> Seems to me that if there is little enough resistance that
> the water back-flows through the dead pump, then they could
> be plumbed in series.  If there is too much resistance to
> plumb them parallel, then back-flow shouldn't be a major
> problem.  Of course if a cheap, simple, reliable check valve
> should appear ...

You're probably right, Jim. Unfortunately, I looked into McMaster-Carr, and found that every check valve that could take the heat and was 2" or so wide was also very expensive.

Somebody suggested two cores, one pump per core. That does tend to eliminate the flow problem, but two new issues crop up. First, now you need two cores. I know a lot of you RV folks already plan to do this, but I'm working on a Cozy and had only planned for one. =) But more importantly, this pump has a 2000 hour or so lifetime. That's not bad considering its price - I'll happily replace it as a maintenance item at TBO if it works. But it's not as simple as that. The way MTBF figures work, if you have two devices both operating at the same time you're roughly TWICE as likely to have a failure, so the MTBF is cut in half. This solution gets four timnes more expensive if you're replacing two pumps every 1000 hours, as opposed to just the one pump at 2000 hours.

The other problem with a series hookup is that since this pump is limited to 88lpm in flow, a series hookup with both pumps operating would probably not boost flow, so its only use would be to solve the problem if one pump fails. And it might overstress a pump if another was pulling the same load - it's possible this might overspeed the motor.

I've written to the manufacturer to see what they say, and will post their response when I get it.

In the meantime, I'd still love to hear from anybody who might have an elegant solution to a parallel arrangement, since I love the concept of being able to double the flow for critical times, such as while on the ground on a hot day, or during climbout, then shut the second pump off and have it act as a backup in case the first fails. 2 pumps == 4lbs, and the mechanical (no redundancy) is 10. Sounds attractive to me!

By the way - one parting shot. Somebody commented that it would be nice to nuke the belt as a risk factor. But these pumps still do draw a fair bit of power, and if your belt goes, so goes your alternator. You can probably run this thing on battery for a bit, but you'd better have a beefy one considering we're all talking about EFI and everything else that goes with a rotary...

On the other hand, if you know that you're descending, and not using much power, you could use a pump speed controller to slow it down and require less juice. But the counter to that is that it's more workload for the pilot during a stressful time. Lots of tradeoffs.

Regards,
Chad