Still on the subject of the idiosyncratic begaviour of the Gripen, I had an interesting, and most apposite email from Howard Metcalfe, who is a self confessed lover of the black art of trimming and has sorted out many a recalcitrant model.
Howard writes:
"It’s a very nice shape; why haven't I built one yet? I would not hesitate to build from the plan as is, with everything straight and no dihedral and little or no reflex.Which is how I built the small all-Depron test model below:
In test glides, I added a tiny bit of weight to the tail to stop it diving slightly so that the cg was slightly rearward (by 1.5mm on my 11cm span model).
Even without dihedral it snaps back immediately to straight level flight when launched in a very steep bank either to the right or left and it climbed straight and true if thrown hard.
I added a small weight to simulate the weight of a motor and the model flew exactly the same.
I suppose I could crank in the dihedral as per Piotr's plan but I don't want to ruin such a fine little model. Perhaps Brian had his CG too as the model definitely gets squirrelly when I moved the CG back to the position you see in the photo. As you can see, the rear of the wing root ties up with the drawing but the pin shows the CG when it was getting squirrelly i.e. Dutch Rolling.
Above: Howard's small all Depron test model, showing the CG at the recommended position. This resulted in the sort of behaviour Brian was was struggling with.
Howard ends his most encouraging email: " What a star design Piotr Tendera has here, it is magic I would not hesitate to build from the plan as is, with everything straight and no dihedral and little or no reflex".
According to the instructions included in the Gripen kit, the CG is 49 mm back fronm the front of the wing, which is (49/132) about 37% root cord:
Above: detail from the Gripen instructions. To my eyes, and according to Howard's experiments, the CG should come forward quite a bit (see arrow)!
The CG of my original (and slightly larger) Gripen prototype (which though I say it myself, fles very nicely indeed) is around 28% of the root cord. This translates to 36-37 mm for the TSP kit model:
Above: corrected CG. In this position it is also in the right place with respect to the TSP L-1 motor!
I did add just a tad of 'up' aileron (no more than 1-1.5 mm) to my prototype, but this, as brian says, is more in the nature of 'washout' rather than 'reflex'.
To sum up: The TSP Gripen should be built WITHOUT the dihedral on the wing. The slight dihedral on the canard can of course be retained for scale purposes.
The CG dfinitely needs to come forward to around 28% of the root cord, i.e.
36-37 mm back from the leading edge.
As I said in a previous posting, I will now build my own TSP Gripen and, hopefully, confirm these ideas by getting the Gripen to beat up the airfield at Old Warden!
Above: my original profile Gripen for L-2 power. Soon, I hope, to be joined by its small L-1 powered TSP stablemate.