planes, especially those of original design, much experimenting is frequently required in order to get the correct combination. In selecting the propeller, the first determination should be to establish the diameter of the propeller. The diameter of the propeller can be calculated by using this formula:

Get The Maximum Performance From Your Airplane

Diameter (D) =-, K N2

V

where D is in feet, K is a constant (use 318 for metal propellers, and 285 for wood propellers), P is the maximum brake horsepower of the engine at sea level, V is the speed of the airplane in miles per hour of maximum hp, and N is the maximum speed of the engine in revolutions per minute. For example, let's assume a typical homebuilt has a maximum speed of 120 mph, with a 65-hp engine turning at 2600 rpm. Using these

values in the above formula, the diameter is: By L. G. Tucker, EAA 11791 109 Lilac Ave., Pittsburgh, Pennsylvania

318^65 26002 x 120 --_= 53.6 in., or approximately 54 in.

AM SURE that most of us in flying light airplanes Iin our have many times wished that we had more horsepower little four-cylinder engines. Naturally, there is very little that we can do toward increasing the power from a given engine, but frequently we can get additional performance by carefully selecting the diameter and pitch of the propeller to suit the power and speed of tho airplane. This selection is quite well established with production airplanes, but with experimental air-

The next step should be to determine the pitch of the propeller after selecting the diameter. Fig. 1 illustrates how the pitch of the propeller can be determined to obtain the maximum efficiency which also means the maximum performance. In using this chart, special care should be taken to make certain that the point on the curve does not slip off on the back side of the chart where the efficiency drops off rapidly. Now, let's determine the desired pitch for this 54-inch propeller. The ratio V/ND, where V is the velocity in feet, N is revolutions per second, and D is the diameter in feet, is: 176/42.3 x 5.33 = .765. Using the ratio of

0.90 —

0.80

0.70

P -Pitch in Ft. at .75r

y

0.60

Dp=Dia. in V=Speed Ft. per sec.

or LU

N—Rev. per sec.

jjj C/L

0.50

0.40

0.30 0.9

1.0

Technical art by Bill Blake

N 20

JANUARY 1970

Dp

1.1

.765, the pitch should be equal to the diameter of the propeller in order to hit the peak of the efficiency curve. The pitch, expressed in inches, for this propeller is 54-inches. Sufficient data is not often available with an untried airplane to make these calculations as described. Under these conditions, the next best thing to do is to obtain a propeller that has approximately the correct diameter and pitch. A test flight will then give the necessary data so that a more accurate determination can be made regarding the diameter and pitch. Again, I would like to caution you in selecting the pitch and diameter to stay on the up side of the curve so that a sharp drop in propeller efficiency does not occur. As a word of caution, do not take off with an untried propeller unless the engine turns up fast enough to develop the required horsepower. A crude estimate is that the full-throttle run-up on the ground should be almost equal to the cruising rpm. For example, if an engine is designed for an economical cruise of 2250 rpm, then the full-throttle run-up on the ground should be almost 2250 rpm. I am sure that many airplanes flying today would operate more smoothly if the propellers were more accurately balanced. The average Homebuilder is not equipped with precision balancing equipment for aircraft propellers. A simple mechanism that does allow one to balance a propeller very accurately is seen in Fig. 2. The first item in making this yoke is to take special care in assuming that the yoke is balanced. The knife edge should be adjusted so that it is very close to the center of gravity of the propeller, but be certain that it is above the center of gravity so that when the propeller is balanced on the knife edge it will be in a stable condition and will not upset. With a smooth level anvil and a sharp knife edge, a small piece of paper will cause the propeller to move. When the propeller is accurately balanced, the hub will be resting in a level position. Usually the final adjustment to get an accurately balanced blade can be obtained by adding another coat of varnish or anti-glare paint to the blade on the light side. To summarize, I am sure many owners of lightplanes would benefit greatly if they would check their propellers to determine if they have the correct diameter, the correct pitch for maximum efficiency, and that the propeller is accurately balanced. ® With Propeller Balanced Top Of Hub Will Be Level Bolt with Knife /Pivot Blade (Polish) Strap Ke"xl"

Set Bolt Vertical ' (Polish Face)

BOOK REVIEW

"Illustrated Flying Basics", by Thorn Hook, is now in its fifth edition and includes the article "Fly Your Own Plane — For Less Than A New Car" which appeared in the June, 1969 "Popular Science." A chart incorporated in the article lists some 25 used aircraft and gives latest "blue-book" prices as well as furnishes other information such as engine, power, cruise speed, range, etc. The 68-page yellow paperback was first published in 1965 as a sketchbook containing actual pointers a student learns during his first 60 hours until he becomes a private pilot. The book is suitable for children or adults, non-pilots or pilots, and copy is lean, terse and crisp with numerous sketches and cartoons. Numerous pages have been updated to make the book current. AIR SHOW is a limited edition hi-fi sound recording of a live classic barnstormer's show, featuring engine sounds, laughs, thrills, and comedy from the acts of famous pilots Bevo Howard, Bill Adams. Frank Sadler and others. A glider aerobatic act is included, in which the "swoosh" of the wind as the sailplane makes its loops is clearly heard over the master of ceremonies' humorous description. The record has a color jacket showing Bevo Howard's Buecker "Jungmeister" making an inverted ribbon pick-up. Bill Adams' 450-hp Stearman provides some sounds guaranteed to liven any gathering of aviation buffs simply by playing it. For further details, contact Thorn Hook Associates at Ferry Farms, NAPO in Annapolis, Maryland 21402. ®

European Air Tour Use inside calipers or dividers to - accurately center pivot locate point just above C. G. of propeller

Technical art by Bill Blake

Fig. 2

The chartered air tour to the European EAA Fly-In and Convention May 7 - 10, already has 101 people signed up, according to Chet Wellman who is in charge of the arrangements. P'ull details of the tour appear on page 55 of the November, 1969 SPORT AVIATION. All those still interested in getting in on this tour

should contact Chet Wellman at 228 S. Main Street in Rockford, Illinois 61101 as soon as possible. ® SPORT AVIATION

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