Ok, here's the situation: I have Goldberg 10-300 Ultimate Bipe 60" WS weighs about 11lbs with a Zenoah G20ei turning a 16x6 Scimitar prop. It flys great, rolls are crisp and will knife-edge with almost 0 coupling. But....its snaps out at the top of a loop. I am baffled. CG is right on, maybe just a tab noseheavy. Could it be too much elevator?

Could be too much elevator, what kind of loop are you trying to do? Is it big and graceful or are you just hammering the up elevator full deflection and hanging on for dear life?

classic weight and angle of attack situation.
The fix is more speed thru a larger loop
easier said than done with some models

As rmh says, too much elevator. The wings are stalling. Fly a bigger loop.

Bliksem

Could you explain that a little more?

It is precisely at the top of the loop where less elevator should be applied.

May it be that the plane stalls and drops the nose at that point just for lack of speed, especially in windy conditions?

http://www.rcflightschool.com/Sport_...ortD27-28.html

Yes, it is right at the apex of the loop, and I am holding full elevator at this point.

Just release back elevator at the first 120° of the loop and float over the top.

the powerplant used and the weight and type model says that power is likely used up at top of loop
I am very familiar with that power setup and at that weight you are maxed out .

In that case, diving before starting the loop and reducing the radius may help reaching the apex with decent speed.

Too little power and too much weight. That airplane really should be in the 8 to 8.5 lb range to fly really well.

What's happening is that you're losing speed up the first half of the loop and the model is slowing down enough that with full up elevator it's literally stalling and then snap rolling out of the loop.

As mentioned already you want to ease up on the elevator over the top or get more speed before entering the loop.

Loops done with constant full up elevator tend to be rather egg shaped anyway due to the speed issue. So to make them a proper round shape you need to ease up on the elevator over the top then apply full elevator on the last part of the downside. Precision aerobatics pilots are using a constantly varying amount of elevator to generate truly circular loops for this reason.

And if your model has a lot of elevator deflection it's likely that you're making it loop so tightly that it's near the edge of a G loaded stall for much of the loop. Try getting up a bit more speed and then do a loop using a bit less then full up so you have something to work with for altering the curve at various points around the loop as the speed alters. The loops will be more round and at the same time less likely to do any high speed/high G stalls.

Prebres... You are getting good advice here from all the posters. I had a heavy Superstar once that would snap in virtually any flight mode with a little up elevator and a touch of rudder. Always had to release back pressure (lower angle of attack) at the top of the loop or it would snap or almost snap. Made for some impressive maneuvers but that problem finally caught up with me and the plane is no more.

Did Goldberg ever make a 60" span version of the Ultimate?.. i thought it was 54" span only?

If it is the 54" span version then you areabout 3 lboverweight which will explain the problem you are having, basically you are stalling.

try doing a loop from a dive and ease the elevator a bit

There's your problem. Ease the back stick as you approach the top of the loop (nose about 30 deg above the horizon) and fly the plane across the top of the loop, applying more back stick as you start down the backside of the loop (nose about 30 deg below the horizon..I usually ease some power, too, to keep from getting excess airspeed/g down the backside). This will make a nice round loop, not one 'pinched' at the top.

BMatt nailed it right on the head.

He is likely maxed out on power as the model approaches top of loop - therin lies the problem
Somehow the speed has to increase relative to size of loop as I mentioned on first post - this is a classic problem with weight/power wing loading.

For sure...proper entry speed into the loop was assumed. Start up too slow and all bets are off.

Well thanks for the good tips guys. I cut down on my elevator in the loop and it is no longer snapping out. It's now making giant graceful loops like a biplane should.

While, I have you guys here, one more question....What would you say would be the best prop for this combo. I am running a 16x6 scimitar, and it's turning almost 9000 rpm. That seems a little much on the high end. Could I do a 17x6 or 18x6 maybe?

any and all advise is welcome

The best way is to get a few props and try them. The first goal is to let the engine operate in the optimum power range. The second is to provide thrust in a way that works with the model. There's a number of diameter to pitch combos which will allow the engine to work in the ideal RPM range. The trick is then matching one of those options to the plane's weight and preferred flying speed. There's no doubt that since it would appear that your Ultimate is a bit overweight if the previous posters are right that your model will do best with a different prop than the same model which is at the optimum lighter weight. So the only decent way to find out is to try some different options. The props that don't work out? Either keep them for some future model that uses the same engine or sell 'em off at a club swap meet somewhere along the way.

You are not far off -for that engine
They were designed to run at about 10000 -typical for Zenoah industrial engines
as a rough aprox, you will pick up about a grand in level flight BUT on climbing stuf you will be closer to the readings on the ground
Anything over most 16x6/8 props will likely reduce power -on that 20

I presume you are not running tuned exhaust - the in cowl mufflers reduce max power so don't expect a larger prop will increase performance - typically on the smaller gassers they have to rev pretty well.
On my 25/26 cc gassers -on tuned pipes - I propped on the ground for about 8500-9000 rpm-props 18x8 wood
These engines both had recording tacs built into the ignition and maxed out readings (no dives!) were10000 + which was about max power on gasoline do not over lean the Zenoah- it should hold rpm from level flight-thru a loop- this means a teensy bit rich for level flight.

Just a thought but my Falcon will snap at the top of a loop and will roll upright at any moment when flying upside down. I fixed this by adding weight to the rear, only because I was told a slightly nose heavy plane has a tendency to fly in a stable and upright position. are the full size acrobatic planes balanced heavy to the tail?

actually the cg is not related - as for aerobatic stuf - typically cg is a bit further aft as some types of flying work best when stability is minimized
NO HARD FAST RULE

Well after a little trial and error I found that was was trying to pull too tight of loop. If I make a huge loop it has no problems. Also tamed down the throw of the elevator just a little. Now she's flying like a Bipe should. Thanks for all the help guys. The Ultimate and i will be at DOGS in Dayton next weekend!

Yup! forcing the wing past the "critical" incidence stalled your airplane causing it to snap out on the top. Too much throw on elevator. Reduce throw and you'll be fine.

Brian