Model This: Joined Wing Airplane

Wondering what these funny looking airplanes are all about?

 

 

 

Pickerel

My Pickerel model airplane design started out as a flying wing. I quickly realized that I would never be able to make that work, unless I compromised on my design goals. Not willing to compromise, I explored alternative wing configurations.

My research with a spreadsheet and a series of half-size prototypes showed me that some sort of tandem wing configuration could work very well to meet my design goals. For strength, I wanted a configuration where the two wings supported each other.

Over the years many airplane configurations with two wings that touch each other have been tried. Some names that have been used to describe them are box wing, diamond wing, rhomboidal wing, and joined wing. I settled on what are commonly called joined wings because of their good looks and good performance.

Joined Wings

Many variations on the joined wing concept have been invented over the years. I will describe here what I consider to be the most common configurations.

A joined wing airplane has a front wing with a sweep angle of zero to sixty degrees. It also has a rear wing with a negative sweep of from zero to sixty degrees. The tips of the rear wing end somewhere along the span of the front wing. Obviously, both wings cannot have a sweep angle of zero at the same time.

The two wings could be located vertically at the same level. More commonly, the root of the rear wing is mounted higher than the front one.

History

Joined wings were originally proposed as a design configuration in 1986 by Julian Wolkovitch. He wrote about them and was granted a patent. For many years the patent kept other researchers away. In the meantime, Mr. Wolkovitch was unable to capitalize on his invention.

After the patent expired interest in the design started growing. The interest in these airplane shapes has exploded in the last ten years. However, no commercial full-size airplanes have been put into production using them.

While doing the research for this article, I was very surprised to learn that there have been very few joined wing model airplanes built. Given their advantages, this must be due to the lack of awareness. Hopefully this article can help change that.

Advantages

Joined wing airplanes have slightly lower induced drag than conventional airplanes of comparable size. Avoiding induced drag is most important when flying at low speeds. I would not get too excited about this benefit. The advantage is only about 5%, and it could easily be slightly worse than a conventional airplane if it is not designed just right.

Because you now have four wing halves, you have twice as many places where to put control surfaces. Using programmable mixes, these can be used to design a highly maneuverable airplane. If you are into extreme 3D, this offers tantalizing possibilities.

The primary advantage of joined wing airplanes is lower structural weight. In fact, a joined wing airplane could be up to 30% lighter than a comparable single wing airplane. Given how important weight reduction is to airplane design, this is an eye-popping advantage. Under similar flight conditions, the tip of the single wing configuration can deflect three times as much.

Better Climb Performance

Having a lower flying weight and a lower induced drag is an interesting combination. We normally use full power when taking off and climbing. Both of these factors improve the length of the take off run and the climbing performance. An interesting option is the possibility of using a smaller power plant for the same performance as a conventional design.

Challenges

A lot of the research work in recent years has gone into learning how to analyze these airplanes. The interactions between aerodynamics and structures are complex. So far it has been mostly large corporations with big research budgets that have tackled the design. Fortunately, with the massive amounts of computing power available to researchers today, much progress has been made in a short amount of time.

I was expecting to run into some problems with stalls, but the problems have simply not materialized. All of my prototypes have had very pleasant and controlled stalls. No tip stalls and the front wing has always stalled first. Given the very clean configurations, airspeed is regained very quickly once the nose drops. There is very little loss of altitude during the stall recovery.

Coplanar Wings

The configuration that I am using in my Pickerel model airplane design is a coplanar joined wing. Neither wing has dihedral and they both lie on the same vertical plane.

I am not the first person to explore this configuration. For years NASA funded a research project that favored a coplanar joined wing design. This was called the SensorCraft UAV concept vehicle. Both McDonnell Douglas and Boeing developed designs. Below is the link to a patent granted to McDonnell Douglas. Boeing called their design the “fluid wing”.

My main motivation for using this wing arrangement was simplicity. Also, without a vertical offset between the wings, there is no compressive buckling force on the rear wing.

Lessons Learned

I have included a link below for the most useful recent research paper I was able to find. They extensively tested many different joined wing configurations and gave specific design recommendations. The primary result table is reproduced at the right.

I would not blindly apply their results to a model airplane. But some guidance is far better than no guidance at all. Their results are also not intended to be point solutions. There are a wide range of configurations similar to their recommendations that work well, too.

They concluded that you generally have two choices. Either put a large vertical distance between the wings and use thin airfoils, or use a low vertical offset between the wings and thick airfoil sections. Of course, thin and thick are relative terms. They used 10% and 20%, respectively.

They also concluded that the joint should be in the 50-75% range of the wing span of the front wing. Other studies have shown that 70% is about optimal.

Finally, flutter is more likely as the sweep angles of the wings increase and as the joint moves forward towards the root of the front wing.

Conclusion

I have been very impressed with the performance of the joined wings in the Pickerel prototypes. I definitely plan to explore them further in future designs. I hope you give them a try, too.

McDonnell Douglas Coplanar Joined Wing Patent (PDF)

Joined Wing Research Paper (10MB PDF)