Disadvantage of Duo-Mono

Disadvantages and ...the solutions.

And yes, there is one. I noticed some things in the articles and my suspicion was confirmed by Dieter Schulz, the designer of the Sunny Ultralight. He wrote me:

"I had a report from a testpilot of an US-company, something like Westland and the Pilot was called Penrose, I seem to remember.
I wrote some words about those planes in my first Sunny informations in the beginning 90th. The pilot Roger Penrose found the craft remarkable stable and well going, except: landing! Under low speed conditions the rudders did not have enough power and the craft was very difficult to hold over the strip.
This report was one of the reasons I used a swept back upper wing (directional stability) and built in "spoilers" instead of rudders. Rudder usually generate a side force and spoilers a longitudinal force parallel to the x-axis. And as one can see by a short look at a Sunny from the top: Longitudinal x-forces at the wings end are the best to turn the craft around the z-axis. Rudders would be of no sense there."

To better understand what a spoiler is, go see the next drawings.

Classic ruddersat wingtips of rear wing: The rudder forces (red push the rear wing sideways). But, remember, the distance of these rudders to the Cg is rather short when compared to other airplanes. The DRAG of the rudder with deflected control-surface (blue) is the same on each side. So they are making no changes in steering.

 

Spoilers: Here only one rudder is deflected. The sideways rudder force is the same, but it is less due to the fact that there is only ONE rudder deflecting. But ... the right rudder has its original drag and the deflected left rudder has more drag. So ... it slows down that wingtip and pushed the airplane around this wingtip. There is a extra force to force the airplane to yaw.

Like you see. There is already a solution. So, don't let this disadvantage of the old Delanne airplanes hold you back from trying it.

Just a extra note: Penrose believed that much in the airplane that he proposed a similar airplane, the Penrose Gremlin. Stress and performance calculations were already done. But the proposal never got build because the intended light engine (Wren) never got materialized. All the others engines at that time with the same performance were too heavy for this proposal. The engine was placed on the rear of the airplane and a heavier engine would have placed the engine needlessly backwards.