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.

[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

[Dan_San_Abbott]

Taz:
If you compare the structure of the Oef D.III wings, you will see they were "beefed up" and most likey better made. I have wondered why the Germans did not build Austro-Daimler 225 ps engine for themselves, it was a damn good engine.
Blue skies,
Dan

[leo]

To say the Austrians had no problem with their DIII's is a bit of an overstatement. They did have some incidents of lower wing failure, but they were very rare. Austrian pilots had great faith in their DIII's-Series 53, 153 and 253. They were far and away the best fighters in the AH armory. The could face up to any Allied fighter.

All of the Austro Daimler engines were very good as were the later Hiero engines.

[Taz]

Leo- Cheers, mate. Agree wholeheartedly. Bow to one who has analysed this a bit more thoroughly than I have. Good aircraft, and much less concern about killing yourself if you pushed the envelope than in the equivalent Luftstreitkrafte fighter.
Terry Phillips
Taz