by Neil D. Bingham (EAA 183801) 1333 N. Oakridge Dr. Centerville, UT 84014 Choosing An Engine After making the choice to build a KR-2 in early 1982, there was little attention given to choosing an engine in the first year. There seemed to be plenty to choose from . . . ranging from 50 to 100 hp. Sometime into the second year, additional thought was given to a viable engine for the project, viable in my mind at least. It turned out to be the most difficult task of the entire project and, frankly, became a pain in the neck. It added about 8 months to build-out. Let me explain. After an experience with a manufacturer who offered, but never delivered, a 90 hp, 2500cc version of his standard 65 hp, 2100cc model, I became enamored with a Wankel rotary. It turned out to cost me $5,700 by the time this whole saga reached epilogue stage. All I have is receipts for my experience. "Let the buyer beware"... oh, how true it is. Then Joe Alvarez, designer of the Polliwagen aircraft, suggested I look into the Limbach engine from Germany. It was viable, certified (in Europe) and available. This started one of the finest relationships of my career. I soon found out that what Mr. Peter Limbach or his U. S. Agent, Jeri Treager in Tulsa, OK, said, you could carry to the bank. The engine was ordered on December 23, 1984 and was on the dock at Salt Lake International ten days later, complete with accessories and test report. Liability There was some hesitance on my part to put together even a semi-technical article on my project, not that we in the EAA community don't need more of them, for we do, but America has turned so lawsuit crazy that one has to be selective with whom he shares design data. 54 MARCH 1986

To clear the air at the outset, let me say this: I'm not in the business of selling design details or data, nor am I guaranteeing that anything I did will turn out the same again. Should any of my ideas or suggestions be utilized, the user is on his own. I'll gladly share anything I might have learned with anyone as long as his intentions are to advance the free exchange of ideas and thus advance the cause of the experimental aircraft movement. If the user's intentions are other than this, either now or in the future, then I reserve the right to treat my ideas and suggestions as proprietary, restricting their use and application.

marginal conditions is far more important than top speed. Yet if we take care of the margins properly, speed will take care of itself. As demonstrated above, I can and do operate safely out of tough airports and at the same time I'm completely satisfied with my 160 mph cruise and 178 mph top. I've had the craft up to 240 mph true on a low pass out of a shallow dive from 1800 ft. AGL in relatively still air. Aside from the emotion of sheer exhilaration, the little craft was a handful 20 feet above the deck! All this can be reduced to engineering numbers. Our Cessna 152, for example, has a power loading of roughly 15 Ibs./hp at take

Power Loading

off gross. My KR has a power loading of 11.25 Ibs./hp at the designer's gross weight. I would not be comfortable in my KR with a power loading like the C-152. It just isn't enough horsepower to suit me for operations in marginal conditions.

What we learn very early on as we test fly our creations is that these light, aerodynamically clean airplanes, such as the KR-2, will go faster than we are comfortable with except in still air. That is, trying to hold altitude and heading at the speeds these birds are capable of in bumpy air is at best difficult and uncomfortable. This being the case, when we set about choosing our engine we probably need to focus on a different set of requirements from sheer speed. May I suggest operation in and out of short, high altitude fields at design gross weight. If we can pick a powerplant that will give us 200 ft. per minute climb rate out of a 7000 ft. high airport, and get off in 70% 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 Oshkosh showed that my KR should not have less than 80 hp. The density altitude was 10,000 ft.! I had a full load (24.5 gals.) of gas and though I was solo, I had tools, books, camping gear, etc. so I was close to my placarded gross weight. As it turned out, the takeoff was uneventful but with 65 hp it would have been much more exciting! The point here is that safe operation under

Weight and Balance In small aircraft such as the KR-2, where the inflight center of gravity winds up relative to the MAC (Mean Aerodynamic Center) is very critical. I suppose a quick way to get into trouble is to put a 216 lb. engine in the same position relative to the datum as the plans suggested 160 lb. engine. It is interesting what this does to the CG. Let's assume a 440 lb. aircraft without engine, and for talking purposes a minus 36 inch arm to the engine CG and a plus 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 forward 3-3/8 inches! This could have serious consequences in the flying characteristics of the finished aircraft.

CALCULATIONS AND COMPARISONS USING A LIMBACH L2000 E01 ENGINE ENGINE SPECS

MAX. H.P.-5 MIN. FOR TAKE-OFF CONTINUOUS HORSEPOWER — CRUISE COMPRESSION RATIO DISPLACEMENT WEIGHT-WITH STARTER AND ALTERNATOR FUEL CONSUMPTION - 3,000 RPM

80 H.P. AT 3,400 RPM 70 H.P. AT 3,000 RPM 8.4 TO 1 1994CC- 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 ON OTHER KR-2 AIRCRAFT. 200r

J^

190

S

TC P ! PE EC — N8 N B

180

-6f tWK

170

-N€ 1-h B-

. }

//

/

3/ f

f

_ 160

/

X

Q.

2 f

Q 150

/

III

/

* 140

/ 1/

130

/ ^

120

'

110

100

10

20

30

40

50

60

70

80

90

100

110

120

HORSEPOWER

FIGURE 1

There is a CG "envelope" or "range" into which all configurations of loading must fall. To solve the above problem we usually have only two choices: 1. Add weight in the tail. This partially negates the benefit of a larger engine by adding to the empty weight and thus reducing the "useful" load the aircraft will accommodate. The additional horsepower will not always increase the gross weight allowed by the designer. Some other factor may control. 2. Move the engine back closer to the firewall. In our example above we would have to move the heavier engine back 10.3 inches and we would be "in" the firewall most likely, i.e., it's not doable. The answer then, unless we are prepared to handle the problem outlined above, is to look for an engine that produces the horsepower we need at a weight closer to the one the designer used in his calculations. Not an easy job sometimes. In the case of the KR-2, I found that the normally aspirated 80 hp Limbach was not significantly heavier than the 65 hp engine

the designer used. I was able to design a

swing-out motor mount so I could get to the

rear of the engine and change the magneto when I moved the engine back the required

One more comment on CG. In discussing the CG envelope with Jen Rand I suggested that the plans CG range (8 to 16 inches aft of the center section wing leading edge) be reduced to 8 to 14 inches. I'm not all that crazy about the aft end of the CG range in an aircraft with nominal elevator power at slow speeds. Ever read the Cherokee 140 reports? There are difficulties getting out of ground effect without nosing up or breaking a stall-spin when loaded near gross with an aft CG. The Cherokee 140 and the KR are not that different in design. In fact, my experience with flying my KR has me believing that, as the gross limit is reached, even though I'm still 1-1/2 inches from the 16 inch aft end of the CG range, the little craft takes constant attention in pitch. The 13-1/2 inches I added to the aft fuselage length helps but may not be the total answer. For sure it reduces pitch sensitivity, but not enough to make one comfortable with an aft CG in a close-to-gross configuration. What the aircraft probably needs is about 24 inches of additional fuselage length and

about 20% more horizontal and vertical stabilizer area. Motor Mounts

amount to bring the CG to where I wanted it.

In the process I picked up 15 hp for about

8 Ibs. in additional weight. That is a better trade-off than in the example above where it cost 64 Ibs. for 35 hp. That's a ratio of 1.88 hp per pound compared to .54 hp per pound.

3-1/2 times better.

Motor mounts perform at least three key

functions:

1. Position the engine such that the prop

swings about a longitudinal center line known 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, because unless it was significantly off it probably still would. However, assuming the designer did a good job in the first place, any deviation from this line constitutes a moment, correctable in flight by some input to the control surface involved. This reduces to more drag. Roughly equivalent to a smaller engine in net result. Even in light aircraft, the thrust energy produced by the prop is substantial. It takes a well designed mount to transfer that energy to the firewall reliably for a long time. Like any other well designed "passive" component, put it in its place correctly and it is never heard from again. To better understand the magnitude and effect of this thrust, a 200 lb. man cannot hold my KR back when I open the throttle in the static position. It will slide his feet on the dry pavement. This is "static" or constant load. Even stronger are some of the "dynamic" forces experienced during pull out from a dive or in a tight turn or those seen in flight through turbulent air. The materials used in light aircraft mounts are not very many. Most are made of 4130 steel tubing, either gas or heliarc welded. The choice of this material is probably because it is readily available, reasonably low in cost, welds easily and has an excellent strength to weight ratio. As strength is mentioned, I'm reminded that during my analysis of the stresses predicted in the Limbach/KR mount I found that smaller, lighter tube than we used would have done the job. However, like so many engineering decisions, another factor controlled. In this case it is the relative difficulty of welding very thin material. A diameter of .625 in. and wall thickness of .049 in. is easy to get and easy to weld so we used it. In the design of the mount I was able to dismiss the static and even dynamic loading as factors which controlled the design. I became more concerned with structureborne vibration transmission to the airframe and resonances in the engine/mount system. There are inherent dynamic forces at predictable frequencies in a reciprocating engine. Among these are: 1. Power impulses in the direction of piston travel at a frequency determined by the number of power impulses on the crankshaft per revolution. In a four-stroke, flat opposed four that turns out to be twice the rpm and side-to-side in direction. 2. Rotational forces caused by asymmetry of rotating parts and imbalance in these rotating parts about their center of rotation. I designed the mount using the configuration one sees in most light aircraft. I obtained the services of one of my engineering associates, a bright young mechanical engineer, Kris Bowers, who could easily put the math model on the big computer. He used the applicon "IFAD" program ("IFAD" = INTEGRATED FINITE ANALYSIS FOR DESIGN)

ELEMENT

The results were impressive. We found

five resonances, only two of which were in the range of concern. The others were so high they would not be excited by our rpm range of 750-3400. The lowest frequency of interest was 86.6 SPORT AVIATION 55

or the hangar gang unless, of course, we

DATA ON FUEL CONSUMPTION L 2000 E01 FROM LIMBACH TEST STAND RPM L/HR GPH 2400 6.7 1.8 2800 9.5 2.5 3000 12.6 3.3 3400 21.6 5.7

77.5

70

I

K

forgot to put enough gas in the tank in the first place. Of course, the latter goof is "fuel exhaustion", not "starvation" — two very different problems. In the installation of the Limbach in th KR-2 I did all that Tony suggests plus couple of other things I'll pass on. Since the Limbach came with a mechanical fuel pump I decided to utilize it as the primary means of getting gas to the carburetor, even though the carburetor is low enough on the L2000E01 to work well with gravity flow. Maybe it was paranoia, maybe good engineering sense, but I realized that there, staring me in the face, was a simple, inexpensive and light means of providing a back up fuel flow system should the mechanical pump fail. On page 120 of the 1984 Aircraft Spruce Catalog is a schematic of a parallel flow system using a check valve to sense when mechanical pump pressure fails. If the fuel pump fails, the check valve opens and allows gravity-fed fuel to flow to the fuel line tee at the carburetor input. The cost is a few dollars and a few ounces and may save a life. In the KR I installed a fuel pressure gage to give me visual indication of when my life is being saved. There has been a lot of talk recently about composite materials and their compatibility

with fuels, especially certain chemical additives to auto fuels. It was well after my three

3

4

FUEL BURN RATE (GPH)

FIGURE 2

Hz and in the vertical mode. Since it was vertical it would not be excited by power impulses and therefore would be at rpm fre-

quency:

86.6 Cycles x 60 sfe =5196 Cycles Ste min min

This also was outside our rpm range. Resonance No. 2 was in the side-to-side direction and at 116.3 Hz and must be analyzed in terms of rpm x 2: 116.3 Cycles x60 rtc =6978 Cycles s«c min min

But since we are dealing with rpm x 2 we must divide by 2 to get the effective rpm: RPM = 6978 =3489 2

This was very close to the 3400 max rpm specified by Limbach so we added another stiffening member across the top of the

mount, opposite diagonally to the one we

had to begin with. Another run was made and it raised the resonance to 127.3 Hz which corresponds to an rpm of 3819. We felt that this was high enough to be safe because the rubber isolators in the mount become very effective in isolating vibrations at these higher frequencies. At 3819 rpm they absorb about 97% of the vibratory energy. Thus we see the third function of a motor mount in action, that of isolating the engine vibrations from the airframe.

Firewall Key to the long term, reliable performance of a motor mount is tying it to the firewall so that the loads are transferred to fuselage 56 MARCH 1986

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

it gets the loads to the longerons.

We've read reports where cracking has occurred in the firewall or separation between firewall and longerons. Gone undetected it could be serious. Ever imagined what it would be like to lose an engine in flight? Goodbye, weight and balance, plus a few other things!

Even though we may be able to show that

we have enough strength built into the motor mount/longeron interface, there is another factor to consider — fatigue. Even small vibratory motions over a long period of time can cause hairline cracks to develop in the members. If we are going to be liberal in application of safety factors and be doubly careful in construction details, this is a good candidate. A nick, scratch, saw blade groove or a bad batch of epoxy are all super critical in this area. The engineers call them "stress

raisers".

Fuel System

I often have asked myself why any accident reports point to "fuel starvation" as the cause for homebuilt aircraft accidents; they should never happen! I have almost memorized certain sections of EAAer Tony Bingelis' books ('The Sportplane Builder" and "Firewall Forward"), like fuel system layout and fuel flow measurements. Apparently there are those around who have failed to get and/or understand that what Tony says should be taken as "Bible". If we do we won't have to explain "fuel starvation" to St. Peter

KR tanks were built out of foam, fiberglass and epoxy that Jen Rand warned me about possible breakdown of the epoxy in the KR kit with auto fuel. Apparently avgas was O.K. I bought 1-1/2 gallons of Randolph "Sloshing Sealer" and triple sealed all three tanks. I've been diligent about checking the gascolator screen for slutting and have found none to date, though I use unleaded auto fuel. If you want a thrill, put some gasohol in your tank and fly on a hot dayl I did, unknowingly, and it is exciting. Here I am over Great Salt Lake, probably a mile off-shore and the Limbach begins to sputter and cough! I backed off on the throttle and headed for shore, diligently looking for a farmer's field close to shore, absolutely convinced I would be using it. By backing the throttle off and opening the cowl flap, I apparently cooled things off enough to stop the vapor lock. I don't think I hurt the tanks for the Randolph Compound is very effective. However, in discussing this with Randolph people at Oshkosh they inform me that the current sloshing sealer is not impervious to continuous use of methanol and ethenol. The new compound they are working on will be, however. After this episode I went back to my regular fuel supplier and conclusively established

that his unleaded auto gas contained no alcohol.

Engine Controls and Instruments In a side-by-side aircraft with full dual controls the only place for the carb heat, throttle, mixture control cluster is the bottom center

of the instrument panel or in a center console. Since my flap handle, wing tank fuel selector and landing gear retract handle takes up all the console area, I had only the former choice. Getting the Bowden Cables from this cluster under the main fuel tank, through the firewall and to the right spot was a job. I wanted 20 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 type and does not use mixture control... I have an old fashioned "choke". I've taken some flak from several who felt one must have manual mixture control to attain decent altitudes. I was told by the Limbachs that the Solex has a certain amount of

EFFICIENCY RATING — CONSIDERING: 1. AIRSPEED 2. FUEL CONSUMPTION 3. TIME TO DESTINATION

ASSUME A RELATIONSHIP:

6

automatic mixture control, being a

plunger, constant pressure design. How much and how effective I did not know. That being the case, about halfway through my 40 hours I decided to see for myself. One morning, early, I loaded up with fuel and took off, climbing over the Wasatch Range of mountains to 14,000 feet. I experienced several things, staying at that altitude for sometime: 1. The Limbach was giving me 150-200 fpm climb. OAT was 56 degrees F. 2. The engine ran smooth and strong, giving no indiciation it was running rich or that mixture was a problem. 3. I got caught up in the euphoria of a beautiful morning in the crystal clear Rockies air, where I could see from southern Wyoming to the Nevada border and from central Utah to southern Idaho. 4. When in the spell of all that beauty, I discovered that I was having a little difficulty mentally computing my fuel consumption rate. I decided that I was near the onset of hypoxia and forthwith reduced the engine rpm to 2000 and spent the next half hour circling and playing the upslope breezes as I descended to pattern altitude. For me, that is "sport flying". A phenomenon I discovered is that my Limbach engine is very sensitive to carburetor air input. I had designed and built a closed ram air, carb input system with a cockpit controlled cutoff valve. When closed off, hot air was to have been drawn from the bottom of cylinders number 2 and 4 through a pleated filter to a carb input plenum. I discovered that the Stromberg-Zenith carburetor is sensitive to any intake air flow modifications. My final design is the same ram air duct and cut off valve which brings cool air to the "vicinity" of the carb inlet, eliminating the pleated filter. When the ram air is cut off, warm air from the rear of the engine compartment is drawn in for carb heat. Limbach tells me the temperature is typically 120 degrees F. or higher in this area. They saw the set-up at Oshkosh and put their approval on it. I have flown the set up in rain and on some pretty humid days with no indication of carb icing. In these conditions I typically fly with the ram air shut off. In order to make the throttle hook up on the Limbach I had to design and fabricate an actuating ami for the right side of the carburetor and turn the throttle shaft around. There was just not enough room on the left side to accommodate the mechanism required to get the throttle arm to go in the right direction.

- AIRSPEED_________ (GPH) 1.4 (ENDURANCE) 1.2

WHERE THE HIGHER THE NUMBER THE BETTER

90

^ JQO .180 -.160 .140

.120 O .100

.2 JQO

70

S

.020

60

3.000

JiQa

.980

.960 .940

50

.900 2.880 130

>.OM

140

150

170

Q.

AIRSPEED CONCLUSION: BEST RPM 2.950 RPM BEST SPEED 158 MPH

FIGURE 3

up, which is not as serious in the KR as other retracts because the wheels don't retract all the way. I'm told, however, that if you do land gear up, you'll make a shorter prop and scare the heck out of yourself. I put a cylinder head temperature thermocouple under each of the 4 spark plugs and ran them to a selector switch on the instrument panel. From here I took a lead to a single CHT gage. This is an excellent, inexpensive way to monitor all cylinders — very important when proving out the engine cooling scheme. The four cylinders are within 25 degrees of each other, rising to about 400 degrees F. on extended climbs on hot days, then dropping to 300-310 degrees F. on cruise. The Limbach representatives strongly recommend such instrumentation for all Limbach powered customized aircraft. The Limbach is no different from any other aircooled aircraft engine. The baffling design and installation has to be right. The high pressure air is typically brought in the front

is to put the gear down in lock position which

wall dropped the oil temperature about 15 degrees F. while having no perceptible effect on the CHT's. I got another 10-15 degrees

across the normally closed contacts. This should help preclude landing with the gear

I 160

8

and forced down past the cylinder fins and into the low pressure area in the bottom/rear of the engine compartment. On the Limbach the oil cooler is at the top center. I found that coupling the rear of this

actuates a similar microswitch but wired up

I m

t .060 uj — .040

Another thing I did with the throttle is install a small cam on the throttle shaft opposite the actuating arm. I positioned a microswitch so that when the throttle is reduced to 2000 rpm the normally open contacts close and light a "Gear Warning" red light on the instrument panel. The only way to extinguish this light

%

rectangular cooler to a correspondingly

shaped, flanged opening in the upper baffle

F. when I changed the prop to a Warnke

"AlmostConstant Speed". Oil temperatures now run from 190 degrees F. in cruise on a hot day to 230 degrees F. on long climbs. Oil pressure is about 20 psi at idle and 40

psi above 2500 rpm.

The Westach quad gage contains CHT, EGT, oil pressure and oil temperature. The EGT is not all that useful since there is no manual means of leaning the mixture, but it came in the gage. I put the thermocouple probe in the main exhaust stack as it exits the muffler. I have three other engine instruments, a fuel pressure gage we talked about earlier and a manifold pressure, both contained in

a single 3-1/8 in. gage. The tach is a 5000

rpm electric and runs off the magneto p-lead. I've been asked why I have a manifold pressure gage on a non-turbo'ed engine. My answer is that it is a good way to tell when you are lugging your engine. I find myself dropping the nose a tad in climbout to let the MP

come up nearer peak.

Cowling and Baffling In the Limbach workshop manual published 9/83 on page 1.3 it specifies 900 liters

of cooling air per second at full throttle for

proper engine cooling. Some simple arithmetic can be employed to predict how close we come to this figure in our cowling design. I'll use the KR in the

example:

SPORT AVIATION 57

INLET OPENING AREA = 3 in. x 6 in x 2 x .75' = 27 in2.

avoiding common flow measurement problems

' (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

hr

ft

x1

ifii

Mr

= 7040

60 min

APPROXIMATE ENTRANCE PRESSURE LOSS IN PERCENT OF VELOCITY

TYPE Or ENTRANCE

fl

PRESSURE AT SECTION 'A'

min.

A.

SMOOTH HELL ROUNDED WlIK RADIUS - 1 DIA. D

g

VOLUME (Q) = 7040 fl x 27ln = 1320 f

min.

144 in2

min

3-5% (2)(4)

CONVERTED TO LITERS PER SECOND:

Q = 1320 f

x 1 jiter

.0353 ft3

min

x 1 min

60 sec.

FLANGED 15' CONE

= 623 liters

sec.

:>

This is not enough. We either enlarge the openings or bring up our climb speed. Take 120 mph:

Q= 120 x623 = 935 liters (OK.) 80 sec.

This simply means — and experience has proved it — you bring the gear up, drop the nose to bring the speed up and climb at 120 mph. The temperatures stay within an acceptable range, even on hot days. In using this type of approximation analysis and their results we must realize that: 1. The back pressure (resistance to flow) in the cooling air flow path is kept low by sizing the total outlet area to roughly 2 times the inlet area for climbout. Incorporating a cowl flap will satisfy this requirement nicely. On the KR the cockpit controlled cowl flap open area is 3 in. x 9.5 in = 28.5 in2. The fixed openings total to another 27 in2 for a total of just over 2 times the inlet area. 2. In cruise mode the speed is up and the back pressure in the cowling can be allowed to rise, so the cowl flap can be closed to reduce drag (about 5-6 mph in the KR). Even in this case the ratio of outlet to inlet shdlild be no less than 1 to 1. Streamlining Cowl Openings There are some relationships from fluid mechanics that are helpful in treating the air inlets and outlets to cut down flow path resistance. They are called coupling losses. In Table 1 are some common coupling configurations that can be related to our cowling openings. For our purposes, what is said of inlets can be said of outlets also. Note that the treatment of the opening in example A is better by over 80% than no treatment. Relate this to the inlets in the typical cowling. Most of the well engineered, premolded cowlings do a pretty good job on the outside but are lousy on the inside. We should take some foam, glass and epoxy

and round off the inside. We can also make our outlet edges rounded and approximate example A. Our radii may not be as large as

example A but as in a lot of things some is much better than none.

If you want to see who has taken note of these relationships, look at the inlet and out-

let treatment on the vacuum Venturis hanging on some of our classic aircraft.

Spinners I'm told by my friendly prop manufacturer that a well designed and properly installed spinner is worth about three mph. Maybe it

is more psychological than scientific, but my

58 MARCH 1986

,

T"

6%

UNFLANGED IS' CONE

t

)

12%

0.

35-50% (2)(4)

E.

!

UNFLANGED PIPE

tr

85-95% (2)(4)

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

TABLE 1

little KR, without the spinner, just doesn't look like it will fly well at all! Put on the spinner and it is half airborne already. I have an idea though that there is more to the story. I can't help but believe that the airflow into the cowling intake openings is enhanced by the presence of the right spinner. Someday I would like to calculate the differences to determine the effects on cooling.

Propeller Choice How do you tell a guy that his beautiful hand polished metal prop is not the best choice for his homebuilt? The fact is that there are some engine

people, and Limbach is one of them, who absolutely specify a wood prop of high quality for use with their engines. There is at least one good and valid reason for this: wood is an inherently resilient

material. It is less apt to either produce or transmit higher frequency vibrations. This has a beneficial effect on the engine. Swing-

ing a smooth running "stick" is just better than swinging one that sends back energy in the form of deleterious high frequency vibrations.

One day I got the chance to measure the

difference. My instruments were my ears. While in the backyard a Cessna 150 flew

over. It sounded very familiar. I knew what it

was without looking up. A few minutes later a Rutan VariEze flew over on approximately the same track. The audible difference was pronounced. It was not the engine because an O-200 is an O-200! It was the prop. Absent from the Eze was the sharp twang of

the metal prop. Put a metal prop on an Eze and, except for the subtle differences between tractor and pusher, it would sound like the 150. A Glasair with a metal CSP sounds to me for all the world like a C-172 with a burned out muffler! Put a wood prop on the Washington Speedster and it sounds different — all engine/stack sound and the prop is essentially quiet. I've wondered for a long time why a guy

with a limited budget should be stuck with one prop. For less than half the cost of a CSP one can get three wood props, tailored to do exactly what he wants to do and with one heck of a lot less weight hanging out there where it shouldn't be! Why not a climb prop, an all around one and a cruise prop for cross country? Maybe he can eliminate the middle one, but they are awfully nice.

Anyone who has flown a real climb prop on a quick airplane knows the thrill of having to ease in the throttle until the tail is up, then coming in with the rest of the throttle and the

rest of the rudder! In a light taildragger it will

teach you WWII fighter pilot skills quickly.

But, as master prop maker Bernie Warnke

PREDICTED PERFORMANCE BASED ON 22 GALLONS USABLE USING WARNKE PROP — 53" x 52" PITCH (ACTUALS 7-19-85) RPM

AIRSPEED MPH

FUEL BURN GPH

MILEAGE MPG

RANGE MILES

ENDURANCE HRS

1

2000

122

1.4

87

1,914

15.7

2

2100

126

1.5

84

1,848

14.7

3

2200

130

1.6

81

1,782

13.8

4

2300

133

1.7

78

1,716

12.9

5

2400

137

1.8

76

1,672

12.2

6

2500

140"™

1.9

74

1,628

11.6

7

2600

145

2.1

69

1,518

10.5

8

2700

150

2.3

65

1,430

9.6

9

2800

154""'

2.5(301

62

1,355

8.8

10

2900

158

2.8

56

1,232

7.9

11

3000

3.3(ltl

49'""

1,080""'

6.7(S"

12

3100

167

3.9

43

946

5.6

13

3200

170"7"

4.2

40

880

5.2

14

3300

174

5.1

34

748

4.3

15

3400

178

5.7

31

686

3.9

162""'

TADI C O

says, "You can't go anywhere!" I have 2 props with matching spinners for the KR. The climb prop is a Hoffmann. Not cheap but good. It didn't start out as a climb prop but when I got it from EAAer and fellow homebuilder, Erv Hamilton up in Canada, it was 6 inches longer than the 53 inch max I could use on the KR-2. I carefully trimmed 3 inches off each tip — carefully so I could retain the same tip shape and make sure run-out and balance were correct. Whereas with the 59 inch Hoffmann I got 2800 rpm, static, with the modified 53 inch Hoffmann I get 3100 rpm, static. I like the Hoffmann prop for short, high altitude runways and the KR type of aerobatics. It is smooth, quiet and very responsive, exploiting the KR's quick, low speed characteristics. But for x-c I've got a Bernie Warnke "Almost Constant Speed" 53 incher. My rate of climb dropped from about 1000 fpm to about 700 (at 5000 ft.) but my cruise at 3000 rpm jumped 35 mph. I typically cruise at 150 mph true at 2850-2900 rpm and burn about 3.8 gallons per hour. The Warnke prop is a 53 in. dia. x 52 in. pitch. Just one more comment on the Warnke "Almost Constant Speed" — many think Bernie says that with tongue-in-cheek, but he doesn't. The rpm range of the prop on the

MIN. CONT. RPM

MAX. CONT. RPM

MAX. RPM

Limbach/KR is astoundingly narrow. I get 2800 static, take off at 2900, cruise at 2900 and go 178 mph true, "flat out", at 3150 rpm. What is more surprising comes in a shallow dive — the engine rpm does not increase substantially, only the speed. You've got to fly one to appreciate it. I can tell you, but you won't believe me!

Noise We are not too popular around our neighborhoods in our "homebuilt" things anyway, so how can we justify making the situation worse by flying our birds with straight pipes? We may not fly through quiet neighborhoods but we fly over them, so in my opinion we should be as quiet as we can be. I've got a muffler system on the KR-2 that is also used on the Damona H-36 commercial motorglider made in Europe. I had to modify the inlet pipes some to make it lay flatter in the lower cowling. If N81NB is anything, it is quiet! The penalty, according to the standard argument, is weight and loss of horsepower. The latter due to additional back pressure in the exhaust system. The penalty I paid was about 6 Ibs. in weight and apparently a negligible amount in horsepower.

I say this because I ran a simple test. I ran static run-up with the muffler hooked up to the header pipes and with it off. There was considerable difference in noise but none in static rpm at full throttle! It would seem that if the muffler/exhaust pipe were robbing horsepower it could be detected in this test. I did not detect such a loss so I believe my conclusions are justified. Even if we are somewhat unconcerned about our neighbor's right to peace and quietude, we should be concerned about our own hearing loss in the cockpit. How many times have we climbed out of our factorybuilt "spam cans" and found ourselves yelling at the line boy — not because we're angry with him but because we think he can't hear either? There are some simple things we can do to quiet things down in our birds. May I suggest three? 1. Apply 1 /2 to 3/4 inch foam to the inside of walls and floor of the cockpit, covering it with a layer of glass cloth. I did this on the KR except I used 1/16 thick birch plywood instead of cloth. The result is noise attenuation, insulation and a lot of additional strength where you need it. The penalty is about 3-1/2 Ibs. 2. Make sure the rubber isolators in the engine mount are not too stiff (60 durometer is about right) and that they are not over tightened. I've seen engine installations where no isolators are used at all — the structure borne noise in the cockpit must be fierce. 3. "Engineer" in a good aircraft muffler and route the exhaust stack out the bottom. We've all been in noisy airplanes where head sets are not a luxury! I don't say my KR is as quiet as a first class seat in a 747 but I regularly fly using the cabin speakers and hand held mic. I have a headset but it is the new Telex "airman" 750 and is built for quiet cockpits. Cabin Heat

In 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 homebuilder is getting a good reliable muff that won't feed him carbon monoxide from the exhaust later on. This gets into the engineering of materials. Stainless steel exhaust pipes and muffs will go a long way toward precluding exhaust pipe burnout inside a muff where it is hard to see. I used a J-3 type heat box Tony Bingelis shows in his first book, except with a simple mod to bring the hot air in on the other side. The Damona H-36 motorglider muffler I use has a built-in heat muff which takes a 2.0 inch dia. flex hose. I used "SCAT" hose instead of "CAT1 because the elastomeric material is silicone rubber instead of neoprene and is rated much higher in service temperature. This is required when you hook up to a hot muffler. The intake is in the front engine baffle where it takes in high pressure air at the left cowl inlet. The outlet is down between the passenger's feet with a simple diffuser plate, a la Bingelis. Flow is controlled by a push-pull control at the passengers right

hand.

Predicting Performance Building an airplane will teach more patience 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 a satellite should malfunction. The new system will be very accurate and can provide position determination up to 100

meters (328 ft.) for civil users. It will provide

nap-of-the-earth capability for helicopter operators and can use the military's NAVSTAR system for non-precision approaches. For precision approaches the new Microwave

Landing System (MLS) may be the accepted

international standard. For enroute communications the L Band (1.5-1.6 GHz) would be used mostly for data link but will have a limited voice capability for emergency situations. In terminal areas the same L-Band transceiver would be used but communications would be direct and not via satellites so that voice communications would be normally employed. According to a report that appeared in Aviation Week and Space Technology ENGINE INSTALLATION . . . (Continued from Page 59)

that is almost impossible to defeat is the desire to know how it will perform long before it does. I took the coward's approach to this "yen". Rather than try to dig through the myriad books and technical papers put out by aero engineers, etc., I scanned the newsletters for flight reports and got a spread of data points. The data points are performance figures from my kind of airplane using different size engines. Figure 1 is the result of that study, showing where the Limbach L2000E01 would fit.

From the Limbach factory engine test stand report, furnished with my engine, I could get fuel burn rate vs. rpm under load and came up with the graph in Figure 2. Then I pulled all this data together to construct Table 2 which predicts efficiency, range and endurance at different rpm and

magazine, there are two possible methods of air surveillance under consideration for the new system. One of these methods is called

Automatic Dependent Surveillance (ADS) and relies upon L-Band communication

satellites to relay aircraft position with data obtained automatically from an airborne

based computer would calculate aircraft positions. The second method of surveillance is an Independent Cooperative Surveillance technique that would use L-Band airborne transceivers as transponders that would respond to ground based interrogations relayed via satellites. By measuring the time needed to get a response a ground based computer could calculate aircraft positions

are talking in terms of 25 years down the road. In the meantime the present VOR/ TACAN, Mode C and the coming Mode S transponders will be useable and will be the primary tools for navigation. The transition

will be gradual so there should be no fears among the users of smaller general aviation

aircraft that their present equipment will suddenly become obsolete and useless.

The one big question is what the coming budget cuts as mandated by the Balanced

very accurately.

No final choice has yet been made between these two systems. Such a satellite system as described above, operating for communications, navigation and surveillance, would have worldwide application and would be expected to be adopted as an International

Budget and Emergency Deficit Control Act of 1985 will do to any plans for improving the

present ATC system and developing an entirely new satellite system for the 21 st century. That is a story in itself and will be discussed in next month's Washington Report. setting is 2950 and that yields 158 mph and

airspeeds. Some actuals are listed in parenthesis. I worked out another graph (Figure 3) on efficiency. The criteria is purely subjective. I worked out a formula that "weighted" things to fit me and my value system. The formula 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 much fuel is burned getting there faster. I weighted this 1.4 in the exponent. 3. With a given load of fuel, how long can I stay up there if I don't care how long it takes (endurance)? I gave this a weighting of 1.2

in the exponent.

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

require new airborne equipment for those aircraft wishing to use it. This will include a NAVSTAR transceiver and an updated

Loran C navigation receiver. This implies an expensive refitting program for the airlines as well as general aviation. Fortunately we

NAVSTAR or Loran C receiver. A ground

isthis:

Civil Aviation Organization (ICAO) standard by the year 2010. Obviously the new satellite system would

burns roughly 3.5 gallons per hour. I'm quite

satisfied with that. I was quite satisfied when I did it several months ago and knew I was going to be pleased with the plane, all other things being right. .

Conclusion The past three years working on my KR-2 sportplane project have been a very satisfying exercise. What I have learned about airplanes and aircraft engineering and construction are all worth the effort and cost even if the KR went away today and was never seen again. I'm now well on my way on my second project, a Baby Lakes single-place biplane of "classic" construction, i.e., wood ribs, welded steel frame and fabric skins. The aim is to take it to the flight line at Oshkosh in 1987 for more of the excitement and fellowship that no other place or time will match.

This KR-2 tri-gear is shown during its first

flight on December 6, 1985. It is owned by Mike Lamb of 5327 W. L-10, Quartz Hill, CA 93536. He says it cruises at 160 mph

and flies great.

90 MARCH 1986