Dorsal Fin

[Doug Johnson]

Dorsal Fin

If anyone is interested. I made the dorsal fin for my helio by hand, I got permission from the owner of N6470V #1421 since he parked right beside me to wrap a piece of paper over the dorsal fin and make a copy. I tried it but it didn't work very well so I waited for a day when the wind wasn't blowing got a piece of clear plastic cut a notch in it to fit up against the tail and wrapped it over the dorsal fin and  taped the opposite side down to hold it while I did the tracing using a felt tip pen to make a full sized copy. Once I had a copy I folded it in half did some trimming to make it symmetrical and checked against the dorsal again.

I took all but a couple screws out and matched the holes on my pattern then I took the dorsal completely off and sat in on a piece of cardboard and made an outline of the bottom of the fin with it setting on the cardboard.

I took a measurements across at about one inch intervals so I could get the same base pattern on my tail empanage. I used a center line to keep it symmetrical and centered. If I remember correctly there was a stamped angle frame for it attached to the tail of the 1400. On my Helio I used pieces of 1/2” aluminum angle cherry max riveted in at least two places for each of the dorsal fin screw attach points. It worked but I later wished I'd bent a an angle out of sheet metal and made a frame by bending it around a block of wood of the proper shape by cutting notches in the bottom of the angle. Where it was about an inch wide I could have squared it off in the front part

Next came the hard  part I cut out a copy using I think 25 or 32 thousandths aluminum I got someone to help me and we clamped a piece of 1” round bar stock between two benches  then clamped  the piece of aluminum we were bending on it's center line to the piece of  round stock. Then using two sets each of  vice grip sheet metal clamps we tried to bend it into shape. We ruined the first one. So we cut out another one and left some extra metal to grab onto with the clamps and got a couple more people to push down in the middle with a block of wood on each side, it got a little crowded and was a struggle to keep even pressure as we bent it down but it did work. Then I cut off the extra metal and did just a little more trimming to get it to fit properly around the tail.

I seem to remember using a couple pieces of the old Royalite gap cover to use as fillets between the vertical and horizontal tail.

When I talked to one of the Jims at Jaars he said that on their speed mod tests that the dorsal fin seemed to make very little difference.  I think I remember the one that they put on the “gray ghost” was made out of fiberglass and was shaped a little differently, a little sharper leading edge and a curve in the up angle. But I liked its looks and thought that, if I was ever to put on floats it couldn't hurt to have it.

Lets hears some comments.

Doug

[gearedone]

The dorsal fin was more of an appearance gimmick as compared to anything speed enhancement-wise. What should be done is plug up some holes back there, noticeably where the stabilator attaches to the vertical fin. There has been a long standing debate as to whether the Helio really needs all the height in the rudder, since it has limited travel, but if it ain't broke don't fix it!!

[Doug Johnson]

I've been thinking about this, there was a post by Louis about needing more vertical area with amphib's and skis. I assume this is for lateral stability.

Wouldn't the dorsal fin help in this area or does it need to be on the bottom of the fuselage for greater effectiveness?

I thought Louis' post about the helio people not wanting you to have greater rudder control so you couldn't force the airplane into a position where it could spin more or less covered the size of the rudder but it did seem to me that with an all moving rudder and the dorsal fin you could reduce its size. But then we come back to the ski amphib thing.

Does it need be bigger or is there a way to make it smaller to reduce crosswind component?

I looked at making some triangular L shaped pieces of aluminum that would bolt on to the stabilator and fit inside the tail but were free to move up and down and to fill those holes but if I remember correctly they were going to hit on structural members.

Has anyone really given some that to filling those holes?

Jaars did some work on speed mods but as far as I know they never looked at the wing intersection to fuselage area on the bottom of the wing. It seems to me about everyone that works on speed mods looks there. Everyone know's about the eyebrow above the window but I don't think Jaars looked at the underside of the wing.

Anyone have any opinions? or am I posting to a blank wall.

Doug