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  #1  
Old 07-10-2019, 07:07 PM
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DanH DanH is offline
 
Join Date: Oct 2005
Location: 08A
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Default Gas Gun and Other Under-wing Things

The first of two related under-wing projects is a gas gun, aka a "machine gun simulator" in the military reenactor and movie prop worlds. The basic operating principle is simple enough. A timer board controls two solenoid valves and an ignition coil. Oxygen and propane are vented into a chamber, the solenoids close, the coil fires, and the result is a shock wave at the tip of the barrel. There is no projectile, just noise and muzzle flash.

Propane supply is typically one of the standard 1 lb, 4" dia bottles from the camping department. Overall, they are low pressure canisters full of liquid, with actual pressure linked to temperature. High pressure oxygen is a bit more involved. The bottle is charged to 2000 psi, just like breathing oxygen. In this case, I selected a new M6 size bottle, 3.2" dia and 11" long, plus valve.

The regulators are new Victor G150's, modified by changing some of the fittings, plus removing the high pressure gauge and plugging the port. The gas regulator requires an adapter between the propane bottle and the regulator inlet. The propane bottle must be upright so only vapor reaches the outlet; liquid propane shuts down the gun, as the mixture is too rich. I found a 45 degree bottle angle was practical, then assembled fittings and a 1/4 turn valve so as to put the regulator alongside the bottle, inside the taper of the aeroshell's tail.

The O2 bottle can be ordered with a CGA-540 outlet valve so it will couple directly to the Victor regulator. The required transfill hose has male CGA-540 fittings at both ends fill the bottle from a shop oxygen tank.

The timer board, solenoids, and some other detail parts were sourced from Steve Smith (No, not our Steve Smith. A different one. Apparently there are a lot of them.) Go here: http://www.ww2steel.com/Gasguns/Gasguns.html

So, how to package the above? I started with CAD drawings, moving key bits around to consider the effects on packaging, aero, and structure. In the end, the best configuration was a 4" wide enclosure laid out as three stacked compartments:



The O2 bottle is in the belly, with the butt end extending through a support bulkhead, and the valve end supported by a custom aluminum fitting. The gun compartment has an annular air intake. Cooling air (cyan) flows along the barrel, past a breech block with a heat sink, through a rear bulkhead, then around the propane bottle and out an exit at the underside of the wing. The flow cools the gun and warms the bottle, good for both. The entire compartment is lined with aluminized insulation, although it's probably not needed. The upper compartment contains the board, solenoids, and the ignition coil.

Structure is a glass/epoxy shell with internal bulkheads. The bulkheads and the mounting plate are 2mm Lanter Soric cores with glass face plies. The uppermost floor is a bolt in, allowing access to the gun compartment. The tail comes off for bottle service.



The base shell was created with 4 plies of 5oz BID laid up over a solid foam block. As always, time spent making good forms and molds pays a large dividend later.

Gotta start somewhere. Draw guidelines and rough it out:



Shape with 80 grit blocks and templates:





The shaped block was epoxy coated for sealing, lightly sanded, waxed twice, and sprayed with PVA. Here it is, ready for layup:



The base is temporary. It was cut off. The stacked BID plies were wet out between plastic sheets, draped and smoothed over the form, then covered in peel ply and bleeder ply. The whole thing went into a plastic bag, where it was placed under vacuum for cure. After cure, it was cut into two parts. A few hits with compressed air and the form popped right out, leaving the raw shells.



The cored bulkheads were cut from flat layups cured on the bench. They were bonded in with flox filets and 2 ply layups. The mount plate was molded right on the bottom of the wing to ensure a matched curve. The wing was protected with gloss tape. The layup position was drawn on the tape. The wetted, stacked layup (bleeder ply, peel ply, glass plies, soric core, glass plies, peel ply) was positioned and covered with a 4 mil plastic sheet, the perimeter of which sealed to the wing with sticky tape. Vacuum ensured conformance.



Determining the correct alignments required some jigging on the wing, while trimming the shell to match the mount plate curve. When it was right, the shell was tacked to the plate with a few dots of five minute epoxy, then taken home for the connecting layups.



More later.
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Last edited by DanH : 07-11-2019 at 03:49 PM.
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  #2  
Old 07-10-2019, 07:22 PM
Ted RV8 Ted RV8 is offline
 
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Dan
Your just having way to much fun!
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  #3  
Old 07-10-2019, 07:46 PM
rightrudder rightrudder is offline
 
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Nice work, Dan! You make me want to build something.
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  #4  
Old 07-10-2019, 08:42 PM
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rv6ejguy rv6ejguy is offline
 
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As always, nice work Dan and something always interesting from you.
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  #5  
Old 07-10-2019, 09:16 PM
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DanH DanH is offline
 
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Ok, where were we? Ahhh...

A key issue was how to attach to the wing. Originally I looked at mounting to the tiedown point, but in the end I moved inboard and installed dedicated mount points. They are simply #10 (AN3) K1000 nutplates installed on the top side of the lower rib flanges, three per rib. AN3 size fasteners go up through the composite mounting plate, the wing skin, and the rib flange. The points are widely spaced:



The completed gun weighs 16 lbs with a full propane tank. If we assume 6 G plus 50%, and load applied equally to all fasteners, the load per fastener is 24 lbs each plus aero load.

The inboard location (covering the 2nd inspection plate) is outside the propeller flow and just inboard of the flap end. The CG of the assembly is about 8" aft of the main spar and perhaps 6" below. I did not attempt to calculate a contribution to torsional frequency or any related concern, nor did I make any serious assessment of maximum load capacity.

I did flight test, first with the empty, bare aeroshell, then with the complete assembly. Primary interests were clean and dirty stalls, slow flight, asymmetric drag, and overall drag. I could find no real difference in clean stalls. Dirty stalls require a bit more rudder to keep the ball centered; I think the shell is a sort of flow fence in front of the flap. A stall while allowing the ball out of center was no different from a stall without the shell.

Asymmetric drag is worth about an 1/8 of a ball in cruise, well within the capability of the rudder trim. It is a bit more noticeable at VNE plus 10 knots. Level flight at max power is a few knots slower, as expected.

At this time, the gun has been bench fired only. I still need to wire the airframe, and just received the wiring parts today. The gas gun needs four wires, two being main power and ground, and two being for the trigger circuit. The plan is to run four 18 ga with an Amphenol AT at each end. I want the wiring to also accommodate a new creamains dispenser design, possibly a pumpkin-related device, and who knows what interesting science experiments. Not all devices will require the same control functions. So, the plan is to assemble a breakout box specific to each device, any of which can be quickly plugged to a two-wire power supply and the four-wire to the wing. The box for the gun will have an ON-OFF toggle and a momentary button. A box for the creamains dispenser might have a polarity reversing switch to drive a linear actuator. You get the idea...just plug in the appropriate box to match the installed device.

Back to the gun. This is the breech block. The barrel is sealed in the block with viton o-rings, and locked by a retainer clip in a groove. The retainer serves double duty as the rear gun mount.



I found a computer heat sink in the junk drawer...just right:


Breech compartment:



The upper compartment is a gear bay:



The bottles and regulators installed....



...and pulled out for service:



So there you have it. It's all subject to tweaking, so I may have more to say later. Comments are welcome. I'll report when I make progress on the other devices.

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Last edited by DanH : 07-11-2019 at 10:55 AM.
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  #6  
Old 07-10-2019, 11:01 PM
gasman gasman is offline
 
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Dan, why not store the cylinders in the cargo area and run HP lines down to the solenoid.

Have you figured out how many rounds you get from a pound of propane.
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  #7  
Old 07-11-2019, 12:23 AM
scsmith scsmith is offline
 
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Quote:
Originally Posted by gasman View Post
Dan, why not store the cylinders in the cargo area and run HP lines down to the solenoid.

Have you figured out how many rounds you get from a pound of propane.
"The whole nine yards"
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  #8  
Old 07-11-2019, 06:15 AM
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DanH DanH is offline
 
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Quote:
Originally Posted by gasman View Post
Dan, why not store the cylinders in the cargo area and run HP lines down to the solenoid.
Excellent question. Two design points.

First, the self-contained approach minimizes modification to the airframe, and allows easy exchange of other equally modular under-the-wing items.

The second reason is safety. The ventilated design minimizes the chance that a propane leak could result in a fuel/air mixture within the combustible range. The same can't be said for a propane bottle in an enclosed baggage compartment, or a nicked propane line in the wing. In addition, there are no significant ignition sources in the pod. For example, there are no switch contacts, and nothing as hot as the required ignition temperature.

Quote:
Have you figured out how many rounds you get from a pound of propane.
No, but I think I'll run out of O2 first. Or friends tired of being "shot down"
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Last edited by DanH : 07-11-2019 at 07:00 AM.
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  #9  
Old 07-11-2019, 07:19 AM
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Danny King Danny King is offline
 
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Default Dawn Patrol

Dan,
I don't think there is anything you can do! Hats off!
If you need extra pilots to fly the dawn patrol at Ripon and Fisk give me a call.
See you at Oshkosh.
Sky
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  #10  
Old 07-11-2019, 07:56 AM
moosepileit moosepileit is offline
 
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Gun camera in pod and an aft facing oblique camera for scoring pumpkin hits?

Toilet paper roll ejector panel? Use the cheap stuff and leave a 6' tail pre-unrolled folded up to be first out the door.
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