AIRCRAFT LOADS AND OPERATING. LIMITATIONS

FLIGHT LOADS-MANEUVERS AND GUSTS

The loads imposed on an aircraft in flight are the result of maneuvers and gusts. The maneuver loads may predominate in the design of fighter airplanes while gust loads may predominate in the design of the large multiengine aircraft. The maneuver loads an airplane may encounter depend in great part on the mission type of the airplane. However, the maximum maneuvering capability is of interest because of the relationship with strength limits.

The flight load factor is defined as the pro­portion between airplane lift and weight, where

n = L/W n — load factor L=lift, lbs.

W= weight, lbs.

MANEUVERING LOAD FACTORS. The maximum lift attainable at any airspeed occurs when the airplane is at CLjnax. With the use of the basic lift equation, this maximum lift is expressed as:

Lmax = CLmaxpV2S

Since maximum lift must be equal to the weight at the stall speed,

W-Ct’J’MS

If the effects of compressibility and viscosity on Ctmax are neglected for simplification, the maximum load factor attainable is determined by the following

ГThus, if the airplane is flying at twice the stall speed and the angle of attack is increased to obtain maximum lift, a maximum load factor of four will result. At three times the stall speed, nine "g s” would result; four times the stall speed, sixteen g’s result; five times the stall speed, twenty-five g s result; etc. Therefore, any airplane which has high speed performance may have the capability of high maneuvering load factors. The airplane which is capable of flight speeds that are

many times the stall speed will require due consideration of the operating strength limits.

The structural design of the aircraft must consider the possibility of negative load factors from maneuvers. Since the pilot cannot com­fortably tolerate large prolonged negative "g", the aircraft need not be designed for negative load factors as great as the positive load factors.

The effect of airplane gross weight during maneuvers must be appreciated because of the particular relation to flight operating strength limitations. During flight, the pilot appre­ciates the degree of a maneuver from the inertia forces produced by various load factors; the airplane structure senses the degree of a maneuver principally by the airloads involved. Thus, the pilot recognizes load factor while the structure recognizes only load. To better understand this relationship, consider an ex­ample airplane whose basic configuration gross weight is 20,000 lbs. At this basic configura­tion assume a limit load factor for symmetrical flight of 5 6 and an ultimate load factor of 8.4. If the airplane is operated at any other con­figuration, the load factor limits will be al­tered. The following data illustrate this fact by tabulating the load factors required to produce identical airloads at various gross weights.

Gross weight, lbs.	Limit load factor	Ultimate load factor
20,000 (basic)…………………………………..	5.60	S. 40
30,000 (max. takeoff)………………………..	3.73	5.60
13,333 (min. fuel):…………………………….	8.40	12.60

As illustrated, at high gross weights above the basic configuration weight, the limit and ulti­mate load factors may be seriously reduced. For the airplane shown, a 5-g maneuver im­mediately after a high gross weight takeoff could be very near the “disaster regime,” especially if turbulence is associated with the maneuver. In the same sense, this airplane at very low operating weights below that of the basic configuration would experience great­ly increased limit and ultimate load factors.

Operation in this region of high load factors at low gross weight may create the impression that the airplane has great excess strength capability. This effect must be understood and intelligently appreciated since it is not uncom­mon to have a modern airplane configuration with more than 50 percent of its gross weight as fuel.

GUST LOAD FACTORS. Gusts are asso­ciated with the vertical and horizontal velocity gradients in the atmosphere. A horizontal gust produces a change in dynamic pressure on the airplane but causes relatively small and unimportant changes in flight load factor. The more important gusts are the vertical gusts which cause changes in angle of attack. This process is illustrated in figure 5-2. The vec­torial addition of the gust velocity to the air­plane velocity causes the change in angle of attack and change in lift. The change in angle of attack at some flight condition causes a change in the flight load factor. The incre­ment change in load factor due to the vertical gust can be determined from the following equation:

д«=0.П5 —VL – V’QLU)

(w/s) K

where

An=change in load factor due to gust те = lift curve slope, unit of per degree of a

<r = altitude density ratio W/S= wing loading, psf Ve = equivalent airspeed, knots KU= equivalent sharp edged gust velocity ft. per sec.

As an example, consider the case of an air­plane with a lift curve slope те = 0.08 and wing loading, (WjS’) = 6Q psf. If this airplane were flying at sea level at 350 knots and encountered an effective gust of 30 ft. per sec., the gust would produce a load factor increment of 1.61. This increment would be added to the flight load factor of the airplane prior to the gust,

e. g., if in level flight before encountering the gust, a final load factor of 1.0+1.61 = 2.61 would result. As a general requirement all airplanes must be capable of withstanding an approximate effective +30 ft. per sec. gust when at maximum level flight speed for normal rated power. Such a gust intensity has rela­tively low frequency of occurrence in ordinary flying operations.

The equation for gust load increment pro­vides a basis for appreciating many of the variables of flight. The gust load increment varies directly with the equivalent sharp edged gust velocity, KU, since this factor effects the change in angle of attack. The highest reasonable gust velocity that may be anticipated is an actual vertical velocity, U, of 50 ft. per sec. This value is tempered by the fact that the airplane does not effectively encounter the full effect because of the response
of the airplane and the gradient of the gust. A gust factor, К (usually on the order of 0.6), reduces the actual gust to the equivalent sharp edged gust velocity, KU.

The properties of the airplane exert a power­ful influence on the gust increment. The lift curve slope, m, relates the sensitivity of the airplane to changes in angle of attack. An aircraft with a straight, high aspect ratio wing would have a high lift curve slope and would be quite sensitive to gusts. On the other hand, the low aspect ratio, swept wing airplane has a low lift curve slope and is com­paratively less sensitive to turbulence. The apparent effect of wing loading, WjS, is at times misleading and is best understood by considering a particular airplane encountering a fixed gust condition at various gross weights. If the airplane encounters the gust at lower than ordinary gross weight, the accelerations

due to the gust condition are higher. This is explained by the fact that essentially the same lift change acts on the lighter mass. The high accelerations and inertia forces magnify the impression of the magnitude of turbulence. If this same airplane encounters the gust condition at higher than ordinary gross weight, the accelerations due to the gust condition are lower, i. e., the same lift change acts on the greater mass. Since the pilot primarily senses the degree of turbulence by the resulting accelerations and inertia forces, this effect can produce a very misleading impression.

The effect of airspeed and altitude on the gust load factor is important from the stand­point of flying operations. The effect of alti­tude is related by the term д/o-, which would related that an airplane flying at a given EAS at 40,000 ft. ((7 = 0.25) would experience a gust load factor increment only one-half as great as at sea level. This effect results be­cause the true airspeed is twice as great and only one-half the change in angle of attack occurs for a given gust velocity. The effect of airspeed is illustrated by the linear variation of gust increment with equivalent airspeed. Such a variation emphasises the effect of gusts at high flight speeds and the probability of structural damage at excessive speeds in turbu­lence.

The operation of any aircraft is subject to specific operating strength limitations. A single large overstress may cause structural failure or damage severe enough to require costly overhaul. Less severe overstress re­peated for sufficient time will cause fatigue cracking and require replacement of parts to prevent subsequent failure. A combat airplane need not be operated in a manner like the “little old lady from Pasadena" driving to church on Sunday but each aircraft type has strength capability only specific to the mission require­ment. Operating limitations must be given due regard.