Курсовая работа: Airfoils and Lift

Another common phenomenon that is misunderstood is that of ground effect. That is the increased efficiency of a wing when flying within a wing length of the ground. A low-wing airplane will experience a reduction in drag by 50% just before it touches down. There is a great deal of confusion about ground effect. Many pilots (and the FAA VFR Exam-O-Gram No. 47) mistakenly believe that ground effect is the result of air being compressed between the wing and the ground.

To understand ground effect it is necessary to have an understanding of upwash. For the pressures involved in low speed flight, air is considered to be non-compressible. When the air is accelerated over the top of the wing and down, it must be replaced. So some air must shift around the wing (below and forward, and then up) to compensate, similar to the flow of water around a canoe paddle when rowing. This is the cause of upwash.

As stated earlier, upwash is accelerating air in the wrong direction for lift. Thus a greater amount of downwash is necessary to compensate for the upwash as well as to provide the necessary lift. Thus more work is done and more power required. Near the ground the upwash is reduced because the ground inhibits the circulation of the air under the wing. So less downwash is necessary to provide the lift. The angle of attack is reduced and so is the induced power, making the wing more efficient.

Earlier, we estimated that a Cessna 172 flying at 110 knots must divert about 2.5 ton/sec to provide lift. In our calculations we neglected the upwash. From the magnitude of ground effect, it is clear that the amount of air diverted is probably more like 5 ton/sec.

Conclusions

Let us review what we have learned and get some idea of how the physical description has given us a greater ability to understand flight. First what have we learned:

· The amount of air diverted by the wing is proportional to the speed of the wing and the air density.

· The vertical velocity of the diverted air is proportional to the speed of the wing and the angle of attack.

· The lift is proportional to the amount of air diverted times the vertical velocity of the air.

· The power needed for lift is proportional to the lift times the vertical velocity of the air.

Now let us look at some situations from the physical point of view and from the perspective of the popular explanation.

· The plane’s speed is reduced. The physical view says that the amount of air diverted is reduced so the angle of attack is increased to compensate. The power needed for lift is also increased. The popular explanation cannot address this.

· The load of the plane is increased. The physical view says that the amount of air diverted is the same but the angle of attack must be increased to give additional lift. The power needed for lift has also increased. Again, the popular explanation cannot address this.

· A plane flies upside down. The physical view has no problem with this. The plane adjusts the angle of attack of the inverted wing to give the desired lift. The popular explanation implies that inverted flight is impossible.

As one can see, the popular explanation, which fixates on the shape of the wing, may satisfy many but it does not give one the tools to really understand flight. The physical description of lift is easy to understand and much more powerful.

Axis of Rotation

Axis of an Airplane in Flight.

An airplane may turn about three axes. Whenever the attitude of the airplane changes in flight (with respect to the ground or other fixed object), it will rotate about one or more of these axes. Think of these axes as imaginary axles around which the airplane turns like a wheel. The three axes intersect at the center of gravity and each one is perpendicular to the other two.

Longitudinal Axis: The imaginary line that extends lengthwise through the fuselage, from nose to tail, is the longitudinal axis. Motion about the longitudinal axis is roll and is produced by movement of the ailerons located at the trailing edges of the wings.

Lateral Axis: The imaginary line which extends crosswise, wing tip to wing tip, is the lateral axis. Motion about the lateral axis is pitch and is produced by movement of the elevators at the rear of the horizontal tail assembly.

Vertical Axis: The imaginary line which passes vertically through the center of gravity is the vertical axis. Motion about the vertical axis is yaw and is produced by movement of the rudder located at the rear of the vertical tail assembly.

DETAILS OF MODERN AIRSHIPS - 1927

Advantages of Rigid Type Airships--Airship Frame Construction--Large Airships Projected--Army Non-rigid Dirigibles--Requirements of Airships for Civilian Flying.

Advantages of Rigid Type Airship. Before describing typical lighter- than-air craft or airships that have received actual commercial as well as military usage, it may be well to briefly review some of the advantages of the rigid type, which is the one that lends itself most easily to large structures and which is also the safest of the three types we have previously reviewed in Chapter II which is devoted to a consideration of the elementary principles underlying airship construction and application. Rigid airships have made longer single flights than other types and have flown more hours and miles without refueling than any other form. The rigid airship is said to be the fastest large vehicle of transportation that engineering ability of man has yet evolved. The Navy Airship Los Angeles, shown near the mooring mast at Lakehurst, N. J. to which it may be anchored is depicted at Fig. 315. A design of the new 6,500,000 cubic foot capacity ship recently authorized by Congress is shown at Fig. 316 flying over a battleship at an elevation of about 1,500 feet. The rigid airship, owing to its large size and light weight can carry more load than any other type of aircraft. It is independent of topography as oceans and continents are but areas to fly over. Land vehicles must stop when they reach water, water transport must stop when the ship is docked.

Airship Frame Construction. The rigid airship, because of its bulkhead system, in which the lifting gas is carried in 16 to 20 cells, has a much greater safety factor than the types in which the gas is carried in only one or two containers. In event of damage to one or two cells, the ship can continue its journey and repairs can be made to a leaky gas cell while in flight.

The rigid ship has a complete metal framework. Girders extend from nose to tail, or in nautical parlance, from stem to stern. Ring girders set at intervals brace the longitudinals and are themselves internally reinforced by cross girders and tension wire bracing. The entire framework is enclosed by a network of wiring and the whole is streamlined or faired to minimize air resistance with a fabric covering.

The view of the crew's quarters on the Bodensee, a German air liner at Fig. 317, shows the triangular keel member with the cat-walk by which the crew can travel from one end of the ship to the other and gain access to the different gas bags. The character of the longitudinal duralumin girders and the way they are braced by the ring girders is clearly shown at Fig. 318. This depicts that portion of the hull where one set of fuel tanks are located. The view at Fig. 319 shows the interior with the deflated gas cells hanging from the top-most longitudinal ready for inflation.

The outer skin is in place and the large size and extreme lightness of the structure is clearly shown. The passenger cabin of the Deutschland, another rigid dirigible of the Zepellin series is shown at Fig. 320. Wicker chairs are used because of their light weight and the interior structure of the cabin can be determined by study of the illustration.

The control of a Zepellin type airship is not as simple as that of an airplane and no one man is at the controls. Special controls are provided for the elevators and still another set for the vertical rudders. The elevator control of the L59 with the instruments for altitude navigation is shown at Fig. 321. Control is by a large wheel similar to the steering wheel of a ship. Directional control is by a similar wheel at another part of the control car.

Large Airship Projected. The largest of the United States Navy airships, the Shenandoah was 600 feet long with a capacity of 2,115,000 cubic feet. The projected airship designed by the engineers of the Goodyear- Zepellin Company, while it has over three times the capacity of the Shenandoah will be only 100 feet longer and will be of such size that it may be housed in the Lakehurst hangar. The illustration at Fig. 322 shows how the new ships authorized by congress will compare with the Shenandoah. The control car will be built into the hull and streamlined. Engines of 4,800 horsepower, giving a speed of 90 miles per hour with fuel for from 5,000 to 8,000 miles will drive the ship. The air screws will be fitted in tilting mountings, which will turn in a 90 degree arc to help force the ship upward or downward as desired and greatly aid in controlling the huge vessel.

It will embody the proved structural advantages of some 135 ships built in the past.