lml@lancaironline.net Mailing List Archive

Angier,

This can get a little involved - all very doable however. To see what is going on under the hood, I would suggest following along and replicating the measurements taken in this report:

http://n91cz.com/Pressure/PlenumPressure.pdf

You are basically looking for two things. First is how much pressure recovery you are obtaining above the engine and then secondly how much leakage you have bypassing the engine. The pressure drop vs. mass flow curve for standard Lycoming cylinders is available. The problem is that one needs an independent measurement of the mass flow to look for leaks. If you just measure pressure drop across the engine, you will get that mass flow, but there might be other paths present.

I used a pitot tube in the throat of the inlet to get a second source for mass flow. My oil cooler uses upper plenum air, so I also had to measure the mass flow through the oil cooler as well. In the end, all the numbers lined up so I had confidence of good sealing.

Frank Motts (Lycoming) has a good paper that walks through all the calculations required to determine the losses in terms of power in the system. Instrumenting the plane is the most tedious part. Testing itself is quick.

Chris Zavatson

N91CZ

360std