Abstract:
An Improvement to enhance the usability of magnetic compasses by eliminating acceleration and turning errors, especially in aviation applications, without compromising simplicity and reliability. The improvement includes the addition of a weightless mass to the bar magnet of a conventional magnetic compass to counteract undesirable inertial forces on the bar magnet.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to magnetic compasses used in aviation, in general, and to improved magnetic compasses which correct for acceleration and turning errors as well as magnetic dip forces, in particular. 
     2. Prior Art 
     A magnetic compass was one of the first instruments to be installed in an airplane, and it is still the only direction-seeking instrument in many smaller aircraft. One great advantage of such a compass is that it is a self-contained unit which does not require electrical or vacuum power. To determine direction, the compass uses a simple bar magnet with two poles. The bar magnet in the compass is mounted so that it can rotate freely and align itself automatically with the Earth&#39;s magnetic field. 
     However, magnetic compasses are subject to a problem called “magnetic dip” because the lines of flux of the Earth&#39;s magnetic field are not parallel to the Earth&#39;s surface (except at the magnetic equator). Since the compass needle aligns itself with the lines of flux, the north-seeking end of the needle tends to dip toward the Earth (in the Northern Hemisphere). This dip angle is caused by the vertical component of the Earth&#39;s magnetic field and increases from zero at the magnetic equator to almost 90 degrees near the magnetic pole. Many magnetic compasses (including those used in aviation) have a small weight on the south-pointing (again in the Northern Hemisphere) end of the needle to counteract this magnetic dip by using gravitational force. That is, a counterweight is added to the end of the compass needle which points away from the magnetic pole. This arrangement does, indeed, almost completely fix the dip problem and has been used on compasses for many years. Unfortunately, this technique introduces two new errors, called the “acceleration error” and the “turning error.” These errors are exhibited as unwanted needle movements when the compass is subjected to inertial forces, viz. due to change in speed and/or direction. The inertial force acts on the counterweight of the needle and produces a torque about the pivot at which the compass is suspended because the mass of the needle is not equal on both sides of the pivot point. This torque causes the needle and the attached card (in aviation compasses) to rotate and thereby give a false reading. 
     During acceleration or deceleration of an airplane on an easterly or westerly heading, an erroneous indication will occur. During acceleration, inertia causes the compass weight on the south end of the bar magnet to lag slightly and turn the compass toward a North indication even though no change of direction has taken place. During a deceleration, inertia causes the weight to move slightly ahead, which moves the compass toward a southerly heading indication. The compass will return to its previous, and proper, heading once the acceleration or deceleration subsides. 
     This acceleration error does not occur when flying on a directly north or south heading because the dip compensation weight is in line with the direction of travel, but becomes more pronounced as the plane&#39;s heading is closer to due east or west. These acceleration error examples are valid only for the northern hemisphere; the effects are reversed in the southern hemisphere. 
     Turning error is also caused by inertial forces acting on the counterweight. It is most pronounced when turning from headings of due north or south. At the beginning of a turn from a heading of north, inertia (in the form of centrifugal force) forces the counterweight to the outside of the turn so the compass initially indicates a turn in the opposite direction. When the turn is established, the compass card begins to rotate in the correct direction, but it lags behind the actual heading. The amount of lag decreases as the turn continues, then disappears as the airplane reaches a heading of east or west. 
     When turning from a heading of east or west to a heading of north, there is no error at the beginning of the turn. However, as the heading approaches north, the compass increasingly lags behind the airplane&#39;s actual heading. When turning from a heading of south, the compass initially indicates a turn in the proper direction but leads the airplane&#39;s actual heading This error also diminishes as the airplane reaches a heading of east or west. Turning from east or west to a heading of south causes the compass to move correctly at the start of a turn, but then it increasingly leads the actual heading as the airplane nears a southerly direction. (As in acceleration errors, these turning errors are only valid for flight in the Northern Hemisphere but in the Southern Hemisphere act in the opposite directions.) A more detailed description may be found in “Private Pilot Manual” by Jeppeson Sanderson Training Products. 
     SUMMARY OF THE INSTANT INVENTION 
     One solution to the error situation in a magnetic compass is to balance the inertial forces on each end of the needle without affecting the weight distribution. This is accomplished by fastening a small capsule to the north-seeking end of the needle which capsule has the same mass as the counterweight, but has zero weight because it is designed to have the same density as the fluid with which the compass is filled. Of course, the capsule could be designed to be integral with the needle, as well. 
     In a preferred embodiment, the capsule is hollow. The result of incorporating such a capsule is that inertial forces cause no torque because the mass of the needle is equal on both sides of the pivot. The balance of the gravitational force acting on the counterweight versus the vertical component of the magnetic force acting on the needle remains undisturbed. 
     The magnetic compass is made more usable and less error prone over a wide range of acceleration and turning conditions. It may, therefore, be used as a stable reference source for a heading Indicator or other instrumentation. 
     The inherent reliability of the magnetic compass is retained inasmuch as no moving parts nor a power source are required. 
     The improved magnetic compass is considerably more useful to a pilot because its reading is more stable and accurate. Since it is not subject to the acceleration and turning errors, it can be read in non-straight-and-level flight maneuvering. This difference could be of extreme importance should the aircraft experience a vacuum failure (which renders the gyroscopic Heading Indictor inoperable in a typical General Aviation craft). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of an existing magnetic compass. 
     FIG. 2 is a schematic representation of one embodiment of an improved magnetic compass of the instant invention incorporating a compensating, weightless mass on the needle. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, there is shown a schematic representation of a known magnetic compass  10 . The compass  10  comprises a compass needle  11  mounted on a conventional pivot  12 . Typically, the compass needle  11  is fabricated of a magnetic material. The method of manufacturing the compass needle is well known in the art. 
     In use, the compass needle  11  tends to rotate around the pivot  12  so that it is aligned with the magnetic flux lines of the Earth with tip  11 A pointing toward the magnetic pole. As is well known, the magnetic tip  11 A of the needle on an aircraft magnetic compass tends to dip downwardly toward the magnetic pole of the Earth along the Earth&#39;s magnetic flux lines. 
     In the prior art compass, an attempt to offset the magnetic dip was made by adding a counterweight  13 , to the tip of the needle that points away from the magnetic pole. However, as noted supra, the counterweight caused complications of its own. 
     Referring now to FIG. 2, there is shown a schematic representation of the improved compass of the instant invention. 
     In this embodiment, the components remain largely unchanged from the prior art. For example the needle  21  rotates on the pivot  22 . Likewise, the needle  21  has a tip  21 A and a counterweight  23  at the opposite end thereof. 
     However in this embodiment, a mass  24  is formed on the needle  21  adjacent to the North-pointing end thereof. The mass  24  can be added to the needle  21  as a separate component. Conversely, the mass  24  can be formed integrally with the needle. 
     The significant characteristic of the mass  24  is that it should be essentially weightless but have the same mass as the counterweight  23 . 
     The shape and fabrication technique for providing the mass  24  is largely a function of design or manufacturing preference. For example, the mass  24  can be a hollow sphere attached to the needle  21 . Alternatively, the mass  24  can be a body or housing formed as a part of the needle  21  during fabrication thereof. 
     Typically, in a magnetic compass used in aircraft the needle is immersed in a liquid within a housing. By using a hollow sphere for the mass  24 , the mass will tend to float in the liquid thereby establishing a “weightless” condition. 
     Alternatively, by forming the mass of a material which is of a density which is equal to the density of the liquid in the compass housing, the mass  24  will be, effectively, weightless. 
     However the mass  24  will be subject to the laws of physics, i.e. F=mA, as is the mass of the counterweight  23 . By making the mass of mass  24  equal to the mass of counterweight  23 , the inertial effects of the counterweight are balanced and no torque about the pivot is experienced. Thus, the inertial effects of the counterweight  23  on the needle  21  during acceleration and/or turning are counterbalanced and operation of the compass is enhanced. It is recognized that in many aircraft magnetic compasses, the “needle” actually comprises two needles or bar magnets acting in concert and mounted within a rotating card. The instant description is intended to include such devices. That is, the drawings and description are a simplified depiction of existing devices and are intended to cover such devices, as well. 
     The improved compass structure enjoys many advantages. For example, the simplicity of the magnetic compass is retained because only one non-movable part is added (or integrated). Thus, the manufacturing cost of the improved compass remains about the same. As a result, once it is certified, the improved compass can be sold without difficulty to current and future customers as standard or upgrade equipment. 
     Thus, there is shown and described a unique design and concept of an improved magnetic compass structure especially useful in aircraft environment. This improvement nullifies the effect of an unwanted force (or force component) by counteracting this force (or force component). While this description is directed to a particular embodiment, it is understood that those skilled in the art may conceive modifications and/or variations to the specific embodiments shown and described herein. Any such modifications or variations which fall within the purview of this description are intended to be included therein as well. It is understood that the description herein is intended to be illustrative only and is not intended to be limitative. Rather, the scope of the invention described herein is limited only by the claims appended hereto.