Between the Funtana threads and the U-Can-Do threads, I'll never leave this chair !! Anyway, rather than sort through nearly 1000 replies, I'll risk the "use the search button" replies.

It seems most of you are quite a bit aft of the recommended cg. I have a YS 91 FZ, with the tank on the CG, But I will still have to cut a hole in the tail, and mount the battery back there, I'd like to only do it once, and get it right the first time, if possible. With those of you very aft, say 6.5" does it fly around with the tail low? Of course this is normal at very low speeds or in a harrier, but A friend of mine has a Laser 3d that just plain looks silly in level flight even at moderate speeds. The wing loading is so light, you can get away with such an aft cg without the usual bad habbits, but it just doesn't look right. Does the UCD exhibit these tendencies ?

I've got my CG about where you're talking about and it doesn't drop the tail during level flight, but it does when coming in slowly for the landing. In fact, it's almost tough NOT to have the tail wheel (Sullivan ) touch first. It flies great there, but I may juggle my battery forward just a touch to see the difference. I was shooting for about 6" after reading those 1000 threads you mentioned, but ended up a little further back.

Oh, I cut the bottom of the fuse to mount my pull-pull servo for the rudder. If you target the hatch just right (between the formers), you may still be able to "adjust a little to fine tune it after installation. It does get tight back there. Even with the elevator servos and wires, it's close back there once you peak inside.

-s

Thanks, I think I will shoot for 6" for the first flight and see how it goes. I was planning on just gluing the "hole" shut again on the bottom of the fuse, But I guess I could make it a removeable hatch, and like you said, move the battery slightly between the formers if need be.

On a Dazzler I've got, I made the entire bottom of the fuse removeable so I could move the battery anywhere. As it is, the battery is ALL the way back. I would go the removeable route. Hate to have to cut the glue when something with the battery needs to be done.

Maybe:

1. cut hatch out
2. recover hatch piece to get edges of wood sealed
3. insert 2 (1 on each opposite side) small pieces of lite ply into the hole and glue flat to bottom of fuse (inside fuse) so that 1/2 of each piece over-hangs the hole.
3. insert hatch, drill 1/16" holes through hatch and lite ply. Unless CA has really dried well - carefull on pushing with the drill bit.
4. insert screws

Shouldn't take more than 15-20 minutes, a little lite ply, and some CA

-s

I read all the threads and was shooting for 6 inches. I cut a removable hatch just forward of my bottom mounted pull-pull rudder servo for my 5 cell 1650 nimah battery. The CG came out to be 6 and 5/16. Darn, too far aft. I haven't flown it yet ... too much snow and cold here in CT. I'll be watching this thread carefully!

I've been flying my UCD at 6.5" back. Engine is a ST 90 spinning an APC 15x4w. Fuel tank is in the stock location.

Until the tank is nearly empty, the plane still falls in hands off inverted flight......which indicates it's still actually nose heavy. When the engine is running on fumes, the UCD comes close to maintaining hands off level inverted flight, but it definitely doesn't climb.....so it still isn't too tail heavy. Deadstick landings with an empty tank require a light touch on the elevator when approaching and flaring for landing, but I don't consider it to be unstable.

With the fuel tank on the CG, I would aim for an initial CG of 6 3/8" back. And I'd be willing to bet that after flying the UCD for awhile, you'll settle on a CG that will be within 1/4" of this mark.

I don't know if thats a good indicator of CG. The incidence and the thrust line affect that as well, as well as speed. I have seen many planes set up "just right" from pattern to Imac, and TOC style, I think most require a slight push to maintain inverted flight.

Just curious, how does the Super Tigre handle the 15x4W? Is the spool-up sluggish due to the prop weight? What's your top rpm?

My cg is at 6-1/8" and I wouldn't want to fly it any more NOSE heavy. The plane is quite stable and a joy to fly at this CG setting. It does couple a little during hovering but the tradeoff is better flight characteristics during all other maneuvers. It doesn't drop the tail during landing at this setting. No weight was needed to achieve this CG with a Saito 91. The prop is a 14x6 and I am pleased with its performance.

The ST 90 with a stock muffler spins the APC 15x4x at 10,800. Experiments with a MCP have boosted the peak RPM to 11,700.

The spool up time with the 15x4x is slower than I would like, but it's not that bad. The wood props I've tried improved the spool up time, but the UCD wasn't as much fun to fly. So I've stuck with the 15x4w. An APC 14x4 might work better, but I don't have one to test.

I started with mine at about 5 1/4" from the leading edge. Powered with a YS91FZ and the tank on the CG. Hovers and spins/snaps are easier when it was moved back to 6 1/4". However, the coupling in knife edge was horrendous. I've since moved it forward again to about 5 3/4". Knife edge takes only a little mixing (unless you are at a high deflection) and hovering and snaps are manageable (a little harder). Landing is more on the main gear at this setting (easier in high wind)

Chris

Ya know, BB. Because of this post, I watched my plane just a little closer today and I think the tail is down ever so slightly at very slow speeds. I'll also agree with Chris about the knife-edge characterstics with the CG moved back. That and you have to get used to a plane where you actually don't need all the rudder to knife-edge. I've always been a fan of enlarging rudders, but Shazzam if this thing doesn't have a rudder and a half relative to the plane size.

-s

I will fly mine at 6" first, I expect a tail low attitude as you get slow, and start pitching up to maintain altitude that is perfectly normal, but I just don't like it like that even at speed, just doesn't look right. I thought I read somewhere that the coupling was real bad on the UCD in general, and that moving the CG aft helped that as well as spins and snaps, I guess it's the other way, Maybe the reason people are complaining about the coupling, is because we all have the CG so far out of the recommended range ?

<<Maybe the reason people are complaining about the coupling, is because we all have the CG so far out of the recommended range ?>>

Although I've not yet moved my CG forward for comparison, I think you nailed it on this one!

-s

Is the coupling really that bad? I have not flown mine yet, but I don't expect it to be a pattern plane, most "ugly stick" or fun fly types do have a lot of coupling. I'm sure it can be mixed out, although that probably only works for one specific speed. I guess if you have to pick the worst of two evils, most like the way it flies with the aft cg and just deal with the coupling.

Is it that bad? Well, I think you can make it that bad... ha, ha. Since I'm flying it with a Fut 6XA which is the pre-mixing model radio , I'm training my thumb. I think the reason it's initially so bad is that one thinks you need a ton of rudder to knife-edge based on previous plane experience. Even my other fun-fly requires about all the rudder it has and I even enlarged it upon building. So, on the UCD, you tend to initially put a ton of it in and there she goes to the belly. Probably do a complete inside knife-edge turn if you let it. Back the thumb off the rudder and you will keep knife-edge while reducing the belly turn a bit. I'm still playing with this and convincing my head and thumb to back off that rudder... ha, ha.

<<I guess if you have to pick the worst of two evils, most like the way it flies with the aft cg and just deal with the coupling.>>

I think you're also right about that!

-s

Interesting, It's been so cold I'm doing a lot of hangar flying So it pitches with rudder, but no rolling? that doesn't sound too bad, I can't wait to try it. You say it doesn't require much rudder input to sustain knife edge, what about real slow "high alpha" knife edge, is it capable of that? I suppose it could get ugly if the coupling gets worse as you slow down and add more rudder.

No, I really don't have too much issue with rolling... it's more pitch - and to the belly. On other threads, they attribute this to the % of rudder above the center line of the plane. Nope, not much rudder for knife-edge. High-alpha & slow - get back to you on that one... ha, ha

-s

Does any one have any pictures of the hatches they've cut to move their battery back? I'm looking for ideas. I started flying my UCD with a 5.75" CG. But that was without cowl and with a wooden zinger prop. I have since added the cowl and an APC prop and the CG has moved forward about 1/4". I definately do not want it at 5.5". I was planning on getting it back to 6 or maybe 6.25". I refuse to add more weight and so I'm relagated to moving the battery back. I'm looking for a nice clean way to do this without hacking up the plane. I don't believe it will need to go all of the way back to the rear servos. Maybe a little more than half way back. I'll check before I do anything drastic, but I'd just like to see what you guys have done.

I flew mine with the APC for the first time today. I picked up about 300 rpm over the zinger, but with the added weight on the nose it didn't fly any better like I had hoped. I need to get that weight back.

A word of caution to you guys planning to cut into the fuselage; be careful.

Mine fell off the wall rack the other night and landed on the corner of a stool, knocking a fist-sized hole in the side of the fuse exactly halfway between the tail and rear of the wind saddle.

I had to cut into the bottom to effect the repair. The fuselage is constructed of VERY light balsa, with lite ply "stiffener strips" running vertically, halfway between each former.

Basically, all you've got back there are stringers and thin balsa sheet.

What a bummer; trashed before the first flight. At least it went back together okay, and I was careful when I cut the covering, so it doesn't look bad.

Maiden flight tomorrow; and naturally gale force winds are forecast. Oh, well...

Steve

Come on fellas. I know a lot of you have moved your battery back in the fuse. Aren't you going to give me a peak at how you accomplished this

How far back do you think the battery will need to go in order to take the CG from 5.5 to 6.0? Total plane weight is 7.6 lbs.

I spent a lot of time cutting a hole, building a hatch, shelf, etc. for the battery, and it flew like Sh**, so after one flight I moved the battery back up under the wing. My CG was 6" maybe 6 1/8", now I'm about 5" maybe a touch more, it's much more stable, and with my insane control throws, it still does all the maneuvers. I think there is less coupling with the foreward CG too, although it's still pretty bad.

I used Saito 100, which is lighter by 2~3 oz. I swapped out the stock motor mount for a Dav Brown 91 mount to save 2 oz nose weight. I didn't need to put battery at the tail to achieve 5.75" CG; I put it above the TE.

To those who are experiencing coupling I have to say the reason for this is simply because you have gone beyond recommended specs. I set my cg back  to 5- 5 1/4 and coupling is non-existent during knife-edge flight. Hovers are decent and landings are 3 point. Flies at a tail high attitude with no drop off in inverted flight. I'm using the normal setup with equipment just behind the cg with an O.S. 91FS.

Coupling problems go away with a 6 pitch prop.