Hi,
I am looking for a way to slightly improve the inverted flight performance of my Cermark Javelin II. Inverted flight needs a bit to much down elevator for my liking. I have played around with the C.G and after putting a little more back the inverted flight improved a bit. Putting the CG even more back will make the plane to critical for me.
I also gave the engine a bit more right thrust and down thrust by adding one spacer behind the engine mount (haven't test flow it yet)

I was thinking about the ailerons now and hope you can help me out with some insight on the following:
How does the trim of the ailerons (both a bit up or both a bit down) effect the plane?
And more specifically, would giving both ailerons a slight bit of 'up' trimming positively influence its inverted flight?

Thanks,
Jock

Thats not the way to correct the problem Jock. You can either correct it by changing the CG or increasing positive wing incidence so that the plane requires less up elevator trim or a little down elevator trim to fly level I believe. I Think www.nsrca.org has a trimchart. Follow it in order and you should be able to correct it without screwing up all of the other trim characteristics. MAybe someone else will comment now that I have messed you all up!


Mike

Isn't it supposed to be moving the CG backward or reduce the wing incidence to reduce the down elevator input during Inverted flight[sm=spinnyeyes.gif]?

I am not sure how the wing is mounted, but wood fuselage can be trimmed a little to change the incidence. I tried it on a couple of planes, but be really careful not to trim too much. I used very slow sanding method. (sanding paper on glass bottle, pvc pipe etc). Took a long time, but worth it.

I suggest having at least two incidence meters (I had one and borrowed one).

Before you do anything, check the incidences of wing and horizontal stab. I believe the plane should have the horizontal stab at 0. Contact Cermark for this info also.

http://www.nsrca.org check the trim chart before you try the change anymore. You may have to put the thrust angles back to where they were.

Good luck.

IP

Well,
ideally you should just barely have to breathe on down elevator when the plane is inverted, you dont really want hands off level flight while inverted.
Theres really a process to properly trimming the plane that is pretty long and drawn out, you really need to start with step 1 and go through the process to get it right. Notice the chart says holding "A LOT" of down elevator while flying inverted. You should have a little down elevator while inverted you should only correct if it requires more than a breath.
Heres the trim Chart at NSRCA
[link=http://www.nsrca.org/TrimA.htm]NSRCA Pattern Plane Trim Chart[/link]



One of my friends I fly with and learn from who came in 3rd at the NATS in Advanced likes to have just a little TINY bit of down elevator trim in his planes when he is finished trimming. I had the same thing in my Prophecy and it flies really well in uplines, downlines and inverted.



I reckon that if your plane has a fixed wing incidence then all you really have to work with is CG and thrust to get it as close as you can...

Okay.. Now I've got a question for you guys. For those of you that say you need to just breathe on the stick for inverted flight, do you do that with your elevator expo set or with zero expo? My point here is most people set their expo in the 30% or so range for a softer neutral. If you were to have zero expo, that would be a slight breath. If you put in expo, then, maybe a little breeze on the stick. And if you use a lot of expo, you may need a tornado blowing on the stick. Get my point? Some radios give you the ability to set up/down expo at different rates. Couldn't you set the up expo rate differently that would give you the results you want?

OR as a friend of mine suggested: If your radio doesn't support setting different expo rates for up and down but does so for servo travel give the servo some extra down travel to get a simular effect.

Jock

This is an interesting topic.. years ago we sat around talking about trimming like this. A symmetrical airfoil has to fly with some positive incidence, there must be lift someplace to keep the plane level. If you change the wing incidence, and retrim to fly level I think the wing still flys at the same original incidence - it can't change or the plane climbs with a change of airspeed. Adjusting wing incidence only raises or lowers the stab while in flight, which puts in more up or down elevator for level flight. Although I understand Jock's comments, and I'd try this, not sure it will work - but give it a try.. and I'd also play with thrust, but not right thrust - up/down thrust.

Scott

"How does the trim of the ailerons (both a bit up or both a bit down) effect the plane?
And more specifically, would giving both ailerons a slight bit of 'up' trimming positively influence its inverted flight"



If you put up in both ailerons thats called reflex which is close to giving down incedense in the wing, I have done this as a way of doing simple test on planes with wings mounted solid to see how much I need to shim wing if you have a incedense meter put it on and just move the aileron by hand and you can see what the incedense meter reads, I know a really good flier that has done this with a design that the wings are solid and it does very well, if you use the NSRCA chart please note there should be a correction to the incedense statement, it says to adjust the wing or stab, which should only read adjust the wing as in the chart on aerobobs site [link]http://www.rcaerobats.net/trim_chart.htm[/link]
Steve Maxwell

Guys, there is really a LOT of "personal taste" in the "down elevator" thing. I have found that EVERY time I try to set a plane for "just a breath of down", I have a tail-heavy, belly-pitching, poor snapping, over-damped TWITCHY handful. Jock and I both fly in WIND...and that is NOT where you want a tail heavy, over-pitch-damped airplane bouncing around.

So - I've found I'm happiest with a slightly nose-heavy condition, which when all is said and done requires a touch of UP trim on the elevator. Which is the WRONG direction to make the inverted elevator as responsive as we like. To work around that, I just increase the travel on the down elevator, and decrease the expo a bit. My brain and fingers "think" it's being more responsive, so it's just a "breath" of down on inverted.

Others may disagree, and some will say there is actually an airplane out there that is "nearly hands off inverted" and NOT twitchy - however - walk around the pits, ask guys questions, and I think you'll find it quite a bit different in practice than in theory.

Solid airplanes that lock on are NOT tail heavy.

When I first flew my Javelin, with CG right on the spot recommended in the instructions, the plane flew way too noseheavy. I plotted the wing to get 33% of mean aerodynamic chord, and came up with 6-1/2" back from leading edge (instructions were for 5-3/4"). I eliminated noseweight and rebalanced at 6-1/2" back and the plane flies great. I could push it back even further, but I'm very happy where it is. The plane is not at all touchy at this balance point--nice and smooth on all inputs. Of course I have very conservative throws on ailerons & elevator, plus 50% expo on these controls. My engine thrust offsets are strictly what comes built in to the firewall. I have thought about messing with side-thrust offset, but so far have been content to play with throttle to rudder mixing. Vertical thrust is fine right where Cermark has it--no sinking or ballooning when I go from full to chop throttle or vice versa.

I set my elevator trim so the plane will come down slightly with no elevator input on straight & level. So I'm always touching the elevator stick--just a touch of pull for straight & level upright, just a touch of push for straight & level inverted.

The Javelin is susceptible to having the wing incidence vary somewhat from one model to another. I notice when I tighten my wing hold-down bolts, the "fuselage" part built onto the bottom of the wing is not perfectly flush with the fuselage at the trailing edge of the wing--it sets in maybe a little less than 1/16". If having these surfaces perfectly level is zero incidence, then I'm probably at negative 0.5Ëš. I'm not recommending that to anyone, just observing how mine went together.

I know a Master class pattern flyer with lots of experience on the Javelin, who's very happy with the recommended CG, and that's where he flies his. When I asked him to show me straight & level inverted flight on his Javelin with no elevator input, my feeling was it came down a lot faster than I like to see. That would imply nose-heaviness to me.

My first recommendation would be to rebalance your plane at 6-1/2" back from the leading edge. 33% of MAC is not tail-heavy, it's very close to neutral. My second recommendation would be no change to the vertical offset of the engine--just mount it straight to the firewall. Side offset--be my guest, but I find it easier to fiddle with throttle to rudder mix, which I set up to kick in only above 3/4 throttle. The rest of the time I'm flying the rudder all the time anyway.

For what it's worth, I have a YS63 in my Javelin, with fuel tank under the canopy (just forward of my CG). I love this plane. Everything I am capable of, it's more than capable.

I would add to the discussion, but I can't! Bob P's response mirrors that of my own experience- every word! A little elevator differential-- giving some more down-than up, helps for a more symetrical feel during inverted.

Best of luck,
Don