**1. Introduction**

During the flight, aircraft is usually loaded by four forces (**Figure 1**)—gravity force *G*, lift *L*, drag *D*, and thrust *T*. The combination of these forces defines the behavior of flight and the performance of aircraft. There are also cases when the side forces *S* act on aircraft due to sideslip (**Figure 1**), which are balanced by the vertical tail of aircraft, and during the analysis of the flight vehicle performance they are usually neglected.

As we can see from the **Figure 1**, the vector of aircraft speed *v* ! does not coincide with its body frame axes *xb*, *yb*, and *zb* and has inclinations from axis *xb* expressed by flow angles—angle of attack *α* and sideslip angle *β*. The point of acting gravity force is the center of gravity *Ocg* and usually the line of acting of thrust passes through the center of gravity to avoid generation of destabilizing torques or moments. Aerodynamic forces act at the point named center of pressure *Ocp*, which is not a fixed and changes its position depending on the angle of attack and the air speed. Therefore, a more stable point—aerodynamic center *Oac*—is introduced, where the changes of aerodynamic forces act, so the aerodynamic moments at that point do not change with the changes of angle of attack. However, aerodynamic center can have variations [1] with Mach number *M* ¼ *v=a*, where *v* is the magnitude of aircraft speed, *a* is the local speed of sound.

In problems of flight vehicle performance analysis, usually a simpler model of the balance of forces is used: the side forces *S* are neglected due to small values of sideslip angle *β* in steady flight regimes, the point of action of all unneglectable forces assumed the same, at the same time, taking the lines of action of thrust *T* and drag *D* forces coinciding (**Figure 2**).

*L* ¼ *cL*

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>>:

reference area, and *cD* is the dimensionless drag coefficient.

From the system of Eq. (1)

*Flight Vehicle Performance*

*DOI: http://dx.doi.org/10.5772/intechopen.92105*

**Figure 3.**

**155**

*Lift coefficient of aircraft versus angle of attack.*

it changes with changes of angle of attack *α*.

using methods of computational fluid dynamics (CFD).

*D* ¼ *cD*

*L <sup>D</sup>* <sup>¼</sup> *cL cD :*

*ρv*<sup>2</sup> 2 *Sref* ,

*ρv*<sup>2</sup> 2 *Sref* ,

where *cL* is the dimensionless lift coefficient; *ρ* is the freestream density; *v* is the magnitude of freestream speed, which is taken equal to aircraft speed; *Sref* is the

The ratio *k* ¼ *L=D* is usually called lift-to-drag ratio and in eastern literature it is defined as aerodynamic quality of aircraft. This ratio has interesting properties and

Let us consider now the lift coefficient *cL* and its relations with angle of attack *α* (**Figure 3**), which is similar to the analogous relation for 2-D airfoils. The function *cL* ¼ *fL*ð Þ *α* can be found using experiments with aircraft model in wind tunnels or

As we can see from the graph of function *cL* ¼ *fL*ð Þ *α* (**Figure 3**) there is an angle of attack *αcr* at which the lift coefficient is maximal, that is, *cL* max . The flight at critical angle of attack *αcr* will lead to stall, resulting aircraft crash. At angle of attack *αts*, the tip stall processes are started and the rate of increase of lift coefficient is decelerated, allowing it to get its maximum value *cL* max and decrease sufficiently. At the point *αts* ð Þ ,*cts* of starting tip stall, the effects of shaking of aircraft are started [3]. The range between angles of attack *α*0, which is called zero lift angle of attack,

(1)

Based on the model of force balance, we will study flight vehicle performance at several steady flight regimes, but before getting there let us consider the aerodynamic forces—lift and drag, and the effect of their relations on the aerodynamic quality of aircraft.
