## Ferrying from Ontario to Alabama

An L-39 ferry flight took place from CYXU to KGAU. Nice experience of an international hopping flight at 300kts ground speed, crossing the always “interesting” US border, watching the climatic change but also the cultural changes on the way to lower latitudes.

## Journal publication as a lone author

My latest paper just received online release and is expected to get published soon in the printed version of the Journal of Aerospace Engineering. Besides being my first post-PhD journal publication, the other reason of being of certain importance is of myself being a lone author.

“The lone author has all but disappeared. In most fields outside mathematics, fewer and fewer people know enough to work and write alone.” [1]

“The effort and initiative required to publish alone suggests an independent and tenacious scientist — both highly desirable qualities in any researcher.” [2]

References

1. M. Greene, The demise of the lone author, Nature 450, 1165 (20 December 2007)
2. K. Hallock, Qualities of a lone author are beneficial to science, Nature 452, 282 (20 March 2008)
Posted in Aviation Science | 2 Comments

## New year, new goals (2016)

Main goals for the new year still remain on the professional side. Increase of productivity through increase of effectiveness, focus, speed and minimization of wasted working time. In addition, education in some new scientific areas and finally upgrade of my pilot licenses.

This blog is (un)fortunately targeted only over aerospace and technical subjects. However, I would like to express my deepest appreciation to some remarkable individuals that in one way or another have expressed personal interest in myself. I just feel it is not my time yet.

## Super Tucano Qual Eval, Reno NV

A qualitative evaluation flight of the A-29 Super Tucano for the Advanced Trainer and Close Air Support (CAS) roles took place in Reno Nevada. Great opportunity to test fly a high performance turboprop with some highly experienced fighter pilots.

Starting from cockpit evaluation and building up to open loop flying qualities, performance, handling qualities and basic aerobatics work. As part of the tactical assessment a number of simulated CCIP profiles were flown using MK82 bombs. A few “peculiarities” were discovered and discussed with the test pilot students and operational pilots, but not significant enough to affect the overall conclusions.

## Visit to the National Museum of USAF

I have been to the National Museum of United States Air Force in Dayton OH a few times. Each and every time is worth it and every time there are new things to discover and learn.

If Philosophy and Democracy belong by definition to Ancient Greece, Aviation and Space Exploration definitely belong to the United States of America.

## Freefalling and body aerodynamics

Beside the adrenaline rush, skydiving involves some aerodynamic challenges which are not directly evident to the distant observer. Beside establishing and maintaining flight stability which is highly dependent on the arc position and the symmetrical application of the aerodynamic forces around the human body, the free fall velocity (terminal speed) has to be adjusted when considering formation skydiving. A look on basic aerodynamics follows:

$W=mg\\ D_{ff}= \frac{1}{2} \rho V^2 S_{exp} C_{D}$

At terminal velocity condition and considering a vertical free fall ($L=0$):

$W=D_{ff}\\ mg= \frac{1}{2} \rho V_{term}^2 S_{exp} C_{D}$

In order to establish a formation with jumpers of various weights, the terminal velocity ($V_{term}$) has to be the same for all. This requires different body exposed surface ($S_{exp}$) for each jumper:

$V_{term}= \sqrt{\frac{2mg}{\rho S_{exp} C_{D}}}$

For example, for myself being currently on the “heavy side” of the jumpers, the exposed surface has to be larger, which is not directly implied by the increased body mass. In order to do so, arms and legs need to be extended in wider angles without sacrificing free fall stability. Drag coefficient might also experience a slight increase due to the more extended arc position, but the vast increase in drag comes from surface. The “light side” jumpers on the other side need to fall in a less extended position and sometimes even add weight zones to increase their mass…

In other words… sounds like fun!!!

## Calculating angle of attack without an air data boom

Angle of attack ($\alpha$) and angle of sideslip ($\beta$) have always been some of the most difficult parameters to measure precisely. A number of methods has been proposed in the past for measuring those angles with the aid of an inertial reference (IRS/INS) [1], most of which include -to various extent- aerodynamic modelling of the vehicle under test.

The present post reviews a method initially developed and used for the YF-16 [2] where $\alpha$ and $\beta$ values can be derived for both static and dynamic test techniques. Considering the high accuracy inertial systems available today, this is a particularly interesting method as only an INS is required without any aerodynamic modelling and can be used in evaluations of aircraft with no available $\alpha$ and $\beta$ data acquisition -common in some TPS exercises.

Attention: By no means does this suggest that an air data boom is not required, it just reviews a method in case one is not available. There are a number of assumptions involved, which -despite being reasonable- can have an effect on the accuracy of the result.

For this method the data needed are inertia velocities (with reference to Earth axes), wind information and the aircraft Euler angles and rates. Basic assumptions of the simplified steps presented below include that aircraft is rigid and the wind is acting only on the horizontal plane.

First the Earth axes INS velocities ($E$) are converted to velocities relative to the surrounding air mass ($am$) by correcting for wind. (Accurate wind knowledge might be another challenge, but it can either be calculated using other techniques, or derived by the onboard air data computer.)

$V_{x_{am}}=V_{x_{E}}+V_w\cos\psi_w \\ V_{y_{am}}=V_{y_{E}}+V_w\sin\psi_w \\ V_{z_{am}}=V_{z_{E}}$

Then the conversion to INS sensed body axes can be applied using the following matrix.

$\left| \begin{array}{ccc} V_{x\prime_B}\\ V_{y\prime_B}\\ V_{z\prime_B} \end{array} \right| = \left| \begin{array}{ccc} \cos\theta \cos\psi & \cos\theta \sin\psi & -\sin\theta \\ -\cos\theta \sin\psi+ \sin\theta\sin\phi\cos\psi & \sin\theta\sin\phi\sin\psi+\cos\phi\cos\psi & \cos\theta\sin\phi \\ \sin\theta\cos\phi\cos\psi+\sin\phi\sin\psi & \sin\theta\cos\phi\sin\psi-\sin\phi\cos\psi & \cos\theta\cos\phi \end{array} \right| \left| \begin{array}{ccc} V_{x_{am}}\\ V_{y_{am}}\\ V_{z_{am}} \end{array} \right|$

The actual body axis velocities can be calculated by correcting for the displacement of the sensed INS axes from the aircraft CG ($l_x, l_y, l_z$) using the body axes rotational rates ($p,q,r$). The latter can either be derived from the Euler angle rates, or can be read directly from onboard rotational gyros.

$p=\dot{\phi}-\dot{\psi}\sin\theta\\ q=\dot{\theta}\cos\phi-\dot{\psi}\cos\theta\sin\phi\\ r=\dot{\psi}\cos\theta\cos\phi-\dot{\theta}\sin\phi$

$V_{x_B}=V_{x\prime_B}-ql_y+rl_p \\ V_{y_B}=V_{y\prime_B}-rl_r+pl_y \\ V_{z_B}=V_{z\prime_B}-pl_p+ql_r$

Using the corrected body velocities $\alpha$ and $\beta$ can then be derived from the following relations:

$\alpha=\arctan\frac{V_{z_B}}{V_{x_B}}\\ \beta=\arctan\frac{V_{y_B}}{\sqrt(V_{x_B}^2+V_{z_B}^2)}$

The accuracy of the method depends highly in the accuracy of the measured data and would require an error analysis. From similar studies it is estimated that an accuracy of 0.5 deg could be feasible.

References
[1] Zeis, J.E., “Angle of attack and sideslip estimation using an inertial reference platform”, MSc Thesis, AFIT, 1988.
[2] Olhausen. J. “Use of a Navigation Platform for Performance Instrumentation on the YF-16”. AIAA 13th Aerospace Sciences Meeting. AIAA-75-32., Pasedena, CA, Jan 75.