3 section fuel tank

I have been contemplating fuel tank layout for a new KR and this came to me as an idea. If the idea is not workable, no need to beat around the bush. I can handle the critical thoughts( I am married you know)

I am curious as to whether this could work well. The common fuel tank setup for taildragger KRs would be a pair of equal sized fuel tanks in the stub wing area(of between 8-12 gallons each). These tanks are located between the main and rear wing spars starting immediately outside the fuse. If a smaller(roughly 5 gallon), triangular cross sectioned fuel tank (filling the void under the seat bottom area) was built to fit between the wing spars nearly the width of the fuse, and connected to both the right and left stub wing tanks, it would essentially create a 3 section fuel tank that would function as a single tank with the center tank being the active sump area.

I heavily lean toward (reasonably light gauge) welded alumunim fuel tanks. (A very good friend of mine builds aluminum fuel tanks for a living with 25 yrs. experience. His skills are absolutely top of the line, and I like the idea of being able to leak test them before installing them in a composite wing structure) My thought is to have a pair of small aluminum tubes (perhaps 3/4") protrude toward the fuse from each of the two stub wing tanks. One up high and all the way forward and one down at the bottom of the tank in the middle fore and aft. Both tubes would be long enough to protrude through the fuselage sides by about an 1 1/2". The center tank would also have a pair of aluminum tubes protruding (matching the location of the stub wing tank tubes). This way, a short flexible fuel hose could connect the tanks together by clamping them over the transfer tubes facing each other.

I would presume that a measure of slosh control would be needed (baffling in the stub wing tanks would be a given). Such as one way flaps on the inboard side of the lower fuel transfer holes to prevent fuel from draining quickly from high to low in a long banking turn, etc. Perhaps filling, or partially filling, the center tank with anti slosh foam could be useful. The wing will have its dihedral starting from the fuse sides, so there would be some measure of gravity promoting fuel to end up in the center tank(the lowest point).

The potential benefits:

1- a single fuel pickup in the center tank (fed by both stub wing tanks)

A- No need to worry about the safety and functional concerns regarding the fuel delivery from multiple independent fuel tanks. A BIG plus, considering that the complications involved with multiple fuel systems have been directly or indirectly involved with a fairly significant percentage of aircraft accidents.
B- Would allow a single fuel pump line and system. Far less complicated that selector valves, multiple fuel pumps, transfer lines, etc)
C- A single fuel sender and gauge (in the center tank) would simplify the calculations of remaining fuel.

2- The center tank could have a gasketed removable inspection port(that the fuel sender/fuel pickup line passes through) large enough (5" perhaps) to be able to inspect/maintain all mechanical parts(flappers, fuel pickup/filter, gauge sender, slosh foam?) This would not be possible with stub wing tanks being enclosed by the wing on its upper surface.

3- All 3 tanks would be in a location where there would be very little wiring(concerning possible sparks in case a tank/transfer tube was punctured in a crash). As opposed to having a header tank inches away from tons of active wiring, not that this has proven to be inherently unsafe or anything.

4- The center tank would increase the overall fuel capacity by a roughly 20-25%.

The way that you describe it, the high tubes are primarily for vent, and can be much smaller. Even the main liquid lines can probably by 3/8" without difficulty given the fuel flows that KR's run at. Well, unless you want to just fill from one wing. With 3/4" connectors, you probably could just fill from one wing with just a bit of patience.

The systems sounds simple and straightforward. One-tank systems like this are heavily used in Long EZ and all of its derivatives. Even with the small amount of dihedral, I would run a vent line between the high spots of all three spaces to make sure that vent pressures would not drive fuel to one wing.

The trap doors off the the wing tanks will help with keeping fuel in the center section during uncoordinated flight, unusual attitudes, spins, etc. Coordinated flight will no more move fuel outboard than straight and level.

I only have one concern over using the center location. If you were to belly in somewhere, you would want it adequately protected from rupture, being as you are sitting on that fuel. To my mind, that might mean a separate skid plate in between the tank and the belly of the bird.

Billski

Thanks for the thoughs.

As you suspected, I was thinking of the possibility of being able to fill the entire tank "system" from one of the stub wing tanks. Even with the mild dihedral of the stub tanks, the upper edges of the outer tanks would only be slightly higher than the center tank. It would seem to be easy to fill them all at one time as long as both the upper and lower(especially the lower) cross/transfer tubes were large enough to allow fuel to flow at a typical fuel pump rate. I threw out 3/4" as this isn't too far off from the ID of the average gas pump nozzle. If I were to go with seperate fuel fills, much smaller upper and lower tubes(such as 3/8" or 1/2") would be plenty I'm sure for a 100hp engine.

You mentioned venting between the tanks. The upper transfer tubes were intended to connect the "high spots" on all three tanks, serving its purpose for venting. That was my intent anyway.

Thanks for mentioning the skid plate idea. I was thinking along the same lines. I was thinking about using something along the lines of a 5/8" divinycell foam on the inside of the plywood fuse bottom skin between the wing spars in the fuse(under the center tank) and covering it with a layer of 3/32" aircraft grade mahogany plywood(to match the 3/32" ply of the underside skin of the fuse. This has shown to be very puncture resistant. It would certainly be much, much stiffer/tougher than the single outer skin of 3/32" ply.

I'll have to look into the Canard fuel systems. I understand that there are fuel tanks in either stub wing. I presume that they connect to a fuselage mounted center tank such as I am envisioning?



George

As I understand, (?) if the tanks is relative large in fore-after direction, it isn't bad to have baffles to prevent fuel to slush in that direction too, to prevent rapid tiresome CG changes. So baffles lay out as a cross in the typical KR stub wing tanks is a good idea.

Others, it seems like a good idea, maybe just because I hade the same idea with a pick up/inverted centre tank

Jan

Jan,

The stub wing tanks would be roughly 21" wide by 22" for and aft, and contoured to the shape of a 18% laminar flow airfoil. You are certainly right, a baffle set sid to side as well as for and aft as is typical with KR stub wing tanks. Some people add even more baffles. Fuel sloshing around freely in a plane as small and lightweight as a KR is certainly not a good thing.


You mentioned that you had the same idea with a "pickup, inverted centre tank". By that do you mean that you intend to have the same interconnected stub wing tank/center tank under the seat(s) arrangement, but with the center tank utilizing a flop tube fuel pickup to prevent fuel starvation during inverted flight?


George

Well, once the anti-backflow flaps are installed in the wing tanks, you will have to fill from both sides...

The vent system must have slight positive air pressure on the inlet, and be plumbed into the high spots on all spaces to ensure that pressures are the same throughout the system. There is much written elsewhere about having vent air plumbed to the high spots...

My point about a skid plate is a redundent sacrificial panel between things on the ground and your fuel tank. I think that is what you were describing, with the PVC foam as a spacer (we are thinking along the same lines there too). As far as what I had in mind - belly/PVC foam/plate/PVCfoam/tank, and perhaps the plate would be a Kevlar lamination.

Your idea will provide considerable protection against ripping the tank on a belly landing. It is all just a matter of how much prtection you are willing to add to the ship.

Billski

Billski,

I understand what you are saying regarding the need to fill from both sides if anti backflow flaps are installed. That was one reason I was thinking about using anti slosh foam in the center tank(and maybe part of the outboard tanks. I was thinking that if the anti slosh foam could serve as an effective means of preventing significant fuel shifting from side to side during uncoordinated maneuvers, perhaps anti backflow flaps would not be needed. If that is the case, I would guess that the tanks could then be filled from one side. I wouldn't think that the anti-slosh foam would slow the transfer of fuel from one side to another enough to slow down the fuel filling process.

I like your thinking on the skid plate concept. I was thinking ply/PVC foam/ply. Kevlar did not come to mind. Something to think about during some of those tough decision making moments, extra weight vs extra safety factor .


George

Kevlar is (in my opinion) overused in composite applications ust because it's "cool"; but for a skid plate, there's truly nothing better. Abrasion resistance is insane, and puncture resistance is competitive with anything else the same weight.

George,
Yes that was mine thoughts, not as much as a centre tank but a hopper Tank? A cylindrical tank placed inline with fuselage, with a floppy hose.

Jan