In April we continued making progress on the DarkAero 1 prototype. The Wisconsin stay-at-home order has temporarily shifted our activities more towards design and documentation. We ordered hardware for the engine installation and completed further design and CAD work on firewall forward systems. Machining tool paths were planned for manufacturing the remaining nose gear components and more planning was completed on the aircraft electrical system. We also put effort into tools and resources to improve the build experience for our customers. 

Firewall Forward - Cylinder Cooling
The UL Power engine is air cooled and uses a downdraft cooling arrangement with air flowing in a top-down direction over the engine cylinders. Included with the engine is a standardized set of air boxes used to guide the cooling air. The air boxes need to be trimmed to shape, closed out on top, and inlets need to be added to interface the air boxes with the cowling. We finalized the CAD for the final air box and inlet geometry and purchased the tools required for trimming the air boxes. Removable fasteners will be used to join the air box assemblies and make them serviceable. We are pursuing having the inlets 3D printed and we expect them to arrive in May.

Firewall Forward - Oil Cooling System
We completed design details and purchased hardware for the oil cooling system which provides a portion of the engine cooling for the UL520iS engine. We are using an Airflow Systems 2006X oil cooler on the prototype. There are a number of options for forcing air through the oil cooler and we opted for an arrangement with a dedicated duct system. This will allow us to tightly control and optimize the amount of air passing through the cooler. Air flows into the oil cooler starting at an inlet near the spinner and then through a diffuser as it reaches the cooler. We are experimenting with a 3D printed diffuser and duct inlet that are connected by a segment of SCEET duct. We are pleased with our first iteration on the diffuser but have identified a few improvements we want to incorporate in the design going forward. Other work on the oil cooling system involved converting the first set of oil cooler hoses to an alternate type with integrated firesleeve. The new hoses trim about a pound of weight from our original set and also have a higher temperature rating and longer lifespan. We are working with an aircraft hose manufacturer to provide a standard fluid line kit for the DarkAero 1 so builders will not have to fabricate their own hoses.

Firewall Forward - Cabin Heat
The cabin heat system in the DarkAero 1 will utilize a heat exchanger that scavenges waste heat off of the cylinder 1 exhaust pipe. Fresh air flows into the heat exchanger through a length of 2” diameter SCAT duct. The air is heated as it passes through the heat exchanger and then it flows to the cabin through another section of ducting. The amount of hot air flowing into the cabin is controlled by a simple valve mounted on the firewall. We purchased all of the required hardware for the cabin heat system that will be installed firewall forward. We plan to test and experiment with the heat ducts within the cabin in order to achieve desirable cabin temperatures in different conditions. If additional heating is necessary, there is space to install an entire duplicate heat system on the other side of the engine.

Firewall Forward - Heat Shield
The firewall heat shield consists of 0.016” thick sheet of titanium with a layer of Fiberfrax ceramic insulation behind it. This material was selected in accordance with the list of approved firewall materials in FAR 23.1191. The titanium sheet was purchased and will be installed once we have all the miscellaneous hardware in final position on the firewall. We plan to do all the heat shield fabrication work manually for the prototype but in production we intend to have this component laser cut to simplify the build process. 

Nose Gear Retract Links
The landing gear for the DarkAero 1 are retracted in flight to reduce drag and the retract links are a key aspect of the design that allow the nose gear to properly extend and retract. Landing gear that efficiently absorb energy and are also retractable can prove to be challenging to design. Even more challenging is doing so with the engine and engine accessories battling for space. To accomplish this with our setup we designed two sets of link bars that attach to the aft trunnion and to a torque tube tied into the engine mount. They are actuated through a stepper motor that drives the torque tube allowing them to fold into one another while in their retracted position.  With the design of these components finalized, we were able to move onto developing the computer aided manufacturing (CAM) tool paths and plan the machining operations. 

YouTube Machining Video
Each time we machine out a new part of the aircraft we use it as an opportunity to learn and improve for our future machining work. In the process we end up collecting a lot of photos, videos, and lessons learned. In March we machined out the first trunnion half for the nose gear. We cut together the footage and lessons learned from machining that component and released it to YouTube in April. We received a lot of encouraging feedback from the aviation and machining community. We have several more parts to machine out and more to learn and share in the future so we look forward to bringing more of these types of videos to you as we progress. Shown below is the second trunnion half resting on top of the machining fixture. The machining fixture was used to machine both trunnion halves for both their 1st and 2nd machining operations. 

Avionics 
Wiring for the center tunnel avionics box was completed at home this month. While progressing through this wiring process we created a spreadsheet based tool that assists with planning out the electrical system. This tool has allowed us to calculate wiring lengths, determine connector bottlenecks, and calculate costs of our system. It also allowed us to virtually reposition avionics in our aircraft and see how these changes impact cost, weight, and wiring logistics. Our plan is to make this tool more widely available to builders who also want a way to dynamically calculate these values while planning their aircraft wiring system.

Improving Canopy Clarity
In February we produced our highest quality canopy form to date. One of the primary defects we have been reducing with each canopy iteration comes from dust contamination. The canopy made at the end of February was created inside of a very crude cleanroom setup with minimal cleanroom practices employed during forming. In March we planned out a room and tested equipment that would take this form a step further to help eliminate dust contamination. This month we received everything we need to build the next version of our cleanroom once the stay-at-home order is lifted at the end of May. Our aim is to build a basic cleanroom inside the hangar that meets a minimum of ISO 5 standards for air quality.

DarkAero Knowledge Base
Over the course of developing the DarkAero 1, we have enjoyed sharing our journey and hearing how others have been inspired. We are thrilled to inspire others, but we also want to enable people to act on that inspiration. With that said, we are excited to announce a new addition coming soon to our website called the DarkAero Knowledge Base!
We have noticed that for many of the projects we have had to tackle, the necessary information has been spread out and difficult to find. We have acquired information from a range of fields and have compiled it internally along the way. To better assist builders of the DarkAero 1, or those that want to design and manufacture around the same processes, we are putting together an initial release of information on these topics on our website within the DarkAero Knowledge Base. We will keep everyone updated on when this release is published to gather feedback for future iterations.

The following are overviews of some of the first sections we will be including in the DarkAero knowledge base: 

Aerospace Hardware
Every aircraft relies on hardware to join components together. Although hardware is small, it is critical to get right. There are guidelines that are part of many different standards established by the FAA, Department of Defense, and other Aerospace bodies for bolts, washers, nuts, rivets, and beyond. In the process of designing the DarkAero 1, we have collected a database of these standards and distilled them down into key documents. One of these is our hardware document which will serve as a reference tool for our builders as part of their work instructions. The document provides an overview of bolt part numbering definitions, torque requirements, bolt lengths, and what nuts and washers to pair with what bolts. Additionally, it includes a full list of lengths, diameters, and weights for AN3 thru AN6. The document links to our CAD and allows our builders to view the CAD hardware for the DarkAero 1 without requiring a CAD license or account. CAD hardware can also be downloaded or referenced if it ever needs to be replaced or updated for servicing and maintenance. 

Aerospace Machining
Each time we machine out a new component we learn a little bit more and improve on our process and techniques. Along the way we have taken diligent notes and compiled them into useful pieces of reference material. For machining we have our own internal spreadsheet we use for calculating our machining cutting parameters both for rough cutting and finish machining. We are adding this worksheet to our DarkAero Knowledge Base for others who may find value in it as well. 

In the News
Simscale blog post:
https://www.simscale.com/blog/2020/04/darkaero-wing-stall-characteristics/

"The team at DarkAero used CFD through SimScale to virtually 'fly' the wing of the DarkAero 1 at a range of different angles of attack to determine the angle of maximum lift and where stall occurs. Once the angle of maximum lift was determined, the Karls further refined their simulation in the region a few degrees before and after stall, in order to then predict how the stall would develop on the wing."
 

May and Beyond
As mentioned at the top of the email, Wisconsin is currently in a statewide lockdown that forces all non-essential businesses to temporarily halt operations. That said, this order does not bar us from doing remote work from our homes and there is still plenty of progress our team will be making over the next month under these unprecedented circumstances. We are hopeful and optimistic that the lockdown will ease up going into June and will not be a significant roadblock to us getting the plane through flight testing this year.