## 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.

## Flying the Commander RC-700

My first flight on the controls of the ITPS RC-700 aircraft for verification of the flight test instrumentation system and a qualitative evaluation. Benign airplane with good flying qualities, though not a great performer.

## Stall speed testing for certification: To idle trim or to PLF trim?

It has been more than once that I have been involved in discussions about stall speed testing for Part-23 certification purposes and, as far as I have experienced, opinions seem to split over a certain part of the testing technique. (To add to this, reference suggest that there have been controversies over stall speed testing even within the FAA organization [1]).

More specifically, “§23.201 Wings level stall” indicates the conditions under which the stall should be demonstrated and 23.201(f)(6) states “Trim: At 1.5 VS1 or the minimum trim speed, whichever is higher.” (same applied for VS0 testing). What is not very clear is whether the regulation require this trim configuration with power idle (off) which would result in a constant descent rate, or trim with power for level flight (PFL) and then bring the power back to idle. Depending which condition is used, the trim setting will be different, with the idle condition resulting in a larger trim tab deflection downwards (reaching even maximum limits), which will result in a larger upwards elevator deflection in order to generate the pitching moment required for equilibrium. So does this small (or not) difference in the trim tab position effect stall speed and is it important?

Running back to basic aerodynamics, for a fixed configuration, stall speed depends on weight, density, and maximum lift coefficient (stall angle of attack). Where does the trim tab get into play? Well, the point is that in certification testing the definition of the stall speed is not always resulting from an aerodynamic stall, but stall speed can be defined by “§23.201(a)(3) The control reaching the stop.” which means that the airplane is actually elevator power limited. So the question now changes to “Does the trim tab position affect the elevator power/effectiveness?”. As in many flight test related questions, the answer is “It depends”. There have been airplanes that have not shown any change in stall speed between max down and max up trim tab position. There have been other (undocumented) cases where a nose down trim tab position provides an increased elevator effectiveness and results in a smaller stall speed than a nose up trim tab position, which would actually mean that we should trim in an idle condition to get the worst case (higher stall speed) scenario – which would be the case in a trimmed approach of an engine-out landing.

Enough analysis for certification purposes, risking to go too far to capture a small difference. The bottom line of this post is that it is my opinion, as well as others as far as I know, for stall speed certification testing at power off conditions, the airplane should be trimmed at 1.5VS at power idle. This would mean that for a stall speed determination at a target altitude, the first descent pass should have trim only purposes and by the time the desired trim tab position is established, power should be added to climb above the altitude band (without changing the trim position) and during the second descent pass on the desired stabilized condition a stall speed test point should be executed at the target altitude.

The next question would be whether trim tab position affects stall characteristics, and while someone might rush to say that those are governed by an aerodynamic stall with the trim tab having minor effect, some undocumented results indicated significant difference [2].

References

[1] Kimberlin R.D., Flight Testing of Fixed-Wing Aircraft, AIAA Educational Series, 2003.

[2] http://www.pprune.org/archive/index.php/t-445920.html, Accessed April 23, 2017.

## AIAA YP Member of the month – April ’17

Glad to be featured in the AIAA Young Professional Committee Newsletter.

http://www.aiaa.org/Secondary.aspx?id=29094

## Night rating complete

Finally! Night rating complete. Best part, the Toronto night tour at 2,000′ MSL (~1,750′ AGL) passing by the CN Tower just above the lighted sky-scrapers. Just magic…

For the last weeks I have been heavily involved in the write-up of technical academic manuals for flight test. Writing of academic manuals (textbooks) is not a simple task, as beside the technical content that has to be included, the most important part is the way it is presented so it can be followed and understood by the students. Beside those primary points, a significant characteristic of an academic manual is the consistency of the technical symbols, equations and figures as it drastically facilitates readability and clearness of the content. I was fortunate enough that during my PhD studies I did extensive work on revision and editing of academic manuals under the supervision of my professor, who was insisting for the slightest technical detail going down to symbolic differences between “ν” and “$v$” – and he was right!
For those who do not know, or for those who know and still insist not to change their habits, the best and only option for technical editing excellence is $\LaTeX$, which I also use in the equations of this blog. The consistency and automation in text formatting and presentation of complicated math equations is incomparable to the mainstream editing tool, which is definitely easier and faster on a short term basis but when you are looking for technical detail it can drive you crazy and even when you finish our documents does not look that good – as with most things in life, the easier option is not the best option. So, for all those out there that want to deliver technical writing excellence, get to use $\LaTeX$, and as soon as you dive in its world, you will comprehend the weakness and inadequacy of all other tools. Peace!