How it Works: F-35 High Angle of Attack Testing
For every student who doesn’t care much for Geometry, there’s a fighter pilot who ‘gets’ its importance. Angles and equations determine speed and position in modern air combat. They become the difference between the ‘quick’ and the ‘dead’ once the fight is engaged. The 5th Generation tactical fighter aircraft men and women will operate in the coming decades must be able to handle the toughest flight regimes for mission success and survival.
Flying at high angles of attack (AoA), or “High Alpha,” a modern fighter aircraft gains enhanced nose-pointing capability, which allows the pilot to track and acquire enemy aircraft in aerial combat. The ability to accurately and rapidly point their aircraft’s nose is key to outmaneuvering their enemy, while locking on to them with their radar or heat-seeking missiles to take the decisive shot, most likely at a beyond-visual range.
‘Angle of Attack’ is the angle between an aircraft wing’s chord line, or the imaginary straight line joining the wing’s leading and trailing edges, and the aircraft’s flight path. It is not to be confused with the relation of the aircraft to the Earth as this is called the attitude, which is seldom, if ever, the same as the angle of attack. When the angle is small, the aircraft is said to be at a low angle of attack. When the angle is large, the aircraft is said to be at a high angle of attack. Two variables can change the amount of lift generated by a wing in a given configuration: the speed of air flowing over the airfoil and the angle of attack. Some military aircraft are able to achieve very high angles of attack, but at the cost of drag. This provides the aircraft with great agility, although such maneuvers slow the aircraft down.
The aircraft, when pushed beyond its control, can seem to defy the laws of physics and the principles of flight. The inadvertent loss of control at high angles of attack happens frequently and the aircraft and pilot can be killed due to their failure to recover from out-of-control flight conditions. ‘Departures’ from controlled flight may occur unintentionally in high-G maneuvers (aka “pulling Gs”). The gravitational load bearing on the pilot can cause him or her to pass out from the rapid onset as it drains the blood flow to their brain, removing oxygen. It could happen from a nose-high deceleration to zero airspeed in trying to gain the geometric advantage over their enemy in combat maneuvering. Or a steep spiral may be mistaken to be a spin, causing the recovery flight control surfaces to be misapplied. Whatever the circumstances, departures from controlled flight all too often end in catastrophe. For this reason, pilots must be familiar with every facet of high angle of attack flying and their aircraft must be checked out to the limits of its endurance.
The F-35’s high AoA testing pushes the aircraft beyond both the positive and negative maximum command limits, including intentionally putting the aircraft out of control in several configurations. This includes initially flying in the stealthy ‘clean’ wing configuration, followed by testing with external air-to-air pylons and missiles, and then with open weapon bay doors, all creating additional drag on the aircraft.
The F-35’s high angle of attack testing began in late October 2012, with the aircraft pushing to its production AoA limit of 50 degrees nose high. Test pilots take the aircraft beyond this limit to evaluate its characteristics in recovering from out-of-control flight conditions.
“High AoA testing produces some of the most challenging environments for the engine because the intake gets bad air,” explained David Nelson, lead F-35 test pilot for Lockheed Martin at the U.S. Air Force Flight Test Center at Edwards AFB, California. “The bad air creates a potential for producing a flameout, which is basically an engine shutdown. For that reason, air start testing preceded high AoA testing.”
For all testing, recovery from ‘out of control’ flight has been 100 percent successful without the use of the spin recovery chute, which was installed to maximize safety.