Drag of Miscellaneous Items

In addition to the basic parasite drag of the major components, the drag due to aft-fuselage upsweep, and control surface gaps, there is usually other parasite drag that must be taken into account. This is the drag associated with the air conditioning system, various cooling systems, and the many necessary protuberances that exist on an airplane. The classification of the items to be included in the miscellaneous drag category-and hence to be separately listed-and of the items to be implicitly included in the roughness drag, is somewhat arbitrary. Neither extreme is very attractive. That is, it is impractical to account for every last protuberance on the airplane separately. yet, on the other hand, some of these items can be significant, so that failure to account for them separately could cause the airplane drag to be underestimated. In the method of this course, such items are included in the miscellaneous drag category. These items include the air conditioning system, flap hinge and track covers, wing fences, and any unique protuberances. Items for which the drag is an implicit part of the 'roughness' markup include cabin leakage, normal antennas, nacelle compartment cooling, canopies, pressure and temperature probes, windshield wipers, and miscellaneous inlets and exhausts.

In accounting for the drag caused by the air conditioning system, only losses associated with the cooling air are to be included. No engine bleed losses are included. However, any thrust recovery resulting from the efficient discharge of cabin air should be included in this evaluation. The parasite drag of any specific protuberance should be calculated by applying the methods discussed previously.

If the design of the airplane has not progressed to the point where a detailed calculation of the drag of the air conditioning system and other items can be made, the drag of these miscellaneous items can be assumed to be about 1.5% of the total airplane parasite drag. This estimate is based on the drag of such items on the DC-8-62, -63, and on the DC-9. The breakdown of the miscellaneous drag for these airplanes is shown in the following table. (Numbers are percent of total airplane parasite drag.)

Item / Airplane DC-8-62 DC-8-63 DC-9-10 DC-9-20 DC-9-30
Flap Hinge Covers 0.12 0.12 0.69 0.97 0.69
Air Conditioning System
(incl. thrust recovery)		 0.84 0.82 0.25 0.24 0.24
Vortilon - - 0.30 0.29 0.29
Fence and Stall Strip - - 0.99 - -
Miscellaneous 0.25 0.25 - - -
Total 1.21% 1.19% 2.23% 1.50% 1.22%