Mar 10, 2014, 09:02 PM Re-kitting planes since 1990 Australia, WA, Beldon Joined Jun 2013 648 Posts JGAF, thanks for your input. I'll crank up the rates of those ailerons, I think they're at about 30 degrees at the moment. Is there a reason why we can't put the pivot for the stab back at the centre of gravity of the stab itself? I realize that if it's too far back (behind the centre of pressure) then a small deflection will get pushed into a large deflection by air pressure and slop, I'm more talking about making it completely neutral so that air pressure doesn't want to either centre or deflect the surface. The plans show the pivot a lot further forward than this point, so air pressure will work to centre the surface. Being a flat plate, the CG of the stab should be about the same point as the centre of pressure (or the neutral point). I've never dealt with full flying stabs before so I'm shooting in the dark here.
 Mar 10, 2014, 09:10 PM Aerobatics! United States, CA, Carlsbad Joined Nov 2007 1,978 Posts I'm pretty sure the pivot point works well at around 25% of the stab chord, keep in mind sweep/root & tip lengths. I don't do any mathematical calculations but for a 6" stab I usually place the pivot point at about 1.75" and that works well.
 Mar 10, 2014, 09:14 PM Flagstaff, AZ USA, AZ, Flagstaff Joined Mar 2003 3,708 Posts Nebbian: Regarding roll rate: My Le Fish light (21 oz) has a 60" WS; 3" tapered to 1 7/8" ailerons; 35° of throw each way. My Le Fish heavy (43 oz) is trimmed to 62" WS; 2" balsa ailerons; and about 30° of throw each way. They both roll fine. Regarding balancing the "Mad" stab: My Le Fish light is not quite balanced with 12 grams of weight on the stab's LE. It did flutter at speed before I added the balancing weight... so that is why I added the weights. However my linkages were also looser then too. It would be interesting for you to remove some weight to see if flutter appears. FYI... my Buzz Evo has no balancing weights and the linkage is very "loose" and does not flutter. So go figure! Also... the pronounced rudder authority is great for snap rolls and spins. Dawson
Mar 10, 2014, 09:17 PM
Aerobatics!
Joined Nov 2007
1,978 Posts
Quote:
 Originally Posted by dawsonh Nebbian: Regarding roll rate: My Le Fish light (21 oz) has a 60" WS; 3" tapered to 1 7/8" ailerons; 35° of throw each way. My Le Fish heavy (43 oz) is trimmed to 62" WS; 2" balsa ailerons; and about 30° of throw each way. They both roll fine. Regarding balancing the "Mad" stab: My Le Fish light is not quite balanced with 12 grams of weight on the stab's LE. It did flutter at speed before I added the balancing weight... so that is why I added the weights. However my linkages were also looser then too. It would be interesting for you to remove some weight to see if flutter appears. FYI... my Buzz Evo has no balancing weights and the linkage is very "loose" and does not flutter. So go figure! Also... the pronounced rudder authority is great for snap rolls and spins. Dawson
I flew the evo I sold fred up at mt. laguna in 35 mph winds and it didn't flutter then either
Mar 10, 2014, 09:20 PM
Flagstaff, AZ
USA, AZ, Flagstaff
Joined Mar 2003
3,708 Posts
Quote:
 Originally Posted by nebbian Is there a reason why we can't put the pivot for the stab back at the centre of gravity of the stab itself? I realize that if it's too far back (behind the centre of pressure) then a small deflection will get pushed into a large deflection by air pressure and slop, I'm more talking about making it completely neutral so that air pressure doesn't want to either centre or deflect the surface. The plans show the pivot a lot further forward than this point, so air pressure will work to centre the surface. Being a flat plate, the CG of the stab should be about the same point as the centre of pressure (or the neutral point).
This is another great area for experimentation. Some folks pushed the pivot point forward to reduce flutter (for example in the Kulbutin). I think these ultralight aerobats bend the rules of regular aircraft design. I say try pushing the pivot point back to the natural balance point.

My Buzz Evo is flying at a CG near 50% of wing chord... and she is super stable with no signs of tucking in a dive. How does that work?

Dawson
Mar 10, 2014, 09:25 PM
Feeling FrSky
United States, CA, Santa Barbara
Joined Feb 2003
19,698 Posts
Quote:
 Originally Posted by nebbian That said, the roll rate didn't really impress me. I've got 40mm balsa ailerons -- what throws would you recommend? I've got the standard 66 inch wings of the LEG kit.
The 60" span can roll a little bit faster than the 66", but first: what are you using for your aileron pushrods? How much throw do you have?

On my original Le Fish (the 38oz prototype) I used threaded rod and metal clevises along with standard size servos. The roll rate was excellent.

On my ultralight Le Fish, I initially went with a very lightweight but somewhat flexbile carbon fiber pushrod and plastic clevises. Roll rate was mediocre. I later upgraded to a carbon fiber tube and it was a night and day difference... with no other changes.

So it could be a pushrod thing... it's at least something to examine and rule out, if nothing else.

Also make sure your aileron servos aren't moving in their pockets and that there's no slop in your aileron hinges. Your build looks nicely done so I doubt that either of these are issues, but again, just something to check.

Regarding throw: there can be a point of diminishing returns with maximum throw, where too much can actually cause so much drag that the plane slows down and hence the roll rate too. I've never gotten (nor wanted) a "moto tool" roll rate from a Le Fish - but it's fast enough to be quite nimble when down close to the ground.

Assuming you've ruled out the pushrods, servo pockets, and aileron hinges, then next step would be to experiment with differing amounts of aileron throw and exponential. I generally run at least 30% expo on all my surfaces, so that they are softened around stick center but I still have enough throw at the max.

Also, please make sure that you're not using any aileron differential. Le Fish is an aerobatics glider, not a TD ship, and aileron differential is only going to cause trouble.

Steve
Latest blog entry: Buzz Evo RTF acro glider review
 Mar 10, 2014, 09:35 PM Re-kitting planes since 1990 Australia, WA, Beldon Joined Jun 2013 648 Posts Thanks Steve. Pushrods and servos are rock solid, however there is some slop in the clevis - horn link, the hole in the horn is bigger than the pin in the clevis. It's on my list of things to sort out. I'm not using differential, I'll have to measure the throw but I think it's around 30 degrees each side. Dawsonh, I'll try making some stabs with the pivot point at 50% and see what happens. If nothing else it will reduce my all up weight, which is always a good thing, right? From flying it the wing loading seems a bit high compared to my wicked wing, it just doesn't seem to have the float that the wicked wing has. It's at around 24 ounces at the moment. Thanks all for the advice, I'll report back when I've made the experiments
 Mar 10, 2014, 10:05 PM Feeling FrSky United States, CA, Santa Barbara Joined Feb 2003 19,698 Posts The slop you're describing in the aileron control horn / clevis interface could very well make the initial roll rate seem less responsive. I had a plane with a sloppy torque rod setup and until fixed it was rather annoying in roll. Afterwards it got very crisp -feeling. The perception of lack of floatiness could be a CG thing. If you're a bit noseheavy, then you may be carrying additional up-elevator trim and this could be creating unnecessary drag. This would make light lift performance worse and overall responsiveness dulled, possibly quite a bit. Dive tests and inverted tests are the way to go as far as dialing that in. Also, not sure what your settings are for snap flaps, but make sure you're not coupling too much flap deflection into your elevator movement, and make sure the flaps are moving the correct direction with regards the elevator ("up" elevator should result in flaps going down, and vice versa). I maidened my ultralight Le Fish with my snap flaps moving the wrong way - having put in an all-nighter to finish it, I simply failed to notice they were going the wrong way. Plane seemed really lousy until that was fixed, as you can imagine. Not saying any of those are what's happening, just that any or all of them could be part of the explanation. That might also account somewhat for the lackadaisical roll rate too. Lots and lots of variables to account for! Steve Latest blog entry: Buzz Evo RTF acro glider review
Mar 10, 2014, 10:14 PM
Feeling FrSky
United States, CA, Santa Barbara
Joined Feb 2003
19,698 Posts
Quote:
 Originally Posted by dawsonh My Buzz Evo is flying at a CG near 50% of wing chord... and she is super stable with no signs of tucking in a dive. How does that work?
I'm guessing the long tailboom and proportionally enormous horizontal stabilizer have a lot to do with it.

My Buzz Evo CG is ridiculously aft, too, somewhere around the aileron hinge point. The plane flies great with no bad habits whatsoever!

Steve
Latest blog entry: Buzz Evo RTF acro glider review
Mar 11, 2014, 01:00 AM
Aerobatics!
Joined Nov 2007
1,978 Posts
Quote:
 Originally Posted by nebbian Thanks Steve. Pushrods and servos are rock solid, however there is some slop in the clevis - horn link, the hole in the horn is bigger than the pin in the clevis. It's on my list of things to sort out.
An easy way to fix the slop in the clevis is to use a little ca + kicker, and then work your clevis back & forth while the glue is going off to fill the gaps.
Mar 11, 2014, 05:18 AM
Pompano Hill Flyers
United States, FL, Coconut Creek
Joined Aug 2007
293 Posts
Quote:
 Originally Posted by Snewk Waaay too pretty to fly at a Landfill!! Have a great Maiden. Ken
Thanks for the good words. Our flying sight is a real dump, but on our hill you can smell the lift.