EFFECT OF DIFFERENT LOCATION OF AIRCRAFT CENTER OF GRAVITY

EFFECT OF DIFFERENT LOCATION OF AIRCRAFT

CENTER OF GRAVITY

In this post I am going to share my knowledge about the effect of different locations of center of gravity(away from the margin specified by designer ) in different directions , so that you can identify the probable cause in your model if your craft show below given character .

1. IMPACT OF LONGITUDINAL POSITION OF C.G

a) When c.g is too forward

When c.g is forword the elevator authority will decrease by large magin. This effect we can see at the time of landing and take off .
ON LANDING aircraft is unable to pitch up at low speed , the aircraft is too nose heavy.
ON TAKE OFF aircraft unable to produce enough moment to rotate the nose resulting aircraft need longer runway for taking off.

EFFECT ON LONGITUDINAL STABILITY :- The forword c.g location will increase aircraft longitudinal stability (why? Explanation below ).
The forward c.g has a greater distance to the neutral point, means greater the static margin, resulting better aircraft attitude after a disturbance due to gust .

PERFORMANCE VIEW. :- aircraft with forward c.g has very poor performance at any given airspeed (why ? reason below).
Due to too forward c.g the downward force on the tail to resist the nose drop tendency will increased ,it increased the trim angle of attack of the aircraft, resulting overall drag will increased.
Also Because o the above reason, the cruise speed for a given thrust and weight will reduced.

b) When c.g is too aft

Because of too aft c.g location , aircraft always have nose up tendency .and aircraft become more likely to stall(its too dangerous, believe me )
ON LANDING the effect is exactly opposite to the what we see in forward c.g location i.e the more nose down elevator is required to counter the nose up tendency of aircraft.
ON TAKEOFF aircraft is likely to nose up prematurely , and reduced the climb performance
Spin recovery becomes more difficult as the CG moves rearward. Reason:-Centrifugal forces acting about the CG, during a “flat” spin, pull the aircraft out of its axis of spinning, making it difficult to nose down and recover.

Possibility of a tip over if the CG is far aft the Neutral Point.

2. EFFECT OF LETERAL POSITION OF C.G

Laterally imbalanced c.g will cause unexpected roll during flight that required an extra effort to make the aircraft balanced .

It has little or no effect on longitudinal stability but affect the aircraft’s lateral stability.

The roll control surface (ailerons, spoilers ) sizing is extremely depends on the expected variation of lateral c.g that arise due to factors like weight asymmetry , one side engine failure etc.

The effect of lateral c.g imbalance on aircraft handling is typically assessed by:-

– Fuel imbalance test

– One engine failure test (for multiengine aircraft ).

The combination of both scenarios provides the maximum rolling moment for the aircraft that is crucial in aileron and spoilers sizing.

3. EFFECT OF VERTICAL POSITION OF C.G

The vertical location of the CG changes the amount of dihedral effect (for more on dihedral effect follow my 22^nd JAN post ). As the vertical CG moves lower, dihedral effect increases. This is caused by the center of lift and drag being further above the CG and having a longer moment arm.
So, the same forces that change as sideslip changes (primarily sideforce, but also lift and drag) produce a larger moment about the CG of the aircraft. This is sometimes referred to as the pendulum effect.
On lateral stability – a c.g elevation leads to a deterioration of spiral stability, dutch roll and roll maneuverability.