On our 912S we have a fuel flow gauge that normally displays fuel_flow =
flow_out
-
flow_back. It has an optional display that shows flow_out and flow_back
seperately.
From
this we can tell if there is any return flow and how much it is. We usually see
about 2.2
gph (gallons per hour) for the return flow. Boost pump on or off makes only a
small
difference, as the pressure only changes from about 4 psi to about 3 psi, as I
recall. It
is my understanding that the return flow numbers for 914 equipped planes is
greater,
presumably due to greater fuel pressures.
A frit type restrictor with many microscopic holes would be more resistant to
clogging
than a single hole type as supplied. We have considered changing ours to a frit
type,
although we have had no trouble with the supplied unit in 250 hours, having just
checked
it just a few days ago.
I don't believe there would be any 'over pressure' issues due to a clogged
return
restrictor, just potential problems with vapor lock. There have been 912 and 914
engines
flown without return flow.
I have heard of vapor lock problems with a 914 installation that had a
non-standard
return flow arrangement, and we believe we have had vapor lock problems with our
912S
that has the 'standard' return flow setup. We believe our problem is related to
flights
in the winter time on auto fuel (more volatiles in the winter formulations), and
following a hot start, where the fuel system had a chance to heat soak before
the
flight.
The problem is rough running at higher power settings, that slowly gets better
in flight.
We are still looking at this one, instrumenting fuel system temps in flight,
etc.
regards,
Terry Seaver
"Ronald J. Parigoris" wrote:
>
> Taking a look at how fuel is returned to tank, it uses a small restrictor to
> keep pressure up.
>
> I have a few questions about this.
>
> What would happen if it clogged?
>
> How would you know if it is clogged? Wait for vapor lock sound of silence?
> Wait
> for black smoke to appear from exhaust (perhaps fuel pressure would overcome
> needlevalve), so black smoke when flying and motor that would flood at idle?
>
> On 914? Running on 1 pump? Running on 2 pumps?
>
> On 912S? Running on 1 pump? Running on 2 pumps?
>
> Is there provisions to put some sort of mini filter just up flow of it? I have
> not looked first hand at oriphis, perhaps it has a cintered bronze style
> filter
> on it?
>
> Thx.
> Ron Parigoris
>
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