determing c.g.

[steve pagano]

How do you know were the center of gravity should be for a plane?
Does each plane have predetermined c.g. before you build it?

[Dwayne]

How do you know were the center of gravity should be for a plane?
Does each plane have predetermined c.g. before you build it?

Here's a can of worms opened up...ah yes the sweet science of CG...LOL I hope a  bunch of guys weigh in on this. I'll start. The basic rule of thumb on a stunt model is to start at the thickest part of the rib usualy where the spare is, then work your way back as you test fly(trim) your model, you will be suprised as to how far back you can move the CG before the plane becomes unflyable. There are two top pilots in this hobby that I look up to, one is Bill Werwedge and the other is Bob Hunt, Bob  used to like his CG a little back of the spare but still a little nose heavy while Bill put the CG as far aft as the plane would stand, one day while flying Bob asked Bill to fly his plane(for what reason I dont remember but that's not important)  on the take role Bill couldnt get Bobs plane to take off he had to actualy pull back on the handle to get it to lift off, that's when Bill gave Bob a very serious talk about CG placement. I read that artical in Flying Models over and over and have tried it and it is true the CG of a model can be moved back alot more than people know and still have a model that flies well and is not overly sensitive...
Hope this helps and doesnt confuse you
Good Luck
Dwayne

[Charlie Pate]

To tweak a little of what Dwayne has said(and I agree with him).
C.G. is usually a personal preference,to a degree;If you get off far enough , no one can fly it.
Example:Ricky Cruz was a Nashville Tn. Speedway track champion. For special events,Sterling Marlin
would come in and drive one of their cars.
He could not drive the car the way it was set up for Ricky.They changed the set up and he could and did
win with the car. Ricky could not drive the car the way they set it up for Sterling,so they shanged it back to his set up and every thing was fine.
Point is ,they changed several things;shocks, gear ect.
C.G, is just one adjustment,and is sometimes just one of the needed changes.
Prop dia. and pitch might work better ,one C.G. setting , than the other .Its just the old change and TAKE
NOTES thing.

[Dennis Adamisin]

Everything said so far is RIGHT ON target.  The only other thing you might want to worry about before you start is: will the bird balance without a lot of ballast?  Put another way, say you are building a Classic stunt model originally campaigned with a F0x 35 without a muffler that weighed 6.5 oz.  Lots of folks use modern engines, larger than original and of course with mufflers.  The new engine and muffler combo can easily weigh twice as much as the old Fox did!

[Jim Thomerson]

You can retrim a nose-heavy airplane.  You may be able to repair a tail-heavy airplane. 

[John Miller]

It's been said by others, more enlightened than I, that there is a "rule pf thumb" method for determining a good starting point for the CG placement.

It goes like this.

Determine the percentage of the wing area, of the stab and elevator. For the sake of this example, we'll use a modern number, 25%. (Thanks Ted.) We'll use a starting CG of 25% of the MAC. If the area of the stab and elevator, had been 18%, we would have used a starting balance point of 18% of the MAC.

Determine the MAC, the chord line approximatley half way out on the wing. Several ways are available to figure this, but the method of adding the legnth of the root chord to the tip chord, and dividing by 2 will give you a legnth close enough for our purposes. For this example, we'll use 10" for this legnth.

Find the location along the wing, where the distance from the leading edge, to the back of the flap, equals our 10" MAC legnth.

In our example, 25% of 10", we should measure back from the leading edge, along this line, 2.5 inches. The indicated spot would be our balance point at 25%, but since this is in the middle of the wing, we can extend the line out to the tip, or to the wing fuse junction.

Now, very good advice, has been previoously been given on adjusting, (trimming) the CG.

Best wishes.

John Miller

[Dwayne]

To tweak a little of what Dwayne has said(and I agree with him).
C.G. is usually a personal preference,to a degree;If you get off far enough , no one can fly it.
Example:Ricky Cruz was a Nashville Tn. Speedway track champion. For special events,Sterling Marlin
would come in and drive one of their cars.
He could not drive the car the way it was set up for Ricky.They changed the set up and he could and did
win with the car. Ricky could not drive the car the way they set it up for Sterling,so they shanged it back to his set up and every thing was fine.
Point is ,they changed several things;shocks, gear ect.
C.G, is just one adjustment,and is sometimes just one of the needed changes.
Prop dia. and pitch might work better ,one C.G. setting , than the other .Its just the old change and TAKE
NOTES thing.

Hey Charlie
What kind of track is Nashvile? Is it paved. How big !/4 mile or 1/2 mile etc. what kind of cars do they run there? I live up here in Kingston Ontario Canada, We are right on the New York State border so there are a lot of D.I.R.T. tracks around here, They run the 358 and big block Modifieds it's alot of fun. But I used to work on the pit crew of an Oswego Super modified...Do you know what they are? These are bombastic short track cars (pavement) that have 468 big block chevys pushing about 800 horses with the engines hanging way out on the left hand side, 70% of the cars weight is on the left hand side, they have big huge tires to keep them stuck to the track and they go like a bat out of hell!! Check this out, Oswego is a 5/8 mile track and they will do a lap in 16.75 secs at about 150 mph on the straights.. Super fast!! and a whole lot of fun!
Google  "Pinners Bar & Grill supermodified racing"  or "Oswegospeedway"
 

[Charlie Pate]

Dwayne
The Nashville Speedway track is a 5/8 mile high bankAsphalt.
It used to be a track with banks and turns just like BRISTOL(Before Bristol added concrete to the turns)
They told them Not to put down more than 4 inches of gravel for an Asphalt base ,.They were having
trouble with the turns ,so they just poured the rock to  it where needed ;7 inches in places.
The last Cup (Winston , Nextel  1969 I believe )they had there,they had to delay the race start, while they patched a man size hole in the middle of the 1-2 turn. During the race the hole reappeared ,about the size of a five gallon bucket       
and it was somthing to see them race and staddle the hole.The track was redone and the banks lowered some.
About 40 mi. east of Nash. there is the all concrete cup track about 1&3/8 mi.
They run Busch, Craftsman truck,and indy car( 200 Mph!on a 1-3/8 mi. track)Fast but not much rubbing like at the old speedway where they now run  everything from school buses to late models