Hi Guys,

Maybe I'm having a "Senior Moment" as Jim Sower would say,  but
I feel I've just gotta butt in here.  Been reading all the to and fro of
this dual EWP thingy.  As you can imagine,  I have devoted much
thought to redundancy of EWPs in an aircraft situation since I first
spoke about EWPs on that "other" place (where HIM ran the
numbers and said they wouldn't work)..

My idea is that the most efficient use of a dual (redundant) EWP
set-up is that they should be run in PARALLEL with two simple
non return valves. Yep,  sure it's a bit more complicated to
engineer,  but I believe it is worth the extra effort.

That way,  you will get nearly double increase in flow (weeeellll ,,,
maybe 70 - 80%?? depending on the actual flow capacity of the
system )  with both pumps running when you need it - for take-off
and climb-out or go-around  - once in cruise mode,  you can turn
one of them off to reduce current draw.  

As an aside,  similarly,  when running dual EFI fuel pumps,  they
are always (well in race cars at least) run in parallel with non return
valves.  What we need is extra  FLOW,  not pressure.  In aircraft
use,  I plan to run two fuel pumps in parallel feeding from a
common sump or header tank,  and have both on for take-off and
climbout and landing (in case of go-around),  but be able to switch
one of them off for cruise to reduce curent draw. Same applies with
the EWPs.

With EWPs,  a switching mechanism can be arranged so that the
second pump will come on and off automatically whenever the
water temp goes over or under pre-set limits.    An ordinary
thermoswitch of the type used for auto radiator fans - especially the
auxillary air cond fan(s) are pretty reliable.  Obviously,  you have a
manual over-ride switch and a nice big coolant temp warning light
in case the thermoswitch decided to go awol.

When you run pumps in SERIES,  you will increase the
PRESSURE considerably,  but not the flow by much.  The reason
you only get a 25% increase in flow when both are going is that the
second pump and the first pump is seeing a bit LESS 'head"  as
the outlet is on the "suck" side of the second pump).  But it's still
pumping through the same size orifice!  

My concern is that if they are run in series,  just when you need
flow most,  (as in one pump failing - which is the purpose of
redundancy - you will get 20% LESS flow than one pump on its
own as per Todd's tests).

See,  . runing just one pump in series with the other freewheeling,  
the other impellor  will cause resistance to the flow as it tries to
spin up the impellor,  and make the electric motor try to act as a
sort of dynamo.

By the same token, the MAX flow capacity has to do with the
minimum diameter of the inlet and / or outlet the size of the hoses,  
and the total internal resistance of the centrifual housing.as well as
any restrictions elsewhere in the cooling system.
  
So my personal opinion is that EWPs should be the same as fuel
pumps -  run in parallel.  (But now I'm repeating myself - I'd better
get out of your way now!)   Comments and criticism welcome.


Cheers,

Leon