Power P = 1 x A
By Jim Sparks
Electronics as applied to aviation" is probably one of the simplest definitions of "avionics." Intimidating as it may be, electronics are still nothing more than the application of electrical principals. With the onslaught of "Next Generation" aircraft utilizing computerized systems such as engines, air-conditioning and pressurization systems, fuel systems, wheel brake ,and steering— not to mention auto pilot and flight deck displays — Computer technology offers numerous advantages over systems of yesteryear.
Size and weight are considered an important advantage in aviation, plus electronic systems have few moving parts, so reliability can be significantly higher. Another benefit of computer technology is "BITE" (built-in test equipment). That is, if a fault is detected, the computer can advise the flight crew and retain the fault in memory until a technician can retrieve it. This self-diagnostic capability, when properly used, can save hours in troubleshooting.
One thing all computerized systems have in common is the need for electrical power. Airframe manufacturers have different specifications on the type and amount of energy needed. General aviation aircraft most frequently use direct current (DC) for primary power while commercial aircraft use alternating current (AC). Each type has benefits as well as drawbacks. Problems with a power source can have dramatic effects on sensitive electronic equipment.
Power may be best described as "the ability to do." Electrical power is measured in wattage (watts). In a DC system, wattage is determined by multiplying the electrical potential or voltage times the amount of electrical current flow or amperage:
P = I x E
P = POWER OR WATTS
I = AMPERAGE
E = ELECTROMOTIVE FORCE or VOLTAGE
A similar principle applies to AC systems; however, current flow and electrical potential are rarely in phase, so an approximation can be made by multiplying voltage times amperage and using 70 percent of the answer as approximate power. Exact calculations require factoring in circuit impedance and determining the phase relationship between circuit potential and flow.
P = I x E x .7
Advisory Circular 43.13-1A dictates that in an aircraft, the size of wire utilized in a circuit is directly proportional to the amount of current flow and the overall distance. Some other factors include whether the wire is in a bundle or free air and if the amperage draw of the circuit is continuous or intermittent.
For example, a No. 18 copper wire in a bundle can carry 10 amps and have a resistance of 6.44 ohms per 1,000 feet. The weight of 1,000 feet of this wire would be 8.4 pounds. In an aircraft using an 115-volt AC power system, the amperage or current flow through a conductor is substantially lower than an aircraft using a 28-volt DC system. A typical, electrically-operated fuel boost pump might require 300 watts of power for operation. An aircraft using 115 volts would require around three amps, while a 28-volt DC system would need about 11 amps to achieve the same result. Although an AC motor will differ internally from a DC motor, the amount of power used to perform a specific task is still similar. Aircraft operating on AC can have a significant weight savings in wire over a DC-powered counterpart.
Turbine-powered aircraft found in general aviation will frequently utilize a combined starter generator. This device can use energy stored in aircraft batteries to produce the necessary torque and speed to get the engine started. Rather than going along for the ride, its internal components are used to produce power to replenish the battery and provide for the electrical system.
The mechanical means of producing electrical power requires a magnetic field, coils of wire, and relative motion between the two. The relative motion is supplied by the mechanical connection between the generator and the engine. Coils are incorporated in the generator case and the commutator (spinning section). The power requirements of various aircraft dictate that generators be able to produce a significant output, in some cases as much as 400 amps with the voltage output remaining relatively constant.
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