36 Steady Level-Flight Operations

Introduction

An airplane’s typical operation involves several phases, including takeoff, climb, cruise, turns or maneuvers, descent, and finally, a landing. As shown in the figure below, a civil aircraft, such as a commercial airliner, spends much of the duration of its flight in the cruise condition flying from one airport to another. This condition is predominantly level flight at relatively constant airspeed and high altitudes. A contingency plan for a possible diversion because of bad weather or other issue preventing a landing at the destination must always be accounted for.

 

Typical phases of flight for a civil aircraft such as a commercial airliner. A contingency plan for a possible diversion must be accounted for, which requires reserve fuel.

A military aircraft may spend much more time in climbs, turns, and maneuvers, and the cruise segment may be much shorter. It may also involve an out-and-return mission profile, i.e., fly to a destination to conduct some operation and return to base without landing, as shown below. Nevertheless, any airplane’s steady level flight performance is important in determining its flight range at normal cruising speeds, ceilings, and airspeeds to attain maximum range and/or endurance.

 

Typical phases of flight for a military aircraft. Military aircraft, more often than not, perform out-and-return or “round-robin” missions.

The analysis pursued to quantify level flight performance depends on the type of airplane, i.e., jet-propelled airplanes or propeller-driven airplanes. For this reason, engines have previously been classified into two main groups; one is propeller-driven engines (i.e., piston-prop and turboprop), and the other one is jet engines (i.e., turbojet and turbofan).

The primary consideration in the analysis of airplane flight performance is that the output of a jet engine is quantified in terms of its thrust production. In contrast, the output of an engine driving a propeller is quantified in terms of its power output. However, it must also be recognized that for propeller-driven engines, the power is converted into propulsive thrust by the propeller. For jet engines, power is required to produce the thrust by increasing the momentum of the flow through the engine in the form of a jet velocity.

Therefore, all engines are examined one way or another as power-producing devices, which is one reason why they are often called powerplants. Another reason is that they also generate electrical, hydraulic, and pneumatic power. Also, recognize that fuel is required to produce the power, so for airplane performance, the fuel flow, and hence the fuel burn, is always at the core of the engineering analysis

Objectives of this Lesson

  • Be aware of the general flight performance characteristics of an airplane in straight and level flight.
  • Understand the basic differences in the performance analysis of jet-powered versus propeller-driven aircraft.
  • Review typical thrust required curves for jet-powered aircraft and power required curves for propeller-driven aircraft.
  • Appreciate the effects of weight and altitude on thrust required and/or power required performance curves and flight characteristics.

Performance Characteristics of an Airplane

The performance characteristics of an airplane depend on its aerodynamic drag, as well as the characteristics of the engines that power it. A large fraction of the total aircraft drag comes from the wing, which depends primarily on its angle of attack and operational Mach number. However, the airframe drag and everything else other than the wing is also significant contributor to the total drag of the airplane. Once the drag (or an estimate for drag) is determined, then the thrust or power requirements for flight can be determined at any given aircraft weight and operational altitude (specifically the density altitude). Using the engine characteristics (thrust developed or power available), then many performance characteristics of the airplane can be calculated such as its fuel burn, maximum and minimum attainable flight speeds, rates of climb, best range airspeeds, ceilings, range, endurance, etc.

Drag Model

The simplest drag model for an airplane is to represent its drag as an average (non-lifting) value combined with a value that varies with the square of the lift coefficient. The average value is independent of lift and, in aggregate, accounts for the profile drag on all airframe surfaces, i.e., the sum of the skin friction and pressure drag over the wings, fuselage, empennage, etc. The other part of the total drag is called induced drag because it is the drag that is induced on the airplane from the creation of lift; the physics behind this component is tied to the trailing vortex system behind the airplane, as previously discussed.

Therefore, the drag coefficient for the entire airplane can be expressed as

(1)   \begin{equation*} C_D = C_{D_{0}} + C_{D_{i}} = C_{D_{0}} + k {C_L}^2 \end{equation*}

where C_{D_{i}} is the induced drag coefficient and k depends on the wing design. Theoretically, the induced drag coefficient can be expressed as

(2)   \begin{equation*} C_{D_{i}} = \frac{{C_L}^2}{\pi A\!R e} = k {C_L}^2 \end{equation*}

where A\!R is the aspect ratio of the wing and e (the value of e is < 1) is called Oswald’s wing efficiency factor.

There are other sources of drag, such as wave drag (from the creation of shock waves) and the effects of stall. The growth of wave drag as the transonic flight regime is approached is highly nonlinear in terms of Mach number and angle of attack, so the development of a suitable mathematical model is more difficult. The effects of flow separation and stall of the thrust-required curves typically manifest at higher gross weights.

Thrust & Power Required for Flight

The total (dimensional) drag on the airplane, D, is then

(3)   \begin{equation*} D = \frac{1}{2} \rho V_{\infty}^2 S \left( C_{D_{0}} + k {C_L}^2 \right) = T \end{equation*}

a representative variation being shown in the figure below. Notice that the drag must be equal to the thrust, T, needed from the propulsive system when lift equals weight, i.e., in balance flight or trim, then

(4)   \begin{equation*} L = W, \quad \quad T = D \end{equation*}

Representative thrust required(to overcome drag) curve for an airplane.

The corresponding power required for flight is then

(5)   \begin{equation*} P_{\rm req} = T V_{\infty} \end{equation*}

as shown in the figure below, which again has a characteristic U-shaped curve.

Representative power required curve for an airplane.

Jet-Propelled Airplane

Consider first the detailed analysis of a jet-propelled airplane. For the steady level flight condition where T = D, then

(6)   \begin{equation*} T = \frac{1}{2} \rho V_{\infty}^2 S C_D = \frac{1}{2} \rho V_{\infty}^2 S \left( C_{D_{0}} + k {C_L}^2 \right) \end{equation*}

which assumes that there is no extra compressibility drag (i.e., wave drag) at high cruise Mach numbers. The equation to be solved is

(7)   \begin{equation*} T - \frac{1}{2} \rho V_{\infty}^2 S \left( C_{D_{0}} + k {C_L}^2 \right) = 0 \end{equation*}

Because in steady flight L = W then

(8)   \begin{equation*} L = \frac{1}{2} \rho V_{\infty}^2 S C_L = W \end{equation*}

where \rho is the density of the air in which the aircraft is flying, S is the reference wing area and C_L is the total wing lift coefficient (the assumption here being that all lift is generated by the wings). Notice that \rho = \rho_0 \sigma where \sigma comes from the ISA model, i.e.,

(9)   \begin{equation*} L = W = \frac{1}{2} \rho_0 \sigma V_{\infty}^2 S C_L \end{equation*}

Rearranging this equation, the lift coefficient that needs to be produced on the wing for a given flight speed can be solved for, i.e.,

(10)   \begin{equation*} C_L = \frac{2 W}{\rho_0 \sigma S V_{\infty}^2} \end{equation*}

Therefore, after some algebra, the drag becomes

(11)   \begin{equation*} D = \frac{1}{2} \rho_0 \sigma V_{\infty}^2 S C_{D_{0}} + \frac{2 k W^2}{\rho_0 \sigma S V_{\infty}^2} \end{equation*}

Notice that the first term in this latter equation (the profile or zero-lift drag) becomes dominant at higher airspeeds, and the second term (the induced drag) becomes larger at lower airspeeds; the resulting drag curve is U-shaped, as shown previously. Therefore, the equation to be solved is

(12)   \begin{equation*} T - \frac{1}{2} \rho_0 \sigma V_{\infty}^2 S C_{D_{0}} + \frac{2 k W^2}{\rho_0 \sigma S V_{\infty}^2} = 0 \end{equation*}

Of course, this problem can be solved graphically.

Thrust Available

Now consider the engine thrust. The maximum thrust from a jet engine is available at sea level. However, it will decrease with a decrease in air density, i.e., for an increase in altitude, the density decreases, and the thrust available decreases for a given throttle setting. Throttle settings are generally specified as takeoff, maximum continuous, cruise, and idle, but military aircraft may have an afterburner selection too.

Clearly, the thrust available (the output of the jet engine) depends on many things, but specifically the airspeed, the density of the air in which the airplane is flying (i.e., the density altitude), and the throttle setting (\delta_{T_A}). Therefore, the available thrust from the engine can be written the general form

(13)   \begin{equation*} T_A = f(V_{\infty}, \sigma, \delta_{T_A}) \quad \text{or} \quad T_A = f(M_{\infty}, \sigma, \delta_T) \end{equation*}

Consequently, for a given density altitude, airplane weight, and engine throttle setting, both the thrust available (from the engine) and the thrust required (equal to the aircraft drag) becomes a function of airspeed or Mach number.

Thrust Curves

To achieve level flight, then the horizontal equilibrium equation must be satisfied (Eq. 7 or Eq. 12) for a given weight and altitude. To solve the equation, both the thrust available from the engine and also the thrust required (i.e., the drag of the aircraft) must be plotted versus the airspeed (or Mach number) and then determine the precise conditions where the curves coincide, an example being shown in the figure below. At that point, Eq. 12 will be satisfied, and both the airspeed and the thrust required for that condition can be determined.

Representative variations in drag and thrust available versus airspeed for a jet-powered airplane.

Notice that the matching thrust available and the required curve can intersect at two points. The intersection at the highest airspeed will correspond to the maximum level flight airspeed for that aircraft at that particular density altitude, aircraft weight, and throttle setting and flight. The possible intersection at the lower airspeed will be the minimum possible airspeed the aircraft can maintain level flight at that altitude and is called the thrust-limited minimum airspeed, which in some cases may be higher than the stall airspeed. In general, an airplane’s actual minimum flyable airspeed at a given altitude will be greater than the thrust-limited minimum airspeed or the stall airspeed. However, in the clean configuration (i.e., no flaps, gear up), the stall speed is usually the higher airspeed.

The thrust available and thrust required curves versus airspeed can be plotted for each altitude of interest. Of course, the exact quantitative relationships between power and airspeed depend on the detailed aerodynamics of the actual airplane as well as the characteristics of the engines, which, as previously mentioned, may not be available other than in numerical form, e.g., tables. As altitude increases and the air density decreases, the thrust available at a given throttle setting decreases, as shown in the figure below, then these points represent the maximum and minimum airspeeds that the aircraft can fly at each altitude for the given weight and throttle setting and so determine the maximum speed flight boundary. Notice that at higher altitudes, the achievable lowest airspeed becomes thrust limited, while at lower altitudes, the achievable speed is generally determined by the onset of stall.

Representative variations in thrust required versus airspeed for a jet-powered airplane at different operating flight altitudes.

There will eventually be some altitude at which the available thrust approaches the minimum drag, where the available thrust curve will touch the required thrust curve at a single point. This condition corresponds (more or less) to the aircraft’s achievable maximum altitude or ceiling. Notice from the figure below that changing the weight of the aircraft significantly affects the thrust curves because the lift coefficient increases, and so the induced drag increases at lower airspeeds. As shown in the figure, increasing flight weight tends to shift the thrust required curve up and to the right.

Representative variations in thrust required versus airspeed for a jet-powered airplane at different operating flight weights.

The effect of configuration also affects the thrust required, as shown in the figure below. The clean configuration is the normal flight condition with flaps, slats (if any), and the landing gear retracted. Lowering the landing gear increases drag, and the deployment of the flaps increases this drag further, which is referred to as the dirty configuration. Notice in the figure that the flight envelope (or corridor) is increasingly constrained between the thrust available from the engines and the stall speed of the aircraft. In the dirty configuration with landing gear down and full flaps, then the thrust-limiting airspeed is reached before the onset of stall. In this case, such narrow allowable airspeed corridors require that the aircraft be flown fairly precisely, such as during landing.

Representative power required curve for an airplane in the “clean” condition of flight versus the “dirty” configuration with flaps and landing gear down.

While these forgoing graphical results are fairly easy to see, the problem can also be solved analytically under some conditions. Take as a further example a jet-powered airplane where it can be assumed that the propulsive thrust is not substantially dependent on the airspeed for a given altitude, i.e., where it is reasonable to assume that T = constant for a given altitude. The airspeed of the airplane can be found from

(14)   \begin{equation*} T - \frac{1}{2} \rho S \left( C_{D_{0}} + k {C_L}^2 \right) V_{\infty}^2 = 0 \end{equation*}

After some algebra and rearrangement of terms, the airspeed for thrust and drag equilibrium can be obtained by solving the quartic equation

(15)   \begin{equation*} c_1 V_{\infty}^4 + c_2 V_{\infty}^2 + c_3 = 0 \end{equation*}

where

(16)   \begin{equation*} c_1 = -\frac{1}{2} \rho_0 \sigma \frac{S}{W} C_{D_{0}}, \quad c_2 = \frac{T}{W}, \quad c_3 = -k \frac{2}{\rho_0 \sigma} \frac{W}{S} \end{equation*}

Notice that these coefficients contain both the thrust to weight ratio and the wing loading. While there are multiple roots to Eq. 15 they are not all physical, and only two roots will have physical significance. It can be shown that the jet airplane can maintain level flight at a given altitude if

(17)   \begin{equation*} \frac{T}{W} \ge 2 \sqrt{C_{D_{0}} k} \end{equation*}

which is naturally and intrinsically tied to the aerodynamic characteristics of the aircraft. The limiting condition of this latter result occurs at the ceiling of the aircraft when the minimum and maximum attainable airspeeds coincide, which is

(18)   \begin{equation*} V_{\infty} = -\frac{c_2}{2 c_1} = \frac{T}{\rho_0 \sigma S C_{D_{0}}} \end{equation*}

However, this value can only be determined explicitly if the engine thrust is known, which, as previously discussed, is a function of density altitude and throttle setting.

In most cases, the preceding type of problem must be solved numerically or graphically. The procedures described would apply to any drag or thrust variations. The propulsion characteristics of engines are often made available for engineering analysis in graphs or tables to calculate the thrust at each airspeed and altitude. Similar procedures are used to develop a more detailed model of the drag on the aircraft, including the effects of wave drag. For high-performance aircraft, such results are usually determined using a combination of calculations, wind tunnel tests, as well as flight tests and are made available as tables as functions of the angle of attack and Mach number.

Propeller-Driven Airplane

Consider now a propeller-driven airplane. Remember that the output of an engine driving a propeller (e.g., a turboshaft or a piston engine) is quantified in terms of its power. However, it must also be recognized that this power is converted into thrust according to the aerodynamic characteristics and performance of the propeller. There may be some jet thrust from a turboshaft, but usually, this is small enough to be ignored.

The power required for flight can be written as

(19)   \begin{equation*} P_{\rm req} = \frac{T V_{\infty}}{\eta_p} = \frac{1}{\eta_p} \left( \frac{1}{2} \rho V_{\infty}^3 S \left( C_{D_{0}} + k {C_L}^2 \right) \right) \end{equation*}

where \eta_p can be considered as the net propulsive efficiency of the propulsive system (engine and propeller combined); notice that this value may not be a constant and will generally vary with airspeed depending on the type of propeller system. Splitting the foregoing equation (Eq. 19) into its two parts leads to

(20)   \begin{equation*} P_{\rm req} = \frac{1}{\eta_p} \left( \frac{1}{2} \rho V_{\infty}^3 S C_{D_{0}} \right) + \frac{k}{\eta_p} \left( \frac{1}{2} \rho V_{\infty}^3 S {C_L}^2 \right) \end{equation*}

the first term in the equation being the non-lifting part and the second term being the induced part. Notice that the power associated with the non-lifting part increases with the cube of the airspeed but the second (induced) part depends on the lift coefficient, which as has previously been shown, reduces with increasing airspeed.

Using Eq. 10 gives

(21)   \begin{equation*} {C_L}^2 = \left(\frac{2 W}{\rho_0 \sigma S V_{\infty}^2}\right)^2 \end{equation*}

so the power required equation now becomes

(22)   \begin{equation*} P_{\rm req} = \left(\frac{1}{2} \rho_0 \sigma V_{\infty}^3 S \right) \frac{1}{\eta_p} C_{D_{0}} + \frac{1}{2} \rho_0 \sigma V_{\infty}^3 S \left( \frac{k}{\eta_p} \right) \left(\frac{2 W}{\rho_0 \sigma S V_{\infty}^2}\right)^2 \end{equation*}

which after some simplification leads to

(23)   \begin{equation*} P_{\rm req} = \left( \frac{1}{2} \rho_0 \sigma V_{\infty}^3 S \right) \frac{1}{\eta_p} C_{D_{0}} + \left( \frac{k}{\eta_p} \right) \frac{2 W^2}{\rho_0 \sigma S V_{\infty} } \end{equation*}

which is simply in the form

(24)   \begin{equation*} P_{\rm req} = A V_{\infty}^3 + \frac{B}{V_{\infty}} \end{equation*}

where

(25)   \begin{equation*} A = \frac{1}{2} \rho_0 \sigma \frac{1}{\eta_p} C_{D_{0}} \end{equation*}

and

(26)   \begin{equation*} B = \frac{k}{\eta_p} \frac{2 W^2}{\rho_0 \sigma S} \end{equation*}

Notice that the non-lifting part increases with the cube of the airspeed but the lifting part decreases inversely with airspeed.

 Power Available

The power available for flight as a characteristic of the powerplant, i.e., all the power that could be delivered from the engine to drive the propeller and propel the airplane forward. If the shaft power available from the engine is P_{\rm bp} (this is called its brake power), then the power available for flight P_A will be

(27)   \begin{equation*} P_A = \eta_p P_{\rm bp} \end{equation*}

which means that the available power for flight is reduced by the propeller efficiency \eta_p, i.e., not all of the power at the shaft of the engine can be delivered as useful work to the air by the propeller.

The actual power required for flight depends on the drag of the airplane so

(28)   \begin{equation*} P_{\rm req} = D V_{\infty} \end{equation*}

For steady level flight at a constant airspeed and altitude, the pilot needs to set the throttle so that the power required for flight is equal to the power available, i.e.,

(29)   \begin{equation*} P_{\rm req} = D V_{\infty} = P_A = \eta_p P_{\rm bp} \end{equation*}

so that the brake power needed from the engine will be

(30)   \begin{equation*} P_{\rm bp} = \frac{P_A}{\eta_p} = \frac{D V_{\infty}}{\eta_p} \end{equation*}

In performance analyses, the brake (shaft) power matters because it is the power that can be delivered at the shaft that ultimately affects the engine’s fuel consumption. Obviously, the higher the propeller efficiency then, the lower the brake power required, and so the lower the fuel consumption will be.

Naturally, the power available (from the powerplant) may be greater or less than the power required for flight. For example, any excess power available over and above required will allow the airplane to accelerate to a higher airspeed and/or climb to a higher altitude. For this reason, the airplane’s initial takeoff and climb performance is strongly affected by the power available from the powerplant.

Power Curves

A representative power curve for a propeller-driven airplane is shown in the figure below. It can be seen that higher power is required at lower airspeeds, a minimum range of power at some intermediate airspeeds, and then a rapid increase in power is required (with the cube of the airspeed) as higher airspeeds are reached. Aircraft that use propellers will have performance charts given in terms of airspeed rather than Mach number. The lowest possible airspeed is generally limited by the onset of wing stall and/or buffeting from the onset of flow separation, causing the aircraft to shake, no matter how much power is available. At higher weights and/or density altitudes, the rapid increase in power required will eventually limit the maximum level flight airspeed of the airplane for a given weight and operational altitude, assuming no other barrier to flight appears.

Power required for a propeller-driven airplane at different flight weights as a function of its airspeed.

The power available typically increases with airspeed and levels off over the airspeed range where the airplane would generally fly. Power available will lapse with altitude, as shown in the figure below. Again, analogous to the manner for the jet aircraft, the level flight solution is the intersection of the available power curves, and the power required curves. The highest airspeed solution is the maximum level flight airspeed (at a given altitude, aircraft weight, and throttle setting). However, the minimum speed solution is only valid if that speed is greater than the aircraft’s stall speed. Again, the aircraft’s ceiling can be determined at the airspeed when the minimum power for flight coincides with the power available.

Power required for a propeller-driven airplane at different flight altitudes as a function of its airspeed.

By assuming that k and the propulsive efficiency \eta_p remain constant for all weights and airspeeds then the airspeeds can be solved for in closed form. This is a special case, admittedly, but a reasonable assumption for a constant speed propeller with the engine operating at wide-open throttle. After some algebra, the needed equation to be solved is

(31)   \begin{equation*} c_1 V_{\infty}^3 + \frac{c_2}{V_{\infty}} + c_3 = 0 \end{equation*}

where

(32)   \begin{equation*} c_1 = -\frac{1}{2} \frac{\rho_0 \sigma S C_{D_{0}}}{W}, \quad c_2 = -2 k \left( \frac{W}{\rho_0 \sigma S} \right), \quad c_3 = \frac{\eta_p P_{\rm req}}{W} \end{equation*}

from which the maximum and minimum speeds can, in principle, be solved for. However, recognize that this is a more difficult problem to solve because the relevant equation is nonlinear.

The effect of configuration, i.e., clean or dirty with the flaps up or down, landing gear up or down, etc., also affects the power required, as shown in the figure below. Just like jet-powered aircraft, notice that the flight envelope is increasingly constrained between the power available and the stall speed. In the dirty configuration with landing gear down and full flaps, such as for landing, then it is possible that the power limiting airspeed can be reached before the onset of stall. In this regard, the pilot must ensure that the airplane carries enough excess airspeed so that in the event of a go-around, there is enough excess power available to climb away from the runway.

Summary & Closure

The analysis used to quantify level flight performance depends on the type of airplane, i.e., jet-propelled or propeller-driven airplanes. The main difference is that the output of a jet engine is quantified in terms of its thrust production, while the output of an engine driving a propeller is quantified in terms of its power output. Furthermore, fuel is required to produce the power, so for airplane performance, the fuel flow and hence the fuel burn is always at the heart of the analysis.

5-Question Self-Assessment Quickquiz

For Further Thought or Discussion

  • Consider the addition of a term to the drag polar to account for the onset of wave drag. How will this change the thrust required curve?
  • If the aerodynamics characteristics of an airplane are available only in table format (i.e., tables of lift and drag), think about how these results can be incorporated into an analysis to find the aircraft’s performance characteristics.
  • Under what conditions might the minimum level flight airspeed of an airplane be higher than its stall speed?
  • Why does the thrust available from a turboprop system decrease quickly at higher airspeeds?

Other Useful Online Resources

To learn more about the level flight performance characteristics, follow up with some of these more practical online resources:

  • An explanation of the power curve and an in-flight example – check out this video.
  • Another video explaining the power curve of an airplane.
  • A good ERAU video from the flight school explaining the drag curves of an airplane.