i have a EPP Viper combat glider, the problem is its very unstable, everyone else's has no nose weight and is smooth and sharp and stable in turns... with no nose weight mine is unflyable, i have added quite a bit, but it still wallows not sure which way is forwards, it wobbles around turns and can loose alot of speed getting into stalls easily, its like the CofG is miles back, but its forwards of most of the others on the slope. is this dutch roll? and how do i stop it. i don't want to add more lead, it already has the equivalent of another batt up front, i recently recovered and kept the back light, no difference. i also use less rates than most to keep it in control

My guess -

Take out most of the lead.

Take out some of the up-elevon.

Try it. Repeat taking out elevon until speed builds up a bit.

I suspect that you are trying to fly it too slow. Every flying wing I have had anything to do with likes to be that bit faster than the corresponding conventional aircraft.

Check too that the fins are at 0* and not toed in or out.

If none of those work, take another look over the whole airframe. There is something there which is wrong. Bit like a mate of mine who put the wing on a rubber model backwards ( I mean TE first). Couldn't work out why it looked funny in the air - it still worked, kinda.

Dutch roll is a function of dihedral and fin area - so does not apply here...

taking out the lead leaves it unflyable, taking out up elevon results in a dive, i can never be accused of flying too slow
i've not checked the fins too closely because they are always being knocked off during combat, if this was an accurately built balsa job i'd be worried, but all these EPP wings are built the same way, the gear is in the same place which means you can fit and fly, the CofG is automaticaly correct.
the only difference with mine is it does have a differnent section compaired to the older Vipers, but that couldnt be enough to do this...
i built it in the cores to insure no warps, tho i do seem to have slight anhedrial.
its been recovered twice and its been the same both times.
at speed things are fine, but a few sharp turns slows it down much quicker than other Vipers turning even sharper and going slower and eventully leads to it stalling compleatly and falling from the sky in a rocking fashon.
maybe i need to pull a bit of tape across the top to give dehideral and cut some fins bigger than kit.

If you've got the equivalent of a second battery pack worth of lead in the nose, the extra weight would explain why you slow down so quickly, but not why it's unflyable without the lead. If it flys fine at speed with the nose weight, maybe try shifting the actual battery pack itself so you need less extra weight.

Hate to say it, but from what you've described I kinda agree with probligo; it sounds like a problem with the entire airframe. I don't have much experience with EPP foam, could one wing be very much denser then the other?

In straight and level flight, with no nose weight, what control input do you have to give it to keep it flying level? What happens if you give it no input?

the pack is as far forwards as is possible, i actully cut the hole further forwards. the weight is in the tiny v shape left and another slab of lead ontop, very ugly. i am slightly heavier than others, but probably because of the weight. overall every airframe is identical. if you meant am i ballanced laterially, i dunno, just checked, slighty heavy on the left wing.
the up tim needed is different for each slope and wind stenght, i normally set mine to very slowly dive at netural to keep the speed up. on a full dive it dosnt tuck in or pull out, it gos where you point it, a feature of most of these wings is when you then pull full up at speed it'll spin because both correx elevons will never do the same thing even if your thumb got it right.

OoooKay!!

Have you used adhesive tape for the elevons.

I did on my one attempt with these little rockets and I had a very similar problem - mine would spin out everytime on a right turn, not left turn just right. And it was totally irrecoverable straight down!!

The only possible cause that we could find was the tape had lifted slightly on the right elevon. This meant that when the right turn was signalled, the servo was pulling the front edge of the elevon above its proper position as full up was applied.

Reading your last sentence as quoted I get the impression that either -

The only other possibility that I can think of is fairly unlikely. Is it possible that one side is twisting under full elevon? Errrmm if you give it full up the te of the wing goes down? I can see it happening but not with EPP and fg reinforced tape. Just a thought forya.

Oooohhh!! One other thing...
Are the elevons deeper (chordwise) at tip or centre? They seem to behave better with the taper from tip to centre rather than outward. It seems counterintuitive but it works.

And hey!!! What is wth that gap thingy at the back - is one elevon different to the other? Look at the photo - see how the te's don't seem to match? What is that? An optical illusion?

the elevons are hinged with glass tape from the bottom and the top, and then gone over with the coloured tape, these babys ain't going anywhere, they taper with the root being thicker, for me this keeps the elevator where its meant to be and i'd be happier with a forward sweep on the TE, the correx can bend when you start going over 80mph which makes the pull out a little fun
i do have allot less rate than most of the other Vipers, and allot less response and turning ability, turn it up and it'll turn as its meant to, but induce the stall death spin if provoked too much.
it is optical, one elevon is down.
its behavior can be best described as walking... the wing tips don't want to remain behind the plane and neither knows which is meant to be in front. just looking through my vids, but i cant find one that shows it well, after stall turns the wing on the outside trys to over take after your already heading down. stalls also are flat and it walks down and back rather than dropping the nose to let you get flying again
would more rudder area help? my flutes are open on the fins and run horizontally, can this be a reason?

"Curiouser and curiouser" said Alice.

Have you talked with the manufacturer?

I would seal the le's on the fins for starters.

With the elevons on the way they are, is the te of the elevon at or inside the te of the fins? Is this the way that the elevons are fitted on the plans? Are the fins fitted according to the plans?

The reasoning here is that it looks like there is little difference between Viper and the Canterbury Jazz that I was trying to fly. On the Jazz a big part of the stability is the fact that the fins act as tip plates over the end of the elevons. The elevons on mine (one of the early ones) were parallel, but I believe that the Jazz Extreme now has the reverse taper elevons.

I am past the end of my meagre knowledge with this - was the kit made on a Monday? No, only joking.

As a matter of interest, how does the airfoil on the Viper look? The Jazz had a long flat on the underside rather than a smooth curve. Looking at the profiles it seems that this is consistent with many of these wings. I must go take a look at a Viper at the LHS...

fin to elevon gap... depending on combat damage about 1/8th of an inch, the TE of both the elevon and the fin are the same untill it moves as the fin has a v shape at the rear with less fin on the bottom, unlike my GF's fins on her smaller Mamba which are mirrored.
the section is differnt from other Vipers, i think it has more reflex, the bottom has a slight curve all the way and the top rear stops comming down and flattens out. when i brought it from Soar Ahead Sailplanes (SAS or sore head sailplanes if you fly with us )
I was told this improved the turns with less speed droped, its true i can turn tight and almost seem to gain speed, but its rare and three turns quick turns later you better give up or crash. unlike the others who push on. i think bigger fins and using lighter correx may help, maybe more nose weight to loose the tail heavy like handling