Aero Chapter 01, General Aerodynamics Review
T-45 Aerodynamics Student Workbook
These are the allowed positive and negative g limits of the aircraft. Structural limits are based on the
strength of the aircraft structure. Under any loading, even 1 g, the aircraft structure will flex. A large
amount of airframe flexure will lead to permanent deformation (a bent airframe) or component failure. A
lesser amount of flexure affects the lifetime of the airframe due to metal fatigue, etc. The aircraft structural
limits are selected to ensure that g-loading induced flexure will not damage the airframe or shorten its
design life. Structural limits are also referred to as acceleration limits or limit load factors. Exceeding the
structural limits (overstress of aircraft) may or may not damage the aircraft. Numerous overstresses will
shorten the service life of the aircraft.
These define the maximum operating speeds in both KIAS and IMN of the aircraft. Above the aeroelastic
limits, structural damage or failure may occur as well as a loss of stability and/or control authority. The
aeroelastic limit is frequently referred to as the Redline Airspeed.
ULTIMATE STRUCTURAL LIMITS
These define the ultimate operating strength limits of the aircraft. Flight operations beyond the ultimate
structural limits will result in structural failure of some component of the aircraft. It should be noted that
the ultimate structural limits are outside the normal operating envelope of the aircraft. The usual aircraft
design rule is for the ultimate structural limit to be 150% of the structural limit. However, this may not be
strictly true since the structural limits may reflect aircraft lifetime concerns while the ultimate structural
limits do not.
This point is located at the intersection of the structural limit and the aerodynamic limit. Cornering speed
is the minimum speed where the limit load factor can be achieved. Cornering speed defines turning
performance of the aircraft at which the aircraft can achieve maximum turn rate and minimum turn radius.
At or below cornering speed, the aircraft cannot be overstressed; stall will occur first. Cornering speed
may also be referred to as maneuvering speed.
The envelope is constructed based on the variables of weight, altitude, configuration, and loading
(symmetrical or unsymmetrical).
Weight will generally affect the structural limits and the aerodynamic limits. The aerodynamic limits are
based on stall speed of the aircraft. Changing weight changes stall speed and the aerodynamic limits.
Likewise, as weight change affects the allowable gs, the structural limits will also be affected. When the
structural limits are affected, the ultimate structural limits are also affected. An aircraft is usually
structurally limited at low altitudes.
Changes in altitude will affect the aerodynamic limits and aeroelastic limits. As altitude changes, the
maximum Mach number and maximum IAS change. The ability to generate the necessary speed to
achieve maximum g is also affected as density decreases and thrust available is reduced. At higher
altitudes, an aircraft is usually aerodynamically limited due to lack of thrust and decreased density.
CONFIGURATION (OR LOADING CHANGE)
This may affect any element of the operational envelope. The elements affected will depend on the
change in either configuration or loading. For example, rolling pullouts (an asymmetrical load) always
reduce allowable g. Extending landing gear or flaps restricts airspeed, and external stores may reduce
maximum airspeed, Mach number, and maximum g.