I had taught myself to use Xfoil so I set about designing a new airfoil. Sticking with MH45 as a starting point, I increased the reflex (following instructions by Mark Drela on the Xfoil user group) so I would need next to no washout. I also took the opportunity to thin the airfoil to 9%. Construction was the same as the previous model. A little more aspect ratio up to 8.3 now, same span.
From the first flight I could tell it was faster. However, it didn't want to stay up. You will see in the photo that I placed the fins in from the tip. This was to stop the flying wing tip stalling death spiral. I new that the cause was something to do with the tips overloading the wing. My idea worked great too, very hard to stall now. But I also realized that fins at the tip would increase the lift. I think you loose quite a bit because of the sweep back. I made fins with a proper airfoil (RG14) and placed them with the camber side facing inwards in their normal position at the tip. This seemed to work well. Lift was better, still possible to stall, but would just have to live with that.
Handling always was a problem with this model. Mainly the fact that when you slowed right down the model would almost lock its direction and angle and you couldn't change it to avoid that last minute stall. What was the problem? I read all I could find on the net and decided it was the dihedral effect generated by the sweep. Anhedral was the answer. It was a real hassle, but in the end I got so fed up with it that I cut off the fuselage and hacksawed the entire wing in half. I glued everything back together again with, from memory, 2 degrees of anhedral. This helped a lot. I'm learning stuff but these swept flying wings are driving me loopey. How many models do I have to build to get things right?
Speed performance was approximately as good as a good foamie like the M60. This was my DS model. It seemed fast at the time. I spent quite a bit of time sanding the wing smooth after the first few flights and this seemed to help. Now that I have the equipment I reckon a bagged one would be ballistic.
After all the problems that I had I swore I would never make a swept wing again. But now looking at these pictures, they do look sexy and are quicker to make than a conventional plane. I've learnt so much. Maybe I shouldn't give up yet.
Conclusion or How To Make A Successful Swept Flying Wing