I just got a Hobbico Avistar 40 Select RTF that comes with the OS .40LA engine. It assembled easily and everything went according to the instructions, but I am a little confused about the cg. The assembly instruction manual says it should balance between 2-1/2" & 3-1/2" back from the leading edge, but the other manual that comes with it titled "When you go to the field..." says it should be at 3-5/8". Since the second manual seemed to be more generic and not specific to this airplane, I went with it where it was without any modification at about 2-3/4" figuring this was a little forward of the center of the range indicated in the assembly instructions for this plane and that a little nose-heavy would be a good place to start.

Once I flew it though, it became very apparent that it was way too nose-heavy. It did not want to lift off and once it did, I had to give it full up elevator trim and still hold the stick back the whole time to keep it up. I kept adding tail weight in 1/2 oz. increments and it kept getting better, but even after adding an ounce it was still nose-heavy. I have since balanced it to 3-5/8" which is just about 1/8" behind the main spar, but to get it there I had to have a total of 1-1/2 oz. of lead on the tail and move the battery from in front of the receiver to behind it, just in front of the servos. I have not flown it yet in this configuration.

I was wondering if anybody else had experienced this with this airplane.

Are you balancing with the fuel tank empty or full? What are the throws on your elevator? (full and dual)

One option to get rid of some of the weight is to build an access panel about 1/2 way between the tail and rear of the wing and install your battery there. I have a 4*40 I did this with and saved adding a lot of tail weight.

I put my CG at 3" or between the two suggested balance points and it flys fine. Another thing to do, is epoxy in the horizontal stab and verticle stab. Also cut off the wires on the vertical stab with a dremel tool and cut off wheel at the balsa.

Tank is empty (good point). Elevator throws are within spec: 3/8" to 7/16" up & down (don't have dual rates at this point-still using the Futaba 4-channel that came with it).

That's a good idea moving the battery back. I've seen in other threads here where some people have put a heavier engine in the Avistar and put the battery behind the servos to counter the extra weight of the engine. I just can't figure out why I'm having this problem with the .40LA.

Balanced the way it is now at 3-5/8", if I lift the nose with it sitting level on the ground it tips back and rests on the tail. Filling the tank with fuel should improve that, I hope. I don't want it tipping back while I'm trying to takeoff. Maybe I'll remove 1/2 an oz. of lead from the tail before I try to fly it again. I sure don't want the cg too far back.

That's what I would have expected too from what the instruction manual says. I also noticed that the specs on Hobbico's web site also say the cg should be at 3-5/8". I sent them an email. Hopefully they will have some enlightening information for me.

I'm curious why you suggest epoxying the horizontal and verticle stabs and cutting the bolts off.

There are two other things that could cause this problem:

1. Wing incidence is off

2. Excessive Engine Downthrust

Sounds like a possible wing incidence problem. Does it like to fly tail down? Wings always like to fly parrallel to the effective airfoil centerline. A shim under the front of the wing may do wonders. Sometimes an eight of an inch makes the plane. For shorter takeoffs bend the rear landing gear down some to get the tail in the air. Cgshould be around 35% of the wing chord.

Thanks for the replies guys.

I put a small level on the horizontal stab and adjusted the position of the plane so that the stab was level. I then held another, longer level along the chord of the wing at the wingtips. I know this isn't extremely accurate but it appears that at least the wing chord is level with the horiz stab. It looks like there's a couple of degrees of engine downthrust in relation to stab level which I checked by holding a level along the side of the fuse at the center of the prop with the prop in a vertical position and using a protractor sitting on top of the level to measure the angle of the prop from vertical. I know this isn't real accurate either. I'm not sure what the reference line is for measuring wing incidence (or thrust angle for that matter), but with the horiz stab still level, I held a longer level against the side of the fuse with one end at the front of the horizontal stab, and it was level with the needle valve on the engine. This line appeared to be between an inch and an inch and a half above the tip of the spinner. I'd have to do some refresher trigonometry to figure out what that angle would be, but it's a rough estimate anyway.

Cogburn, I'm not exactly sure what you mean by 'does it like to fly tail down'. To me, inexperienced as I am, I think it likes to fly nose down since I have to hold lots of up elevator to keep it level. Raising the tail makes for shorter takeoffs? That's very interesting. I would have thought the opposite. I'll have to try that. I did see where somebody recommended a smaller nose wheel for this plane. Learn something new every day. The wing chord is 10 inches.

If it "likes to fly tail down" the tail will appear low relative to the wing in level flight(taking some consideration for the difference in heights of the horizontal stabilizer and wing from the fuselage centerline). I like them to fly slightly tail up.
If the tail is low for takeoff the plane won't be able to quickly rotate(wing wants to be level and stuffs the nose into the ground) and will basically have to achieve climbing speed before takeoff. Rotating the nosewheel off the ground decreases the effective drag also.
Standard cg usually runs from around 33% for nose heavy and stable to 40% for the really adventurous (inverted flat spins to the ground??). 35-37% is good for aerobatics -adjust to your taste and have fun!
Yes there is a little more to it but that will get you to a happier place.

I started flying mine at the 3 5/8, and moved it back almost another 1/2 inche. I would start out with it right on the main spar along the wing. Thats the starting postion and should make it balance slightly nose down. When I moved the cg back, it was balanced nose up about 20-25 degrees off of the main spar.

Thanks, that's good to hear.

I flew it quite a bit today with the cg at 3-5/8". The top surface of the elevator is level with the top of the stab. It flew very well and I never touched any trims. The main spar is right about 3-1/2" and when I had it balanced there, it still took some up elevator to fly level, which agrees with what you say.

How much weight did you have to add to the tail to get it to balance at 3-5/8"?

I have a new problem now. The tail pipe part of the muffler is spinning around loose and I can't tighten or loosen the nut. It looks like it's painted or loctited on. I've tried penetrating oil with no luck yet. I'm afraid I'm going to strip out the bolt head if I try any harder.

I think I used 2oz. of lead in the tail, and 1/2oz. in the left wing tip for total balance. BUT I was using an OS .46AX. Im not sure how much heavier that is than the .40LA. And the PBox muffler might have a different weight that would affect latteral balance. I'm not sure about the muffler, sorry.

I have an Avistar 40 with the evolution 46 motor.
I added 4.4 oz in the tail to start, to balance it at 3 5/8.
Later I moved my 6 volt battery just infront of the rudder and
removed the tail weight. It just happened to weigh 4.4 ounces too.

The plane flys great.

With a 11 x 7 1/2 I can hang it on the prop.
Takes off in 20 feet or less with a 10 x 6, It flys fast and fun.
With either prop I can do endless loops.

With the 46 motor its a good 1st and 2nd plane.

Av8r1, on this model for training go 3-1/4 inches from the leading edge. For some sweet flying with that 40 LA go 3-1/2 on the CG. Lose the additional dead weight and with a little effort the battery will fit in the first section of the tail through the access hole in the former. No need to cut a hatch. With the battery there, fine tuneing can be acheived with moving the RX around. On wing incidence, this model has a lot of positives going on right from the manufacturer. The main wing is 1-1/2 degrees positive, front is up. The stab is 1-1/4 degrees positive, again, front is up. The engine, fire wall angle, is 2-1/2 degrees negative, angled down. These opposing forces is what makes this model so stable. If you have a cg machine, at 3-1/4, the model should tilt nose down a little. About 5 degrees. At 3-1/2 it should sit about level. Good luck, Joe

I've been flying the Avistar Select quite a bit with the cg at 3-5/8 and it flies beautifully. It's very smooth and stable and does nice clean rolls and loops. It also flies inverted very easily with just a little down elevator. But it takes 1-1/2 oz. of lead in the tail to get the cg at 3-5/8. If I balance it at 3-1/4 or 3-1/2, it takes a good amount of up elevator to keep it flying level. I will probably try moving the battery farther back to get rid of the dead weight in the tail. I just wonder why this is necessary.

I emailed Hobbico asking which cg spec is correct, 2-1/2 to 3-1/2, or 3-5/8 and they recommended going with the manual's spec of 2-1/2 to 3-1/2. When I replied that it's way too nose-heavy in that range and that it has to be at 3-5/8 to fly level with no elevator compensation, they said, well then go with that if it works there. When I asked them why it's necessary to have to add so much weight to an "RTF" airplane, they stopped replying to me.

I also took some people's advice and bent the rear landing gear wires down to raise the tail. This did seem to help some with takeoffs plus it looks pretty cool with the tail sitting up a little higher. I may try to pursue this a little further with a smaller nosewheel or larger rear wheels. I've also discovered that the plane will takeoff pretty short if I just give it some up elevator after it gets a little speed built up, unlike what I'm used to with my little tail-dragger electric Crazy Max that will take right off on its own as soon as it gets to flying speed without any elevator input required. The Avistar is my first trike gear plane so I'm not quite sure what to expect.

I purchased my Avistar 40 as a kit. I added the evolution 46 motor with a 11 x 6 prop and a 6volt battery mounted just in front of where the rear stabilized comes up from the fuse. With no fuel it balances at 2 1/2 ". It's a little nose heavy but I like it better than when it was light in the nose.

At 5.5 pounds this plane does most everything. It will hang on the prop, knife edge endless loops,ect.

The weakness is it doesn't fly real slow. I've been thinking about flaperons.
Has anyone with an Avistar converted theirs to flaperons?

I fly with a dozen people who have different trainers and the Avistar seems to be a little more versatile than the others.

I have just finished my first gallon of fuel, so I'm pretty new at this, But boy do I like it.

Ralph

Ralf, I went with flaperons a while back. Interesting experiment, but found it to be useless on a plane this light. Your model will slow down easier if you move your CG aft a bit. Joe

I have been wondering if the steel rod wing joiner in my RTF version could be contributing to the nose-heaviness of the plane. As I understand it, the ARF kit uses a wooden spar instead. I was thinking they might use the same fuselage for both the RTF and the ARF versions and the difference in the wing joiners could cause the cg to be different between two.

My cg is pretty far back (3-5/8) and the plane can slow down pretty well. I can land in much less than half of the club field's runway sometimes as is. I am considering flaperons to try to slow it down even more. I have two Hitec HS-81 servos that I'm thinking about installing in place of the single standard aileron servo.

Thats what I did. Two mini's in the single servo bay. Worked great without any wing mods. The steel rod should be on or around the CG of the wing, I don't know if that would effect CG too much. Also I place the engine as close to the firewall as possible. Plastic spinners are the lightest of all offered and another weight save is a wooden prop. Joe

All good points. I haven't checked the actual position of the steel rod. Next time I separate the wing halves I'll see where it is. It looks like the HS-81's will be just a little wider than the opening that the standard servo is mounted in. It may require a little widening to get them in. The Avistar Select came with an all plastic spinner and a Top Flite Power Point wooden prop. I've since put an APC prop on it and although it's a little heavier, it didn't make too much difference in the balance. It does seem to give the plane a little more power, though.

The APCs are more effecient, but that small difference is way out on the business end and is a contributor. I started modifying mine way back and havn't stopped. It all started with removing the dyhedral in the wing. From there I removed all none essentials includeing the cowl cheeks for weight savings. Then turned it into a tail dragger and in doing so assumed another weight save with the nose wheel. Its now down to 4.5 lbs. With a .46LA it is airborn in 15 feet and will hang on the 11/4 Zinger prop. Entire cost was minimal aside from the optional covering change. Its now a more advanced model with the ability to float right to your feet. Joe

Yeah, I know. It may have moved the cg just a touch more forward, but it still flies very well. Maybe I needed just a bit more forward cg. Those are cool mods you've made, Joe. I've thought about doing the same things ever since I saw a Hangar 9 Alpha that someone in the club had done the same things to plus he put a .61 Magnum in it. It's a very cool looking and flying plane with lots of power and it doesn't even have the advantage of a semi-symmetrical airfoil like our planes. I thought I might try a .46LA in mine. Was it pretty easy to change from the 40LA to the 46LA? Also, was it a lot of work to take the dihedral out? Was yours the RTF or ARF?

Mine was the ARF, MKll. I'm a cabinet maker, so the dyhedral was easy. Ran it through the radial arm saw. It only took 1/32" from each side. It might work with sanding, but you want square results. You can do it with a wedge also. Re-inforce the wing bottom, once joined, with some balsa sheeting. Swapping out the engine is no big deal, again, I like to keep it tight to the firewall. I'm a firm believer in keeping this model with an engine around 10 oz. If I were to replace this engine, the Russian made 'Norvell' .40 would be it. Light and strong with .2 more BHP than the new OS.46AX. Joe

P.S. on the mods I forgot to include that I swapped out the aluminum mount for a composit one. Big difference in weight.

Thanks for the info. I'm not sure what I'd have to do to remove the dihedral with that steel rod joiner, but it's not that much dihedral anyway. I might just leave it as is.

I was looking at the .46AX too but it's really heavy compared to the LA's.

Nice picture, BTW. The covering scheme looks nice. I may have to get into that someday. I also have a Zinger prop and a wooden Master Airscrew that looks really cool, but I haven't tried either of them yet. I need to figure out how to balance them without messing them up. The APC one balanced perfect brand new. The Top Flite one was way out of balance and I balanced it by sanding down the tip of the heavy blade. I don't know if that's a good idea but it runs smooth and flies fine.