Aeronautics and astronautics – Miscellaneous
Reexamination Certificate
1998-12-16
2001-06-12
Eldred, J. Woodrow (Department: 3644)
Aeronautics and astronautics
Miscellaneous
C324S330000, C324S331000
Reexamination Certificate
active
06244534
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an airborne electromagnetic system of a kind which is suitable for geophysical prospecting using electromagnetic fields.
In electromagnetic prospecting a primary alternating electromagnetic field is transmitted from one point and the resultant field, (i.e. the primary field plus the secondary field from the earth), is received at a second point, which is spaced from the first point. Anomalies in the earth, e.g. conductive substances, can be detected due to the secondary fields which are generated in them by the primary field and which are received at the second point.
In order to measure the secondary field accurately it is important that the primary field at the receiver should be as constant as possible. Generally, for large scale towed bird systems, the primary field is produced by a transmitting coil in an aircraft and the primary plus the secondary field from the earth is measured at the centre of a receiving coil or coils in a bird which is towed by the aircraft. In a system which is towed by a helicopter, the primary field at the receiver is kept as constant as possible by rigidly mounting the receiver and transmitter to a relatively short rigid boom.
It is apparent that aerodynamic stability of the bird is highly desirable. A large transmitting antenna or loop is also desirable and, where necessary, it should be possible to space the transmitting and receiving coils from each other by a significant distance.
SUMMARY OF THE INVENTION
The invention provides apparatus for use in an airborne electromagnetic geophysical prospecting system which includes an aircraft, a transmitting antenna on the aircraft, means on the aircraft for generating electrical power for operating the transmitting antenna, and means for attaching at least one tow rope to the aircraft for towing the aircraft.
The aircraft may be of any suitable kind and, for example, may include a fuselage, at least one wing or foil for generating lift, and a stabilising element.
The aircraft may be made, at least principally, for non-conductive and non-magnetic material such as timber, plastic or the like in order to minimise induced magnetic fields which are a source of noise.
The tow rope attachment means may be any suitable fixture e.g. eyelets, clamps or the like positioned at selected locations e.g. on the fuselage and wing.
The stabilising element may be any appropriate device and for example may include a rudder and a horizontal stabiliser or a drogue, or both these components. These items are given merely by way of example and are non-limiting.
The power generating means may include an engine which is fuel driven. Thus the aircraft may include a fuel tank.
The power generating means may, besides its primary purpose of producing electrical power, additionally be used to produce thrust and, for example, may drive at least one propeller or fan.
The invention also extends to an airborne electromagnetic geophysical prospecting system which includes a towing aircraft, and apparatus of the kind described attached to the towing aircraft by tow rope means.
The towing aircraft is, preferably, a helicopter.
The thrust which is produced by the power generating means may produce a significant proportion of the thrust which is required for self-sustaining flight of the aircraft. For example the power generating means may produce at least a quarter and preferably a third of the thrust which is required for self-sustaining flight of the aircraft.
The tow rope means may be attached to the foil or to the transmitting antenna at least at two spaced locations using any suitable fasteners. Two of the spaced locations are preferably situated at right angles symmetrically on either side of the aircraft fuselage and are preferably at or near respective ends of the foil. In order to minimise flexing of the foil and transmitting antenna during flight, an additional tow rope may be attached to a third point, e.g. at the centre of the foil or at the fuselage such that the three attachment points are essentially on a line which is at right angles to the fuselage. These three tow ropes may meet at a point approximately seven meters above the wing (in one particular example) and then from this point there is a single tow rope which goes up to the helicopter.
The system may include means for limiting the pitch of the aircraft relatively to the towing aircraft.
The pitch limiting means may include at least two pitch limiting ropes which are connected fore and aft to a fuselage of the aircraft at respective locations and which limit pitching of the aircraft upwards and downwards.
The pitch limiting ropes are connected to the aircraft at locations which are fore and aft respectively the aforementioned line on which the tow rope attachment points lie.
The pitch limiting ropes may extend upwardly from the fuselage of the aircraft towards a location at which the tow ropes, which are fixed to the aircraft, meet.
The pitch limiting ropes may be sections of a single rope which passes over a pulley positioned at the said location. The extent to which this rope can move about the pulley may be limited in any suitable way e.g. by placing or fitting stop fixtures to the rope. A convenient way of controlling or directing this movement is simply to knot the rope at two points which are positioned on opposing sides of the pulley.
However any other means or mechanism for controlling the pitch of the aircraft may be employed.
Thus, an alternative arrangement eliminates the pulley and uses two slack ropes attached at the point where the three tow ropes meet and to the fuselage and which become taut alternately as the aircraft pitches considerably upwards or downwards during liftoff or landing operations.
The system may include a receiving coil or antenna. For example a receiving coil may be mounted on the aircraft preferably at a location which is spaced by the maximum possible extent from the transmitting antenna. The receiving coil or antenna may be of any appropriate construction and for example may comprise a three-component device of a kind which is known in the art. The receiving coil may be hard mounted to the aircraft or it may have a suspension system between it and the aircraft.
The receiving antenna may be mounted on or be attached indirectly to the fuselage of the aircraft. For example the receiving antenna may be mounted on a receiver bird which, itself, is attached by means of a tow rope to the aircraft which, in turn, is towed by means of a helicopter.
Preferably the secondary craft i.e. the receiver bird, is attached to the aircraft at its centre of gravity, e.g. at the intersection of the wing or foil and the fuselage thereof.
The arrangement may be such that the secondary craft is towed at a substantially constant selected angle relatively to the aircraft. For example the secondary craft i.e. the receiver bird may be at an angle of approximately 14° below the horizontal plane in which the aircraft is positioned or flies. The preferred angle is determined by the airspeed, cable density and cable diameter and is the angle at which the cable flies essentially straight behind the aircraft when it is not attached to the receiver bird at the end.
The invention also extends to an airborne electromagnetic geophysical prospecting system which includes a towing aircraft, a towed aircraft which is attached to the towing aircraft by first tow rope means, a transmitting antenna on the towed aircraft, a receiver bird which is attached to the towed aircraft by second tow rope means, and a receiving antenna which is mounted to the receiver bird.
The system may include means for limiting upwards and downwards pitch of the towed aircraft relatively to the towing aircraft.
The system may also include power generating means on the towed aircraft for powering the transmitting antenna.
REFERENCES:
patent: 2955251 (1960-10-01), Shaw et al.
patent: 3617866 (1971-11-01), Dowsett et al.
patent: 3836841 (1974-09-01), Morrison
patent: 3872375 (1975-03-01), Ronka
patent: 3976937 (1976-08-01), Hearn
patent: 449292
Arent Fox Kintner & Plotkin & Kahn, PLLC
Eldred J. Woodrow
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