## Evaluation flight in the Jet Provost

As part of an ITPS student project on the Jet Provost Mk.4, a qualitative evaluation flight of 1.5 hrs was assigned to me as the supervising instructor. Instead of performing a standard qualitative evaluation flight gathering comments and qualitative ratings, a more flight test engineering approach was followed, targeting the flight towards the collection of quantitative data. Possibly a presentation on the flight to take place in the next Annual SFTE Symposium. Following images include a sample of the raw data collected from portable instrumentation.

## TM demonstration and display optimization

A telemetry (TM) demonstration took place at ITPS facilities. In the mobile control room myself with the SETP Vice President monitoring relevant airplane parameters during a standard training test flight. The technical parts of data transfer and radio comms worked very well, but a number of deficiencies were identified on the monitoring real-time displays, to be fixed in the very near future.

## Back to basics: Vortex lattice method for stability derivative estimation

The Vortex Lattice Method (VLM) roots back to the principles of lifting line theory and is thus also limited by the incompressible, inviscid and irrotational flow field assumption.

Basically the method works on splitting the geometry to a number of panels and calculating the contribution of all panels in terms of velocity potential (φ) on a control point in each panel. The resulting velocity, in combination with the free stream speed (Vinf) and the Kutta condition provide a solution for the vortices’ strength (γ) and through the integration of those, lift and induced drag forces are calculated. An output of the VLM is the stability derivative estimation used for an initial assessment of the aircraft stability and flying qualities.

As with many other subjects, while this method is taught during standard aerospace courses, students most often lack deeper understanding due to not going through the method’s application principles by writing a part of the computational code required to solve a panel method problem.

Below some images of applying the VLM for a B737 model using the AVL program.

## Beach reading on the first man

Back to fatherland for short vacations, reading on the life of the first man and gathering inspiration to continue the trip towards my own moon.

## ITPS Instructors’ photo – 2017

Being part of an amazing team.

## Wind-up turns: TLF vs. MAX thrust and more…

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 ($F_{net}$) and the thrust axis angle ($\delta$).

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

$L+F_{net}\sin(\alpha+\delta)=n_zW$

For the case of turning on the lift boundary:

$n_{z_{max}}=\frac{q C_{L_{max}}}{W/S}+\frac{F_{net}}{W}\sin(\alpha_{max}+\delta)$

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

$\Sigma F_{c}=\cos(\theta-\alpha)W-L-F_{net}\sin(\alpha+\delta)=-n_rW$

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

$n_{r}=n_{z}-\cos(\theta-\alpha)$

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

Using the law of cosines we get:

$n_r^2=n_z^2+g^2-2n_zg\cos(\phi) \Rightarrow$

$n_r=\sqrt{n_z^2+1-2n_z\cos(\phi)}$

The above analysis indicates the following:

1. Thrust setting affects $n_{z_{max}}$ and $n_{r_{max}}$. Higher thrust settings resulting to higher g values.
2. $n_{z_{max}}$ does not depend on plane of maneuvering, but $n_{r_{max}}$ 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 $n_{z_{max}}$ in any maneuvering plane we can translate it to horizontal ITR through standard data analysis. (Radial g in a horizontal turn is given by $n_r=\sqrt{n_z^2-1}$.)

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.