On Dec 10, 11:06 am, GeorgeB wrote:
On Mon, 10 Dec 2007 09:23:52 -0800, Richard Riley

wrote:
If you're flying a homebuilt you can burn whatever you want - but the
alcohol restriction wasn't put there at random, it increases vapor
lock problems dramatically,

How do the planes which do fly on ethanol handle that problem,
pressurized tanks?

and is incompatable with many of the
materials commonly used in aircraft fuel systems.

The sealant sloshed in the tanks is one, I think ...

Do automobiles with flex-fuel capability do anything to minimize the
vapor lock issues? I'm sure the materials were selected to be ok.

Most autos use electric in-tank fuel pumps now. When the
fuel is pushed to the engine there's little vapor-lock risk. Aircraft
still often use pumps on the engines that pull the fuel, so that the
pressure on the fuel in the lines drops and the vapor pressure of the
fuel will cause vapor lock under the right conditions. Low-wing
airplanes that have the tanks in the wings will have boost pumps
somewhere low in the system, but when they're turned off after takeoff
the risk of vapor lock rises with autofuels.
Remember the old high-school science demonstration of water
boiling at room temperature when a bell jar is placed over a bowl of
it and the air sucked out of the jar? The lowered atmospheric pressure
lowers the boiling point of the water. Gasoline has a higher vapor
pressure than water, so lowering the pressure on it will make it give
off vapors quickly, and those vapors displace the fuel in the lines
and prevent the fuel flow. The pump will be quite happy to pump
vapors, but carburetors and fuel injectors don't deal with vapors very
well, and the engine gets hungry and goes on strike.
A pump that sucks the fuel to lift it from the tank lowers the
fuel pressure between the tank and pump. A pump that pushes it upward
from the tank avoids that.


Dan