It currently has several potential problems...it is not covered so is very flexible...

A friend gave me his Noseeum, it's a small (about 30") EPP flying wing, designed to be hand launched. He was not able to get it to fly right, so he gave it to me.

From : Don Stackhouse

Be careful. Is the stiffness less than you FEEL comfortable with, or is it less than what the model actually NEEDS? The two are not necessarily the same. Many builders beef things up unnecessarily. If it flutters on launch or at high speed, then it needs more stiffness. However, if it's performing ok in this regard at all normal operating speeds, then you gain nothing by making it stiffer than necessary.

...the elevons are also very flimsy and don't have enough throw...

Based on what criteria? What you need to ask yourself is "are they getting their job done?" Once again, unnecessary stiffness improves nothing, it just adds weight. More throw than necessary to get good control response just adds drag and makes the model touchy to fly. Don't second-guess the designer unless you're already very familiar with their design, not just the what's and how's, but also (especially) the WHY's. If the model has a good reputation (and I agree that there are some on the market that don't), I would be inclined to give the designer the benefit of the doubt and try it their way first.

...and it appears to have a fair amount of anhedral.

It also has a lot of sweep, and it may NEED that anhedral. Read on...

I don't know what effect the anhedral has on it's flight characteristics, but the other issues are easy enough to fix. Would you be able to offer a suggestion as to whether I should make the wing more flat or leave it as designed?

There is a balance between dihedral and vertical fin effect. Too much fin and the model becomes spirally unstable (in a turn it wants to tighten up into a "graveyard spiral"). Too much dihedral and the model wants to dutch roll (rocks side-to-side like a falling leaf). In some models it can be a very delicate balance.

Now the catch: to some extent, at higher angles of attack sweep acts just like dihedral. On a swept flying wing like the Noseeum, the sweep is necessary to get some semblance of a non-zero tail moment arm so you can get at least a little yaw stability. Unfortunately, as you add sweep you are also increasing its dihedral effect. The tip fins can add additional dihedral effect. It's very easy to end up with enough dihedral effect that the model gets into a dutch roll problem. By adding the anhedral, you cancel out some of the sweep's dihedral effect, and help get the system back into balance.

I recommend that you try it first the way it shows on the plans. If it shows problems that way, THEN AND ONLY THEN should you start modifying it. The designer of the Noseeum spent a fair amount of effort getting the model to behave the way he thought it should, and chances are he made it the way it is for a reason.

Don Stackhouse
DJ Aerotech