Is every part of a fuselage tank above the level of the carburettor or the fuel servo? For that to be achieved, the bottom of the tank would need to be well off the bottom of the fuselage. And you would not be able to climb 
So surely a pump is always needed in practice, in any low wing configuration. If you have a header tank then you solve the immediate problem but need a transfer pump to feed that.
ChuckGlider wrote:
I know about vapour pressure and vapour lock but in simple aircraft with a simple, gravity fed fuel supply, ie. no pump or convoluted plumbing – liquid downhill – vapour uphill – no vapour traps, it works just fine.
I have a low wing, electric fuel pump, carburettors above fuel tank (Rotax 914) and went up to FL200 without any problems on pure EURO 95 in ISA+15°C. Other aircraft with similar configuration did that as well….
If running Mogas the UK LAA Permit approval is for flight no higher than 6000feet.
@alanr is that for every aircraft type?
With a suitable fuel system design, the issue can be avoided, as europaxs shows. But maybe the LAA didn’t want to get into that too deeply?
surely a pump is always needed in practice, in any low wing configuration
All Sonex aircraft are built with gravity feed from fuselage mounted tanks. I think even the Lancair originally had that config. Sonex themselves recommend using only avgas though, probably due to vapor lock reasons. According to them, the more gadgets you can do without, the more reliable and trouble free the aircraft is. Lots of truth in that, no reason to complicate things if that complication isn’t needed. Running with mogas, maybe that complication is in fact needed, but there are other options also.
Peter wrote:
Is every part of a fuselage tank above the level of the carburettor or the fuel servo? For that to be achieved, the bottom of the tank would need to be well off the bottom of the fuselage. And you would not be able to climb So surely a pump is always needed in practice, in any low wing configuration
Two solutions for fuselage tanks that’ve been used are locating the fuel tank between the instrument panel and the firewall, and locating the fuel tank high behind the cabin, at the level of the pilot’s head in a plane with no rear windows. Both have been certified without fuel pumps.
This is one reason the carb is typically at the bottom of the engine on a Lycoming or Continental. The other reason is so that any fuel leaks don’t drip on a hot engine.
But why a restriction? Other than in mountainous terrain, surely if you get so high that you get vapour issues, you just descend and they go away?
Having ridden a plane with the vapor lock to the ground once, as a passenger, I can say that vapor lock doesn’t always go away. Also if the propeller stops and you’ve drained the battery trying to restart at altitude, restarting the engine is a problem regardless.
The eventual solution in that case was a fuel pump to pressurize the warm fuel line, plus a landing gear rebuild, and it didn’t happen again.
Ok. I just have experience with vapor lock in a PA32, which can suffer from it at oxygen altitudes, and it starts with rough running. It goes away when you sheepishly switch on the fuel pump you have forgotten… guess how I know.
Two solutions for fuselage tanks that’ve been used are locating the fuel tank between the instrument panel and the firewall, and locating the fuel tank high behind the cabin, at the level of the pilot’s head in a plane with no rear windows. Both have been certified without fuel pumps.
That, however, is feasible only with very small fuel tanks.
The crashworthiness of either of these is also mind-boggling
Peter wrote:
That, however, is feasible only with very small fuel tanks.The crashworthiness of either of these is also mind-boggling
About 3 hours range for a small engine.
I’d say 20,000 Piper J-3 Cubs (just one example) can’t be entirely wrong! The design choice is a matter of what you’re trying to do. One of my planes came with 3 hours range in a fuselage tank and was modified to 6 hours in wing tanks. There was obviously a reason for doing the mod. but for my use, for various reasons some of which aren’t relevant here, I wish it had been left as it was.
Small-engined Luscombes have a rather robust fuel tank located behind your head, mounted in a steel tube subframe within the aluminum fuselage. Thousands were built that way. People think it must be hazardous in a crash but the record doesn’t show that to be the case. There is an issue with climbing attitude fuel flow that is enough but no more than enough, and that you have to look behind your right shoulder to see the mechanical (Ford Model A, I’m told) fuel gauge. But no demonstrated crashworthiness issue, the fuel pump can never fail, and the plane rolls a little more responsively than with wing tanks.
