Airfoils and Lift (стр. 3 из 5)

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.

(a) Multiple gas cells which function like bulk-heading on a steamship, so that if one or more cells fail the ship will still remain aloft: (b) The triple cover system, one cover to hold the lifting gas, one consisting of the shape-forming duralumin frame-work, and an outer cover to shed rain and snow, to reflect rather than to absorb heat, and to present a fair surface; (c) invulnerability against lightning; (d) accessibility to inspection and repair.

It will however present certain new features as well of far reaching importance: (a) A double or triple keel giving added longitudinal strength comparable to the breaking strength of one length of metal, as against two or three bolted together; (b) a new type of ring girder each internally braced and structurally self sufficient, which (c) will permit the control car and even the power cars to be built within the hull; (d) even fuller accessiblity to continuous inspection and permitting repairs to be made even in flight; (e) the use of new fuels to conserve helium and reduce weight.

Army Non-Rigid Dirigibles. The non-rigid dirigible is the smallest of the three types as the largest now being built in the United States for the Army and Navy service have a gas capacity of about one-tenth that of the Los Angeles. Under ordinary conditions a 230,000 cubic foot non-rigid has a cruising radius of from 500 to 1,000 miles and an air endurance of from 18 to 24 hours. Such airships are essentially motorized free balloons and the engines are carried in a car attached to the lower side or bottom of the bag. The Pilgrim, a small non-rigid previously described with a gas capacity of 50,000 cubic feet has a speed of 50 miles per hour and is propelled by a Wright "Gale" three-cylinder engine as shown at Fig. 323. This small ship was built to carry four passengers. The gas in non-rigid ships, as in the army TC types, as shown at Fig. 324 is contained in a single bag, but an inner two compartment bag, called the ballonet, is filled with air to keep the main container properly distended because the air pressure can be made to compensate for variations in gas pressure in the bag. These ships have a capacity of about 200,000 cubic feet, are 196 feet long overall and 47 feet in extreme height. The hull diameter is 33.5 feet. The fineness ratio is 4.4 to 1. The total lift is 11,584 pounds of which the useful lift is about 4,000 pounds. The gross weight per horsepower is 38.6 pounds. Two Wright Type I water-cooled engines of 150 horsepower each were provided on the first ships of this series but these have been replaced on later types with two Wright J1 engines, which are nine-cylinder radial air-cooled types driving tractor propellers 9 feet 10 inches in diameter. It is claimed that the saving of 400 pounds over the water-cooled installation permits an increase of speed from 54 to 60 miles per hour; with an increase in range of 10 per cent.

Flight Control Surfaces - Elevons

Delta winged aircraft use elevons as primary flight controls for

roll and pitch.

Elevon: Delta winged aircraft can not use conventional 3 axis flight control systems because of their unique delta shape. Therefore, it uses a device called an elevon. It is a combination of ailerons and elevators.

The elevon is used as an aileron. Ailerons control motion along the longitudinal axis. The longitudinal axis is an imaginary line that runs from the nose to the tail. Motion about the longitudinal axis is called roll.

The elevon is also used as an elevator. Elevators control motion along the lateral axis. The lateral axis is an imaginary line that extends crosswise, from wingtip to wingtip. Motion about the lateral axis is called pitch.

Fokker DR.I - Thoughts on Wing Failures

The recent WWI AERO article (#165, Aug, 1999) concerning wing failures in the Nieuport 28 prompted me to put some ideas to paper, regarding those more familiar failures of the Fokker triplane. The reputation of Fokker aircraft for fragility was mainly the result of structural problems with the Dr.I triplane and D.VIII cantilever monoplane. The D.VIII wing problem was due to flexural failure (ie, they broke in bending); and the evidence indicates that this was due to production quality-control inadequacies rather than deficiencies of design or technical understanding. The Dr.I, however, is a different "kettle of fish" in that it experienced failures very like those of the Nieuport 28, namely that of "wing stripping." Unlike the D.VIII, the triplane was grounded not because of spar failure, but because of the disintegration of the secondary structure- wing ribs etc- whilst the spars remained intact. The similarity of the failures in the N28 and Dr.I is intriguing because the 2 aircraft are fundamentally different: one a biplane of almost sesquiplane proportions, the other a triplane of equal-chord wings. The N28 had thin-section wings, wire-braced; the Dr.I had thick sections and a cantilever structure very different animals. For me, the most interesting fact of all (and the most difficult to explain) has been that the failures always occurred in the upper wings of either aircraft - to my knowledge there are no reported incidents of failures in the lower planes. In the case of the 2 most notable triplane failures, the extent of the upper wingstripping was almost total, with fatal consequences for Lieutenants Gontermann and Pastor. It is of particular interest that, after the triplane was reissued with modified wings, the same type of failure still occurred - but to a more limited (and survivable) extent. At the time of the Dr.I grounding, after the 2 crashes mentioned, various theories were proposed to account for the failures. Sand loading of the Fokker F5 (the Dr.I prototype) had shown that the triplane cantilever wing cellule had excellent strength for its period; and it fell to those interested to create new (and unlikely) aerodynamic phenomena to account for the fatal discrepancy between experiment and practice. Because the ailerons of both Gontermann's and Pastor's aircraft were seen to detach, interest centered on the aileron supporting-structure and related internal componentry. Various reinforcements were introduced, and emphasis was placed on better internal protection of the glued structure by varnishing. (The peculiarity of upper wing failures had not, of course, gone unnoticed at the time. The possibility of the casein glue deteriorating, due to weathering, gave cause for concern - the lower wings being considered to be somewhat protected - debatable, of course.) Also poor workmanship was extensively uncovered in grounded aircraft and Fokker was urged to improve on this aspect of his production of further aircraft. However, as noted, failures continued to occur in the reissued aircraft.

In the case of the Nieuport 28, the fabric of the upper-wing top-surface together with the entire leading edge would detach. On this aircraft, however, damage appears to have been selflimiting at this point: the rib tails and undersurface, for instance, always seem to have held up. This is just as well for the pilots concerned, since the (almost) sesquiplane proportions of the N28 could not have tolerated complete loss of the upper lifting area. Fortunately, the Nieuport carried its ailerons on the lower plane so that roll control was available - no doubt this helped survivability.

Of all WWI aircraft, these 2 are the only ones I am aware of that suffered this type of failure as a generic fault. "Ballooning" of wing fabric was a known risk resulting from wing leading-edge damage. Wings failed simply through lack of strength. Wings failed due to a lack of stiffness. (True sesquiplanes- V-strutters, notably other Nieuport and Albatros models- are known to have occasionally lost a lower plane due to a lack of torsional stiffness) - but wingstripping seems mainly recorded for the 2 models in question. Since wing reinforcement better weather protection and better-built quality did not fully cure the triplane ills, then there was another factor at work. So what was it?