Aircraft Performance
13. Aircraft Performance
In this chapter we will make the connections between aircraft performance and propulsion system performance. For a vehicle in steady, level flight, as in Figure 13.1, the thrust force is equal to the drag force, and lift is equal to weight. Any thrust available in excess of that required to overcome the drag can be applied to accelerate the vehicle (increasing kinetic energy) or to cause the vehicle to climb (increasing potential energy). Figure 13.1: A schematic of the forces on an aircraft in steady level flight
13.1 Vehicle Drag
Recall from fluids that drag takes the form shown in Figure 13.2, being composed of a part termed parasitic drag that increases with the square of the flight velocity, and a part called induced drag, or drag due to lift, that decreases in proportion to the inverse of the flight velocity. Figure 13.2: Components of vehicle drag.
where
and
Thus
or
The minimum drag is a condition of interest. We can see that for a given weight, it occurs at the condition of maximum lift-to-drag ratio,
We can find a relationship for the maximum lift-to-drag ratio by setting
from which we find that
and
and
13.2 Power Required
Now we can look at the propulsion system requirements to maintain steady level flight since
and
Thus the power required (for steady level flight) takes the form of Figure 13.3. Figure 13.3: Typical power required curve for an aircraft.
The velocity for minimum power is obtained by taking the derivative of the equation for with respect to and setting it equal to zero.
As we will see shortly, maximum endurance (time aloft) occurs when the minimum power is used to maintain steady level flight. Maximum range (distance traveled) is obtained when the aircraft is flown at the most
In this chapter we will make the connections between aircraft performance and propulsion system performance. For a vehicle in steady, level flight, as in Figure 13.1, the thrust force is equal to the drag force, and lift is equal to weight. Any thrust available in excess of that required to overcome the drag can be applied to accelerate the vehicle (increasing kinetic energy) or to cause the vehicle to climb (increasing potential energy). Figure 13.1: A schematic of the forces on an aircraft in steady level flight
13.1 Vehicle Drag
Recall from fluids that drag takes the form shown in Figure 13.2, being composed of a part termed parasitic drag that increases with the square of the flight velocity, and a part called induced drag, or drag due to lift, that decreases in proportion to the inverse of the flight velocity. Figure 13.2: Components of vehicle drag.
where
and
Thus
or
The minimum drag is a condition of interest. We can see that for a given weight, it occurs at the condition of maximum lift-to-drag ratio,
We can find a relationship for the maximum lift-to-drag ratio by setting
from which we find that
and
and
13.2 Power Required
Now we can look at the propulsion system requirements to maintain steady level flight since
and
Thus the power required (for steady level flight) takes the form of Figure 13.3. Figure 13.3: Typical power required curve for an aircraft.
The velocity for minimum power is obtained by taking the derivative of the equation for with respect to and setting it equal to zero.
As we will see shortly, maximum endurance (time aloft) occurs when the minimum power is used to maintain steady level flight. Maximum range (distance traveled) is obtained when the aircraft is flown at the most