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Subject: RE: [FlyRotary] Re: EWP - Success at last?
Date: Sat, 11 Oct 2003 11:00:00 -0400
Message-ID: <0B27ED95697C4D4CBC82D79E790FE567086F86@exchange.rfgonline.com>
Thread-Topic: [FlyRotary] Re: EWP - Success at last?
Thread-Index: AcOPqJWuZeAIuPCWQ1mDqIFMSlb9GgAXZbew
From: "Robinson, Chad" <crobinson@rfgonline.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>

> Seems to me that if there is little enough resistance that=20
> the water back-flows through the dead pump, then they could=20
> be plumbed in series.  If there is too much resistance to=20
> plumb them parallel, then back-flow shouldn't be a major=20
> problem.  Of course if a cheap, simple, reliable check valve=20
> 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. =3D) 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 =3D=3D =
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