﻿

# Tait-Bryan rotations

The Tait-Bryan rotations, named after Peter Guthrie Tait and George Bryan. are three elemental rotations about each one of the principal axes of a body (but not necessarily the principal axes of inertia). For a craft moving in the positive "x" direction, with the right side corresponding to the positive "y" direction, and the vertical underside corresponding to the positive "z" direction, these three angles are individually called roll, pitch and yaw.

They can be used to describe a general rotation in three-dimensional Euclidean space using usually the order "once about the "x"-axis, once about the "y"-axis, and once about the "z"-axis". They are also called "nautical rotations".

In aeronautical and aerospace engineering they are often called Euler angles, but this conflicts with existing usage elsewhere, because Tait-Bryan rotations have differences with Euler angles described below.

They are intrinsic rotations and the calculus behind them is similar to the Frenet-Serret formulas.

Definition

The three critical flight dynamics parameters are rotations in three dimensions around the vehicle's coordinate system origin, the center of mass. These angles are "pitch", "roll" and yaw:

*Pitch is rotation around the lateral or transverse axis&mdash;an axis running from the pilot's left to right in piloted aircraft, and parallel to the wings of a winged aircraft; thus the nose pitches up and the tail down, or vice-versa.

*Roll is rotation around the longitudinal axis&mdash;an axis drawn through the body of the vehicle from tail to nose in the normal direction of flight, or the direction the pilot faces.
**The roll angle is also known as bank angle on a fixed wing aircraft, which "banks" to change the horizontal direction of flight.

*Yaw is rotation about the vertical axis&mdash;an axis drawn from top to bottom, and perpendicular to the other two axes.

Composition of intrinsical rotations

To perform a rotation in an intrinsical reference frame is equivalent to right-multiply its characteristic matrix (the matrix that has the vector of the reference frame as columns) by the matrix of the rotation

Proof

The composition of rotations in the fixed axes is a left-multiplication, because the usage of matrices as operators (left-multiply rotates all that is at the left of the operator). Suppose that you write the characteristic matrix of the frame as product of fixed axes rotations. Then, let "x"(φ) and "z"(φ) denote the rotations of angle φ about the "x"-axis and "z"-axis, respectively. In the moving axes description, let "Z"(φ)="z"(φ), "X"&prime;(φ) be the rotation of angle φ about the once-rotated "X"-axis, and let "Z"&Prime;(φ) be the rotation of angle φ about the twice-rotated "Z"-axis. Then:

:"Z"&Prime;(α)o"X"&prime;(&beta;)o"Z"(&gamma;) = [ ("X"&prime;(&beta;)"z"(&gamma;)) o "z"(α) o ("X"&prime;(&beta;)"z"(&gamma;))−1 ] o "X"&prime;(&beta;) o "z"(&gamma;)
:::: = [ {"z"(&gamma;)"x"(&beta;)"z"(−&gamma;) "z"(&gamma;)} o "z"(α) o {"z"(−&gamma;) "z"(&gamma;)"x"(−&beta;)"z"(−&gamma;)} ] o [ "z"(&gamma;)"x"(&beta;)"z"(−&gamma;) ] o "z"(&gamma;)
:::: = "z"(&gamma;)"x"(&beta;)"z"(α)"x"(−&beta;)"x"(&beta;) = "z"(&gamma;)"x"(&beta;)"z"(α) .

Therefore rotations in the intrinsical frame can be performed right-multiplying the matrix of the frame, as we wanted to prove.A simpler way to see this is changing the rotation operator in the intrinsical basis to the external basis. To change an operator to a basis given by a matrix P, we have the expresion R'=P−1.R.P and we need its inverse R=P.R'.P−1, and applying this to the rotation matrix we have P'=P.R.P−1.P=P.R

Differences and similarities with Euler angles

The main difference is that the set of rotations defined by Euler angles (precesion, nutation and intrinsic rotation) are commutative, and this set of rotations is not.

Other difference is that, as rotation axis are not fixed, their position depends on the first rotation. This complicates calculus, but allows us to reach any final position with only two of the three elemental rotations [Development of a two-wheel contingency mode for the MAP spacecraft, Scott R. Starin and James R. O’Donnell, Jr. [http://lambda.gsfc.nasa.gov/product/map/dr2/team_pubs/TwoWheel.pdf] ] . For example, a satellite could be stabilized with only two reaction wheels. The existing MAP Safehold/CSS (CSS:coarse Sun sensor) controller can work with only two wheels if the system momentum bias is small.

You can visualize why this can be achieved with an example. An aeroplane doesn't need to perform a yaw to turn. It is enough to make a roll. Then the lift on the wings will force a pitch upwards. At the end, the plane will perform a roll in the opposite direction of the previous one to get the wings horizontal. The whole maneouvre is equivalent to a yaw, but only pitch and roll were performed.

Another good example that any final position can be achieved with only two inertial wheels could be the following picture. With a wheel on the z axis and other in the y axis, we can prove that any position can be reached because starting with z over Z we can perform with a single inertial wheel the first and third Euler angles.

Tait-Bryan rotations, as any other intrinsic rotation, can be composed with each other with no limit. Nevertheless, some compositions of three of them are equivalent to the Euler angles, and share with them their properties. Euler angles therefore can be considered a particular application of Tait-Bryan rotations when the moving frame initial position is the same as the external reference frame and the order in which the rotations are applied is the proper one (if xyz are the reference frame and "XYZ" the moving frame, the first rotation (yaw) around "Z" leaves the line of nodes over "y", so that the rotation around "y" (pitch) may be taken as equivalent to rotation about "N")

As in a moving frame all these things are true only for an instant, we can only assert them in the limit when time goes to zero. Thus, in a frame co-moving with the rotating system, Euler angles are equivalent to a special combination of Tait-Bryan angles in the limit when delta time goes to zero,

Applications

The main usage is in a part of flight dynamics, called attitude control, because the three angles can be controlled separately. If we correct small errors in yaw, roll and pitch individually, then we have achieved the nominal attitude of the aircraft. In case of a unmanned spacecraft, this can be performed automatically with a gyroscope and a reaction wheel controller in each axis.

ee also

*Yaw angle
*Euler angles
*flight dynamics
*attitude control
*Moving frame

References

* Wright Air Development Center Technical Report 58-17: "On The Use of Quaternions In Simulation of Rigid Body Motion", Dec. 1958 by Alfred C. Robinson (Appendix B)

Wikimedia Foundation. 2010.

### Look at other dictionaries:

• Euler angles — The Euler angles were developed by Leonhard Euler to describe the orientation of a rigid body (a body in which the relative position of all its points is constant) in 3 dimensional Euclidean space. To give an object a specific orientation it may… …   Wikipedia

• Flight dynamics — is the science of air and space vehicle orientation and control in three dimensions. The three critical flight dynamics parameters are the angles of rotation in three dimensions about the vehicle s center of mass, known as pitch , roll and yaw… …   Wikipedia

• Yaw axis — is a vertical axis through an aircraft, rocket, or similar body, about which the body yaws; it may be a body, wind, or stability axis. Also known as yawing axis. [cite web|url=http://www.websters online… …   Wikipedia

• List of mathematics articles (T) — NOTOC T T duality T group T group (mathematics) T integration T norm T norm fuzzy logics T schema T square (fractal) T symmetry T table T theory T.C. Mits T1 space Table of bases Table of Clebsch Gordan coefficients Table of divisors Table of Lie …   Wikipedia

• List of aerospace engineering topics — This page aims to list all articles related to the specific discipline of aerospace engineering. For a broad overview of engineering, see List of engineering topics. For biographies, see List of engineers.compactTOC NOTOC AAblation cascade… …   Wikipedia

• Poinsot's ellipsoid — In classical mechanics, Poinsot s construction is a geometrical method for visualizing the torque free motion of a rotating rigid body, that is, the motion of a rigid body on which no external forces are acting. This motion has four constants:… …   Wikipedia

• Orientation (geometry) — This article is about the orientation or attitude of an object in space. For orientation as a property in linear algebra, see Orientation (vector space). Changing orientation of a rigid body is the same as rotating the axes of a reference frame… …   Wikipedia

• Charts on SO(3) — In mathematics, the special orthogonal group in three dimensions, otherwise known as the rotation group SO(3), is a naturally occurring example of a manifold. The various charts on SO(3) set up rival coordinate systems: in this case there cannot… …   Wikipedia

• Conversion between quaternions and Euler angles — Spatial rotations in three dimensions can be parametrized using both Euler angles and unit quaternions. This article explains how to convert between the two representations. Actually this simple use of quaternions was first presented by Euler… …   Wikipedia

• Rotation matrix — In linear algebra, a rotation matrix is a matrix that is used to perform a rotation in Euclidean space. For example the matrix rotates points in the xy Cartesian plane counterclockwise through an angle θ about the origin of the Cartesian… …   Wikipedia