DIRECT DRIVE

vs. V-BELTS By Ted Barker (EAA 45177) Barker Experimental Engines

Palomar Airport, Bldg. 5E Carlsbad, California 92008

J. HERE HAS BEEN considerable discussion about the merits of reduction drives for the Volkswagen aircraft engine. Being a builder of engines, I have watched and listened to this theorizing with dismay as only a small portion of the facts are presented to interested homebuilders. Never to my knowledge has a more light weight, trouble free, and inexpensive engine been so raped by would be experts in the field. The homebuilders potential dollars are tossed like dice — by theories. In all of the articles, grandiose publicity, and in hangar flying sessions across the nation it has been preached over and over like a tired record, "the slower turning propellers are so much more efficient". This may well be true on paper, but these same reduction unit prophets neglect to give the homebuilder some extremely important, pertinent facts that can't be overlooked and ignored. I speak here of weight, complexity and increased frontal area. Another comment that gets belabored is, "the VW engine is not suitable for aircraft use because propeller rpm is too high for good efficiency". Contrary to these notions, several hundred men are happily flying direct drive VW engines in the U. S. and an unestimated number in Europe and other countries are doing likewise. The propeller has no idea how many rpm's it is turning. It can't read. All it realizes is tip speed. If this tip speed is kept below 950' per second, with a properly DESIGNED propeller, efficiency will remain constant. Ask men like Hegy, Trigg, Troyer, Steinhilber, Reese and others. The fable started it seems with some people having sad experiences with their homebuilt VW aircraft engine. It was a one of a kind, with their own eyeballed thoughts on proper propeller needs. Their engine was fine, but they selected the wrong propeller. The VW power curve is such the engine must be turned about 3800 rpm, full throttle, level flight to obtain close to the rated power. A 54" diameter propeller has a tip-speed of 895'/sec. at this rpm, which is below our criteria of 950'/second maximum. A 60" propeller at 3800 rpm has a tip speed of 975'/sec. which is slightly over our maximum. This suggests a diameter somewhere in between as the ideal. Let's face it — the reduction unit is heavy. The lowest figure I have been able to obtain is 18 Ibs. for the basic unit. Not too bad you say? What about the propeller 20 MARCH 1973

itself? A 72" propeller has to weigh more than say a 56" model. Right? (We won't even talk now about the added cost of a 72" model over a 56" model.) My guess is the added inches to that larger propeller adds another 7 Ibs. and brings our 18 Ibs. penalty up to 25 Ibs. Now we have increased the engine and the propeller weight by 25 Ibs. at this point, so we had better make the firewall stronger to compensate. O. K., let's add another 1 Ib. for this and bring it up to 26 Ibs. Now let's not forget, that to maintain the same ground clearance on our airframe we must extend the gear by some 8" vertically to provide for this larger and heavier propeller. (V6 of 72" equals 56"). In a Cessna type spring gear this amounts to about 5 Ibs. Our original 18 Ibs. has now grown to 31 Ibs. On a single place airplane weighing 500 Ibs., this is an increase of 6%. Your airplane now weighs with a reduction unit 531 Ibs. — but we are not through yet! Theoretically, to maintain the same structural integrity of the airframe, we will have to beef up to support this additional weight. So tack on about 2 Ibs. of integrity — to a total of a now plump 33 Ibs. Are we through gaining weight now? Not by a long shot! Let's look at the aerodynamics of the thing. Say we designed to a wing loading of 8.5 Ibs./sq. ft., at a gross of 770 Ibs. Wing area would be 90 sq. ft. The new airframe is 803 Ibs. gross, wing area would have to be increased to 94.5 sq. ft, to maintain equal wing loading. This increase in wing area just cost you another 4.5 Ibs. Figuring 1 Ib./sq. ft. (normal engineering practice), our baby has now grown to 37.5 Ibs. Finished yet? No! We have to consider the cowling. To cover up this bulky V-Belt unit the cowl will have to have a bulge. The best estimate of this bulge for excess materials is an additional 1 Ib. Our slim baby is not only heavy now but it is bulky and the increased frontal area of the cowling and landing gear is probably best estimated at an additional .5 sq. ft. This naturally causes an increase in drag and approximately 15 Ibs. at 100 mph. Look now to the problem of transferring, say, 60 hp at 4000 rpm's through a set of belts to a propeller in the reduction unit arrangement. There is a mechanical loss here of 5 to 10%. Let us use a median of 7.5% for our discussion. Therefore, power delivered to the propeller is 92.5 x 60 hp equaling 55 hp. Now as propeller efficiency would be 88% maximum (all would agree with this) it leaves us a total power transmitted by propeller to be .88 x 55.5 hp. This gives us a 48.8 useable horses using the reduction unit on a 60 hp rated engine. Now let's look at the direct drive. We have a prop efficiency of 75% so with the engine of 60 hp turning 4000 rpm's, we have a .75 x 60, equaling 45 hp.

Using the reduction unit your total gain is now 48.8 minus 45 hp — A GAIN IN HORSEPOWER of 3.8 — or 10 Ibs. per horse. In my book this is pretty lousy horse trading. I have been deliberately conservative in my figures, giving the reduction unit advocates every benefit of doubt and use of the lightest materials. The Volkswagen aircraft engine is a fantastic and worthy power plant for the properly designed airplane. The ultimate single place airframe should not weight over 400 Ibs. There are several really fine airframe plans available to the homebuilder — let's look together now at the classic Taylor Monoplane as an example. This English design is a good sample of what can be accomplished by homebuilders. The Taylor Monoplane is light — about 410 Ibs. empty. With a 170 Ib. pilot and 10 gallons of gasoline she grosses at 640 Ibs. However, by adding the weight of the belt drive penalties, just look at her: 18 Ibs. worth 1. Basis Belt Drive Unit 7 Ibs. worth 2. Longer diameter propeller 5 Ibs. worth 3. Longer Landing Gear 30 Ibs. worth (We are not counting the 8.5 Ibs. of beefs and bulges for the reduction unit application.) The weight would be 440 empty; an increase of 7% and giving it a gross of 670 Ibs., maintaining 230 Ib. payload. The additional wing loading as we are maintaining constant wing area in this discussion, would use up the additional 3.8 hp you so expensively and innovatively worked to obtain. Indeed an exercise for paper engineers and rich homebuilders with lots of time. Two of my esteemed customers are Ed Flint, Martinez, Georgia, and Les Trigg of Propeller Engineering and Duplicating of Manhattan Beach, California. I've known both men for some years and we can share experiences freely. They both have a Taylor Monoplane, and are competent and honest men in their evaluations. Ed wanted what he considered the best for his meticulously built craft. (Ed is an ex-FAA man.) He built his 1600 cc VW engine himself, using a good many of my conversion parts, and a reputable and well advertised reduction unit was installed, plus a big slow swinging propeller was tacked on the end. Ed reports the following information for your aid: Rate of Climb 800 fpm Top Speed 120 mph Les bought Jim Kerly's Taylor Monoplane for some fun flying and as a test bed for some of his propellers. His craft is about seven years old and not what you call clean aerodynamically — but fun to fly. I exchanged his old 1200 cc engine for a 1700 cc engine a couple of years ago. He turns the engine at a modest 3400 rpm (about 400 less than my recommended 3800 rpm's). Rate of Climb 1000 fpm Top Speed 118 mph Now in preparing this article for you and discussing the merits of reduction units for the VW aircraft engine vs. direct drive, I have for the most part left the problems of cost, additional materials, labor, esthetics and reliability to your imagination. Let me just say that the reduction gear bulge is NOT FOR FREE! You have in the past been given and heard 50% of the reduction gear story — you now have the other 50% of the picture. I leave the decision of "to gear or not to gear" up to you — THE JURY.

Notes On Modifying Cessna 150 Spring Steel Gear Legs By Tony Bingelis (EAA 2643) 8509 Greenflint Lane Austin, Texas 78759 I cut down a set of Cessna 150 gear legs for the Turner T-40 without using a cutting torch or even getting the gear annealed. What a surprise. My handsaw already had a used Vfe" flexible metal cutting blade (18 teeth per inch). Of course the bandsaw is converted for metal cutting through one of those Sears gear boxes. A big 12" pulley replaces the regular handsaw pulley. This combination gives me a nice speed for most work. All that was necessary was to draw my cutting lines with a silver pencil and just feed that heavy old gear into the blade. It is important to keep the blade cutting by feeding the work as needed. You can't rush it. I had to make 4 different cuts of about 5" each. The saw cut through the %" material in a most gratifying manner. It cut at a rate of about 1" per minute. The whole cutting operation took only 30 minutes. I was amazed. Still am. The cut was very straight and clean. Also surprising was how simple it was to get a straight smooth perfect edge on the cut-off area merely by running them across my bench disc sander. It had a 10" aluminum oxide sanding disk glued on at the time. (Medium grit). If I had used a cutting torch (which I don't know how to) there would have been one long job of grinding and smoothing of the cut edges. Remember, the cuts were made with the gear in its normal hardened condition.

BATTERY BOX CORROSION By Jim Peale (EAA 15132) 511 Ashby Way Warner Robbins, Georgia 31093 Corrosion around and in battery boxes has always been a problem. I've found that this can be eliminated through the miracle of modern chemistry. The battery box should be cleaned with baking soda and water to neutralize the acid and the corrosion and all old paint should be removed. The box should be painted with a two-part epoxy paint, both interior and exterior. The two-part epoxy paint is usually marked for marine use and found at most local hardware stores, Sears, or boat dealers. This will stop battery box corrosion. But what about the battery cable terminals corroding? Well, that's been taken care of too. There are several companies that are making a product that you spray from a can that protects the battery cable terminals. One such product, Dow-Corning Z4 Silicone Lubricant, is sprayed on the battery cables after installation. It is a sticky, usually red in color, coating that protects your battery cable terminals. This product is sold at many of the electronic supply houses (is also used in TV sets to keep corrosion off wire terminals), some hardware stores, and at Sears. For one dollar extra, Sears will spray your battery cable ends when you buy their battery, and also will guarantee that the battery cables will not corrode for the life of the battery, so it must be fairly good. SPORT AVIATION 21