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.40 Size Stuka Bomb

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Old 09-23-2019, 03:56 PM
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H5606
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Default .40 Size Stuka Bomb

This 20 year-old scratch-built, Stuka bomb had seen its better days, so I decided to do a total restoration and - in the process - get it to do something else than just tumble when it hits the ground.
As originally designed, the dummy bomb could be dropped from the airplane and - being one-piece - would remain intact and either tumble when hitting hard ground or impale very soft (rain-laden) turf.
Ideally, I would have liked the bomb to be able to pop out a bouquet of flowers or unfurl a big flag that has the word BANG! written on it but likely would take a magician's touch to pull off.

What I decided to try to do was incorporate an old idea into the existing bomb and make it "explode" a cloud of particulate matter like flour, lime, or talcum powder. Not really sure if it will work well but thought this thread might be a resource for sharing ideas on this topic.

Pictures are included to document the process of restoration and conversion to a talc-dispenser of sorts...

#1 shows the bomb with center body in such poor condition that the bomb cannot even be secured to the airplane anymore.
#2 shows the component parts separated into the nose cone, center body, and aft cone with fin assembly. The dowel pin with notch above the center body is a means of securing the bomb to the cradle on the airplane.
#3 shows the new, replacement center body with the original nose and aft sections.






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Old 09-24-2019, 03:40 PM
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The nose cone and tail cone were both fabricated from balsa blocks: first rough shaped with a bandsaw in two dimensions and a knife to knock the corners off, then closer with a coarse sanding tool, and finally turned in a drill press and sanded. The tail cone block was slotted with a cruciform pattern to accommodate the plywood fin assembly before shaping. The plywood fin assembly is a two-piece unit - one piece slotted halfway from the LE the other slotted halfway from the TE and then slid together to form the "X" - the union is supported by the tail cone.

When my wife finishes up a roll of something, the discard filters through me and I've collected plenty of discarded cardboard tubes over the years. They come in handy for projects like this one. The center body is a 1 5/8" Dia x 3.5" length tube.

#1,2,3,4: In order to make the bomb come apart, I needed to find a tube that nested neatly inside this center body tube. I found one that was close but a little too tight and would also need to be cut down to length so I started by sanding the diameter down with a sanding tool checking the fit until it would slide into the center body easily.

#4,5,6 shows marking of the center body tube with a poor man's surface gauge (a stick attached to a base with a pencil clamped at the appropriate height) and another tool made to match the height and used to cut the tube precisely for a separation point. The longer piece will attach to the nose cone; the short piece will attach to the tail cone. This cut now makes the bomb a two-piece unit instead of the original one-piece bomb.













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Old 09-26-2019, 03:17 PM
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By sliding the longer-in-length but smaller diameter tube inside of the center body tube, I not only have an overlapping, stepped union to allow coupling together the forward section to the aft section, but also a reinforced body for impact resistance. Furthermore, I wanted the inside to be a smooth bore and unobstructed so that I can incorporate a piston-plunger to eject the contents.

The tip of the nose needed to be reinforced with a bearing of sorts to support the plunger shaft I wanted to use to drive the piston. The 1/4X20 bolt that used to absorb impact was removed, the nose tip was sanded down and a 1/4" piece of plywood was bonded to the nose before blending to shape.

#1 shows the modified basic component parts to make up the new two-piece bomb - note overlapping union on the forward section.
#2,3,4 is the process used to create the carry-thru hole in the nose for the piston-plunger rod.








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Old 09-28-2019, 06:27 AM
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Discards from a hole-saw carbon/balsa/carbon laminate serve as the basis for a disk to attach to a 1/4" dowel to create the piston-plunger. Turning down the laminated disk to fit inside of the bomb body with some clearance so there is no drag is accomplished by turning the disk on a fixed pivot against a sander. Locating the piston on the pushrod (dowel) is accomplished by inserting the dowel rod with piston disk positioned on the rod into the bomb nose, attaching the aft section, and - ensuring the dowel contacts the aft section, without disturbing the dowel position - pull off the aft section and push the piston disk against the back of the balsa nose section. A fillet of epoxy is applied to the union of the piston disk with the dowel at this location. Determining the length of dowel rod to leave out beyond the nose is simply a matter of pushing the piston out until it emerges the back of the center body and cutting the excess off the front even with the nose.

#1 & 2 are fabrication of the piston disk.
#3,4,5 shows process of locating piston disk on the dowel rod and bonding in place.
#6 shows bomb in complete form before "detonation".
#7 shows bomb after "detonation" - the sanding bar I'm holding represents the ground; by virtue of the relative movement of the nose and center body with respect to the piston plunger, the aft section and contents should be ejected. (you can just make out the piston peeking out at the back of the center body)













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Old 09-29-2019, 06:30 AM
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Latching the bomb in the cradle on the airplane is accomplished by fitting a dowel pin with a notch in which part of the servo output arm nestles into. The servo serves double duty by driving the trapeze as well. The trapeze throws the bomb clear of the prop arc in vertical dives. Trial found that the trapeze by itself would not hold the bomb in the cradle with the engine running, hence the need for the latching dowel pin.

Since the new bomb needed a clear bore for the piston-plunger, the dowel pin required a blind hole for bonding to the center body. A new, extra short dowel pin was fashioned and attached to mate up with the servo output arm. The original "U" shaped scab plates (used to connect and release the bomb from trapeze) were removed from the discarded center body, the surface gauge was used to draw center lines on the new center body and the scab plates were bonded to it.

#1 the dowel pin with notch originally used is shown upside down here to help explain the concept without bomb for clarity. (notch is ramped to pull bomb in tight against cradle)
#2 shows the output arm nestled into the dowel pin notch.
#3 shows relative positions of the output arm and dowel pin during bomb release sequence - the bomb with dowel pin would actually be moving out and away with the trapeze at this point. (Note: pins in cradles keep bomb from sliding fore and aft)
#4 are the original "U" scab plates for trapeze interface salvaged from the old center body.
#5 shows center lines added to locate the scab plates.
#6 & 7 show latching dowel pin and scab plates bonded to new bomb center body.














Last edited by H5606; 09-29-2019 at 03:58 PM. Reason: captions added to finish post
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Old 09-30-2019, 04:07 PM
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Bondo was used to fill voids and impact rash in the balsa and white glue mixed with water was used as a sanding sealer on the exposed balsa and cardboard areas before using a rattle-can primer. The first coat of primer highlights defects that need more attention so more Bondo is applied as seen in #4.

Since the black bomb could be difficult to find at times from errant drops in tall grass, I decided to go red this time around.

#5 bomb fit to airplane to check clearance of piston-plunger dowel rod.
#8 shows bomb in cradle on belly of airplane ready to go.
















Last edited by H5606; 10-01-2019 at 03:45 AM. Reason: caption addition
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