Old Wives' Tales of Power Management

Pilots are taught to believe their handbooks or manuals and tend to believe that the "facts" shown are absolute.  Once in a while the opportunity presents itself, like in our FAA test flights, to prove that some of the testing by the manufacturers was inaccurate and of poor quality.  The engineers, however, historically have recognized the inaccuracies of early testing and have built in broad safety tolerances to cover these testing shortfalls.

A perfect example is the CHT green arc and redline on your Mooney panel.   Ours shows 475°F as the top of the green and 500° as red line.   These temperatures are confirmed in the Lycoming Handbook.  Fuel flows are recommended as 125°F rich for Best Power and 25°F rich for Economy Cruise.   After flying with possibly the best test pilot in the business and the CEDI computer, these are seen as patently wrong.   They appear to be temperatures set either by conservative engineers or protective attorneys.

Our engines proved they prefer to be flown at TIT Peak.  At that point all the like item temperature numbers on our engine analyzer came closely together and the CEDI recorded the lowest level of operating vibration.  One test pilot, airline captain remarked that the smoothness was like flying a turbine.

Unwilling to go against the numbers put out by both Mooney and Lycoming, M-20 compromises by recommending cruising at 25° Rich as Best Power (their Economy Power).  Then, not only is most of the fuel converted to power and speed, but the CHT stays as cool as Lycoming recommends "for maximum engine service life".  On the #3 CHT, that is 40° cooler than the top of the green, 65° below redline.  Why should a green line include a 40 degree range of engine life shortening temperatures and over-rich fuel flows?  Whose tail is this Old Wives' Tale protecting?

Also, some work is being done flying Lean of Peak in violation of another Old Wives' Tale.   Results seem quite logical.   Three M-20 Turbo'd J models flying these tests find the engines tolerate 30° lean with CHT's dropping 20° across the board, free of extra vibration.    Of course speed bleeds off from lower fuel flow, but it can be restored by bringing the throttle up a bit.   Not enough is known as yet to make a recommendation, but it is clear that these engines do better than the Old Wives would lead you to believe.

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