how do you work out the size on the vertical finn is there a formula or do you just take what looks rigth?

For a conventional aircraft (with tail arm about 2.5*MAC) the fin area is usually about 7% of the wing area.
The fin area should be increased with a shorter tail arm and vice versa.

what about the side area of the fuselage ? it must change the picture a great deal .. there must be some way of calculating it and it may also be diffrent from a funflyer to slope soarer ( more or less stability ?

The side area does affect it and if you have a wildly odd looking model then you need to worry. But for more or less normal designs you can forget it. The side of the nose cancels out the side area of the tail generally.

What we are actually considering here is the vertical tail volume coefficient. That's the factor that uses the size of the fin and length of the tail to arrive at a number. So a short tail needs a bigger fin and a longer one a smaller fin. Dihedral and wing placement also comes into play. A polyhedral or old timer V dihedral with lots of anglel needs more fin area to avoid dutch rolling and a high wing model can use a couple of % more than a low wing model.

Also it's not totally specific down to a single % point. There's actually a fairly broad happy range for most designs. So call it anywhere from 5 to 10% for a more or less normal fuselage based on wing placement and dihedral. And then there's that bit that Dick Hanson alludes to. With enough power and at very low speeds you can get away with murder. 3D parkflyers are using some huge fin areas.

As for fuselages the parkflyer and ProBro designers are making the fuselage shapes such that the 25% side area point of the fuselage matches the 25% point of the wing so the model will more or less fly in knife edge with the same trim as it does when "flat".

You can work it out but consider this. The airplanes that are winning in any competition of any sort are better than all of the calculations or wind tunnel tests you can have. They have gone through the evolutionary process in the hardest way. Look at your airplane and determine what you want it to do. I am sure that there is a model of that sort somewhere. I would copy it with many thanks to the designer/flyer. You can always give it a bit of self by changing some non critical lines. It's the way most of the models are designed today.

Andy Lennon's designs being an exception, he marches to a different drummer.

thank you for all info, i sucked it in too my brain (hope somthing good will come out off it..)

Some stuff we fly simply needs -nor tolerates anything more than a breath of rudder in knife edge -
My 30% Dalotel needed just a little rudder
the old skinny fuselage pattern planes of yore with huge fins and rudders were awful - they fell sideways and adding more rudder just added more drag and upset things.
One of the beautiful things about getting proper rudder/fin to total lateral area sizing, is that snaps are much nicer as the plane easily -instantly yaws with little input.
signed
Murder Inc.

Where do you get plans for a 30% Dalotel? I would love to build one. Randy Wallace [email protected]