Albatros DIII, why the sesquiplane wing?

[Romani]

Basically, I think it's an issue of personality and philosophy, not physics. If aircraft designers had based their machines on aerodynamic principles - or indeed if they had understood those principles well enough to even be able to design around them - we never would have seen combat aircraft that shed their wings in a dive or stubby low-aspect ailerons. As effective as some of these kites were, I have to believe that it was more by trial-and-error and sheer luck than science and theory. With the D.III, Albatros got lucky - and with their Dr.I they didn't.

Well, let's dont underestimate the engineers of the time. Aeronautics was a experimental science, and much remained to be discovered, sure, but the Wright brothers knew what they were doing, and they had a good grasp of the basics, they used wind tunnels to test their designs, planes weren't designed on the back of a coaster, as Ernst Heinkel exaggerated once! Another German, Hugo Junkers, did indeed knew what he was doing and where he wanted to go. Though of course Junkers was a genius and 20 years ahead of his time.

I agree with the point of imitating success. Also, in wartime a lot of crazy ideas are explored in the off chance that some work, ideas wich would have been rejected in peacetime after thoughtful consideration. Case in point, the triplane madness.

The biggest constraint is the materials and technology available at the time. There are no biplane birds, but wing surface had to be maximized to compensate the low power of the engines, wich in turn brought the added baggage of struts and bracing wire. The Albatros fighters with its rounded lines are a recogniction of the importance of aerodynamics and an effort to improve the airflow, while on the other hand the SE5a sacrifices aerodynamics in favor of expediency, for easier manufacture, and compensates drag with raw power.

It may seem ilogical that the advantages of the monoplane configuration weren't realized, but taking into account planes had no brakes, and a higher landing speed were troublesome, it has a twisted logic to it. Maybe the authorities at the time were right in their distrust of monoplanes as structurally fragile, and that only a solidly braced pair of wings was robust enough. I think of the Fokker Eindekker, a wing that has to be warped to bank can't be safe

So though a large deal of trial-and-error was involved, I believe most of it was science, though imperfect, and that technology constraints played a large influence in design decisions.

[Dan_San_Abbott]

Ladies and Gentlemen:
The seat of the pants designers disappeared when the various governments specified strength analysis and destruction testing to prove the design.
Government physical laboratories NPL in England, Eiffel in France and Gottingen in Germany, and wind tunnels defined airfoils, fuselage shapes to achieve tohe greatest lift with minimum drag. The crazy designs came from those individuals that did not have the engineering training to design a good airplane. The classic the the 'Christmas Bullet". The same is true with the engines. The aircraft were not designed on a by two guys doodling on a table table cloth during dinner. Designs came about in response to a government design specification which stipulated the type of aircraft, engine and guns and load, performance and range.
For example, the Morane-Saulnier Mos27c1, SPAD XV and the Nieuport 28C1 were designs in response to the same STA design specification. What was common with these three machines was the Gnome Monosoupape 150 hp rotary engine. Every company in every country had graduate engineers that
designed the aircraft.
Blue skies,
Dan-San

[Romani]

This thread has raised a lot of questions, thank you all for your interesting contributions. There are so many things to comment on! Here's the next round of comments.

Originally Posted by Tripehound

I think it all started with the belief that the lower wing was less efficient, due to upper wing interference, than it actually was.

Belief or experimental fact? The Albatros fighter career bears a striking paralellism to that of the Messerschmidt Bf 109 of WWII. A fighter ahead of its time, that enjoyed a superiority over technically obsolete opponents (I am referring to the pusher types) and whose superiority against the others (Nieuport 17, Sopwith Pup) was a question of speed and altitude performance, and firepower.
Yet the edge was very narrow, German pilots admired captured SPAD VIIs, and let's not forget the panic that a handful of Sopwith Triplanes caused.


With the DIII, the plane had reached the maximum potential of the design, the DV and DVa were just attempts to keep it going because lack of a suitable replacement (the Pfalz DIII being a dud). I have the feeling that the increase of weight in the DVa (50 kilos) resulted in it losing turning ability (more weight, higher wing loading, higher stall speed , correct?) and becoming more sluggish, handling sacrificed in favor of speed and altitude to go in hand with the "hit and run" tactics used to counter the Allied planes superiority both in performance: outran by the SE5a and SPAD, outturned by the Sopwith Camel, and numbers. It's the same story as with the change from the Bf109F to the G series!

The biplane configuration was a given due to bracing requirements.

Hmm, I wonder if given the limitations of technology and materials, it was difficult to build a monoplane with a robust enough wing. The bias against monoplanes must have had some basis to it.

I also think the effect of rigging on drag is not as great as generally supposed.

Well, this is a revelation. I have read a lot blaming rigging for drag, and claiming that British planes were faster than German ones because they used streamlind flat wires.

A wing thick enough to obtain the required strength also generated as much, and probably more, drag than a thin wire-braced wing.

Aha! This is the explanation of the apparent paradox that the braced Pfalz DXII was as fast or slightly faster than the Fokker DVII (Mercedes engine, 192 km/h vs 185), coupled that with a plywood fuselage having less drag than a fabric covered one, of course.

And this help to explain why the Junkers DI was not what it was supposed to be. (More on this later, or perhaps in another thread)

Pfalz tried fully rigged and “wireless” wings on one of their experimental fighters in 1918 and found no appreciable difference. I also think the British rigged a captured D.VII to see what effect the wires had and came to the same conclusion. Witness all the thin rigged wings post war.

I didn't know this. But you are right, I should have realized it. The last of the biplanes, the Polikarpov I-15 , had rigged wings. Clearly the structural reinforcement was worth the penalty in drag.