James & Rossco - Make Sure You're Sitting Down ...

[larry cottrill]

James, Rossco et al -

I just thought of the ultimate way to show the value of flight-worthy valveless pulsejets:

The world's first PERPETUAL INTERNATIONAL PULSEJET ENDURANCE CHALLENGE !!!

ONE MAN AND ONE PLANE FOR AS LONG AS YOU CAN TAKE IT!

Guys, what is it that we can do that no valved engine can match? The answer is: WE CAN RUN FOREVER, AS LONG AS FUEL IS SUPPLIED!

Your mission, should you wish to accept it, is to develop a plane AND fuel system for your favorite valveless engine that will do the following:
1. Keep running through takeoff, normal flight and landing;
2. Be safely and instantly refuelable on the ground WITHOUT POWER INTERRUPTION, as many times as needed, by means of "quick change" fuel tanks (yet to be developed - see below)

For you guys, this would be just the OPPOSITE of what you're working on now, i.e. a plane that is big and relatively slow with moderate wing loading and effortless R/C flying. For me, it would mean developing a big, fairly slow U/C plane and long-term trouble-free control system. In both cases, there would need to be an internal fuel reservoir that could keep the engine running at idle for up to a minute while your HIGHLY TRAINED ground crew swaps out the main tank without spilling a drop or igniting any vapor, OR creating an air bubble in the fuel line to the engine. That's all there is to it. We can design this system together and come up with a standard main fuel tank design and size, so the only basic difference in our efforts will be our engine/plane combination. Our fuel tank design could become the "de facto standard" for pulsejet endurance competition for anyone that wants to try it.

Now all we need is a big ugly trophy to be handed off to the winner every year (or whatever) in some honking big hyped-up international ceremony.

There you have it, gentlemen - I'm throwing down the gauntlet! (Take THAT, you young punks! Ha! ;-)

So are we going for it, or what?

Remember, folks - you heard it here first!

L Cottrill

[larry cottrill]

OK, guys, since I KNOW you'll be fired up about this, here's my first grab of how to make a simple interchangeable tank setup with a fuel sump the engine will run from. In this configuration, only the sump would need slosh baffling, because it will act as an "air separator" that relieves any gathered air back to the main tank so it can't ever get to the fuel extractor pipe and on to the engine. This is just a "schematic" view - in real life, I can see the two pipes to the sump positioned side-by-side at the rear of the main tank, with the same arched curve in both pipes.

Drawing 1 shows the typical fuel layout early in normal flight. The sump siphon line continually refreshes fuel in the sump, and the sump stays flooded except for a tiny amount of air at the top. The fuel level in the sump vent line will stay the same as the main tank fuel level, except when occasional trapped air bubbles up through it.

In Drawing 2, we're near the end of practical flight time. The siphon line still remains full, providing fuel from the tank. At this point, the level in the sump will decrease along with the level in the main tank, with no fuel at all in the sump vent line. Yes, it's definitely time to land for re-fueling!

In Drawing 3, the plane is on the ground and the main tank is quickly pulled and set aside. As soon as the siphon line connection is broken, the fuel in that line will fall immediately back into the tank by reverse siphon action, and there will be practically zero fuel spillage. The engine is running on the fuel in the sump alone at this point, so now we have to hurry!

Drawing 4 shows a new main tank plugged in. The plane is momentarily tipped sharply nose up to flood the sump siphon and sump vent lines and begin siphoning action into the sump. Once siphoning into the sump is started, the plane can be re-launched immediately. Acceleration during the takeoff run will aid in filling the sump rapidly.

Simplicity itself, gentlemen. The only harrowing part I can see is that the fresh tank has to be held fairly level when putting it in place, to prevent spillage from the sump vent and sump siphon lines before they get pressed into the hoses. Not too tough with some good practice on the part of your pit crew. In real life, of course, there would need to be some kind of hold-downs over the swappable main tank.

Come on, guys ... you know you want to!

L Cottrill

[Irvine.J]

Well Larry, records were made to be broken, we'll see what we can do eh Gimme some time to think about the best way we can go about it.

[larry cottrill]

Well Larry, records were made to be broken, we'll see what we can do eh :) Gimme some time to think about the best way we can go about it.

Great!

Here's an important first revision to the fuel setup schematic. The earlier Drawing 4 would not have worked, because with both forward pipes of the sump closed off with fuel, there was no way for the air in the sump to be displaced. Duh. The solution is to extend the sump vent pipe clear to the front of the main tank - just takes a longer pipe and a lot more solder. I've also added a spillage dam and rear drain to lower the overall anxiety level during the pit stops.

Incidentally, for my U-control version of this setup, my fuel tank and sump would be rectangular and my outlet pipes would be toward the outside of the flying circle, in other words, in the lower outside rear corner. Also, the sump would be located to the outside of the tank CL. This is because for me, the outward G-force will be MORE important than gravity while in flight. For an R/C job, you could make the bottom of your tank and sump V-shaped to get good drainage to the outlet, or use a rounded bottom with an internal flexible hose and 'clunk' tank gimmick.

Of course, we're not talking about something that would stand acrobatic maneuvers, in either case. It will be very boring flying to set this record - the only probable excitement being in the throttled-down landings and pit stops. And maybe weather, for the real die-hards among us ;-)

L Cottrill

[Al Belli]

Hi Larry,

How about re-fueling the tank with a third control line of Teflon spaghetti tubing so that You don't have to land !!!!!!!!!! ( continuous re-fueling )

Al Belli

[Anders Troberg]

How about re-fueling the tank with a third control line of Teflon spaghetti tubing so that You don't have to land !!!!!!!!!! ( continuous re-fueling )

How much would that length of tubing, full of fuel, weigh? It could be a problem.

[Al Belli]

Hi anders,

I am thinking of using !/16" OD. X 1/32" ID. tubing, which will contain less than one fluid Oz. of fuel weighing less than one ounce.
A reasonable pressure will be required to force the fuel through the thin line at the proper rate and into the tank on the plane.

Al Belli

[larry cottrill]

Hi anders,

I am thinking of using !/16" OD. X 1/32" ID. tubing, which will contain less than one fluid Oz. of fuel weighing less than one ounce.
A reasonable pressure will be required to force the fuel through the thin line at the proper rate and into the tank on the plane.

Al Belli

I don't know ... to me it sounds like more hassle than it's worth. The other thing is, maybe the pilot would like some occasional pit stop time, too!

L Cottrill

[larry cottrill]

All right, here's what I think would be a good candidate for valveless pulsejet enduro - the justly famous MiG-15 fighter. This is just semi-scale, so I got away with a few mods without screwing up the scale appearance too much (except on the inboard side, which isn't seen much except by the pilot).

I wanted the wing to pass through more-or-less unbroken, so the engine CL is raised way above the scale centreline. Because I want to use a pretty thick stunt airfoil, the wing still had to be lowered (but only slightly) so the engine can pass over the top. If you look at Section A-A, you'll see this pretty clearly. The fuel sump is the only clunky part that cuts into the wing, and the wing can be strengthened around it easily. The large swappable main fuel tank is accommodated easily on the inboard side of the cockpit canopy area. The horizontal tail surfaces are enlarged considerably from scale, to increase controllability and sensitivity. Unfortunately, the tail moment arm is much too long for snappy stunt work, but lazy stunts like loops and wingovers should be available, to occasionally break up the dull weariness of endurance flying. Wingspan is about 48 inches or about 1.2 metres - a reasonable size airplane for a Lady Anne type engine to yield easy, relatively slow flight.

The plane could actually be built as a profile fuselage craft, because the basic structural unit is a vertical hardwood truss, probably faced with thin birch plywood on both sides, and with special attention given to the wing and engine mount points. This would be a very strong lightweight structure. Adding on the built-up fuselage is just a matter of soft balsa carved blocks and/or medium balsa thin sheet structure. The wing can be very light construction if stressed-skin fabrication is used - for example, triangulated rib truss design covered with either 3/32-inch medium balsa or 1/16-inch birch plywood - practically indestructible. The tail planes would be similarly built, but with almost-flat airfoils. The most difficult part would be control system rigging, because of the highly swept-back fin. The wing-mounted main gear would have to be anchored really carefully, because the stance is so wide (impact will have lots of leverage around the wing center mount point).

Though I eliminated a LOT of scale detailing, there's still enough left to make an impressive-looking fighter model: nose guns, wire and mast antennas, wing-top vanes, landing gear doors, etc. Any of this can be eliminated to get the model into the air quicker, of course, and flying would be unaffected.

I show an R/C type servo and valve for throttling, but there are several different ways that issue could be handled, of course. Batteries, etc. could be part of the much-needed nose ballast.

Would this be a great-looking ship, or what? Won't quite go up against James and Rossco's Sabre, but I'm after endurance, not combat performance.

Comments?

L Cottrill

[Anders Troberg]

Beatuiful, at least from the outboard side.

Some minor comments:

* Remember to rig the fuel system so that it's compensated for the centrifugal force. There may be problems with teh tank/sump interaction otherwise.

* Am I correct in that you are planning to do a retractable gear? How is that going to be operated?

[larry cottrill]

Beautiful, at least from the outboard side.

Thanks. That MiG was really a great design, and very distinctive with its huge fin & rudder. Unmistakable silhouette in the air.

Remember to rig the fuel system so that it's compensated for the centrifugal force. There may be problems with teh tank/sump interaction otherwise.

You're correct that the centrifugal effect from going around the circle is crucial. Even though this is nothing like a speed model, the outward G force will still be several times the downward gravitational force acting on the fuel.

That should be handled correctly with the tank vs sump orientation shown here. The siphoning action will be downward from the tank into the sump when she's sitting on the pad, but outward from the tank into the sump while she's going around the circle. As in all sport U-control models, the fuel outlet (on both tank and sump, in this case) are at the bottom and the outside wall of the tank, for good drainage in both ground and flight conditions. The engine has to "lift" fuel both upward and inward - its ability to do so has yet to be ascertained, but should be adequate to handle this setup.

Am I correct in that you are planning to do a retractable gear? How is that going to be operated?

No, that's not my intent. I show the bottom view with the doors closed solely for the purpose of providing flat plan patterns for the door pieces. Retractable gear have been occasionally used on scale U-control models, but that sort of rankles against the spirit of simplicity for which U-control is well-known and loved. One problem I have is that with U-control you're usually only a metre or two off the ground, and in case of engine failure you need to get your nose down and get into a landing NOW - you can't afford to wait while some mechanism sets your wheels down. The other issue is the extra weight of the retraction mechanism. In this case, the whole airframe needs to be built with absolute minimum weight in mind, for a good light wing loading. So, this one will fly around "dirty" with the silly wheels hanging out. At least that will help keep the speed down to something manageable.

All in all, this should end up being a reasonable jet sport flyer, even at times when you don't care about testing your endurance, but just want something good-looking and easy handling to fly.

L Cottrill

[larry cottrill]

This should clarify the main landing gear design. With the MiG's center wing design, we have the problem that if we (ideally) build the wing as a single unit, it has to pass through the fuselage center, which we can't do with both main gear fastened in! So, what we do is build a plywood socket for each gear strut that spreads the landing gear forces smoothly into the stressed skin of the wing (1/16-inch plywood sheeting) and make right and left torsion bar mount blocks that mate with the sockets. Then, the gear torsion blocks can be "plugged in" with lots of epoxy as one of the final steps in construction of the airframe. The small plywood shear gussets shown help to carry the landing loads into the wing harmlessly. These end up glued to both the socket plates and the installed torsion block, as well as the wing sheeting.

Torsion bar suspension is highly effective, and is easily accomplished with music wire with a little metal tubing slid over it before finally bending to shape. This is sandwiched between plywood plates epoxied together as shown in the detail sections here.

This should be full size when you print it on your 96 DPI printer.

L Cottrill

[larry cottrill]

This shows how the main landing gear torsion mount fits in place in the bottom of the right wing.

L Cottrill

[dustyattic]

There are header tanks we use on high performance R/C helicopters to try to keep air out of the carb while the heli is doing stunts. Is this the idea?

http://www.heliproz.com/DBCheaders.html

Also available is a "bubbless clunk". Supposedly fuel can seep in but not air. See here:

http://www3.towerhobbies.com/cgi-bin/wt ... XXF25&P=ML

Also, regarding the endurance plane... Does it have to be liquid fueled? Why not have 2 of the slender, hand torch cylinders with manifold and switchover valve? Fly with right hand tank till timer says land. Then just screw in a full left hand tank and hit the switchover lever. Meanwhile assistant unscrews and removes the right hand tank for refilling on site, ready for the next landing.

How will you slow this plane down on the ground with the engine running?