content_image/588171.jpg content_image/588170.jpg
<img src='content_image/588171.jpg'>

Pendulum stability is the result of a number of PPC design characteristics: there is no downward force from a horizontal tail that must be counteracted by the wing producing more lift; there is no weight of the tail that must be lifted; and there is no tail to impose extra drag on the aircraft. Gravity is the primary force that stabilizes the aircraft using pendulum stability. 

The “dynamic pendulum effect” can be demonstrated by swinging the weight around and then stopping the swinging to notice that the weight keeps swinging from the momentum. The swinging weight is known as the “dynamic pendulum effect” which will be dis- cussed in detail later. 

## Thrust Line Moments 

PPC designs can have different moments caused by thrust; the propeller thrust may be above the CG (Figure 2-21 left) or go through the CG (Figure 2-21 right). Since the cart swings free in pitch, this CG thrust moment can slightly affect the pitch of the cart in relation to the wing. 

PPCs with an attachment point above the thrust line have a pitching nose up moment as shown in Figure 2-21 (both diagrams), with a thrust line, wing attach point, and arm “b.” 

## Gravity Moment 

There is a moment arm from the CG of the cart, to the cart/wing attachment (see “arms” in Figure 2-22). The longer the distance from the CG to the 

<img src='content_image/588170.jpg'>

wing attachment point, the more stable the cart pitch is from thrust moments. The higher the wing attach- ment point above the CG, the more stable the cart is from swinging underneath and the less the cart will pitch up when thrust is applied. 

A higher hang point will also better stabilize the PPC cart in turbulence because the moment of the weight and the larger “d” arm creates a larger stabilizing mo- ment. [Figure 2-22] 

For ground operations, the moment arm distance “c” cannot be too great or the front wheel would lift off the ground prematurely, trying to inflate the wing (see right side of Figure 2-22). 

During flight, the advantage of a high attachment point arm “c” is less swinging around of the cart un- der the wing, and less cart “pitch up” when throttle is applied to climb. This high attachment point creates thrust that is now pointed slightly down to the rela- tive wind, which has two significant effects. First, it creates a negative P-factor, counteracting increased torque. The second effect is a disadvantage: less climb rate or more thrust required for the same climb rate. This is due to the increased load on the wing from the thrust, requiring more speed and/or angle of attack to lift the total load. [Figure 2-23] 

## Wing Attachment to Cart 

If flying straight and level over a perfectly flat landing strip, then with the rear wheels 1 inch above the run- 

