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Many aircraft have the horizontal stabilizers mounted in front of the main wings (this is called a canard configuration); however, to the best of my knowledge, there don't seem to be any aircraft with vertical stabilizers in front of the wings.

Why is this?

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marked as duplicate by Peter Kämpf aircraft-design May 25 '18 at 22:31

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  • $\begingroup$ @Peter Kämpf These might be different questions unless a rudder is the same thing as a vertical stabilizer and possibility the same as practicality. $\endgroup$ – Pilothead May 26 '18 at 20:34
  • $\begingroup$ @Pilothead: Calling any surface forward of the center of gravity a stabilizer is a misnomer. It can only stabilise the aircraft when actively and intelligently moved, like a rudder. So yes, a forward-mounted vertical "stabilizer" is a rudder. $\endgroup$ – Peter Kämpf May 27 '18 at 4:55
  • $\begingroup$ @PeterKämpf So then aren't these different questions? Forward mounted vertical stabilizers aren't used because they don't stabilize, and forward mounted rudders can stabilize if actively controlled. $\endgroup$ – Pilothead May 27 '18 at 21:14
  • $\begingroup$ @Pilothead Where's the difference? Since it makes no sense to add a forward "stabilizer", it has not been done (except for pterodactyls and missiles, in a way - they have actively controlled rudders). $\endgroup$ – Peter Kämpf May 27 '18 at 21:41
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Stabilizers create directional stability - they keep the pointy end pointed forward. To do that, they must be mounted behind the rotational pivot point that the aircraft rotates about, the center of gravity.

So when a airfoil or control surface is mounted forward, in front of the center of gravity, (canard), it cannot, by itself, be functionating as a stabilizer, (even though we may refer to it as a stabilizer). It is, in fact, only a horizontal or pitch control surface.

In this case, (a forward mounted canard), the canard is not creating directional stability, the rear-mounted main wing creates this directional stability (pitch stability) about the lateral axis. The canard itself actually detracts from directional stability. It is there to provide control about the lateral axis.

A vertical stabilizer is there to provide directional stability about the vertical axis (yaw or sideslip stability). You can't mount a vertical stabilizer forward because there is no other vertical airfoil at the back providing directional stability about the vertical axis. Or perhaps it might be more accurate to say you could, but only if there were a larger vertical stabilizer in the back. In which case it would be pointless to put another one in the front. that would be reducing stability and requiring the rear-mounted one to be bigger.

An example here to illustrate the point might be the AIM-7 sparrow missile. It does have it's horizontal and vertical control surfaces in the front. It has fixed wings at the aft end of the missile (which are the stabilizing airfoils), and movable control surfaces somewhat forward of the C.G. It maintains a constant* roll attitude and can maneuver in any direction (left/right/up/down), without rolling or changing its bank angle. It does this by deflecting the forward control surfaces as necessary to create lift in the desired direction. So, it effectively does have it's horizontal and vertical "stabilizer" (actually, control surfaces) forward of its fixed stabilizing airfoils.

*NOTE. although actually, in the F-4 at least, the AIM-7 was programmed to always initially roll and then maintain a roll attitude where its wings and control surfaces are oriented in an "X" configuration relative to the launch aircraft, not as a "+" the way you might imagine.

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From the image you can see that a vertical stabilizer mounted behind the aircraft CG imparts a restoring moment to a yaw disturbance which increases as the yaw movement increases. If the fin were placed on the nose of the aircraft, ahead of the CG, the moment would amplify the yaw disturbance, which is destabilizing.

An actively controlled rudder could be located ahead of the CG, but cost and complexity normally limit this to things like missiles.

Directional Stability

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All the above answers are excellent, but I do need to post this:

enter image description here

This the famed HOTOL, SSTO spacecraft. Note the forward vertical stabilizer! In this case I believe the reason for this layout was that during the nose-high entry the rear of the fuselage had basically no airflow. Why they didn't use two wing-tip mounted fins... well you'll have to ask him.

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    $\begingroup$ +1 for mentioning HOTOL. A major reason was the much smaller vertical needed for control - this saved precious mass! $\endgroup$ – Peter Kämpf May 25 '18 at 22:33
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Because having the vertical fin fwd of the CG causes static and dynamic instability and requires an augmented flight control system in order to prevent the aircraft from spinning. Nature has done this kind of arrangement; birds with large beaks such as pelicans and the extinct Pteradactyl used their large heads as rudders during flight and rely on their central nervous system to provide an augmented control system.

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