Can someone give me the formula for calculating the CG?

How it is done is you calculate a neutral point for the wing-body shape. Then the effect of the horizontal tail on that is calculated. Depending on desired stability the CG is selected.

There isn't necessarily a one formula answer. Could I ask what airplane or shape are you trying to do?

For instance a flying wing with no sweep flies OK with the CG at 20% of the wing chord.

For a "standard normal" type of airplane like we all know and love a CG at around 30% of the wing chord is OK.

Rather than try to calculate the CG anymore I just make a small foam glider of the configuration and adjust elevator trim and nose weight until it will glide nice. So far using the same percentage chord on the model has worked fine.

I would especially use the technique for anything that is strange or wierd in shape. It is a good approach that works.

I found some formula in Martin Simons book Model Aircraft Aerodynamics. I recommend you buy it - for the small cost it is a really well done book written for the beginner who wants a little more meat in the subject.

On page 222 through 225 are the set of equations and how to use them. I hesitate to copy them as it is a lot out of his copyrighted material. Do buy the book though, worth it.

Foam - what a wonderful tool--
We purchased a grundle* of Depron in various thicknesses and spent the morning out flying various permutations of one design.
They are all between 6-8ozs . wing loadings 4-5 --4-3 oz sq ft
The only positive finding-once again -is that given calm air - you can't beat lower wing loading .
Even moving cg around -- the model (four of em ) with lightest loading --was most controllable when flown at low throttle (I guess we should call it a rheostat) and pulled and held right at the "stall".

all were flat foam -all braced the same all same scale etc..- so no airfoil variables - just wing loading differences.
Too bad the boys who pi--ed all the money away on the OSPREY didn't have a a few hectares of foam and some latitude to use their imaginations.
I think the government approach is hectares of money and no imagination.
* great big bundle -- old western folk expression

Agreed Dick, the only thing you have to watch out for is in the application of the knowledge to the big ones. The light foamys can give a new comer a false sense of security.

I still like throttle for a name, rheostat reminds me of the big foot diameter things we had in EE power labs. It brought up a memory. We had a DC power lab at Purdue where you used these big 1930's (if not older) equipment and connected them with big cables and the like and measured stuff with monster meters. It was all fun but I was the only guy in the lab group who had ever handled circuity, I had been a ham since 8 years old (child prodigy - what happened to such a promising start :-(

Anyway we needed a lower voltage than was available out ot the patch panel. I asked the grad student instructor what could we use? He pointed to this big something on wheels and said to hook it into the line and it would give up variable voltage. OK I said and hooked it up. When your circuit was ready you went over to a wall sized panel and used big power plugs to tap into the voltage available.

So I checked it over, went to the board and started to plug in the plug. ZZZZaaaaPPPP, flames and arcs and there was a big black smear where the plug almost welded itself to the panel. Luckily I yanked it back (I had made lots of zaps as a kid but on a smaller scale).

The instructor came running over looking all flustered and looked things over and said woops, sorry about that, the variable voltage thing was AC. It was a direct short across the DC voltage instead of being a variable inductance on AC.

A rheostat would have been a better choice.

But I still like throttle, or speed control. You have a speed control, you open up the throttle.

My latest airplane a P-51 is around a 800 watt system. Insert a Tim the tool man Taylor grunt here. The electric stuff is fun and a lot lighter. Batteries at $150 a small bundle sure need to come down in price though.

P51 - Arf?
kit , scratch?
6 lb? what??
My next leap up in electrics is a babystep - 250 watts and 20 ozs--
going for a silent, park flyable - precision aerobat. Also I am helping a friend do one of the "biggie" electric pattern models - with the $500 battery setup (sheesh)
PS- we have no false sense of security - or any other sense for that matter -
I love to use the "No dogs allowed" section of our local new dogpark. Only problem is the dog owners - A lady came over yesterday andasked if her big ol friendly Shepard could sniff the model -to make sure it was not a threat of some kind
I asked if I could sniff her --

It is the Hangar 9 Miss America. It will be 20 volts or so at 40 amps or so. 8 to 9 pounds.

I have a couple in the 250-300 watt range, the latest is a conversion of a Goldberg Falcon, the original built in 1966 for my Dad. I have had it in the shop for 15 years and decided to elictrify it. Put on a bigger rudder and elevator and went to a tail dragger and it looks like a different airplane.

I have a couple in the 100-150 watt range also that are nice but none of them has the freedom of fear that the little foamys have. It is nice to know the foam breaking will protect the gear and you can get back to flying in just a few minutes.

I just ran across the plans for my old Ambroid Charger while cleaning the basement. The block under the tank is about 16 pieces of 1/4 inch lamination and the wind shield is the same. It took forever to buld the beast with hundreds of bits of wood and laminations using Ambroid (no zap, etc), granted it is still around, but the amount of emotional content invested in it is a lot more than a little foamy.

$500 for batteries is toooo much. Mine will have $300 worth in it and it starts to make me nervous. The costs of motor and ESC is just about equal to a gasser and all else is the same. But I figured I had to fly 200 flights (if memory serves) to break even on fuel costs. I am not sure I can help from crashing in that length of time!

.... if I could sniff her....... no wonder we get thrown out of parks sometimes. Did she offer to show your butt her Shepard's teeth close up and personal :-)

Airacobra - I also found several more articles in older magazines (cleaning out the basement) that suggest various ways of calculating CG but most of them basically end up with something around 25-30 percent of the Mean aerodynamic chord. It gives a very conservative CG location to start with for the average tail in the rear airplane.

The H9 --pretty model - I did the big H9 for a friend - kept cg forward as wing loading was on the "don't go there " side of reason

One can locate the CG without any calculating. Rule: suspend aircraft from any point and theCG will lie EXACTLY below where that suspension line is theoretically extended thru plane. Perform this suspension from 2 or 3 different places to create an intersection point and one will have established the true CG of that plane.
Sounds easy but it isn't. Notice the true CG must be controlled not only along a wing chord BUT its height above or below lift point. Locate CG below lift point and one gets a very stable flying machine. Locate the CG above the lift point and one gets a plane that would rather fly inverted. Put lift an CG together and "hang on to your hat."
This will answer your questions re: dihedral vs flat wing. Dihedral simply raises lift point above CG and adds stability.
Glen

Glenn it wasn't about where the CG is but where the CG should be based on the physical layout of the wing-body-tail. I found a calculator that will give Neutral Point that is really neat

http://www.geistware.com/rcmodeling/cg_super_calc.htm

The question of dihedral vs flat wing isn't just raising the lift point above the CG. In a steady state turn for instance the CG can be along the lift vector and the airplane will not tend to roll out of the turn.

The actual dihedral effect or sweep effect addition is calculated without needing the CG location.

Hey Ben where can I find info on bi-plane bal. and cg.

Thanks Kent