it gives information about designing RC planes.

1. CG
LOCATION

2. WHY WE NEED TO LOCATE
CG?
LONGITUDINAL STABILITY
SELECTION OF HORIZONTAL TAIL’S
ANGLE OF INCIDENCE
MODEL AIRCRAFT’S
MANEUVERABILTY

3. STEEL BALL AND SAUCER
THEORY

4. FORWARD CG
Position of CG in front of aerodynamic
centre.
Extent of this position depends upon
downward lifting capability of horizontal
tail.
In the designing of swift, the tail
download ,tail download needed to offset
its wing airfoil’s pitching moment was
calculated at 15.4 ounces at 60mph level
flight.
At 5% of MAC , a CG revs up the tail
download.

5. Results of surged tail
download
Increases the weight the airplane’s
wing must support.
Plunges the horizontal tail’s
manoeuvrability.
This limited capacity makes achieving
a full stall attitude difficult.

6. Ebbs and flow of forward CG
High longituidinal stability.
Good recovery from pitch
disturbances.
Easy to fly but not highly
manoeuverability.

7. Neutral point and rear CG
Point where the total lift (and drag)
forces of two airfoil’s(wing &tail)
effectively act.
NP lies between the aerodynamic
centre of two airfoils.
Any perturbance in pitch causes the
AoA of both airfoils to change ,
resulting in change in lift(and drag)
forces at NP.
Nose up disturbance increasing lift

8. Static margin
Degree of inherent longitudinal
stability governed by distance
between CG and NP aft of it.
For given NP , moving CG aft would
reduce inherent stability.
CG and NP coincide to give neutral
stability.
Moving CG aft of NP would cause
serious instability.

9. Factors involving complex
calculation of NP
Tail plain efficiency.
Areas of wing and tailplane.
Distance between aero dynamic
centres of wing and tail.
Slopes of respective airfoil’s lift curve.
Fuselage area distribution in plan
view.
Downwash variations
Propeller’s rotation

10. Is the stability margin
restricted.....?
Yes , at least a 5%margin is needed.
Should not cross the NP.

11. Does the fuel consumption
related to lower bound of
5%......?
 Yes, as the fuel content gets low CG
gets shifted to right and might cross
over NP if it is too nearer(5%) and
causes serious pitch instability to the
aircraft.
 Pattern ship designers recognised this
and position their fuel tanks on
model’s CG.

12. Use of ballast to correct CG
 Add ballast up front for tail-heaviness.
 Add it aft for nose-heaviness.
 However adding much weight doesn’t
improve the model’s performace.

13. Suggestions for installation of
control components
 Position the servo’s wirings as far away from
the receiver and antenna.
 Keep the antenna away from control cables.
 Position engine , rudder and elevator servos
close behind the tank.
 Position servos for ailerons and flaps in the
open wing centre section, between the main
and aft spar.
 Receiver’s battery should be easily
accessible.
 All in fuselage and wing equipment should be
easily accessible.

14. Weight analysis in CG
 Power components and fuel.
 Control components.
 Landing-gear components.
These are the items over which
designer has no weight control.

15. Position the CG’s of the variable
weight items
 Wings with flaps-50% MAC.
 Wings without flaps-40% MAC.
 Horizontal tail-40% MAC.
 Vertical tail-eyeball the CG.
 Fuselage-normally 40%of the distance
from bulkhead to rudder post.