Albatros DIII, why the sesquiplane wing?

[Romani]

I have always wondered at the sesquiplane wing, if it gave so much trouble and was so dangerous in dives, why the Germans bothered with it, and if it gave any advantage to the DIII and successors, what it was?

The advantage of the early Albatros DI and DII was they had enough horsepower to bear the weight of 2 machineguns without hampering their perfomance. The SPAD VII and later the SE5 had even more horsepower and could have mounted twin machineguns if it hadn't been for short sighted design, (maybe dictacted by insufficient machinegun production).

It seems that the DII was a fairly maneuverable aircraft with a good turn radius, so what exactly is so superior in the sesquiplane configuration for the Germans to copy it from the Nieuport 17?

I can think of a few things: less wing surface, less drag, more speed. One single spar and V struts, less weight , better power/weight ratio.

But there doesn't seem to be much difference. DII 886 kilos, 160 ps, DIII 884 kilos, 176 ps. Max speed DII, 175 km/h at sea level, DIII 176 km/h at 1000 meters.

It seems that the improvements on the DIII in keeping engine power at higher altitudes and climb rate are more the result of a improved higher compresion engine than the changes in structure and aerodynamic refinements.

I had thought that the simplified struts and smaller lower wings meant less mass away from the longitudinal axis, so that might result in a faster roll rate. That makes sense in basic rotation momentum physics, but it seems control surfaces effectiveness is a far more important factor than mass distribution.

(example, the 2 long narrow ailerons in the Fokker DrI being more efficient than the six ailerons in the Sopwith Triplane in achieving a quicker roll)

I think that the DIII became more of a climb and dive plane that a dogfighter in turning fights. The DIII was better than the Sopwith Pup and Nieuport 17 because it was usually faster at altitude and could fly higher. Against a SPAD VII it would be the opposite, have the advantage of a lower stall speed and tighter turning radius because the SPAD thin airfoil wings and the interference between them resulted in it being unsuited to a low speed turning fight.

In view of this it seems that the DIII also enjoyed a better rate of climb, despite the superior speed and higher power/weight ratio of the SPAD ( 150 CV for 704 kilos). And of top of all this you have the firepower advantage, of course.

What could be the technical advantage of a smaller lower wing? I have heard that high aspect wings (those that are long and narrow) are more efficient in lifting than a short, wide wing. Perhaps this is what happened with the lower wing? Despite the reduced wing surface, it was almost as efficient as the wider longer wings of the DII? Maybe reduced interference with the upper wing plays a part too.
In conclusion, that despite the smaller wing surface, the DIII retained good maneuverability.

An interesting question is if the Nieuport 17 and DIII designers knew this, or the former just tried it and the latter, noting the success, copied what it obviously worked.

This all is speculation on my part since I know very little about aerodynamics, but I think I can point with confidence one clear advantage of the sesquiplane configuration: visibility. Not only downwards, since there's not so much wing getting in the way, but it must be advantageous to not have your vision blocked by a cage of struts and wires as in the SPAD or to a lesser degree in all other paralell strut planes. So maybe the biggest advantage of the DIII was in a higher situational awareness of its pilots.


Of course, in the final analysis it might very well that the Albatros DIII was nothing extraordinary and it just enjoyed a temporary advantage over underpowered rotaries and obsolete pusher types, and that if the RFC had been equipped with SPADs or the French had been more active, Bloody April would have never happened. The other explanation is that at this time, German pilots were better trained, organized and had better tactics than their opponents.

Maybe the sesquiplane configuration was a folly in the first place, and all the aerodynamic refinement the Germans put into the Albatros (removal of side radiators, raked windtips, rounded rudder and elevators , oval cross section fuselage in the DV, plywood covering creating less drag than fabric cover, nose spinner, single bay V struts ...etc) didn't mean squat because aerodynamic refinement under a speed of 200 km/h don't have much of a impact. By contrast, the SPAD and the SE5a were drag monsters but could overcome it with brute force. It all comes down to the engine, , more powerful engine for a given weight can make the airplane smaller, wich reduces weight and drag, increasing the power/weight ratio and speed in a virtuous circle. Ah, and on top of all that there was the technical advantage of the RAF flat bracing wire, wich allowed the SE5a to get away with murder. (maybe this was also used on French planes like the SPAD ?)

I am coming to the conclusion that bracing wire was the invisible drag killer, causing a disproportionate increase in drag through turbulence that is belied by their apparent small surface and cross section. It didn't matter that the Germans came with such stylized and refined fuselalages as in the Albatros and Pfalz DIII if the bracing wires ruined it.


In conclusion, I am seeking understanding to answer 2 questions:
- 1) What did the Albatros DIII have to achieve air supremacy in the first half of 1917?
- 2) What part did the sesquiplane configuration had in it?

[EricGoedkoop]

Romani -

As far as Albatros adopting the sesquiplane layout, I think it was simply a matter of following orders and Thelen's method of doing so. Throughout the Pioneer Era and WWI, aviation design was a game of follow-the-leader. Success bred imitation - Bleriot's success brought the tractor monoplane into fashion, the Sopwith Tabloid's record-setting performance did the same for the small, tractor biplane. As you know, the Nieuport Scouts gave a very good performance in combat and this led to the German manufacturers being ordered to "copy" the design. Some manufacturers (SSW, Euler) took this directive literally, while Albatros chose to adapt the wing layout to it's already-successful basic design. I doubt the aerodynamical pros and cons were very well understood at the time; I think the philosophy was "If it works for them it'll work for us." A similar approach was taken following the Sopwith Triplane's debut, except in that case the Albatros/Thelen system of evolutionary design was a total miscalculation.

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.

[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

[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. Best to put the wing area in the upper wing and minimize the lower. The biplane configuration was a given due to bracing requirements. A smaller lower wing would also improve visibility downward. What was unknown, and unanticipated, at the time was aerodynamic flutter. The heavily under cambered and lightly built lower wing on the Albatros was particularly prone to flutter. The strong lift vector in front of the spar at high speed forced the leading edge up; material strength resisted the movement and brought the leading edge back down. This up-down sequence happened VERY fast. I would guess the last thing many Nieuport and Albatros pilots probably heard was a low-pitch buzz from one or the other lower wing tip and BAM the wing fails and that's all she wrote.

I also think the effect of rigging on drag is not as great as generally supposed. It was well known at the time that in order to attain the required structural strength a thick wing section was needed to contain a deep enough spar (or spars) to handle the loads. A wing thick enough to obtain the required strength also generated as much, and probably more, drag than a thin wire-braced wing. What seems to be unappreciated at the time was the much gentler stall characteristics of a thick wing with a relatively blunt leading edge. This is one of the major things that gave the Dr.I and D.VII their edge in combat. 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.

[Taz]

Romani- You hit it on the head. The downward visibility was much better.

Terry Phillips
Taz

[StephenLawson]

It was a mimic of the Nieuport 11-27 type layout and the reason was downward visibility.

[Dan_San_Abbott]

Taz;
From what I have read the sesquiplane was the a means of providing rigid bracing to a parasol wing configuration and instead of fairing the beam a narrow wing was used to compilment the upper wing. With the single spar in the lower wing creats a different problem in when in a dive, the center of pressure moves aft and it causes the wing to rotate about the spar. The failure occurs in the attachment of the wing to the fuselage or the V type interplane struts. Pfalz resolved the problem by increasing the chord of the lower wing that would allow a two spar structure. I don't classify the Pfalz D.III/D.IIIa as a sesquiplane, but a biplane.
I understand the the V strut Nieuports had the same problem as the V struts Albatros aircraft.
I had a Great time at the Bar-B-Que, I wish you could have been ther.
Blue skies,
Dan

[Taz]

Dan-San- Sorry I missed the barbeque, too. One of the complaints against the Albatros D.I, which came out virtually simultaneously with the D.II, was the upward and downward vision. With the D.II it was downward vision. Makes sense that a parasol would have the best downward vision and lower drag, and a sesquiplane layout would be a compromise between the two. Funny thing is, the Austro-Hungarians never had any trouble with their Oef D.IIIs and the Series 253 D.IIIs with the 225 hp Daimler were the equal of any fighter the Entente threw at them.

Terry Phillips
Taz

[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.