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Gas Gun and Other Under-wing Things

DanH

Legacy Member
Mentor
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:

Layout.JPG


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.

Structure.JPG


The base shell was created with 4 plies of 9oz 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:

Foam%20Block%20Start.jpg


Shape with 80 grit blocks and templates:

Long%20Board.jpg


Template.jpg


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

Coated,%20Sanded,%20Waxed,%20PVA.jpg


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.

Shells.jpg


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.

Mount%20Plate%20Layup.jpg


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.

Added%20Structure.jpg


More later.
 
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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:

Mounting%20Points.jpg


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.

Breech%20Block.jpg


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

Breech%20Block%20and%20Heat%20Sink.jpg

Breech compartment:

Breech%20Compartment.jpg


The upper compartment is a gear bay:

Upper%20Compartment.jpg


The bottles and regulators installed....

Mounted%20Bottles.jpg


...and pulled out for service:

Servicing.jpg


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.

Finished%20and%20Mounted%204.jpg
 
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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.
 
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.

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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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
 
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.
 
You need a good lead computing Gunsight

Come join our HUD project to complete your project!

A good gunsight is pretty easy to do in our HUD software.

TRON
 
A long time ago, I had a look at the ?guns? made for Airwolf. They used a very small air compressor with a small fuel pump for jet fuel, readily available from the helicopter fuel tank. They found that propane flames didn?t show up well when filming in full sunlight.


The ?gun? looked very similar otherwise.
 
This project wouldn't have anything to do with this year's Petit Jean balloon bust contest would it? :) Pretty cool, Dan.
 
Game, set and match!

Dan you win it all. I?m always amazed by you talents and creativity!! Very cool project.
 
Nice work! What fun! One thing, though: Are the mounting holes in the honeycomb reinforced with flox or metal or phenolic to transfer the load from the bolt head to the layups? The reason I ask is that there are a number of phenolic or metal "hard points" on my Lancair.
 
Nice work! What fun! One thing, though: Are the mounting holes in the honeycomb reinforced with flox or metal or phenolic to transfer the load from the bolt head to the layups? The reason I ask is that there are a number of phenolic or metal "hard points" on my Lancair.

Good question John. I'm familiar with flox hard points, but did not install any here. The Soric XF core is not foam, nor is it a typical open cell honeycomb. It is polyester fiber pressed into something like a firm felt. The stuff was designed for the resin infusion market as a flow media, one which can remain incorporated in the layup as core material. The honeycomb pattern is actually a network of very shallow surface channels in which the resin flows across the layup during infusion.

http://lantor.com/wp-content/uploads/2018/02/LAN0005_Datasheet_SORIC_XF_WEB.pdf

The above datasheet offers a clue as to the need for hard points. Note compression strength at 8 MPa, 1160 lbs per sq in. If we assume the ID and OD of a #10 fastener head as similar to the same AN960 washer, bearing area under the head would be 0.11824 sq in. So, load to crush would ballpark at 0.11824 x 1160 = 137 lbs.

That's possible because is soaks up resin, similar to glass mat. The datasheet suggests about 7.5 times its own weight.

The values are for polyester resin. Epoxy values would probably be better.

So, I'm just going to keep an eye on them. If the mount holes appear to be overloading, I'll flox core them using the old bent wire method. I have some of those in my cowl.
 
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Really cool invention. Being a gadget guy I love to see ingenuity at work.

I do have a question that nobody else seems to have asked though. How close do you think someone will need to be to the airplane to actually hear the "machine guns" firing? Will they only be for strafing runs or do you envision mock aerial combat? Just wondering.
 
How close do you think someone will need to be to the airplane to actually hear the "machine guns" firing? Will they only be for strafing runs or do you envision mock aerial combat?

A quick check at 1/2 mile, done to see if it was likely to irritate or alarm airport neighbors, was not loud at all. All else remains TBD. For a given fuel flow, increasing fire rate reduces sound power.

Strafing runs might be considered improper, so we won't go there.

I recently built a killer smoke system for my buddy's Rans S-7. He can't wait to be "shot down" on video...you know, muzzle flash, smoke on, roll over, cue the diving Stuka scream...
 
A quick check at 1/2 mile, done to see if it was likely to irritate or alarm airport neighbors, was not loud at all. All else remains TBD. For a given fuel flow, increasing fire rate reduces sound power.

Strafing runs might be considered improper, so we won't go there.

I recently built a killer smoke system for my buddy's Rans S-7. He can't wait to be "shot down" on video...you know, muzzle flash, smoke on, roll over, cue the diving Stuka scream...

What about laser to initiate the smoke trail?

That way actual dog fights would be interesting.

All I know is I want want two, one for each wing....

How interesting would it be to have 4-person teams from different areas that meet to have dog fights? Two on two starting from a Lufbery Circle.

Red Bull and Monster would probably sponsor as it would probably be as much fun to watch as participate.

Wonder how many could become Aces before their first shoot down happens...

Best regards,
Mike Bauer
 
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Yeah, no way Dan would do that, uhuh...

Always a model of propriety.

Seriously, there is no rational justification for this thing. Yes, gun development has addressed practical issues (for example, vacuum bagging on the wing, the mounting points, and aerodynamic changes) with an eye toward future devices like the creamains unit. However, it's mostly an exercise in Shop Madness, a chronic disease with recurring symptoms. It's also an amusing toy given the right setting.

The other thread ("Oshkosh Arrival Enforcement") is an obvious spoof, a fun way of further promoting the idea that we all need to straighten up and fly right during next week's inbound. In truth I'm not planning to bring the gun. No one has invited the "RV COIN fighter" into the Warbird show, and I imagine our EAA friends would have a cow if someone ripped off a few bursts during the HBC Beer Tasting.

There can be safety issues too, some obvious, some not so obvious. Want to build one? Fine, but be aware there are ways to screw up very seriously, and further, I have not yet fully tested everything seen here. Proceed with caution, please.
 
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As they’re relatively easy targets, I’ve taken out many an ice fisherman during winter doldrum strafing runs. I’m pretty sure my victims all remained clueless to my exploits, but should they hear faux machine gun fire, I would think there would be anxious calls to the local authorities. Doesn’t seem to be a great idea for that particular activity, but I would LOVE to have one of those nonetheless Dan! Really cool and it fits right in with my man-child mentality!
 
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The concussion to the skin rivets given enough cycles is one item.

Introducing a flame / ignition source near the fuel cap and vents on the ground could really make a guy wish he had just gone flying.
 
As they?re relatively easy targets, I?ve taken out many an ice fisherman during winter doldrum strafing runs. I?m pretty sure my victims all remained clueless to my exploits, but should they hear faux machine gun fire, I would think there would be anxious calls to the local authorities. Doesn?t seem to be a great idea for that particular activity, but I would LOVE to have one of those nonetheless Dan! Really cool and it fits right in with my man-child mentality!

Just start with a little extra altitude, and pull the power back to idle before you start your strafing run, like the Night Witches did on their bombing runs. That way, they'll almost certainly hear the gun, and be a bit more surprised, too.
 
Just start with a little extra altitude, and pull the power back to idle before you start your strafing run, like the Night Witches did on their bombing runs. That way, they'll almost certainly hear the gun, and be a bit more surprised, too.

Do that in Texas and you?re the one who?s going to get surprised when they shoot back at you !! :D :)
 
Introducing a flame / ignition source near the fuel cap and vents on the ground could really make a guy wish he had just gone flying.

Fair point. A dribbling tank vent with fuel on the ground may have some ignition potential, assuming the formation of a vapor cloud within combustion limits outboard and upward some distance.

Not very concerned about inflight combustion. Pretty sure I'd need a missing fuel cap to put enough fuel into the air to be within combustible limits. The tank vent is just too far away.

POSTSCRIPT 7/16: So far, inflight testing has been a non-event. My nose did catch a slight whiff of muzzle exhaust, as the vent intake for the rear seat of an RV-8 is under the right wing, inboard of the simulator. I will pick up a few CO detectors at OSH next week.
 
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Left OSH late morning Wednesday. Dropped into NW Chicago (Lake In The Woods) for some deep dish pizza (see the recent Pete Howell thread), then non-stop back to Alabama.

Good opportunity to better quantify cruise speed loss. Turns out it's not bad, perhaps 3 knots max. Here it's slightly LOP, where it would typically clock about 183 true. The 16 lb package does make that wing heavy, but not enough to bother the autopilot. Ball is in the center without changing the normal bungee trim.

Cowl door shut, about 30 sq inches exit area.

Economy%20Cruise%20with%20Gun.jpg
 
Wait for it . . . .

I can see the next generation, repurposed laser tag hardware that activates the smoke. Coming out of the sun . . .
 
Slick.
Any guesses on what causes the slight ≈300hz tone when it is firing? Are we hearing the gas flow through the valving?
 
Slick.
Any guesses on what causes the slight ≈300hz tone when it is firing? Are we hearing the gas flow through the valving?

My guess was some sort of electronic noise picked up by the GoPro. Cannot hear anything like it when operating the gun on the ground, maybe because everyone in the vicinity has their fingers in their ears.
 
Outstanding !

This might give another bubba the idea to strap a couple of AR-15's or AK-47's to the lift struts of his C-172 and try some duck hunting at the next rally. I'll give credit for nice workmanship.

Have a nice Sunday
 
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