[Just thinking (OP)]

The fin is the fixed vertical bit. The wobbly bit behind it is the rudder.

I think the term fins refers to the vertical and horizontal sections as a whole just like the fins or flights on a dart the vertical fin consists of a vertical stabiliser and ruder the horizontal fins consist of a horizontal stabiliser and it is also the elevator consisting of two main parts low speed elevator the stabilizing horizontal fin and a separate elevator for high speed use as it is much smaller. Basically, stabilisers and fins are the same things. I believe you are correct in saying that stabilisers are the more common terminology for them. leaving the underbelly fitting as the fin that is if the aircraft has one.

[wolfekeeper]

I believe that one of the limits is the engines. You have to compress the thin air so you can reach a combustible mixture. But when you compress the air, the air gets hotter. Eventually you have problems just because it gets too hot at the inlet/compressor and the air's nitrogen starts reacting with the oxygen. I believe nitrogen burns endothermically, so you're losing lots of energy at the compressor. That's really bad, by way of contrast the SR71 and Concorde recover almost all the energy from the inlet. I think that sets an altitude limit.

Can anyone shine some light on something that has fascinated me for a long time I have tried to research this but with no luck. OK, If you look at the horizontal fin/elevator on a 747 and on almost all large jet planes you will notice that the profile of the fin is like a wing only inverted.

That's completely right. Those tail wings push the tail downwards!!! Well observed!

The reason is because the aircraft has to be stable when moving through the air, so the centre of mass has to be as far forward as possible, so that the aircraft sort of shuttlecocks and keeps itself pointed into the airstream. In fact the CofM has to be ahead of the centre of lift. The centre of lift lines up pretty much with the wings, and the CofM has to be ahead of that. So if you think about it, the nose of the aircraft always wants to drop, and it should go into a screaming dive. This would be bad.

Instead, the empenage section has a wing (the horizontal stabilizer) which pushes the tail downwards, and balances out the torque from having the centre of mass ahead of the centre of lift. Because the tail is quite a long way back, it doesn't take a lot of downwards lift to balance the torque, and although it's not terribly efficient, it's not horribly inefficient.

Some 'tailless' aircraft designs, such as Concorde, don't do this, they sort of rely on just having a very long wing. Early tailless designs were horribly unstable, but newer ones are much better. Also, a lot of newer aircraft are actively stabilized, so that the centre of mass and the centre of lift are more or less inline and a computer is constantly fighting to keep the aircraft pointing forwards into the air stream using the control surfaces.

[Just thinking (OP)]

Because the tail is quite a long way back, it doesn't take a lot of downwards lift to balance the torque, and although it's not terribly efficient, it's not horribly inefficient.

Thanks, Wolfekeeper for your reply. That is a very interesting point you make regarding the overall size of the stabilisers. If we look closely at the tail section on a 747SP we can see that the vertical and horizontal surfaces are much larger than on the standard 747 models this is due to the SP having a shorter body and needs more assistance to hold the aircraft in line as the torque on the fuselage is decreased with length with an outboard engine out we would need a larger ruder to correct with.