EFFECT OF HIGH SPEED FLIGHT

Many different factors may be of structural importance in high speed flight. Any one or combination of these factors may be encount­ered if the airplane is operated beyond the limit (or redline) airspeed.

At speeds beyond the limit speed the air­plane may encounter a critical gust. This is especially true of a high aspect ratio airplane with a low limit load factor. Of course, this is also an important consideration for an air­plane with a high limit load factor if the gust should be superimposed on a maneuver. Since the gust load factor increment varies directly with airspeed and gust intensity, high airspeeds must be avoided in turbulent conditions.

When it is impossible to avoid turbulent conditions and the airplane must be subject to gusts, the flight condition must be properly controlled to minimize the effect of turbulence. If possible, the airplane airspeed and power should be adjusted prior to entry into turbu­lence to provide a stabilized attitude. Ob­viously, penetration of turbulence should not be accomplished at an excess airspeed because of possible structural damage. On the other hand, an excessively low speed should not be chosen to penetrate turbulence for the gusts may cause stalling of the aircraft and difficulty of control. To select a proper penetration airspeed the speed should not be excessively high or low—the two extremes must be tempered. The “maneuver” speed is an im­portant reference point since it is the highest speed that can be taken to alleviate stall due to gust and the lowest speed at which limit load factor can be developed aer©dynamically. The optimum penetration speed occurs at or very near the maneuver speed.

Aileron reversal is a phenomenon particular to high speed flight. When in flight at very high dynamic pressures, the wing torsional deflections which occur with aileron deflection are considerable and cause noticeable change in aileron effectiveness. The deflection of an aileron on a rigid wing creates a change in lift and produces a rolling moment. In addition the deflection of the control surface creates a twisting moment on the wing. When the actual elastic wing is subject to this condition at high dynamic pressures, the twisting mo­ment produces measurable twisting deforma­tions which affect the rolling performance of the aircraft. Figure 5 5 illustrates this process and the effect of airspeed on aileron effective­ness. At some high dynamic pressure, the

twisting deformation will be great enough to nullify the effect on aileron deflection and the aileron effectiveness will”be zero. Since speeds above this point create rolling moments op­posite to the direction controlled, this point is termed the “aileron reversal speed.” Oper­ation beyond the reversal speed would create an obvious control difficulty. Also, the ex­tremely large twisting moments which produce loss of aileron effectiveness create large twist­ing moments capable of structural damage.

In order to prevent loss of aileron effective­ness at high airspeeds, the wing must have high torsional stiffness. This may be a feature difficult to accomplish in a wing of very thin section and may favor the use of inboard ailer­ons to reduce the twisted span length and effectively increase torsional stiffness. The use of spoilers for lateral control minimizes the twisting moments and alleviates the reversal problem.

Divergence is another phenomenon common to flight at high dynamic pressures. Like aileron reversal, it is an effect due to the inter­action of aerodynamic forces and elastic deflec­tions of the structure. However, it differs from aileron reversal in that it is a violent instability which produces immediate failure. Figure 5-5 illustrates the process of instability. If the surface is above the divergence speed, any disturbance precipitates this sequence. Any change in lift takes place at the aerody­namic center of the section. The change in lift ahead of the elastic axis produces a twist­ing moment and a consequent twisting deflec­tion. The change in angle of attack creates greater lift at the a. c,, greater twisting deflec­tion, more lift, etc., until failure occurs.

At low flight speeds where the dynamic pressure is low, the relationship between aero­dynamic force buildup and torsional deflection is stable. However, the change in lift per angle of attack is proportional to V2 but the structural torsional stiffness of the wing re­mains constant. This relationship implies that at some high speed, the aerodynamic force buildup may overpower the resisting torsional stiffness and “divergence” will occur. The divergence speed of the surfaces must be suf­ficiently high that the airplane does not en­counter this phenomenon within the normal operating envelope. Sweepback, short span, and high taper help raise the divergence speed.

Flutter involves aerodynamic forces, inertia forces and the elastic properties of a surface. The distribution of mass and stiffness in a structure determine certain natural frequencies and modes of vibration. If the structure is sub­ject to a forcing frequency near these natural frequencies, a resonant condition can result with an unstable oscillation. The aircraft is subject to many aerodynamic excitations while in operation and the aerodynamic forces at various speeds have characteristic properties for rate of change of force and moment. The aerodynamic forces may interact with the structure in a fashion which may excite or negatively damp the natural modes of the structure and allow flutter. Flutter must not occur within the normal flight operating en­velope and the natural modes must be damped if possible or designed to occur beyond the limit speed. A Typical flutter mode is illus­trated in figure 5-5.

Since the problem is one of high speed flight, it is generally desirable to have very high natural frequencies and flutter speeds well above the normal operating speeds. Any change of stiffness or mass distribution will alter the modes and frequencies and thus allow a change in the flutter speeds. If the aircraft is not properly maintained and excessive play and flexibility exist, flutter could occur at flight speeds below the limit airspeed.

Compressibility problems may define the limit airspeed for an airplane in terms of Mach num­ber. The supersonic airplane may experience a great decay of stability at some high Mach number or encounter critical structural or engine inlet temperatures due to aerodynamic heating. The transonic airplane at an excess і ve speed may encounter a variety of stability, con­trol, or buffet problems associated with tran­sonic flight. Since the equivalent airspeed for a given Mach number decreases with altitude, the magnitude of compressibility effects at high altitude may be negligible for the tran­sonic airplane. In this sense, the airplane may not be able to fly at high enough dynamic pressures within a certain range of Mach num­bers to create any significant stability or control problem.

The transonic airplane which is buffet lim­ited requires due consideration of the effect of load factor on the onset of buffet. Since critical Mach number decreases with lift coef­ficient, the limit Mach number will decrease with load factor. If the airplane is subject to prolonged or repeated buffet for which it was not designed, structural fatigue will be the certain result.

The limit airspeed for each type aircraft is set sufficiently high that full intended appli­cation of the aircraft should be possible. Each of the factors mentioned about the effect of excess airspeed should provide due respect for the limit airspeed.