When I build all my Combat planes, I take care to balance them correctly. On an R&B Rip-off plane, which I use, that balance point is between at the back of the main spar and 1/8” behind the spar with an empty bladder.
I lost at least 10 matches last year as a result of tailheavy planes after I changed engines and/or props. I should know better. At the F2D team trails, in 2009, Jeff Rein and I both had tail heavy airplane because we used a Profi 15 engine instead of a normal PC6 which is heavier. We also used lighter High Performance F2D props instead of the heavier APC 7x3 we use in 80mph Combat here in the Northwest. This not only made the plane more tailheavy but lowered the spinning mass. We both should have known better.
A tailheavy airplane will turn too tight and cause the lines to go slack and that trips the H&R shutoffs, killing your engine. You can't win a match on the ground restarting your engine. If your plane is very tailheavy, you will not be able to fly level, as happen to Tristan McKay in one match at the last Bladder Grabber.
In 80mph Combat it is good to go near 80mph and nice to use a muffler to save yours and your pit man's ears. Many do not use a muffler to get closer to 80mph. I like to take care of our ears so prefer using a muffler. You can use a “better” prop, like a true F2D prop or the new “Scimitar” shaped Master Air Screw (MAS) 6.5x3.5 that is cut down to 6 inches rather than the usual APC 7x3. If you do this, then you can get about the same speed with a muffler as you do without it while using an APC 7x3 prop. True F2D props cost $4 to $5. APC 7x3 and the MAS 6.5x3.5 cost less than $2 and your local hobby shop can order them.
Engine weights
So it makes a big difference to the balance point which engine you use.
Prop weights
Keep in mind that a “lighter” prop has less spinning mass and your plane will turn tighter even without taking into account the overall weight savings or the balance point. I found a couple of washers that can be added in front of the prop that each adds 4.0 grams. This washer makes the new Scimitar MAS prop weigh almost as much as the APC. Even with the washer added the plane still turns a little tighter as the spinning mass of the washer has less affect near the crank shaft, where I added the washer, as it does out near the prop tips.
To modify the MAS 6.5x3.5 props after cutting them down to 6”, I reshape the props using a belt sander which only takes about 2 minutes a prop after you get a feel for it. After rough shaping, I finish sand with 400 grit paper. These props cost less than $2 and perform very close to the Blue F2D prop some of us think is best. Be sure to rebalance the props after cutting and shaping them. I'm even going to try this prop in F2D Proto Speed where I hold the record using a Blue type F2D prop now. If you have trouble shaping and balancing your MAS props, I can do it for you.
This page was upated March 22, 2011