REFLECTOR: First taxi test, a bit too exciting

Scott Derrick scott at tnstaafl.net
Sun Sep 13 22:36:23 CDT 2009


Got everything buttoned up today!  Well, everything thats needed to fly
anyway.  All paper work is signed off also.

Decided a taxi up to the wash stand for a badly needed airplane washing,
was a great way to start taxi testing.  This would be the 4th time I
have run the engine cowled up with the ram scoop on. If the taxi back
and forth was fine I was going to do a few mid speed taxi's down the runway.

Started fine, let her run for a couple minutes and then time for big
wind! I can't believe I've already forgotten how to taxi a Velocity? 
Just getting it going straight was more than frustrating!  After weaving
to and fro I finally remembered  the gentle taps and made my way to the
other side of the ramp in a semi stabilized manner.  Pulled up to the
hose rack and shut down.

Washing went as predicted, shammied her off and man did she look good! 
An airplane again!  Finally!

Got back in and did my normal hot start procedure, but all I got was a
short stutter and a  muffled back fire.  Tried  a flooded start, no
joy.  Tried a normal start,  another stutter and died.  This was weird! 
It had never done this before.  I tried another hot start, nada.. 

Hmmm....  Got out and looked and saw what looked like mist in front of
the induction scoop?   What the heck is that.  Got down and smelled, and
it smelled like smoke!!  felt the scoop and it has hot hot hot!  DAMN,
induction fire!!! And going on as I sat there! 

I ran back to the cockpit door, full throttle, idle cut off and held the
starter switch on for 30 seconds, seemed like minutes! 

Crawled back under the cowling with my flight bag fire extinguisher in
my hand, no smoke now in the scoop, and the scoop was cooler though
still really hot!  Looked into the engine cowling next to the prop and
saw no flames or light of any kind in the area of the induction system. 

Grabbed my screwdriver and tried to quickly take the back side of the
scoop(its in two pieces)  off. Finally got it off and  everything in
side the scoop was covered with black soot!  The back side of the filter
was browned and the rubber was melted.  Black soot going up into the air
servo...

Man, I almost burned up my airplane with an induction fire!  And I was
probably throwing on more and more fuel with every start I attempted!

After thinking things over there are two problems I built into the  new
cowl/scoop. 

1.) no drain hole in the scoop for excess fuel.  The scoop is so tight
it just pools up in the scoop next to the filter if i get too much fuel
when priming.  Gonna have to have to drill a couple bleed holes for
that. It will suck  a minuscule amount air through the holes at slow
speeds. The scoop is pressurized at cruise speeds though.

2.) The air servo inlet is about an inch below the level of the lower
cowling.  I had to make a small box like structure in the lower cowl
that protrudes an inch below the cowl that the servo inlet bolts to. 
The scoop then fits over this box on the outside of the cowl. When
washing the airplane(or flying through rain for that matter) the water
that entered the upper NACA's dripped through the engine and found the
low spot, the servo box at the lowest level.  This channeled some/most?
of water into the induction scoop which pooled inside already for that
first start to suck some of it into the engine for my first aborted
start attempt.  I need to make a  small  1/4 inch  raised lip  around
the  box inside the  cowl  with  drain holes  so most of the water can 
drain  out of the  cowl without  flowing into  induction scoop.  The
small drain hole in the scoop mentioned above can allow any water that
makes it into the scoop to drain away.

Well, I guess thats why we call them experimentals.

Scott

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