Tri Gear Yaw Stability

[pauldblackmore]

Hi All,

I have noticed that there are at least two Tri Gear aircraft out there with an additional dorsal fin (in front of the main vertical fin). Can anyone comment on the reasons why these have been done and the effect of this addition on stability if any. We are test flying a tri gear and are finding it a little less stable in yaw than the tail dragger that we are used to. At the moment we are flying it without wheel pants so that may be a factor too.

Any thoughts or experiences appreciated.

Thanks

Paul

[Area 51%]

If all else fails............you might consider towing a banner.

Hey.........you said any thoughts appreciated.

[SonexN76ET]

I believe the fin on the nose wheel pant was something on the earliest Sonex kits.

The adverse yaw can be reduced by aligning the nose wheel with the rudder. You want to make sure that the nosewheel and rudder are both exactly aligned with the center line/direction of flight of the aircraft. Any deviation will result in adverse yaw.

That being said, the nose wheel does act as a sort of a rudder as it is directly in the propellers thrust line and varies with throttle and airspeed so you do have to work the rudder a little more than a tail dragger. But it is not bad. You do want to be aware of keeping the ball centered on takeoff and climb to get maximum climb performance.

On the plus side, cross wind landings up to the maximum cross wind component can be done with confidence in a properly configured nose wheel Sonex.

Jake

[daleandee]

I have noticed that there are at least two Tri Gear aircraft out there with an additional dorsal fin (in front of the main vertical fin). Can anyone comment on the reasons why these have been done and the effect of this addition on stability if any.

Hi Paul,

Can't tell you what difference it made on stability but Robert Eley did it on his because he wanted "the look" it gave to his Sonex ...

https://plus.google.com/photos/104825542669454751314/album/5956358146004533793/5956374352354556658

You might also notice the "full pressure recovery" wheel pants (from an RV) that he used on his plane. I think it looks fantastic. Not sure whether the different wheel pants helped with the yaw stability of the nose roller version or not.

Mine has the third wheel on the back now. 8~)

Dale Williams
N319WF @ 6J2
Myunn - "daughter of Cleanex"
120 HP - 3.0 Corvair
Tail Wheel - Center Stick
Signature Finish 2200 Paint Job
168.7 hours / Status - Flying
Member # 109 - Florida Sonex Association
https://www.youtube.com/watch?v=1VP7UYEqQ-g

[Onex107]

I don't know what effect it had on yaw, but after I added a gap seal to the rudder the ball stays centered much better.

[peter anson]

I have noticed that when climbing hard in bad turbulence even my tail wheel Sonex doesn't feel very stable in direction. The fin is not all that big and it is pretty short coupled, but it feels fine in better conditions. You see plenty of aircraft that have grown dorsal and sometimes ventral fins in later developments, the P51 for example. It would be nice to hear from someone who has added the dorsal after flying without it.

Peter

[gammaxy]

I seem to recall at least one dorsal was added to hide some damage that was caused by a strap over the top of the fuselage when transporting it to the airport.

I suspect it would be difficult to feel any difference with/without the dorsal sized as seen on Sonexes. As I understand it, the dorsal is beneficial at high sideslip angles where the tail or rudder would otherwise stall (I've never experienced this). To make a noticeable change on stability, you'd probably need to make the tail taller or otherwise increase its size.

[pauldblackmore]

If all else fails............you might consider towing a banner.

Hey.........you said any thoughts appreciated.

Well the Rotax 912S will certainly have no trouble pulling it :)

[pauldblackmore]

Thanks everyone, I think that answers my question - the dorsal is cosmetic. I'm going to double check that I have the nose wheel perfectly aligned with the rudder, also any info on the rudder gap seals would be of interest - how did you do it?

Thanks everyone

Paul

I believe the fin on the nose wheel pant was something on the earliest Sonex kits.

The adverse yaw can be reduced by aligning the nose wheel with the rudder. You want to make sure that the nosewheel and rudder are both exactly aligned with the center line/direction of flight of the aircraft. Any deviation will result in adverse yaw.

That being said, the nose wheel does act as a sort of a rudder as it is directly in the propellers thrust line and varies with throttle and airspeed so you do have to work the rudder a little more than a tail dragger. But it is not bad. You do want to be aware of keeping the ball centered on takeoff and climb to get maximum climb performance.

On the plus side, cross wind landings up to the maximum cross wind component can be done with confidence in a properly configured nose wheel Sonex.

Jake

[Onex107]

Gap seals on the tail feathers are not difficult. It looks a lot better, and you don't get those goofy questions as to why your rudder is offset. The material I used is .010 alum. flashing from Bob's Ace Airplane Store. Cut a piece about three inches wide and drill/punch holes in the inside edge so it will fit over the rivet tails of the vertical stab skin. It helps to put a slight bend in the seal, or the seal edge, to make it lay tighter on the rudder. The first one I put on with double stick tape, but later changed it to a more permanent install by making a piece of 1/8 X 1 with notches on the rear side so it can lay behind the rivet tails and clamp down the gap seal. A couple of rivets in the clamp piece hold it all in place. My theory is the gap was killing the air flow on the right side of the rudder and letting the tail wag that direction. I can't prove anything but the tail looks better and it stays in line better. I also closed the gaps on the elevators and saw a change in the trim setting, telling me that the same thing was happening there.