Fuel sumps and tank layout

[addaon]

Hey all, I plan to monitor this thread and add some sketches and drawings later to clarify what's not quite clear, but let's start with English and see how far we get.

Context: My design is a "true low-wing" (wing bottom skins is lowest surface of fuselage) retractable-gear monoplane with quite limited dihedral. It has two wing tanks, which start just outboard of the fuselage and continue to approximately a heavy rib that also acts as the gear mount point. The forward limit of the fuel tank location is the wing main spar; the aft limit is a stub spar that extends out to that same rib to help handle gear loads. The total volume constrained this way is about 25 gallons, and the desired tank volume is anywhere between 20 and 25 gallons. See approximately the green volume with the inboard skins removed:

(Moving the fuel tank to another location is greatly restricted by CG limits for a flying wing; the tank as shown is centered on the aircraft nominal CG. I'm also trying to avoid fuel in the cabin pretty strictly.)

Regulations: Part 23 doesn't directly apply, but I believe that deviations should only be taken with an understanding of why. I quote the following:

Sec. 23.971 — Fuel tank sump.​

(a) Each fuel tank must have a drainable sump with an effective capacity, in the normal ground and flight attitudes, of 0.25 percent of the tank capacity, or1/16gallon, whichever is greater.

(b) Each fuel tank must allow drainage of any hazardous quantity of water from any part of the tank to its sump with the airplane in the normal ground attitude.

(c) Each reciprocating engine fuel system must have a sediment bowl or chamber that is accessible for drainage; has a capacity of 1 ounce for every 20 gallons of fuel tank capacity; and each fuel tank outlet is located so that, in the normal flight attitude, water will drain from all parts of the tank except the sump to the sediment bowl or chamber.

(d) Each sump, sediment bowl, and sediment chamber drain required by paragraphs (a), (b), and (c) of this section must comply with the drain provisions of §23.999(b)(1) and (b)(2).

Sec. 23.999 — Fuel system drains.​

(a) There must be at least one drain to allow safe drainage of the entire fuel system with the airplane in its normal ground attitude.

(b) Each drain required by paragraph (a) of this section and §23.971 must—

(1) Discharge clear of all parts of the airplane;

(2) Have a drain valve—

(i) That has manual or automatic means for positive locking in the closed position;

(ii) That is readily accessible;

(iii) That can be easily opened and closed;

(iv) That allows the fuel to be caught for examination;

(v) That can be observed for proper closing; and

(vi) That is either located or protected to prevent fuel spillage in the event of a landing with landing gear retracted.

Concerns: The bottom of the defined volume is very close to flat, especially in the chordwise direction. (There's a couple of degrees of dihedral spanwise.) At cruise AoA, the forward portion of the volume is a fraction of a centimeter below the aft portion. At ground AoA, there's about a 3° slope with the aft portion lower. At climb AoA, this obviously increases. The intended fuel pickup point is in the aft inboard corner.
By 23.971, there should be an 8 oz sump. I see three major categories of ways to achieve this:

1. Use the entire volume for a fuel tank (likely a wet wing), and have the pickup a few millimeters above the bottom, such that 8 oz is unusable at the ground AoA. This is a very low, wide "sump", and also leaves a significant amount of unused fuel in the cruise condition.
2. Offset the bottom of the fuel tank from the volume by a few centimeters, set to be approximately level at the ground AoA and have a ~1 cm diameter x 3 cm height sump in that corner, with the pickup just above the sump. This is trivial if using a separate tank in the volume, but would require a "false floor" or some other approach for a wet wing.
3. Use the entire volume as a tank, and put a separate sump at a lower point. The only available point below this is the lip area around the radiator duct; and while it's possible to get enough volume there, this is the expected contact point in a gear-up landing and I don't see an obvious way to comply with 23.999(b)2(vi).

Skins-removed view showing what reduced tank volume looks like for (2), and shows placement of duct lip:


Request: If you have opinions on options 1, 2, or 3 (or an unknown 4), talk me through why! I'm interested in wet wing vs internal tank, and naïvely favoring the former for weight and usable volume reasons. I'm very much concerned about crash safety, most obviously for (3) but also for any wet wing setup. Brainstorming requested!

 

[Marc Zeitlin]

 

[Tiger Tim]

 

[don january]

 

[addaon]

 

[Marc Zeitlin]

 

[addaon]

 

[Chopndrag]

 

[addaon]

 

[Dana]

 

[rv7charlie]

 

[addaon]

 

[rv7charlie]

 

[rv7charlie]