On the Wing of an Albatros

[Ira_Silverman]

After reading and hearing so much about lower wing failures on Albatros Vee-strutters, especially the experience of H.J. von Hippel of Jasta 5, I’ve built a finite element computer model of the Albatros D.Va wing cellule, with the intent of determining the ultimate torsional load of the lower wing spar and the wing loading necessary to generate such a load.

Thanks to the plans in the wonderful Smithsonian Institution monograph on the D.Va (Stropp), I have full details of the wing and strut structures for this particular model. Generating the numbers will take some time (I’m doing this on the occasional evening and weekend), but all the information I need is available.

What I’d also like to do is determine if the Oeffag built version was actually stronger, as has often been alleged. Can anybody lead me in the direction of details of the lower wing spar of the Oeffag-built Albatros?

One other point has been bothering me. By mid-1917, most of the leading powers had feasible parasol monoplane designs on the drawing boards. Both the Fokker E.V and the Morane A.1 saw limited service by war’s end, and Sopwith had the Swallow ready for introduction. Even Nieuport was shrinking the sesquiplane lower wing to the point where the Nieuport-Delage NiD 622 was effectively a parasol monoplane.

The Albatros sesquiplane design would appear to be a natural for development as a strut-braced parasol monoplane. It would have eliminated the lower wing problem altogether, and the availability of up-rated Mercedes and BMW engines could have resulted in a very potent fighter aircraft.

I have heard nothing of Albatros ever having considered a monoplane design, occupying its design bureau instead with triplane and rotary-engined biplane developments.

Has any of our esteemed colleagues heard of a late war Albatros monoplane design, either in prototype form, or on the drawing board?

Inquiring minds want to know (well, at least one does).

Your finite element technique sounds very interesting. I'm also curious about this as it would apply to other WW1 aircraft, more specifically some of the more rare planes used in WW1.

Is it possible that I could get a copy of your source code.

M.Sc. Applied Mathematics University of Toronto.

[Darryl]

Hi Ira,

>>I’ve built a finite element computer model of >>the Albatros D.Va wing cellule, with the intent >>of determining the ultimate torsional load of >>the lower wing spar and the wing loading >>necessary to generate such a load.

Do you suspend that in a strong, brownian, motion producer?*G*

Seriously, the stress is, correct me if I am wrong, due to a couple of factors. The first is the twist and the second what I would call the "flap" (yes, my aero engineering degree IS still in the mail). The total of these forces can be varied by acceleration and decelaration as well as the pressure caused by G's of turn (which, let's face it, is just another type of accelaration.) As I understand it it is perfectly feasible for an aircraft to break up in LEVEL flight with no control input, if its engine is capable of propelling it to VNE at that attitude.
The forces exceed the strength of the airframe in whatever 'moment' is being applied?

Now given that, it makes sense to this 'enquiring mind' that the force necessary to break the wing would not even have to be sufficient to break the main spar if applied in a vice on the ground? Vis, a glass can be broken by 'high c' notwithstanding that an even higher note will NOT break it?

Now I am an Accountant, not an Engineer ("oh really" they all cry in unison.) but I seem to recall that this is in layman's terms the theory as to why Albatros didn't pick up the failures in their testing process?

Somebody, anybody, save me....

Darryl

[Darryl]

Oh,

Monoplanes...power plus streamlining equals speed, speed plus lift efficiency of the wing equals climb rate.

Engine design / power and wing design had not reached such a stage as to deliver the lift from a single wing of acceptable length/chord desirable for a scout until very late in the war. Two wings meant somewhat less than twice the lift (more drag and the robbing by the top wing of some of the bottom wing's lift effect by its very being there, can't remember the damned name of that effect)but was an acceptable compromise.

Conclusion, the advantages of monoplanes over biplanes at that stage (and well into the thirties) were not significant enough to pursue the 'new order' at the expense of proven designs.

Just a tuppeny 'appeny opinion

Darryl

[Ira_Silverman]

Hi John:

My model is built using Algor Finite Element Analysis software. Please contact me through e-mail and we can discuss this further. I look forward to it.


Hi Darryl:

In simplest terms, wing stresses are totally due to pressure acting along the wing. In level flight, the center of pressure is approximately at the ¼ chord point. When an angle of attack is introduced, that center of pressure tends to move forward. Also in level flight, the total lift is equal to the weight of the aircraft, and the equivalent pressure equals the lift divided by the wing area.

As acceleration ‘G’ increases, so does the effective weight of the aircraft, resulting in proportionally higher effective pressures to keep the craft in the air.

In a multi-spar wing (the upper wing of the Albatros), the center of pressure lies between the two spars (in the Albatros, the front spar is actually the leading edge), and there is little net torque applied to the spar. In a single spar wing (the lower wing of the D.III/D.V series), the center of pressure is ALWAYS ahead of the spar, and a significant torque is introduced, which increases with an increasing angle of attack. Add to this the effect of high G maneuvers, and that spar sees a significant torsional stress.

Bear in mind that this torsional load is NOT the only load the spar sees. Pressure loads along the span will also introduce a vertical shear force and a bending moment (forces applied near the wingtip result in a much higher bending moment and stress than the same load applied further inboard). Forward motion will impose a horizontal shear force due to drag. My hypothesis is that these are “normal” loads. The single spar sesquiplane design introduces the torsional load as an “abnormal” load not present in more conventionally designed wings.

Fortunately my computer model allows multiple load cases, so I will be able to see the effect of individual loads as well as multiple loads superimposed upon one another. At a later point, I’ll look at alternating loads, which will reduce the allowable stress for the spar because of material fatigue.

Several other points:

(1) Wood is actually rather strong from a fatigue standpoint, being non-crystalline, as opposed to metals that are crystalline and do fatigue easily.

(2) The Albatross wing used a box spar assembly. The glue used to assemble the box is yet another variable. For my first go-round, I’ll assume the glue is as strong as the wood itself. Torsion can cause de-lamination at a joint – something to consider later.

(3) Different aircraft woods, e.g. spruce, fir, pine, ply and linden (The German aircraft industry made extensive use of linden) have significantly different properties.

What led me to investigate the monoplane alternative was largely the sesquiplane lower wing, which wasn’t much of a wing at all. In my estimation, based on relative wing area, and interplane interference drag, the lower wing contributed approximately 25% of the total lift. Removing it would reduce total drag, reduce overall weight, increase the efficiency of the upper wing and reduce the overall complexity of the aircraft, in addition to eliminating the A-number-1 problem of the craft to begin with. Fundamentally, the Albatros was already 75% monoplane; why not go the final 25%.

I was under the impression that the aversion to monoplanes was largely due to fear of speed, especially high landing speeds, and possibly stall characteristic.

As a final note (no, not really) I have the feeling that the Albatros-Werke, by early 1918, was just cruising on existing contracts, having given up new design and development once the Luftstreitktafte settled on Fokker as their prime supplier. They appear to have been quite content with building Fokker D.VIIs under subcontract.

Rrrrring. Enough for now. That was Tolstoy on the phone – he thinks I’m moving in on his territory <G>.

VBR,

Ira

[leo]

The Oef Alabatros was a DIII, not a DV or DVa. Austro-Hungarian Army Aircraft Of World War One has information on what Oef did. It obviously worked as Kuk LFT pilots reported that the Oef DIII's could be dived and manouvered in perfect safety. There were few reports of wing failure.

From p 248 "1. German sparw and ribs were appreciably weaker than those on the 53.2 seriers.
2.Ribs between the main and auxilary spar and constructed of heavier plywood. 3. Spar flange thickness is increased from 10 to 20 mm at stress points. 4. Metal reinforcing is added between the main and auxilary spar. 5. The front auxilary spar is prevented frpm twisting by a metal fixture at the fuselage juncture."

leo

[Ira_Silverman]

Hi Leo:

Thanks for the reference. It’s very much appreciated.

I’m aware the Oeffag model was a D.III variant and very succesful, but I am of the opinion that the fundamental wing design (number of ribs, spacing, span, chord, etc) was similar enough that adjusting the effective pressure for the difference in loaded weight should give useful results.

Your excerpt from page 248 was just the sort of thing I was looking for. I suspect item 5 was the most significant difference. If the front auxiliary spar (which was the leading edge and not prevented from rotating about the fixed main spar on the D.Va) was tied by the metal fixture to the fuselage, then the torsional stresses disappear.

I will go on the assumption that if the fix was effective on the D.III, it would be equally effective on the D.Va. Interesting, though, that no mention is made of a change to the spar itself, the implication being that there was none.

VBR,

Ira

I have some additional technical comments, I AM NOT AN engineer, but a formally trained biochemist now working as a computer technician, but believe it or not I know some about this topic. My wife and I build furniture kits from the Bartley Collection (they are made of fine solid cherry wood). These kits are reproductions of Chippendale designs and use bone glue. In talking with Bartley Collection designers they mentioned that bone glue (white wood glue) is far, far, stronger than wood. It often leads to the wood giving way before the glue does. I would wager that the Germans used bone glue. Please factor that the glue would be stronger than the wood. I hope someone can find out exactly what glue the Germans used.

Ira,

Who was H.J. von Hippel, and what exactly happened to his plane? I've heard of him, but I don't know anything about him.

Thanks,
David

[Ira_Silverman]

Hi David:

Hans Joachim von Hippel was a Jasta 5 pilot who, during combat with 56 squadron RFC on 18 Feb 1918, lost the entire starboard lower wing of his D.V, and managed to fly the ship down to a succesful crash landing. Photographs of the crashed plane, including wing spar details, are among the best available of such wing failures.

Von Hippel lived into the late 1960s or early 1970s and often spoke about the incident.

VBR,
Ira