24Carrot,
I don't have POH for normally aspirated aircraft to hand. However, what you should see is a table for a given cruise power setting (say 75% Best Power Mixture)
The tabulated values should have the following pattern of change with regard to altitude up to about 6000 feet
- RPM should increase (as per the lengthy posts that you need more rpm to dissipate the same power in the propeller because it is spinning in thinner air)
- Fuel flow should remain the same (because we remain at 75% power)
- TAS should increase (but not by quite as much as one would expect -see next point)
- IAS should decline. This is the important one - power = drag * speed (At higher altitude you TAS will typically be higher therefore the drag must be decreased. The way this is achieved in cruise is a reduction in IAS and a corresponding drop in parasitic drag (with a small offset due to increased induced drag)
So the range is improving with altitude, but it is almost exclusively due to the reducing IAS and the corresponding reduction in drag.
As a note we all cruise at speeds way above optimal L/D (Vy) and therefore slowing down (no wind) will invariably reduce drag and improve MPG.
Range is determine by energy consumption which is drag*distance travelled
Power is determine by drag *velocity (TAS in this case)
above 6000 feet or so you should see no change in rpm (as you are already max achievable power), a more rapid decline in IAS and a less rapid increase in TAS until the IAS gets low enough that the increasing induced drag becomes material)
finally IAS will reduce to Vy and you are at your ceiling.