Monoplane vs Biplane

[CjBobrow]

Good day all... particularly anyone who wishes to indulge in responding to this query...

Looking for information that details why during the war the move from monoplane to biplane was pushed.

I've seen passing reference to maneuverability, structural, flight characteristics, field of vision, etc... I'm hoping that someone has written about this and looked into the question more closely than anecdotal remarks.

Any references would be appreciated

[Volker_Nemsch]

.
… soon made the experience that the Taube type aircraft in use (A-class) were inferior to the newest B-type biplanes from Aviatik and Albatros.

While it was no problem to add a 150 or 160 hp engine to one of the new biplanes, this power would have overstrained the relatively light structure of the Taube type aircraft (including the slightly more robust Stahltaube ("steel pigeon")).

Also the Fokker and Pfalz Eindeckers were made to be used with a light rotary engine. The 160 hp twin-row Oberursel engine showed that there was no future for this lightweight pre-war aircraft design.

In addition there was the experience that the newer biplane 2-seaters and fighters were more suitable for the rough handling along the frontline, due to the better stability of their additional struts and bracing.

For the German side the "old" monoplane idea again became an interesting concept after the successful development of the cantilever wing, as used in connection the Fokker D.VII and later the all metal monoplanes made by Junkers, but by then the Allied side had an immense advantage to produce high-performance engines in large numbers. So the Allies were able to use older concepts for a longer time by simply adding more power to structurally reworked biplanes.


Just an idea...

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[ViktorRenquist]

I don’t think we should discount the prejudice against monoplanes, either; most especially that of the RFC. The rejection of the Bristol M1C and D was just that, given that it beat all the listed criteria for ‘Scout’ aircraft

[AndersJ]

From a flight performance perspective the monoplane is superior in every way: For a given wing loading, usually determined by the required landing speed, the monoplane will give you a higher speed due to less drag and in addition better climb, turn and ceiling performance due to lower induced drag.

As far as I have been able to determine, what held the monoplane back until late WW1 was an erroneous belief that the thick wing profiles necessary for cantilever wings had more wing profile drag than the thin profiles typical of the time. This was due to wind tunnel testing at the time being done at too low wind tunnel speeds (e.g. circa 9 m/s in Göttingen “low speed” tunnel) with too small models resulting in a low Reynolds number (Re) which “hid” the benefits of a thick profile for many designers at the time (Not only in Germany but in France and Britain as well) since thin profiles looked good and thick looked bad at these far below actual in real life Re number tests.

This in turn brings up an interesting question: What inspired Hugo Junkers to design the thick winged J4 etc. (preceding Fokker’s V-series monoplanes) in the first place? As I understand it the “high speed” wind tunnel (speeds of circa 30 m/s) in Göttingen had not yet been commissioned at this time so was Junkers simply willing to accept the slightly higher wing profile drag indicated by low speed wind tunnel tests to do his cantilever designs or did he somehow have prior knowledge of the Re effects? If so does someone know the story here?

Best regards,

Anders

[YavorD]

May be I see the question from a somewhat different angle

An early examples of a monoplane design were quite successful (Antoinette, Bleriot, Nieuport II and IV, Morane Saulnier, Bristol, Etrich and other Taube designs, just to name a few) but the biplane wing cellule provided rugged and practical solution with a good strength for a given weight and not too much excess drag in comparison with the wire-braced monoplanes.
The biplane wing cellule was the preferred solution until the "internally-braced" cantilever wing design was worked out and perfected as a wooden or metal structure.

The answer was not obvious from an aerodynamics point of view either. The rules of the scaled down experiment (including Reynolds number effect) were not fully understood and not widely accepted but the research was going on and results being accumulated pending comprehension. Even before the war, an experiment on a long straight stretch of railway with a Bleriot monoplane rigged in a flight attitude on a railway car and driven to a meaningful speed was done. Quite large whirling arms were used for propeller tests, too.

The theoretical research was not without problems, too. A 'perfect' (inviscid) fluid approach produced a zero drag theoretical result and it took time until Prandtl's practical approach was accepted leading to a design of thick wing sections and a cantilever wing.

Regards,
Yavor

[CjBobrow]

Originally Posted by YavorD

May be I see the question from a somewhat different angle

An early examples of a monoplane design were quite successful (Antoinette, Bleriot, Nieuport II and IV, Morane Saulnier, Bristol, Etrich and other Taube designs, just to name a few) but the biplane wing cellule provided rugged and practical solution with a good strength for a given weight and not too much excess drag in comparison with the wire-braced monoplanes.
The biplane wing cellule was the preferred solution until the "internally-braced" cantilever wing design was worked out and perfected as a wooden or metal structure.

The answer was not obvious from an aerodynamics point of view either. The rules of the scaled down experiment (including Reynolds number effect) were not fully understood and not widely accepted but the research was going on and results being accumulated pending comprehension. Even before the war, an experiment on a long straight stretch of railway with a Bleriot monoplane rigged in a flight attitude on a railway car and driven to a meaningful speed was done. Quite large whirling arms were used for propeller tests, too.

The theoretical research was not without problems, too. A 'perfect' (inviscid) fluid approach produced a zero drag theoretical result and it took time until Prandtl's practical approach was accepted leading to a design of thick wing sections and a cantilever wing.

Regards,
Yavor

My understanding is that the Junkers team (1914) developed the thick wing profile independently of the Göttingen research which came later wonder if this is correctly stated.

Also the issue of monoplane and downward visibility has been stated though I'm not sure if this was in fact an issue or post change to biplane rhetoric.

[AndersJ]

Originally Posted by YavorD

May be I see the question from a somewhat different angle

An early examples of a monoplane design were quite successful (Antoinette, Bleriot, Nieuport II and IV, Morane Saulnier, Bristol, Etrich and other Taube designs, just to name a few) but the biplane wing cellule provided rugged and practical solution with a good strength for a given weight and not too much excess drag in comparison with the wire-braced monoplanes.
The biplane wing cellule was the preferred solution until the "internally-braced" cantilever wing design was worked out and perfected as a wooden or metal structure.

The answer was not obvious from an aerodynamics point of view either. The rules of the scaled down experiment (including Reynolds number effect) were not fully understood and not widely accepted but the research was going on and results being accumulated pending comprehension. Even before the war, an experiment on a long straight stretch of railway with a Bleriot monoplane rigged in a flight attitude on a railway car and driven to a meaningful speed was done. Quite large whirling arms were used for propeller tests, too.

The theoretical research was not without problems, too. A 'perfect' (inviscid) fluid approach produced a zero drag theoretical result and it took time until Prandtl's practical approach was accepted leading to a design of thick wing sections and a cantilever wing.

Regards,
Yavor

Always good with multiple angles

However, the “successful” monoplanes you mentioned had two major flaws: One is that they had the same thin wing profiles as the biplanes of the time with an excessive camber making then aerodynamically inefficient except in a very narrow angle of attack (aoa) range as compared to round nosed thick wing profiles that have a wider good aoa range. The other was structural: Since they were monoplanes with a rather short bracing post, the low bracing angle led to high compressive loads on the spars. The only way around that is building hefty heavier spars that weight a lot or limiting the permissive load factor, neither of which are good design features.

From the aerodynamic standpoint I have to disagree: The cantilever monoplane is aerodynamically superior since for a given wing area you get a longer span leading to less induced drag and you do away with the bracing wires which even though the dimensions may be small cause very high drag. As a rule of thumb, a round wire causes as large drag as a wing profile 10 times as thick.

Inviscid fluid flow is not useful for practical aircraft design and is more of a theoretical exercise which was also understood at the time and why they tested in wind tunnels. Regarding Prandtl influence on the thick airfoils developed in Germany at the time, as far as I know the thick Göttingen profiles stem from trials on samples of airfoils submitted by German aircraft manufacturers like Junkers and Fokker and not the other way around.

Best Regards,

Anders

[AndersJ]

Originally Posted by CjBobrow

My understanding is that the Junkers team (1914) developed the thick wing profile independently of the Göttingen research which came later wonder if this is correctly stated.

Yes, this is my understanding as well and why I’m so interested in understanding how Junkers came to develop his thick cantilever wing designs. In Michael Tate's excellent book "Fokker Design & Development to 1919" he references a US patent from 1911 already with thick profiles so one wonders were Junkers got the inspiration/insight?

Best regards,

Anders

[YavorD]

Originally Posted by AndersJ

Always good with multiple angles
...
Inviscid fluid flow is not useful for practical aircraft design and is more of a theoretical exercise which was also understood at the time and why they tested in wind tunnels. Regarding Prandtl influence on the thick airfoils developed in Germany at the time, as far as I know the thick Göttingen profiles stem from trials on samples of airfoils submitted by German aircraft manufacturers like Junkers and Fokker and not the other way around.

Best Regards,

Anders

I am trying to stress two aspects of the problem.
(1) A good working theory is a necessity to comprehend the experiment and to look for an improved technical solution. Inviscid fluid models are most used even today with a better understanding of their limitations.
(2) The Prandtl theory provided a practical tool to analyze and calculate lift and drag of an aircraft and to study the design before building it - no matter monoplane or biplane and much faster even in comparison with a wing tunnel test of a scale model.

I have no answer to the question where the Junkers team got the idea about thick wing sections. Prof. Junkers was already a respected authority in thermodynamics and engineering mechanics but not specifically in applied aerodynamics.

Regards,
Yavor

P.S. Patent Application filled 1911 (based on a German Patent from 1910) ...

[CjBobrow]

Originally Posted by YavorD

I am trying to stress two aspects of the problem.
(1) A good working theory is a necessity to comprehend the experiment and to look for an improved technical solution. Inviscid fluid models are most used even today with a better understanding of their limitations.
(2) The Prandtl theory provided a practical tool to analyze and calculate lift and drag of an aircraft and to study the design before building it - no matter monoplane or biplane and much faster even in comparison with a wing tunnel test of a scale model.

I have no answer to the question where the Junkers team got the idea about thick wing sections. Prof. Junkers was already a respected authority in thermodynamics and engineering mechanics but not specifically in applied aerodynamics.

Regards,
Yavor

P.S. Patent Application filled 1911 (based on a German Patent from 1910) ...

Colin Owers book https://www.goodreads.com/book/show/...rplanes-of-wwi among others written as well as a variety of research paper I have read show that Hugo Junkers assembled a remarkable team including Theodor von Kármán https://www.centennialofflight.net/e...arman/TH21.htm to look into the question of flight dynamic and wing design in particular building a quite sophisticated wind tunnel