As I've indicated, I'm not much of a prop jock, Nancy. And if you're riding a turbine, whether it's hooked to a prop or not, you're right--keep climbing as long as the OAT is going down. But that's not the case with an air pump like a recip. If the throttle isn't wide open, the cylinder has to suck harder to get air into it. That sucking takes energy away from shaft driving the prop, so you burn more gas. Altitude isn't nearly as important in a prop as throttle setting and mixture. This is a cool site where you can play with the variables:
http://selair.selkirk.ca/Training/Aerodynamics/range_prop.htmHere are some quotes. Specific range is miles per gallon or lb. of fuel:
"Effect of Altitude on Specific Range and Endurance
Previously we saw that altitude had a huge effect on SR for jet airplanes. But it actually has almost no effect on propeller airplanes.
As the drag curve shifts to the right with altitude the power curve shifts up because power drag x velocity (and shifting to the right represents an increase in velocity even though drag does not change.)
How does altitude affect endurance of a propeller airplane?
In the graph to the left you can see the bottom of the FF curve moves UP with altitude. Therefore maximum endurance occurs at sea level for a propeller airplane.
Of course this does not take changes in SFC that occur with altitude into account. Previously we saw that altitude has only a minor effect on piston engines, so we expect a piston airplane to achieve maximum endurance at or very close to sea level.
NOTE: piston engines are usually manually leaned while jet and turboprop engine fuel flows are electronically controlled. The pilot is the most likely culprit causing high SFC for piston engines. If the pilot does not properly lean the engine then everything said below is completely invalid.
How does altitude affect the Range of a propeller airplane?
The graph to the left shows that SR does not change with altitude. Recall from above that SR is maximum when the angle "R" is minimum. You can see that R does not change with altitude, therefore SR does not change.
As with the discussion of endurance, this analysis assumes that altitude does not affect SFC. We know that for a piston airplane altitude has no substantial effect on SFC although we should fly high enough to operate at full throttle. This leads to the conclusion that piston airplanes achieve maximum - or the same - range at any altitude from about 6000 or higher.
The most important point, and one that cannot be emphasized too much is that the main factor determining cruise altitude for a propeller airplane is wind. On most days wind gets stronger with altitude so it would be foolish to climb into a stronger headwind given the graph for range shown above. It would be equally foolish NOT to climb if a stronger tailwind is available. Pilots who do not heed this advice are being wasteful with fuel.
On days when the wind is light any altitude will do, but it is IMPORTANT to note that while the airplane achieves the same SR at all altitudes, TAS increases with altitude. In other words you will get to your destination faster at higher altitude. Since the old saying that time is money is pretty close to being true where airplanes are concerned it "pays" to fly higher. It does NOT pay in fuel, but it does pay in other ways."
Me again: Per Chater, the Nauru weather observation, received as Earhart was taking off, included the following: "NARU 8 AM UPPER AIR OBSERVATION 2000 FEET 90 DEGREES 14 MPH 4000 FEET 90 DEGREES 12 MPH 7500 FEET 90 DEGREES 24 MPH."