# How do X-shaped control surfaces work?

I watched this video of the Raytheon SDB II and noticed that it seems to have no control surfaces on its wings, just the four fins on the tail. How do these fins work to alter the attitude of the SDB and other missiles or aircraft using the same scheme? For example, if it were to pitch downwards, which fins would need to move and in which direction?

• Why is this question getting down voted? It is clear, specific, and on-topic. It is a good question! Jan 23 '16 at 14:45

First, the precise terms is not fins, but control surfaces. Fins would be fixed but the four rear surfaces can rotate around a spanwise axis. By rotating, they can be positioned at an angle to the airflow, thus deflecting it and creating an opposing force.

Compare that to a V-tail on an aircraft. They work in the same way by creating a force normal to their plane. While this force has a vertical and horizontal component, the horizontal components of the single surfaces cancel each other when all surfaces are rotated by the same angle. Their vertical components will add up, creating a pitching moment.

When the left and right surfaces are rotated by the opposite angle, their vertical force component will cancel while the horizontal component will add up, thus creating a yawing moment which swings the tail around.

If it were to pitch downwards, which fins would need to move and in which direction?

All four surfaces will move trailing edge down, so the resulting force will point up and pitch the whole craft nose down.

• Finally someone answers the actual question! Glad you beat me to it; you are better at explaining these things. Jan 23 '16 at 16:49

The Raytheon SDB (Small Diameter Bomb) has four fins at the rear end for control of the bomb during flight.

Image from defenseindustrydaily.com

The basic operation of the fins for control is similar to the aircraft control surfaces like the stabilizer. The fins are located way from the cg so that any change in their lift will cause a moment at the cg, helping in changing the missile orientation. For example, for turning the misile, the top and bottom fins are rotated.

Image from Principles of Guided Missiles and Nuclear Weapons by USN

Note that, in case of SDB, the entire fin rotates rather than a part, but the principle is the same. For pitching up and down, the fins on either side are rotated (rotate up for pitch down and rotate down for pitch up).

Image from Principles of Guided Missiles and Nuclear Weapons by USN

A number of missiles have 'X' shaped control surfaces so as to mount on the aircraft rails so that they extend on either side of the rail. The operation is similar to what is outlined above- the deflection (rotation) of the control surfaces create pitching moment about the cg.

For 'X' shaped control surfaces, the rotation of control fins cause forces in both the horizontal and vertical axes (as opposed to the '+' shaped ones, where only two can be rotated for creating a pitching moment in one plane); Here, if all the control fins are rotated by same angle, the horizontal components cancel each other and the vertical component causes the missile (SDB) to pitch. For yaw, the surfaces are rotated in opposite directions, resulting in the vertical component being cancelled, and the horizontal components causing yaw.

• The SDB seems to be closer to a V-tail than a conventional one.
– mins
Jan 23 '16 at 1:53
• Having control surfaces exclusively at the rear results in a long arm indeed - bot only for pitch and yaw. What about roll? Is coordinated flight less important for 'aircraft' like this? Jan 23 '16 at 9:48
• Roll control is easily obtained by deflecting all surfaces in the same (relative) direction, canards can be used too.
– mins
Jan 23 '16 at 10:25
• @RobVermeulen They have some roll control, but looking at the picture it is probably not a coincidence center of lift is high with dihedral wing. Jan 23 '16 at 10:29
• Nice graphics, but doesn't answer the question: How do the X-config control surfaces work to alter the attitude, etc Jan 23 '16 at 14:43