HOW TOBy BRIAN E. MEYER

EAA 1270182585 Seneca Dr.Troy, OH 45373

INTRODUCTION

There comes a time in nearly everyaircraft construction project when thebuilder is faced with a bare engineflange and must decide on the pro-peller to be installed. There are manyfactors to be taken into account whenchoosing a propeller including perfor-mance, weight, cost, noise, diameterlimits and so on. Selection of a properpropeller is also important since therehave been a number of serious acci-dents in the past due to improperapplication or modification of pro-pellers. This article discusses some ofthe primary considerations in pro-peller selection. It will also acquaintaircraft builders with some of the in-

formation that is available to helpthem make a good propeller choice.

The first decision made by mostbuilders is whether to go with wood ormetal. Wood propellers are generallylighter, less expensive and hard to beatfor aesthetics. For builders using non-certificated engines they may be theonly available choice. For aircraft us-ing certificated aircraft engines, manybuilders choose metal propellers forimproved performance and durability.For higher powered aircraft , metalmay be the most practical choice.

Much of the following article fo-cuses on aircraft which use certificatedengines and metal propellers. If youare a die-hard wood fan (pun intended)you may want to quit reading here.However, there are experimental pro-pellers on the market which use woodor composite blades in metal hubs somany of the following comments maystill be of interest.

PEAKEFFICIENCY

E aLU <oc xi «

UJ

CONSTANT SPEEDPROPELLER

FIXED-PITCHPROPELLER

\J FLIGHT SPEEDPROPELLER RPM

FIGURE 1 TYPICAL PROPELLER EFFICIENCIES

A Few Words On Performance . . .

Performance is obviously importantto safety since we all want to get overthe trees at the end of the runway.However, since our concern here iswith "safe" rather than "optimum,"our performance discussion will bevery brief. Propeller performance isanother subject in itself.

Fixed pitch propellers offer re-duced weight, cost and maintenancerelative to constant speed propellers,at the expense of performance. Thepeak performance for a fixed pitchpropeller occurs at only one combina-tion of rpm and airspeed.Performance estimation is furthercomplicated because rpm, horse-power developed and aircraft speedare all interdependent when using afixed-pitch propeller.

The most important performanceparameter which should be checkedwhen using a fixed-pitch propeller isthe "static rpm" range. The staticrpm limits are reference conditionsintended to make sure that the en-gine/propeller combina t ion willproduce sufficient power and thrustfor takeoff and climb, yet will notover-speed in cruise. All fixed pitchpropellers installed on certificatedaircraft have static rpm limits. Theyare measured with the aircraft at restin calm wind conditions with thethrottle wide open.

Constant speed propellers offerhigher efficiency over a wider speedrange compared to fixed-pitch pro-pellers, as shown in Figure 1. Theyalso allow the engine to develop morepower relative to an engine driving afixed-pitch (load) propeller (Figure 2)since rpm and power can be con-trolled independently at any airspeed.Both advantages are due to the bladeangle being variable.

When using a constant-speed pro-peller, the most important criteria forachieving acceptable performance arethe blade angle limits. Proper lowblade angle allows the engine to de-velop maximum power for takeoff,while the proper high angle limit pre-vents the engine from over-speedingat high airspeeds.

Before flight testing your airplane,SPORT AVIATION 41

RATEDHORSE-POWER

CONSTANT SPEEDPROPELLER

EXTRAPOWER

AVAILABLE

FIXED-PITCHPROPELLERLOAD CURVE

ENGINE RPMRATEDRPM

FIGURE 2 TYPICAL POWER CHARACTERISTICSit is a very good idea to check yourinstallation against a similar produc-tion airplane (one having the sameengine, propeller and similar cruiseand climb speeds) for static rpm or

blade angle criteria. Sources for thisinformation will be discussed later.If needed, the propeller can be re-pitched or have the angle limits resetby your nearest propeller shop.

STRESS(S)

FAILUREOCCURS

O.K. ALLOWABLESTRESS

NUMBER OF CYCLES (N)

FIGURES S-N CURVE

What About Vibration?

Probably the most importantsafety consideration in selecting ametal propeller is checking for vibra-tion approval. To understand whyvibration approval is important, let'slook at an example. Let's say abuilder that we know happens tohave another flying buddy wi th ahangar full of used propellers. Ourfriend is fortunate enough to be givenhis choice of a propeller for his soon-to-be-completed aircraft project.

He selects a metal propeller withthe proper hub to fit his engine. Thepropeller appears to be in good shapeexcept for a little stone damage onthe tips. This damage does not botherhim since he needs to cut down thediameter to get more ground clear-ance. Besides, he also plans toover-speed his engine a little to getmore power and does not want a lotof extra noise due to excessive tipspeed. So, out comes the trusty hacksaw, off go a few inches from eachtip, and it is ready for a quick staticbalance check. While he's at it, hedecides he's always wanted to have asexy nosebowl and spinner. He goesto a friend with a lathe and a nicelong spacer is created to fit the en-gine and propeller with the nosebowland spinner.

The great day arrives and our friendmakes his first flight. Back on theground he is ecstatic, and rightfully so.He has completed his project, and itperforms and handles well.

But what's wrong with this pic-ture? Unfortunately, plenty. All thedecisions he has made regarding hispropeller can have a profound effecton the longevity of the airplane andits builder. Along with the lack ofany check to find out what engineand airplane the propeller may havebeen originally installed on, thechanges in diameter, operating rpmand inclusion of a custom spacermake this airplane a very high riskinstallation.

Build An Airplane, Not A Time iBomb . . . '

Propellers, like all structures havecertain "natural" frequencies at whichthey will vibrate excessively with verylittle coaxing. The example of a tuningfork comes to mind. If a propeller isinstalled on an engine which excites(shakes) it at or near a natural fre-quency, a condition called "resonance"is created. When operating in or neara resonance condition stresses in thepropeller are very high. Metal fatigueand eventual structural failure becomea real possibility.

42 JULY 1994

TYPE CERTIFICATE DATA SHEET NO. 3A12

This data sheet which is part of Type Certificate No. 3Al2 prescribes conditionsand limitations under which the product for which the type certificate was is-sued meets the airworthiness requirements of the Federal Aviation Regulations.

Type Certificate Holder Cessna Aircraft CompanyP.O. Box 7704Wichita, Kansas 67277

I - Model 172. 4 PCLM (Normal Category). Approved November 4.1955: 2 PCLM(Utility Category). Approved December 14.1956

Engine '" '

'Fuel

* Engine Limits

Propeller andPropeller Limits

Continental 0-300-A or 0-300-B

80/87 minimum grade aviation gasoline

For all operations, 2700 rpm (145 hp)

1. Propeller •..•-..„(a) McCauley 1A170

""" Static rpm at maximum permissible• ' throttle setting:

Not over 2360, not under 2230No additional tolerance permittedDiameter: not over 76 in., not

.-, . under 74.5 in.(b) Spinner, Dwg. 0550162

2. Propeller(a) Sensenich M74DR - ——

Static rpm at maximum permissiblethrottle setting:

Not over 2430, not under 2300No additional tolerance permittedDiameter: not over 74 in., not

under 72.0 in.(b) Spinner, Dwg. 0550162

3. Propeller(a) McCauley 1C172/MDM 30 lb. (-39.0)

Static rpm at maximum permissiblethrottle setting:

Not over 2350, not under 2250No additional tolerance permitted

Diameter: not over 76 in., notunder 74.5 in.

(b) Spinner, Dwg. 0550216

FIGURE 4

Most of us have, at one time or an-other, flexed a paperclip back andforth until it broke. A propeller bladeor hub can behave like the paperclipon a larger scale. The engineers read-ing this are probably visualizing an"S-N" curve right about now. Forthose readers who are not, please referto Figure 3. The S-N curve describesthe number of times (N) a paperclip orpropeller blade can endure a givenamount of flexing (S or stress) beforeit breaks.

As you can see, the lower the stress,the more cycles it takes to break thepart and the longer it will last. If thestresses are low enough (below the al-lowable stress) the propeller will lastalmost indefinitely. This is the waymost propellers are designed whenused in a proper installation.

Meanwhile Back At The Ramp . . .

Let's return to the example of ourfriend with his newly completed air-

plane. Our friend has taken offand is cruising along at 3000rpm with an engine rated for,say, 2700 rpm. One reason pro-peller vibration can be sodestructive is that high stressescan occur at frequencies of up to8 or 9 cycles per crankshaft rev-olution. This can put a largenumber of cycles on a propellerin a short time. A little arith-metic shows that 3000 rpm x 9cycles per revolut ion of thecrankshaf t x 60 minutes perhour = 1.6 million cycles perhour. In 600 hours of flyingthis works out to ONE BILLIONCYCLES. If the stresses at thiscondition are high enough, itmay not be long before some-thing sudden and unpleasanthappens.Our friend could also have sev-eral other problems. Cutt ingdown the diameter below ap-proved l imits can increasestresses in the propeller; it alsochanges the locations and rpmat which they occur. Even if hehad used an approved propellerat its proper diameter , over-speeding his engine may haveexposed him to "new" resonanceconditions lurking out past themaximum rated rpm. Anythingthat can change the vibrationalcharacteristics of the propellercan have major effects on thelongevity and safety of the in-stallation. In addition todiameter and rpm that we havealready discussed, these vari-ables can also include propellerblade thickness and chord di-mensions, spacer length,crankshaft damper configura-tion, and sometimes theindexing of the propeller on thecrankshaft.Some builders assume that justbecause the propeller installa-tion is "smooth" (lack ofinstrument panel, cockpit andcontrol stick vibration) that it issafe from destructive propellervibration. This is definitely nottrue. It is possible to fly along

in a perfectly smooth cockpit while thepropeller is gradually destroying itself.On the other hand, a particular instal-lation could be very annoying to flybut could actually be quite safe (atleast until the instruments vibrate outof the panel and land in your lap). Thereason that you cannot feel a good in-stallation in the seat of your pants isbecause the frequencies at which pro-peller vibrations occur are typicallymuch higher than airframe structuralfrequencies.

SPORT AVIATION 43

TYPE CERTIFICATE DATA SHEET NO. P12EAType Certificate Holder McCauley Accessory Division

,.,' '.. * Cessna Aircraft Company, . , , ' , - . ' . ' • Vandalia, Ohio 45377 -",• .

- • ' . - ' - ' ' ' » . ' . / . ; , . -. . . . . . _ ?

Type ':' 'MaterialNumber of blades . ;,

Model(See Note 2)

1B235/BFA1B235/DFC1P235/PFA1P235/AFA1A230/AFC

Takeoff&Max. Cont.HP RPM

260 2700260 2800260 2700260 2700230 2600

* Weight includes integral.. .: i. '! '. • •' r t j : • j ; ", '

Diameter

84"-74"84"-72"84"-76"84"-76"84"-76"

doweled•• -

1 ' • • • . - • , -T i

Fixed pitch, single-pieceAluminum alloyTwo

StandardPitch

80"-40"80"-40"80"-40"80"-40"80"-40"

spacer.

Hub DrillingNo. Dia. Dia. BoltHoles Holes Circle

6 33/64" 4-3/4"6 33/64" 46 33/64" 4-3/4"6 33/64" 4-3/4"6 33/64" 4"

FIGURE 5

DiameterPilotHole

2-1/4"2-1/4"2-1/4"2-1/4"2-1/4"

Hub DimensionsDia. Thickness

6" 7-7/16"5-1/4" 3-7/16"6" 4-11/16"6" 3-7/16"5-1/4" 3-9/16"

Weight(lb.)

(Max. Dia.)

42.0*36.238.0*35.035.0

The Certification Shuffle

To prevent fatigue problems, allmanufacturers of certificated pro-pellers perform the work required toobtain certification. Although youmay be thinking, "My airplane is ex-perimental; why should I care aboutcertification?" . . . remember that

even though your airworthiness cer-tificate says "experimental" the lawsof physics are still in effect.

Certification is done in two stages,"basic" approval (FAR Part 35) and"specific" approval (FAR 23.907).Basic approval confirms that the pro-peller meets certain strength anddurability criteria. However, the fact

NOTE 9 Table of Propeller-Engine Combinations ApprovedVibrationwise for Use on Normal Category Single Engine Tractor AircraftThe maximum and minimum propeller diameters that can be used from a vibration standpoint are shownbelow. No reduction below the minimum diameter listed is permissible since this figure includes the diam-eter reduction allowable for repair purposes.

PropellerModel1P235/AFA1P235/PFA

1B235/BFA

1B235/BFA

1A230/AFC

1B235-DFC

- n* •

Max. Dia. . , Min. Dia.• • ' • ' • ' . ' • ' Engine Model (Inches) . _ • (Inches) PlacardsLycoming O-540 & IO-540 series, 84 77 Nonewith one-5th and one-6th ordercrankshaft damper configuration(up to 260 hp @ 2700 rpm)

Lycoming O-360 series (up to 78 74 None180 hp @ 2700 rpm) r ? < ;

Lycoming IO-360 series (up to 78 fTC''-1 74 None200 hp @ 2700 rpm)

Continental O-470 series, with <: 84 : - j§0? > • M,; Noneone-5th and one-6th order crank- , . •: : . . « '••-, ' .-.Ir, - <; ;:shaft damper configuration (up to . • . . : ••<'.•:230 hp @ 2600 rpm) ,,r >/ ; , a), > . " . , ' . ' . . . , ; . • . ' .- • •••:* -ir. • . ^r!

Continental IO-360 series 82 ; 72 ' i f U-. Nonewith two-6th order or one-6th < - ; i . • • . - • , • . _ - ; ^ ! >and one-4. 5 order crankshaft !;•, :;>'•- • • . . . - . . . . . . - - / / •damper configuration (up to , , - , . . ; ,- '• ' . ; d l210 hp @ 2800 rpm) •-.:: . -; .-,: ' .-;•, •.-. . .- MIT, ' J , ;,

FIGURE 6

44 JULY 1994

that a propeller is "certified" does notmean it is O.K. for installation on aparticular airplane. The airplane thatit is going to be installed on alsoneeds specific vibration approval.

Vibrat ion approval of the air-plane/engine/propeller combinationgenerally involves flight testing astrain-gaged propeller and measuringvibratory stresses in various areas ofthe propeller while the airplane isflown throughout its flight envelope.Generally the propeller is also testedat several different diameters to al-low field repair for blade erosion andminor tip damage.

The stress levels measured in flightare compared to the strength of thepropeller at various locations. If allof the in-f l ight stresses are lowerthan those allowed, the propeller isready for vibration approval. Occa-sionally a resonant condition will befound to exist at a specific rpm.These propellers require placardswhich warn the pilot not to operateat the resonant condition. An accu-rate tachometer is important whenoperating with a placarded propellerto ensure that the condition is trulyavoided.

O.K., I'm Still Aware, Now What?

There are several sources for infor-mation on the approved installationsfor a part icular propeller. All themanufacturers of certificated pro-pellers (McCauley and Sensenich forfixed pitch; Hartzell and McCauleyfor constant speed propellers) pub-lish application guides.

These guides make an excellentstarting point when shopping for apropeller. They list a variety of air-planes and, most importantly,engines that a particular propeller isapproved on. Your EAA Chapter li-brary may have copies. Although theapplication guides are convenient,they are a collection of informationfrom other sources. There may beother propellers which are also suit-

able for your a i rp lane . The finalsources for propeller informationshould be:

T.C.D.S.

No, we're not talking about the yo-gurt shop (that 's T.C. somethingelse). T.C.D.S. stands for Type Cer-tificate Data Sheets, and they come inthree flavors: airplane, engine andpropeller. Your local IA mechanicmay have some of them, and of coursethey are always available from theFAA.

The airplane type certificate datasheets contain the propeller staticrpm or blade angle limits for the air-plane. They can serve as a referencefor a similar homebuilt airplane. Fig-ure 4 shows a typical example, thefirst page of the Cessna 172 datasheets which list the approved mod-els and static rpm limits.

Engine data sheets include rele-vant information on the propeller

such as rated horsepower and rpmlimits, compression ratio and thecrankshaft damper configuration (ifany). They are similar in format tothe airplane data sheets.

A real "gold mine" of informationcan be found in the propeller typecertificate data sheets. These sheetsinclude not only the basic approvalinformation such as diameter, spacerconfiguration, horsepower and rpmlimits (Figure 5), but they also oftencontain "NOTE 9" vibrat ion ap-provals such as those shown in Figure6.

The Note 9 installations that thepropeller manufacturer chooses tolist contain vibration approval for theentire d iameter range when usedwith a particular engine on normalcategory single engine a i rcraf t .Note that twins, aerobatic and un-conventional installations are notcovered. Any placard or engine lim-its are also listed. Such broadapproval is possible because the par-

ticular engine/propeller combinationis most critical from a propeller vi-bration standpoint, while the exactairframe is generally less important.However, use of the same "factoryairplane" shock mounts and a similarengine mount structure is stronglyrecommended.

How Is This Useful?

Armed with the above informationwe are now prepared to do some pro-peller shopping. Let's suppose you'reout at the airport one day. Yourbuddy with a factory airplane men-tions that dur ing his annualinspection it was determined that hispropeller needed to be replaced. It iseroded below the minimum approveddiameter limits for the airplane. It istherefore illegal for his certificatedairplane (diameter limits are usuallyset fairly tight for a particular air-plane because the airplane must meetthe handbook performance even atminimum diameter). As luck wouldhave it, you just happen to need apropeller for the same engine in yourairplane that is under construction.His "illegal" propeller may still beO.K. for your project. You may havefound a bargain if the propeller is vi-bration approved on your engine atthe reduced diameter, the blade chordand thickness dimensions are withinlimits, and the performance is accept-able on your airplane. Verification ofthis information is the builder's re-sponsibility.

With a little research it is possibleto come up with a safe and even ver-satile propeller choice. Whilepropeller shopping recently for mylatest project, I obtained a fixed-pitchpropeller for my engine which hadthe desired spacer, was free of plac-ards and offered several inches ofdiameter repair. It is also vibrationapproved on the next larger engineshould I choose to upgrade the air-plane in the future. Such an upgradewould also require notification of theFAA (since this would be a major

SPORT AVIATION 45

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IBER46 JULY 1994

change) and possibly re-pitching ofthe propeller, but the expense of anew prop would be avoided.

Some builders are fortunateenough to buy the "firewall forward"off of a storm damaged factory-builtairplane or one which is being up-graded to another engine. Alongwith getting the engine, exhaust sys-tem, baffling and so on, often at aconsiderable savings, propeller selec-tion is not much of an issue here.Using an approved engine/propellercombination will also usually allowyou to obtain a 25 hour flight test pe-riod for your completed airplanerather than the usual 40 hours foruncertificated engine/propeller com-binations.

Whatever propeller you choose, itis strongly recommended that it beoverhauled. Especially if it's a con-stant speed model or you are at alluncertain of its history. Even if itlooks O.K. at first glance, it will runsmoother and perform better after ithas been freshly balanced and hadthe blade angles and other dimen-sions checked. At the very leastyou'll get a shiny new paint job to gowith your shiny new airplane and, ofcourse, peace of mind.

In Conclusion . . .The uniqueness and creativity ex-

hibited by aircraft builders are someof the things that make "homebuild-ing" so enjoyable. However, mod-ifying or mis-applying a propeller is apoor place to express your individual-ity. Why not take advantage of theavailable information that someoneelse spent the time and money to ac-quire? Operating your propellerwithin its approved diameter and rpmrange when installed on the properengine can go a long way toward en-suring a safe, dependable aircraft. Allit takes is a little awareness and re-search. *

About the Author

Brian Meyer is an engineer in-volved with propeller perfor-mance and vibration. An EAAmember for 16 years, he has builtand test flown two airplanes andis now working on his third pro-ject (with the gracious indulgenceof his charming, intelligent, andbeautiful wife .. . who added thisphrase in the final draft).