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<< Lancair Builders' Mail List >>
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Hello Dave Riggs:
>From the symptoms you describe, you have run out of turbo (your deck
pressure in falling). Deck pressure is the pressure after the intercoolers
but before the throttle (manifold pressure is after the throttle) and is the
pressure altitude the engine "thinks" it is flying at. With a MAP of 28.5" I
would guess your Deck pressure is about 30". You still have cabin
pressurization because at FL230 the cabin altitude is about 8,000 feet or
22" Hg.
As I recall the B models have a different, less efficient, compressor than
the later models which would make your engine more sensitive to restrictions
in the induction and air filtration system.
DANGER DANGER WILL ROBENSON!!! You may be on the verge of compressor stall
and flame out. Use the prop control to run higher RPM. If the engine RPM
keeps falling you may cross the stall surge line on the compressor map. Once
flamed out you may not get the engine to relight until 10,000 feet or lower.
I suspect you have one or perhaps two problems. Either you have a high
pressure drop across your air filter or you have enough of a drop to cause
the auxiliary air doors to open.
As you climb the pressure drop across your air filter increases. This is
because filters offer kinematic resistance to air flow (the air has to
change direction a lot to get through) and this resistance is proportional
to velocity. As you climb your engine is requiring a constant flow, and
volume, of induction air at sea level conditions. The turbo is spinning it's
little heart out to take larger and larger volumes of air and compressing
them to feed to the engine. At sea level and 2,500 RPM the air filter needs
to pass about 400 cubic feet per minute. At FL230 it is more like 800 CFM!
Solution. Find a K&N air filter that fits your plane. They have much smaller
pressure drops.
The second part of your problem may be leaky or prematurely opening
auxiliary air valves. The auxiliary air valves bypass the filter and admit
air from the lower plenum (area under the engine) into the induction system.
This is supposed to happen only if the filters are clogged (ice, volcanic
ash, feathers etc.). The air in the lower plenum is at about the same
pressure altitude as the outside air BUT it is much hotter and therefore at
a much higher density altitude. When the aux air doors open the turbos see
air with a density altitude of sever thousand feet (could be 8,000+) higher
than your pressure altitude.
What to do. Inspect your auxiliary air doors for correct function, sealing
and spring tension. I used a small postage scale to measure the force
needed, applied at the center of the door, to crack open the aux air doors.
Divide this by the door area to get the pressure differential needed. Less
than 1 PSI (~2" Hg) and your doors are probably opening in flight.
Three tests you can do:
1) Put a small piece of masking tape across the opening of the auxiliary air
door so that if the door opens one end of the tape will get sucked into the
opening, indicating the door has opened. Fly the airplane and inspect.
2) Cover and seal the aux air doors and test fly the airplane.
3) Remove the air filter and test fly the airplane (on a calm day).
Your problem could be as simple as not enough tension on the aux air door
spring and/or a too restrictive air filter. All this is based on the
assumption that Magnum is correct and the engine itself is fine.
Please post the results of your investigations.
Regards
Brent Regan
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