Raoul J. Hoffman

Surprisingly, experimentersand engineers are very ad-

verse to making tests withoutthe aid of the latest scientifictesting units. The main reasonfor this is the necessary appli-cation of mathematics and basiclaws of physics. With a fewodds and ends, however, thefinest test results may be obtain-ed that will give at least com-parative values; such would be

the case testing engines forhorsepower.

To grasp thoroughly themeaning of power (horsepower)the mechanics of rigid bodieswill be explained:

A push, pull or thrust on abody (without moving it) isnothing else than a force actingon that body; this force may beinternal or external. Only ex-ternal forces will be considered.

The force is usually denotedin pounds, but other units maybe used, such as ounces, kilo-grams and others. Graphicallyit is shown as a line having acertain length drawn to anyconvenient scale. (Fig. 1). Theforce to lift a body is equal tothe weight of the body. Whenno paticular locality is giventhe standard weight of thepound body is one lb.

By pushing a body withsteady force for a certain dis-tance, work is delivered thatis equal to the force times thedistance. The work is inde-pendent of the time required.Fig. 2 illustrates the work doneto push a packing case a dis-tance of 10 ft. The force is 55lbs. and the work delivered willbe 55 times 10 or 550 ft. - lbs.The same amount of work isrequired to lift 10 lbs. to aheight of 55 ft. or 55 lbs. to aheight of 10 ft.

In some test-books, the wordpound-feet is used to representwork to avoid confusion withfoot-pounds of torque or bend-ing moments.

A law of physics — the con-servation of energy — statesthat work can only be createdby equivalent work deliveredand that no energy is lost. Abody lifted to a certain heighthas potential energy that isequal to the work required inraising it. If it falls to theground the moving body haskinetic energy, which can dothe same work as it took tolift the body. Potential or kin-etic energy may be convertedinto heat, light or chemical en-ergy, but the sum of all will bethe same.

Force and work of a pro-peller are very often erroneous-ly used. Static thrust of a pro-peller should not be comparedwith the thrust delivered duringflight. The static thrust is aforce and the thrust duringflight is work done in a certaintime, which is called power.

Power is the rate at whichwork is delivered, or the unitsof work done in unit time. Theunit employed by engineers isthe horsepower, which is equalto 550 ft. - lbs. per second, or33,000 ft. - lbs. per minute.

Fig. 4 shows the lifting powerof one horsepower. Fig. 5shows the formulae for calcu-lating power and its applicationto two simple problems. Thepower indicated is for climb-ing only; it does not includethe power necessary to keepthe airplane in the air.

But not all forces act in astraight line. In Fig. 3 a forceis acting at the end of a crankhandle that will produce a turn-ing moment. This torsionalstrength, expressed by the ra-dius times the load, is called thetorque. The units of torqueis in ft. - lb or in in. - lb. Thework delivered in one revolu-tion will be two times the torqueor 6.28 times the torque.

The determination of thetorque and the revolutions inunit time is the main object offinding the horsepower of anengine, using formulae noted inFigs. 6 and 9.

The instruments for findingpower are dynamometers. Theyare of two kinds; those ab-sorbing power by friction and'dissipating it as heat, and thosetransmitting the power theymeasure for further use, losingonly part of ' it by friction.Usually the torque, or the loadat the end of a torque arm, ismeasured. If standard weight-ing scales are used, the length ofthe torque arm is chosen soas to give the power in decimalmultiple of the load. For labora-tory work, however, directreading torque meters are em-ployed. They may be actuatedby the gravity (weight) or bythe torsional displacement of asteel spring.

The simplest dynamometer,shown in Fig. 10, has a pulleyor drum attached to a rotatingshaft and a rope passed halfway around the circumference.Attached to one end is a springscale and to the other endweights are added until the cor-rect revolutions per minute arereached. The torque is foundby multiplying the difference ofthe weights by the distance ofthe center of the rope to theaxis of the rotating shaft.

Another design has the ropewith oak or maple cleats passedaround the whole circumfer-ence.

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Figs, 6 and 7 show Pronybrakes, which create torqueloads by pressing wood blocksor cleats to the drum surfaces.The attached arms then indi-cate the torque force on a plat-form scale. Care must be takenthat the line passing throughthe center of the shaft and thepoint of the torque arm is hori-zontal.

Very often the great heat de-veloped by such brakes must becarried away by cooling water.To overcome this inconveniencedynamometers are used thatdrive an impeller, in a casingthat is rotatable, with an at-tached torque arm, shown in asketch in Fig. 8. The mediummay be water or air. The varia-tion of resistance is accomplish-ed by changing the distance ofthe vanes or the discharge pres-sure. The brake horsepower iscalculated in the usual way by alever arm and scales.

The use of air is very advan-tageous in testing air-cooled en-gines by having part of the airpassing over the engine to giveconditions similar to flying con-ditions.

Another type of dynamometeruses the eddy current created bya number of electromagnets andone or more copper disks as in-dicated by a sketch in Fig. 9.Varying the distance betweenthe magnets and the diskchanges the torque that may bemeasured any convenient way.

One method used in measur-ing high powered engines is toconnect an electric generator tothe main shaft. Knowing theefficiency of the generator, theoutput indicated (kilowatts)times the efficiency is the powerdeveloped. The produced cur-rent may be utilized by feedingit a power line of the factory.

This system is costly to installbut it is very economical in therunning cost, especially in test-ing electric motors of largepower that use the same kindof current that is generated.Feeding it back to the line, the

losses are only a few percent ofthe total power developed.

For approximate methods offinding the horsepower of high-speed engines, air-brake dyna-mometers are suitable. Theplates, having an area of onesquare foot, are adjustable andthe resistance calculated withstandard aeronautical formula.The fan brake is the most com-pact unit but it will not givevery reliable results. (Fig. 12)

Fig. 11 shows a testing as-sembly for measuring the staticthrust of a propeller and thetorque of the driving motor.The motor swings on two ballbearings that rest on brackets ofa stand. Attached to the basefigures is a torque arm fromwhich a silk line runs over apulley to a tray for weighingthe torque load. Placing thestand on a platform scale thestatic thrust may also be mea-sured.

Testing airplane engines forpower can be simplified by usingpropellers of similar designs.Knowing the power co-effici-ents, the power developed maybe calculated if the rpm are alsoknown.

Another method of using pro-pellers for testing engines issketched 'n Fig. 14. The engineis mounted on a swing;ng frameplaced on a stand. The torquecreated by the propeller maybe measured with the torquearm in the regular way. Theengine should be mounted inthe same manner it is mountedin the airplane in order to givethe same power.

Finding the power deliveredby the engine and also by thepropeller during flight is ac-complished by attaching hydrau-lic cylinders radially and axiallybetween the propeller and theengine shaft. The rpm, thepressure in the cylinders and theflying speed will give the ef-ficiency of the whole unit.

This scientific method of as-certaining performance quali-ties of engine and propeller

cannot be compared with thecrude methods applied to pre-war pushers by using a springscale for determining the cor-rect power of an engine.(Fig. 15)

There are torsion dynamome-ters that use the angular twistof a long rod or long shaft. Theirmost important application ismeasuring horsepower of ma-rine propeller drives.

In the above mentioned meth-ods the inch, foot and poundunits were used. Neverthe-less, other units of variousbranches of engineering may beemployed to simplify mathe-matical operation. Some ofthem are noted on the conver-sion table in Fig. 15.

Lester Zehr, Route 7, FortWayne, Ind., who is vice-

president of the Fort WayneChapter 2, is making good pro-gress with his Baby Ace. Leswrites:

"I have been working on myairplane every spare minute. Istarred approximately a yearago after looking at severalplans. The Baby Ace seemedto be about what I wanted soI purchased a set of plans andbegan working. I bought allnew 4130 tubing and startedmaking the fuselage, changingmany of the drawings to mysatisfaction, such as installing adorsal fin and using all-metalfairing strips on the fuselageinstead of wood. I rounded thefuselage sides instead of havingthem straight, thus adding tothe appearance.

"For the wings I purchased

a set of Luscombe panels andremoved the excess length fromthe inboard end. This has savedmany hours of work and willmake me a faster airplane. Iam using Cub landing gear com-plete with wheel pants. Theelevator control wires werechanged to a push and pull tubesystem. The engine I am usingis a Continental 65 hp, newlymajored.

"I might add that the interestand assistance of the membersof our local Chapter 2 has help-ed to keep my enthusiasm high."Below are some dimensions ofmy Baby Ace:Wingspan . . . . . . . . . 25 ft. 3 in.Length . . . . . . . . . . 18 ft. O in.Height . . . . . . . . . . . . 6 f t . 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-placeparasol monoplane. When theDepartment of Commerce dis-couraged homebuilding withstringent regulations, he aban-doned his project. Now the air-plane is underway again, onlythis time Young's 19 year-oldson Gerald is a partner in theproject. Using the fuselagetacked together at the start somany years ago, the front endhas been revised in order touse Cub gear, engine mount andgas tank. Tail surfaces are cutdown from a T-craft and thewing uses an NACA 23012 air-foil. B

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