Letter by Ron Ligeti.

In November 2003 I got in contact with Ron Ligeti, the son of the designer of the Stratos. I asked him to explain a bit more about the stability of this concept. He sent me this letter:

α1 = The max angle of attack of the wing (related to air flow) α2 = The angle between the two wings

"If the max angle of attack of the airfoil is greater than the angle between the wings, The wings stall before the turbulent wash of the canard will hit the main wing.

As the air hits the canard you get a natural upwash the air curves over the canard and then hits the main wing at a reduced angle of attack causing the canard to stall first.

Difference in angle of attack = upwash angle (canard) + downwash angle (main wing)

On a BADLY designed tandem wing aircraft the stall may create a uniform turbulent wash over the rear wing causing a deep stall. Research has shown that you might even get  the aircraft in a condition where you can not get the rear wing back out of a stall once you are in it.

A GOOD design usually has the wing loading on the canard higher than that of the main wing. The vertical /horizontal distance between the wings is set so that the wings stall before the turbulence of the canard effects the main wing. The incidence angles of the two wings can be different so the main wing is at a lower angle of attack. The main wing can have extended tips that are in clean air just in case.

*There are a couple really good tandem wing aircraft out there.

If you did get the Stratos in a disruptive airflow state (condition A). You do not create a uniform turbulent wash over the whole main wing but only in the center part. Even if you increase the angle again (condition B) You only partially disrupt the airflow over the main wing. This is purely due to the sweep of the Canard."

It might be sometimes hard to see, but Ron added some notes on the drawings. Look closely!