From: peon@pacific.net.au
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Date: Sat, 18 Oct 2003 20:35:36 +1000
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Subject: EWP Series vs PARALLEL pumps & flowmeter 
Reply-to: leon@aerota.com
Message-ID: <3F91A418.19800.1CF143C@localhost>
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In-reply-to: <list-2640625@logan.com>

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