Pilot Training - Theory of Flight: General

1. Purpose.-The purpose of this manual is to provide-

a. The technical training necessary to understand and obey the technical orders and instructions covering the use and operation of military aircraft.

b. A working knowledge of the mechanical and -physical laws which govern airplane performance, creates a proper respect for the limitations of the airplane, and a corresponding increase in the efficiency of operations.

c. A working knowledge of the progress of aeronautical research sufficient that in conversation an Air Corps officer's remarks may reflect credit upon himself and the military service.

2. Mathematics: The student will be required to understand problems involving the squares and square roots of numbers, but equations involving powers higher than the second and fractional powers have been reduced to a minimum. Likewise, calculus is excluded from the, discussion, and trigonometry only at the rare intervals when avoiding it would appear to lead to complications..

b- That the airplane flies is a physical reality. but what makes it fly involves a knowledge of fundamental physical conceptions.. Students whose previous educational experience has not fitted them to cope with physical conceptions,  such as force mass, length, motion, and dimenion, must acquire a working knowledge of them before an understanding of what makes an airplane fly can be acquired.

3. History-a. At approximately half past ten o'clock on the morning of December 17, 1903, Orville Wright piloted the fanmous Wright powered biplane a distance of a little over 120 feet in 12 seconds, at a maximum altitude of 10 feet and at a speed of slightly over 31 miles per hour. That flight, which has gone down in history as man's first successful attempt to stay aloft in a heavier-than-air craft, was one of the four flights made that historic morning at Kitty Hawk, N. C., the longest of which was made by the late Wilbur Wright for a' distance of 852 feet in 59 seconds. During the 5 years following the first flight no fundamental changes were made. in the original airplane, which controlled lateral stability by warping the after portion of the wing section in such a way as to vary the camber at the tips.

b. Until the outbreak of the World War, aircraft design and construction had very little, scientific or engineering data as a basis. 8tresses in flight were largely unknown and the aerodynamics of balance and control were vaguely understood. The theory of stability was quite unappreciated and wing sections in use were inefficient. Great strides, however, were made in experimentation. Important patents were taken out,. The industrial field was scoured for suitable materials and familiar structural principles were gradually applied to this new field. In this early period the records of failure and of achievement in aeronautical activity served as the backbone of the science rather than the supporting theory, which later was quickly built up as a result of wind tunnel investigations, flight tests, and application of physical laws.

c. The academic interest evidenced by scientists in aeronautical problems was supplemented by energetic application shortly after the outbreak of the World War. The employment of airplanes as a valuable asset in warfare made accessible resources that private interests, stimulated by pioneering enthusiasm or ultimate financial Profit, could never have effected. Eng,ineers; and scientists at home and abroad in a veritable orgy of investigation set about making aircraft a new weapon.

4. Airplane-  The airplane in order to be considered as a useful  vehicle must perform a certain duty. The duty of an airplane is the expeditious transportation of of the "pay" load, the military passenger or cargo parts of the so-called "useful" load.

a. Requirements.-(I) In general, problems involving  air  and water craft are fairly similar. Both must have the following essential requirements:

(a) A medlum of support for the craft which in the case of the airplane is a dynamic reaction with the air.   

(b) A proper housing and protection for the crew and equipmenti which is carried.

(c) A means of propulsion or driving force.

(d) A provision for navigation of the craft, that is, the crew must be able to steer it in the desired direction and keep it under control at all times.

(e) The airplane, in addition, must be provided with a means of taking off from or alighting on either land or water, without damage to itself or to the crew and cargo carried.

(2) A successful airplane will at best be a compromise, owing to conflicting requirements. Thus an airplane designed for ease of overhaul may offer parasite resistance greater than that occurring where this feature is neglected. Again visibility must be sacrificed for aerodynamic efficiency. On the whole then, the successful airplane must, meet, the requirements which are listed below in the order of their relative importance:

(a) The design must be such that the airplane will best perform the functions for which it is built.

(b) Repair and maintenance in the field must require a minimum of time and expense.

(c) The airplane must be built as cheaply and simply as possible.

(d) The airplane must be aerodynamically eflicient or must have a maximum of speed and climbing ability with a minimum of power.

b. Essential parts.-The essential parts that meet the requirements in a above are divided into five groups:

(1) The first requirement is a means of sustentation. The wings of the airplane are the surfaces which are designed to take the forces lifting the airplane when it is moving rapidly through the air. The hull of a ship, on the other hand. is subject to the buoyant forces of the water displaced and the ship is sustained on the Water. However, the wings are more comparable to the board of an aquaplane. which is sustained on the surface by dynamic rather than static forces.

(2) The second requirement, proper housing, is met by the fuselage, nacelle, or boat hull of the airplane, just as the ship's hull houses the crew and cargo.

(3) The gasoline engine and the airscrew or, as the combination is generally known, the "engine-propeller" unit fulfill the third requirement, giving power to pull the airplane through the air. In the case of the aquaplane, the engine-propeller unit is the towing boat, applying the pulling force through the towline.

(4) The fourth requirement is fulfilled by the "control surfaces" which are required to enable the crew to direct the flight of the craft.

(a) The airplane is unique in that the pilot must be provided with devices to control it about its three principal axes. A ship operating on the surface of the water needs but one control, the rudder, for changing direction. The submarine when submerged needs two controls, the rudder for changing the direction in the horizontal plane and the elevators for changing direction in the vertical plane (rising or diving).

(b) The airplane needs both rudder and elevators, and, in addition, a third or lateral control, primarily for keeping the airplane on an even keel in straight flight and for maintaining a proper attitude in circling flight.

(5) The landing gear fulfills the fifth or distinctive requirement for the airplane, supporting it when at rest or when taking off or making a landing. It has no useful function once the airplane is in flight, whether it is of the wheel or float type.

c. Vital characteristics-The designer of an airplane, having available an engine of given power to carry a specified weight, does his best to embody the following characteristics in the completed airplane:

(1) Lowest feasible landing speed for a given area of wings.

(2) Greatest possible high speed.

(3) Best rate of climb.

(4) Desirable, amount of stability.

(5) Maximum visibility.

(6) Minimum structural weight for adequate strength.

(7) Minimum parasite resistance.

(8) Low cost.

The first three characteristics mentioned hinge primarily on the selection of the airfoil or airfoil combination. Stability and visibility depend largely on the arrangement of the structure or lay-out. The remaining characteristics are associated with the selection and fabrication of structural materials.

d. Structural qualities.-In addition to the characteristics vital to performance, there are certain structural qualities essential to successful operation. These are-

(1) Sufficient rigidity to prevent vibration and distortion.

(2) Enough flexibility to absorb and distribute, sudden shocks and uneven strains.

(3) Fabricated parts of type such as to show definite signs of deterioration prior to absolute failure.

(4) Material of high strength-weight factor to insure lightness.

(5) Material which is homogeneous, resistant to fatigue, shock, stress reversal, etc.

(6) Maximum resistance to deterioration due to corrosion, decay, etc.

(7) Structure readily accessible for inspection and repair and also designed for ease of overhaul of power plant and equipment.

5. Nomenclature.-In order to "speak the language" of aeronautics complete familiarity with aviation terms is necessary together with the accepted definitions of these terms. The current publication of the National Advisory Committee for Aeronautics entitled "Nomenclature for Aeronautics" should be considered as part of this manual and reference to that publication should be made often. (A copy of the NACA Report can be found here)

6. Notation.-The following notation which is used in this manual is consistent with the standard adopted by the National Advisory Committee for Aeronautics and the usage of the Materiel Division, Air Corps.

Greek symbols