Let's say the POH power setting tables say 65% power is 2,400 RPM. When leaning to best power or best economy, is it assumed that one ensures RPM remains at 2,400 after leaning? If so, why does one get lower airspeed when leaned to best economy?
Are you reporting this as an experimental observation in a fixed pitch prop aircraft? That you:
* start at 2400 rpm and airspeed V
* lean the mixture to best economy
* increase the manifold pressure (i.e. push the throttle forward) until the rpm returns to 2400
* observe an airspeed
less than V
?
With a fixed pitch prop at a particular altitude, for a particular aircraft, in a particular configuration/weight, and assuming you're well above minimum power airspeed, any one of of power, airspeed and rpm determines the other two. Hence I would not expect the observation above.
With a constant speed prop the relationship is different. The variable pitch of the prop means that the different combinations are possible. Hence for example the same airspeed can be achieved with several different combinations of engine power and rpm, which have different efficiencies.