Thanks Howard. Question: how did this downthrust tab work in practice? Did you have to bend it down (or up) for different motors? Could the trough be modified during construction to incorporate downthrust?
Yes I see no reason why that shouldn't work, the tail piece anyway, it would be a suck it and see job and may still require a small tab, I think Stave Bage did something like this on his B&V? tailless design. The part above the motor as you have drawn it would entail quite a bit of messing around with the formers as this is where the wing root comes into the centre of the fuselage.
I had tried a smaller tab initially but it was insufficient for an L2X hence the broad one shown. I cannot alter the angle without making a new slot as I use .5mm ply for the tab. I have reduced the wing tips reflex I added and also reduced the size of my tailplane to the one shown on the plane - don't ask!! I am hoping I will now need less nose weight and a smaller down thrust tab, we will have to see next time out. Whenever we can get back on a flying field that is in all this windy weather.
I'm no expert in rubber power, but I don't think the Cougar would be suitable at all. It is a great shame low thrust long duration motors suitable for model jets are not available in the US .
Dr Jones has developed a Rapier Analogue, but volume production and distribution are a real huurdle.
Have you thought of 'micro' EDF? A 'power train' is available from SAMS Models in the UK. Here's what it looks like:
This gives about 22g of thrust - good enough for a good, if not stellar, performance.