“You are not normal, that is not allowed,” responded the wife of a test pilot Marjan Jelen when he came home that day. Not having it among his tasks, yet believing in the aircraft J-22 Orao, Marjan Jelen was the first to break the sound barrier on November 11, 1984, with the speed of Mach 1.04 with a Yugoslav-made plane.

DUTY TO TRY

Test pilots are known to have an internal need for pushing boundaries. Joining their love for calculated risk with practicality, they end up testing aircraft in many different ways for the safety of those who follow. Pushing the envelope to break the speed of sound was first done by an aviator ace, Chuck Yeager who did so on October 14, 1947, when he flew the X-1 over Rogers Dry Lake in Southern California. His own words confirm the passion for the challenge: “We didn’t know if we could break the sound barrier. But it was our duty to try.”

Since numerous followed. The fastest airplane to date is unmanned NASA’s X-43 which almost reached the approximate Mach 9.6, 1 Mach(ma) being 1225.04 km/h.

YUGOSLAV AIR FORCE

In what was once Yugoslavia, the first pilot to break the speed of sound was Nikola Lekić, who did so on July 31, 1956, with an F-86 Sabre over Batajnica airport, Serbia. After many followed him with MIG-21 aircraft. Yet, November 22, 1984, was the only moment in the history of the Yugoslav Air Force that a domestic plane broke through the sound barrier.

J-22 ORAO

There are many tasks for the test pilot when pushing an aircraft envelope and Marjan Jelen too had a handful of them, when testing the J-22 Orao(Eagle). In his own words, his thoughts were:” Above all how far to go and in what way, how to meet all safety criteria and quality of testing, and yet achieve the goal?”

He was assigned to test the manoeuvre limit at the following Mach numbers M = 0.40; 0.50; 0.60; 0.65; 0.70; 0.75; 0.80; 0.85 and 0.90, record the amount of fuel consumed at each set speed, plane behaviour etc.

After many successful tests, the aircraft’s last achievement was M= 0.97. 

In the post-analysis, Marjan stated that the tasks were completed in their entirety, but he thought that the goal of the compressibility test was not completed and that there were possibilities to continue as this was not the maximum for the J-22 Orao airplane. Engineer Vujović agreed with him so it was decided that in the next flight the acceleration would be repeated, but with a maximum allowed altitude of 12,000 meters and acceleration of aircraft in horizontal flight to maximum possible speed before the dive.

THE BREAKTHROUGH

“I accelerated the plane to M = 0.85, and then energetically translated the plane into a dive at an angle of about 30º. The speed started to increase, again quickly reaching the M = 0.97 number. I had a feeling for 2-3 seconds (or it just seemed so long) that the Mach meter would stay at that value again, and immediately after that, the plane calmly and quickly continued to accelerate to M = 1 and crossed that value into the area of ​​the Mach meter display, which I had never had a chance to see before. From the theoretical description of the instrument, and following the instrumental speed and Mach number, I thought that I achieved the maximum M = 1.04, since the divisions behind M = 1 are not numerically marked. At the moment of passing the speed of sound, I registered that the instrumental speed was Vi = 750 km / h and that there was a disturbance – rough jumps in showing pitot-static instruments (speedometer, variometer, and altimeter), caused by the shock wave on the plane and pitot tube, with the re-establishment of the normal indication in supersonic flight. The phenomenon is characteristic of all supersonic planes. 

I did not register the exact height when passing the sound barrier. After the observed acceleration to M> 1, I turned off the additional combustion and started by gently pulling the plane into horizontal flight. At about 9000 meters, the plane calmly, without accompanying negative phenomena, went into subsonic flight. The first breakthrough of the sound barrier with a domestic plane was made exactly at 12:12 on November 22, 1984.”

GOLDEN FLYING BADGE OF YUGOSLAVIA

Rarely in his career does a pilot have the opportunity to experience such a feeling of happiness, and satisfaction, but also pride, as Marjan felt at that moment.

Later in December 1984, as a flight instructor, Marjan received the Golden Flying Badge, the highest recognition in aviation of former Yugoslavia.

Marjan Jelen has also been a flying instructor at Aeroclub Galeb in Belgrade. In the photo, flying Galeb G-2 together with Primož Jovanović.


WHAT IS A SONIC BOOM?

When flying faster than the sound pilot can only tell from the instruments. However, people on earth do hear “a boom” of a plane flying over at supersonic speeds.


The sonic boom problem- TEDEd


Source:
-Пробијање звучног зида авионом домаће производње J-22 Орао, Marjan Jelen
https://www.ekapija.com/people/274525/marijan-jelen-pilot-avion-mi-se-otimao-kao-konj-u-galopu
-Marjan Jelen personal photo archive
https://tangosix.rs/2014/02/12/ladevci-slavili-dan-brigade-i-40-godina-od-prvog-leta-orla/
https://boomsupersonic.com/flyby/post/how-exactly-do-you-break-the-sound-barrier

https://en.wikipedia.org/wiki/Supersonic_speed

Categories: Sky Stories

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