Now climb to 10000 feet at the same power settings and you see
25/2500. 1100. 194
25/2100. 1270. 170
You'll only be able to get 25" up to 5,000. Above that altitude MAP decreases at 1"/1,000. At 10,000 the best you can expect is 20". Anything above 5,000 will be WOT. All spelled out on the OPs graph.
The effect of altitude on the range of a propeller powered aircraft may be appreciated by inspection of the attached graph. An increase in altitude has the effect of moving the graph up and to the right. If a given configuration of aircraft is operated at constant gross weight and the lift coefficient for (L/D)max, a change in altitude will produce the following relationships:
V2/V1= SQRT(O1/O2)
Pr2/Pr1= SQRT(O1/O2)
where
condition (1) applies to some known condition of velocity and power required for (L/D)max at some original, basic altitude condition (2) applies to some new values of velocity and power required for (L/D)max at some different altitude
and
V= velocity, knots (TAS, of course)
Pr=power required, h.p.
O=altitude density ratio (sigma)
Thus, if flight is conducted at 22,000 ft. (O=0.498), the airplane will have:
a 42 percent higher velocity
a 42 percent higher power required
than when operating at sea level. Of course, the greater velocity is a higher TAS since the airplane at a given weight and lift coefficient will require the same EAS independent of altitude. Also, the drag of the airplane at altitude is the same as the drag at sea level but the higher TAS causes a proportionately greater power required. Note that the same straight line from the origin tangent to the sea level power curve also is tangent to the altitude power curve.
The effect of altitude on specihc range can be appreciated from the previous relationships. lf a change in altitude causes identical changes in velocity and power required, the proportion of velocity to power required would be unchanged. This fact implies that the specific range of the propeller powered airplane would be unaffected by altitude. In the actual case, this is true to the extent that powerplant specific fuel consumption and propeller efficiency are the principal factors which could cause a variation of specific range with altitude. If compressibility effects are negligible, any variation of specific range with altitude is strictly a function of engine-propeller performance.
The aircraft equipped with the reciprocating engine will experience very little, if any, variation of specific range with altitude at low altitudes. There is negligible variation of brake specific fuel consumption for values of BHP below the maximum cruise power rating of the powerplant which is the auto-lean or manual lean range of engine operation. Thus, an increase in altitude will produce a decrease in specific range only when the increased power requirement exceeds the maximum cruise power rating of the powerplants. One advantage of supercharging is that the cruise power may be maintained at high altitude and the airplane may achieve the range at high altitude with the corresponding increase in TAS. The principal differences in the high altitude cruise and low altitude cruise are the true airspeeds and climb fuel requirements.
(Courtesy of Aerodynamics for Naval Aviators)