So I wasl telling you all that I put together my Graupner Cub ARF. The usual bugs, getting things to work right. Anyway, I go to check the CG (not with my fingertips, but with a TF unit), and the thing is very tail heavy.

I was expecting this, b/c someone who built this plane
http://www.rcuniverse.com/reviews.ph...ew&reviewid=20 used a Saito 80, I believe, which is heavier than my OS 46 FX, and his too was still tail heavy.

I only had 6 oz of weight to add, but I'm figuring it is going to be double that weight (yes, like 12 oz of lead).

The battery and receiver are all the way up against the fuel tank.

Is something very wrong here?

If it's NOSE heavy you need to move the weight BACK, try moving the RX, battery (and any thing else) towards the back

I just edited my post Woodsy--I meant tail heavy

No idea if this is "normal" with the model but you could use a solid Ally spinner and bolt the lead at the engine mount, this would help minimise the extra weight required

Paul,
While it's not unheard of, 12 oz. of lead is a lot. Where are your servos located? Can the servos be moved further forward?

General rule of thumb; every oz of extra tail weight takes 3 oz in the nose.
Dennis-

Dennis, I put the servos where the plans called for, which is in a "standard" location I feel. Moving them forward would be tough, because that would remove the little bit of room that I have for the batt/receiver.

I haven't tried 12 oz, but 6 oz didn't budge things much. We'll see how it goes. Maybe measuring the CG with a half tank of fuel would be more realistic. This would put the CG forward. Anyway, if I do need 12 oz, I'm worried I've overpowered the 46 FX, as strong of an engine as it is.

IMHO balancing with 1/2 tank of fuel is asking for trouble.
i've just read the report and they also say it's tail heavy, and they have an 80 4 stroke and 2700mah pack up front?? so no way the 46fx is going to add enought weight.

theres another report of an elec conversion at e-zone
http://www.ezonemag.com/articles/2003/feb/cub/cub.shtml

they also had CG trouble and had to cut a hole in the fire wall to shove the 16 cells further forward.


a couple of things

mont the engine as far forward as possible
get a good heavy spinner
mount the battery on the fire wall in the engine bay.
add the lead under the engine mount as far forward as possible

Good stuff, Woodsy. I will put on a Harry Higley Heavy Hub. The engine cant go any further forward (I just spent the day on that cowl!!). Again, battery IN the engine compartment with a cowl = melted battery. I will put the lead way up front.

If all this doesn't work, I may get a bigger engine at some point. I wonder if I can buy an aftermarket cowl, since mine is all carved up for the 46 FX?

Balance it and fly it, its a cub. You'd be surprised how much weight they will take.

I hate adding dead weight (lead) to my planes. Instead try using some of these options:


Bigger battery (maybe go 6v)

brass prop nut (these are much heavier than the aluminum type)

change the prop-APC is pretty heavy, wood is light

Change to heavier or lighter wheels

Change your throttle servo to a lighter one. You don't need a standard servo to operate that little throttle.

Install lights and a seperate battery. There are several brands that run on a regular 9v battery.

Change the engine mount to a soft mount. Its aluminum so its heavier plus the soft mount puts the engine further forward.


Remember, functional weight is always the best kind!

Ditto

I was recently considering a GP Slow Poke 40 and from the reviews and owner comments I came across they seem to require 16 oz +/- of nose weight.

To put it in some perspective: during a flight you lose 8 or 10 ounces to fuel consumption for a .40 to .46 size aircraft. In some designs this change is unnoticeable as far as changes in flight characteristics. (At least to duffers like me).

I have heard it said many times.

Whatever weight it takes to balance the plane is the right amount of weight.

I flew the cub in the review when it was tail heavy. NOT FUN!.
felt like I was fighting it every step of the way.

You can also get a spinner weight. This is a round doughnut shaped weight that will fit between the prop nut and the prop. This places useful weight as far forward as possible, and improves the low end idle.

Thank you all. Chris, I hope you can come down in a few weeks or so when I get this thing sorted out (see new post for new problem :-(