Power And Its Measurement

In some test-books, the word pound-feet is used to represent ... has potential energy that is equal to the work ... to two simple problems. The power indicated is ...
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Raoul J. Hoffman

Surprisingly,

experimenters

and engineers are very adverse to making tests without

the case testing horsepower. To

grasp

engines

thoroughly

for the

the aid of the latest scientific testing units. The main reason

meaning of power (horsepower)

for this is the necessary application of mathematics and basic

will be explained:

laws of physics. With a few odds and ends, however, the finest test results may be obtained that will give at least comparative values; such would be

8

the mechanics of rigid bodies A push, pull or thrust on a body (without moving it) is nothing else than a force acting on that body; this force may be internal or external. Only external forces will be considered.

The force is usually denoted in pounds, but other units may be used, such as ounces, kilograms and others. Graphically it is shown as a line having a certain length drawn to any convenient scale. (Fig. 1). The force to lift a body is equal to the weight of the body. When no paticular locality is given the standard weight of the pound body is one lb. By pushing a body with steady force for a certain distance, work is delivered that is equal to the force times the distance. The work is independent of the time required. Fig. 2 illustrates the work done to push a packing case a distance of 10 ft. The force is 55 lbs. and the work delivered will be 55 times 10 or 550 ft. - lbs. The same amount of work is required to lift 10 lbs. to a height of 55 ft. or 55 lbs. to a height of 10 ft. In some test-books, the word pound-feet is used to represent work to avoid confusion with foot-pounds of torque or bending moments. A law of physics — the conservation of energy — states that work can only be created by equivalent work delivered and that no energy is lost. A body lifted to a certain height has potential energy that is equal to the work required in raising it. If it falls to the ground the moving body has kinetic energy, which can do the same work as it took to lift the body. Potential or kinetic energy may be converted into heat, light or chemical energy, but the sum of all will be the same. Force and work of a propeller are very often erroneously used. Static thrust of a propeller should not be compared with the thrust delivered during flight. The static thrust is a force and the thrust during flight is work done in a certain time, which is called power. Power is the rate at which work is delivered, or the units of work done in unit time. The unit employed by engineers is

Fig. 4 shows the lifting power of one horsepower. Fig. 5 shows the formulae for calculating power and its application

the horsepower, which is equal

with oak or maple cleats passed

to 550 ft. - lbs. per second, or 33,000 ft. - lbs. per minute.

around ence.

to two simple problems.

The

power indicated is for climbing only; it does not include the power necessary to keep the airplane in the air. But not all forces act in a straight line. In Fig. 3 a force is acting at the end of a crank handle that will produce a turning moment. This torsional

strength, expressed by the radius times the load, is called the torque. The units of torque is in ft. - lb or in in. - lb. The work delivered in one revolution will be two times the torque or 6.28 times the torque. The determination of the torque and the revolutions in unit time is the main object of finding the horsepower of an engine, using formulae noted in Figs. 6 and 9. The instruments for finding

power are dynamometers. They are of two kinds; those absorbing power by friction and 'dissipating it as heat, and those transmitting the power they measure for further use, losing only part of ' it by friction. Usually the torque, or the load at the end of a torque arm, is measured. If standard weighting scales are used, the length of the torque arm is chosen so

as to give the power in decimal multiple of the load. For laboratory work, however, direct reading torque meters are employed. They may be actuated by the gravity (weight) or by

the torsional displacement of a steel spring.

The

simplest

dynamometer,

shown in Fig. 10, has a pulley or drum attached to a rotating

shaft and a rope passed half way around the circumference. Attached to one end is a spring scale and to the other end weights are added until the correct revolutions per minute are reached. The torque is found by multiplying the difference of the weights by the distance of the center of the rope to the axis of the rotating shaft. Another design has the rope the

whole

circumfer-

Figs, 6 and 7 show Prony brakes, which create torque loads by pressing wood blocks or cleats to the drum surfaces. The attached arms then indicate the torque force on a platform scale. Care must be taken that the line passing through the center of the shaft and the point of the torque arm is horizontal. Very often the great heat developed by such brakes must be

losses are only a few percent of the total power developed.

For approximate methods of finding the horsepower of highspeed engines, air-brake dynamometers are suitable. The plates, having an area of one square foot, are adjustable and

the resistance calculated with

standard aeronautical formula.

The fan brake is the most com-

that is rotatable, with an attached torque arm, shown in a sketch in Fig. 8. The medium may be water or air. The variation of resistance is accomplish-

pact unit but it will not give very reliable results. (Fig. 12) Fig. 11 shows a testing assembly for measuring the static thrust of a propeller and the torque of the driving motor. The motor swings on two ball bearings that rest on brackets of a stand. Attached to the base figures is a torque arm from which a silk line runs over a

the vanes or the discharge pressure. The brake horsepower is calculated in the usual way by a lever arm and scales.

the torque load. Placing the stand on a platform scale the static thrust may also be measured.

gines by having part of the air

power can be simplified by using propellers of similar designs.

carried away by cooling water.

To overcome this inconvenience

dynamometers are used that drive an impeller, in a casing

ed by changing the distance of

The use of air is very advantageous in testing air-cooled enpassing over the engine to give

conditions similar to flying conditions. Another type of dynamometer uses the eddy current created by a number of electromagnets and one or more copper disks as indicated by a sketch in Fig. 9.

Varying the distance between the magnets and the disk changes the torque that may be measured any convenient way. One method used in measuring high powered engines is to

connect an electric generator to

the main shaft. Knowing the efficiency of the generator, the

pulley to a tray for weighing

Testing airplane engines for

Knowing the power co-effici-

ents, the power developed may be calculated if the rpm are also known. Another method of using propellers for testing engines is sketched 'n Fig. 14. The engine is mounted on a swing ; ng frame placed on a stand. The torque created by the propeller may be measured with the torque arm in the regular way. The

engine should be mounted in

the same manner it is mounted

in the airplane in order to give the same power.

Finding the power delivered output indicated (kilowatts) by the engine and also by the times the efficiency is the power propeller during flight is acdeveloped. The produced cur- complished by attaching hydraurent may be utilized by feeding lic cylinders radially and axially it a power line of the factory. between the propeller and the This system is costly to install engine shaft. The rpm, the but it is very economical in the pressure in the cylinders and the running cost, especially in test- flying speed will give the efing electric motors of large ficiency of the whole unit. This scientific method of aspower that use the same kind of current that is generated. certaining performance qualiFeeding it back to the line, the ties of engine and propeller

Lester Zehr, Route 7, Fort a set of Luscombe panels and Wayne, Ind., who is vice- removed the excess length from president of the Fort Wayne the inboard end. This has saved Chapter 2, is making good pro- many hours of work and will gress with his Baby Ace. Les make me a faster airplane. I writes: am using Cub landing gear com"I have been working on my plete with wheel pants. The airplane every spare minute. I elevator control wires were starred approximately a year changed to a push and pull tube ago after looking at several system. The engine I am using plans. The Baby Ace seemed is a Continental 65 hp, newly to be about what I wanted so majored. I purchased a set of plans and "I might add that the interest began working. I bought all and assistance of the members new 4130 tubing and started of our local Chapter 2 has helpmaking the fuselage, changing ed to keep my enthusiasm high." many of the drawings to my Below are some dimensions of

satisfaction, such as installing a dorsal fin and using all-metal fairing strips on the fuselage

instead of wood. I rounded the fuselage sides instead of having them straight, thus adding to the appearance. "For the wings I purchased cannot be compared with the crude methods applied to prewar pushers by using a spring scale for determining the correct power of an engine. (Fig. 15)

There are torsion dynamome-

ters that use the angular twist of a long rod or long shaft. Their most important application is measuring horsepower of marine propeller drives. In the above mentioned methods the inch, foot and pound units were used. Nevertheless, other units of various branches of engineering may be employed to simplify mathematical operation. Some of them are noted on the conversion table in Fig. 15.

my Baby Ace: Wingspan . . . . . . . . . 25 ft. 3 in. Length . . . . . . . . . . 18 ft. O in. Height . . . . . . . . . . . . 6 ft. 3 in. Wheel tread . . . . . . 6 ft. 4 in. Tire size . . . . . . . . . . . . 8.00 x 4

A number of years r go Mem-

ber LeRoy Young of Viola, 111.

started to build a single-place parasol monoplane. When the

Department of Commerce discouraged homebuilding with

stringent regulations, he aban-

doned his project. Now the airplane is underway again, only this time Young's 19 year-old son Gerald is a partner in the project. Using the fuselage

tacked together at the start so

many years ago, the front end has been revised in order to use Cub gear, engine mount and gas tank. Tail surfaces are cut

down from a T-craft and the

wing uses an NACA 23012 airfoil. B 9