It remains to mention the tandem arrangement, used in all airplanes, where the tail is a tandem surface in conjunction with the wings. A surface located in the position of an airplane tail is at a disadvantage and shows low efficiency for flight purposes. This is because the main wings deflect the air downward and when the tail comes along it meets air which has a more or less downward trend, instead of encountering fresh, undisturbed air (see Fig. [16]).

Resistance of an Airplane to Motion.—Earlier in this chapter the support of an airplane was explained and it was seen that the weight was exactly equalled by the lift or support; it was also explained that the production of this lift required considerable force in moving the wings rapidly through the air. It is not only the wings, however, which require force to overcome the resistance to motion. In order to have any wings at all it is unfortunately necessary to supply also struts, wires, etc., for bracing these wings, also a motor and seat for the passenger, which are usually included inside a fuselage, also wheels for landing and various control surfaces. None of these accessories to the wings contribute material lift, but they involve a large amount of resistance which is therefore a dead loss. Note carefully that there are two distinct sorts of resistance: (1) that of the wings, which is the necessary price paid for securing lift; (2) that of all the rest of the machine, in return for which nothing beneficial is received, and which therefore has sometimes been called “parasite” or “deadhead” resistance.

In a typical training machine the total resistance to be overcome if forward motion is maintained is as follows: (See Fig. [26].)

At 72 miles per hour:

Wings160 lb.
Deadhead
resistance		Fuselage75
Wiring70
Struts20195 lb.
Miscellaneous
Balance30
Total355 lb.

At a speed of 57 miles per hour:

Wings158 lb.
Deadhead resistance130 lb.
Total288 lb.

At a speed of 43 miles per hour:

Wings350 lb.
Deadhead resistance125 lb.
Total475 lb.

It is seen that the above resistance values total to the highest figure at the lowest speed, and that the lowest value of resistance occurs at an intermediate speed; the resistance decreases as the speed decreases from 73 to 57 miles per hour; but a further decrease in speed finds the resistance running up rapidly so that at minimum speed the resistance is very great again. This is due to the fact that at high speeds the deadhead resistance exceeds that of the wings but at slow speeds although the deadhead resistance is very small, the wings being turned up to a large angle within the air, have a resistance which is at its maximum. This seems clear enough when we remember that the lift of the wings remains the same as the angle decreases (and speed goes up) but that the efficiency of the wings increases so that the wing resistance is a smaller fraction of the lift at high speed than at low speed.