Keeping the Wings in One Piece

King Posts, Struts and Cantilever Part I

By: Norm Goyer

Let’s start with some real basic facts: of the terms airplane and aircraft; airplane is the most accurate. The airplane has wings that plane through the air. A wing can be as flat as a barn door and the aircraft will still fly. Bernoulli’s Principle has nothing to do with a wing generating lift. You can have the most sophisticated airfoil in the world and without AOA (Angle of Attack) the plane will not take off. Some airfoils like the Clark Y and the modified version on the Piper Cub have several degrees of incidence (AOA) built into the airfoil. So the wing can sit at zero incidence on top of the fuselage and still have AOA so the plane will take off.  Airfoils are a whole different subject which we will leave to another edition.

Today the subject is very basic: what prevents the wings from snapping in two when they are overstressed? The design of a wing which has no struts or guy wires is called cantilever (without external bracing.)  

Cantilever wings are seen on most modern low wing and high-performance aircraft. The internal structure of a cantilever wing is quite complex to overcome the stresses generated in flight, lifting loads, landing loads and drag loads.

A Cessna 172 is strut braced; a Piper Warrior has a cantilever wing. Eliminating the high drag of cables and struts to counteract the stresses has been the goal of aeronautical engineers even before there were aeronautical engineers. Early aircraft designers, like the Wrights and Glenn Curtiss, used enough guy wires, turnbuckles and struts to stretch around the world. Most of the early aircraft used both struts and cables. Anthony Fokker and his designer Reinhold Platz were among the first to design an aircraft that did not need struts or cable bracing. The Fokker, with its true cantilever wing, was the 1918 Fokker D-VIII. The aircraft never had a real chance to show its worth, as it was released very late in the war and saw very limited use.                                                                    
I filmed this Fokker D-VII at the annual Geneseo, New York Air Show a few years ago. This aircraft was considered so advanced that they became part of the war reparations after the Armistice. Many were still flying well into the late 1920s. It had a superior Mercedes six-cylinder in-line overhead valve engine.

This high wing parasol had a thick airfoil, so the wing could be internally braced to avoid the use of struts. The wing was covered with plywood, and then covered again with linen for weather protection and ease of painting. This revolutionary wing design was also used on the late 1920’s  Fokker Trimotor airliner, similar to the one that broke apart in the air while carrying Knute Rockne, the football hero.
 
Note the familiar design of the wing on this 1920s Fokker Trimotor transport. It was the same design as that used on the earlier WWI Fokker D-VIII.

Even though the wood wings had nothing to do with the crash, the public blamed them. In reality, a severe thunderstorm the plane flew through was the culprit, not the wood wing. The publicity from this widely reported crash caused the hammer to drop on cantilever wood wings on airliners. Lockheed took up the challenge and produced the outstanding Lockheed Vega series that has been mentioned in previous editions. This plane also had a cantilever wood wing and broke numerous long distance and altitude records. Again, properly designed and stored wooden airplanes are just as strong as those with any other construction techniques. DeHavilland proved that with their Mosquito twin-engine bomber/fighter during World War II.

The ultralight movement that started in the 1970s revived the king post as one means of controlling landing and lifting loads of wings.
 
A large number of early ultralights used king posts to handle landing loads. Bracing was via cables.

Again an explanation is in order: cables can only supply strength in one direction, pulling. Wing struts can supply strength in both directions, pushing and pulling.
The Aeronca C-2 and C-3 also used a king post on top and the fuselage on the bottom for cable attach points.

In the 1930s, there were two certified aircraft that used king posts, the Aeronca C-2/C-3 Collegiate and the Buhl pup.                                                     
The Buhl Pup used a king post in front of the cockpit on top and the fuselage on the bottom to secure the cables.

The cables going to the top of the king post were for landing loads and the cables going to the bottom of aircraft were for lifting loads. Many airplanes used the landing gear to attach the lifting load cables. Numerous king post and cable-related wing-failures caused the CAA not to approve any more certified aircraft with king posts. They disappeared, only to appear once again on ultralight vehicles, which were not considered “airplanes” by the FAA. But struts also caused problems. There were a series of accidents involving older Piper aircraft with wing struts that failed due to internal rust, caused either by rain or condensation gathering in the bottom interior resulting in failure of the struts. When the struts failed, the wing would fold. An AD was issued with new, harsher inspection methods and a mandatory replacement. Cessna brought out their partially wood constructed Airmaster C-34 cantilever four-passenger plane with a small radial engine in 1937.                                
The Cessna 1937 Airmaster was considered the epitome of personal transpiration with its strutless cantilever wings. The Lockheed Vega also used this type of wing very successfully many years earlier.

 One of the main causes of fatal crashes during the Thompson Trophy and other racing events were wing failures due to huge “G” loads occurring during the tight pylon turns. Post-war aircraft production saw both strut braced and cantilever wings used by manufacturers of high wing aircraft.

Next week, Post-War Wing Technology