Why do modern aircraft tend to have angular tails?

Question

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.

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.

What is the reason for this change in trend?

You say the P-51 is an "exception" but it looks to have a very rounded tail to me. It also seems like older passenger planes (e.g. DC-3, Lockheed Electra and Constellation) had rounded empennages as well, so I'm curious as to what caused the change. The fact that slow GA airplanes also have an angular empennage indicates it isn't just a speed-related design. — zymhan, Feb 13 '20 at 15:28
I guess we've found out later that rounding the tip is pointless. — user3528438, Feb 13 '20 at 15:32
@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. — flyingfisch, Feb 13 '20 at 15:33
@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. — zymhan, Feb 13 '20 at 15:40
@quietflyer why do high-speed fighter aircraft seem to be the first to adopt the angular tail? I doubt their design decisions were based primarily around looks... — flyingfisch, Feb 13 '20 at 15:48
@user3528438 care to elaborate? what was the perceived benefit to rounding the tail that was later found to be pointless? — flyingfisch, Feb 13 '20 at 16:04
@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. — AEhere supports Monica, Feb 13 '20 at 16:10
@AEheresupportsMonica I think that makes sense for GA aircraft, but again, I doubt the F-86 was given an angular tail just because it looked cooler. — flyingfisch, Feb 13 '20 at 16:21
@flyingfisch I'm saying there might have never been any benefit to begin with. — user3528438, Feb 13 '20 at 16:23
@user3528438 if that's the case, I wonder why it took so long for common aircraft designs to adopt the more angular tail. You'd think there would be some aircraft with rounded tails and some with angular tails in the era from 1903 to 1940. In addition I could be completely off-base but it seems like it's more difficult to mass-produce a round tail than an angled one? — flyingfisch, Feb 13 '20 at 16:27
Dupe of this: https://aviation.stackexchange.com/questions/65169/why-is-the-tail-group-of-virtually-every-airplane-swept-instead-of-straight/65172#65172 — John K, Feb 13 '20 at 16:33
@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. — John K, Feb 13 '20 at 16:38
@JohnK I don't think it's an exact duplicate. I'm not referring to swept tails in general, the bf 109 and P-80 both have swept, but round, tails. I'm referring to the move from a rounded tail shape to a sharply angled tail shape — flyingfisch, Feb 13 '20 at 16:38

score 15 · Answer 1 · answered Feb 13 '20 at 18:30

15

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.

answered Feb 13 '20 at 18:30

Romeo_4808N

73,674
7
150
274

3

" 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? https://en.wikipedia.org/wiki/Supermarine_Spitfire_(early_Merlin-powered_variants)
– flyingfisch Feb 13 '20 at 19:17
They may have additional benefits but one undeniable features a clip wings is it simply cheaper and easier to manufacture the things. When you’re a nation fighting for its survival against a hostile power, and needs as many fighters as it can get its hands on, that tends to win out. – Romeo_4808N Feb 13 '20 at 19:57
1

do you have a source on that being the reason? It seems odd that the RAF would start clipping wings to save costs once they were already winning the war – flyingfisch Feb 13 '20 at 20:20
4

Against this explanation, no modern glider has elliptical surfaces. – Martin Argerami Feb 14 '20 at 03:12
2

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. – John K Feb 14 '20 at 04:18
2

Elliptical wing planforms have the lowest coefficients of drag out of any planform for subsonic applications. Source: Elger, D., Engineering Fluid Mechanics. – Romeo_4808N Feb 14 '20 at 04:38
As to their use in manned gliders, one can speculate on that, though there are plenty of scale gliders which make use of elliptical planform wings. It is reasonable to suspect that a rectangular or tapered wing capped with winglets is more aerodynamically efficient and easier to build that an elliptical planform on large scale. – Romeo_4808N Feb 14 '20 at 04:41
@John K and they're lighter and cheaper to build, too. Just at the expense of higher approach speeds and degraded slow speed handling. – Romeo_4808N Feb 14 '20 at 04:50
@flyingfisch the smaller wing span increases the roll rate and reduces mass (thus increasing speed), whether the wing tips are square or elliptical is largely irrelevant for that. – jwenting Feb 14 '20 at 05:21
@MartinArgerami Modern gliders and indeed planes such as the 747 and 777 and A360 achieve elliptical lift distribution by varying the angle of attack from wing root to wing tips - this is more commonly known as washout. This achieves the same effect as an elliptical shaped wing but is far cheaper and easier to manufacture – slebetman Feb 14 '20 at 12:07
4

@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 – slebetman Feb 14 '20 at 12:10
2

And considering RJ Mitchell and his team only had slide rules available to them, and minus another 85 years of aerodynamic research, an elliptical platform was an easy solution here. – Romeo_4808N Feb 14 '20 at 13:15
@JohnK the test results quoted here seem to suggest that the speed increase was marginal but the roll rate increase was massive (at least at low speeds, 105 deg/sec to 175 deg/sec @ 200 mph): https://www.quora.com/Why-did-some-Spitfires-have-clipped-wings-in-WW2 – llama Feb 14 '20 at 17:39

score 10 · Answer 2 · answered Feb 13 '20 at 17:04

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.

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". — Peter Duniho, Feb 14 '20 at 03:01
@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. — John K, Feb 14 '20 at 04:16

score 2 · Answer 3 · answered Feb 14 '20 at 13:44

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.

Why do modern aircraft tend to have angular tails?

3 Answers3