ORIGINAL: da Rock

There is a better, easier way. 9 measurements with a yardstick and choice of static margin: http://www.geistware.com/rcmodeling/cg_super_calc.htm Run it with a 5% then a 15% and you've got a safe, reliable CG range. It includes everything on the airplane that affects the CG. And tells you where the NP is along with a lot of other things.

You do understand that the problem is the CG affects both how the plane flies and how it handles on the ground also. Since it matters more how it flies, that's the deciding factor on where the CG get's moved to. With planes like the Decathlon, you then have to arrange the gear to match. And that's where the guys who've moved CGs forward to be extra safe have screwed their ground handling.



13x6 too big? They were for my 55AXs. My planes that have 55AXs on them have done best with smaller. I did a prop test session awhile back with the help of a buddy who ran the stopwatch. That's when I had just retrofitted a new 55AX on my Calmato Sport that'd had a 46AX on for a couple of seasons. So I was pretty experienced with how that plane flew. The watch was for vertical comparisons.

The two 13x6s tried gave the least rpm on the ground (as expected) and did the poorest in the vertical tests. RPM figures won't mean anything much and especially since the DensityAltitude on that day was abnormally high, highest of any tests so far.

Anyway, the 13x6s both gave rpms under 10k and shortest climbs.
A 12x6(3) was next slowest to those two. As expected, it still turned in the 3rd highest climbs.
A wooden Zinger 12x6 was about as slow and did average climb numbers.
An identical Zinger cut down to 11.5x6 was slightly better but still average.
A wooden 12x5 made from a cut down 13'' prop was the star of the show. RPM was around 11.8K but the climbs times were significantly higher than next best.
An 11.7x6(3) was next best and 2nd overall. Interestingly, it gave the about the same RPM as the 12x5. They probably have the same overall blade areas. It was made by slightly trimming a 12x6(3) identical to the one that'd done the 3rd highest climbs (but was at least 1K slower, which shows how little you can tell from ground rpm).

The climb times aren't the deciding factor, btw. But they're almost always good for the props that show up best in the maneuvers part of my tests. They're subjective

ORIGINAL: prr6100

used 11 x 6 prop for the first flight on mine, also left the cowl off too check everything out

ORIGINAL: da Rock

Take more than one and test 'em. The props we get often vary so much that two of the same dia/pitch of the same brand will sometimes differ. Not usually too much, but enough to tell. Also, the altitudes at our fields make a difference too. That usually makes the most difference in what everyone gets for results.

ORIGINAL: da Rock

Ground rpm reading comparisons are fun to do but really aren't very informative. They'll tell you if each prop is turning in an rpm range that OUGHT to work or not, but the difference between different props is vastly overrated. About all you can count on when one is faster than the other is that it's probably got less pitch or efficiency. And about half the time the slower one flies the plane better.

One of the best prop tests is to do two consecutive loops and simply judge how well the prop pulls the plane over the top on the 2nd one. It's completely subjective, but works pretty good in spite of that.

ORIGINAL: prr6100

The main thing is to get it in the air!! Fall starts tomorrow, summer flying weather will be over with soon, enjoy!!

ORIGINAL: prr6100

work, what is work? retired last Jan, sorry to hear you still have to work!

ORIGINAL: fcomer84

[link]http://www.myspace.com/ashvilleairshow[/link] What a coincidence! A buddy of mine sent this to my email. And I am supposed to fly a pattern event that day!

ORIGINAL: billd76
I compared weight to HP between all three, and for the HP vs weight the 55 AX won the battle.

ORIGINAL: wyo69cowboy

da Rock, thanks for the tip on the baffling. I too had an OS .46 FXi in my Decathlon, and though it flew it ok, at my elevation(5000') it was somewhat underpowered. I am in the process of replacing it with an Aviastar .53;

ORIGINAL: nitro-pilot

Actually it didn't. You cannot calculate with false advertised data and get any true answer. Tower shows OS specs:

Power Output: 1.75 ps/16,000 RPM

How long do you think the nickel liner will last at 16,000 rpm on that engine? Did you use a tach to produce that 1.75hp @ 16k?
What size prop do you think Tower used to achieve 16k RPM? Probably had a pitch of 1, or less. How much flying do you do in a Decathlon at 16k rpm?

The Decathlon flies fine with a .46, it's basically a trainer in disguise at low rates. I thought the goal would be to fly it scale?as Watch out for those weak tail wheel assemblies on all Phoenix models.

ORIGINAL: wyo69cowboy

Well, my Decathlon met its maker last weekend. [&o] Actually had flown about 5 flights with the Aviastar in it that day, and though it wasn't tuned great, it did seem to make a fair bit more power than the .46 FXi. Ran two flights with an APC 11x6, then switched to a Xoar 12x4. Better pull, more scale speed. I did seem to have some overheating issues, so I pulled the cowl for the last two flights, and it seemed better. Was gonna do the baffle trick, and enlarge the exit hole in the cowl bottom, but in my eagerness to enjoy the new power, pulled up from a low & slow pass into full throttle for a stall turn, and didn't get high enough (air density had to be bad; it was 85 and our elevation is 5000'), stalled as intended and added elevator and throttle to try to recover from the low altitude. Good plan for a spin; bad plan for recovery at that spot! She spun in and smashed everthing back to the servo tray. Wing damaged too, so it's toast. I like the plane well enough though, that I plan on ordering another one in the near future.