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MORE THOUGHTS ON COOLING
In the last two weeks, we have tested 6 EZs, two VariEzes and
4 Long-EZs, using a water manometer. We used a stock Cessna 150 as a kind of
"baseline". We found that a standard ram inlet EZ compares very closely to an EZ
equipped with a flush NACA inlet, provided both have well-built, close fitting baffling
and both have the same size, stock cooling outlet. Changing the size of the outlet will
change the pressure drop across the cylinders.
Of course, there is a lot more to cooling than the pressure drop across the cylinders. "Blockage", or the resistance to the flow of cooling air caused by the baffled engine is a big driver. Very poor baffling or no baffling at alI , obviously will result in a very low pressure drop. Very tight baffling forcing the incoming high pressure air to slow way down will obviously result in a large pressure drop. This differential is called the delta 'p' and is measured in inches of water.
Lycoming says that for a Lycoming 0-235 engine, you need a delta 'p' of about 4" of water. The curves shown below are the results of our recent tests.
Click to enlarge
Note that the two Long-EZs with the lowest delta 'p' across the cylinders (only 3" delta 'p' at 160). do in fact, have good cooling. Both are well equipped with 4 probe calibrated CHT gauges. What does this prove? Only that if the baffling is excellent, tight with absolutely no leaks, the cylinders will cool acceptably even with only 3" of water delta 'p', also, note that both of these Longs have smaller than normal cooling air outlets.
The temperatures on the above airplanes are measured at the bayonet cylinder head fitting on the Lycomings and on the top spark plugs on the Continentals. One of the Lycoming engines is really well instrumented, with probes on all four cylinders at the bayonet fittings, and on the bottom spark plugs as well as on the top spark plugs. The results of this test are as follows. Maximum power setting (mixture slightly rich for the climb) results in the bayonet probes averaging 360' 380' F. Bottom spark plugs average 400' - 420'F. Top spark plugs average 440' - 460'F. At 10,000 feet, OAT= +10', in level flight at maximum continuous power, (mixture leaned to best power max. EGT), the bayonet probes average 330' - 350'F. The bottom plugs average 360' - 380'F and the top plugs average 430' - 450'F. Inspite of an average difference of 70' - 100'F from the bottom to the top of each cylinder, this is probably about as good as you can do and is quite acceptable, according to Lycoming.
The optimum baffling for an EZ engine is probably not possible due to the mechanical difficulty of building it, but you can come close. For a Lycoming 0-235 or Continental 0-200, try to baffle as close to the sketch below as you can.