Elliptical planform spitfire fins and stalling behaviour.

The revolution' spitfire fins which are the usual subject of spitfire fin tests have a few problems when used as examples of elliptical planform fins. 

One is that they are not even close to an elliptical planform, the tip are in planform is way too tnarrow and is almost pointed.

Another is the thickness distribution, which is not elliptical. in that respect it follopws the spitfire wing whose thickness distribution was also not elliptical ( see the picture below). The revolution spitfire fin and the spitfire wing itself were both too thin in the tip area. this encourages tip stalling and is not the fault of the elliptical planshape, which as has been pointed out was not even used,

The revolution spitfire fin is also markedly raked, which is another deviation from the true elliptical planform.

Therevolution fin is also exceptionally thin overall in chord ratio as well as being too thin in the tip, this reduces the angle of attack capability of the entire fin and the tip in particular.


In short the revolution spitfire fin's stalling characteristics are not a fair test of the elliptical planform fin's capabilities.


Wing twist used to correct the poor thickness distribution:

 


With my elliptical planform fins the thickness distribution is either elliptical or slightly thicker in the tip area than a true elliptical thickness distribution, I also make sure that the chord ratio is not unusally high, this means that the fin panel used must be thicker than is usual.



 

http://thoughtality.com/the-spitfire-wing

If there is a tendency for elliptical planform wings to stall at the tip first, then it is utterly stupid to make the thickness to chord ratio at the tip less than that of the rest of the wing i.e. a thickness to chord ratio which diminished towards the tip.... which is exactly what was done with the spitfire wing AND the revolution 'spitfire fin'.

Although I have never experienced tip stall with my elliptical planform fins, I keep the thickness to chord ratio constant or increase it towards the tip, I do so because I've visualised the flow rather than by calculation.

I also move the position of the thickest part of the chord forward towards the tip, this also increases aoa capability and helps to prevent tip stall. Again I do this because I've visualised the flow rather than because of any tip stall problem.

The addition of leading edge tubercules makes the elliptical planform less relevant from a drag point of view but it is still a good planform in my opinion because it is upright and also because it keeps excessive area out of the tip where it inhibits rail to rail movement.

Here is an essay on elliptical lift distribution:


http://www.djaerotech.com/dj_askjd/dj_questions/ellthoughts.html

Besides, raked greenough fins feel gutless and vague.