The last time I flew on a military plane was 20 years ago, but I still remember the incredible emotion I felt. It was exactly the same way I felt recently while scrolling the check-list from the rear seat of an L39 Albatros as I got it ready for take-off and flight.
Picture this: All the caution-panel yellow and red lights in front of me are on, since this is a routine test. Even so, I can’t avoid thinking about the complexity of the plane, which is a basic jet aircraft compared to today’s standard: It was designed in the 1970s as an advanced trainer for pilots. There is no fly-by-wire system. It incorporates a simple avionic and no embedded software. This means that the pilot has to do all of the work.
The ground specialist helps me in arming the ejection seat, shows me the red-flagged safety pin, and closes the canopy. Thumb up, and we are ready to start the Ivchenko AI-25TL turbofan engine.
Did you know that the patent for a stationary gas turbine was granted to an Englishman in 1791? But it was 1939 when we first saw an aircraft using this kind of propulsion; it became widespread only in the second half of 20th century.
The air is taken in from front inlets and compressed by a series of rotating blades attached to a shaft. When this compressed air reaches the last section of the engine, it is sprayed with fuel and lighted by an electric spark. The burning gas expands and flows out through the nozzle, providing thrust.
The mechanical and thermal stresses of these engines are very high, and there is a stringent maintenance program that includes checking the engine after a defined number of working hours. Typically, several blades are replaced with new ones, even if they are still in perfect working condition, to provide a high safety margin for critical parts. The cost of this is very high, not only because of the parts that are replaced, but also because the aircraft is grounded to a hangar for hours, if not days. If this were a passenger aircraft, it could not fly and produce money for those days.
I was really surprised when I discovered that several turbine producers are using ANSYS software to perform fluid–structure analysis on these blades to predict failure probability and to properly set up the maintenance cycle. Volvo Aero is one manufacturer that is using this technique. (Read the full article.)
Now we are taxiing to our assigned runway. Throttle to full power … All the instruments are on green status and the engine is working at 102% with an EGT (exhausted gas temperature) near 600 C. In another few seconds, we release the brakes and we are airborne, scrambling in the sky.
Within five minutes, we are cleared to enter our assigned operation zone. My flying mate, an expert test pilot, let me start my “stick time” with a wide security turn to spot any inbound traffic. I scroll the list of the pre-acrobatic checks. Here we go!
Another check of the parameters: throttle at 90%, check speed … 250 knots … Start pulling till reaching 40 degrees pitch … Rolling to reach the top at 120 knots with 100 degree roll angle and then dive, closing this wingover turn. I give it some more throttle: 95 % at 300 knots now, start pulling and keeping 4 Gs for the first quarter of this loop. Almost at the top, we are quite near to the stall speed and 0 Gs, in that incredible situation where you can see the world upside down while you hang at the seats, kept there only by the belts.
The plane is very sensible in this moment, and is better to be gentle on the stick. If I pull too much I can enter a stall, but if I release the stick I can put the plane in a negative Gs acceleration. I look back over my shoulder to find the reference points and check that the wings are leveled. The plane is diving again. My copilot takes back the control and we stress the aircraft a bit more with a hammerhead stall turn.
If we can do all this in a very safe way, it’s because engineers have designed this aircraft to perform all these maneuvers and identified flying parameters for each of them, highlighting its operational limits. Test pilots have verified these parameters and procedures by flying the aircraft in every possible condition, pushing it to unusual altitudes to spot the plane’s behavior and write recommendations about how to deal with them.
In the Air Force, they say that the EMERGENCY CHECKLIST is written on red pages because the pilots who wrote them were risking their lives. Casualties still occur today, of course. But these days, performance is analyzed in detail through numerical simulation far before the first flight of the first prototype of a new aircraft. Thousands of simulation runs allow engineers to explore the aircraft’s design, optimize it, and forecast its behavior in different condition, taking into account even icing phenomena or bird strike. You can see a lot of these cases in the recent edition of ANSYS Advantage magazine.
Suddenly the master alarm light starts blinking. We are at “bingo fuel” and we need to head toward our airbase. Flaps are on LD position, the gear is down, and I’ve checked that the three green lights on my left console confirm just that. While the wheels touch the runway, I enjoy the emotions of such a great time, wondering when will be the next time, and on which plane.
In the meanwhile, I will go on helping aerospace companies to design better aircraft, making them safer, increasing fuel efficiency, extending their operational life … and helping both engineers and test pilots in their demanding jobs.