4130 Steel Airframe
Part 4
By Greg Gremminger
Traditionally, gyros have been constructed with bolted and gusseted aluminum airframes. For more durable and long‐lasting reliability, and with a century of experience in the processes, 4130 “Chromoly” (Chrome‐Moly) steel structures have been refined and adopted as a primary structure technology for the entire aerospace industry. Lately some gyroplane manufacturers have begun using welded stainless steel as the primary structure in order to try to save weight and cost above standard aircraft steel technology.
Magni may be the only major manufacturer that has applied aircraft standard 4130 “Chromoly” steel and associated aerospace industry standards to its primary gyroplane airframe structure. Magni steel airframes are professionally welded with varying tube wall thicknesses engineered to address distributed strength and fatigue stress requirements while minimizing total weight. Stainless steel has a low specific gravity (low weight per unit volume) and may be less expensive than 4130 Chromoly. But, Magni, and most in the aerospace industry, consider stainless steel to be unacceptable for airframe structures because of its much lower resistance to fatigue cracking, difficulties in specialized welding and weld stress relief processes, and need for thicker tube walls (defeats perceived advantage in total weight) to offset its strength and fatigue deficiencies. So far, gyroplane industry experience with stainless steel airframes has incurred structural fatigue cracking in critical areas (the mast!!). Additional welded components have been added to address cracking in specific high stress areas, but typically such remedies simply transfer the high stresses to new areas.
4130 “Chromoly” steel is an alloy steel, containing Chromium and Molybdenum that is widely used in the aerospace industry because of its superior strength and fatigue and corrosion resistance. Chromium increases the hardness, elastic limit, tensile strength, and resistance to corrosion and wear while reducing thermal conductivity – a welding process advantage. The Molybdenum further increases the strength and the hardness and improves the response of the metal to the various treatments post‐welding processes. The steel Magni uses is professionally “normalized”, an aerospace standard treatment that improves the grain of the steel returning it to its original condition after being worked. This leads to an improvement in the strength and performance of the welds. Such normalization is difficult and less proven with welded stainless steel.
4130 Chromoly has much greater strength in both compression and tension, and with a lower specific gravity than other steels, including stainless steels. The 4130 alloy steel Magni uses is the aeronautical steel par excellence. It is particularly strong (many other steels, including stainless steels, crack and break well before the 4130 even bends!) and is more corrosion resistant than even many of the stainless steels.
Magni uses 4130 steel for the complete airframe structure, all rotor head components, all control linkage, and all metal parts that have any structural importance. While we appreciate experimentation with new materials and structural innovations, Magni does not consider stainless steel airframe technology to be properly matured and appropriate for production aircraft structures. The truth be told, with proper engineering, 4130 Chromoly steel still achieves superior strength per weight performance over all other materials ‐ with mature and decades proven superior resistance to fatigue cracking and corrosion. With proper engineering, 4130 Chromoly steel still far exceeds the presumed weight advantage of lighter steels and even aluminum – which may anecdotally be demonstrated with the necessity of additional airframe structural components added to later evolutions of gyros constructed with these less proven materials. And the fact that those manufacturers specify relatively short life limits on AIRFRAMES and critical. ROTOR COMPONENTS suggests that perhaps even those producers feel those technologies are not quite ready for prime time use in aircraft. In particular, fatigue strength is critical in rotorcraft. Magni 4130 airframes have no life limits. Magni has experience with airframes that have over 3000 rugged student training hours, even operating on more rugged turf runways throughout the world. As with rotors, doubts about structural reliability, especially concerns with ‐ often hidden ‐ fatigue cracks in more brittle materials, can really take the fun out of flying. Here again, the old adage, “you get what you pay for”, is more than appropriate.
