Chuck wrote:

        I returned from a trip to visit my sister near Seattle .... SNIP

    I had a blast, the trip went real good, except for two problems. First,
I

had a trailing plug fail , it broke the insulator and would misfire

badly

with full throttle after a few seconds. I think the broken pieces of
insulator would glow and cause preignition. So I just used less than

full

power, fortunately don't need full power to take off.
        Second, I noticed that on take-off at the low elevation fields,

which was

most of them on this trip, the RPM would be at 5200 which is normal,

then it

would drop off after 3 or 4 seconds into the takeoff roll to maybe 4700.

It

was "interesting" at first! My established minimum T.O. RPM is 4700, so

it

still lifted off fine.

Ed Anderson - I remember you posting about similar occurrences with the

prop

speed, but cannot recall what the resolution (if any) was.

Chuck Dunlap
N616RV - RV6 with 13B

Ed Anderson wrote:

Great hearing about your successful trip, Chuck.

Yes, regarding the rpm drop.  In my case, particularly with cooler OAT
(increased air density)  the engine will rev as high as 5600-5800 static,
but during take off roll the rpm will decrease approx 400 rpm at some point
after starting takeoff roll.  Best I can estimate it happens around 40-50
IAS.  Although, I can not prove it, several folks have suggested that  my
prop blade is partially stalled at the higher rpm and air loads sitting
still.  Once rolling and picking up airspeed the prop apparently unstalls
and loads up causing engine rpm to drop temporarily.  I broke the ceramic
cone (around the center electrode) on two spark plugs when I had the old
engine ignition timing overly advanced.  It just chewed the plugs to pieces
and cause their failure in as little as 10 hours.

Glad your trip was safe and fun - way it should be.

Ed

 
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