flybyjohn

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I have always talked about doing a build thread but never got around to doing it so here it goes. I took lightend version of Ken Willard's plan and after makeing the mount in the fuse for the engine pod, decided that I wanted to try 25 engines instead. I will make a plug for the pod hole so that if the twin does not work or I want to convert back to single engine I can easily just by making another wing or cutting off the nacelles of the one I build. I have the fuse sides and bulkheads glued in and the tail feathers ready for sanding. I want to run the push rods before adding the top or bottom sheeting. The plans call for 1/32 inch ply for the front bottom but all I have is 1/8 light ply that I plan to use. Hope this won't be too heavy. I have cut the wing nacelle sides out of 1/8" light ply and they are locked into the top and bottom main spars. I hope this will be strong enough, I have never built a twin before and so I don't know much about attaching nacelles. Well lets get some pictures of what I have so far.

flybyjohn

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I extended the rudder down a bit lower so that it can act as the water rudder. I will get to the shear webs tonight I hope. I plan to shear web the front and back of the main spars out to the nacelles and then just in the middle of the two spars from there out to the wing tips. I think the shear on front and back will give some rigidity to twisting. I am going to sheet the wing out to one bay past the nacelles, this is only a couple more bays than what was on the plans. The plans say this is a 5.5lb plane but I think it is going to be a bit heavier especialy with the twin engines. I am hoping to keep it under 7 pounds. I don't plan on glassing the bottom so that might keep it a little lighter.

flybyjohn

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Well I decided that there is a difference in polyurethane glues. I built the fuselage with gorrilla glue and it took a bit of water to make it foam. It dried fairly hard and the foam around the edges form somewhat of a fillet weld. I ran out of gorrilla and bought the cheaper titebond polyurethane glue and used it for the wing. It does not need any water to start foaming and starts foaming within minutes. The foaming that is produced around the edges peals off easily and is very soft and can be squished like foam. I am not happy with it. If you use water with it, it gets really brittle and the foam crumbles between your fingers. I wonder just how strong the joints will be. I am going to go around some of the edges with CA just to make me feel more at ease.

I put the shear web, top spar, and leading edge on with this glue. I will get pictures tonight.

Wayne22

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glue up some scraps with your new glue, and let them set/cure/dry for a good period then see how much force it takes to break them apart. The wood should fail before a good glue joint fails...

The twin sounds like a neat mod...will the props be high enough out of the water so the bow spray will not hit them?

flybyjohn

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I am hoping that the props will be high enough. I did a rough calculation with the target weight of the plane and the volume of the lower front of the fuse. and came up with a estimated water line. I then went 1 1/2 inch higher with the props. Rough water may be a problem and water from the bow may also give me some problems. I am hoping that it is light enough to keep the props clear of water though.

With the glue, I don't have a problem with either glue as long as the joint is solid tight and is clamped until dry, but in some cases where the may develop a little gap, I believe the gorrilla glues foam itself would hold tight where as the titebond polyurethane would pull apart. Don't get me wrong, I don't build gaps into my project to be filled but sometimes things just happen and the gorrilla glue would work.

flybyjohn

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Well, looks like I took too long to write the last post and the system timed me out. All that writing wasted. I guess from now on I will write in word and then paste it in here just before posting. Anyway some photos from the last two nights. The servo trays are in with two servos a piece, aileron and throttle. Throttle pushrod will go under the tank and provide almost a strait shot to the control horn on the carb. The aileron will run out the rear of the nacelle (not built yet ) through a hole and control the aileron with a horn mounted on the top of the aileron. I wanted it on the bottom but I figured on the top would keep it dryer. I am ready to place the top, center sheeting/sheathing now. I think I will glue in the firewalls before I flip the wing over and sheet the bottom.

I just figured out that with the rear of the nacelles in place, I will have a hard time getting to the throttle linkage. I may have to put a small access hatch in the bottom of the wing for hookup.

flybyjohn

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Some more progress to show. I turned into an old man now just recently, the big 40. The kids had a great time making the cake for me. Anyway, firewalls in with 5 degree out thrust on each. Wing just needs the wingtips installed, ailerons built, and sanded. I need to finish extending the nacelles to the rear. The props will be 3 inches higher than the step. If needed I could go to a 3 blade 8" prop and save 1/2" I am still concerned with the water spray. I was going to mount the engines 90 degrees to the side. I have a small muffler clearance problem. I will need to extend the engine 1/2" forward or make 1/4" muffler extensions. This is something I overlooked with the newer engines I got.

flybyjohn

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more pictures

flybyjohn

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more

Lifer

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Nice progress on your twin. Happy birthday!

SIMCO

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If you are worried about water spray on the airplane, install a plexiglass strip on the edge of the hull. Doesn't have to be real wide(1/4") maybe. Deflects the water outward vs. straight up along the hull. If it's clear hardly ever notice it. project looks good. Building right now the 120 version. Picked up a G38 cheap. Going the 30's US NAVY colors.

flybyjohn

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Well, good news. I got the wingtips on and decided to get a rought estimate on the weight. I set the wing on the fuse, and loaded all the electronics, fuel tanks, extra balsa wood needed to finish it servos, and engines and mufflers. total weight is 4.95 lbs. Covering will be about .33 lbs, battery 5 oz, and pushrods. I should come out under 6 lbs.

jrf

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John:

Your concerns about the prop clearance over the water are very valid. I can't tell you how many scale PBY's I've seen over the years that simply would not get on the step because the props were sucking up water. I believe you will have the same problem. Give some thought to a solution now, before you go to the trouble of building nacelles and covering everything.

The rule of thumb for float planes is to have 1-2" of clearance above the top of the floats, and the top of the floats in usually 2 or 3" above the surface of the water with the airplane at rest. If the prop gets much closer than that (about 4" above the water), the prop will suck up water.

Possible solutions? Three blade props will help. The deflectors that were mentioned will not solve the problem by themselves because your props are low enough to suck water from directly below the engines. You might try making the nacelles 1 or 2" higher plus adding the deflectors. You could build up the bottom of the hull by adding an inch of styrofoam on the first flight just to make sure it will get off the water. Or add it permanently and skin the bottom with balsa and or fiberglass.

Or hand launch it.

Jim

flybyjohn

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Been busy lately but have worked on the plane a bit in between activities. I got the top and bottom sheeting on the rear of the plane and the ply on the front bottom. I used carbon fiber arrowshaft pieces in place of the front hold down dowels, and have the wing mounted. I have been holding off on the front sheeting because I didn't know if I needed to put the battery up there or not. I loaded up the plane with all the accessories minus the rx and checked the balance. Without the covering, the battery has to go right behind the wing to balance. I hope with covering, that it will end up right at the rear of the wing. I still have to finish the nacelles and put the floor in the fuel tank compartment, and run the throttle cables. I will get the front buttoned up now that I know the battery won't have to be up there. I need to still make the tip floats also. I keep forgetting about those. I think I am just going to hot wire some foam and then cover it with 1/16" balsa and attach them with silicone sealant. I also have the engines mounted as high as possible without having to modify the mufflers. I had to mount them about 105 to 110 degrees to clear the mufflers. I hope this will not be a problem when starting them. They also were mounted as far inboard as possible on the firewall so that with the 5 deg. outthrust, the prop nut will be in the middle of the nacelle and will not look so funny with the outthrust. I am going to use plastic soda bottles for the cowls. I have made a practice mold and heat shrinked the bottles over the mold and they turned out great. I sanded them with 400 grit and painted them with rustoleum paint and they seem to be holding paint quite well. I bent them around and have not lost any paint. I hope the hold up to the heat of the engine. I weighed everthing again and am still coming up at 5.35 lbs minus the covering, tipfloats, and remaining nacelle pieces. I will have about an ounce more balsa here and there also. So it is looking like 6 lbs dry. Target flying weight is 6.75lbs with fuel. I am 1/2 lb heavier dry than the plans call for and 3/4lb heavier rtf (extra fuel).

Jim, I think you have a good point on the water clearance, however at this point I think I will just finish it up and take it to the water and try it out. if I have a problem then I would have to do the same amount of work to get the engines higher anyway so I might as well see if it will work as is before doing the work. I am keeping my fingers crossed. Well here is some pictures.

flybyjohn

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another

flybyjohn

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more, I need to learn how to resize my pictures.

flybyjohn

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again.

flybyjohn

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last one

Lamoilleriver

I'm going to join in with the motors are too close to water crowd. Water will spray out from under the hull and will be sucked right onto the props.

JimCasey

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Very nicely done.
Isuspect strongly that you only need out thrust on the right engine. With the direction of prop rotation the left engine is pulling from almost straight down the centerline of the plane.

If you do find that the props are getting spray, it would be worth it to try the deflector trick which also adds dynamic lift to the hull. Expanding upon that thought would be to artifically widen the hull with a 3/8"(arbitrary value) sheet of foam bonded to both sides of the hull, sheeted and covered to match. That would add static floatation as well as dynamic lift.

Roger Gray

Flybyjohn-I'm building the PBM by Keith Sparks on the forum close to yours, and couldn't help to notice your build. With the conserns shared, could I make a suggestion. Looking at the point of construction on your last photos you might try adding a balsa triangle strip to the bottom outer edge as I did on my PBM. This will diplace the force of the water shed off of the hull down as it slips out from under the hull. So easy to do and not even noticeable when completed. This might make a big difference. Good luck

flybyjohn

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I looked at your PBM thread several times over the last month. I may try the spray rails starting just behind the nose to just past the props. I read about the outthrust and was suprised by the differents opinions people had as far as right and left sides. I finally decided to go with both sides to get a more symetrical look. I thought that the pull to the side, commonly called take off roll was due to the twisting of the fuse becuase of the the propeller resistance to the air causing a torque in the fuse, in which case the airplane on take off would start banking to the left. Once the airframe was up to speed and the propeller was not biting the air so much I thought that the torque eased off quite a bit. I have many single engine models with no right thrust and do not notice any pulling to the side when the airplane is up to speed.

I got the front top sheething on and the throttle cable sleeves run throught the tank compartment. I just came to the conclusion that the tanks may not be removeable once in the nacelles. I know that this is not good practice because silicone needs to be replaced from time to time. I may be able to make them just slip out through the back but that will make my access door to the servos pretty large however this will be my only option for removeable tanks.

flybyjohn

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One more thing I forgot to mention is I scaled the photo of Carlos Murphy's twin seamaster that has 2 36's and his thrust line looks to be almost 1 inch higher than mine. Hmmm. well we are just going to push on and worry about this later, when It doesn't fly. This may just be a calm water plane.

Lifer

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I put a set of spray-rails on a Sig Sealane because it had a reputation of doing submarine duty on occasion. The rails are 5 inches long, 1 inch wide and set at a 20 degree angle (approx.) They sit on the waterline at the forward 1/3rd of the distance from the bow to the prop.

It was all a best guess but it kept the nose up, and the spray went distinctly off to the side.

Hope it helps. Your twin is a neat project!

flybyjohn

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I did a little more work on her last night. I have the throttle cables run to the carbs and the fuel tank compartment bottoms in place. The tanks will definently be not removeable without a little bit of cosmetic surgery. If I need to get them out every once in a while, I will have to make some cuts in the top balsa sheeting and then recover after repairs have been made. I think that if I keep the tanks cleaned out good before storage and filter the fuel going in to them, they should stay pretty clean and trouble free. Before long storage, I can fill and empty the tank with something that will get all the caster out so I don't have any caster cake cloging my carbs.