Maximum Limit of Load Factor
This is the required maneuver load factor at all speeds up to VC. (The next section defines speed limits.) Maximum elevator deflection at VA and pitch rates from VA to VD also must be considered. Table 5.1 gives the g-limit of various aircraft classes.
For military aircraft applications, in general, the factor of safety equals 1.5 but can be modified through negotiation (see Military Specifications MIL-A-8860, MILA-8861, and MIL-A-8870).
Typical g-levels for various types of aircraft are shown in Table 5.2. These limits are based on typical human capabilities.
5.6.1 Speed Limits
The V-n diagram (see Figure 5.2) described in Section 5.7 uses various speed limits, defined as follows:
VS: Stalling speed at normal level flight.
VA: Stalling speed at limit load. In a pitch maneuver, an aircraft stalls at a higher speed than the VS. In an accelerated maneuver of pitching up, the angle of attack, a, decreases and therefore stalls at higher speeds. The tighter the maneuver, the higher is the stalling speed until it reaches VA.
VB: Stalling speed at maximum gust velocity. It is the design speed for maximum gust intensity VB and is higher than VA.
VC: Maximum level speed.
VD: Maximum permissible speed (occurs in a dive; also called the placard speed).
An aircraft can fly below the stall speed if it is in a maneuver that compensates loss of lift or if the aircraft attitude is below the maximum angle of attack, amax, for stalling.
Table 5.2. Typical g-load for classes of aircraft
5.4 V-n Diagram
To introduce the V-n diagram, the relationship between load factor, n, and lift coefficient, CL, must be understood. Pitch-plane maneuvers result in the full spectrum of angles of attack at all speeds within the prescribed boundaries of limit loads. Depending on the direction of pitch-control input, at any given aircraft speed, positive or negative angles of attack may result. The control input would reach either the CLmax or the maximum load factor n, whichever is the lower of the two. The higher the speed, the greater is the load factor, n. Compressibility has an effect on the V-n diagram. In principle, it may be necessary to construct several V-n diagrams representing different altitudes. This chapter explains only the role of the V-n diagram in aircraft design.
Figure 5.2 represents a typical V-n diagram showing varying speeds within the specified structural load limits. The figure illustrates the variation in load factor with airspeed for maneuvers. Some points in a V-n diagram are of minor interest to configuration studies – for example, at the point V = 0 and n = 0 (e. g., at the top of the vertical ascent just before the tail slide can occur). The points of interest are explained in the remainder of this section.
Inadvertent situations may take aircraft from within the limit-load boundaries to conditions of ultimate-load boundaries (see Figure 5.2).