I am almost done scratch building a mini cruiser, but I have a question about thrust angle of the power pods. It looks like the motor pods have a pronounced downward thrust angle. Am I correct this is intentional, or am I supposed to use the neutral/square side of the motor pod? Is the downward thrust angle the correct way to set up this plane?
OK, I found out the thrust angle in the plans is correct and just fine. But I have a question about CG. I have tried it from the mark on the plans (1.75" back from leading edge) up to ~1.25" back (the number on the store web page), and I'm still trying to dial it in.
For those of you that have built and flown mini cruisers, what CG works for you?
My scratchbuilt FT Mini Cruiser is now one the best flying planes in my hangar, but that was not always the case. In fact, for a few months after the maiden, it was the worst-flying plane in my hangar. In case this is helpful to others of you building/owning this plane, here are some tips that worked for my plane.
1) Center of gravity. My plane only flies well if the CG is set to 1 1/8" back from the leading edge. The plane is very sensitive to small changes in CG. With CG at 1 1/8", long or short loops are perfectly clean, inverted is easy, turns are carved, and it generally just flies great.
2) Down-Thrust and throttle-to-down-elevator-mix. This plane has a ton of power for its tiny airframe. I fly it with 5 x 4.5 x 3 props on twin A-pack motors. This is a monstrous amount of thrust for this little plane---it would be easier to handle with less prop, but I eventually figured out how to make these props work well. It always wanted to go "belly up" and point its nose at the sky if I went past 50% throttle, regardless of how far forward I put the CG. After watching the FliteTest episode on thrust angle, I started adding down-thrust and life got better. My (limited) understanding is that my plane needed a lot of down-thrust-angle because of the relatively large amount of thrust (compared to the weight) and how close the props are to the CG on this plane (resulting in a short lever-arm between prop and CG).
I added as much down-thrust as I could (see picture), then added a throttle-down-elevator mix to finally get it flying level at all throttle settings. It will now scream through the sky on full throttle or cruise around at 35% throttle, all at level flight. The throttle-elevator mix uses a -70% offset and a 10% rate.
Throttle-down-elevator mix on a Spektrum transmitter. You'll need to vary the "10%" rate depending on how your prop/motor/power-pod-thrust-angle setup.
3) Cockpit "glass". It's worth installing the seemingly optional cockpit "glass" cover (we cut up a plastic water bottle bottom). My plane flies better with it installed, presumably because it smooths out what is otherwise an air damn behind the CG.
4) Wing tips. For the first 3 months of its life, this plane would tip stall like crazy---giving it a mean, nasty, and angry personality. It would bit me hard if I even thought about slowing down. That's now cured with a simple modification. The idea came from a retired Boeing engineer who I met at the flying field. I showed him my angry mini cruiser, and he assured me that all I had to do to tame it was smooth out the airflow over- and under- the under-cambered wing tips. He told me to replace the square cut foamboard leading edge with a thicker, more rounded shape created by adding a piece of 3/16" x 1/2" balsa (or a 2nd piece of foamboard) to the underside of the wing tips, shape this piece into a smooth wedge-like shape to meet the underside of the wing, then sand this 2-layer leading edge to be smooth and rounded (see pictures below).
This small, easy fix completely changed the character of the plane. This plane is no longer angry, and has completely stopped biting me. Stalls are now gentle, nose-down, no-tip-stall affairs, and it now lands nice and slow. I now do this on all of my foamboard wings that have elliptical wingtips. For example, I did this on all 4 wingtips on my foamboard Beech Staggerwing.
Here are pictures of this modification using a small strip of foamboard (these first few pictures are actually from my Staggerwing). I make the strip of foamboard into a wedge shape by ironing one side of it with a laminating iron set to ~330 degrees Fahrenheit.
Then hot glue the shaped strip to the underside of the wing tip.
Then sand both the 2-layer leading edge to a smooth rounded shape.
Here is the same modification done to my Mini Cruiser wing using a 3/16" x 1/2" strip of balsa wood instead of foam board, sanded into the wedge shape, glued to the underside of the wing, then the foam + balsa leading edge is sanded smooth/round.