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?