The main technique to determine the lifting boundary of an airplane is the Wind-up turn (WUT) (with the Split-S being a second option). This is a mandatory item in producing the instantaneous turn rate (ITR) performance of a fighter airplane. I encountered recently a suggested modification to the WUT that indicated that after establishing the trim shot at the desired target speed with thrust for level flight (TLF), then while establishing the turn at constant targeted M, throttle can be advanced to MAX in order to minimize altitude loss, with no effect on the test results, considering that the lifting boundary is by definition related to wing’s maximum lift capability only. Before adopting or rejecting this approach, let’s see what is the effect of the thrust component in the WUT.

Let’s start the analysis by considering a turn in the vertical pane only. As the figure below shows, in a pull-up, the net thrust exerts a significant component on the vertical stability axis, which relates to the thrust magnitude () and the thrust axis angle ().

The “cockpit g” () is measured on the axis – normal to the flight path – and is related to the total lifting force applied.

For the case of turning on the lift boundary:

The above equation indicates that the load factor in a vertical turn at specific speed, weight and density altitude, depends on the thrust magnitude () and thrust angle (). Similar dependency is true if the “cockpit g” is read directly by the pilot from a body fixed accelerometer (aligned to the axis).

However, the load factor that is used to determine turning performance (turn rate and turn radius) is not the cockpit g, but the “radial g” which is related to the centrifugal force:

Radial g can be related to cockpit g and for a vertical turn it is:

For the case of a WUT which considers maneuvering in the oblique plane, the expressions are the same as they are weight component independent, but the expression becomes:

Using the law of cosines we get:

The above analysis indicates the following:

- Thrust setting affects and . Higher thrust settings resulting to higher g values.
- does not depend on plane of maneuvering, but and the equivalent turn performance parameters depend.

ITR as depicted in standard doghouse plots regards principally maneuvering in the horizontal plane, so as long as we obtain in any maneuvering plane we can translate it to horizontal ITR through standard data analysis. (Radial g in a horizontal turn is given by .)

With the above in mind, the technique of applying MAX thrust during the WUT for the purpose of creating standard ITR curves should be avoided.