how to find the cg

[Lammergier]

How woud one find the cg on a scratch built plane?

[FlyByMike]

If you are searching for where the CG is currently located (as opposed to where it should be) just get it level while it is balanced on your finger tips. That may be crude but ususally is good enough. Do a low wing inverted and a high wing in a normal flight attitude.
If you want to know where the CG should be, check the plans. Or read the eZone FAQ -it says something like a typical CG is 1/3 of the chord behind the LE.
Hope this helps.
-Mike

[Ollie]

CG location in relation to the aerodynamic center of the whole aircraft determines how much stability and control response in pitch and yaw. The aerodynamic center of the whole aircraft is known as the neutral point (NP). The distance of the CG ahead of the NP is called the static margin and is a measure of stability. The larger the tail area in relation to the wing area and the longer the tail moment arm in relation to the wing chord, the more aft will be the loaction of the NP. These basic principles hold for all configurations including flying wings, canards, tandem, etc. In the case of a pure flying wing, the NP is at about 25% of the mean aerodynamic chord and the CG must be ahead of that for pitch stability.

There is a range of acceptable CG locations. Where the CG should be located in that range depends on the purpose of the aircraft, the skill of the pilot, the desired control response and the pilots flying style. The best CG is determined by trial and error adjustment to meet a balance of all the above considerations.

[vintage1]

Originally posted by Ollie

There is a range of acceptable CG locations. Where the CG should be located in that ranghe depends on the purpose of the aircraft, the skill of the pilot, the desired control response and the pilots flying style. The best CG is determined by trial and error adjustment to meet a balance of all the above considerations.

Not to mention the design of the plane in terms of wing positions, relative areas of stabiliser to wing, and whether the smaller wing is at teh front, or the back.

Example. Given two identical wings one at the front and one at the back the CG would need to be a little forward of the center line between them...

[Ollie]

When the CG shifts during flight due to fuel consumption, that must be taken into account when determining the static margin so that the CG never goes aft of the NP where stability is lost.

When the aerodynamic center of a model wing shifts with angle of attack due to changes in the boundary layer like laminar seperation bubbles, it affects the NP. Such shifts must be allowed for too in order that stability be retained.

The rigging of the model (angles among the thrust line and the chord lines of the wing and tail) should be adjusted to establish a trimmed flight condition. The CG location should be reserved for establishing stability and not used for trimming the airspeed or pitch attitude.

The development of model plans should reflect the experience of prototype flight testing to establish a safe CG or range of CG's and proper rigging angles. Then the user of those plans can adjust the CG and rigging angles to their particular needs through flight testing.

[Sparky Paul]

There is such a thing as too much information.
Without knowing the airplane shape, getting involved in ALL the details of c.g. placement such as NP is premature.
MOF, the next time I know where the NP is on one of my airplanes will be the first time.

[Dick Huang]

Lammergier,
Search for a thread "Determining the CG" dated March 25,2002 for a CG analysis program.
Dick Huang:)