I have a tower hobbies 40 trainer. I put a super tiger gs-40 engine on it. The engine is heavy at 19.08 oz, but is listed as a recommended engine for this plane. The CG is supposed to be 3.5 inches aft leading edge. At 3.5 inches, I have to add 4.5 oz of weight on the tail fin to balance it ( fingertip method). My other engine thats in a spad is a GMS47 and is 16.75oz. The OS 40LA is even lighter than this and would probably be just the right weight on it. The battery is as far back as possible without moving it to the rear of the servo tray. So , What would be the best way to balance this thing? I hate to change to a lighter engine as this was ordered for this plane, and I hate to have to add 4.5oz to the tail either!

Did you have fuel in the tank when you balanced? Should always be empty.
Where's the reciever?

Most important, fingertips are NOT reliable when balancing. Either buy or build a balancer, simple one uses two pieces of 2x2 cut and squared of, then drill holes such that you can insert two pieces of 3mm piano wire vertically to balance on. Far more accurate, and you'll probably find that some of your tailweight disappears.

Fuel tank was empty. The receiver is in front of the battery I would have to move in behind the servo tray to move it any more towards the rear.

You may be able to squeeze a larger, heaver battery pack in your trainer.
If you need to ad weight to your plane, it may as well be useful.

Here is my cheap balancer... it works. You could try a servo extension on the battery if you really need to move it (if you do, make sure you use about an inch of electrical tape to keep the connector from coming apart accidentally) , but 4 or 5 oz. of weight won't make that much difference. It just sounds like a lot.

I've been flying for over 40 years, and I still use my fingertips for balancing. It's as accurate as you need it to be. IMHO using a fancy mechanism for balancing is about as needed as buying a plastic "Rubber Band Puller" so you don't have to use your fingers.

But as to your statement:

What's wrong with putting the battery behind the servo tray? Heck, if you need to, put an extension on it and stick it in the tail.

My experience is that the following method works well and it is easier to move stuff around to balance the airplane.

You might try hanging the airplane from above with a mason twine cradle from engine to tail. Then drop a plumb bob from the suspension point. When the plumb bob stops swinging it will point to the balance point. The airplane should be approximately level or at flight attitude.

I lay the battery (or weight) at various locations on top of the airplane to find the best balance location. There is nothing to buy or make and the wheels can be a few inches off the floor or table just in case something slips. The point of attachment of the cradle to the airplane is not critical.

Bill S

bills,
I'd like to see a picture of what you're talking about. It sounds interesting. Next time you balance a plane, would you post some pics of that setup?

Hi!
Agree with Minnflyer...all you need is your fingertips to balance an airplane.
This has worked for me for 30 years, on all types of airplanes. Don't be fooled to get a lot of gadgets!![>:].
Why not mount the OS LA instead !! Better newcomer engine than the Super Tigre .40
Just put an 11x5-11x6 APC prop on and enyoy.
second alternative is to mount the batteries further back. But spreading weight is not so good.
I would go with the OS LA .40!
Regards!
Jan K
Sweden

Bingo: How do you like your tiger 60? I have one with a Saito 100 on it. I do love it. Great flight characteristics!

I got this one, I just threw up my combat plane for the pic and the plumb bob string is short but you can get the idea. Just hang the plane and the plumb bob from the same point and the plumb will point right to the place the CG is at, Just put a mark on the wing where the CG is wanted and add till the plumb bob points the mark on the wing.

Dauntae

Dauntae

Minnflyer, jaka, try this,

Balance a plane on your fingertips as in the normal way. Once it's settled, bend your fingers forward. Notice how much the plane moves in accordance with your fingers??

The 'gadget' I described is almost the same as Bingofields, just split in half, and I'm sure no1 here will say thats terribly hi-tech. For that few pence/cents(not even pounds/dollars), you get a lot of extra assurance, and confidence in your plane.

On no less than 3 occasions have I fingerbalanced a plane only to find that its at least 3/4inch out.

Human error is often the biggest form, just like golf, the more we can remove it, the more consistent we'll be.

Well, I moved the battery pack as far back as It would go, which is about half way back towards the tail. I still need about 2 oz to balance it. I think it'll just have to be nose heavy, but of course it would be with a full tank anyway . The Buhor spad I built is very tail heavy and I think it flys ok, so maybe the nose heavy will be OK. I can't bring myself to tear open the fuselage to add weight to it where it's most effective, which is in the tail. ( leverage you know). Maybe the OS 40 LA would be the best choice. How good is that engine? The ST GS40 ringed runs great!

Nose heavy is always better to start with. Can you get your hands on some self-adhesive weights(great-planes?), that way you don't have to open the fuselage til you're certain.

Yes.

Now try this...

FLY the plane. THAT will tell you where you want your CG to be.

Many people seem to think that the CG is this magical place where the plane MUST balance. True, there is an AREA where it will fly best, and a few areas where it will not fly at all, but anyone who thinks that if they balance their plane exactly where the manufacturer suggests it will be perfect is, well to be honest, a rookie.

The initial balance point should be withing the manufacturers suggested range (BTW, note the words SUGGESTED and RANGE). So to get it in this ballpark area, a ballpark method is all that's needed. Now, FLY the plane and determine if you want to change the CG.

If a MFG says "Set the CG at 4 1/2" behind the LE" they don't mean that that's the best place for it. they mean that's a place where it will fly without being too nose or tail heavy.

But if you balance it exactly at 4 1/2" that doesn't mean it's in a good place - nor does it mean you'll like the way the plane flies with the CG there. It is just a reference point that should probably be changed once the plane it flown.

So why get anal over getting it perfect?

Yes, this is pretty simple, a piece of 2x8, and 2 pencils with good erasers. It puts the plane up where I can add or subtract weight and see the balance effected. That's all. Works whether you are balancing right side up, or upside down. Like Minn said, the recommended cg is the starting point.

Dauntae,
Thanks for the pics. That helps a lot, and looks very interesting.

Hi!
All aircraft with a constant cord wing have their CG at around 25%-30% of the wing cord...
where is it on your airplane? As Minnflyer said .The recommended CG location is just a starting point.

Regards!
Jan K
Sweden

2slow2matter,

Here are pictures of a trial balance of a three-engine project under construction. The battery is not in the airplane and the balance is to the rear. The right tanks are mounted in the wing and the left tank and cover is on top of the wing. The engine boxes are not far enough along to allow trial fitting the engines.

I like the cradle method since it allows trial fitting parts. The wheels are not on the floor and the oil bottle is to prevent rotation.

I prefer to pay attention to detail. Enough things will go wrong without my help. A lot of people are very casual about balance and control throws and the approach works for a while. A US Air commuter crashed on take off and killed about 80 people a few years ago. The loading created a rearward CG that was thought to be within tolerance.

Bill S

BillS,
Would incidence or angle change anything when you are checking cg in that rig? IE>> nose up or down attitude have any effect on where the plumb would be ? Thanks..

I'm not getting anal about anything, but the very words you're using should remind you where we are, THE BEGINNERS FORUM, i.e. for people who don't have your 40 years experience. I didnt say it had to be perfect, but closer to the recommended makes it easier to eliminate problems if something goes wrong when the plane does fly.

Before I put this to bed, I had a Precedent flyboy as my first plane. Built absolutely straight by my father who already had about twenty kits under his belt, and verified by at least 8 experienced flyers(not rookies). With everything as in the manual, it would take off, climb vertically and then snap roll back into the ground for seemingly no reason. After months of head-scratching and me almost giving up the hobby because it was getting me down, we stuck 6oz of lead in the nose, and after that it flew fine. So no, I don't think the manu. knows 'where the plane MUST balance', but it's a bloody good place to start.

Back to topic.

Bingo field,

The attitude (of the airplane) makes only a miniscule difference in the plumb bob location but I prefer to have the horizontal stabilizer level. If one wanted more accuracy an incidence meter could be put on the stabilizer but eyeball is probably more than good enough.

Bill

I didn't figure it made a major difference, but you know what they say when you "ASSUME". Since the plane is suspended more like a pendulem, the weight will go under the bob, and the center of gravity is exposed rather than the bob goes to the center of gravity. I have to put these things into perspective before I can understand them..... With the pencil stand, once the cg is marked, it would show tail or nose heavy, and you add weight like on an old scale until it balances closely to where you want it. Nothing fancy, just gets you to where you know where it balances out at.... Thank you for the explaination.

Don't know fi the original poster is still around, but if you are looking for a light 40 size, you won't get much lighter than the Norvel AX-40 at 13.1 oz with muffler for a dual ball bearing engine. I don't have a Norvel 40 but I have several others from .061 to .15 size and am happy with all of them. A little more expensive at $110 though than you standard sport 40 size. However, shaving 6 oz off the nose should help with your balance issue.

Duke

And I was thinking of buying one of those...

http://www2.towerhobbies.com/cgi-bin...?&I=LXRS89&P=7