Diseño De Aeromodelo

Lab 8 Notes – Basic Aircraft Design Rules
Nomenclature x, y longitudinal, spanwise positions
S reference area (wing area)
b wing span
c average wing chord ( = S/b )
A R wing aspect ratio
CL lift coefficient
Υ wing dihedral angle
β sideslip angle
φ bank angle
R turn radius
Role of Simple Design Rules Sh Sv ℓh ℓv A h R Vh Vv B α V

6 Apr 06

horizontal tail area vertical tailarea horizontal tail moment arm vertical tail moment arm horizontal tail aspect ratio horizontal tail volume coefficient vertical tail volume coefficient spiral stability parameter angle of attack velocity

Aircraft must have a certain amount of inherent stability and controllability to be flyable. It is therefore important to consider these characteristics when designing a new aircraft. Accurateevaluation of the stability characteristics of any given aircraft is a fairly complicated process, and is not well suited for preliminary or intermediate design. Fortunately, we have alternative criteria which give reasonable estimates and are vastly simpler to apply. The criteria involve basic dimensions, shown in Figure 1. Longitudinal x locations are typically

y
MAC c

x

b

c
S

lvSv

xcg xnp

lh

Sh

Figure 1: Lengths, areas, and angles used in simple stability criteria.

1

measured relative to the leading edge of the wing’s Mean Aerodynamic Chord, or MAC, which is the root-mean-square average chord. For most wings this is very nearly equal to the simple-average chord c. The ℓh and ℓv tail moment arms are the distances between the Center of Gravity (CG)and the average quarter-chord locations of the horizontal and vertical tail surfaces. The criteria which will use these dimensions are estimates, so it’s acceptable to estimate the CG position and to “eyeball” the tail average quarter-chord locations when measuring ℓh and ℓv . Center of Gravity Position An aircraft’s horizontal tail size and position, and the CG position are the dominant factorscontrolling the aircraft’s pitch stability, which is the tendency to automatically maintain an angle of attack and airspeed. The basic effects of moving the CG position are: Decrease xcg /c (move CG fwd.): Increase xcg /c (move CG back): increased stability; more resistance to α and V changes. decreased stability; less resistance to α and V changes.

There is one particular CG position which givesneutral stability, which is called the Neutral Point (NP). This is shown as xnp in Figure 1. The degree of pitch stability or instability is traditionally specified by the Stability Margin. xnp − xcg (1) S.M. = c Figure 2 illustrates the natural behaviors of an airplane after a pitch disturbance, for different values of S.M. The unstable behavior occurs when S.M. is negative, i.e. when the CG isbehind the NP. Because pitch instability makes the aircraft very difficult or impossible to control, the NP position is considered to be a practical aft CG limit.
neutral (S.M. = 0) weakly unstable (S.M. = −0.05)

strongly unstable (S.M. = −0.4)

weakly stable (S.M. = +0.05)

nose−up pitch upset strongly stable (S.M. = +0.4) trimmed level flight strongly stable (S.M. = +0.4)

weakly stable(S.M. = +0.05)

nose−down pitch upset
weakly unstable (S.M. = −0.05)

neutral (S.M. = 0)

strongly unstable (S.M. = −0.4)

Figure 2: Natural aircraft responses to a pitch disturbance, for different amounts of pitch stability. Making the S.M. strongly positive by moving the CG far forward will give plenty of pitch stability and a strong resistance to pitch upsets, but it also has undesirableside effects. One large drawback 2


of a large S.M. is that it causes large (and annoying) pitch trim changes with changing airspeed. Figure 3 shows the flight paths of airplanes with different nonnegative S.M., immediately after an airspeed increase caused by a power increase. The straight-ahead acceleration of the weakly stable or neutral airplane is more desirable for the pilot.
strongly...