Dorsal Fin

[FoxBob]

I am at the stage of fabricating the dorsal fin for my Model V Vixen. Not crazy about the design, sitting on top of the fabric, flimsy, and likely to be grabbed and damaged. I spoke to Merle Williams, who said that an easy mod was to replace the dorsal fin with a tube welded between the turtledeck and the vertical stabilizer. Any pix available of this conversion, anyone who has done it? It seem stronger than the aluminum dorsal fin and would be covered by the fabric. Any other ideas of how to smooth the transition from the aft fuselage to the vertical stabilizer? Is anyone just building without the dorsal fin?

Foxbob

[jtpitkin06]

It would be easy to make a dorsal fin like the Model 7SS. It's just an aluminum tube joined behind the turtle deck and extending aft curving up to blend into the vertical stab. It uses a few plywood braces for extra support and is held in place with Hysol. It's actually pretty sturdy and covering it was not difficult.

The forward end of the tube slips over a stub weldment that is Hysol attached to the cross member aft of the turtle deck. The aft connection to the vertical stab is merely tapered and filleted with Hysol.

JP

[DesertFox4]

Murle welded in a tube on my model 4 back in 2001 as he described to you. It is the same tubing used in the airframe construction and has a couple triangle gussets welded beneath the tube to the airframe for strength. It is strong enough to be able to pull down on it and raise the nose off the ground without any deflection to it or the fabric.
It also made it so much easier to put the fabric on in that area.
Sorry I don't have any photos right now of the uncovered airframe. I will look through my build photo album and see if I took some pictures of it before covering.

Here is one of the finished product. 100_6109.jpg

[jwlt500]

check cainbirds album on a IV. I did the same thing but with CRMoly tubing and welded it in place.

[dholly]

Just make certain there is no interference with the counter balance weights when the wings are folded (please don't ask... ).

[mr bill]

Here are a few pictures of what I did with half inch diameter 4130 steel tubing welded in place.

[HighWing]

Mine was done similar to John's. I used 5/8" aluminum tube Hysoled to the structure at each end. I suppose my second priority after fairing everything was weight saving so I eliminated the plywood truss. With nothing but tension from the fabric, I felt that the worst I would see is a sliight sag of the straight part. I also terminated it aft of the antenna mounting plate common on Model IVs as I wanted to utilize that for the com antenna. I see also that I opted for a more gentle sweep up to the vertical stabilizer. Once again, I used my favorite stuff - Micro - to fair in the joints - 5/8" aluminum to 5/8" steel.

A note on the possiblity of using the dorsal fin for a handle and damaging it. With the handle on the left side for moving the airplane on the ground, the habit of going to that side and grabbing the handle becomes automatic.
Lowell

[FoxBob]

Does the dorsal fin accomplish any functional purpose (like yaw stability), or is it just for looks? Has anyone just left it off of a Series V Vixen?

[Eric Page]

The Series 5 doesn't have a dorsal fin. There's a relatively small radius curve from the flat top of the fuselage to the leading edge of the vertical stab, which is formed by a steel tube that's part of the frame, but no real fin. Some builders have added a tube above the fuselage top to enlarge that radius and form sort of a fin; you'll find most references to it on the forum are called a "razor back."

[jrevens]

Does the dorsal fin accomplish any functional purpose (like yaw stability), or is it just for looks? Has anyone just left it off of a Series V Vixen?

In my opinion it looks better, but it also makes it easier to install fabric in that transition area between the fuselage and the vertical stabilizer.