Is it important

[chefwarthog]

I look to many photos of water bird, and I wondering why some planes have an extend of there horizontal stabiliser underneath there tale?

Is it necessary or it is there just to look like The ''Beaver''?

[jiott]

It is an increase in side surface area behind the CG. Not all float planes need it, and the size of it depends on the airplane model and the specific floats you are using. If the plane needs the added fin, you can tell when you are flying because a crosswind will try to push the nose downwind (due to too much side area on the floats forward of the CG). You want to add enough fin so that a crosswind always makes the plane weathervane slightly into the wind. There are formulas and calculations for this, which I do not have, but they are available. It is also very important when taxiing on the water-you always want to weathervane into the wind.

Jim

[Geowitz]

The extra vertical stabs are fixed. They are only there to add stability. Floats tend to destabilize airplanes as most are typically designed for best performance on wheels. The extra vertical surface helps with this.

[chefwarthog]

Thanks guys for those answers, now it is clearer in my mind and it make sense. With this fin underneath the tale and a Rotec R2800 Radial Engine on float it should look like a little Beaver, in a less expensive package!

[akflyer]

It is called a "Ventral" fin, just like on a fish.

The side area of the floats forward of the CG act like a rudder up in the front of the plane. Some planes have a large enough vertical fin to over come this. Others, Like the AVID (mine is even extended forwards from stock) dont have a large enough fin and the nose will "hunt and peck" side to side keeping your feet very active on the pedals to keep it flying in a straight line. To put the fin on and it just pins to the two tubes that the tow bar would pin into. This give you addequate area so the nose wont hunt on you.

With out it , it is not something that is un-managable, but it does make a big difference when you put it on there.

[Jfquebec]

Yes it's because the weight change with float and without that the airplane slip in turn..

Some beaver have no fin like that but he can put 2 other one on the elevator..