Back to basics – Pre-TPS course 2015

pre-course class

Refreshment course starting in order to prepare the new class students for the demanding flight test year. A 5-week accelerated aeronautics course with calculus, trigonometry, statistics, series, ODEs, root-locus, aerodynamics, compressible flow, aircraft design, propulsion, materials strength, navigation principles and so on, as well as some flying, aims in preparing the pilots and engineers for the theory of the flight test principles and data analysis that follow. Undoubtedly I also find very valuable going again through the basics as every time the understanding goes deeper and the connection of the number crunching to the physical meanings becomes stronger.

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Thank you and happy new year

thank you

Through this post I would like to express my thankfulness to some individuals who think and pray for me. I do feel your thoughts and your prayers as I go through my journey. I don’t forget anyone and I smile every time. Thank you so much!

Wishing you all the best, and a very happy new year! :-)

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One year further, one year closer…

A demanding but successful year reaches an end and already looking forward to the next one, while back in Greece.
During this year’s graduation ceremony and seminar at ITPS I had the pleasure to meet fellow flight tester Colonel (HAF) Apostolos Doukellis.  I was highly honored that Apostolos presented me with a beautiful plaque which carry’s an aircraft aluminium patch specially painted with the emblem of the Hellenic Air Force.


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Abstract accepted at NATO UAS Flight Testing Symposium in Ottawa, May 2015


My submitted abstract was accepted this week and Ι will be presenting at a NATO Symposium in Ottawa next May. Unmanned Air Systems are becoming more widely used by NATO in the major conflict areas around the world. Content and more details can not be released at this time.


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Propeller effect on yawing and rolling moments in a sideslip

This post was triggered by a relevant discussion on the subject and the asymmetries in a SHSS that might be encountered in a prop aircraft.

There are a number of different propeller induced effects on the weathercock stability (Cn_beta) and dihedral effect (Cl_beta) of an aircraft. Although no rule of thump is  applicable, the following provide some general tendencies.

Yawing moments (Prop effect on Cn_beta)

i) Prop normal force
Due to the airflow momentum directional change in a sideslip, the prop normal force (or sideforce)  is the most significant contributor to the yawing moments. For the case where prop is fwd of CG the force is destabilizing both for left and right sideslip. No significant asymmetries are expected due to this effect.prop_sideslip
ii) Spiraling slipstream
It is the result of the air circulating around the aircraft because the propeller imparts such motion to it and its magnitude and effect varies greatly between aircraft designs and flying conditions. The rotating air changes the direction of the local airflow at the side of the fuselage and vertical stabilizer, causing a yawing moment to the left (for clockwise prop rotation).
During wind-tunnel studies spiraling slipstream yawing moments are found to be much more significant to other asymmetric prop phenomena (p-effect). In straight level flight this asymmetry is counterbalanced by rigging, in side-slip flight some asymmetry is expected.

iii) P-effect
When the airflow into the propeller is n’t perpendicular to the propeller plane or disc, the thrust produced is n’t symmetrical about the disc. Again this varies significantly with flight conditions. Its asymmetric effect during most flying conditions is expected to be smaller than slipstream, and it could be argued than due to side-slip (side AoA) the p-effect is now encountered over the pitch axis, as the left going blade sees a different AoA than the right going blade.

iv) Other effects
Engine torque, gyroscopic precession and thrust axis offset. These effects are generally minor for our case, however it is important to note the coupling between roll and yawing moments, meaning that an asymmetry in roll moments due to torque or other effects resulting in different aileron deflections during the L and R side-slip will eventually be translated in yawing moment asymmetries due to aileron drag. Same with rudder deflection and rolling moments.

Rolling moments (Prop effect on Cl_beta)

A side-slip may induce a rolling moment with power-on because the slipstream
strikes more wing. Spiraling slipstream may contribute to a rolling moment if it alters the direction of the airflow over the wing.prop_sideslip2

In conclusion from all the above, my opinion is that the most significant factor in SHSS asymmetries encountered in prop aircraft is the spiraling slipstream and its interaction with the aircraft geometry.

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Circular Error Probable approximations for weapons testing

Circular Error Probable (CEP) is defined as “the radius of a circle centered at the target or mean point of impact within which the probability of impact is 0.5“.
CEP is used significantly in measuring weapons systems’ impact accuracy and some navigation systems accuracy. Approximations for CEP include non-parametric and parametric methods.


Non-parametric approximations make no assumptions about the underlying distribution of the impacts. One such method is simply to consider the sample median for CEP. The non-parametric methods can provide good results when the sample size is greater than 30, but are of limited use to flight testing especially when considering missile tests where the sample size usually not greater than 15.

Parametric approximations make an underlying assumption that the impacts are normally distributed. The CEP can then be computed either by closed form integral calculations, or by numerical calculation of integrals, or by algebraic approximations of the bivariate normal function.
The latest method can give very fast results, practical for flight tests and is very accurate in certain areas of the parametric space. Three parameters to consider for the impact distribution are bias, eccentricity and correlation. These parameters are of importance and should not be overlooked as they might provide some insight on the characteristics of the weapons system under test. Various algebraic approximations exist and they are discribed extensively in literature.

In case the distributions are zero mean Gaussians (bias=0) the main parameter to be considered is eccentricity and a simplified approximation can be used:

[1] An examination of CEP approximation techniques
[2] AFOTEC Technical Journal CEP
[3] Use of Circular Error Probability in Target Detection

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Visit to the Kennedy Space Center, Florida


A visit to the Kennedy Space Station is an experience of a lifetime. Getting exposed to all those technological marvels blows and in the same time humbles ones mind. Seeing with my own eys and trying to conceive the egnineering work required and the hundreds of thousands of people who worked on these projects towards space exploration, leaves me amazed… A great salute to all those individuals either ground technicians or astronauts and of course the USA; the country that made it all possible.

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