Following the successful initial Special Issue on "Aircraft Design (SI-1/2017)" and the relaunch with "Aircraft Design (SI-2/2020)", this is already the third SI in sequence named "Aircraft Design (SI 3/2021)". Activities in the past showed that aircraft design may be a field too small to justify its own (subscription-based) journal. A continuous open access special issue may fill the gap. As such, the Special Issue "Aircraft Design" can be a home for all those working in the field who regret the absence of an aircraft design journal. SI-3/2021 contains six papers (original research articles) about 1.) Oil Fumes in the Cabin and Flight Safety, 2.) Closed-Loop Flying Qualities Requirements, 3.) Preliminary Design of a Medium Range Box Wing Aircraft, 4.) Influence of Novel Airframe Technologies on the Feasibility of Fully-Electric Regional Aviation, 5.) Design and Optimization of a Large Turboprop Aircraft, 6.) Sources of Onboard Fumes and Smoke.