Progress continued on the DarkAero 1 prototype in May with efforts focused primarily on systems tasks that must be completed prior to the first engine start. The keys to the new pilot production shop were officially received which allowed the move-in of equipment to begin. The airframe flutter analysis moved forward as well.

Main Landing Gear
The main landing gear struts were recently painted. Several coats of gray primer were applied for UV protection on the carbon fiber struts and corrosion protection on the aluminum strut ends. After the paint dried, the hardware and fasteners that interface with the strut were installed. The fit of everything is tight enough that the paint thickness had to be accounted for in the tolerance stack up. Too much paint would mean the parts would not fit together properly. An additional sacrificial thickness of carbon fiber was added in critical areas to allow the assembly interfaces to be fine tuned by sanding material away. Potential interferences were checked in between coats of paint to see how the fit was trending with successive layers of paint. This process is iterative and could be improved upon, but alternative plans are being made to integrate the parts into a more unified strut design and eliminate much of the hardware altogether. 

Nose Landing Gear
Several outstanding tasks related to the nose landing gear were wrapped up this past month. A small amount of play was identified in the bracket assembly that attaches to the end of the nose gear strut. This was corrected by machining the mating faces in the bracket components to eliminate the play and achieve a tighter interface with the strut. The Click Bond nut plates that are used to constrain the drag link mount were permanently bonded onto the nose gear wheel well. Previously, temporary hardware had been used which required two people to be involved when installing the nose gear. Additionally, testing was completed to validate some potential heat shield solutions that were being evaluated to prevent an engine fire from damaging the structure of the nose gear wheel well.

Firewall Heat Shield 
The DarkAero 1 uses a firewall heat shield made from a thin sheet of titanium that is backed by a layer of ceramic blanket. This material combination protects the major flat regions of the firewall, but a different fire barrier solution was required in the exhaust tunnels that extend aft from the firewall due to their compound curvature. Several candidate fire barriers were considered for the exhaust tunnels including fireproof intumescent paint and adhesive backed composite fire blankets. 

Samples of these fire barriers were recently tested to the FAA standards for certified aircraft firewalls in FAR 23.1191, but none of the candidate materials produced acceptable results. This was troubling considering these materials have been observed as a firewall solution in a few experimental category aircraft. The decision was made to modify the shape of the exhaust tunnels, eliminating their compound curvature and enabling the use of the same proven fire barrier employed on the flat area of the firewall. The previous exhaust tunnels have already been removed, and the new exhaust tunnels are under construction. Beyond achieving better fire protection, the new exhaust tunnel design will also be more compatible with adjustable cowl flaps that will enable more optimized engine cooling across a broad speed range.

Fuel System
Many accidents in general aviation are caused by fuel system related issues, so the design of the DarkAero 1 fuel system has been given extra attention. An important part of the fuel system is the fuel lines. The UL520iS engine in the DarkAero 1 requires both a supply line and a return line between the engine and the central sump tank. The FAA has established guidelines for proper fuel line support and strain relief, as well as limitations on their proximity to wire bundles and abrasion hazards. Additionally, the fuel lines are competing for their space claim with other hardware and airframe structures, which makes establishing acceptable lines routings challenging. A routing had previously been developed for the fuel lines, but after some modifications were recently made, a new path was required. An updated routing was agreed upon, and new aluminum lines were fabricated and installed. Additionally, several remaining plumbing tasks were completed on the test tank which is now ready for first engine start. 

Electrical/Avionics
A few updates were made to the center tunnel avionics box to prepare it for installation for the first engine start. Several connectors plugged into the Vertical Power solid state breaker box were updated to provide better strain relief for their wires. Locking hardware was installed throughout the box to prevent loosening of connections due to vibration. Wiring was added for sensors that will be important to monitor during the first engine start. These include the fuel level sensor for the test tank, oil temperature sensor, and oil pressure sensor. Temperature sensors were installed inside the box to allow temperature monitoring during ground tests. These sensors will help determine if additional cooling is required to keep temperatures in the center tunnel box stable.

Flutter Analysis
A project that has been advancing in parallel to tasks on the prototype is the flutter analysis. Flutter, more technically known as aeroelasticity, is an interaction between the mass and stiffness of the airframe with aerodynamic forces that results in vibration in the airframe. The objective of the flutter analysis is to predict whether or not the airframe will vibrate apart at any speeds or configurations within the operating envelope of the aircraft. Flutter is a concern in all aircraft designs, but it is of particular interest in the design of the DarkAero 1 because of its combination of performance and airframe optimization. Generally, flutter is more and more of a risk at higher and higher airspeeds. The airframe of the DarkAero 1 is designed to be flutter resistant, but a rigorous flutter analysis had remained an incomplete task due to limitations in resources. Sam Jaeger was brought in to move this analysis forward. Sam has a background in aeroelastic analysis and testing that he developed while working at ATA Engineering in San Diego, CA. His initial efforts at DarkAero will involve developing an analytical model of the aircraft to characterize its aeroelastic behavior prior to flight testing. The wing is the first structure being analyzed, with the deflection data from the static wing load tests being used to improve the accuracy of the model. Ground vibration tests will also be performed to measure the natural vibration modes and frequencies of the airframe, but this will occur in the upcoming weeks. There will be more to share on this project as the analysis and testing advances.

Pilot Production Shop
The last tasks to complete the build-out of the pilot production shop were wrapped up in the first week of May. After the keys to the new space were officially received, equipment was transferred there from the hangar. A 26 foot U-Haul truck was used to transport the majority of the production molds, tools, and equipment that will be used to manufacture DarkAero 1 kits. A few larger pieces of equipment remain to be moved. These include the three section cure oven and the lower wing skin mold which is 24 feet long. A new 3-axis CNC router was recently obtained from AVID CNC and is currently being set up in the new shop. This machine will be used in the DarkAero Mold Making Course and also in production to cut soft materials such as honeycomb sandwich panels and carbon fiber billet plates. Setting up molds and equipment in the new building has enabled more firm plans to be established for the layout of the shop. Preliminary arrangements were explored through 2D floor plan drawings, but there is no replacement for getting an in-person feel with physical equipment. The classroom was set up as well, which enabled the first session of the DarkAero Aerospace Composites Course to be conducted in the new building.

Aerospace Composites Course
The May session of the DarkAero Aerospace Composites Course was held in the new building. This was the largest class size for the course yet, with 11 students attending. Students in the May session were working on aircraft and automotive related projects, so relevant lecture examples were selected for these applications. The July 7-8 session is already full, but there are a number of seats available in the August 4-5 session. Seats can be reserved through the DarkAero course webpage here: https://www.darkaero.com/courses

YouTube
Two new YouTube videos were released in May centered around setting up the new shop and moving manufacturing equipment. These videos can be viewed via the links provided below:

Building an Aircraft Factory | New Shop Tour

More than a prototype | The tools for DarkAero 1 production

Looking Ahead
In June we plan to perform the first engine start and continue setting up the new pilot production shop. The first session of the DarkAero Mold Making Course will be held as well.