1) To get the effective prop swinging horsepower for small direct drive four-cycle engines, divide the cubic inches by two. That makes an O-200 Continental a 100 horse engine. The hopelessly weak Corvair rated at 150 hp in a car has 145 cubic inches like the 65 hp Lycoming, and that’s about all it can do. The 750 cubic Hisso engine is rated at 150 horses but has an effective propeller power of 375 hp. That’s why it can turn an eight-foot propeller and fly a big airplane that a 150 hp Lycoming can barely taxi. The source of this rule was Hobart Sorrell (now deceased) of Tenino, Washington. His crafty wits produced the Guppy biplane that flew on a Cushman golf cart engine.

2) Using the effective power described above, you can multiply times 4-1/4 to get the approximate thrust with typical propeller efficiency of 85%. Source of this rule was John Thorp (deceased) of the T-18. See "Cowling and Cooling of Engines" which appeared in SPORT AVIATION some years ago (Ed. Note: This series appeared in the November and December 1963 issues).

Hopefully, these two have left a legacy that we can all use. When a reduction gear is applied to the engine, the effective power is multiplied by the gear ratio.

I haven’t tried to extrapolate the formula to two stroke engines because I have very little experience with them. I can’t tell what the effective power is of the little 70 horse drone engine that one sees buzzing away on a Bensen Gyrocopter for instance.

Yours truly - Bertran Copp, Monmouth, Oregon

Q.  Dear Mary - In the September 1995 EXPERIMENTER, Jim Delaney of Coralville, Iowa asks whether a Continental A-65, a 65 hp Rotax and a 65 hp Volkswagen all produce the same thrust. Assuming the same airplane and properly matched propeller/engine/airplane combination for best cruise, the answer is yes, theoretically. Propeller efficiency differences will produce the only variation in performance. This usually amounts to only 2 or 3%. Thus, horsepower is a valid method of comparison for correctly matched propellers because thrust is directly related to horsepower.

Often static thrust is erroneously thought to be an indicator of flight performance. A propeller designed for maximum static thrust will severely limit cruise speed. This type of design is only suitable for helicopter rotors. It is an over-simplification if applied to airplanes. Proper propeller design involves matching thrust to drag and finding the best cruise to climb tradeoff. Putting it all together involves difficult and tedious calculations. I am a consulting aeronautical engineer and author of a computer program called "prop optimizer" that matches propeller to engine and airplane automatically.

Many design studies I have done indicate that there is an optimum propeller size and rpm for any engine/airplane combination. Airplanes, even of one design, can vary considerably in drag due to manufacturing differences. Therefore, if best performance is desired, the propeller should be individually matched for each airplane. Specific questions may be directed to me at 916/622-1886.

Sincerely - Don Bates, Placerville, California

Q.  Dear Engine Q&A - I would like to know if there is a four-stroke engine out there in the 65 hp range that would work on my new S-12XL, other than the Rotax 912 - one that doesn’t require a second mortgage to purchase.

The RANS S-12XL is an excellent aircraft, I believe. The engine to use on it has stumped me. The only two engines that I know of that might work are the new Yamaha four-stroke engine, which is unavailable at this time, and the Moto Guzzi, which is unproven on a pusher aircraft. What is your opinion? Who answers these questions? What about a Subaru?

Sincerely - Don Herwander, Everett, Washington

A.  Dear Don - We’ll toss your questions out for our readers and see if, per chance, another S-12 owner is powering his aircraft with an alternate engine.

Thanks for writing! - Mary

Q.  Dear Don - May I congratulate you on your column in the EXPERIMENTER as I feel it is a very important part of that magazine providing the type of information that we all want regarding the Subaru engine.

In the May edition, regarding the tip speed of propellers, was the decimal point in the wrong place? Should it have been .00436 and not .000436?

I am building a Pober Pixie lookalike under our 95/10 category and will be powering it with a Subaru EA 81 imported from Japan with approximately 34,000 miles on it. (It appears to be in very good condition.)

I will be using one of the carburetors that you recommend, but is there any reason we can’t use the one that is on it - a Hitachi DCM 306-1 downdraft.

With respect to the engine, I will be leaving it exactly as is with direct drive and a 52 x 26 propeller. However, I will need a thrust bearing and propeller extension. Could you please advise me where I can obtain these.

I also believe with this combination I should get in the vicinity of 70 hp. Is this correct?

Thanking you once again for all your help in the past and looking forward to meeting you maybe one day at Oshkosh if we can make it again. (We were over there in 1982 - a wonderful show.)

Kindest regards - Joe Anderson, Queensland, Australia

A.  Dear Joe - It’s nice to receive a letter all the way from Australia. Your Pober Pixie lookalike is going to be a good looking aircraft. There’s so many different aircraft to choose from, it’s hard to select one you would really like.

Regarding the Hitachi carburetor, it will work fine. The venturis are small and functional, but at this application, you’ll want to use a larger venturi-type carburetor to give you more power, like the Ford Pinto carburetor I’ve mentioned before. If you look at them both together, you’ll use the difference. You’ll need more air to give more perforrmance because you’re running the engine at a higher rpm. The engine has to breathe.

Regarding the thrust bearing, I don’t think you’ll need one. On the 52 x 26 you should get about 3600 rpm at about 60 or more hp because the engine doesn’t get into the power curve, but you should get 325 - 330 lbs. of thrust.

What kind of prop adapter are you using. If you don’t have one, you need to get one that you can drill a hole in the center and tap the crank shaft to make a good secure connection.

The comment on tip speed on prop was made by Ed Sterba and I can’t answer that (Ed. Note: Yes, you’re right, the figure should have been .00436 - we apologize for this error and any confusion this has caused our readers. The correct formula, then, is: Tip Speed = .00436 x diameter x rpm (with the tip speed in ft./sec. and the diameter in inches.)

I hope I’ve helped in some way. Feel free to write any time.

Yours truly - Don

Q.  Greetings, Don - I’ve found your work very informative and wonder if I could avail myself of some of your information services, while keeping same as short as possible.

I’ve always been impressed with the formula for torque, rather than horsepower. In endeavoring to keep a Subaru conversion as simple as possible the thought occurred to me to try drive direct and with half rpms. I’ve often thought about a blower but with enough input, like a unit from an emission system, to keep a plenum full of intake air. Possibly a special cam shape may help in improving induction while exhausting more fully.

I realize this sounds extreme, but it appears the engine has enough flywheel weight to ensure smooth running at low rpms. And, with the extant oil pump, has any thought been given to oil cooling? I would imagine any number of highly developed oil coolers might be available cheaply off of unused large airplane engines.

Any thought or information in this very conceptual stage will be greatly appreciated. By the same token, if you have any special interests, I’d appreciate knowing them.

All the best - Norman Benedict, Santa Maria, California

A.  Dear Norman - Nice receiving your letter and the interest in Subaru powerplant conversions. If you use the EA-81 for small aircraft, straight drive, a 54 x 26 prop would give you 325 lbs. of thrust in the pusher type. You didn’t mention the aircraft you wanted to use. I have no idea what horsepower you want, so we’ll cover all the bases.

EA-81 - 72 hp (ideal for small aircraft). With drive would give around 450-475 lbs. of thrust with a 2.2:1 ratio. Weight with redrive 180 lbs.

Legacy - 130 hp, straight drive would give you 450-475 lbs. of thrust. With redrive, 550-570 lbs. of thrust with a 2.1:1 ratio. The weight of the Legacy with drive would range from 180-210 lbs.

Oil coolers have been used on Subarus and are easy to hook up. There are discharge fittings on oil pumps that you can use.

I hope I’ve helped you. - Don

Q.  Dear Don - I have read with great interest the various responses to letters in the EXPERIMENTER concerning the use of Subaru engines in homebuilt aircraft, and you obviously have an extensive knowledge of these powerplants.

In the November 1994 issue of EXPERIMENTER there was a table published which defined general engine specifications, which I must assume was from official Subaru sources.

When I study the table, I find that the highest horsepower developed by an EA-81 OHC engines in naturally aspirated forms do not achieve close to the advertised 98/100 hp advertised by all the major Subaru engine convertors for the EA-81.

Any ideas how this massive discrepancy occurs other than a communal dose of advertising licence?

I have already ordered a converted EA-81 engine (SHO) version from what I judge as a professional engineer to be the premium quality source, but I will most certainly be very pleasantly surprised if the actual power output is in excess of 100 rather than the advertised 118 hp. Your article appeared two months after I had placed my order after much deliberation on the relative merits of the Subaru versus the Rotax 912 or 914.

Incidentally, there would appear to be equal optimism regarding fuel consumption of Rotax engines. A study of the genuine Rotax fuel flow charts will quickly reveal that one would have to lean the mixture to unburnable ratios to achieve the popular quoted figures. It was quite refreshing to read a report in the latest issue of KITPLANES of a flight test of the 914 in the Pelican which quoted realistic fuel consumption figures which do match the fuel burn rates shown on the Rotax charts for this engine.

Any comments would be most welcome.

Yours sincerely - Fred H. Tayler, Hoffman Estates, IL

A.  Dear Fred - Was nice receiving your letter and very interesting. The EA-81 engines that they advertise with higher horsepower is accomplished by different cams, shaving heads and polishing intake and exhaust ports,and sometimes increasing the size of pistons. That’s how they accomplish the claimed horsepower.

The fuel consumption of the Rotax engine probably involves an in-cockpit adjustment for the carburetor to lean out for better fuel consumption. You can also do that with the Subaru.

I feel that the EA-81 stock works very well without adding cams, etc. I feel if it works, don’t mess with it.

I hope I’ve answered your questions.
Yours truly - Don

 
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