engine-out landing. It certainly lost the "lawn dart" effect! Direction changes are easier too.
My Mossie practically has the NorVel .074s in the leading edge (well, not quite so far back).
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Re: Brodak F-82 Twin Mustang

Post  Ken Cook on Mon 16 Jul 2018 - 3:54

Brodak offers 3 types of nylon/plastic mounts. The larger mounts is what is on my model which only is drilled for integral tanked Bee engines. The other 2 mounts are identical with the exception of what they're made of. One is a harder beige colored plastic offering product backplate application or tanked Bees. The 3rd is a softer opaque nylon which is quite flexible used on the 1st gen 1/2A Baby Clown ARF. My initial plan was to use the tanked Bees and then switch to the product engines. Then I discovered that not only is the Goldberg mount identical to hole location, so is the placement of the cylinder off of the firewall. If I was to use a product engine , not only would it be lighter, it would also pull the cylinder back approx 3/8". Unfortunately, the mounts required to do this are considerably smaller and their bolt patterns aren't the same.

I can bury the lead in the tail, I just should've explored this rather than guess but time is not something I seem to have a lot of. I use lead lined drywall at work. The entire back of the sheet is covered in lead of various thickness depending on what were shielding. This is quite nice due to being able to get sheet lead in 3 different dimensions.
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Re: Brodak F-82 Twin Mustang

Post  getback on Mon 16 Jul 2018 - 10:03

Thats sucks you're haven those same issues even knowing from past experience and trying to make changes to compensate while building . If you add weight will it bee split between the two tail booms ?
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Re: Brodak F-82 Twin Mustang

Post  Ken Cook on Mon 16 Jul 2018 - 15:07

That's what I'll do. The bummer is that I already put the plane in clear. I knew that was not a good thing . I had to take the chance due to the humidity we were having.
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Re: Brodak F-82 Twin Mustang

Post  roddie on Mon 16 Jul 2018 - 21:22

PM sent...
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Re: Brodak F-82 Twin Mustang

Post  944_Jim on Wed 25 Jul 2018 - 23:35

Ken,
Did you still want the CG mount?
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Re: Brodak F-82 Twin Mustang

Post  Ken Cook on Thu 26 Jul 2018 - 4:24

Jim, sorry I didn't get back to you. After my findings, I went in yet another direction. At this stage, I won't be needing them. I do appreciate the offer. On my next builds, there will be no question in terms of what direction to approach. My gut instinct was to just do my own thing. I could've just left things stock but I just wasn't happy with the end result. Thanks again, Ken
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Re: Brodak F-82 Twin Mustang

Post  roddie on Thu 26 Jul 2018 - 19:15

Hi Ken, Please keep us posted on your progress. This model is going to "eat-up the sky".
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Re: Brodak F-82 Twin Mustang

Post  Ken Cook on Fri 27 Jul 2018 - 4:12

I'm looking forward to flying it. Looking over my pictures, the approx time that I built my first one was nearly 15 years ago. Time does fly by.
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Re: Brodak F-82 Twin Mustang

Post  ticomareado on Sat 28 Jul 2018 - 8:07

Two good Testor Macs stripped of starters (or maybe not) on plywood firewall with wedgetanks would probably solve all nose heavy issues.
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Re: Brodak F-82 Twin Mustang

Post  Ken Cook on Sat 28 Jul 2018 - 12:33

I don't want to instigate a bickering battle, those engines would never be considered by me. There's no place for those engines in my airport. I sold all of that stuff a few years back and I don't miss it a bit. I currently have one left in a Testor's Fly Em Zero. When it does start, it's a gutless engine that barely flies a plane with a wingspan less than 12". I have owned all of the Testor's series engines and I had one pipe bomb that stood out from the rest that was until the crank broke in 2 places. The conrod pins on the crank are extremely small and this is another failure point where the crank pin and web fracture off. I did enjoy that pipe bomb until the crank took a dump. Nonetheless, the quality in my opinion was always lacking.
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Re: Brodak F-82 Twin Mustang

Post  ticomareado on Sat 28 Jul 2018 - 13:17

I'm talking about an FRV, not a pipe bomb.
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Re: Brodak F-82 Twin Mustang

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