Dive Brakes and Spoilers Drag
To decrease aircraft speed, whether in combat action or at landing, flat plates – which are attached to the fuselage and shaped to its geometric contour when retracted – are used. They could be placed symmetrically on both sides of the wing or on the upper fuselage (i. e., for military aircraft). The flat plates are deployed during subsonic flight. Use CDnbrake = 1.2 to 2.0 (average 1.6) based on the projected frontal area of the brake to air stream. The force level encountered is high and controlled by the level of deflection. The best position for the dive brake is where the aircraft moment change is the least (i. e., close to the aircraft CG line).
9.14.1 Undercarriage Drag
Undercarriages, fixed or extended (i. e., retractable type), cause considerable drag on smaller, low-speed aircraft. A fixed undercarriage (not streamlined) can cause
Figure 9.12. Drag polar with single-slotted Fowler flap extended (undercarriage retracted)
up to about a third of aircraft parasite drag. When the undercarriage is covered by a streamlined wheel fairing, the drag level can be halved. It is essential for high-speed aircraft to retract the undercarriage as soon as it is safe to do so (like birds). Below a 200-ft altitude from takeoff and landing, an aircraft undercarriage is kept extended. Again, it is cautioned that the data in this book are intended for coursework so readers have some sense of the order of magnitude involved.
The drag of an undercarriage wheel is computed based on its frontal area: An_wheei product of wheel diameter and width (see Figure 7.15). For twin side-byside wheels, the gap between them is ignored and the wheel drag is increased by 50% from a single-wheel drag. For the bogey type, the drag also would increase – it is assumed by 10% for each bogey, gradually decreasing to a total maximum 50% increase for a large bogey. Finally, interference effects (e. g., due to doors and tubing) would double the total of wheel drag. The drag of struts is computed separately. The bare single-wheel CD_wheel based on the frontal area is in Table 9.6 (wheel aspect ratio = D/Wb).
For the smooth side, reduce by half. In terms of an aircraft:
CDp_wheel — (CDn_wheel X An_wheel)/SW
A circular strut has nearly twice the amount of drag compared to a streamlined strut in a fixed undercarriage. For example, the drag coefficient of a circular strut based on its cross-sectional area per unit length is CDn_jtrut — 1-0 because it
Table 9.6. Bare single-wheel drag with side ridge (Figure 7.15)
operates at a low Re during takeoff and landing. For streamlined struts with fairings, it decreases to 0.5 to 0.6, depending on the type.
Torenbeek  suggests using an empirical formula if details of undercarriage sizes are not known at an early conceptual design phase. This formula is given in the FPS system as follows:
Cd_uc = 0.00403 x (MTOW0’785)/Sw (9.36)
Understandably, it could result in a slightly higher value (see the following example).
worked-out example. Continue with the previous example using the largest in the design (i. e., MTOM = 24,200 lb and SW = 323 ft2) for the undercarriage size. It has a twin-wheel, single-strut length of 2 ft (i. e., diameter of 6 inches, An_strui ^ 0.2 ft2) and a main wheel size with a 22-inch diameter and a 6.6-inch width (i. e., wheel aspect ratio = 3.33, Anwheel ^ 1 ft2). From Table 9.6, a typical value of CDnwheel = 0.18, based on the frontal area and increased by 50% for the twin-wheel (i. e., CD0 = 0.27). Including the nose wheel (although it is smaller and a single wheel, it is better to be liberal in drag estimation), the total frontal area is about 3 ft2:
fwheel = 0.27 x 3 = 0.81 ft2 fstmt = 1.0 x 3 x 2 x 0.2 = 1.2 ft2
Total fUc = 2 x (0.81 + 1.2) = 4.02 ft2 (100% increase due to interference, doors, tubing, and so on) in terms of CDpmi„_Uc = 4.02/323 = 0.0124. Checking the empirical relation in Equation 9.36, CduC = 0.00403 x (24,200a785)/323 =
0. 034, a higher value that is acceptable when details are not known.