Originally Posted by Jim Woodward
Aio 1 - congrats on your Index II design! It looks like you are installing a strip of depron to the top and bottom of the LE of the wing. Are you doing this for strength, or to replace a flat CF strip in that area?
There's a couple of reasons for the strips. the following is in no particular order.
It locally stiffens the LE. There's less tendency fo the LE to flutter or bend due to flight loads. Also depron doesn't always stay straight over time. If you look along the LE of a single thickness wing some time after you built it there tends to be some ripples. Even when the wing is well braced bends tend to develop between the supports. (This doesn't necessarily have a noticable effect on flight).
It makes the plane a lot more resistant to hanger rash and handling damage which usually occurs at the LE more than anywhere else.
There's also an argument for turbulating the airflow over the wing to make the flight characteristics more predictable. Since these planes fly at very low Re. numbers there's a good chance of attached laminar flow which separates sooner than turbulent flow as you change the conditions. Efficiency isn't really important on these planes so it should make sense to avoid laminar flow. that's the theory. I haven't done enough work on it to be sure it holds water!
I tried a similar strip on my original Index design and rounded it off to a smooth LE profile. I expected this to help with a delayed and more gradual stall (not that these planes have a noticable stall so much as a significant increase of drag). The Index 1 did have really nice high angle of attack characteristics but i can't be sure this was the cause.
For the Index 2 I left the LE square so far and it flies great. My father also built one but rounded the LE. I haven't decided if there's a significant difference yet.
What is the dia. of the rods you are using for stiffness on the bottom of the plane?
It's all 1mm diameter on the original (except for the landing gear which is 1.5mm) but you could probably get away with 0.7 or even less for the fuselage bracing. Since the total carbon in the tail weighs only a gram or so I stayed with 1mm all around.
And last but not least, my "rookie" question of the day - what is the best foam-safe CA to use? I have some BSmith foam-safe CA that gells when I apply accelerator, but doesn't really kick-off like I would hope for. Hopefully there is a better foam safe CA product.
I'm still auditioning new candidates. I've been using a German one which I can't remember the name of but it's not as consistent as I'd like. Sometimes cures very quickly with activator and sometimes doesn't. I think I'll try ZAP next.