PROGRESS IN FLYING MACHINES

Eventually we hope to be able to present a complete list of Chapter projects but we are adding one or two each month. We had not previously listed Kurt Schrader's Kitfox V. Kurt has the airframe just about complete and is looking forward to the cool (we hope) fall weather to get the covering done. The Kitfox seems to be, from what we have read and seen, an excellent choice. A recent article in the EXPERIMENTER tells of one builder who first flew his aircraft in 1999 and already has almost 1000 hours on it !!

Other previously listed projects: RV-6A by Jim Woodward, Murphy Super Rebel by Don & Joy Jeffries ( That Super Rebel with the Russian radial at OSH looked mighty good but Don said he is going to stay with the Lyc IO-540), Wittman Tailwind by Rich Wells, Nieuport by E. J. Schickli (first flight yet ?), Nieuport by Eliot Willoughby, Curtiss Jenny by Chris Davis, Kitfox Outback by Berry Bannon , Sonerai 2 by Tom Hubbuch.

Dream Wings Inc. has ended up just a dream after all and Gary Graham is now dreaming of a Titan Tornado. Gary has already acquired the Jabiru engine for his Dream works machine hopes the engine will be a nice match for the Titan.

Email info and photos about your project to projects@eaa110.org

TECH NOTE

How to pick the proper fixed pitch propeller for a homebuilt aircraft seem to be one of the greatest mysteries of all time. Statistics show that the average homebuilder buys at least 1 1/2 props for his project before getting the right combination. ( I know that you can't buy half a prop but you know how statistics are. How about the old one about the one pregnant woman and the nine virgins !! )

Anyway there is an interesting article in SPORT AVIATION this month that gives us some of the basics. The author gives us two important definitions:

NOMINAL PITCH- What the manufacturer designates as the pitch of his propeller based on mathematical calculations. For the trigonometrically un-challenged this is equal to the tangent of the blade angle measured at 75% of the prop radius, multiplied by pi x 75% of the prop diameter.

EFFECTIVE PITCH - What the prop actually delivers. This depends on the prop-aircraft combination. A clean design at high cruise will deliver very close to 100% of the nominal pitch. Ole Rickety Struts with your head and shoulders hanging out in the breeze may deliver as little as 85%.

Simplifying the formula given in the article: To find the effective pitch in inches multiply your hoped for cruise speed in miles per hour by 1056, and then divide the result by what you expect your engine rpm will be at cruise.

Rate your aircraft for "draginess", ranging from 15% for an open cockpit biplane with lots of flying wires, landing wires and struts, to about 3% for a nice clean strutless monoplane, and add this percentage to the nominal pitch.

EXAMPLE: Ole Rickety Struts is going to be a legal ultralight with a cruise speed of 63 MPH while the little 1/2 VW whirs at 3000 RPM. 63 X 1056 = 66528. Dividing this by 3000 gives us an effective pitch required of 22.176 inches. Ole Rickety Struts is an open cockpit parasol monoplane, not a clean aircraft but it doesn't have a second wing and all those extra wires and struts to drag around so we will assign it a "draginess" rating on the high side, but not the highest, about 10%. Multiplying 22.176 X 110 % = 24.4 inches.

Okay, now that we have the pitch nailed down, how about the diameter ? The ideal wooden prop is the largest diameter that will let your engine develop its horsepower without the tips exceeding about 85% of the speed of sound. At sea level, this figures out to about 850 ft. per second. For Ole Rickety Struts we might want to climb out at about 3200 RPM which limits us to about a 60 inch diameter prop ( 5 ft X pi X 3200/60 = 837 ft/sec.) The next step, of course would be to determine if our little engine has enough power to turn a 60 inch prop at 3200 RPM.

At Oshkosh, this year, we saw an interesting demonstration of how Wilbur & Orville solved this problem. They affixed two wooden beams a couple of inches square and several feet long, calculated to be equivalent to the drag of their proposed propeller (s), to the prop hub of their 1908 engine, and from the RPMs attained, calculated the horsepower within a few percentage points !

We don't wish to have to repeat W & O's experiment or on the other hand end up as one of the statistical 1 1/2 prop buyers. Fortunately , we have enough predecessors who have already found that the little 1/2 VW won't cut it with a 60 inch diameter prop and have settled on props of about 52/ 54 inch diameter - which hopefully is a good and also a final starting point !

But of course it's not always necessary to re-invent the wheel . If your project is a common airframe and engine combination, there's plenty of info already out there. However, if you decide upon prop specs based on the performance of a successful flying project, it would probably be wise to purchase your prop from the same manufacturer, since props, especially wooden ones, do vary !

Last Updated September 10, 2001

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