FOCUS Trimming Question - need experienced help
 

I'm busily trying to get my Focus trimmed before the weather gets too bad, and I'm having some difficulty getting a perfectly straight down line. The thrust seems to be OK, but I've adjusted the wing incidence several times and am unable to comletely eliminate a slight tendency for the plane to pull toward the canopy on a long downline - maybe in a 200 foot drop it will pull out 1 or 2 degrees, especially at the bottom of the line when it seems to have picked up some speed. Another club member noticed what I was doing and suggested that, even though I'm dropping the front edge of the wings, I correct back to level flight with the elevator trim each time, and in so doing put the angle of attack back to what it was before I started. I've done about a full turn and a quarter on the adjusters and still have a slight pull out. Is it possible I'm overlooking something? Am I asking too much of this ariplane?

Thanks for any help.

FOCUS Trimming Question - need experienced help
 

Nope you are not asking too much.

Check your stab incidence. Downlines are mostly in the wing/stab alignment, not thrust. Assuming the stab is fixed, you'll have to keep dialing the wing, or live with it.

This is assuming your CG is correct of course. The flying CG should always be the first thing established.

Check the stab incidence first. If it's off tremendously, you could be fighting a losing battle.

You may also have a warp problem. That's not unheard of. Check that too.

Lastly if it has fixed gear, make sure the gear are not mounted at a positive angle relative to the flight path. This can also play havoc with trim in some cases.

Check all of that and see how it goes.

-Mike

FOCUS Trimming Question - need experienced help
 

All of Mike's suggestions sound good to me. After a quick ground check of your stab incidence for a significant error I'd suggest that you try adjusting your CG as your first adjustment. The reason I'd do this first is that you probably have a fixed stab and you said that someone observed that each time you change your wing incidence you're simply offsetting it with elevator.

CG and wing incidence are related. If your CG is too far forward it makes your plane nose heavy which means that you have to have excess elevator to keep the nose from dropping. When you go into a downline the gravitational forces on the plane are different which results in the additional elevator causing the plane to move towards the canopy. By moving the CG back you don't need as much elevator in level flight which can reduce the drift to the canopy.

Some other questions:

1. When trimmed for level flight does you elevator seem to be trimmed to a level position or is it trimmed up slightly?

2. When you fly inverted does it require a lot of down to keep the nose level?

It may be wise to try moving your CG back and setting your wings back to a 0 deg incidence and then re-trim it from there.

BTW, did you ever work out the excess right trim needed in your ailerons?


KeithB

FOCUS Trimming Question - need experienced help
 

Howdy Keith - how's the Aries coming?

It turns out that I started with the CG where it should be and I moved it back (2 oz of weight in the tail). As a result it will fly nearly hands off inverted - exactly the opposite of what would seem to cause my problem, and the elevator is neutral in level flight. I'm going to check out the other issues and then bring the wing back to "zero."

I've been trying to neutralize the ailerons by moving the right wing leading edge down more than the left when I make adjustments, but I'm not there yet. I've never had wing adjusters before and I guess I'm a little surprised at how little effect they seem to have on things. I think I'll mark the leading and trailing edges and fuselage to see how much they're actually moving as I make adjustments.

Thanks

FOCUS Trimming Question - need experienced help
 

Well it certainly doesn't sound like it's nose heavy. Keep at it, I'm sure you'll get it dialed in eventually.

The Aries is coming along very well. I've got the wings, stab and rudder all sheeted and 95% complete. Just need to bevel and hinge all control surfaces then decide what method I'm going to use to hold the wings in place. Rather than using a bolt in the wing tube I'm considering using a bolt on the wing root that goes into the fuse. Lance is doing this on his new Aries and I really like this approach. Since it has a removable canopy it will make the wings really easy to install.

I got real industrious and made the stab removable. I followed Bob Noll's tapes pretty much to a tee and it came out awesome. The only difference is I used a carbon fiber wing tube and used Titebond Polyurethane Glue rather than epoxy to hold the wing tube in place, this should make it much lighter. This was about one to one and a half full days worth of work but I think it will be well worth it in convenience and flexibility later. Now that I've done one I could do another one much more quickly.

As for the fuse I've gone over all seams to get everything nice and smooth and I'm now ready to prime it. After that it's just a matter of final assembly, painting and covering.

Keith

FOCUS Trimming Question - need experienced help
 

The CG as per instructions was far to far forward for me...same problem on my Hydeout.
I've got it set 1.5cm behaind the wing tube and the wings set a zero.
Hands off inverted... straight, 45 degrees etc.
I aways set zero on the wings and correct down lines with CG. Don't understand the the principle of changing wing incidence. Surely, this is going to effect the set up between upright and inverted?

I prefer the zero setup and a neutral CG.
Mine you, a neutral setting it takes abit of getting used to.

Keith & bla bla
 

I agree about the CG being too far forward to start with - needed lots of down to hold inverted. Having the CG back far enough to hold inverted hands off has it's own set of problems too. It's real easy to climb while inverted, especially if you deploy top rudder on the way around.

As I think about the wing incidence solution to downline correction, I think you're right. Each wing adjustment must be followed by a corresponding elevator correction to maintain level flight, thus negating that adjustment. Why then do the trimming charts suggest doing this?

The one variable which may be my solution is airspeed. The pullout increases with airspeed, so if I can keep the speed down, maybe the line will stay straighter - I'll give it a try.

Thanks

FOCUS Trimming Question - need experienced help
 

Put the focus back on the bench and set up the wings 0/0 degrees. Set elevator at neutral.
Test fly and shift CG backwards to compensate for up elevator trim.... keep doing this until the aeroplane flys without any trim or any need to change incidence. Roll test inverted hands off(ish) and knife edge hands of/ horizontal fall attitude.
Do this until things get pretty damed good and then and only then start playing with the wing incidence in very small ammounts (to eliminate any building cock-ups) taking a written note of changes so you can go bacwards.

You'll be getting very close but you'll never find the absolute perfect set up...even between 2 Focus's. I'm getting happy when I take a simple 45 degree upline, roll invered and the aeroplane follows exactly the same track.
Yep it's difficult when thing get neutral...
try flying straight and level, then reduce te engine to idle... the aeroplane just keeps flying straight and slowing down!!!! untill it just drops, falls out of the sky... very strange.

FOCUS Trimming Question - need experienced help
 

Here is a good overall link on how to trim a plane. The CG stuff is interesting. Good luck.

http://www.rcaerobats.net/trim_chart.htm