TOWARD BETTER PERFORMANCE

should provide an increase in the maximum lift coefficient (see NASA TR 482). One area that needs fairing and is seldom treated properly is a wing-strut juncture area. This is especially

important on a compression strut braced low wing because the influence of the strut over the wing can trigger a stall in the area under the struts. You can make a study of the solution to this type of problem by comparing an early Cessna 188 (Agwagon) strut fairing to the fairing used on current production airplanes. The basics of proper fairing of this area are shown in Figure 5. This type fairing should be the best arrangement for a high wing airplane, and the fairing is easier to develop. This type of fairing was used on the high wing Howard DGA series airplanes.

Article III BREAKS AND FILLETS

By Al Backstrom (EAA 1162) Rt. 1 Frisco, Texas 75034

FIGURE 1

A Well-Faired and Filleted Flying Machine

-L HE LINES OF a flying machine should be smoothly' faired with no sharp breaks or junctures. Figure 1 shows a near ultimate approach to proper fairing of lines and fillets. Our airplane design problems are different than those of our friend the bird and so they must have a different solution. Figure 2 is a tuft study on a really bad example of fairing lines and lack of fillets. Figure 3 shows the solution to these problems. Most airplanes do not have problems that are this bad, but I do not believe that any abrupt juncture has been designed that would not be improved with a good fillet. Figure 4 shows the basic ingredients of typical, good

fillets. If these fillets are not practical, a leading edge pie section or filleting may be used to reduce intersection drag. The leading edge type of filleting should be avoided on low wing designs since the fillets as shown in Figure 4

12 JANUARY 1975

An example of the importance of smooth flowing external lines is shown in Figure 6. This example is especially interesting, since a reduction in drag was achieved in spite of increased frontal and wetted areas. It is fortunate for those of us who like simple structures that sharp external breaks parallel to the airflow direction are not so critical. If they were, we would never get a rectangular fuselage airplane to fly. I hope that from the words and pictures in these articles that you have been able to appreciate the importance of detail design features in achieving better airplane performance. As you look at your airplane, walk down a flight line, work at the drawing board, or work on your airplane, look for details that you can change to improve performance. FIGURE 2

Wing/Pod Intersection Flow in the Original Configuration of the Backstrom EPB-1 Flying Plank Glider.

FIGURES

Wing/Pod Intersection Flow on the Modified Configuration of the Backstrom EPB-1. Note the Tufts Stuck on the canopy due to a Static Charge of the Nylon Fiber-glass.

R = .05c - .08c For FWD 25 - 30% C

Max R = .10 To .20c Top View A. Low Wing Type

Strut Chord Minimum

Front View

Side View

FIGURES Fairing For Strut Braced Low Wings

B. Airfoil To Flat Plate (Dimensions Same As A. Above)

R. = .05c - .08c

Const C. High Wing

Upper Fuselage Lines

FIGURE 4 Typical Good Fillet Design FIGURES

Canopy installation on a Vought F4U. A well rounded canopy was installed to eliminate the sharp peak of the original canopy. Although the modified canopy was larger in order to afford the pilot greater visibility, the canopy drag coefficient was decreased by 0.0004 and a 2 mph speed increase was realized. From National Advisory Committee for Aeronautics Report WRL-108.

Modified Canopy Original Canopy

SPORT AVIATION 13