Bjorn’s Corner: New aircraft technologies. Part 37. First flight test prototype

By Bjorn Fehrm

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.

Figure 1. The development plan for a new airliner. Source: Leeham Co.

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.

Figure 2. The FAA 14 CFR Part 25 airliner flight envelope. Source: FAA.

The test aircraft shall gradually fly through the full envelope in Figure 2, where;

  • The maximum positive load factor is 2.5G with flaps up and 2G with flaps down. 1G negative is the maximum allowed.
  • For speed and altitudes, the aircraft shall fly to typical cruise speed (Vc), maximum dive speed (Vd), design maneuvering speeds at different altitudes (Va), maximum flap speed (Vf), and stall speeds at different loaf factors (Vs).

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.

Flight Flutter

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:

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:

Measurement equipment

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.

Everything orange in this first test aircraft for the CSeries (today A220) is test equipment and installations.

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 fleet

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.

7 Comments on “Bjorn’s Corner: New aircraft technologies. Part 37. First flight test prototype

  1. One problem is modifications rolling into the production and when to redo test due to changes incorporated. Changes keep rolling in due to mass, cost, software, durability modifications and each new combination needs to be evaluated for a requirement for new test flights superseding old test flights. The FAA/EASA must be on top of this as well to put additional requirements on the tests and analyses to be performed and its documentation. (Lex 737MAX)

    • Claes:

      I am not sure where the line really lies between Prototype and Initial Low Rate production (aka military term)

      The LRP are not really prototypes, they are supposed to be production though the reality is that there is a line change between 15-20 that changes are made.

      The right term is concurrent production (or so I think).

      The Boeing Dash 80 would have been a prototype in my view. That lead in turn to what we think of as the 707 as well as the KC-135, both different widths.

      The Aircraft mfgs know the aircraft will fly and they don’t expect massive changes.

      Add in there is a difference in the LRP and Industrialization that goes on with mods made on a regular basis so a few years down the road a lot of changes have been made to the mfg of the current aircraft.

      • You try to decide how many test aircrafts and modification updates you plat to put into the project timeline and budget. Then as you reach the end of the project phase and start to certify you easily get into these troubles as modifications keep rolling into the test aircrafts and their supplier delivered components like engines, hence making no test aircraft is like the other and they change quickly. You must decide what test need to be redone or not as you incorporate changes. Once you are certified with a Type Certificate you must issue SB’s to make new pn’s approved to install and mass, cost, reliability changes keep on rolling in and the work does not loose speed until after a few years. Just look at what P&W experiences now with its type certified PW1100G.

  2. The Airbus Beluga flies low over my house almost daily; I’m very happy it does not flutter!

  3. One small correction: the flutter accident that you mention was on one of the Grob SPn flight test aircraft. This small business jet program had nothing to do with the Learjet 85.

    Bombardier and Grob did originally have a agreement whereby Grob would be responsible for the design of the Learjet 85 composite structure and the construction of the first three prototypes, but this agreement was terminated early in the development process.

    • No wonder the Learjet 85 was a financial hole for Bombardier at the time.
      Which came first the 85 or the SPn and was Bombardier not happy a design and build supplier would have its own small business jet ?

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