by Neil D. Bingham (EAA 183801)1333 N. Oakridge Dr.Centerville, UT 84014

Choosing An EngineAfter making the choice to build a KR-2 in

early 1982, there was little attention given tochoosing an engine in the first year. Thereseemed to be plenty to choose from . . .ranging from 50 to 100 hp.

Sometime into the second year, additionalthought was given to a viable engine for theproject, viable in my mind at least. It turnedout to be the most difficult task of the entireproject and, frankly, became a pain in theneck. It added about 8 months to build-out.

Let me explain. After an experience witha manufacturer who offered, but never deliv-ered, a 90 hp, 2500cc version of his standard65 hp, 2100cc model, I became enamoredwith a Wankel rotary. It turned out to cost me$5,700 by the time this whole saga reachedepilogue stage. All I have is receipts for myexperience. "Let the buyer beware"... oh,how true it is.

Then Joe Alvarez, designer of the Polliwa-gen aircraft, suggested I look into the Lim-bach engine from Germany. It was viable,certified (in Europe) and available. Thisstarted one of the finest relationships of mycareer. I soon found out that what Mr. PeterLimbach or his U. S. Agent, Jeri Treager inTulsa, OK, said, you could carry to the bank.The engine was ordered on December 23,1984 and was on the dock at Salt Lake Inter-national ten days later, complete with acces-sories and test report.

LiabilityThere was some hesitance on my part to

put together even a semi-technical article onmy project, not that we in the EAA commu-nity don't need more of them, for we do, butAmerica has turned so lawsuit crazy that onehas to be selective with whom he shares de-sign data.54 MARCH 1986

To clear the air at the outset, let me saythis: I'm not in the business of selling designdetails or data, nor am I guaranteeing thatanything I did will turn out the same again.Should any of my ideas or suggestions beutilized, the user is on his own. I'll gladlyshare anything I might have learned withanyone as long as his intentions are to ad-vance the free exchange of ideas and thusadvance the cause of the experimental air-craft movement. If the user's intentions areother than this, either now or in the future,then I reserve the right to treat my ideas andsuggestions as proprietary, restricting theiruse and application.

Power LoadingWhat we learn very early on as we test fly

our creations is that these light, aerodynam-ically clean airplanes, such as the KR-2, willgo faster than we are comfortable with ex-cept in still air. That is, trying to hold altitudeand heading at the speeds these birds arecapable of in bumpy air is at best difficult anduncomfortable.

This being the case, when we set aboutchoosing our engine we probably need tofocus on a different set of requirements fromsheer speed. May I suggest operation in andout of short, high altitude fields at designgross weight. If we can pick a powerplantthat will give us 200 ft. per minute climb rateout of a 7000 ft. high airport, and get off in70% or less of the runway length at full gross,we're getting close.

As an example, pulling out of Rawlins, WY(6900' msl) on the trip back from Oshkoshshowed that my KR should not have lessthan 80 hp. The density altitude was 10,000ft.! I had a full load (24.5 gals.) of gas andthough I was solo, I had tools, books, camp-ing gear, etc. so I was close to my placardedgross weight.

As it turned out, the takeoff was uneventfulbut with 65 hp it would have been much moreexciting!

The point here is that safe operation under

marginal conditions is far more importantthan top speed. Yet if we take care of themargins properly, speed will take care of it-self.

As demonstrated above, I can and do op-erate safely out of tough airports and at thesame time I'm completely satisfied with my160 mph cruise and 178 mph top. I've hadthe craft up to 240 mph true on a low passout of a shallow dive from 1800 ft. AGL inrelatively still air. Aside from the emotion ofsheer exhilaration, the little craft was a hand-ful 20 feet above the deck!

All this can be reduced to engineeringnumbers. Our Cessna 152, for example, hasa power loading of roughly 15 Ibs./hp at takeoff gross. My KR has a power loading of11.25 Ibs./hp at the designer's gross weight.I would not be comfortable in my KR with apower loading like the C-152. It just isn'tenough horsepower to suit me for operationsin marginal conditions.

Weight and BalanceIn small aircraft such as the KR-2, where

the inflight center of gravity winds up relativeto the MAC (Mean Aerodynamic Center) isvery critical.

I suppose a quick way to get into troubleis to put a 216 lb. engine in the same positionrelative to the datum as the plans suggested160 lb. engine. It is interesting what this doesto the CG. Let's assume a 440 lb. aircraftwithout engine, and for talking purposes aminus 36 inch arm to the engine CG and aplus 18 inch arm to the airplane CG:

X, = 440 lbs, x 16 in. - 160 lbs, x 36 in. = 3.6 in.600 Ibs.

X2 = 440 bs. x 18 in. -216 Ibs, x 36 in. = .22 in.656 Ibs.

The final combined CG has shifted forward3-3/8 inches! This could have serious conse-quences in the flying characteristics of thefinished aircraft.

CALCULATIONS AND COMPARISONS USING ALIMBACH L2000 E01 ENGINE

ENGINE SPECSMAX. H.P.-5 MIN. FOR TAKE-OFFCONTINUOUS HORSEPOWER — CRUISECOMPRESSION RATIODISPLACEMENTWEIGHT-WITH STARTER AND ALTERNATORFUEL CONSUMPTION - 3,000 RPM

80 H.P. AT 3,400 RPM70 H.P. AT 3,000 RPM8.4 TO 11994CC- 121.68CU. IN.72 KG - 1 58.4 LBS.12 L/HR - 3.17 GPH

PERFORMANCE (KR-2) — POINTS 1,2,3 AND 4 FROM FLIGHT DATA ONOTHER KR-2 AIRCRAFT.

200 r

190

180

170

_ 160XQ.

Q 150III

* 140

130

120

110

100

TC

-6f

P!

tWK

PE EC — N8

^'

N

-N€

1/

B

1-h

//

B-

2/

/

/f

.}

f

3f/

S

J/

/^

/

10 20 30 40 50 60 70

HORSEPOWER

80 90 100 110 120

FIGURE 1There is a CG "envelope" or "range" into

which all configurations of loading must fall.To solve the above problem we usually haveonly two choices:

1. Add weight in the tail. This partiallynegates the benefit of a larger engine by ad-ding to the empty weight and thus reducingthe "useful" load the aircraft will accommo-date. The additional horsepower will not al-ways increase the gross weight allowed bythe designer. Some other factor may control.

2. Move the engine back closer to thefirewall. In our example above we wouldhave to move the heavier engine back 10.3inches and we would be "in" the firewall mostlikely, i.e., it's not doable.

The answer then, unless we are preparedto handle the problem outlined above, is tolook for an engine that produces the horse-power we need at a weight closer to the onethe designer used in his calculations. Not aneasy job sometimes.

In the case of the KR-2, I found that thenormally aspirated 80 hp Limbach was notsignificantly heavier than the 65 hp enginethe designer used. I was able to design aswing-out motor mount so I could get to therear of the engine and change the magnetowhen I moved the engine back the requiredamount to bring the CG to where I wanted it.

In the process I picked up 15 hp for about8 Ibs. in additional weight. That is a bettertrade-off than in the example above where itcost 64 Ibs. for 35 hp. That's a ratio of 1.88hp per pound compared to .54 hp per pound.3-1/2 times better.

One more comment on CG. In discussingthe CG envelope with Jen Rand I suggestedthat the plans CG range (8 to 16 inches aftof the center section wing leading edge) bereduced to 8 to 14 inches. I'm not all thatcrazy about the aft end of the CG range inan aircraft with nominal elevator power atslow speeds. Ever read the Cherokee 140reports? There are difficulties getting out ofground effect without nosing up or breakinga stall-spin when loaded near gross with anaft CG. The Cherokee 140 and the KR arenot that different in design. In fact, my experi-ence with flying my KR has me believingthat, as the gross limit is reached, eventhough I'm still 1-1/2 inches from the 16 inchaft end of the CG range, the little craft takesconstant attention in pitch. The 13-1/2 inchesI added to the aft fuselage length helps butmay not be the total answer. For sure it re-duces pitch sensitivity, but not enough tomake one comfortable with an aft CG in aclose-to-gross configuration.

What the aircraft probably needs is about24 inches of additional fuselage length andabout 20% more horizontal and verticalstabilizer area.

Motor Mounts

Motor mounts perform at least three keyfunctions:

1. Position the engine such that the propswings about a longitudinal center lineknown as the "thrust line".

2. Transfer thrust energy from the en-

gine/prop system to the aircraft.3. Isolate vibrations inherent in running

engines from the airframe.Placing the propeller shaft centerline on

the aircraft designer's thrust line is important.Not that the aircraft would fail to fly, becauseunless it was significantly off it probably stillwould. However, assuming the designer dida good job in the first place, any deviationfrom this line constitutes a moment, correct-able in flight by some input to the controlsurface involved. This reduces to more drag.Roughly equivalent to a smaller engine innet result.

Even in light aircraft, the thrust energy pro-duced by the prop is substantial. It takes awell designed mount to transfer that energyto the firewall reliably for a long time. Likeany other well designed "passive" compo-nent, put it in its place correctly and it is neverheard from again.

To better understand the magnitude andeffect of this thrust, a 200 lb. man cannothold my KR back when I open the throttle inthe static position. It will slide his feet on thedry pavement. This is "static" or constantload. Even stronger are some of the"dynamic" forces experienced during pull outfrom a dive or in a tight turn or those seenin flight through turbulent air.

The materials used in light aircraft mountsare not very many. Most are made of 4130steel tubing, either gas or heliarc welded.The choice of this material is probably be-cause it is readily available, reasonably lowin cost, welds easily and has an excellentstrength to weight ratio.

As strength is mentioned, I'm remindedthat during my analysis of the stresses pre-dicted in the Limbach/KR mount I found thatsmaller, lighter tube than we used wouldhave done the job. However, like so manyengineering decisions, another factor con-trolled. In this case it is the relative difficultyof welding very thin material. A diameter of.625 in. and wall thickness of .049 in. is easyto get and easy to weld so we used it.

In the design of the mount I was able todismiss the static and even dynamic loadingas factors which controlled the design. I be-came more concerned with structurebornevibration transmission to the airframe and re-sonances in the engine/mount system.

There are inherent dynamic forces at pre-dictable frequencies in a reciprocating en-gine. Among these are:

1. Power impulses in the direction of pis-ton travel at a frequency determined by thenumber of power impulses on the crankshaftper revolution. In a four-stroke, flat opposedfour that turns out to be twice the rpm andside-to-side in direction.

2. Rotational forces caused by asym-metry of rotating parts and imbalance inthese rotating parts about their center of ro-tation.

I designed the mount using the configura-tion one sees in most light aircraft. I obtainedthe services of one of my engineering as-sociates, a bright young mechanical en-gineer, Kris Bowers, who could easily putthe math model on the big computer. Heused the applicon "IFAD" program ("IFAD"= INTEGRATED FINITE ELEMENTANALYSIS FOR DESIGN)

The results were impressive. We foundfive resonances, only two of which were inthe range of concern. The others were sohigh they would not be excited by our rpmrange of 750-3400.

The lowest frequency of interest was 86.6SPORT AVIATION 55

DATA ON FUEL CONSUMPTIONL 2000 E01

FROM LIMBACH TEST STAND

RPM2400280030003400

L/HR6.79.5

12.621.6

GPH1.82.53.35.7

IK

77.5

70

3 4

FUEL BURN RATE (GPH)

FIGURE 2

Hz and in the vertical mode. Since it wasvertical it would not be excited by power im-pulses and therefore would be at rpm fre-quency:

86.6 Cycles x 60 sfe =5196 CyclesSte min min

This also was outside our rpm range. Re-sonance No. 2 was in the side-to-side direc-tion and at 116.3 Hz and must be analyzedin terms of rpm x 2:

116.3 Cycles x60 rtc =6978 Cycless«c min min

But since we are dealing with rpm x 2 wemust divide by 2 to get the effective rpm:

RPM = 6978 =34892

This was very close to the 3400 max rpmspecified by Limbach so we added anotherstiffening member across the top of themount, opposite diagonally to the one wehad to begin with. Another run was madeand it raised the resonance to 127.3 Hzwhich corresponds to an rpm of 3819.

We felt that this was high enough to besafe because the rubber isolators in themount become very effective in isolating vib-rations at these higher frequencies. At 3819rpm they absorb about 97% of the vibratoryenergy. Thus we see the third function of amotor mount in action, that of isolating theengine vibrations from the airframe.

FirewallKey to the long term, reliable performance

of a motor mount is tying it to the firewall sothat the loads are transferred to fuselage56 MARCH 1986

members designed to carry those loads.Some designs around do not do that well,i.e., the span between the firewall pads istoo short. The firewall flexes too much beforeit gets the loads to the longerons.

We've read reports where cracking hasoccurred in the firewall or separation be-tween firewall and longerons. Gone unde-tected it could be serious. Ever imaginedwhat it would be like to lose an engine inflight? Goodbye, weight and balance, plus afew other things!

Even though we may be able to show thatwe have enough strength built into the motormount/longeron interface, there is anotherfactor to consider — fatigue. Even small vib-ratory motions over a long period of time cancause hairline cracks to develop in the mem-bers. If we are going to be liberal in applica-tion of safety factors and be doubly carefulin construction details, this is a good candi-date. A nick, scratch, saw blade groove or abad batch of epoxy are all super critical inthis area. The engineers call them "stressraisers".

Fuel SystemI often have asked myself why any acci-

dent reports point to "fuel starvation" as thecause for homebuilt aircraft accidents; theyshould never happen! I have almostmemorized certain sections of EAAer TonyBingelis' books ('The Sportplane Builder"and "Firewall Forward"), like fuel system lay-out and fuel flow measurements. Apparentlythere are those around who have failed toget and/or understand that what Tony saysshould be taken as "Bible". If we do we won'thave to explain "fuel starvation" to St. Peter

or the hangar gang unless, of course, weforgot to put enough gas in the tank in thefirst place. Of course, the latter goof is "fuelexhaustion", not "starvation" — two very dif-ferent problems.

In the installation of the Limbach in thKR-2 I did all that Tony suggests pluscouple of other things I'll pass on.

Since the Limbach came with a mechani-cal fuel pump I decided to utilize it as theprimary means of getting gas to the car-buretor, even though the carburetor is lowenough on the L2000E01 to work well withgravity flow.

Maybe it was paranoia, maybe good en-gineering sense, but I realized that there,staring me in the face, was a simple, inex-pensive and light means of providing a backup fuel flow system should the mechanicalpump fail.

On page 120 of the 1984 Aircraft SpruceCatalog is a schematic of a parallel flow sys-tem using a check valve to sense whenmechanical pump pressure fails. If the fuelpump fails, the check valve opens and allowsgravity-fed fuel to flow to the fuel line tee atthe carburetor input. The cost is a few dollarsand a few ounces and may save a life.

In the KR I installed a fuel pressure gageto give me visual indication of when my lifeis being saved.

There has been a lot of talk recently aboutcomposite materials and their compatibilitywith fuels, especially certain chemical addi-tives to auto fuels. It was well after my threeKR tanks were built out of foam, fiberglassand epoxy that Jen Rand warned me aboutpossible breakdown of the epoxy in the KRkit with auto fuel. Apparently avgas was O.K.I bought 1-1/2 gallons of Randolph "SloshingSealer" and triple sealed all three tanks. I'vebeen diligent about checking the gascolatorscreen for slutting and have found none todate, though I use unleaded auto fuel.

If you want a thrill, put some gasohol inyour tank and fly on a hot dayl I did, unknow-ingly, and it is exciting.

Here I am over Great Salt Lake, probablya mile off-shore and the Limbach begins tosputter and cough! I backed off on the throt-tle and headed for shore, diligently lookingfor a farmer's field close to shore, absolutelyconvinced I would be using it.

By backing the throttle off and opening thecowl flap, I apparently cooled things offenough to stop the vapor lock. I don't thinkI hurt the tanks for the Randolph Compoundis very effective. However, in discussing thiswith Randolph people at Oshkosh they in-form me that the current sloshing sealer isnot impervious to continuous use ofmethanol and ethenol. The new compoundthey are working on will be, however.

After this episode I went back to my regu-lar fuel supplier and conclusively establishedthat his unleaded auto gas contained no al-cohol.

Engine Controls and InstrumentsIn a side-by-side aircraft with full dual con-

trols the only place for the carb heat, throttle,mixture control cluster is the bottom centerof the instrument panel or in a center con-sole. Since my flap handle, wing tank fuelselector and landing gear retract handletakes up all the console area, I had only theformer choice.

Getting the Bowden Cables from this clus-ter under the main fuel tank, through the fire-wall and to the right spot was a job. I wanted20 gallons in the main tank but could get no

more than 14-1/2 and still clear the cables,legs, feet, etc.

The Limbach carburetor is the Solex typeand does not use mixture control... I havean old fashioned "choke".

I've taken some flak from several who feltone must have manual mixture control to at-tain decent altitudes. I was told by the Lim-bachs that the Solex has a certain amountof automatic mixture control, being aplunger, constant pressure design. Howmuch and how effective I did not know.

That being the case, about halfwaythrough my 40 hours I decided to see formyself. One morning, early, I loaded up withfuel and took off, climbing over the WasatchRange of mountains to 14,000 feet. I experi-enced several things, staying at that altitudefor sometime:

1. The Limbach was giving me 150-200fpm climb. OAT was 56 degrees F.

2. The engine ran smooth and strong,giving no indiciation it was running rich orthat mixture was a problem.

3. I got caught up in the euphoria of abeautiful morning in the crystal clear Rockiesair, where I could see from southern Wyom-ing to the Nevada border and from centralUtah to southern Idaho.

4. When in the spell of all that beauty, Idiscovered that I was having a little difficultymentally computing my fuel consumptionrate. I decided that I was near the onset ofhypoxia and forthwith reduced the enginerpm to 2000 and spent the next half hourcircling and playing the upslope breezes asI descended to pattern altitude. For me, thatis "sport flying".

A phenomenon I discovered is that myLimbach engine is very sensitive to car-buretor air input. I had designed and built aclosed ram air, carb input system with acockpit controlled cutoff valve. When closedoff, hot air was to have been drawn from thebottom of cylinders number 2 and 4 througha pleated filter to a carb input plenum.

I discovered that the Stromberg-Zenithcarburetor is sensitive to any intake air flowmodifications. My final design is the sameram air duct and cut off valve which bringscool air to the "vicinity" of the carb inlet,eliminating the pleated filter. When the ramair is cut off, warm air from the rear of theengine compartment is drawn in for carbheat. Limbach tells me the temperature istypically 120 degrees F. or higher in thisarea. They saw the set-up at Oshkosh andput their approval on it.

I have flown the set up in rain and on somepretty humid days with no indication of carbicing. In these conditions I typically fly withthe ram air shut off.

In order to make the throttle hook up onthe Limbach I had to design and fabricate anactuating ami for the right side of the car-buretor and turn the throttle shaft around.There was just not enough room on the leftside to accommodate the mechanism re-quired to get the throttle arm to go in the rightdirection.

Another thing I did with the throttle is installa small cam on the throttle shaft opposite theactuating arm. I positioned a microswitch sothat when the throttle is reduced to 2000 rpmthe normally open contacts close and light a"Gear Warning" red light on the instrumentpanel. The only way to extinguish this lightis to put the gear down in lock position whichactuates a similar microswitch but wired upacross the normally closed contacts. Thisshould help preclude landing with the gear

EFFICIENCY RATING — CONSIDERING:1. AIRSPEED2. FUEL CONSUMPTION3. TIME TO DESTINATION

ASSUME A RELATIONSHIP:6 - AIRSPEED_________

(GPH) 1.4 (ENDURANCE) 1.2

WHERE THE HIGHER THE NUMBER THE BETTER

.180

-.160.140

.120O .100

t .060uj— .040

.020

3.000

.980

.960

.940.900

2.880

^ JQO

>.OM

.2 JQO

JiQa

90

70

60

50

%ImS

130 140 150 I 160Q.

170

8

AIRSPEEDCONCLUSION: BEST RPM 2.950 RPM

BEST SPEED 158 MPH

FIGURE 3

up, which is not as serious in the KR as otherretracts because the wheels don't retract allthe way. I'm told, however, that if you doland gear up, you'll make a shorter prop andscare the heck out of yourself.

I put a cylinder head temperature ther-mocouple under each of the 4 spark plugsand ran them to a selector switch on the in-strument panel. From here I took a lead to asingle CHT gage. This is an excellent, inex-pensive way to monitor all cylinders — veryimportant when proving out the engine cool-ing scheme. The four cylinders are within 25degrees of each other, rising to about 400degrees F. on extended climbs on hot days,then dropping to 300-310 degrees F. oncruise. The Limbach representativesstrongly recommend such instrumentationfor all Limbach powered customized aircraft.

The Limbach is no different from any otheraircooled aircraft engine. The baffling designand installation has to be right. The highpressure air is typically brought in the frontand forced down past the cylinder fins andinto the low pressure area in the bottom/rearof the engine compartment.

On the Limbach the oil cooler is at the topcenter. I found that coupling the rear of thisrectangular cooler to a correspondinglyshaped, flanged opening in the upper bafflewall dropped the oil temperature about 15degrees F. while having no perceptible effecton the CHT's. I got another 10-15 degreesF. when I changed the prop to a Warnke

"AlmostConstant Speed". Oil temperaturesnow run from 190 degrees F. in cruise on ahot day to 230 degrees F. on long climbs.Oil pressure is about 20 psi at idle and 40psi above 2500 rpm.

The Westach quad gage contains CHT,EGT, oil pressure and oil temperature. TheEGT is not all that useful since there is nomanual means of leaning the mixture, but itcame in the gage. I put the thermocoupleprobe in the main exhaust stack as it exitsthe muffler.

I have three other engine instruments, afuel pressure gage we talked about earlierand a manifold pressure, both contained ina single 3-1/8 in. gage. The tach is a 5000rpm electric and runs off the magneto p-lead.I've been asked why I have a manifold pres-sure gage on a non-turbo'ed engine. My an-swer is that it is a good way to tell when youare lugging your engine. I find myself drop-ping the nose a tad in climbout to let the MPcome up nearer peak.

Cowling and BafflingIn the Limbach workshop manual pub-

lished 9/83 on page 1.3 it specifies 900 litersof cooling air per second at full throttle forproper engine cooling.

Some simple arithmetic can be employedto predict how close we come to this figurein our cowling design. I'll use the KR in theexample:

SPORT AVIATION 57

INLET OPENING AREA = 3 in. x 6 in x 2 x .75' = 27 in2.' (the inlet openings are estimated to be 25% restricted with

engine profile "cluttering" the flow path).

AIR VELOCITY INTO OPENING 'a 80 MPH:

V = 80 ffii x 5280 ft x 1 Mr = 7040 flhr ifii 60 min min.

VOLUME (Q) = 7040 fl x 27lng = 1320 fmin. 144 in2 min

avoiding common flow measurement problems

CONVERTED TO LITERS PER SECOND:

Q = 1320 f x 1 jiter x 1 minmin .0353 ft3 60 sec.

= 623 literssec.

This is not enough. We either enlarge theopenings or bring up our climb speed. Take120 mph:

Q= 120 x623 =80

935 liters (OK.)sec.

This simply means — and experience hasproved it — you bring the gear up, drop thenose to bring the speed up and climb at 120mph. The temperatures stay within an ac-ceptable range, even on hot days.

In using this type of approximationanalysis and their results we must realizethat:

1. The back pressure (resistance to flow)in the cooling air flow path is kept low bysizing the total outlet area to roughly 2 timesthe inlet area for climbout. Incorporating acowl flap will satisfy this requirement nicely.On the KR the cockpit controlled cowl flapopen area is 3 in. x 9.5 in = 28.5 in2. Thefixed openings total to another 27 in2 for atotal of just over 2 times the inlet area.

2. In cruise mode the speed is up and theback pressure in the cowling can be allowedto rise, so the cowl flap can be closed toreduce drag (about 5-6 mph in the KR). Evenin this case the ratio of outlet to inlet shdlildbe no less than 1 to 1.

Streamlining Cowl Openings

There are some relationships from fluidmechanics that are helpful in treating the airinlets and outlets to cut down flow path resis-tance. They are called coupling losses.

In Table 1 are some common couplingconfigurations that can be related to ourcowling openings. For our purposes, what issaid of inlets can be said of outlets also.

Note that the treatment of the opening inexample A is better by over 80% than notreatment. Relate this to the inlets in the typ-ical cowling. Most of the well engineered,premolded cowlings do a pretty good job onthe outside but are lousy on the inside. Weshould take some foam, glass and epoxyand round off the inside. We can also makeour outlet edges rounded and approximateexample A. Our radii may not be as large asexample A but as in a lot of things some ismuch better than none.

If you want to see who has taken note ofthese relationships, look at the inlet and out-let treatment on the vacuum Venturis hang-ing on some of our classic aircraft.

SpinnersI'm told by my friendly prop manufacturer

that a well designed and properly installedspinner is worth about three mph. Maybe itis more psychological than scientific, but my58 MARCH 1986

TYPE Or ENTRANCE

A. SMOOTH HELL ROUNDEDWlIK RADIUS - 1 DIA. D

FLANGED 15' CONE

:>, UNFLANGED IS' CONE

T"t )

0.

E. UNFLANGED PIPE

tr

APPROXIMATE ENTRANCE PRESSURELOSS IN PERCENT OF VELOCITY

PRESSURE AT SECTION 'A'

3-5% (2)(4)

6%

12%

35-50% (2)(4) !

85-95% (2)(4)

Typical Air Entrance Losses Resulting from Accelerating Flow Into a Duct.

TABLE 1

little KR, without the spinner, just doesn'tlook like it will fly well at all! Put on the spin-ner and it is half airborne already.

I have an idea though that there is moreto the story. I can't help but believe that theairflow into the cowling intake openings isenhanced by the presence of the right spin-ner. Someday I would like to calculate thedifferences to determine the effects on cool-ing.

Propeller ChoiceHow do you tell a guy that his beautiful

hand polished metal prop is not the bestchoice for his homebuilt?

The fact is that there are some enginepeople, and Limbach is one of them, whoabsolutely specify a wood prop of high qual-ity for use with their engines.

There is at least one good and validreason for this: wood is an inherently resilientmaterial. It is less apt to either produce ortransmit higher frequency vibrations. Thishas a beneficial effect on the engine. Swing-ing a smooth running "stick" is just betterthan swinging one that sends back energyin the form of deleterious high frequency vib-rations.

One day I got the chance to measure thedifference. My instruments were my ears.While in the backyard a Cessna 150 flewover. It sounded very familiar. I knew what it

was without looking up. A few minutes latera Rutan VariEze flew over on approximatelythe same track. The audible difference waspronounced. It was not the engine becausean O-200 is an O-200! It was the prop. Ab-sent from the Eze was the sharp twang ofthe metal prop. Put a metal prop on an Ezeand, except for the subtle differences be-tween tractor and pusher, it would sound likethe 150.

A Glasair with a metal CSP sounds to mefor all the world like a C-172 with a burnedout muffler! Put a wood prop on the Wash-ington Speedster and it sounds different —all engine/stack sound and the prop is es-sentially quiet.

I've wondered for a long time why a guywith a limited budget should be stuck withone prop. For less than half the cost of aCSP one can get three wood props, tailoredto do exactly what he wants to do and withone heck of a lot less weight hanging outthere where it shouldn't be! Why not a climbprop, an all around one and a cruise prop forcross country? Maybe he can eliminate themiddle one, but they are awfully nice.

Anyone who has flown a real climb propon a quick airplane knows the thrill of havingto ease in the throttle until the tail is up, thencoming in with the rest of the throttle and therest of the rudder! In a light taildragger it willteach you WWII fighter pilot skills quickly.But, as master prop maker Bernie Warnke

PREDICTED PERFORMANCEBASED ON 22 GALLONS USABLE

USING WARNKE PROP — 53" x 52" PITCH

(ACTUALS 7-19-85)

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

RPM

2000

2100

2200

2300

2400

2500

2600

2700

2800

2900

3000

3100

3200

3300

3400

AIRSPEEDMPH

122

126

130

133

137

140"™

145

150

154""'

158

162""'

167

170"7"

174

178

FUEL BURNGPH

1.4

1.5

1.6

1.7

1.8

1.9

2.1

2.3

2.5(301

2.8

3.3(ltl

3.9

4.2

5.1

5.7

MILEAGEMPG

87

84

81

78

76

74

69

65

62

56

49'""

43

40

34

31

RANGEMILES

1,914

1,848

1,782

1,716

1,672

1,628

1,518

1,430

1,355

1,232

1,080""'

946

880

748

686

ENDURANCEHRS

15.7

14.7

13.8

12.9

12.2

11.6

10.5

9.6

8.8

7.9

6.7(S"

5.6

5.2

4.3

3.9

TADI C O

MIN.CONT.RPM

MAX.CONT.RPM

MAX.RPM

says, "You can't go anywhere!"I have 2 props with matching spinners for

the KR. The climb prop is a Hoffmann. Notcheap but good. It didn't start out as a climbprop but when I got it from EAAer and fellowhomebuilder, Erv Hamilton up in Canada, itwas 6 inches longer than the 53 inch max Icould use on the KR-2. I carefully trimmed 3inches off each tip — carefully so I couldretain the same tip shape and make surerun-out and balance were correct.

Whereas with the 59 inch Hoffmann I got2800 rpm, static, with the modified 53 inchHoffmann I get 3100 rpm, static.

I like the Hoffmann prop for short, highaltitude runways and the KR type of aerobat-ics. It is smooth, quiet and very responsive,exploiting the KR's quick, low speed charac-teristics. But for x-c I've got a Bernie Warnke"Almost Constant Speed" 53 incher. My rateof climb dropped from about 1000 fpm toabout 700 (at 5000 ft.) but my cruise at 3000rpm jumped 35 mph. I typically cruise at 150mph true at 2850-2900 rpm and burn about3.8 gallons per hour. The Warnke prop is a53 in. dia. x 52 in. pitch.

Just one more comment on the Warnke"Almost Constant Speed" — many think Ber-nie says that with tongue-in-cheek, but hedoesn't. The rpm range of the prop on the

Limbach/KR is astoundingly narrow. I get2800 static, take off at 2900, cruise at 2900and go 178 mph true, "flat out", at 3150 rpm.What is more surprising comes in a shallowdive — the engine rpm does not increasesubstantially, only the speed. You've got tofly one to appreciate it. I can tell you, but youwon't believe me!

NoiseWe are not too popular around our neigh-

borhoods in our "homebuilt" things anyway,so how can we justify making the situationworse by flying our birds with straight pipes?We may not fly through quiet neighbor-hoods but we fly over them, so in my opinionwe should be as quiet as we can be. I've gota muffler system on the KR-2 that is alsoused on the Damona H-36 commercialmotorglider made in Europe. I had to modifythe inlet pipes some to make it lay flatter inthe lower cowling. If N81NB is anything, it isquiet!

The penalty, according to the standard ar-gument, is weight and loss of horsepower.The latter due to additional back pressure inthe exhaust system. The penalty I paid wasabout 6 Ibs. in weight and apparently a neg-ligible amount in horsepower.

I say this because I ran a simple test. I ranstatic run-up with the muffler hooked up tothe header pipes and with it off. There wasconsiderable difference in noise but none instatic rpm at full throttle! It would seem thatif the muffler/exhaust pipe were robbing hor-sepower it could be detected in this test. Idid not detect such a loss so I believe myconclusions are justified.

Even if we are somewhat unconcernedabout our neighbor's right to peace andquietude, we should be concerned about ourown hearing loss in the cockpit. How manytimes have we climbed out of our factory-built "spam cans" and found ourselves yel-ling at the line boy — not because we'reangry with him but because we think he can'thear either?

There are some simple things we can doto quiet things down in our birds. May Isuggest three?

1. Apply 1 /2 to 3/4 inch foam to the insideof walls and floor of the cockpit, covering itwith a layer of glass cloth. I did this on theKR except I used 1/16 thick birch plywoodinstead of cloth. The result is noise attenua-tion, insulation and a lot of additionalstrength where you need it. The penalty isabout 3-1/2 Ibs.

2. Make sure the rubber isolators in theengine mount are not too stiff (60 durometeris about right) and that they are not overtightened. I've seen engine installationswhere no isolators are used at all — thestructure borne noise in the cockpit must befierce.

3. "Engineer" in a good aircraft muffler androute the exhaust stack out the bottom.

We've all been in noisy airplanes wherehead sets are not a luxury! I don't say myKR is as quiet as a first class seat in a 747but I regularly fly using the cabin speakersand hand held mic. I have a headset but itis the new Telex "airman" 750 and is built forquiet cockpits.

Cabin HeatIn my country if you don't have cabin heat

you only fly half the year! It is that simple.One of the practical problems to the home-

builder is getting a good reliable muff thatwon't feed him carbon monoxide from theexhaust later on. This gets into the engineer-ing of materials. Stainless steel exhaustpipes and muffs will go a long way towardprecluding exhaust pipe burnout inside amuff where it is hard to see.

I used a J-3 type heat box Tony Bingelisshows in his first book, except with a simplemod to bring the hot air in on the other side.The Damona H-36 motorglider muffler I usehas a built-in heat muff which takes a 2.0inch dia. flex hose. I used "SCAT" hose in-stead of "CAT1 because the elastomeric ma-terial is silicone rubber instead of neopreneand is rated much higher in service temper-ature. This is required when you hook up toa hot muffler. The intake is in the front enginebaffle where it takes in high pressure air atthe left cowl inlet. The outlet is down be-tween the passenger's feet with a simplediffuser plate, a la Bingelis. Flow is controlledby a push-pull control at the passengers righthand.

Predicting PerformanceBuilding an airplane will teach more pa-

tience than anything I know, but one thing(Continued on Page 90)

SPORT AVIATION 59

\\WlNGTON REPORT...(Continued from Inside Back Cover)

added capability of alerting flight crews if asatellite should malfunction.

The new system will be very accurate andcan provide position determination up to 100meters (328 ft.) for civil users. It will providenap-of-the-earth capability for helicopter oper-ators and can use the military's NAVSTARsystem for non-precision approaches. Forprecision approaches the new MicrowaveLanding System (MLS) may be the acceptedinternational standard.

For enroute communications the L Band(1.5-1.6 GHz) would be used mostly for datalink but will have a limited voice capability foremergency situations. In terminal areas thesame L-Band transceiver would be used butcommunications would be direct and not viasatellites so that voice communicationswould be normally employed.

According to a report that appeared in Avi-ation Week and Space Technology

magazine, there are two possible methodsof air surveillance under consideration for thenew system. One of these methods is calledAutomatic Dependent Surveillance (ADS)and relies upon L-Band communicationsatellites to relay aircraft position with dataobtained automatically from an airborneNAVSTAR or Loran C receiver. A groundbased computer would calculate aircraft pos-itions.

The second method of surveillance is anIndependent Cooperative Surveillancetechnique that would use L-Band airbornetransceivers as transponders that would re-spond to ground based interrogations re-layed via satellites. By measuring the timeneeded to get a response a ground basedcomputer could calculate aircraft positionsvery accurately.

No final choice has yet been made be-tween these two systems.

Such a satellite system as describedabove, operating for communications, navi-gation and surveillance, would haveworldwide application and would be ex-pected to be adopted as an International

Civil Aviation Organization (ICAO) standardby the year 2010.

Obviously the new satellite system wouldrequire new airborne equipment for thoseaircraft wishing to use it. This will include aNAVSTAR transceiver and an updatedLoran C navigation receiver. This implies anexpensive refitting program for the airlinesas well as general aviation. Fortunately weare talking in terms of 25 years down theroad. In the meantime the present VOR/TACAN, Mode C and the coming Mode Stransponders will be useable and will be theprimary tools for navigation. The transitionwill be gradual so there should be no fearsamong the users of smaller general aviationaircraft that their present equipment will sud-denly become obsolete and useless.

The one big question is what the comingbudget cuts as mandated by the BalancedBudget and Emergency Deficit Control Actof 1985 will do to any plans for improving thepresent ATC system and developing an en-tirely new satellite system for the 21 st cen-tury. That is a story in itself and will be dis-cussed in next month's Washington Report.

ENGINE INSTALLATION . . .(Continued from Page 59)

that is almost impossible to defeat is the de-sire to know how it will perform long beforeit does.

I took the coward's approach to this "yen".Rather than try to dig through the myriadbooks and technical papers put out by aeroengineers, etc., I scanned the newslettersfor flight reports and got a spread of datapoints. The data points are performance fig-ures from my kind of airplane using differentsize engines. Figure 1 is the result of thatstudy, showing where the LimbachL2000E01 would fit.

From the Limbach factory engine teststand report, furnished with my engine, Icould get fuel burn rate vs. rpm under loadand came up with the graph in Figure 2.

Then I pulled all this data together to con-struct Table 2 which predicts efficiency,range and endurance at different rpm and

airspeeds. Some actuals are listed in paren-thesis.

I worked out another graph (Figure 3) onefficiency. The criteria is purely subjective. Iworked out a formula that "weighted" thingsto fit me and my value system. The formulaisthis: e= airsoeed

(gph) 1.4 x (endurance) .Basically it says:1. I like speed. Otherwise I'd drive a car

or take the bus!2. Speed isn't everything, however. Some

consideration has to be given to how muchfuel is burned getting there faster. I weightedthis 1.4 in the exponent.

3. With a given load of fuel, how long canI stay up there if I don't care how long it takes(endurance)? I gave this a weighting of 1.2in the exponent.

It's not a very sophisticated formula andprobably full of holes but it tells me a story.From it I get that, for me, an optimum rpm

setting is 2950 and that yields 158 mph andburns roughly 3.5 gallons per hour. I'm quitesatisfied with that. I was quite satisfied whenI did it several months ago and knew I wasgoing to be pleased with the plane, all otherthings being right.

ConclusionThe past three years working on my KR-2

sportplane project have been a very satisfy-ing exercise. What I have learned aboutairplanes and aircraft engineering and con-struction are all worth the effort and costeven if the KR went away today and wasnever seen again.

I'm now well on my way on my secondproject, a Baby Lakes single-place biplaneof "classic" construction, i.e., wood ribs,welded steel frame and fabric skins. The aimis to take it to the flight line at Oshkosh in1987 for more of the excitement and fellow-ship that no other place or time will match.

This KR-2 tri-gear is shown during its firstflight on December 6, 1985. It is ownedby Mike Lamb of 5327 W. L-10, Quartz Hill,CA 93536. He says it cruises at 160 mphand flies great.

90 MARCH 1986