Originally Posted by bobmij
So if I had a trim of 25% MAC with a wing of 100cm chord then that refers to 25cm with respect to the leading or trailing edge?
You don't have a trim of 25% MAC. You have the CG at the 25% of the MAC, which gives you a specific angle for the THS. And yes, if your CG is at the 25% of MAC, if MAC measures 100cm. you will have the CG 25cm. behind the LEMAC.
In little acfts (the ones I fly
) we don't work with MAC. For us the CG position is measured from the datum (generally at the nose). But in big acfts the CG is measured by %'s of MAC. By aerodynamics you can only have the CG in the MAC, but also if must be within limits (which are also measured in %'s of the MAC).
To sum up, you should know the position of the CG (in % of MAC) at T/O to assure is within limits and also to know what is the required angle of the THS.
Do you understand?
PS: What plane do you fly?