Flight Characteristics
Launch.
Minimum control speeds are not determined by a maximum lift coefficient, but by directional stability and estimated regions of buffet.
Intentional spins in this airplane are prohibited.
Ventral Attached – Fully developed spins are not apt to occur with the ventral attached, except possibly due to reduced directional stability at an angle of attack in excess of 20 degrees. If there is any yawing or rolling due to this reduced stability and a spin is entered, the stick should immediately be moved in the direction of spin rotation and full opposite rudder (against rotation) should be applied. When the spin has stopped, the stick should be centered and moved forward to prevent a spin in the opposite direction, and the rudder neutralized. Do not immediately attempt to level the wings by moving the stick against the direction of rotation.
Ventral Jettisoned – With the ventral jettisoned and the airplane at an angle of attack in excess of 17.5 degrees, a spin may be developed from which recovery may be difficult or impossible; therefore, jettisoning of this surface should be delayed as long as possible before entering the landing pattern. Recommended recovery is the same as when the ventral is attached: The stick should be moved in the direction of spin rotation and full opposite rudder should be applied. When rotation stops, the stick should be centered and moved forward to prevent a spin in the opposite direction, and the rudder neutralized.
Pitch Control – For flight above approximately Mach 1.4, the maximum normal acceleration obtainable can be limited by the maximum stabilizer deflection available. Maximum normal acceleration can also be limited by an estimated initial buffet effect between Mach .5 and Mach 1.0. Below Mach .5, the limit is imposed by the reduced directional stability at angles of attack above 20 degrees. At low speeds, the angle of attack for buffet onset due to airflow separation on the wing is about 13 degrees. Turbulence behind the vertical stabilizer may cause considerable buffet at any angle of attack. Therefore, buffet onset should not be considered as a stall warning until buffet characteristics have been determined from flight experience.
Roll Control – Adequate roll control is available throughout the Mach number range from high-speed flight to landing speeds. Rate of roll is limited to 50 degrees per second when the SAS roll gain is set on high gain. At lower angle of attack, there is slight favorable yaw (yaw in direction of roll application). As the angle of attack increases above Mach .6, there is a decrease in this yaw. At low Mach numbers and in the transonic region, there is very little change in roll effectiveness with angle of attack. Roll effectiveness increases with an increase in angle of attack above Mach 2.6. Roll coupling does not present a problem with or without SAS, except at extremely high Mach numbers at high altitudes. Roll coupling does not occur at as low an altitude with SAS in as it does with SAS out, and in either case, roll coupling would not occur below approximately 100,000 feet. Maximum rates of roll at other Mach numbers and altitudes are limited by the maximum differential horizontal stabilizer deflection.
Yaw Control – Pilot feel is supplied as a function of surface deflection, and the variation of pedal force with pedal travel is linear. The upper and lower (ventral) vertical stabilizers give good directional control throughout the flight range. The ventral is effective in retaining stability at high angles of attack where the upper vertical stabilizer is partially blanketed by the wing and fuselage. At high Mach numbers and high angles of attack, this increase in effectiveness of the ventral produces an appreciable amount of roll in a direction opposite to the yaw. Although the ventral is jettisoned before landing, the upper vertical stabilizer supplies adequate directional control for low-speed flight up to 17.5 degrees angle of attack.
At supersonic Mach numbers, in addition to increasing the effectiveness of the vertical stabilizers and improving directional stability, the speed brakes increase longitudinal stability. As angle of attack increases, the effect of the speed brakes on longitudinal and directional stability also increases. In addition to improving stability at high Mach numbers, the speed brakes obviously provide additional drag to the airplane. At subsonic Mach numbers, the speed brakes reduce both the longitudinal and the directional stability. During the glide, which would occur at lower altitudes, the basic drag of the airplane gives high rates of descent, and the increase in rate of descent due to use of the speed brakes is undesirable.
Launch – Launch from the carrier airplane can be accomplished satisfactorily for 1 G flight conditions during carrier airplane acceleration, climb, and cruise as well as the design launch conditions.
Glide – Refer to "Landing".
Flight characteristics of the X-15A-2 in the clean configuration will be similar to those of the other X-15 airplanes. The flight characteristics discussed herein reflect those of the design mission only. Flight characteristics during missions that are significantly different than that of the design mission are not included.
Launch (external tanks attached) – Engine thrust will produce a nose-down pitch tendency, since the airplane center of gravity with the external tanks attached is lower than the engine thrust line. However, the horizontal tail is very effective and more than adequate to counteract this condition.
The angle-of-attack limit for directional stability can easily be exceeded in the pull-up following launch. Caution should be exercised to avoid exceeding this limit. The airplane tends to pitch nose-up if loss of thrust is experienced shortly after launch. Immediate pilot action may be required to prevent the angle-of-attack limit from being exceeded.
External tanks attached – With external tanks attached, the flight characteristics remain satisfactory throughout the design mission. However, the airplane exhibits reduced levels of both longitudinal and directional stability. The longitudinal control forces required to produce a desired change in airplane attitude are reduced, as compared to the clean configuration (external tanks released) for the same flight condition. Therefore, the maximum recommended angle of attack is reduced, because of this reduction in directional and longitudinal stability.
External tanks released – The external tanks are released at Mach 2.1 at 70,000 feet in a zero-G normal load factor condition. However, wind tunnel results indicate that the external tank release characteristics are relatively unchanged between Mach 1.9 and Mach 2.4 as long as the jettison envelope is observed.