November 2000

Flight testing continues...

Airspeed Calibration:

One of the early tasks in any flight test sequence should involve confirming the operation of the flight instruments. The altimeter ground check (+/- 50 ft) gives us an idea of the altimeter's accuracy prior to flight but the airspeed indicator is a bit more challenging. The pitot/static system had been checked for leaks but errors due to placement are near impossible to check on the ground.

A factory certification program would involve a swiveling pitot/static head on the end of a long boom similar to this fellow...

It is possible to obtain reasonable results without going to all that trouble however, provided you have access to a GPS.

The URL below takes you to a Java applet, which will calculate your ground speed from 3 GPS runs made on headings 90 degrees apart. It will give you an idea of how close your airspeed system is to accurate without the time and expense of mounting a test fixture like the above.

The Gadget's airspeed indicator worked out within 3 mph at cruise speeds. At a later time I plan to cross check these results against another aircraft (and get a few air to air shots for the pages).

Engine difficulties:

After about 7 hours the engine seemed to begin running leaner which I attributed to the colder temperatures. After another hour or so the engine began to run rough at full power.

The valve clearances were checked and recorded. The exhaust valves on the back cylinders were down to 0.000 while the other valves were 0.003 - 0.004. The clearances on these rear exhaust valves should be closely monitored for further reductions.

When the plugs were checked it was obvious there was a serious problem. The heads were pulled for further inspection.

The most likely causes of this kind of damage are detonation (usually caused by an excessively lean mixture) and pre-ignition (usually indicates timing too far advanced). While I will not rule out the possibility of a timing problem (and plan to re-check it) the most likely culprit is the mixture.

I am now beginning to wonder if the mixture on the Rev-Flow can be opened wide enough by my mixture control. I am using a standard push-pull mixture control cable (similar to the old style Cessna unit). Since fuel flow to the carb is in excess of 20 gph and the tank vent is clear (located 4" above the level of the forward deck) the next step would seem to be to confirm the engine can get enough fuel through the Rev-Flow mixture control.