Need your help fellows - fighting elevator on final

[skunkwurk]

When they give you the cg point in the manual are you supposed to measure it with the aircraft upright or inverted?

They call for it upright at the wingtips in the manual. I did that initially but it was really sensitive. For example, if I moved a 1/8 of an inch in the range, forward or aft, the effect would be tail or nose heavy. Very inconsistent. To try and confirm my balance point using that method, I also measured across from wing tip to tip, with a string, to identify a wing root mark, and have tried that balance point inverted. It was easier to get the model to settle into a slight nose down attitude using that method. Does that make sense?

To balance it at the wing root, inverted, I used a method jet modelers use, where they hang the plane from a hook in the shop to two (L) brackets taped to the cg location using a light, high strength, string.

 

[skunkwurk]

By the way guys, I just want to say thank you for all of the comments and help thus far. I sincerely appreciate it, this is why I love this hobby, the people are awesome and working through the mechanics of the plane until it's right just kicks butt.

Then you get to fly them...

sc

 

[Kevin-Young]

yeah, realistically on the cap you should be balancing it inverted because upright you are fighting the pendulum effect, that is why I asked about what the manual said. Is the balance roughly 33% back from the leading edge?

 

[Pistolera]

Lots of good suggestions here, although quite a bit of stuff to chase around. Personally...(and this is my opinion only)....you already have the model's CG close to where you want it via flying it. From there I would test for any up/down thrust adjustments (already described) and any left/right thrust adjustments (in vertical up lines). After THOSE 3 THINGS have been established, then test for any tendencies to pull to the canopy on vertical down lines (with NO throttle). Add in any throttle to elev mix needed to keep it tracking straight down.

If you still have ballooning issues, then look at more expo on your elev for landing. If that doesn't fix things then start digging into incidences, etc. But start with the basic (and simple) things first.

I couldn't tell you where (on the model) on any of my planes their CG's are located....and really don't care. I started each in the ballpark of the manf recommendations and moved things around to fine tune they way I wanted it to fly. Never did remeasure anything after the maiden...just flew them and made small changes until I got the desired effect.

 

[skunkwurk]

yeah, realistically on the cap you should be balancing it inverted because upright you are fighting the pendulum effect, that is why I asked about what the manual said. Is the balance roughly 33% back from the leading edge?

Kevin,

The root is 22 5/8 and I'm @ 7 3/8, it's about 32%.

 

[Terryscustom]

Houston, I think we found a solution. OK, first, thanks for your dimensions, there is at least one of your dimensions that must not be correct because the wing tip chord does not calculate out correctly. I used all the other dimensions as they are more important.

At 7 3/8" that you are at now, at the root of that wing, you are behind neutral CG by a fair amount, so I would recommend you move your CG forward by minimum 1" to get you just a touch in front of neutral MAC CG. Then re-trim the plane as you did at maiden and re-check everything. With luck this makes sense but you do not want to be at 32% of root chord, neutral CG is actually at 32% of MAC (Mean Aerodynamic Chord). So, your neutral CG is at 6 11/16" at the root from leading edge (check inverted) or 4 5/16" at tip (check inverted). Personally I prefer to check at root just to avoid the stress on the wings.

 

[skunkwurk]

Hi Terry,

Thanks for working this out. It's interesting, 6 11/16 puts me just a touch (1/8), in front of the wing tube center point. Might it be that H9 placed neutral MAC CG on the wing tube center-line? Is this common? Sorry for the ridiculous question, if it is one. I'm definitely tail heavy when I use the wing tube as a baseline, I tested that mark once to check where It would put me, as it's so much easier to do. I believe you and I exchanged a message on this once a month or so back.

An other interesting point is that 6 11/16 puts me almost exactly at the 4 1/2 mark on the wingtip. This is 3/16 off for some reason, could be my measurements. The manual calls for a range of 4 1/2 to 5 3/4 at the wingtips, I ended up at 7 7/8 which is ~ the middle of the range. It's odd to be within the manuals range and experience the behavior.

Should I simply use the wing-tube? It would be incredibly easier for me to measure, it doesn't require my wife's help either. Thoughts?

sc

 

[Terryscustom]

What I calculated is the neutral center, this is a starting point and you can adjust from there to determine your preference. Many ARF's put the tube on the CG just to make it easy but it is not a requirement (yes we talked about this quite some time ago). Some just plain mis-calculate or make changes to the airframe and forget to update the manual. Either way, CG is not a hard and fast number you need to find dead nuts on but as you see if you are an inch off it does throw some wrenches at you.

Basic rule - Nose heavy will land a little faster on landing but fly more clean, too far forward it can fly poorly........tail heavy is more maneuverable and can have bad habbits, very tail heavy flies once.

 

[Pistolera]

.........very tail heavy flies once.

Love that one!

 

[witchfingers]

Hello,

Would that create a nose down condition?

In our case it created an up down fight while slowing down to land. It could not find a comfort zone until it was almost stalling. It also had a bad pull to the left under full power that was unaffected by changing thrust angles. When we changed to a prop diameter one size smaller it helped everything.