How many homebuilts fly "high"?

Peter wrote:

The crashworthiness of either of these is also mind-boggling

The Pawnee also has fuselage mounted tank, in front of the dash board. Later versions have wing mounted tanks. They went to wing mounted tanks due to fire when they crashed, which they did from time to time as crop dusters. However, what was wrong with the tank was not in particular where it was placed, but how it was made. It was made of thin fiber glass. In a crash, the tank ruptured and started leaking, even if the crash was survivable. A mod is to install a nylon tank on the inside, and this fixes the problem. This nylon tank has it’s own problems, I had a dead stick landing due to it.

Modern tanks like this are made of a special plastic, same as cars use. I don’t remember the name, but it’s almost crash proof. If it ruptures, the crash is so hard, you have no chance of surviving in any case. Very ductile aluminium will also do I guess.

Vapor lock requires a combination of low pressure and high temperature. The only place you have that combination is inside the cowling. If you have no pump, you must be sure to make enough cooling for the fuel lines, and also make the fuel lines so that any vapor cannot lock the fuel. That’s how the default fuel lines in the Sonex line of airplanes are designed. Fuel goes continuously down to the gascolator from the tank, and continuously up from the gascolator to the carb. I’m also installing a cooler for the cascolator, since I will be using mogas from time to time. Any eventual vapor formed will either just rise up to the tank and condensate, or will go up to the carb where it cannot do any damage in any case, except maybe some limited “purping” when the vapor gets sucked into the carb. If you think about it, a real vapor lock can only exist if you have an engine driven fuel pump, and vapor is formed upstream the pump. The only thing that will help, is an auxiliary fuel pump mounted somewhere else, so the fuel pump is primed with fresh fuel and/or removing the vapor by the pressure from the auxiliary pump.

Taking off in an O200 powered Jodel DR1050, with gravity feed from the front tank (over legs, between firewall and instrument panel), and engine mechanical pump and electrical pump on, I’ve had vapour lock with mogas. Run-up checks were OK, but on opening the throttle, the revs rose to 1500, then decreased, and the engine stopped. It was quickly restarted with enough power to clear the runway. Draining the gascolator produced froth.
Front tank fuel temperature had been checked as below 20°C. Aircraft had been parked into wind after a flight, on a warm afternoon. Presumably engine heat had reached the electric pump and gascolator, low behind the engine.
Flown without problems in cool evening air.
A Jodel with no electric pump, and a failed mechanical pump, was successful flown home, with virtually no climb capability, but able to maintain level flight on gravity feed from the tank. Might not work if the tank was losing fuel.
Would the suction of the pumps increase the possibility of vapour lock in the gascolator?

Maoraigh wrote:

Would the suction of the pumps increase the possibility of vapour lock in the gascolator?

Was the electrical pump also placed downstream the gascolator, inside the cowling? If so, that seems very counter productive to me regarding preventing vapor lock.

I think it is upstream of gascolator, but I’m not sure. I live 80 km from the hangar, and haven’t been there for weeks, as the one-piece wing is off and away for wood repair and recovering.

LeSving wrote:

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.

I’m afraid the use of AVGAS itself might cause problems. It doesn’t do any good in a Rotax (the lead settles everywhere and reduces the maintenance intervals) and I can imagine it doesn’t in an AeroVee (VW-conversion) neither. Hence I doubt, that the usage of AVGAS contributes to a more reliable and troublefree aircraft(engine) other than avoiding vapour lock.

BTW the lifetime of the electric fuel pump in a car normally exceeds the lifetime of the car.

Maoraigh wrote:

Would the suction of the pumps increase the possibility of vapour lock in the gascolator?

I recall reading an article in the EAA magazine many years ago about vapour lock. Piper had built a test rig of the fuel system for one of their models, I forget which, but built entirely from transparent materials so they could see what was going on. The most reliable place for vapour to form was on the supply side of the fuel pump. This had nothing to do with temperature and was all to do with the pressure drop caused by the suction of the pump.

My own little 50 year old machine, low wing, single fuel tank ahead of the panel, is entirely gravity fed, downhill to the filter and downhill again to the carb. Gravity has never been known to fail.

As to the possibility of vapour lock stopping an engine at higher altitudes and the pilot draining the battery attempting a restart, for many (most?) engines I’d say not very likely. Disregarding takeoff from a high altitude field and therefore low height above ground, the pilot will (a) have a lot of height to play with and (b) very probably not be faced with a stopped prop so no battery drain necessary. I have seen this demonstrated and very aggressive action was required to stop the prop.

ChuckGlider wrote:

This had nothing to do with temperature and was all to do with the pressure drop caused by the suction of the pump.

That doesn’t sound right. Vapor forming is a function of pressure AND temperature.

LeSving wrote:

That doesn’t sound right. Vapor forming is a function of pressure AND temperature.

Why not? You don’t necessarily need both. A pressure drop at constant temperature could be enough.

Yes, but lower the temperature, and you suddenly will not get any vapor forming. It has much more than nothing to do with temperature.