Patent Application
20060108472
1. Field of the Invention
The invention relates to the field of aircraft control surface systems and, in particular, to a roll control system for an aircraft which eliminates adverse yaw during such maneuvers.
2. Description of Related Art
When ailerons are deflected one goes up as much as the other goes down so there is no net change in the angle of attack of the airplane. The dominant change is the wing's camber. On one side the camber is increased so that the wing lifts more, while on the other, less. The key to understanding adverse yaw is the fact that the wing is already holding the weight of the airplane. With aileron deflection there is a reduced lift on one side and an increased lift on the other. The down-going side is not negative lift, but merely less lift. Any wing that lifts more will always have more drag than the same wing that lifts less. That's how a differential of drag is introduced and it is this differential drag that causes the adverse yaw.
Proverse yaw, yaw going the same direction as roll, is often assumed to be a good thing. Indeed, yaw with the turn can be made faster, or conversely, the surfaces can be made smaller. But yaw in any direction can cause a spin and should therefore be avoided.
This problem has been recognized. For example, U.S. Pat. No. 6,641,086 B2, System And Method For Controlling An Aircraft by W. Clark, the inventor of the present invention, discloses a system wherein two different size ailerons are mounted on each wing. The smaller outermost aileron on one wing moves when the larger innermost aileron on the other side moves. On both sides, the smaller one is arranged to move up when that wing is to go down. Conversely the larger innermost aileron moves down when that side is to go up. This form of asymmetry allows the control surface on the wing that goes down to deflect over a larger angle. With it deflecting over a larger angle it spends most of its time in negative lift. Increasing its negative lift increases its drag. The down-going side hence has drag, which is proverse with roll. The down-going side of normal ailerons does not move as far, so they rarely go into negative lift; merely less positive lift. Less lift means less drag and less drag on the down going side means adverse yaw.
However, this approach has limitations. The ailerons are constrained to move down only. When the roll rate is very low, that is, if the pilot moves the controls laterally only a small amount, the small aileron (even though it deflects farther than the large aileron on the other side) doesn't move far enough to get into negative lift. Although adverse yaw is not a problem at small roll rates, this older invention did not produce cancelled yaw moments. In fact it was only balanced at maximum roll deflections.
Thus, it is a primary object of the invention to provide a control system for an aircraft that eliminates adverse and proverse yaw during all roll maneuvers.
It is another primary object of the invention to provide a control system for an aircraft that eliminates adverse and proverse yaw during roll maneuvers by means of stabilizers; the very same stabilizers that control pitch.
The invention is a system for controlling an aircraft, the aircraft having a longitudinal axis, vertical and horizontal axis and right and left wings and right and left ailerons mounted on the right and left wings. In detail, the system includes a right stabilizer mounted to the right wing, the right stabilizer canted downward from the right wing such that the outer end thereof is down and able to deflect about an axis of rotation parallel to the span axis of the stabilizer. A left stabilizer mounted to the left wing, the left stabilizer canted downward from the left wing such that the outer end thereof is down and able to deflect about an axis of rotation parallel to the span axis of the stabilizer. A control system is provided to simultaneously deflect the right aileron and stabilizer downward and the left aileron and stabilizer upward to turn left and to deflect the right aileron and stabilizer upward and the left aileron and stabilizer downward to turn right.
The control system further includes simultaneously deflecting the left and right ailerons downward and the left and right stabilizers upward to create a pitch-up moment and to simultaneously deflect the right and left ailerons upward and the right and left stabilizers downward to create a pitch-down moment. It should be noted that the stabilizers in the process of providing up and down forces also produce drag forces and because of their large lateral displacement from each other can operate as rudders automatically in canceling adverse yaw and by deliberate application of rudder inputs from the pilot's controls.
One embodiment includes a left boom extending rearward from the wing on the left side of the longitudinal axis with the left stabilizer mounted on the end of the left boom. A right boom extends rearward from the wing on the right side of the longitudinal axis with the right stabilizer mounted on its end. In a second embodiment of the invention the stabilizers are mounted on the tips of a swept back wing.
The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description in connection with the accompanying drawings in which the presently preferred embodiments of the invention are illustrated by way of examples. It is to be expressly understood, however, that the drawings are for purposes of illustration and description only and are not intended as a definition of the limits of the invention.
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The stabilizers create a roll moment with proverse yaw for two reasons; 1. the stabilizers are normally providing a pitch-up moment and 2. because they are separated from each other laterally. When a stabilizer is said to move up, it is actually moving more up from its normal trim position. And it is not merely moving down but moving less up from its trim position. Moving less up is less drag. More up is more drag. In that way there is a proverse yaw associated with the stabilizers when they assist the ailerons in rolling the airplane.
There is a correctable problem when the stabilizers assist in the rolling of the airplane. On a normal airplane, when ailerons function to roll the airplane, any residual pitch moments are unfelt because the short longitudinal distance from aileron to center of lift. This is not true with stabilizers when they function to roll the airplane. Because the stabilizers are trimmed to provide sufficient pitch-up moment to get the wings to provide lift, when operated differentially from that position, the non-linear characteristics of the control surfaces make the up-going surface less effective (for a given increment of deflection) than the down-going surface, as shown in
The canceling of adverse yaw of the wing-and-ailerons with the proverse yaw of the separated-stabilizers is achieved through changes in drag as the stabilizer deflects to change its aerodynamic lift force. The drag force on the stabilizers produce a yaw moment because they are separated. This phenomenon exists whether the stabilizers are parallel to the wing or canted down as described above. The canting down provides a vertical component, which will cancel the destabilizing vertical component of the fuselage ahead of the CG. Vertical stabilizers can be used with separated horizontal stabilizers if the increased radar cross-section is acceptable.
While the invention has been described with reference to particular embodiments, it should be understood that the embodiments are merely illustrative as there are numerous variations and modifications which may be made by those skilled in the art. Thus, the invention is to be construed as being limited only by the spirit and scope of the appended claims.
The invention has applicability to the aircraft industry.
