Climb performance is a measure of excess thrust which generally increases lift to overcome other forces such as weight and drag
This is true for most aircraft although some high performance aircraft can function like rockets for a limited time, utilizing thrust to lift away from the earth vertically, with no lift required
Excess power or thrust, terms that are incorrectly used interchangeably, allow for an aircraft to climb
Power vs. Thrust:
Power and thrust are not the same, despite their use as such
Power is a measure of output from the engine while thrust is the force that actually moves the aircraft
In a piston aircraft, power is converted to thrust through the propeller
In a jet aircraft, the engine produces thrust directly from the engine
When you are moving the throttle controls inside of the aircraft, you're controlling the engine and that is why they are referred to as power levers
Therefore the best angle of climb (produces the best climb performance with relation to distance, occurs where the maximum thrust is available
The best rate occurs where the maximum power is available)
Propulsion vs. Drag:
The relationship between propulsion and drag is such that it takes a certain amount of power/thrust to overcome drag both on the high end (the faster you go) and also the low-end (the slower you go)
This is noticeable during slow flight where you find yourself adding extra power to overcome all the increases in drag that are necessary to sustain lift
If you fall "behind the power curve" however, you're in a position where you cannot generate immediate performance by simply increasing power
The increase in power must first overcome the increased drag and then the expected performance will occur
