In March, the team worked through remaining control system tasks on the DarkAero 1 prototype including modifications to increase stiffness in the aileron, flap, and rudder controls. The armrest covers were also revised to better enclose the control system hardware. Session 22 of the Aerospace Composites Course was held and new team members were onboarded.

Cabin Work
One of the remaining tasks on the control system involved revising the armrest covers so they extend to the floor of the cabin. Although the new design offered improved aesthetics, the change was implemented primarily to protect the controls against interference from occupants and loose objects in the cabin per FAR 23.685. This became especially important after installing the outboard seatbelt attach brackets as these are positioned particularly close to the flap controls. A pocket was recessed into the new cover so the seatbelt connection had room to rotate and remain separated from the flap actuation mechanism. 

The revised cover was CNC cut from a flat pattern of infused sandwich panel stock, and the recessed pocket was folded into its final shape rather than being molded. A machined HDPE jig was used to hold the folded structure in its correct shape and allow bonding in a single operation. Installing the new cover in place has really made the cabin feel more complete and less like a prototype.

Control System Refinement
Over the past few months the team has 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, the system hardware was assembled to a point where it could be directly loaded and measured. This allowed the characteristics of the torque tubes and push rods to be evaluated as well as seeing how the entire system interacts with the surrounding airframe structure it is mounted to.

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. For the DarkAero 1, this guideline was modified to a more specific criteria stating that when load is applied to the control sticks, the compliance in the system is not so high that the pilot will be unable to achieve enough control authority to properly maneuver the aircraft through typical flight scenarios.

Beyond measuring the total compliance in the system, the breakdown of individual sources of compliance was also studied. In the case of the ailerons, this was accomplished by placing deflection indicators at key locations throughout the system and filming these areas while applying load. After applying simulated aero loads, the footage was reviewed to see which parts of the control system experienced the most deflection. A significant portion of the total compliance was traced to the armrests where load was being applied at an access opening that was included in the design to allow for easy installation of control system hardware.

To address this issue and increase stiffness in the system, a redesigned version of the torque tube bearing bracket was implemented 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. Since this bracket is removable, it will not limit the serviceability of the control system hardware. The team will be ground testing the new setup once again to confirm that the compliance of the ailerons is within desired limits.

Carbon fiber torque tubes are used extensively throughout the control system of the DarkAero 1 for their combination of low weight and high stiffness. Off-the-shelf filament wound tubes were selected for the prototype control system. In testing, a fair amount of system compliance was traced to these torque tubes and their torsional stiffness proved to be much less than expected. Despite being made of carbon fiber and sourced from a reputable tube manufacturer, these tubes were not specifically optimized for torsional loading. To achieve maximum torsional stiffness, the fibers in a carbon fiber tube must be oriented at positive and negative 45 degree angles relative to the axis of the tube. Off-the-shelf carbon fiber tubes are generally not built like this. Filament wound tubes often use a wide range of fiber orientations and roll-wrapped tubes commonly use fibers at 0 and 90 degrees to the tube axis. 

This led the team to manufacture torque-specific carbon fiber tubes in-house with the fibers aligned at the optimum 45 degree orientation. The new tubes were also designed to be easily retrofitted to the existing torque tubes to eliminate the need to remake any control system hardware that interfaces with them.

Rudder Return Spring
The DarkAero 1 is designed with left and right split rudders that can be deflected individually for yaw control as well as simultaneously for speed brake function. The rudders are designed to only deflect outward and cannot deflect inward past their center neutral position. Springs restore the rudders to center and prevent undesired movement. As part of the control system refinement, the team completed work on a new return spring design that will offer increased restoring force.

The initial design leveraged a single torsional spring for each rudder. This approach was simple, lightweight, and compact enough to fit in the available space claim. However, ground testing proved that the spring force was not sufficient for the full range of deflection angles, and it was concluded that a new return spring design would be necessary.

The new design was developed last month that replaces the original torsional springs with a single vertically mounted tension spring, pulley, and cable system. This arrangement will provide the necessary restoring force for both rudders while also remaining compact enough to fit in the rudder hinge area. A test rig is currently under construction which will be used to confirm the system functions as intended before it is installed in the prototype. 

DarkAero Growth
DarkAero has been growing to accelerate flight testing of the DarkAero 1, manufacturing of DarkAero 1 kits, and expansion of engineering and manufacturing services. Interviews for new engineers and technicians have been ongoing, and the team has already gained new members. If you’d like to join DarkAero, check out the DarkAero Careers page and apply today! Interested candidates can inquire at careers@darkaero.com.

Looking Ahead
In April, the team will continue working through the remaining ground tests for the control system while transitioning over to remaining ground tests for the main landing gear. Expanding the team and getting new team members up to speed will continue to be a focus.