„L-39 is a light trainer aircraft, extremely common. I think over 30 countries in the world have it. This is an aircraft that has proven itself over time. The plane itself has specifics. It was designed about 50 years ago in the Czech Republic, when there was no computer power to make more perfect aerodynamics. And naturally, the plane carries these scars and these difficulties.
This is what Dr. Nedislav Veselinov recalled to bTV. He is a teacher at the Technical University.
„One of the features of the aircraft is that it is unstable at higher angles of attack”, he explained, adding that then a flow collapse can occur.
„If the so-called critical angle of attack is reached, this flow collapse occurs. Naturally, the designers anticipated this when they designed the plane, and made the tip of the wing with an angle slightly greater than that at the base of the wing,“ explained the aeronautical engineer.
„Thus, as we approach a critical angle of attack, the flow collapses first at the wing tip. The collapse gradually moves towards the base of the wing before it collapses completely, he further explained.
According to his words, a vibration also begins, which is transmitted to the pilot's body.
„The point is that these critical modes are not reached because the plane crashes and you are in a mode where you can hardly influence it. You can get it out of such a mode, but not from a limited height”, pointed out Dr. Veselinov.
He commented on an amateur photo he saw. According to him, the plane is making a combat turn that has a very small radius. They tried to get the most out of the plane. And that's one of the likely reasons – that they entered a critical flight mode, the jet collapsed and the plane crashed”, commented the expert.
Dr. Nedislav Veselinov uses terrain that approaches the terrain of the “Graf Ignatievo“ airbase.
In the flight simulation, the plane has an initial speed of 450 km/h and a height of 300 meters. The time indicated is 12:00.
„When the angles of attack are not large, the plane turns and flies calmly. No vibration. At a critical angle of attack, you can feel how it trembles, commented Dr. Veselinov.
„The sharp movements that are happening are really not my moves. They are caused by instabilities in the jet. The machine becomes uncontrollable. In a fraction of a second of the pilot's inattention, a critical angle of attack can be reached, he adds
According to the aeronautical engineer, pilots are superheroes. And the deceased Major Petko Dimitrov and Senior Lieutenant Vencislav Dunkin probably directed the plane to the place where it crashed in order to avoid casualties outside the airport.
Dr. Nedislav Veselinov also believes that there are two possible mistakes – unexpected technical or pilot error. What happened – it will become clear from the black box data.