November was a productive month for the DarkAero team. Test pilot Sean VanHatten visited DarkAero to refine plans for flight testing, and work continued on painting the airframe. The last portions of the main landing gear systems moved forward as well.

Flight Testing Plans
An important component of flight testing the DarkAero 1 is the engagement of an experienced test pilot. A half dozen test pilots, both civilian and military, have inspected the prototype DarkAero 1 at different stages in its construction process. One of these test pilots is Sean VanHatten. Sean first connected with the DarkAero team at AirVenture 2019 and has remained in contact with the team ever since. In early November, Sean made a formal visit to DarkAero to familiarize himself with the latest progress on the DarkAero 1 prototype and further refine plans for flight testing. The prototype was disassembled for paint during Sean’s visit, so it was an ideal opportunity to display the structures and systems of the aircraft and reveal how everything was built.

The aircraft and systems were discussed in detail, and a number of ground tests were identified to build further confidence in the design prior to flight testing. Some of these tests such as drop testing the landing gear were already planned, but a few new tests were added. One of the new tests will involve constructing a test rig to validate how fuel in the wet wing behaves. The objective of this test is to explore edge case scenarios that could lead to fuel starvation or fuel cross feeding between the wings. 

A preliminary flight test plan has been outlined to be conducted in three campaigns. A return visit from Sean is planned for early 2023 after paint and systems work are complete. The goal of the next visit will be to move into high speed taxi testing and potentially the first flight!

Paint
Painting the airframe continued in November. The first layer of primer on the wing revealed a few minor variations in the wing skin surfaces. These areas were filled with body filler, and then all the painted areas were sanded to refine the contour of the wing surface. After sanding, the wing was returned to the paint booth for two additional coats of primer. The wing was then sanded once more, which brought the contour of the wing surface to within the desired specification. The objective of this iterative sanding and painting approach is to eliminate any small waves or discontinuities in the surface and make the airframe as aerodynamically clean as possible.

The process for painting the airframe was proven out on the wing and then carried forward into painting the fuselage. The cabin and wheel wells were masked off, and then two coats of primer were applied. After the paint cured, the contour details at the seams between the fuselage panels were smoothed out. This part of the process involved some filling and sanding, but progress moved quickly as the work was limited to small areas. Once the seam work is complete, one more layer of primer will be required to finish up the fuselage before applying a top coat of paint.

The final color scheme of the airplane has not been finalized. The prototype will likely be flown in primer and then a final paint scheme will be applied once it is proven that no major changes to the airframe are required. 

Clear Coat
The epoxy resin matrix that allows the carbon fiber airframe to maintain its shape is susceptible to photodegradation from UV light, and paint is the standard solution for UV protection. Several customers have expressed interest in preserving the carbon fiber look of the DarkAero 1 airframe, rather than covering it with paint. A potential alternative solution for UV protection that still shows off the carbon fiber weave pattern is UV resistant clear coat. This option has been used in automotive and marine applications but has not been employed extensively in aerospace applications. A few products are under consideration and are currently being tested in parallel to painting the prototype aircraft. Many UV resistant clear coats actually degrade through UV exposure, so it is important to validate that the clear coat will provide sufficient UV protection for the life of the aircraft. 

Main Landing Gear
The landing gear doors for the main landing gear were manufactured through an infusion process identical to the other airframe structures in the DarkAero 1. The doors will be segmented, given their complicated shape. These doors will be installed on the fuselage and will sequence with the main landing gear retract cycle.

Work resumed on the retract mechanism for the main landing gear as well. The main landing gear uses a similar architecture to the nose landing gear for its retract mechanism. Specifically, it is electrically actuated up and down through a gearbox. The final design of this gearbox had been an outstanding task, but the design and fabrication of this component is now moving forward. Hardware and material for the gearbox were purchased, and assembly and testing of the gearbox is planned for December.

Ultrasonic Testing
Ultrasonic inspection is a non-destructive test (NDT) method commonly used to find defects, voids, disbonds, and delaminations in composite structures. Ultrasonic inspection tools work by sending a high frequency sound signal through the composite structure and listening for reflections. Defects such as voids or delaminations will cause the sound signal to reflect differently compared to structure without defects. This principle can be used to locate defects and determine both their size and depth within the laminate. 

RCON-NDT and Sonatest visited the DarkAero facility to demonstrate their ultrasonic testing (UT) equipment on the DarkAero 1. Other forms of NDT such as thermal measurements, tap testing, and visual inspection had been used on the DarkAero 1 at different stages of the manufacturing process, but the airframe had not been previously inspected using UT. RCON-NDT was able to scan portions of the wing structure and bonded sections of the fuselage. The data showed results with little to no void content, no indication of delamination, and no debonding in any of the scanned areas of the aircraft. Scans were also taken around fastener holes which can be troublesome delamination areas, but all scanned holes showed no indication of delamination. The inspection session was insightful, and plans are being made to incorporate ultrasonic testing in production.

Seat Bottom
New seat bottom bulkheads were fabricated in November. Previously, a single section of folded Soric panel was used for the seat bottom. The recent updates to the fuel system arrangement mandated changing the seat bottom, but the previous design would have required more time than it was worth to modify it. Instead, a new design was 3D modeled, manufactured, and installed. One of the additional modifications implemented was a hinge that allows for easy access to the fuel sump and flap controls under the seat.

Aerospace Composites Course
The 13th session of the Aerospace Composites Course was held in November. Traditionally, one of the class demonstrations involves manufacturing a small wing through an infusion process. During Session 13, the infusion process was demonstrated by manufacturing the main landing gear doors for the DarkAero 1 prototype. Going forward, the plan is to demonstrate infusion using other airframe parts from the DarkAero 1 such as the cowling or flaps. 

The next session of the Aerospace Composites Course will be held on January 27-28, 2023. Other sessions are available as well. More information about the Aerospace Composites Course can be found here:

DarkAero Aerospace Composites Course

The Aerospace Composites Course is also available in an online format to reach those who cannot attend in person. It is offered at a reduced rate relative to the in-person course, making it a great option for college students or individuals who are just beginning to explore composite materials.

The online course can be accessed here:

Online Aerospace Composites Course

The next session of the Mold Making Course will be February 23-24, 2023. This course was born out of interest from students who attended the DarkAero Aerospace Composites Course and expressed interest in developing a deeper understanding of building tooling to create their own composite parts. The goal of the two-day course is to provide a foundation of knowledge and hands-on training so that building molds does not become a barrier in the process of creating composite parts. Learn more and sign up for the Mold Making Course here:

Mold Making Course

YouTube
A new YouTube video was released highlighting the process to paint the airframe of the DarkAero 1 prototype:

Why no one wants to paint carbon fiber

A livestream was recently held for the DarkAero Mach 2 Community Members. An overview of the flight test plan was discussed as well as many of the lessons learned from the painting operations. The event was wrapped up with a Q&A session to close out the night. A replay of the livestream can be viewed here:

DarkAero Q&A Livestream - November 29, 2022

Looking Ahead
In December we will continue with the paint work and begin reassembling the airplane to finalize systems work. We are also planning to apply for the airworthiness certificate for the DarkAero 1 prototype.