Airfoils and Lift

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.

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?

I began by looking for a common factor. What is it that both aircraft possess which can cause almost identical failure in a wing- and why only the top plane? There are in fact, 2 unusual structural features present in both. Firstly, the main spars are very closely spaced so that the rib noses project unusually far forward of the spar group. The N28 spars are closely spaced, but maintain an orthodox drag-bracing arrangement of steel tube and piano wire. The Dr.I located the spars with a small separation, so that plywood closing-skins top and bottom formed a single-spar system, accounting for both drag and to a limited extent, torsion.

The other critical feature present in both aircraft was the use of a plywood leading-edge contour panel. This was relatively unusual in WWI. British aircraft seem not to have used it at all, preferring intermediate riblets as leading-edge support; and from a quick appraisal of my library, I have identified only 5 aircraft which had this feature (I don't suppose this to be at a definitive.). These are the Pfalz D.XII Fokkers Dr.I, D.VI and D.VII, and the Nieuport 28 (possibly also the 27).

Some aircraft wings were, of course, totally skinned in sheet plywood or aluminum; but with these exceptions, at least, complete fabric cover was the norm. The use of plywood leading-edge covering presents a problem in the attachment of fabric since stringing (ie, the through-wing stitching normally used) would be required to stop at the plywood-covered surface. This may account for the fact that both the triplane and N28 are reported as originally having the fabric tacked to the rib flanges rather than being sewn (which was considered to be the correct way). The fabric attachment itself is therefore suspect but the test still remains; why only failures of the upper wing? If the fabric attachment was the critical factor, then failures could have occurred in any wing with this feature, which would have included lower planes of both the triplane and the N28.

Both aircraft have structurally suspect features in their wing leading-edges. In the case of the N28, the long rib-noses would produce large bending stresses (during violent manoeuvres) at their main-spar attachment locations. Large bending stresses can have attendant large shear stresses; and on the N28, these would exist in the thin poplar rib-webs (typical of the period). This is a very risky arrangement, since timber is not particularly strong when subject to shear loading along the grain - plywood is much better. (The N28 rib-noses had very little shear material anyway)

The other suspect feature is that of the omission of rib-capping referred to in the recent WWI AERO article. These details appear peculiar to the N28, and are at the most extreme in the upper wing. There is little doubt that the upper wing leading edge was simply of marginal strength; and at first sight it seems odd that sandloading did not reveal this weakness. But of course this reveals a weakness of sand-loading. The chordwise distribution of lift, at high angles of attack, will not normally be represented by a heap of sand, since dry sand slumps to approximately 45 deg- forming a triangular load distribution with a centrally-located center of gravity. (This can be modified within limits by constructing walls along the wing edges.) Sandloading therefore successfully tests the wingspar adequacy, but is insufficient to the task of testing the rib nose strength (and remember that here we have 2 aircraft which resolutely held on to their spars, whilst liberally shedding secondary structure). This proof-loading problem is exacerbated by the fact that wing lift (particularly at large angles of attack) is largely generated by the negative pressure zone existing on the forward upper surface (see Fig 18- taken from SIMPLE AERODYNAMICS (1929), by Charles N Monteith.).

The critical structural requirement under these loading conditions is to have adequate "peel" strength between the upper skin and the substructure (ribs and/or stringers etc). Both the N28 and the Dr.I were deficient here. The Nieuport was devoid of rib cap-strips or spanwise stringers at the critical location; the Dr.I leading-edge plywood was severely cut away at each rib, had no supporting stringers, and had only minor connection to the main spar. With this arrangement, a significant amount of the local lift- would have been transmitted in a peel condition from the plywood skin to the supporting ribs - there was no other load-path. Again, this is a very unreliable form of joint. Today, the attachment of wing skins to substructure remains a critical factor; in fact, where fuel is carried inside a wing much of the wing design is overridingly determined by this consideration.

So, the Nieuport had a weak upper-wing leading edge and larger chord to boot. This could (as suggested in the WWI AERO article) be the complete answer to the N28 failures. But the Triplane had the same design condition on all wings, but only the top wing ever failed. So there was something else.

It is not common to see a biplane or triplane wing cellule in which equal-chord wings are of differing span, although some famous aircraft such as the BE- 12, RE-8 and Curtiss Jenny are exceptions. Typically, where an upper wing is of greater span, it is often of greater chord also. This has the virtue of approximately maintaining constant aspect-ratio for each wing in the complete wing system. (To what extent this represented a design objective at the time I have no information.)

The fact that real wings are of finite span (as opposed to the theoretically infinite span wing which is implicit in aerofoil section data) means that a real wing will attain a particular lift coefficient at an angle of attack somewhat greater than that apparent from he section-data. It also follows that wings of differing aspect ratio, but identical section, will generate different lift-intensities, to one another, when operating at the same angle of attack.

The Dr.1 had aspect ratios of 6.8, 5.9 and 5.1 for the upper, middle and lower planes respectively. The wing section (tested as the Gottingen 289 section after the war) had a maximum lift coefficient of about 1.4. Making estimates for each of the triplane wings (working as independent surfaces), the planes would require 19.2, 20 and 21 degrees respectively to reach the maximum lift coefficient. When working at the same angle of attack (as in the aircraft alignment), the upper wing would produce a lift intensity about 9% greater than the lower wing. So could aspect-ratio be the cause of the Triplane wing failures? Well no, I am afraid not. A 9% increased lift intensity cannot be considered sufficient to always fail the upper wing before one or the other planes. Variations in material strength and build quality would both have similar (or greater) tolerance, which would occasionally bias the failure to one of the other planes. There has to be something else - something more emphatic.

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