Fin/rudder area for a delta ?

[Terry Rigden]

Is there a rule of thumb for the size of the fin/rudder for a delta something along the lines of X% of the wing area ?

Regards

Terry

[vintage1]

Not really. It depends on the moment arm from the CG.

The primary function of the fin is to stabilise in the yaw plane: in that respect you can do a simple analysis and as long as the center of gravity is ahead of the center of area of the side view, you SHOULD be stable...that's not the complete story, but its quite close.

[Terry Rigden]

Hi Vintage good to hear from you again

Surely there has got to be more to it than that, most of the deltas I've seen have got HUGE fins (Peter Russel's 363 for instance). But if all we need is the CLA behind the CG then it could be tiny.

And once the thing starts to sidelslip or roll quite a lot of the fin gets masked by the wing at the high AOA deltas fly at.

I suppose I could hack bits off the back till it misbehaves.

Regards

Terry

[Brandano]

if the fin gets blanked at high AOA the obvious solution is to add fin area BELOW the wing. that said, the same rules apply as for any other plane. Perhaps two smaller fins further outboard from the fuselage can take advantage of high AOA vortexes as they do on the F18 ?

[vintage1]

Deltas end up with quite rearward CG's to the fin is shorter coupled.

Many 'swept wings only' end up with tip plates..look at a zagi..known to be spirally unstable if the CG moves back.

[HugePanic]

you have to consider the vertical fin's volume when comparing two planes.

volume = area x length *lol*

length is the lever from AC (or CG) to the fins center of area
area is the fins area

P.S.: i think the reference should be the AC, but the CG should be fine too if you compare planes with a similar AC to CG distance.

[vintage1]

err..think about it.. the aerodymanic center - which is hugely affeccted by the tail volume, has to be behind the CG.

Otherwise any yaw will have the centre of pressure ahead of the CG, and you will get 'terminal oversteer' haha. :D

A crude approximation to the AC is the effective center of area..i.e. where the sum of all the areas times the distnace from the AC net to zero.

That doesn't wholly take onto account the fact that e.g. a flat fin offers more force than e.g. a streamlined wing/fuse side area, but its probably erring on the stable side of things.

The easiest way to test is to make a depron/balsa chuck glider....

[Terry Rigden]

I cut the fin area down by 25% and it made no difference at all to the way the model flew. So I must have had more hanging off the back than I needed and I guess it aint a critical parameter.

Regards

Terry