September 3, 2021, ©. Leeham News: Last week, we looked at the Certification Compliance Planning we do concurrently with Detailed Design, Figure 1.
It’s now time for us to work on our Test rigs and systems for ground and flight testing. We need to get these defined before we freeze the aircraft’s configuration and start making our flight test aircraft.
As we work on our detailed design for the aircraft, all our system suppliers are working on the systems we will integrate into the aircraft.
The suppliers will do system testing of their parts before delivering the system to us. But we must integrate these systems with other systems, and make sure they work together. First on the ground, then in the air.
The most spectacular ground test rig we need to plan and produce is the static strength and fatigue test rigs. With the static strength rig, we must verify that our design can withstand the design’s Limit loads (the highest loads expected in service) and the Ultimate load (limit load times 1.5).
Our rig will test these loads for the wings, fuselage, and empennage structures. As we have an un-pressurized aircraft our fatigues testing (high number of cyclic loads on our structure mimicking flights) focuses on the same areas as our static strength tests. As the static test structure can pass the Ultimate load test with plastic deformations (it only has to stay together and be flyable), we use a second structure for the fatigue tests.
Figure 2 shows the static testing of the Airbus A350 wing, taken from this video https://www.youtube.com/watch?v=B74_w3Ar9nI that also discusses the ultimate pressurization loads for this pressurized aircraft.
To understand why we need ground test rigs for our systems, let’s take the example of the flight control system. Our basic control system is a mechanical concept with Yoke and Pedals, pulling wires over bellcranks and pushing pushrods back and forth to move our flight control surfaces.
But we also need a yaw damper, which is part of our autopilot, to make the aircraft fly comfortably for our passengers (the normal compromise between spiral stability and dutch roll for a passenger aircraft is to allow a bit of dutch roll which you then control with a yaw damper).
The yaw damper is a servo system that is part of our Avionics system. The autopilot/yaw damper must interface with the flight control system, pulling on the wires/pushrods just before our flight control surfaces for pitch, roll, and yaw.
It does this with electric servos that work in serial for the yaw damper (i.e. the pilot can’t feel its interventions in the Pedals) and in parallel with the mechanical flight control system for the autopilot in pitch and roll (i.e. the pilot can follow the autopilots work by watching/touching the Yoke).
The electrical servos for the Yaw damper/Autopilot use electric power from our electric power generation system. We must make sure the servos don’t have time delays or lower slew rates when our electric system is at its lowest power position, for instance, as the pilot throttles back the engines to flight idle because he’s too high and fast in an approach. Lagging servos can cause dangerous positive feedback situations that can jeopardize the aircraft.
The flight controls also command flaps and spoilers, both operated hydraulically as there we have high forces involved (hydraulics can generate high forces from small physical devices). We must make sure all this works together smoothly even when the hydraulic is at its lowest supply position.
We must create ground test rigs for all critical systems and first test them in isolation and then gradually adding them together.
We must, as an example, make sure the landing gear is not stealing the hydraulic power from the flaps as the pilot commands gear-up and flaps to start position as he does a go-around after a botched approach.
Avionics/systems integration test rig is another important tool for a development program. The avionics system communicates with almost every system on the aircraft. Engineers can use this rig to test out “beta” software to identify and fix issues before loading it onto an aircraft. Professionals in this industry test as much as possible in a low-risk environment instead of allowing beta software onto flying aircraft.
For all ground test rigs we need to have sensors at the right places to log pressures, flows, slew rates, forces, current, voltages, and temperatures. All these values must be recorded and stored in a well-thought-out manner, so the results can be conveyed to those who need them in the project and to form part of our proof of compliance for the Certification documents we produce.
We need to get these rigs to arrive so we can do tests before we have come too far in our detailed design. This means creating all test plans, work with our manufacturing team, work with suppliers, and of course the FAA as we need test plans with agreed requirements and setups to ensure the data we acquire can be approved.
We also need to design the flight test rigs and devices, the ones that shall be mounted in our flight test aircraft.
Our first aircraft will verify our flight envelope and flying characteristics. It shall be as close to the final production specification for structure, propulsion, flight control systems, etc as possible. The test rigs and systems must log all the relevant parameters for these tests. It must also log all the pressures, flows, movable slew rates and angles, and structural stresses that we measured in our ground rigs.
An important part of flight tests is flutter tests, a very dangerous checking that we don’t have an aerodynamically caused self-resonance in our structure during flying. For this, we have installed rate gyros, g-meters, and stress gauges on different parts of the aircraft (wings, tail surfaces, engine nacelles…).
Here we must have a real-time evaluation of results. Shall this be done from the ground? In such a case, we need a reliable data-link system. Or, shall the flight test engineer in the cabin check that we don’t enter a dangerous situation while exploring the envelope?
Unexpected structural flutter is the most common cause of test aircraft break-up in the air. Therefore flutter test and clearing of the flight envelope for flutter is something all experienced OEMs and flight test organizations treat with the utmost respect.
Our second test aircraft is equipped with a full ECS system and cabin. This shall be a production conform aircraft so that we can finish the tests with the required Function & Reliability testing. On this aircraft, we check all aspects of passenger comfort such as airflows, temperatures, vibrations, sound levels, and sound effects from our systems when we deploy flaps, spoilers, and landing gears.
Everything must be measured with sensors, routed via a special flight test EWIS to the test rigs where the data is recorded and concurrently routed to the flight test engineer’s place and there presented on his laptops.
With all the sensors and their data, we check that we get the same results during flight as the values from our ground simulations and test rigs. Flight testing has migrated from finding problems to be a verification of our analysis and simulations, and to gather compliance evidence.
We need to conclude our research and system work for these rigs and installations in time so we can get the test results we need for the project before the detailed design has come too far. If there are changes needed because of ground testing we want these changes to make it to our test aircraft.
Otherwise, we need expensive modification periods for the test aircraft to final configuration or we might even need to produce a third test aircraft to have a final configuration for our certification flights.
Eventually, we will need a third test aircraft as we plan to offer a cargo variant, but this can be scheduled so this aircraft can be produced on our production tooling (more on this next week) to get the cost of manufacture down.
This is all tricky stuff and now any misses in our work cost major money. This is where project busts their budgets and run into major delays.