Control System Testing on the DarkAero 1

An important part of validating a new aircraft design like the DarkAero 1 is proving that the control system is able to produce the desired response throughout the full flight envelope. Getting this right is not just about providing the pilot with an enjoyable flying experience, it’s safety critical. Maintaining control of the aircraft is a mandatory requirement, equal in importance to maintaining structural integrity of the airframe.

Within the quantification of the control system response is the “feel” of the system, and this is partly tied to compliance, or how rigid the control system is. That is, do control surface deflections match the pilot’s intentions with the control stick. When it comes to compliance, the mounting structure that supports the control system hardware is just as important as the system itself. An infinitely rigid torque tube would still introduce significant compliance into the control system if it was supported with Silly Putty.

Over the past few months we have been working through an iterative process of testing the flight controls of the DarkAero 1 prototype and measuring the compliance of each control axis. To more quickly quantify compliance, we fully assembled the system hardware to a point where it could be directly tested and measured. This allowed us to understand not only the characteristics of the torque tubes and push rods, but also how the entire system interacts with the surrounding airframe structure it is mounted to.

We first tested the aileron controls, which consisted of applying simulated aerodynamic loads to the deflected ailerons as commanded by fully deflected control sticks. We then measured how much the ailerons deflected back towards neutral as the load was applied. If the system was infinitely rigid, it would not move at all no matter how much load was applied. Conversely, a system with significant compliance would display full deflection back to the neutral position.

A general guideline exists stating that no more than 25% of control surface deflection should be lost under expected aerodynamic loading conditions (see Air Commerce Manual 04 - Airplane Airworthiness). This rule of thumb may need further refinement depending on the mission of the aircraft though. In the DarkAero 1, we want to know that when load is applied to the control sticks, the compliance in the system is not so high that we are unable to achieve enough control authority to properly maneuver the aircraft through typical flight scenarios.

We not only wanted to measure the total compliance of the system but also determine where the largest sources of compliance were. In the case of the ailerons, we accomplished this by placing reference tabs at key locations throughout the system and filming these areas while applying load. After applying simulated aero loads, we analyzed the footage to see which parts of the control system experienced the most deflection. For the ailerons, we found that a significant portion of the total compliance came from the armrests where load was being applied at an access opening that was included in the design to allow easy installation of control system hardware.

To address this issue and add stiffness to the system, we created a redesigned version of the torque tube bearing bracket that now ties into the upper portion of the armrest. This connects the upper and lower portions of the armrest together, which reduces deflection in the lower portion of the armrest under control system loads. Since this bracket is removable, it doesn't hinder the serviceability of the control system hardware. We’ll be ground testing the new setup once again to confirm that the compliance of the ailerons is within our desired limits.

In part II of this post, we'll discuss how we are addressing compliance in other areas of the control system and share more of the lessons learned. Stay tuned!