Bomb Bay Doors

Bomb Bay Door I decided at the start of this project that the bomb bay doors would work and look scale. A bomber has to drop bombs. Ever since then I've kept my eye out for ideas on how to make it work. I never found the simple answer and finally had to sit down and experiment. When I was building the fuselage I had to come up with at least the tracks so I could continue with the build. I figured that making the tracks similar to the full size would be a good idea since that's what I wanted them to be. So I did that and continued on but never felt comfortable with the few ideas I had or that others had done to make the doors or to actuate the doors, that is, making them move simply & reliably.

Bomb bay door

8/25/05 - Through experimentation I settled on a pretty good door style that looks scale. I cut some 1/4" strips of 1/16" balsa and using a jig I made with pins for spacers, I put packing tape on them. I did paint an earlier test door with Rustoleum chrome (it's around 95% aluminum) right on the packing tape and it looked pretty good, but I will use laminating film or even Flite-metal for the final edition so I don't have seams (The test door needed three pieces of 3" tape).

I made a template to fit the fuselage opening and then used that to cut out the door. I added a strip of 1/32" ply to the top edge.


I still didn't have an actuator I liked, i.e., a way to make them open & close. One idea I came across was to use arms attached to the doors that are connected to servos and one I contemplated was to make it very scale and use a cogged gear engaging each runner, just like the full size. But both push the door from one end and that causes problems if the door is not stiff enough. The problem with a stiff door is it doesn't want to bend and the door needs to bend easily. And then it came to me while laying in bed one night. I need the door to be pulled up AND pulled down and the way to do that is with cables. So the next day I went to the hobby store and bought some "Sullivan Very flexible cable" and made a test bed. Using the Sullivan flex cable sheath and some 10 lb. fishing line.

I rigged up a test and it worked beautifully (with no wind). For the track runners I have tried brass channel - worked OK but is labor intensive to make; little pieces of carbon fiber rod glued to ply that's glued to the 1/4" wide balsa strips - that worked great, but is labor intensive; and butyrate plastic that I molded with the vacu-form. The plastic worked good because it slides easy and is easy to make and is on the door in the pictures but I'm afraid it might not hold up in a 50 mph wind (flight). So maybe more testing on the runners later when I need to mount the doors.

Bomb Bay Door actuator test. Door closed- the blue tape is the door's bottom edge.

Bomb Bay Door actuator test. Door open. Only one door for test but the actuator arm will work both sides.

The next thing to do was to make the actuators for all four doors but I decided not to glue it all in yet. I'm confident that it will work fine and will install it all after the finishing is done. One servo will power all 4 doors.

That's it for now on the bomb bay doors. I feel good enough about it that I can now move onto finishing. One reason for stopping now is that I would like to use the Flite-metal on the BB doors but I want to make sure I have enough for the airframe first and also I want to string them all up only once. Also later I will need to make a bomb drop mechanism. Later, later.. I want to get some Flite-Metal on!

Bomb Bay Doors

4/11/06 - I installed the lever shaft and then cut 8 upper and 8 lower cable guides and CA 'd them in. I glued the ends of the guide to cross pieces with CA but that was too fragile so I drilled holes in scrap wood, glued those to the cross pieces and put the guides through the holes. Should have done that first!

4/26/06 - I can't put it off any longer, it's now or never for the bomb bay doors. I really want to get them working partly just to prove to myself that it will work but I should just get the craft flying first and then mess around with the doors. I decided to go for it. I need to at least try and get them working and maybe fine tune to get them working well later on after it flies. I dread the thought of stringing all the fishing lines through all the guide tubes and getting them fastened down and it's been tough to get going on it. Once the doors are on there will not be very much room to work inside but I need to get that done before putting in the air tank, servo and valve, I think.

I started by gluing 10 lb. fishing line to the doors. Trying to get the lines inside the guide tubes proved to be difficult for the upper end since the tube is buried in the slot and I can't get to the end. I had to fish a line through the open end, glue the door's line to that line and pull them back through the tube. Sounds easy but what a pain at first. I sort of got the hang of it by the end of the second door. After I got the first door on I did a mini wind tunnel test on it to see if it would blow off in flight, I blew on it with my air compressor's air gun. It didn't blow off or even move.

5/5/06 - I had to start over. There was too much of a gap at the top where the door seats against the fuselage. This is it. I either get them working in the next couple of days or I give up for now and just screw them in place and fly.

I cut all the lines and pulled off the two doors I got on. I glued two strips together on each door to create a curve at the top end of the door so it pulls itself against the fuselage and attached new line to the doors. I had glued four pieces of styrene angle stock to the doors to ride in the slots. I cut the two off at the top edge. (To backtrack a bit- Originally the plan was to have the doors captured by the slots, little guides would ride in the slots and would hold the doors in, but I found that with the cable system, that is not needed, the cables hold the doors down, so all that's needed is something in the slot to keep the door in it's place. Further refinements eliminated the top side slot guides. So now there is only slot guides at the bottom edge of the door, one on each end.)

I cut slots in plywood strips and glued them to the door linkage pivot rod to secure the lines. The lines run through the guide tubes, into a hole in the end of the door linkage arm, down to and rap around the linkage pivot rod and then over to the slotted ply. Four lines at each end and eight lines through the center arm. After that the lines come out of the fuselage bottom so I can pull it tight when the doors are closed. The biggest problem I had was getting the lines through the top end door guides because to guides could only be accessed from the wrong direction. I had to feed a thin cable through, glue the door line to it and pull it back through and what a pain that was. That took most of the time and a lot of my patience, especially when two of the guide tubes were blocked and I had to ream them out with a sharpened cable.

I got all the lines tight and tried moving them all for the first time. I got a piece of piano wire and attached it to the center arm and pulled to open the doors and it was tough to move. Maybe this won't work. I loosened up some of the lines and that helped a bunch. I could now cycle the doors. It was still not as smooth as it should be but I have a strong servo. I mounted the servo, a Hitec 615MG, hooked up the linkage, plugged it into the receiver on AUX2 and they were moving pretty good. The two rear doors were seating perfect at the bottom, the front doors needed some adjustment at the bottom and all four doors were seating very well at the top edge. The problem now was that thinking I had all the doors where they should be, I CA 'd the lines down. Now I need to do some adjusting and can't. I need a better was of securing the lines. Something that will hold under stress but still allow me to make adjustments when needed. For now I have to wait until I get to the hobby shop and get some CA debonder and then tighten up the front door's gaps.



The good thing is the doors do work. I can rest now. They could be better and I would like to refine it so it's smoother and super reliable but that will be after it flies. They look good when they're open or closed. It was about 10 hours to get the doors working.

12/12/06 - I worked on getting the bombs to drop from this heavy bomber. I originally wanted to have all four doors open and then drop a bunch of scale 500 pounders out. Realizing that while that would be neat, it's not really necessary to have scale round bombs and a lot easier and economical to use silhouette bombs. I played around with some bomb designs and came up with a bomb that is very easy to make, a bunch of them fit in the bomb bays and the doors only need to open 1" to drop a bunch of bombs.


A 500 lb bomb made from construction paper, a 6/32 machine screw and hot glue. A toy cap gun cap fits perfectly on the 6/32 screw and makes a loud BANG! when it hits something hard.



The main problem I've had with the bomb bay doors is that they are very hard to get moving and also to get them all to close at the same time so I decided to re-do the bomb bay doors. I started by cutting all the lines to the front bay doors and to my surprise the back bay doors were able to open and close real easy. Since I need to get into the front bay area once in a while to adjust or fix things -it's the only way into the fuselage- I decided to use only the rear bays for dropping bombs at this time and to use the front bay doors as access hatches. I built boxes 1" wide , as deep as they could go and a little shorter than the bomb bay, from 1/16" balsa and screwed them into the rear bays. I hooked up the servo which I already had in there and the bomber finally had working bomb bay doors. When the bay doors open the bombs drop out. Right now I have no way of having a sequencial bomb drop. The doors open and everything should fall out. I'll see how well it works and make adjustments if neccessary. The boxes weighed .33 oz. each and each bomb only weighs 2 grams so a full bomb load only weighs 1 oz. The only downside is that the added 1.6 oz. of weight is about 6" behind the CG so it will be slightly tail heavy before the bombs drop but I don't think it will be a problem. I only wish I would have done this at the beginning of summer so I could have been dropping bombs on each flight. Now I can't wait to get out and see how well it works in the air.

Bomb Bays I can get eight flat bombs in each bomb bay. I can also load up with anything light enough to not effect the balance too much.




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Updated July 14, 2009