In aeronautics, inertia coupling, also referred to as inertial coupling and inertial roll coupling, is a potentially catastrophic phenomenon of
high-speed flight which caused the loss of aircraft and pilots before the design features to counter it (e.g. a big enough fin) were understood. It occurs when the
inertia
Inertia is the idea that an object will continue its current motion until some force causes its speed or direction to change. The term is properly understood as shorthand for "the principle of inertia" as described by Newton in his first law ...
of a heavy
fuselage
The fuselage (; from the French ''fuselé'' "spindle-shaped") is an aircraft's main body section. It holds crew, passengers, or cargo. In single-engine aircraft, it will usually contain an engine as well, although in some amphibious aircraf ...
exceeds the ability of the aerodynamic forces and moments generated by the wing and
empennage
The empennage ( or ), also known as the tail or tail assembly, is a structure at the rear of an aircraft that provides stability during flight, in a way similar to the feathers on an arrow.Crane, Dale: ''Dictionary of Aeronautical Terms, third e ...
to stabilize the aircraft. The problem became apparent as jet
fighter aircraft and research aircraft were developed with narrow
wingspan
The wingspan (or just span) of a bird or an airplane is the distance from one wingtip to the other wingtip. For example, the Boeing 777–200 has a wingspan of , and a wandering albatross (''Diomedea exulans'') caught in 1965 had a wingspan o ...
s, that had relatively low
roll
Roll or Rolls may refer to:
Movement about the longitudinal axis
* Roll angle (or roll rotation), one of the 3 angular degrees of freedom of any stiff body (for example a vehicle), describing motion about the longitudinal axis
** Roll (aviation) ...
inertia, caused by a long slender high-density
fuselage
The fuselage (; from the French ''fuselé'' "spindle-shaped") is an aircraft's main body section. It holds crew, passengers, or cargo. In single-engine aircraft, it will usually contain an engine as well, although in some amphibious aircraf ...
, compared to the
pitch and yaw inertias.
The term inertia/inertial coupling referring to divergence during a rolling maneuver has been described as misleading because the complete problem is one of aerodynamic as well as inertia coupling. Contributions to the maneuver are complex and include inertial coupling, aerodynamic coupling and the inertia ratios about the three axes, all of which occur simultaneously. However, inertial coupling has also been defined as essentially a gyroscopic effect, i.e. a tendency of a fuselage, when rolled rapidly, to swing away from the direction of flight and become broadside to the wind, and analyzed as such by Phillips. Inertial roll coupling has been defined as a resonant divergence in pitch or yaw when roll rate equals the lower of the pitch or yaw natural frequencies.
Description
Inertia coupling will tend to occur when an aircraft with the weight distribution described above is quickly rolled about an axis other than its roll axis. The tendency can be countered by a number of strategies which include increasing directional stability and reducing allowed roll rate and duration and limiting angle of attack for performing rolling maneuvers.
The cause of the disturbing motion may be visualised by thinking of the aircraft mass being concentrated in two locations, "dumbbell-fashion" on its roll axis, one in front of the centre of gravity and the other behind. The aircraft will be flying along its aerodynamic or wind axis with the "dumbbell" at some angle of attack. Rolling about the aerodynamic axis will tend to cause the "off-axis" masses to move outwards.
The trend in fighter aircraft design through the 1950s of short wing spans, fuselages of high density and flight at high altitude all tended to increase the inertia forces due to rolling in comparison with the aerodynamic restoring forces provided by the longitudinal and directional stabilities. Rolling motion introduces coupling between longitudinal and lateral motions of the aircraft.
Although a typical jet aircraft has most of its mass distributed close to its centerline, and the aerodynamic forces and moments in planes which provide some stabilization (such that small fluctuations in control tend to return it to attitude equilibrium), it is important to remember that aircraft realistically always fly with a small non-zero random rate of yawing and pitching.
Early history
Inertial roll coupling was predicted and analyzed as a gyroscopic effect in 1948 by William Phillips who worked for the
NACA
The National Advisory Committee for Aeronautics (NACA) was a United States federal agency founded on March 3, 1915, to undertake, promote, and institutionalize aeronautical research. On October 1, 1958, the agency was dissolved and its assets ...
.
[ His analysis predated the aircraft that would experience the violent motions he predicted, the X-series research aircraft and Century-series fighter aircraft in the early 1950s. Prior to this time, aircraft tended to have greater width than length, and their mass was generally distributed closer to the center of mass. This was especially true for propeller aircraft, but equally true for early jet fighters as well. It was only when the aircraft began to sacrifice aerodynamic surface area in order to reduce drag, and use longer ]fineness ratio
In naval architecture and aerospace engineering, the fineness ratio is the ratio of the length of a body to its maximum width. Shapes that are short and wide have a low fineness ratio, those that are long and narrow have high fineness ratios. Ai ...
s that reduced supersonic drag, that the effect became obvious. In these cases, the aircraft was generally much more fuselage-heavy, allowing its gyroscopic effect to overwhelm the small control surfaces.
Inertial roll coupling was one of three distinct coupling modes which followed one another at Mach 3.2 killing pilot Captain Mel Apt in his first flight in the rocket-powered Bell X-2
The Bell X-2 (nicknamed "Starbuster") was an X-plane research aircraft built to investigate flight characteristics in the Mach 2–3 range. The X-2 was a rocket-powered, swept-wing research aircraft developed jointly in 1945 by Bell Aircraft Co ...
on 27 September 1956. Inertial roll coupling had nearly killed Chuck Yeager
Brigadier General Charles Elwood Yeager ( , February 13, 1923December 7, 2020) was a United States Air Force officer, flying ace, and record-setting test pilot who in October 1947 became the first pilot in history confirmed to have exceeded the ...
in the X-1A
The Bell X-1 (Bell Model 44) is a rocket engine–powered aircraft, designated originally as the XS-1, and was a joint National Advisory Committee for Aeronautics–U.S. Army Air Forces– U.S. Air Force supersonic research project built by B ...
three years earlier. The roll coupling study of the X-3 Stiletto (first flown in 1952) was extremely short but produced valuable data. Abrupt aileron rolls were conducted at Mach 0.92 and 1.05 and produced "disturbing" motions and excessive accelerations and loads. The first two production aircraft to experience inertial roll coupling were the F-100 Super Sabre and F-102 Delta Dagger (both first flown in 1953). The F-100 was modified with a larger vertical tail to increase its directional stability. The F-102 was modified to increase wing and tail areas and was fitted with an augmented control system. To enable pilot control during dynamic motion maneuvers the tail area of the F-102A was increased 40%. In the case of the F-101 Voodoo
The McDonnell F-101 Voodoo is a supersonic jet fighter which served the United States Air Force (USAF) and the Royal Canadian Air Force (RCAF).
Initially designed by McDonnell Aircraft Corporation as a long-range bomber escort (known as a ...
(first flown in 1954), a stability augmentation system was retrofitted to the A models to help combat this problem. The Douglas Skyray was not able to incorporate any design changes to control inertial roll coupling and instead had restricted maneuver limits at which coupling effects did not cause problems.[Airplane Stability and Control - Second edition, Abzug and Larrabee, Cambridge University Press, , p.119] The Lockheed F-104 Starfighter
The Lockheed F-104 Starfighter is an American single-engine, supersonic air superiority fighter which was extensively deployed as a fighter-bomber during the Cold War. Created as a day fighter by Lockheed as one of the "Century Series" of fi ...
(first flown in 1956) had its stabilator (horizontal tail surface) mounted atop its vertical fin to reduce inertia coupling.
See also
* Upset Prevention and Recovery Training
References
{{DEFAULTSORT:Inertia Coupling
Aviation risks
Aerodynamics
Chuck Yeager