Aviation power curves

POWER CURVES

DEFINITIONS:

Check the explanation of the different forces in the  post:  reciprocating engine take off

Total Drag (D): As we will see later, on the principles of flight subject posts the total drag is the result to sum two different kinds of drags, called parasite and induced drag.

As you can see from the picture above the minimum of the total drag curve is where the induced drag is the same as parasite drag, and it is only reach at certain speed called minimun drag speed.

Thrust (T): force generated by the power plant, which decreases slightly with speed due to slippage of the propeller.

Power (P) : is the work per time unit ( P=W/t). Let´s change this formula in an useful way for us. Work is Force multiplied by distance ( W=F*d) so P=F*d/t, we also know speed(v) is distance (d) divided by time (t). Our power ecuation finish beeing the force multiplied by speed P=F*v.

Available power (Pa) : Is the power of our engine. Its ecuation P=F*v, where the force in this case is the thrust and the speed is the true air speed. P=T*TAS.

Power required (Pr): It is the power required to win the drag in a specific speed. P=D*TAS

FACTORS THAT AFFECTS THE POWER CURVES :

Weight: if the weight increases  

• Pa remains the same ( for example our car has the same power without people inside or full of people).
• Pr increases because we need more power to move the aircraft ( Drag increases).

Density Altitude : ( check Density altitude definition)  If DA increases .

• Pa decreases, because the density decreases so the behaviour of the engine is worse.
• Pr Increases, because the True air speed increases so the drag increases. ( you need much more power to move the aircraft)

Configuration : Flaps and landing gear.

• Pa : Reamaind unchanged ( A can has the same power with the door open and close)
• Pr: As drag increases, the required power increases.

Wind : It doesn´t have any effect neither the Pa nor Pr