Because in most aircraft the 4 forces do not act through one point, they are slightly offset. When the forces are offset they cause a couple, the tail produces another couple (a down force by a moment arm to the CG) to equalize the moments.
It is just an upside down wing, or a symmetrical airfoil with a slight negative incidence on other aircraft. On other aircraft they might put fuel in the tail which also produces a force via additional weight.
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The horizontal stabilizer is just a wing that produces lift in the downward direction relative to the airplane (aerodynamically, it's basically a wing that's attached to the airplane upside-down. So as you're flying along, it generates force downward.
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Roseflyer From United States of America, joined Feb 2004, 8737 posts, RR: 52 Reply 3, posted (9 months 3 days 21 hours ago) and read 2035 times:
It changes throughout a flight. It's really a spectrum. Giving a specific force is very hard, so I'll just give an angle. Typical horizontal stabilizer is trimmed at 1 to 4 degrees trailing edge up relative to the fuselage (please not that this is not the same as units of stabilizer trim).
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KPWMSpotter From United States of America, joined Dec 2006, 325 posts, RR: 2 Reply 4, posted (9 months 3 days 21 hours ago) and read 2034 times:
On a perfect ideal airplane there won't necessarily be any tail downforce.
The elevator and the horizontal stabilizer exist to counteract any un-balanced moments created by the aircraft's aerodynamics and loading. If the CG and the center of pressure are at the same location, no additional downforce is required to balance the aircraft. Since this ideal case almost never happens, the horizontal stab and elevator create down (or up) force as needed.
On most light aircraft the horizontal stabilizer is attached at an angle of incidence relative to the airstream, this angle is determined so that minimal elevator deflection is required to trim the aircraft at the expected cruise condition. Larger aircraft have the ability to adjust this angle as needed to suit the flight conditions (see Mach trim). Other aircraft may have a cambered horizontal stab, which uses an airfoil to generate the required downforce. Other simple aircraft only rely on the elevator and trim tab, using the control surfaces to effectively camber the horizontal stabilizer. This is especially apparent on the Cessna 182, where the rudder horn is bent to reflect the normal trimmed position of the elevator (ie, not aligned straight with the stabilizer.)
Down force itself is created in the same way a wing generates lift; the camber or AoA of the stabilizer produces an aerodynamic force. The only thing that makes downforce different than lift is the fact that it is facing down...