hchharawalla wrote: ↑September 18th, 2023, 10:33 pm
Hi Coach Brian and Coach Chuck- this is my first time trying to coach and help my kids out in Flight, and I appreciate all the fantastic NFFS resources so much! This event no longer seems as terrifying to comprehend
. I think I get that the main area for improvement is good prop design and trimming.
I was comparing Gowen's Finny to the SuperFinny design and his flight notes (
https://indoorarchive.github.io/0811/4855.html), and after seeing some great planes last year I was curious about something. How much time and payoff is worth experimenting with larger wing chord lengths? And at what stage would it be recommended to try? It seems to be a trend I've noticed in some planes with high flying times.
I agree with Brian's direction here. First step is to get in the gym with something decent. Our first year we were in the gym with the equivalent of Brian's "super Simple" design, and taking lots of data. We then built 12 planes, and eventually probably 25 props.
With this year's rules (and last year), much less is defined on the layout of the plane, so experimenting with larger chord is certainly on the table. You do not indicate if you are B or C division. In B division, there is plenty of room to experiment. In C, because of the fairly short tail moment (better than last year), you could run into stability issues. As you make larger chords on the wing and stab, you end up with a shorter effective tail moment, which makes a more troublesome plane to trim (sensitive to very small changes).
Once you have a basic layout, build it and fly it. Work first on getting it optimized with a commercial prop, if you have any available. Once trimmed, change things such as CG, and retrim. Adjust torque until you get to the ceiling, or nearly so, and then compare times to see what was better.
As far as props, I would not get too hung up on perfection here. We typically, on years we went to Nationals, built 25-35 props adjusting flare, shape, pitch, etc. This year we add in diameter as a variable, but the box will set your limits on that. We did "Bucket props" where you form the prop shape on a cone or cylinder, and eventually made our own prop blocks. Bucket props can be a very close approximation to a helical pitch progression prop.
We still use the Ikara shape, forward of the spar, with nothing behind, for our planform. Even in LPOP we use that, slightly enlarged on a copier. The LPP has a diameter limit of 12", so we used that experience to benefit last year's Div B props, whereas C div we used the stock Ikara flaring prop size. We used 1/32" C-grain wood for the blades, and did not bother to thin them as we needed mass up front anyway. We did make some with A grain, but they proved to be too flexible (over-flared), which would result in poor climb after the initial burst.
Be sure to keep detailed logs for your flights. You may even want to start a second log for props, with columns for each variable you can control on the prop. Adding prop building adds a number of variables. and that experimental space needs to be explored carefully and systematically. You can only do this with good logs.
We used the Basswood spars for simplicity, and could wet the inner part of the spar, twist it a bit, and dry with a heat gun to make pitch changes. With the Gowen prop hub, you could make small pitch changes by wetting the blade near the spar and bending it slightly, at the risk of changing your airfoil of the prop. When we did those hubs we typically made multiple props with identical features except varied the pitch over a range.
Coach Chuck
