Fuel Tank

[Tom XL-7]

never  considered hitting my head on the underside of a center wing tank. That top fuselage tube would give my head a.75 inch warning. This isn't anything I have to concern myself with at this time and it is always reversible. 
I have been more concerned with the wisdom of storing fuel  directly overhead. I guess a ruptured tank wouldn't be a good thing whether  it was overhead or in your lap. At least, overhead is not adjacent to the hot "ignighty" parts. It very well may be safer in the wing. Although it is mighty close, just over a shoulder. 
 William Wynne of flycorvair fame was involved when his pietenpol was stalled and spun directly into the ground.  Fuel from the center wing tank leaked and caught fire. This was a high speed auger - nose into ground. He was not in command but was a passenger. The engine quit and the pilot decided to turn the piet into a sailplane. 
 His story focuses on pilot skill -lower the nose and maintain airspeed rather than the fuel tank location, but I have always thought of it when considering the center wing design.
Not so sure a J-3 type tank would have been better , very likely worse. 
If you can predict what type of crash you are going to have perhaps you shouldn't fly. But we should always look for any safety edge we can find . Just like those ounces and grams you can leave off.
 Tom XL-7

[Steve]

never  considered hitting my head on the underside of a center wing tank. That top fuselage tube would give my head a.75 inch warning.

 Tom XL-7

Tom:
There is a material difference in wing attachment arrangement between the LE and the XL... Vertically it amounts to inches in difference... Only an inadequate tank can exist above the fus tube on the LE... On the XL are you planing to take take the fuel out at the front of the tank with no sump?... There might be some lost (any) fuel availability in a sustained climb... Sump at rear of tank takes you below the fus tube with plumbing and into the pilot's head territory...

Again, I mention that fuel system glitches are one of the higher frequency fails in home-building and they happen to many people across the engineering skill spectrum... Build a proven fuel sys design and enjoy flying the airplane - you will be thrilled with your achievement...
Steve

[Tom XL-7]

I think a sump would be prudent and the rear location would support climb out best. But I get your point. While running out of fuel in an initial climb out is the worst time to happen one should not be leaving an airport empty.  But running low on fuel does require a landing. so a front port might seem a good idea. I was thinking of rear sump but yes my head has a similar address. 
Luckily while I like the idea I am not committed to a center wing tank. There will be time to mock one up and sit in the seat. Use a spike to represent the fuel petcock. That will get my attention.  No way.
 Maybe a better use of time than making airplane noises.
 Good point - as I said I had never considered it a problem.  Now it is another constraint that has to be solved.
Thanks
Tom XL-7

[Steve]

Tom:
I think you are completely ready to have a peek at Pat Main's[slowride] run at building a metal Spratt 103 - those wing halfs are just a tad over 20 lbs - no ailerons ofcourse... Pat and builder/flyer friend have matching XL's and they did the total conversion of the 1/2 engines cutcase method including the crank adaptions - you should see their prop duplicator machine - a rework of Pat's Dad's axe handle making machine... Are these guys gear heads or what!...

My message, build your dreams!...
Steve

[Tom XL-7]

So after all that on center wing tanks, there is one heck of a convenience of fueling a fuselage mounted tank. The weight of 5 gallons of fuel seems to be variable because there seems to be a range of 5.8 to 6.5 pounds per gallon. So we can compromise to approx 31.25  pounds.  The center wing tank is mostly centered on lift, so is not an issue.
I never read Sam or Les complain about the changing balance.  Much easier to keep an eye on fuel levels as well.
Swivel head
Tom XL-7

[scottiniowa]

So after all that on center wing tanks, there is one heck of a convenience of fueling a fuselage mounted tank. The weight of 5 gallons of fuel seems to be variable because there seems to be a range of 5.8 to 6.5 pounds per gallon. So we can compromise to approx 31.25 gallons.  The center wing tank is mostly centered on lift, so is not an issue.
I never read Sam or Les complain about the changing balance.  Much easier to keep an eye on fuel levels as well.
Swivel head
Tom XL-7

Tom,
You mean 31.25# in weight, yes.
Yes, if you have a "centered wing tank" the CG will change the least. Though in right or left wing, has not been an issue as it is pretty close to the center LONG axis of the aircraft.
Yep, filling a fuselage mounted tank, is easier than a wing tank.
Yep, if you have a tank right in front of you, you can watch the fuel level on your fuel float (bobber rod) vs looking slightly up and observing a clear line that you have barred line behind.
attached is a center tank photo of 5.3 gal. before the center tube notch. With a 6'2" pilot below.

[Tom XL-7]

I modified the original post from 31.25 gallons which was wrong to the corrected 31.25 pounds
good catch Scott
Tom XL-7

[dz1sfb]

... attached is a center tank photo of 5.3 gal. before the center tube notch. With a 6'2" pilot below.

So to make up for the loss of volume across the top tube, the tank could be extended fore and aft above the wing mount to some degree. This also means there are sumps on each side of the top bar where one would want some kind of balance tube fuel pickup.

 I could see mounting bosses from underneath the extensions to the wing mount posts.

Great work Scott! What CAD system/s do you use?

Ken

[scottiniowa]

Yes, exactly Ken!
  And very simple angles, (bosses-attached to the tank) resting on the fore and aft wing attach squares-  Keeping in mind, the tighter this location gets, the harder it will be to install wing attach bolts.  But many planes are done, with much tighter conditions than these.

AS for keeping fuel even, between sides- yes I think is a must- can either be done by two sumps fore and two aft. Or the simple sump with a cross connection to the other side.  And yes, there are those that say they will never take off with 1.5 gals of fuel left- But then you will also have to say you WILL NEVER LAND  with 1.5 gals of fuel.  To that I will simply say, never say never! When the fuel flow becomes unported, it is a bit late for wishing for more fuel, or to get level and hope it will restart by magic.  To run out of fuel with the tank empty-is much acceptable to me. though still not a good thing.

SolidWorks '16 is the CAD
cheers

[dz1sfb]

Thanks Scott,
Certainly want to leave enough finger and plier room for wrenches and cotter pins install/removal. I still like the addition of a header tank as well.

I'm a Unigraphics NX (day job), TurboCAD, and SketchUp (home business) user.

Ken

[joecnc2006]

So what exactly are the pros and cons for the fuse tank? is this correct so far?

Pros:
1. Easy to fill
2. easy sight fuel level

Cons:
1. CG gets pushed forward
2. have to manufacture a tank

[Dan_]

So what exactly are the pros and cons for the fuse tank? is this correct so far?

Pros:
1. Easy to fill
2. easy sight fuel level

Cons:
1. CG gets pushed forward
2. have to manufacture a tank

Con: Trim change as fuel is burned off
Con: Very low head pressure
Con: Tank volume (See S. Kiblinger post below)

[Will Weidner]

I believe Les and Sam have both mentioned that landing is more challenging when the tank is full.  Perhaps they can weigh in on this.

[scottiniowa]

have to manufacture a tank

I don't think we could/should list this as a con- as to get "any" tank the way we really want it to fit the space, we will or would have to mfg it.
And if your capable of welding the frame, building the wings or tail feathers, the tank is easy.

Of course there are easy and hard ways to build the tank. So a bit of R & D  goes a long ways in this area.  But a little creative welding, and tab making can go a long ways to building a alum, tank in a few hours.

[Sam Buchanan]

I believe Les and Sam have both mentioned that landing is more challenging when the tank is full.  Perhaps they can weigh in on this.

I never made that statement and it doesn't reflect how my plane flew.

There was negligible difference between full and partial full load on my plane because of the limited about of fuel used and it being located fairly close to the CG. If there had been a difference in handling, landings would have been easier with full tank because pitch would be less sensitive.