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The Messerschmitt Bf-109 had struts holding the stabilizer in place up and including the E version. I marked it with a red circle in the picture below of the 109 E-3 of the Flying Heritage Collection.

Bf-109 E-3 in flight

Bf-109 E-3 in flight (picture source)

Given that Willy Messerschmitt was fanatical about reducing drag, and contemporaries like the Hawker Hurricane, the Supermarine Spitfire, the Curtiss P-40, the Heinkel 112 or the Dewoitine 520 had cantilevered stabilizers, why would he tolerate a strut on the tail of his new fighter aircraft?

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  • $\begingroup$ The same on Nord 2002 (source). $\endgroup$ – mins Nov 1 '15 at 16:36
  • $\begingroup$ Related on PPRuNe. Apparently the struts were removed later, when the tail was redesigned and the horizontal stabilizer was made more aerodynamic (it looks like there was a negative dihedral at the beginning). $\endgroup$ – mins Nov 1 '15 at 16:50
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The design was carried over from the Bf 108. This would've enabled a lighter structure. The ME-109 versions 'B' through 'E' had them, which was finally removed from the 'F' version. It came back in the 'H' version, which was cancelled.

When the struts were removed in 'F' version, it was found that high-frequency oscillation in the tailplane spar was overlapped by harmonic vibrations from the engine, resulting in failure; this required stiffening of the structure.

To be fair, the Hurricane prototype had struts too.

Hurricane Prototype

Source: network54.com

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  • $\begingroup$ Then the question becomes: Why did both the Bf-108 and the Bf-109 have those struts? The RWD-9, the direct competitor to the 108, had a cantilevered tail even though the wing had struts. $\endgroup$ – Peter Kämpf Nov 2 '15 at 8:02
  • $\begingroup$ What about the fact that the struts are on the bottom? Shouldn't they be on top to counter the download from the tail? It looks too thing to provide much compressive strength; wouldn't it buckle? $\endgroup$ – ptgflyer Feb 15 '16 at 23:59
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To understand that particular design decision, some background may be necessary.

In the 1920s, Willy Messerschmitt had started a small aircraft factory in Bamberg with limited economical success. When he requested financial help from the Bavarian state, he was forced to merge with another aircraft factory in Augsburg, into Bayerische Flugzeugwerke. Since few of his designs enjoyed commercial success, he was forced to accept the production of other designs under license to keep the workforce of the combined companies occupied.

The only Messerschmitt design which had been accepted by Lufthansa was the M.20, and a close friend of a Lufthansa director was killed in a test flight of the M.20. When that director was promoted to the German air ministry and became responsible for selecting aircraft for building up the new Luftwaffe, he made it clear to Messerschmitt that he would avoid purchasing his designs because he disapproved of his reaction to that fatal accident of the M.20.

Messerschmitt M.20

Messerschmitt M.20 (picture source)

The most successful type of Messerschmitt's designs was the M.23, a two seater sports plane. It had suffered two accidents in which the tailplane broke off in flight, so struts for the tailplane were added while initially produced aircraft had an unbraced tail.

Messerschmitt M.23

Early Messerschmitt M.23a with ABC Scorpion engine (picture source)

Messerschmitt had gained a reputation for aerodynamically clean and very lightweight designs, and accidents of the M.20 and M.23 in which the tail section failed made pilots wary of the structural soundness of his designs. Nevertheless, his aircraft achieved surprising performance with very limited engine power, so he was unofficially asked to contribute a design when a competition for a new standard fighter was announced in 1934. As an outsider, he had to make sure that his design would be accepted by his critics, so he played it safe and braced the tail, as he had done on the M.37 (which was to become the Bf-108 and was in many ways the dress rehearsal to the Bf-109).

That is the real reason for the struts: Messerschmitt had to play it safe to not threaten his chances as an outsider for a very lucrative contract.

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  • $\begingroup$ But shouldn't that be on top, because the tail produces a download for stability reasons? I can't imagine that a WWII-era fighter would have a lifting tail, it would get so tiring fighting it without fly-by-wire. $\endgroup$ – ptgflyer Feb 15 '16 at 23:57
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    $\begingroup$ @ptgflyer: Yes, but there is little structure to attach it to. In a taildragger the vertical cannot be mounted very low, it has to be on the upper half of the fuselage. Now you get more fuselage structure below, and attaching the strut to the vertical means beefing this up. Many early tails were braced on both sides, so there was always one loaded in tension. $\endgroup$ – Peter Kämpf Feb 16 '16 at 7:11
  • $\begingroup$ Hmm, never thought about the ground clearance. Still, wouldn't it be better to mount the other end on the vertical stabilizer? $\endgroup$ – ptgflyer Feb 16 '16 at 18:00
  • $\begingroup$ @ptgflyer: Look at the struts of the M-20: They are more for stabilizing the vertical than for bracing the horizontal. With a propeller in front, you will have asymmetric loads, and those need to be carried into the fuselage. So it is better to use lower struts which can directly lead the torque into the fuselage tube, the only thing which provides torsional stiffness. Relying on upper struts only would need a much stronger and heavier vertical. $\endgroup$ – Peter Kämpf Feb 16 '16 at 19:58
  • $\begingroup$ Oh, I didn't know that it was for the vertical stabilizer. $\endgroup$ – ptgflyer Feb 18 '16 at 15:54

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