C of G effects on Aircraft performance

2 Answers

1. 
   

   
   

   Skyfox on Nov 21, 2016

   These clips are all taken from the Pilot’s Handbook of Aeronautical Knowledge [FAA-H-8083-25A]:

   Loading in a nose-heavy condition causes problems in controlling and raising the nose, especially during takeoff and landing. Loading in a tail heavy condition has a serious effect upon longitudinal stability, and reduces the capability to recover from stalls and spins. Tail heavy loading also produces very light control forces, another undesirable characteristic. This makes it easy for the pilot to inadvertently overstress an aircraft.

   It is important to reevaluate the balance in a rotorcraft whenever loading changes. In most aircraft, off-loading a passenger is unlikely to adversely affect the CG, but off-loading a passenger from a rotorcraft can create an unsafe flight condition. An out-of-balance loading condition also decreases maneuverability since cyclic control is less effective in the direction opposite to the CG location.

   The forward CG limit is often established at a location that is determined by the landing characteristics of an aircraft. During landing, one of the most critical phases of flight, exceeding the forward CG limit may result in excessive loads on the nosewheel, a tendency to nose over on tailwheel type airplanes, decreased performance, higher stalling speeds, and higher control forces.

   In extreme cases, a CG location that is beyond the forward limit may result in nose heaviness, making it difficult or impossible to flare for landing. Manufacturers purposely place the forward CG limit as far rearward as possible to aid pilots in avoiding damage when landing. In addition to decreased static and dynamic longitudinal stability, other undesirable effects caused by a CG location aft of the allowable range may include extreme control difficulty, violent stall characteristics, and very light control forces which make it easy to overstress an aircraft inadvertently.

   The aft CG limit is the most rearward position at which the CG can be located for the most critical maneuver or operation. As the CG moves aft, a less stable condition occurs, which decreases the ability of the aircraft to right itself after maneuvering or turbulence.

   0 Votes 0 Votes  0 Votes

2. 
   

   
   

   Brian on Nov 24, 2016

   Forward is more stable in pitch making a spin less like, which also is felt with heavier control forces in pitch. Cruise speed decreases so fuel efficiency decreases. Stall speed increases so both takeoff and landing distances increase.

   The opposite is true for aft.

   To understand the stability imagine the tail down force and main wing lift force are fighting each other; similar to a group of people playing tug of war. If just two people are playing then you joining in would have a great effect on the game (lighter forces like seen in an aft cg arrangement).

   Conversely, if 20 people were playing and you joined in you’d have a very small effect on the game comparatively speaking. So if the cg moves forward the arm between the cg and center of pressure is greater. This means the tail down force must increase as well. So the two forces struggling to rotate the airplane are now bigger making the pitch stability stronger.

   Lift also must overcome both weight and tail down force. Knowing that means if the tail down force is greater, as is the case with a forward cg, then the total lift needed to fly is increased. Needing more lift to fly means needing to fly faster to become airborne. In other words, the stall speed has increased. Higher stall speed means you must land faster as well.

   If you like math stall speed relationships are easy enough to see by rearranging the lift formula:

   Vs = sqrt (2 x lift / air density x wing area x Clmax)

   If the lifting force has to increase then the stall speed must follow suit; they are directly proportional to each other.

   0 Votes 0 Votes  0 Votes