do you really need the wingtips on a flying wing "type zaggi" will the swebt wings not give yaw stabillity in it self ?

I can't answer your question. But I can say, never do a hammerhead with a flying wing. They go straight into a flat spin.

You need the wingtips. As to hammerheads, I have done many with the Zagi, no problems.

They are not always needed for all modes of flight. This is proven by both some full sized versions along with a few models. However while Northrop and Hortten didn't use any on their bomber or sailplane flying wings it's interesting to note that all combat aircraft flying wings DO use some form of fin. I suspect it's because the simple wing shape is only stable in yaw over a limited range of aerobatics..... much as kingwoodbarney found.

Swept wings will provide a very limited yaw stability due to simple drag. As the model yaws more of the leading wing is presented to the airflow and that should make it pull back into place. But I'll suggest to you that it's a weak force that can easily be overcome by other factors.

As a previous post referred, yaw control is a big problem, but also the coupling to other axes. Using a proven design will give you a fighting chance. Even Northrop started with a tail then gradually deleted it. Modern designs (B2) have an active flight management system to keep the rubber side down. A good reference is "Tailless Aircraft in Theory and Practice" by Karl Nickel and Micheal Wohlfart. It is rather pricey new, but used copies can be found from time to time. It is a good read and will give you something to start from. I have experimented with flying planks for years, some work better than others. I needed a fin on all of them, yaw stability is especially noticable when the bird is getting slow, as in the 180 to final. Good luck.

I resurrected an old Hustler Delta a few yrs. ago. Flew around a few minutes, one rudder blows off. Flew fine for a few more min's. till the other rudder blew off. Went into the fastest rotating flat spin I've ever seen, but the sink rate was about three MPH. No damage other than my pride. Can't trust that Ambroid after 40 yrs.

I do it all the time, never flat spins.. might have a CG problem.
Chris

In streamer combat, all wings have vertical stabs for one BIG reason. You might be stable running around at first. But snag 30' of crepe paper on a wing tip, and see how stable you are NOW. We also don't have rudders, so you need something to overcome the yaw from all the extra drag coming from who knows where.

For foamie combat, and streamer combat to a lesser extent, you need that vertical to get out of a flat spin induced by contact with another airplane. In the Open B class, I've hit wing-to-wing head on at about 150mph closing speed. This results in the coolest, fastest, and flattest spins you've ever seen. Getting out of them with out a rudder takes a lot of vertical area.

And finally, tons of vertical area allows you to fly closer to the stall with out snapping. When turning radius is the name of the game, big verticals allow you to turn tighter. I believe this works because the large vertical area works as a yaw damper, and gets the wings to both stall together. Coupled with an airfoil with a soft stall, you get a plane that can fly right at the critical angle of attack, or slightly beyond it, and not snap or fall out of the sky.

A great exampe is the Falcon, designed by Lee Liddle. It's a flying with, with a moderate taper and no sweep at all. It has winglets and a small vertical tail on a boom. I saw the vertical tail get cut clean off, and the plane stay in the fight. Right up to the point where it scored a cut way out on the wing, and the yaw that resulted was rather comical.