Rate of Climb from Excess Power
Rate of Climb from Excess Power
An aircraft climbs by converting power beyond what it needs just to maintain level flight into a gain in altitude over time — this "excess power" concept is the cleanest way to understand climb performance and is the basis for every climb-rate chart in an aircraft flight manual. Because rate of climb is excess power divided by weight, the same engine producing the same excess power will make a heavier aircraft climb more slowly — this is exactly why maximum takeoff weight restrictions exist for hot, high-altitude, or short-runway airports where climb performance margins are already reduced.
Rate of climb is RC = Pexcess/W, the power in excess of level-flight requirement divided by aircraft weight. where P_exc is the excess power available above what is needed for level flight, and W_ac is the aircraft weight.
Inputs
P_exc := 150 kW=150000 W
W_ac := 20000 N=20000 N
Dividing excess power by weight converts a power surplus directly into a vertical velocity, since power is force times velocity and weight is the force being lifted.
RC_climb := P_exc/W_ac=7.5 m/s
Results
A rate of climb of several metres per second here is a normal, healthy climb performance for a light-to-medium aircraft at this excess power and weight. As altitude increases, available engine power typically falls while weight stays essentially constant, so rate of climb naturally decreases with altitude until it reaches zero at the aircraft's absolute ceiling, where excess power drops to nothing.
Assumptions & limitations
- Steady, unaccelerated climb — excludes transient/pull-up dynamics.
- Excess power assumed known at the given flight condition (speed, altitude, configuration).
- Does not account for the small climb-angle correction to thrust-drag balance.
- Weight treated as constant over the short climb segment considered.