<... 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 ...>
Which is not exactly the point. The point is the likelihood of
a [complete] cooling failure. That would go up roughly infinitely
if you had two units. Further, there would be no need to "change
at TBO" - you could run until failure. As for check valves, I can
envision a couple of ways to build a "dirt simple, dirt cheap" valve that
would be more than sufficient for our purposes. Ed or whoever just
bought a toy mill could start a cottage industry. With check valves,
one could switch water pumps periodically in flight, thus keeping them
both "exercised" and promptly detecting any failure. As for high
demand moments, if the pumps were plumbed parallel, you could use them
both for takeoff, climb, etc.
<... can probably run this thing on battery for a bit, but you'd
better have a beefy one ...>
Anyone with an all-electric airplane MUST have redundant alternators
or batteries. You've GOT to have at least an hour GUARANTEED after
alternator failure. I regard that as a given, regardless of whether
your water pump is engine driven or electrical.
<... use a pump speed controller to slow it down and require less
juice ...>
You already have that in your temp driven controller. When you're
going down, the temps cool off and the pump slows down automatically.
I think parallel pumps with check valves is the "Ultimate Solution"
Now listen, one and all, to my latest bran fart:
.... Build yourself a small plenum. You could use a 2" (or whatever)
PVC butt connector - we'll start with that for the sake of argument.
You ream out the inside for half its length so there is a small "step"
in the diameter of the connector in the middle. The larger diameter
end is downstream. Now, bore a small hole just downstream of the
"step". Now, obtain a couple inches of Al piano hinge. Replace
the pin with a smaller piece of stainless piano wire. Cut the hinge
in a circle so that the hinge line forms the diameter of the circle and
the diameter is smaller than the big end of the housing, bigger than the
small end. Put the hinge (assembled but with no hinge pin) just downstream
of the step, push the pin through the hole you drilled in the housing,
close up the pin hole with epoxy and - Viola' a dirt simple,
dirt cheap check valve.
--
Jim Sower
Crossville, TN; Chapter 5
Long-EZ N83RT, Velocity N4095T
"Robinson, Chad" wrote:
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
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