This year, we continued with flight tests on the fs33 with the new winglets as part of the Idaflieg Summer Meet in Stendal. Since these new winglets alter both the aerodynamics and mass of the wings, flutter tests at speeds above the maximum permissible airspeed of 280 km/h (174 mph) must be done, specifically up to 310 km/h IAS (193 mph).
Till now, we’ve only done flutter tests up to 200 km/h (124 mph), meaning we only need to run tests between 200 km/h and 310 km/h.
To perform the test, the pilot flies the plane at a certain airspeed and tries to induce vibrations through specific control inputs. There are many ways to do this: The first is by repeatedy moving a control surface back and forth with a specific frequency. Another method is to hit the control stick, which allows for a larger induced frequency.
Under normal circumstances, such a vibration usually ends quickly as long as one doesn’t make more control inputs. During a flutter though, the opposite occurs. The vibration intenifies on its own, producing larger and larger amplitudes. This can quickly damage the plane, which is why during flight tests, flutter behaviour must be checked.
Of course, before we started with the tests, we checked the plane, especially the control surfaces for play, to make sure everything is in top shape. The tests were then scheduled for Thursday, the 29th of August 2019.
Since flutter tests are quite risky, they are performed at high altitudes. This will give the pilots, in the event of an emergency, to bail out safely from the aircraft.
The flight began with an aerotow, which brought us to an altitude of 3000 m (~10000 ft) AMSL. We set 1000 m as the minimum altitude where the tests would end.
We started the flutter tests at a speed of 200 km/h IAS. We attempted to induce vibrations in all three axes, meaning we gave aileron, elevator, and rudder control inputs separately. All vibrations were well damped and quickly ended on their own accord.
During this first flight, we were able to increase the airspeed gradually in 10 km/h increments up to an airspeed of 270 km/h. By that point, we reached our set minimum altitude and so we ended the test there. Even with the fs33’s excellent glide performance, the altitude loss when flying at such high speeds is still quite large. This meant that we needed at least a second flight to wrap up the flutter tests.
We started the second flight directly after ending the first one. After arriving at 3000 m, we quickly accelerated to 270 km/h and repeated the tests again, noticing that the vibrations were still well damped.
At a speed of around 300 km/h however, after the control stick was hit to induce pitch vibrations, the aircraft started to flutter. Thanks to a quick reduction of airspeed, the fluttering ended after a few seconds. The test had to be called off as it was unknown if the fluttering caused any damage to the plane, meaning an examination was necessary. After a careful descent and landing, all accessible parts of the fs33’s control system were examined. No damages were found but a more thorough check of the system inside the fuselage was still needed.
All in all, the attempt to wrap up the flutter tests was unfortunately unsuccesful. Our next task is to find methods to increase flutter resistance. These include among other things the mounting of mass balances to the control surfaces. Afterwards, the flutter tests must be repeated. We hope to finish this soon and fully qualify the new winglets of our fs33.
Author: Jonas Schmidt
