November 3, 2023, ©. Leeham News: We are discussing the different design phases of an airliner development program. After covering Conceptual, Preliminary, and Detailed design, we now discuss prototype manufacturing and testing, Figure 1.
Today, we look at the role of the first flying prototype.
We saw last week that the first prototype airframes were built for ground testing of structural strength. Before we can fly a prototype of a new aircraft, we need a flight testing permit from our regulator.
Part of the proof that our aircraft is ready for flight testing is to put it through Limit load tests (Limit load is the predicted highest load the aircraft will meet in its operational life) to prove that the structure will not fail during test flights.
Before the aircraft can be certified, it must pass the Ultimate load test, which is 150% of the Limit load.
The next airframe built is for the first flight test aircraft. A flight test program has several aircraft, all with their role and specific test programs.
The first flight test aircraft is built to test the basic flying characteristics and to clear the flight envelope for the aircraft, Figure 2.
The test aircraft shall gradually fly through the full envelope in Figure 2, where;
The testing of the basic flying characteristics is with modern design knowledge and tools (CFD, Wind tunnel tests, Simulators), not the challenging or dangerous part.
What is dangerous is the clearance of the flutter characteristics for the aircraft through the flight envelope.
Aircraft flutter is an oscillating movement of flight surfaces like wings, stabilizers, movables, or even engine pylons/nacelles that can diverge in amplitude during flight and cause structural failure.
To see how it looks in flight, watch this short NASA clip of a Pa-30 Twin Comanche horizontal tailplane that enters flutter: https://www.youtube.com/watch?v=iTFZNrTYp3k.
I have flown the Twin Comanche, and I’m happy NASA found this tendency before I flew the aircraft and had Piper Aircraft make changes before certification!
There are numerous test aircraft that have gone down because of flutter; the latest I remember is the Grob Business jet prototype (corrected from Grob Learjet 85 prototype, see comments). Its horizontal tailplane entered flutter like in the movie clip and broke off. The aircraft and test pilot perished.
Flutter happens when the aerodynamic force center ends up ahead of the point or line where an aerodynamic structure flexes. You do ground vibration tests of the airframe parts to measure the resonance frequencies of different aircraft parts and also see how the parts flex.
This clip shows the ground vibration test of the Airbus Beluga prototypes: https://www.youtube.com/watch?v=tlbGhKGpthM
The aircraft is heavily instrumented to capture all the test data for the first flying prototype. It has no cabin, just a naked aircraft with a cockpit and test stations for the test engineers, Figure 3.
The airframe is equipped with hundreds of stress-measuring “strain gauges”; there are vibration accelerometers to pick up flutter in all the flight surfaces, temperature sensors on different parts, extra gyros to measure movements, etc.
All these sensors are polled by the test computers in the racks, whereafter a subset is sent in real-time to the ground test center, to have real-time analysis of the results of different tests.
For the dangerous flight envelope expansion with its flutter risk, the tests are done in small steps, where the ground test center analyzes data in real-time from artificial excitations of flutter. If the response is well damped, the flight test crew are allowed to go to the next test point in the envelope.
The flight test of a new airliner program has several test aircraft, often between five and eight craft. Each aircraft has its test role and program. We will look into this more in the next Corner.