Fluid reaction surfaces (i.e. – impellers) – Method of operation
Reexamination Certificate
2000-03-03
2001-11-27
Ryznic, John E. (Department: 3745)
Fluid reaction surfaces (i.e., impellers)
Method of operation
C416S023000, C416S500000, C244S017130
Reexamination Certificate
active
06322324
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to systems for detecting and analyzing mistracking rotor blades of a helicopter rotor and to trim tab systems used for adjusting the aerodynamic characteristics of individual rotor blades so as to correct their tracking and thereby reduce rotor vibration caused by aerodynamic unbalance of the rotor. The invention relates more particularly to systems for remotely adjusting trim tabs on rotor blades during operation of the helicopter.
BACKGROUND OF THE INVENTION
Vibration in helicopters caused by unbalance of the main rotor is a problem that has long concerned those in the helicopter field. Excessive rotor-induced vibration can shorten the life of the airframe and installed components such as avionics, and is generally annoying and uncomfortable for the crew and passengers. Helicopter rotor vibration is caused by two primary mechanisms. Inertial unbalance of the rotor is one source of rotor vibration, and results when the center of inertia does not coincide with the rotational axis of the rotor. Inertial unbalance is primarily caused by differences in mass and/or mass distribution between the rotor blades. Accordingly, it is common practice to analyze the inertial balance characteristics of helicopter rotors and to add weights to one or more of the blades so as to inertially balance the rotor. The inertial balancing operation is performed as a maintenance procedure.
Complicating the analysis of inertial unbalance is the fact that rotor vibration can also be caused by aerodynamic unbalance of the rotor, which results when the aerodynamic forces and moments acting on the rotor blades are not the same among all of the blades. This can be due, for example, to differences in blade shape, such as differences in blade twist and/or differences in airfoil shapes, from one blade to another. Such blade shape differences can cause a blade to “mistrack”, i.e., to rotate in a plane that deviates from the plane in which other blades are rotating. Ideally, for minimizing aerodynamic unbalance, all blades should rotate in the same plane. Blades that deviate from this plane generate increased rotor vibration and reduced performance. Thus, it is common practice to analyze the tracking of rotor blades and to adjust any mistracking blades in some manner to bring their tracking back into alignment so that all blades track in the same plane.
It will be appreciated that in order to properly correct both inertial and aerodynamic unbalance, it is necessary to determine to what extent a detected vibration is caused by inertial unbalance and to what extent it is caused by aerodynamic unbalance. For this purpose, rotor vibration and tracking analyzers have been developed for use in maintenance procedures to correct rotor unbalance. Typically, rotor vibration and tracking analyzers are maintenance equipment items that are operated when a balancing procedure is to be performed, and are deactivated after the balancing procedure is completed. A rotor vibration and tracking analyzer obtains signals from vibration sensors placed on the rotor and/or airframe, and from a tip path plane sensor that detects to what degree a given rotor blade's tip path plane deviates from a reference plane, which is usually defined as the tip path plane of one blade designated as a “master” blade. The analyzer processes the vibration and tracking data and determines what vibration component is due to inertial unbalance and what component is due to aerodynamic unbalance. The analyzer typically also recommends corrections (e.g., addition of weights to the blade tips) that should be made to one or more identified blades to correct the inertial unbalance, and blade orientation changes that should be made to one or more identified blades to correct the aerodynamic unbalance. Thus, in conventional rotor balancing procedures, maintenance personnel use a rotor vibration and tracking analyzer to analyze the vibration and tracking data while the helicopter is operating, and then the blades are corrected as a ground maintenance operation according to the recommended corrective actions provided by the analyzer. Exemplary systems for analyzing rotor tracking data are shown in U.S. Pat. Nos. 4,531,408; 4,112,774; 4,053,123; and 3,945,256, the disclosures of which are incorporated herein by reference. Additionally, U.S. Pat. No. 3,938,762, incorporated herein by reference, describes a system for discriminating between inertial and aerodynamic unbalance forces using installed electronics with data inputs from a rotor shaft position sensor and airframe vibration sensors, and for recommending mass additions and pitch link adjustments to correct the unbalance.
Inertial unbalance is usually corrected by adding mass to the identified blade(s), as already noted. Aerodynamic unbalance is often corrected by adjusting a pitch link that is attached to the blade and provides rotor cyclic and collective pitch control through cooperation with a swashplate located at the rotor hub. The pitch link is a manually adjustable variable-length link, similar to a turnbuckle. Shortening or lengthening the pitch link by turning it one direction or the other causes the blade's pitch to be increased or decreased so that the blade tracks either higher or lower relative to the airframe. Thus, in helicopters wherein blade tracking is adjusted by adjusting the pitch link length, each mistracking blade is adjusted by turning the pitch link a number of turns from a reference position, and the number of pitch link turns is noted and is often marked on the blade.
In other types of helicopters, aerodynamic unbalance is corrected by making deflections to a trim tab that is attached to the blade, usually at the trailing edge near an outboard position. Rotor track and balance analyzers used with this type of rotor are operable to recommend the amount of angular deflection to be made to the trim tab of each mistracking blade. A maintenance person adjusts the trim tabs according to the analyzer's recommendations.
A significant drawback to the conventional maintenance-based approach to rotor balancing described above is that the rotor must be operated in a number of conditions ranging from ground run, hover, to various forward flight conditions, so that vibration and tracking data can be acquired at each condition. It will be appreciated, however, that a tracking adjustment that might be optimum in hover may very likely not be optimum in forward flight or ground run. Thus, the analyzer determines an across-the-board compromise correction that will reduce unbalance generally, but may not be optimum for some flight conditions. A further drawback of this maintenance-based approach is that it takes a great deal of time to make the initial flights for data acquisition, adjust the blades, and then make validating flights to confirm that the corrective action has produced the desired result. Yet another disadvantage is that the trim tab position that is set can wash out over time, requiring readjustment.
Accordingly, efforts have been devoted toward developing systems capable of making in-flight tracking corrections. For instance, the Kaman SH-2G Super Seasprite helicopter has the ability to perform rotor track adjustments during flight. The Seasprite employs a mechanically controlled aerodynamic servoflap to vary blade pitch, rather than the more-typical direct blade root pitch control through a swashplate and pitch link. The servoflap for each blade is attached to an electromechanical actuator in series with a link in the control system that controls the servoflap position. The electromechanical actuator acts as a variable-length link. Tracking adjustments are made to a given blade by activating the actuator for that blade to increase or decrease its length, thereby changing the pitch of the blade. A drawback to this approach is that because the tracking adjustment actuator is in the primary flight control linkage, it must react the primary flight control loads, and hence must be relatively heavy and robust.
Another helicopt
Hassan Ahmed A.
Kennedy Dennis K.
Straub Friedrich K.
Alston & Bird LLP
Ryznic John E.
The Boeing Company
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