u-key c/g

[Bootlegger]

   Just assembled a u key and need the c/g, don't know which one it is but the wing is 500 sq in or there about's.
 Want to install an os 25 engine if wing isn't too big.
  Thoughts?? and c/g suggestions...

[john e. holliday]

At first if CG is unknown is go to highpoint of wing.  I usually try for 25% of mean chord myself on first flights.  It is mucho better to be nose heavy than tail heavy.  At least after first flights you can add tail weight.   

[Larry Fulwider]

At first if CG is unknown is go to highpoint of wing.  I usually try for 25% of mean chord myself on first flights.  It is mucho better to be nose heavy than tail heavy. 
 . . .

Whoa! 25% of MAC on U-Key will be unflyable  .  25% of MAC is ~ 2 13/16" back from the LE at the root.
       The only planes I know that are comfortable at 25% MAC are full-blown high TVC (> .40) flapped competition trimmed stunters.

Gil -- At 500 squares, you have the U-Key 40. I have a couple U-Key 40s from circa 2000-2001 (Joe Just era), but I don't think the wings have changed (?). Anyway, here is what I have:

       LE Sweep = 1/2"
       MAC = 10.391
       LE Adjust (LE of MAC back from LE of wing at root) = .235

So, a CG 2 7/32" back from LE at root calcs out to ~ 18.8% of MAC. I'd go 1/8" further forward for first flights, and ease back towards that number. If you "measure" CG with fingertip support at the tips, perhaps a little more forward yet, depending on the shape of your fingertips.
       A U-Key 40 isn't too bad even with the CG at 15%. (1 13/16" back from LE)

       Larry Fulwider

[Dwayne]

What Larry said, I think...lol      Just balance it 2 inches back from the leading edge that will give you a good starting point.

[Bootlegger]

       Thanks guy's, something that I didn't mention is that I am planning to try an OS 25 LA or FP.  Any thoughts or suggestions?
   Will I be able to fly it with these motors? I plan to use .012 lines about 58'--59'.
  All suggestions appreciated...

[Larry Fernandez]

At first if CG is unknown is go to highpoint of wing.  I usually try for 25% of mean chord myself on first flights.  It is mucho better to be nose heavy than tail heavy.  At least after first flights you can add tail weight.  

25% OF MEAN CHORD ON FIRST FLIGHTS???
Yes its mucho better to be nose heavy than tail heavy, but I highly doubt that 25% of mean chord is nose heavy.

I know I gonna come off sounding like a weenie here, but this is why I have said in the past, that as well meaning as everyone on these forums are, there is a lot of misinformation.

Some poor beginner sets up his new U-Key CG 25% and reports back the next day that the U-Key sucks and he could not control it. Plants it in the ground and tells his pals not to buy the U-Key.

Larry, Buttafucco Stunt Team

[Larry Fulwider]

     Thanks guy's, something that I didn't mention is that I am planning to try an OS 25 LA or FP.  Any thoughts or suggestions?
   Will I be able to fly it with these motors? I plan to use .012 lines about 58'--59'.
  All suggestions appreciated...

Gil --

You mentioned the engine in your first post. I didn't comment as either engine should be fine, my guess; although I haven't tried the LA / FP 25 on a U-Key.
       A Fox 35 and 10/5 APC is fine on 60 * .012s (which I have tried), so you shouldn't have any problems. That combo would power through vertical 8s and overhead 8s with good tension, even with some wind.
     The small empennage and "combat style" airfoil helps keep speed up in maneuvers, so the engine does not need to "pull you through a stall" if you fly it right.

       Larry Fulwider

[ray copeland]

Gil, you need to check the total weight. Those .25's are more comfortable under 35 oz. , and actually my la's have a little more power than my fp's. I do have a couple .25 fsr that will haul a few more oz. I have an oversized skyray at 36 oz and the .25 fp just wouldn't do it. Good luck, hope ya get it goin!

[Dwayne]

     Thanks guy's, something that I didn't mention is that I am planning to try an OS 25 LA or FP.  Any thoughts or suggestions?
   Will I be able to fly it with these motors? I plan to use .012 lines about 58'--59'.
  All suggestions appreciated...

The LA with a 8X6 or 9X6 prop on 58' .015 lines should be a good flying combo, I'm not sure about .012 lines I think they might be to thin.

[Larry Fulwider]

Gil, you need to check the total weight. Those .25's are more comfortable under 35 oz. , and actually my la's have a little more power than my fp's. I do have a couple .25 fsr that will haul a few more oz. I have an oversized skyray at 36 oz and the .25 fp just wouldn't do it. Good luck, hope ya get it goin!

Ray --

Good point  . The new U-Keys may not be as light as the older ones. The one with the Fox 35 was right at 32 oz.

       Larry Fulwider

[Joe Just]

We designed the Ukery series to balance on  the spar by holding you fingers about half way from the root to the end of the wing.  A .25 is a very good choice for the Ukey you have.  The original Ukeys had a cedar body. I believe Jim is now making them with a pine body, or so I've heard.  Having the Ukey a bit nose heavy is a good idea with the first few flights.

Have Fun!

Joe Just

[john e. holliday]

How far back is the spar?

[Kim Mortimore]

I agree that 25% of MAC is way too far back.  I also agree with Larry Fernandez that someone who tries to use this misinformation is likely to end up discouraged.  I would go with the 15% that Larry Fulwider recommends to start with.  This is the same % Ted Fancher recommends as a starting point for flapless planes in general.

Warps.  I recommend checking the wing for warps.  I bought this same size Ukey when I first got back into modelling and the wing was warped too badly to use.  

Weight and motors.  I agree with Ray about weighing the model and matching the weight to the motor. Also, check to see what size motor best fits the engine mounting space provided, and what size motor gives the best balance without having to add a lot of noseweight or tailweight (especially noseweight, since it takes a lot more to move the CG the same distance).  In general it's easier to tame a motor with too much power than it is to squeeze more power out of a less than adequate motor.  I personally like the LA.40 and Brodak .40.  The larger engine makes it easier to start out a bit noseheavy, which we all agree on.

Lines.  58' to 59' sounds about right.  .012 is legal under the new rules up to 40 oz, but I don't recommend pushing it.  .012s are more delicate than .015s, and the Ukey is a bit of a knockabout plane. As carefully as we handle our lines, we still managed somehow to completely trash a set of .012s that Larry Fernandez had expertly wrapped a few weeks ago and have no idea how it happened.   .015s are especially recommended if you go with a larger engine.

I've heard more than one guy say things like, "All my other planes are crashed and unflyable except my Ukey, which takes a lickin' and keeps on tickin'."

Hope this helps.  Happy Flying!

[Lester Nicholson]

I'm not an expert by any means, but have been flying for 45+ yrs. An airplane that is a bit nose heavy will fly when a tail heavy airplane will probably crash.    Nick

[Larry Fulwider]

. . . Warps.  I recommend checking the wing for warps.  I bought this same size Ukey when I first got back into modelling and the wing was warped too badly to use.  
 . . .

Kim --

Thanks for another "U-Key reminder". You can get a unique "U-Key warp" if the holes in the cores are off, or the mounting dowels are crooked. That is, the outboard wing can have different incidence than the inboard wing.
         That potential misalignment is likely the biggest weakness in the original U-Key design.

       Larry Fulwider

[Jim Thomerson]

If you balance the airplane at 15% MAC, it will fly safely.  If it is too sluggish to suit you, move the CG back in small steps until it is just right.