Spar loading is another consideration. Without getting into boring mathematical details, adding fuel to the outboard section of the wing actually decreases both the shear load and the bending load of all spar sections inboard of the added fuel, provided the gross weight is not increased. Increasing the gross weight by adding outboard fuel is a slightly more complicated issue best left for another time. The above comments apply with the airplane in flight, but things are quite different when the airplane is being supported by its gear. On the ground, shear load is increased significantly by adding outboard fuel, but bending load is increased MASSIVELY! A hard landing or even taxiing on a rough surface is going to put huge loads on the wing spar if there is any fuel outboard. For this reason, I never land with fuel in the outboard tanks.
If an outboard tank is simply plumbed with a direct flow or even a one-way valve, it will never be emptied until the main tank is down to about 1/3 full. This was not acceptable to me. My plumbing system insures that the outboard tank is completely empty before any significant fuel is burned from the inboard tank. The geometry of the vent lines insures that even if the airplane is significantly uncoordinated or on the ground on an uneven surface, fuel cannot flow out of the vents. Additionally, the geometry of the fuel transfer vent from the outboard to the inboard results in no negative pressure head in normal, coordinated flight. The transfer pump may be used to eliminate the negative pressure head even in uncoordinated flight.
To address a few of the comments by other posters:
I do not think that it would be a good idea to eliminate the transfer pump. The transfer pump will prime the siphon on the vent/transfer line, which is why I always run both pumps for a few minutes before flight. Also, in addition to maintaining a positive head pressure in uncoordinated conditions, this pump can be used to address several emergencies I can think of. In particular, if there is any air leak in the inboard tank, whether due to bird strike, cap leaks, or deterioration of a seal, there may be insufficient negative head to draw fuel from the outboard tank.
Valid concerns, Pat. In my case, this aircraft (while I own it, anyway) will be operated only off paved surfaces, so the uneven ground and rough ground handling should be a non-issue, with the exception of landings with fuel in the outboard tanks. I read through your missive on your website about your operating limitations with respect to the outboard tanks before ever launching into this project, and you addressed every scenario I had in mind, plus a few I had not considered, and I take exception with none of your logic. I agree with your assessment of the shear and spar bending loads on the wing during ground ops and flight, and accept the restrictions inherent with auxiliary fuel ops. I had not considered the aspect of an atmospheric pressure leak from impact damage or seal detoriation on the main tank, but that is a valid reason to consider keeping the transfer pumps, if only a remote possibility. A much more driving reason, in my opinion, is the ability to prime the transfer lines as you referenced to eliminate any negative head pressure in the mains which could bite you at the least opportune time.
Greg, I can see three issues with your adaptation which give me some concern. I hope you have already thought through these, but I am presenting them here anyway for anyone else reading this thread:
1. All of the lines through which fuel actually flows in my system (or Van's standard system) are 3/8 inch tubing. Your vent/transfer line appears to be 1/4 inch. This is obviously adequate for venting air, but have you made any calculations for running fuel through it?
2. My vent/transfer line runs to the inboard end of the main tank for two reasons; first to prevent the outboard flow of fuel with the tip lower than the root, and second to provide a siphon system for the transfer of fuel. With your system, if the outboard tank has some fuel but is nearly empty there will be 4 or 5 inches fuel column negative pressure head applied to your inboard tank. This will be in addition to any other negative head in your system and if not properly addressed could lead to vapor lock problems. Frank addresses this by having an electric pump in his wing root, but a standard Van's fuel system could have issues with this, especially in high temperatures or using auto fuel.
3. If your vent leads overboard at the wing tip with a tank fitting at the upper outboard end of the tank, there will be nothing but suction keeping fuel in the tank if the tip is lower than the root. In uncoordinated flight or when operating on uneven surfaces this could lead to significant fuel loss. This is the reason for the vent line routing Van designed into the standard tanks. My routing is not as good as Van's, but it is better than a straight run out the wingtip.
Remember: this may be experimental aviation, but more experimental accidents are caused by fuel system problems than any other building error. Please be careful.
Pat
1: Agreed - the photo shows 3/8" lines for the fuel transfer pickup and 1/4" lines for the vent feed pickup. I reconsidered this before closing the tank (after shooting the picture) and replaced the 1/4" line for the vent feed with 3/8". 1/4" line would work just fine for air venting, but may present too much friction loss for flowing fuel at full power - especially considering the smaller openings in the fuel pickup and any possible FOD clogging that pickup.
2: My plan from the origin was to run wing-root fuel pumps as Frankh has, more for the aspect of solving potential vapor lock issues firewall-forward than anything else, but it will still solve this issue. I will eliminate the engine-driven fuel pump. As for the venting of fuel during non-level ground ops, refer to my opening comments - while I own the aircraft, it will not be operated in this manner. My next airplane is going to be a coin-flip between a Harmon Rocket and a Super-Six, I'll worry about aerobatics and grass strips with that bird. This one is a business travel airplane and will be used to pay for the next one.
3: Part 3 would only apply during uncoordinated flight or uneven ground ops WHILE carrying full outboard fuel. I intend to minimize this opportunity for fuel loss by every possible means within my control, but I also made the 1/4" fuel vent line into a 3-loop coil in the final bay before feeding to the exterior pickup. Below is a photo of the vent coil in the outboard tip of the right aux tank, the exterior vent connection is in the bottom right of the photo just off screen. While this won't completely prevent fuel loss in the event of a significant non-level gravitational vector, the loop will slow it down (for the event of temporary uncoordinated flight) and present an impediment to flow initiation in the event of a non-level parking surface. In addition, the 1/4" line itself will serve to limit the amount of fuel that could be lost in any given period of time, and this condition would only apply while the aircraft is in uncoordinated flight or on a substantially non-level piece of ground - one would hope that both these conditions would be very temporary.
Again, as I've said before, please don't feel like I'm shooting down your points. I
THINK I've considered all the implications with this mod, but it would be the high point of hubris to believe that this was actually true. All comments and objections are welcome and will be considered.