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In the WWII-era and earlier, many (most?) planes had rounded tails, even high-speed and high-performance fighters such as the Bf 109 K, Ta 152 (though it is more square-ish), P-38, Hawker Typhoon, and even the F-80. A notable exception to this trend is the P-51.

Image of a P-38, featuring rounded tails and wingtips

Modern aircraft, on the other hand, tend to have very angular tails. High speed fighters, to airliners, even relatively slow aircraft like the Cessna 152 or Piper Archer have very angular tails.

Image of an F/A-18, featuring it's very angular tails

What is the reason for this change in trend?

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    $\begingroup$ I guess we've found out later that rounding the tip is pointless. $\endgroup$ – user3528438 Feb 13 at 15:32
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    $\begingroup$ @zymhan I suppose where you draw the line between "round" and "angular" is subjective to some extent, as it could be argued that the Ta 152's tail is angular with rounded corners, or round with straight edges. While the P-51's tail has rounded corners, I'm more referring to the overall shape versus corners. The Spitfire and P-38 have almost oval tails, while the P-51 has very distinct angles. $\endgroup$ – flyingfisch Feb 13 at 15:33
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    $\begingroup$ @flyingfisch Oh you're right, the last plane you listed was P-51, and then I saw the image of the P-38 and had a brain fart. Indeed, the P-51 has a much squarer/more angular tail. $\endgroup$ – zymhan Feb 13 at 15:40
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    $\begingroup$ @verandaguy I recall an anecdote about the C172 vertical stabilizer being swept for the looks, albeit after calculating that the efficiency penalty versus an unswept design would be small. Looks sell aircraft as much as performance; especially to low-hour, high-income pilots. $\endgroup$ – AEhere supports Monica Feb 13 at 16:10
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    $\begingroup$ @verandaguy on low speed airplanes yes it is mostly for looks. Cessna switched their whole line except the 180-185 to swept tails in the mid 60s, basically for styling reasons. There was no performance benefit significant enough to justify the massive retooling required. Nobody sweeps horizontal tails of light a/c (the Aerostar is one exception) because nobody cares about the look from below. $\endgroup$ – John K Feb 13 at 16:38
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The rounded or elliptical design was found to be the cleanest aerodynamically in non-compressible subsonic airflow, which is why it's very common on aircraft of that era. While the design is very slippery for speeds of Mach < 0.6 or so, it is more difficult to manufacture, which is why, near the end of the war, Spitfires had the 'clipped' wingtips as opposed to the original pure elliptical planforms.

The modern low aspect, sweptback, trapezoidal tail design is a better choice for higher airspeeds and supersonic flight ie Mach > 0.8 and mitigating flight control flutter due to compressibility effects, the same reasons we use sweptback wing. Note as well that modern fighters use the same kind of planform for their wings and tail planes as they offer a compromise between acceptable supersonic aerodynamics and good trans sonic maneuverability.

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    $\begingroup$ " While the design is very slippery for speeds of Mach < 0.6 or so, it is more difficult to manufacture, which is why, near the end of the war, Spitfires had the 'clipped' wingtips as opposed to the original pure elliptical planforms." I thought the wings were clipped to increase roll rate and low altitude airspeed? en.wikipedia.org/wiki/… $\endgroup$ – flyingfisch Feb 13 at 19:17
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    $\begingroup$ Against this explanation, no modern glider has elliptical surfaces. $\endgroup$ – Martin Argerami Feb 14 at 3:12
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    $\begingroup$ The wings were clipped for the same reason Reno air racer wings were clipped, to make them go faster at low alt and improve roll rate, but mostly go faster. Conversely, for the ultra high altitude interceptor version of the Spit, they made the elliptical tips longer. $\endgroup$ – John K Feb 14 at 4:18
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    $\begingroup$ Elliptical wing planforms have the lowest coefficients of drag out of any planform for subsonic applications. Source: Elger, D., Engineering Fluid Mechanics. $\endgroup$ – Carlo Felicione Feb 14 at 4:38
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    $\begingroup$ @CarloFelicione Actually, wings with elliptical lift distribution have lowest drag coefficients. This can be achieved 3 ways - make the planform elliptical, vary angle of attack from root to tip so that when you graph the lift distribution it is elliptical or change the airfoil from root to tip so the lift distribution is elliptical. The planform solution was the simplest to get right when you don't have CAD. With CAD or just a lot of math you can do it without making the planform elliptical $\endgroup$ – slebetman Feb 14 at 12:10
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Aside from any subtle aerodynamic benefits of elliptical surfaces vs straight tapered ones, the move away from rounded profiles was mostly for ease of manufacture. A rounded profile has much more complex lofting demands, especially for the stabilizer part, and is more expensive to produce. Properly done, there are going to have to be compound curves, requiring skins to be stamped in 3-dimensional dies, which has to be done in an annealed state and heat treated after, with rib flanges bent in mating contours. Compare that to a straight edge surface that only needs skins with simple 2 dimensional bends that can usually be done in the final heat treat state (for larger radius bends).

Some light aircraft, like the Cessna 170, were able to get rounded looking profiles with mostly flat skins by using just a compound curve formed leading edge skin. But even here going to a straight tail eliminated that requirement and a stabilizer could be skinned with a single wrap instead of a separate upper and lower or left and right skins and a die stamped compound curve leading edge skin. As well, think of the reduction in rivet count and labour hours in general.

Over time the straight edged shape also came to be associated with modern, so "modern looking" and "cheaper" in combination become pretty hard to beat and round surfaces disappear except in special cases.

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  • $\begingroup$ While discussing the lower-speed GA part of the equation (the original question appears to be not concerned with those types of planes, but it's still relevant to mention them, as your answer does), it's worth pointing out that marketing has its influence as well: Cessna models like the 172 actually had almost-square tail shapes early on, probably closer in efficiency to an elliptical shape, but later were changed to a swept shape, for no apparent reason other than so it "looks faster". $\endgroup$ – Peter Duniho Feb 14 at 3:01
  • $\begingroup$ @PeterDuniho yes I've been saying that in other posts about swept tails. On low speed airplanes it's mostly for looks. Which is why the horizontal tail is rarely swept; it's the side profile that matters. In fact if anything, sweeping the rudder hinge imparts a slight nose up pitching moment when the rudder is displaced, although it's too subtle to notice. $\endgroup$ – John K Feb 14 at 4:16
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Yes, in the case of light aircraft, backward slanting tails were the direction of marketing departments, not aeronautical engineers. The reason they are not good is that in low speed, high angle of attack such as the pre-landing flair, the rudder is actually increasingly canted out of the wind. It's rearward slant adds to the angle of attack to make the projection of the rudder's area in in the direction of flight smaller. One counter example is the Mooney M20 series series. These aircraft have vertical leading edges on the vertical stabilizer, and the rudder actually slopes forward. This gives the rudder a larger projection in the landing flair, resulting in it having increasing effectiveness. Whether or not it makes the Mooney M-20 series of aircraft look faster is irrelevant. Mooney airframes are noted for low drag, resulting in high speed at lower fuel consumption compared to most other light aircraft designs.

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