Don't aircraft use symmetrical tailplanes so as to not produce lifting forces when not needed? Im confused by people saying that they use upside-down cambered airfoils.
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$\begingroup$ Why are you confused? Do you think cambered airfoils have no zero-lift angle of attack? (assuming zero-lift is required in horizontal flight, which is not the case) $\endgroup$– minsCommented Jul 8 at 15:33
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$\begingroup$ What do you mean with "upside-down cambered airfoil"? Do you mean reflexed airfoil? $\endgroup$– sophitCommented Jul 8 at 16:57
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$\begingroup$ Very related: Does the tailplane have a positive/negative camber? $\endgroup$– Peter KämpfCommented Jul 8 at 17:25
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$\begingroup$ @sophit My fault, I meant a cambered airfoil turned upside down. $\endgroup$– ageek245Commented Jul 8 at 18:57
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$\begingroup$ @mins Okay, I'm probably stupid, but wouldn't that angle be totally perpendicular? or otherwise at stalling level? $\endgroup$– ageek245Commented Jul 8 at 19:07
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2 Answers
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Most statically stable aircraft with the tail aft have a download on the tail. The most efficient way to do that is with a horizontal tail with a cambered airfoil -- placed upside down. This configuration is quite common.
You will sometimes see symmetrical airfoils used, but this is not because they run at zero load. These horizontal tails are typically placed at negative incidence relative to the wing (the relative incidence between the wing and the horizontal tail is called declage, positive when the wing is at larger leading edge up angle).
I do not know of any aircraft whose vertical tail is not a symmetrical airfoil. Some vertical tails (on propeller aircraft) are mounted at an angle to build in some offset for p-effects.
Edited to correct that negative tail download is not absolute. Thanks Peter Kämpf.
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$\begingroup$ The Me-109 had a cambered vertical tail. When MBB built a 109 G from parts of a Spanish HA-1112, they initially used a symmetrical airfoil on the tail, only to later find out how the original tail looked. When the old symmetrical tail had been switched for the cambered one, takeoff characteristics greatly improved. $\endgroup$ Commented Jul 8 at 17:21
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1$\begingroup$ "Statically stable aircraft with the tail aft always have a download on the tail" - no, that is not always true. Many gliders have positive tail lift at low speed. $\endgroup$ Commented Jul 8 at 17:23
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$\begingroup$ @PeterKämpf Interesting. Won’t any cambered airfoil create a torque in which the H-stab needs to provide downforce to counteract it? (Probably depends on if the wing’s airfoil has camber) $\endgroup$– WyattCommented Jul 9 at 3:17
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$\begingroup$ @Wyatt Like mounting the vertical tail at an angle, I'm sure using a cambered vertical tail was done to offset P-effects. P-effects is a generic term for aerodynamic effects of the propeller slipstream on the aircraft. Not only does the propeller increase the velocity of the streamtube, it also adds a significant swirl component to the velocity. That swirl hits the lifting surfaces as a local change in angle of attack that can cause the aircraft to yaw or roll. $\endgroup$ Commented Jul 9 at 5:05
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1$\begingroup$ @Wyatt A counter rotating center-line prop would. Likewise, many multi-engine propeller aircraft use counter rotating propellers. The effect of camber or incidence of the vertical tail on the load distribution of the horizontal tail will be very small -- but it is added in to try to reach neutral behavior (when including P-effects). So we don't really worry about breaking it all down, but adjustments seek neutral overall behavior. $\endgroup$ Commented Jul 9 at 18:04
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I think your doubts arise from confusing the aerodynamic characteristics of a cambered airfoil versus an uncambered aka symmetrical airfoil.
The following plot shows the lift coefficient for a symmetrical and a cambered airfoil i.e. the classical NACA 0012 and NACA 4412 respectively (source):
The main aerodynamic effect given by cambering an airfoil is basically to shift the lift curve upward. Comparing the previous two plots you can see how the $C_{l_{max}}$ increases from some 1.3@17° for the NACA 0012 to 1.5@15° for the NACA 4412 while the plot intercept the x-axis at 0° for the symmetrical airfoil (obviously) and at some negative value for the cambered one (-3°).
(Even if it is not shown, the plot for the symmetrical airfoil continues - symmetrically - at negative AoA reaching -1.3@-17° while the NACA 4412 should reach -1@-19°).
So the main reason to camber an airfoil is to achieve higher $C_{l_{max}}$ @ lower AoA.
What type of airfoil should be used for horizontal tailplanes?
The main purposes of the horizontal tailplane is to trim, to manoeuvre and to give stability around the longitudinal axis of the airplane.
Being on the tail is a sufficient condition to provide the airplane with stability, no matter if the airfoil is symmetrical or cambered. Conversely, to trim and to manoeuvre the airplane both a positive and a negative lift is normally needed with a zero (or even slightly positive) lift being the ideal condition: this implies that in general a simple symmetrical airfoil like a NACA 0012 or 0009 is a good choice for any horizontal tailplane.
