Aeroplane Construction and Operation

by John B. Rathbun

Aerodynamic Properties of the R.A.F. Sections. Table 1 gives the values of Ky, Kx, L/D, and the center of pressure movement (C. P.) for the R.A.F.-3 section through a range of angles varying from -2° to 20°. The first column at the left gives the angles of incidence (i), the corresponding values for the lift (Ky) and the drag (Kx) being given in the second and third columns, respectively. The fourth column gives the lift-drag ratio (L/D). The fifth and last column gives the location of the center of pressure for each different angle of incidence, the figure indicating the distance of the C. P. from the entering edge expressed as a decimal part of the chord. As an example in the use of the table, let it be required to find the lift and drag of the R.A.F.-3 section when inclined at an angle of 6° and propelled at a speed of 90 miles per hour. The assumed area will be 300 square feet. At 6° it will be found that the lift co-efficient Ky is 0.002369. From our formulae, the lift will be: L = KyAV² or numerically, L=0.002369 × 300 × (90 × 90) = 5,7567 lbs. At the same speed, but with the angle of incidence reduced to 2°, the lift will be reduced to L = 0.001554 × 300 x (90 × 90) = 3,776.2 pounds, where 0.001554 is the lift co-efficient at 2°. It will be noted that the maximum lift co-efficient occurs at 14° and continues at this value to a little past 15°. The lift at the stalling angle is fairly constant from 12° to 16°.

The value of the drag can be found in either of two ways: (1) by dividing the total lift (L) by the lift-drag ratio, or (2) by figuring its value by the formula D = KxAV². The first method is shorter and preferable. By consulting the table, it will be seen that the L/D ratio at 6° is 14.9. The total wing drag will then be equal to 5,756.7/149 = 386.4 lbs. Figured by the second method, the value of Ky at 6° is 0.000159, and the drag is therefore: D = KxAV² = 0.000159 × 300 × (90 × 90) = 386.4. This checks exactly with the first method. The lift-drag ratio is best at 4°, the figure being 15.6, while the lift at this point is 0.001963. With the same area and speed, the total lift of the surface at the angle of best lift-drift ratio will be 0001963 × 300 × (90 × 90) = 4,770 lbs.

At 4° the center of pressure is 0.385 of the chord from the entering edge. If the chord is 60 inches wide, the center of pressure will be located at 0.385 × 60 = 23.1 inches from the entering edge. At 15°, the center of pressure will be 0.29 × 60 = 17.4 inches from the entering edge, or during the change from 4° to 15° the center of pressure will have moved forward by 5.7 inches. At -2°, the pressure has moved over three-quarters of the way toward the trailing edge -0.785 of the chord, to be exact Through the ordinary flight angles of from 2° to 12°, the travel of the center of pressure is not excessive.

The maximum lift co-efficient (Ky) is very high in the R.A.F.-3 section, reaching a maximum of 0.003481 at an incidence of 14°. This is second to only one other wing, the section U.S.A.-4. This makes it suitable for heavy seaplanes.

Table 2 gives the aerodynamic properties of the R.A.F-6 wing, the table being arranged in a manner similar to that of the R.A.F.-3. In glancing down the column of lift co-efficients (Ky), and comparing the values with those of the R.A.F.-3 section, it will be noted that the lift of R.A.F.-6 is much lower at every angle of incidence, but that the lift-drag ratio of the latter section is not always correspondingly higher. At every angle below 2°, at 6°, and at angles above 14°, the L/D ratio of the R.A.F.-3 is superior in spite of its greater lift. The maximum L/D ratio of the R.A.F.-6 at 4° is 16.58, which is considerably higher than the best L/D ratio of the R.A.F.-3. The best lift co-efficient of the R.A.F.-6, 0003045, is very much lower than the maximum Ky of the R.A.F.-3.

The fact that the L/D ratio of the R.A.F.-3 wing is much greater at high lift co-efficients, and large angles of incidence, makes it very valuable as at this point the greater L/D does not tend to stall the plane at slow speed. A large L/D at great angles, together with a wide stalling angle tends for safety in slow speed flying.

Both wing sections are structurally excellent, being very deep in the region of the rear edge, the R.A.F.-6 being particularly deep at this point. A good deep spar can be placed at almost any desirable point in the R.A.F-6, and the trailing edge is deep enough to insure against rib weakness even with a comparatively great overhang.

Scale corrections for the full size R.A.F. wings are very difficult to make. According to the N. P. L. reports, the corrected value for the maximum L/D of the R.A.F.-3 wing is 18.1, the model test indicating a maximum value of 15.6. I believe that L/D = 17.5 would be a safe full size value for this section. The same reports give the full size L/D for the R.A.F.-6 as 18.5, which would be probably safe at 18.0 under the new conditions.