Re: Fuel dump switch in homebuilt

Thanks, Bob K, or your insightful contribution to the discussion.

Thats good data to have about the fuel pump requirements for the pump
that feed the engine.  Have recently done some looking into of pumps
for a diesel to veg oil conversion project and I've discovered that
you really design pumps differently depending on what your goal is. 
For a fuel pump, you want to deliver small volumes at high pressure. 
In contrast for dumping fuel you want to move large amounts to a
negative pressure.  Have you seen any of the pumps they use in washing
machines?

But since its a negative pressure system, technically speaking, you
don't need a pump at all to drain the tank.  The only question, is how
fast do you need to let it go, that will dictate how big a pipe do you
need from the tank to the valve to the low pressure port on the
surface of the fuselage.  Of course the bigger the pressure
differential you can find the better.


[EMAIL PROTECTED] (Bob Kuykendall) wrote in message news:<[EMAIL PROTECTED]>...
> Earlier, [EMAIL PROTECTED] (Jay) wrote:
> 
> > ...The solution I suggested adds
> > another port on the fuel selector valve,
> > and a piece of plastic tubing to the
> > low pressure port.
> 
> Well, even with a bit of help from a low-pressure area, you're going
> to need additional pumping capacity to get the fuel overboard in a
> reasonable amount of time.
> 
> According to _Firewall Forward_, FAR part 23 guidelines specify that
> pumped fuel system be capable of delivering 125% of the takeoff power
> fuel flow, and gravity flow systems 150%. That's open-port flow with
> zero backpressure.
> 
> For example:
> 
> Say you're using a 180 h.p. or so motor that draws 18 gallons per hour
> at max takeoff power (let's say). The most you can expect the
> appropriately-sized fuel pump to move is about 25 gallons per hour.
> 
> But, more likely, you haven't got an hour. The vast majority of
> concievable small aircraft emergencies are likely to be over in 15
> minutes or less. That means you only have time to send 25/4 or about
> 6.25 gallons (37.5 lbs) of fuel overboard.
> 
> And that's if the pump runs wide open at the outlet. With any
> substantial backpressure, the flow rates will be much lower. And you
> might happen to need some of that pressure to apply fuel pressure to
> the engine fuel system inlet. I suppose that you could add a rate
> restrictor to the overboard port, so that fuel pressure is maintained
> even while dumping. But that's going to cut the dump rate
> substantially.
> 
> Of course, both of these points can be easily addressed. You can add a
> separate dumping pump just to pump fuel overboard, and you can size it
> to achieve the desired rates. You can also give the pump its own fuel
> supply and overboard plumbing, again sized for the desired dump rate.
> You can even plumb the dump system with a standpipe so you can't
> inadvertantly run the tanks completely dry with it.
> 
> However, that pump weighs something, and the fuel, electrical, and
> mechanical connections that service it also weigh something. And when
> you add up all that weight, it is substantial, and it will have a
> measurably deleterious effect on takeoff, cruise, and landing
> performance. It also adds many points of potential failure, both
> mechanical and human. And for those accidents caused by fuel
> exhaustion (a big slice of the pie), it means that the airplane hits
> the ground with more weight and energy, not less.
> 
> What you end up with is a compromise that balances constant and
> measurable penalties (weight and complexity) against hypothetical
> gains (safety).
> 
> Thanks, and best regards to all
> 
> Bob K.