When I walk around an airplane, I find it interesting to try to understand its aerodynamics. My attention is on both the overall configuration (flying surfaces, airfoils, fuselage shape, fairings, engine installation, etc.) and small details. Often, it is the small details that are the most interesting and revealing.
Many airplanes include at least one aerodynamic device that was either designed into the original configuration to avoid a problem or added later to fix characteristics encountered during flight test or certification. Some such devices have become relatively well-known (for example, vortex generators), and some are more obscure.
During my years of working as an aerodynamicist, walking flight lines, and attending Oshkosh (AirVenture) and Sun ’n Fun, I have accumulated a library of such aero devices. Starting with this article, and in times to come, I will describe some of these devices, the aerodynamics behind them, and how they are used to affect the flying qualities and other characteristics of an airplane.
“Speed Bumps,” i.e., Shock Control Bumps
The first device we will look at is used on airplanes that cruise at high subsonic speeds. They are spanwise-oriented bumps on the wing with either a triangular or rounded cross-section. These “speed bumps” are typically installed aft of 50% chord to stabilize the flow over the wing’s upper surface when flying at high subsonic speed. They control the effect of local shock waves that appear on wings when the flow on part of the wing’s upper surface becomes supersonic.
Shock control bumps are used on multiple types, including several Learjets, the HondaJet, and the Cirrus Vision Jet.
Mach Number Effects
At low airspeeds, the airflow over the entire airplane is subsonic, and the effects of Mach number on the aerodynamics of the airplane are negligible.
At higher subsonic speeds, Mach number affects aerodynamics, and the faster we fly, the more pronounced that effect. Mach effects can first appear at airspeeds above about 300 knots, when free-stream Mach number exceeds about 0.5.
The effects of Mach number on the flow over the wings and control surfaces first became a significant concern on WW-II fighters. Several types (notably the P-38) exhibited serious Mach number effects (called “compressibility” effects at the time). If the airplane dove to a higher airspeed than it could achieve in level flight at high altitude, it would tend to tuck under and steepen the dive. The elevators did not have enough control power to raise the nose, and the airplane would be locked into a high-speed dive until it got to a lower altitude where the speed of sound is higher. This higher speed of sound causes the Mach number-induced effect established at high altitude to dissipate. It was then possible to recover. The pullout was still very risky because it was easy to break the airplane by pulling too hard at very high airspeed and overloading the structure.
This “Mach tuck” or “compressibility dive” was dangerous and also hurt the combat capability of these airplanes. If the pilot dived too aggressively trying to attack or escape, he could find himself locked into diving helplessly out of the fight.
Critical Mach Number
As the air flows over the surface of the airplane, the local air velocity changes. At some places on the configuration, the air is slowed down, and in other places, it flows faster than free stream.
On the upper surface of a lifting wing, local airspeed is faster than free stream over a significant portion of the chord.
At low airspeed, the flow over the wing is subsonic everywhere. As speed increases, there is a free-stream Mach number at which the fastest local flow on the wing becomes supersonic. This Mach number is called the “critical Mach number.”
Mach numbers higher than this are called “supercritical” Mach numbers, and fast subsonic airplanes normally cruise at supercritical Mach numbers.
The term “supercritical” is now generally used to refer to a class of airfoils designed for efficient cruise at supercritical Mach numbers. These were pioneered by Dr. Richard Whitcomb and were initially called “advanced supercritical” airfoils. “Advanced supercritical” has since been abbreviated to “supercritical,” leading to the common usage of “supercritical airfoil” as a term of art in aerodynamic design.
On wings flying at supercritical Mach numbers, the flow over the forward portion of the wing is locally supersonic. This bubble of supersonic flow ends with a terminating shock. Airspeed drops to subsonic across the shock.
While it is possible to design airfoils that are shock-free at a supercritical Mach number, it turns out that airplanes achieve their best specific range flying at a Mach number high enough so there is a weak shock on the wing.
The strength and position of the shock change the aerodynamic characteristics of the wing by altering the chordwise distribution of air pressure.
Shock Stability
The strength and position of the shock on the wing are a function of the wing geometry (airfoil and sweep), Mach number, and angle of attack (AoA). The wing planform is fixed, but Mach number and AoA change in flight, and control surfaces like ailerons effectively change the airfoil section when they are deflected.
Ideally, the position and strength of the shock are uniquely determined by Mach number and AoA, and vary relatively progressively as these change. There are, however, some situations where this is not the case.
The stability of the shock structure is a function of the chordwise distribution of air pressure on the wing. Problems can arise if the pressure distribution allows the shock to move significantly in response to a small change in flight condition or a small deflection of a control surface.
The pressure distributions on airfoils designed for efficient, low-drag flight tend to be susceptible to large shock movement in response to small perturbations, so wings with low sweep and high-performance airfoils are quite vulnerable to shock-stability issues.
Shock-Movement-Induced Issues
Changes in the shock structure change the aerodynamic forces acting on the wing by changing local air pressure. These changes can cause significant problems that can range from concerning to potentially catastrophic:
Control Surface Buzz: Deflection of a control surface affects the strength and position of the shock, which affects pressures, which in turn affects hinge moments. This can lead to an aerodynamic feedback loop where the shock oscillates in phase with control surface deflection, and pressure changes drive the oscillation. This phenomenon presents as a high-frequency limit-cycle oscillation of the control surfaces. It is most common on ailerons, but it also shows up on rudders on occasion.
Aileron Snatch: Aileron buzz can be the warning of the onset of a much more dangerous condition. As Mach number increases, instead of driving an oscillation, shock movement causes forces that drive the aileron farther and increase its deflection. The ailerons become unstable, and if the forces are high enough, they can overpower the pilot, drive the ailerons hard over, and lead to loss of control. On early Learjets, the maximum allowable Mach number was set by having a sufficient margin from the onset of aileron instability. Unfortunately, the airplane had enough thrust to exceed the limiting Mach in level flight, and there were several fatal accidents caused by pilots disabling the Mach warning system, deliberately overspeeding the airplane to “see how fast it would go,” and losing control due to aileron snatch. In these accidents, the airplanes rolled uncontrollably and dove into the ground at high speed because the pilots could not center the ailerons and level the wings.
The Fix
Shock-position instability is difficult to predict either computationally or in wind-tunnel testing, although experienced aero-designers can often anticipate what configurations might be prone to the problem.
Both control-surface buzz and aileron snatch are more likely to be detected in a flight test during envelope expansion.
When an aerodynamic problem shows up in a flight test, it is too late to change the primary structure of the airplane, so it is not possible to change the overall airfoil. Even if it were possible, it would not be the preferred solution because the airfoil design was chosen to optimize performance.
Instead, the solution is to add one or more sets of shock control bumps to “pin” the shock in a stable position. The bumps might exact a small drag penalty, but the combination of an optimized airfoil plus bumps is still more efficient than an alternative airfoil with a more stable initial shock structure.
The air is decelerated by the forward face of the bump and then accelerates over the peak of the bump. These local perturbations in the flow trigger the shock at the bump and tend to keep the shock stably positioned at the bump.
Often, a single spanwise bump is enough to alleviate shock oscillations satisfactorily. Sometimes, however, there might be a need for two or more rows to tame the shocks sufficiently to be acceptable. The HondaJet has a single bump with a rounded cross-section, while the Learjet 24 and Learjet 25 use two rows of bumps.
Thanks, Barnaby. I always enjoy reading about the engineering side of aerodynamics.
Thanks Barnaby, I’ll keep it below Mach.5 then.
Glad to see you escaped the AI swamp!
I cannot recall enjoying any article I’ve read recently as much as Barnaby’s “journalistic stroll” past a few airplanes we see everyday. Russ: Please do not let this man escape!
Barnaby: MORE, please!
John Caulkins
I see a post by Mr Wainfan, and I know I am going to learn something today!
This was completely new to me, thanks!
Excellent read, Barnaby! Thought for sure you would bring up the mother of all anti-shock bodies, on the Convair 990 wing. It was supposed to cruise at 0.89M. Hard to think about now when you are forced to cruise a third of the way to LA at 0.72 in a noisy 737…..and watch Klaus pass by in his EZ on the way to Santa Paula.