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I'm sure there are many other examples like this one: Embraer EMB 120 with an almost straight wing and an obviously swept horizontal stabilizer.

Why is there a difference in sweep angle between the main wing and the horizontal stabilizer, since both surfaces fly at the same speed?

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The tail sweep helps to extend its range of angles of attack.

For both stability and control, it is very important that the tail doesn't stall before the wing in any circumstances (for a classic design; for canards it may be beneficial). Generally, for high positive angles of attack, the horizontal tail already has an edge because it is set at a lower angle of incidence for statically stable airplanes. Plus, the change of downwash with AoA helps.

Still, to delay stall further, particularly when control surfaces deflection is considered, a higher sweep angle can be used. The lift slope vs AoA decreases with the [cosine of] sweep angle, and the stall angle increases. Note that the maximum lift coefficient also decreases, so the tail may need to be sized larger.

You might notice that even for the airplanes with swept wings, the sweep angle of the tail is nearly always higher (typically by 5°), primarily for this reason.

The leverage advantage (mentioned by John K), may be important in practice only for naturally short-tail airplanes, typically with rear-mounted engines. A classic solution is to put the horizontal tail on top of a swept vertical tail. But in other circumstances the additional lever arm of a higher sweep will likely be offset by the lower effect per AoA and thus larger size (as mentioned above), and heavier design due to additional torsional loads.

And let's not forget looks! It's fashionable to have at least the vertical tail sweep. Compare different versions of C175, for example.

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    $\begingroup$ I'm quite sure that fashion is less of a design decision than aerodynamics. $\endgroup$ – J... Sep 4 '18 at 12:17
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    $\begingroup$ @David, in this case I was talking about the vertical stabilizer. Fundamentally, the question is about both: sweep is generally associated with high (transonic) speeds, and otherwise better be avoided, so why would you design a swept tail for a very slow aircraft (like this Cessna)? $\endgroup$ – Zeus Sep 5 '18 at 0:20
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The tail is swept to move the aerodynamic center aft to provide more "leverage" for the tail in generating down force. Same as sweeping the fin, although that is also for looks. If the airplane had a straight fin and straight horizontal tail, the tail surfaces would have to be larger to have the same tail "volume" (area x moment arm) or the fuselage itself would have to be longer.

A bit of a free lunch by providing slightly more tail volume than a straight tail, with almost no weight penalty.

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