THRUST REQUIRED
Recall that the total drag curve is the sum of parasite and induced drag. In equilibrium flight total thrust must equal total drag. Therefore, the amount of thrust that is required to overcome drag can be found on the total drag curve. This amount of thrust is called thrust required (TR), and is expressed in pounds. As with the drag curve, the thrust required curve is for one specific weight, altitude and configuration. L/DMAX AOA is the point of minimum thrust required, and is obtained at some specific velocity. Flight at greater velocities requires a reduction in AOA
(to maintain a constant lift/weight ratio) and an increase in thrust (to match the increase in parasite drag). Flight at lower velocities requires an increase in angle of attack and an in- crease in thrust (to match the increase in induced drag).
POWER REQUIRED
Power is the rate of doing work, and work is a force times a distance. Power required is the amount of power that is required to produce thrust required. Power required is the product of Thrust required and velocity.
To find L/DMAX on the thrust required curve, draw a horizontal line tangent to the bottom of the curve. By applying the power equation to this line, the result is a straight line from the origin that is tangent to the power curve at L/DMAX. Unlike on the TR curve, L/DMAX is not at the bottom of the PR curve, but is to the right of the bottom of the curve. L/DMAX still represents minimum total drag, but minimum PR is to the left of L/DMAX. It should be noted that the velocity and AOA for L/DMAX are the same on the PR curve as on the TR curve.
THRUST AVAILABLE
Thrust available (TA) is the amount of thrust that the airplane’s engines actually produce at a given throttle setting, velocity, and density. The most important factor is the throttle. Maximum engine output occurs at full throttle. As the throttle is retarded, thrust available decreases. Since the propeller can only accelerate the air to a maximum velocity, as the velocity of the incoming air increases, the air is accelerated less through the propeller, and thrust available decreases.
Turbojets do not suffer a decrease in thrust available with velocity because ram-effect overcomes the decreased acceleration (Figure 1-6-4). Therefore, TA is approximated by a straight line. As the density of the air decreases, thrust available decreases
.
POWER AVAILABLE
Power available (PA) is the amount of power that the airplane’s engines actually produce at a given throttle setting, velocity, and density. The most important factor is throttle setting. Maximum power available occurs at full throttle. As the throttle is retarded, power available decreases. As velocity increases, power available for a jet will increase linearly, while power available for a prop will initially increase, but will then decrease due to a decrease in thrust available (Figure 1-6-6 and Figure 1-6-7). As thrust available decreases with a decrease in density, power available will also decrease.
THRUST EXCESS AND POWER EXCESS
A comparison of the TR and TA curves on one graph allows us to predict airplane performance. To maintain equilibrium level flight, thrust available must equal thrust required for a specific angle of attack and velocity. This is depicted on a graph where the TR and TA curves cross. The right-hand point of equilibrium will produce the maximum velocity in level flight. This is the greatest airspeed that the aircraft can maintain without descending.
A thrust excess (TE) occurs if thrust available is greater than thrust required at a particular velocity. A positive TE causes an acceleration, a climb, or both, depending on angle of attack. A negative TE is called a thrust deficit and has the opposite effect. Maximum thrust excess occurs at a full throttle setting, and is depicted on a graph where the distance between the TR and TA curves is greatest. For a turbojet, max thrust excess occurs at L/DMAX. For a turboprop, max thrust excess occurs at a velocity less than L/DMAX.
Power excess (PE) is calculated in a similar manner as TE and will also produce an acceleration, a climb, or both. Likewise, a power deficit will cause a decent, a deceleration, or both. For a turbojet, maximum power excess occurs at a velocity greater than L/DMAX. For a turbo- prop, max power excess occurs at L/DMAX. It is important to note that maximum power excess is achieved at a greater velocity and a lower angle of attack than maximum thrust excess. It should also be noted that a power excess cannot exist if thrust excess is zero.
