Aeronautics and astronautics – Aircraft sustentation – Sustaining airfoils
Reexamination Certificate
2000-07-07
2002-04-23
Jordan, Charles T. (Department: 3644)
Aeronautics and astronautics
Aircraft sustentation
Sustaining airfoils
C244S203000, C244S219000
Reexamination Certificate
active
06375127
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a dynamic load alleviation system and, more particularly, to the design, manufacture, and operation of an active control surface modal device.
2. Description of the Prior Art
Civil and military aircraft encounter a number of dynamic load conditions resulting from fluctuating air loads. Such environments lead to ride discomfort, structural fatigue damage and degradation of flight performance. Hence, there is a need to improve the fatigue life and flight performance of civil and military aircraft.
Buffet is a turbulence phenomenon that originates from flow separated wake created behind any aerodynamic lifting surfaces or bodies. In a typical example illustrated in
FIG. 1
, a modern high performance aircraft
20
incorporates a strake
22
on a fuselage
24
extending into a leading edge
26
of a wing
28
. In a typical fashion, the release of strong vortices
30
behind the leading edge extension, or strake, of the aircraft are depicted. At some distance from its origin, as depicted at
32
, the vortex core bursts and engulfs the tail
34
. In this manner, micro vortices are generated which cause a fluctuating pressure field of random nature and severely excite the tail of the aircraft.
Such dynamic environments have caused fatigue failures at the root and the mid-section of the vertical tail or tails of aircraft on which they appear. Tail skin fasteners around the rudder hinge line often disappear. Buffet induced excitation can also lead to dynamic problems of engine mounts that are dose to the tail root section. Consequently, these problems increase the life cycle cost of an aircraft. As a result, the Government procuring agencies and aircraft manufacturers are interested in finding solutions to these problems.
Buffet load problems have been extensively studied by a number of its investigators. These studies were actually conducted in two parts. The first part of these studies was devoted to the understanding of the physical characteristics of the fluctuating pressures, while the second part was focused on the remedial procedures. The remedial procedures offer passive and active control methods. A brief account of these studies will now be discussed.
Passive Buffet Load Control Methods
The passive methods include the design of various configurations of the leading edge extension (LEX) with and without fences on the LEX. The fences serve to break up the vortex core and consequently reduce the vortex strength. On the other hand, strong vortex cores are required to generate suction pressure to achieve super maneuver performance of high performance aircraft. Although fences reduce the root bending moment on the vertical tail, these were not recommended for the production series aircraft for two reasons: (1) they are expensive to install, and (2) they degrade the quality of flow intended for high angle of attack maneuvers. One study reports an alternate passive method that uses blowing and suction of air around LEX to suppress buffeting.
Active Buffet Load Control Methods
Two different principles are used in active buffet load control technology.
Principle No. 1, so-called, uses aerodynamic effectors (control surfaces or active control surface modes generally of the presented below in this disclosure) to generate aerodynamic damping that reduces buffet induced oscillations. The deployment of these effectors is achieved by means of actuators, either of conventional hydraulic actuators having low frequency bandwidth or smart actuators having broad band frequency range. The power requirement is directly related to the amount of damping required, or in other words, deployment amplitude of the effectors. This is a positively robust approach.
Principle No. 2, so-called, uses an anti-wave generation method in which the structure is excited at its natural frequencies and out-of-phase with the forcing signals. In this approach, cancellation can be achieved only at discrete frequencies of the structural modes. At other frequencies, enormous power is required to excite the structure to generate aerodynamic damping. Since buffet is a broad band phenomenon, it can force all structural modes at the same time. The wave cancellation method can be effective only at one frequency at a time, which is the principle behind surface mounted piezoactuators.
One investigator and his associates employ the first principle to reduce buffet induced structural stresses. They activate the rudder using conventional hydraulic or pneumatic actuators to generate out-of-phase unsteady aerodynamic loads to suppress vibration of the tail. Unfortunately, there are two main problems in this approach. The first problem is that the flight control system and the buffet control system use the same control surface, which reduces the availability of the control surface for either purpose. Also, interference with the flight control is an undesirable aspect that pilots do not like. In addition, the conventional actuators are limited in the frequency bandwidth which makes it difficult to swing the massive rudder at higher frequencies about the hinge line. For example, the rudder of an F/A-18 weighs about 64 pounds. Hence, the ability of such a typical airfoil and its actuators to function in a wide-band buffet spectrum is significantly limited. One wind tunnel study sponsored by NASA, reported a 60% reduction of bending moment in a buffet load environment. This particular wind tunnel study employed a ⅙-scale F-18 model with actively controlled surface mounted piezoelectric actuators on the vertical fins. The disadvantage of this approach will be discussed shortly.
While surface mounted piezoelectric actuators proposed for some of these programs are good candidates for wind tunnel models they have no practical value for production scale aircraft for the following reasons:
surface mounted piezoelectric actuators cannot produce anti-mode waves to counteract the buffet excitation and cannot provide large surface strains;
the model studies assumed that actively controlled piezoelectric actuators provide necessary mechanical damping to suppress vibration. This assumption does not hold well for full-scale aircraft. In reality, aerodynamic damping plays a greater role than the mechanical damping. The merits of aerodynamic damping have been demonstrated in active flutter suppression technologies.
surface mounted actuators and electrical contacts may fail due to fatigue and erosion;
surface mounted actuators cause flow separation;
there is a weight penalty if the surface mounted actuators are used in large quantities; and
high voltage input may be required which would cause the risk of arcing across structural joints.
Gust environment is another important aspect of aircraft dynamic loads that arise from atmospheric turbulence. Here, aircraft penetrate a sinusoidal gust wave resulting in loss or gain of vertical lift force. Thus, in this instance, the aircraft is subjected to external excitations causing ride discomfort and structural fatigue damage.
Still another design criterion that requires a careful consideration is flutter, or aeroelastic instability. Flutter is a self-excited oscillatory phenomenon that results in structural instability leading to catastrophic destruction when the flight speed exceeds the design speed limit.
A small number of patents are typical of the known prior art attempting to reduce dynamical loads. For example, U.S. Pat. No. 4,706,902 to Destuynder et al. discloses an active control method of reducing the buffet loads. A device is used to detect buffet onset and active control system is used to actuate a number of control surfaces about their hinge lines to generate aerodynamic damping forces to oppose buffet excitation. Since buffeting is a broad band excitation phenomenon containing a high frequency spectrum in the range of 100 to 300 Hz (cycles per second), the control surface cannot be set in high frequency motion. Consequently, its utility is necessarily limited to low frequency modes of vibration. U
Dinh T.
Hilburger Albert W.
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