# CoG Vs CoP

## Recommended Posts

I was just reading some comments on a skydiving forum and there are some comments on the interaction between Centre of Gravity and Centre of Pressure. I'm a long way from my books so can i have some explanation on the affect of Fwd and Aft CoG and it's affects on the CoP.

Ozzie

##### Share on other sites

A Lazair is a long way from the centre of pressure.

I would imagine, without hitting the books, that the dimension from the CoG to the CoP is a moment which can be calculated. It would be nice and simple of the line was parallel to the flight path, but a bit more maths if not. Thrust and Drag centres will also change the picture.

##### Share on other sites

Ok normal cessna type.

I load the aircraft with a very forward CoG. How does this affect the CoP?

##### Share on other sites

I'm very rusty Oz, but I believe the CoP always relates to the aerofoil.

If you make the ac nose heavy, you will react by pulling the stick back. This will add drag, but the CoP will be in the same place.

If you fly a little nose down, the CoP will be in a slightly different place on the aerofoil.

More than happy for someone to get a book and correct me.

##### Share on other sites

I would rather call it the centre of lift. If it and the Cof G don't coincide then there is a couple that will rotate the plane nose up or nose down.. The elevator opposes this to maintain the desired pitch attitude. This requires either a down force, ( the most normal situation which gives more stability but the down load has to be added to the mass of the aircraft to calculate the lift required to hold a level height or an upload which reduces the lift required from the wing but is invariably pitch unstable in varying degrees.

The CoL will move depending on the airfoil shape and it's angle of attack (actual). This then changes the force requirement from the tailplane to hold the pitch to the required angle. It's not a large amount in practice but can require significant changes to pitch trim on some aircraft. I am led to believe that a symmetrical airfoil has less movement of CofL than other designs.. Nev

##### Share on other sites

Usually the C of P will move forward as the angle of attack increases. This moves requires forward stick pressure to counteract it, otherwise it would just keep moving forward. As speed is increased with forward stick, the C of P will move aft and once again keep moving, giving more speed. It is just a matter of keeping it all in trim usually, but can get interesting if the C of G is out of its safe envelope. So Ozzie your forward C og G is possibly going to increase your speed and move the C of P aft, giving more speed, might make for an interesting landing.

I think I would prefer an aft C of G to a forward one. At least with an aft C of G you can slow the plane down and just have to be very careful not to get too slow, but the alternative could be frightening.

##### Share on other sites

I guess that you've read the excellent tutorials at this website so the next bit of light reading is here.

##### Share on other sites

Thanks. Revision is the key here. How many years has it been? Too many.