Aero-Dynamics

[DanB]

This one is for any of our many aviation engineering types...Or if you have tried to solve this same problem.
I want to bounce this off of you to find out if my aerodynamic thought processes are correct.
Problem:
My Kitfox IV has been flying very well for the last year. When I first got the AW ticket, I went up and realized right off I needed a trim-tab on the rudder as I was needing to hold some right rudder to fly straight. The trim tab is about 6" long and bent off the trailing edge of the rudder to the port side. I ended up needing to bend this tab into the airflow at Approx 45 degrees to fly without right rudder. I know this generates quite a bit of drag which I would like to get rid of.
Solution ?
My thoughts lean towards shimming the engine mount canting the engine slightly to port side. If this thinking is correct, how much would one start shimming? Or is this even a practical solution?
Thanks in advance for your thoughts.

[Dave S]

Dan,

This is not an answer to your question, however, your comment inspired one I had.....you said the rudder tab is 6" long, what is the other dimension...i.e. how far does it extend back from the rudder trailing edge?

Curious because I am in the middle of engineering an adjustable rudder tab for our KF and trying to determine how big to make the tab.

Thanks,

Dave S

[DanB]

Dan,

This is not an answer to your question, however, your comment inspired one I had.....you said the rudder tab is 6" long, what is the other dimension...i.e. how far does it extend back from the rudder trailing edge?

Curious because I am in the middle of engineering an adjustable rudder tab for our KF and trying to determine how big to make the tab.

Thanks,

Dave S

Dave, It sticks out about 1" into the slipstream.

[Av8r3400]

Tab needs more chord, IMO. Mine is 2-3" aft of the trailing edge of the rudder by 5" high.

[jamesmil]

I tried to shim the motor on our ss7 also to center the ball with no help.
do a search under " rudder trim tab " where this was discussed with some good pic's of way's to do a trim tab.

[DanB]

I tried to shim the motor on our ss7 also to center the ball with no help.
do a search under " rudder trim tab " where this was discussed with some good pic's of way's to do a trim tab.

Thanks Jim,
I appreciate the reply. Was curious if someone else had gone down that road. How much shim did you try? I have a trim tab and it works fine...just trying to eliminate it.
Sure seems like there would be something that could be done. I know we have much more expertise on this list and it sure would be nice to get a discussion going about this learning more about the good, bad and ugly alternatives. I have a feeling that too many folks on here have stopped replying to technical questions because it too quickly turns into a shooting match. Is it that or more along the lines of knowing where the good fishing holes are and not telling? :rolleyes:

[jamesmil]

hi danB, I used 3/16 shims at the motor mount. the problem I could see coming with more shim is spinner to cowl interference with enough shim to fix the problem.

[High Country]

I'm no engineer but how about using a wedge rather than a tab is there any benefit to this aside from cosmetics? this one might be worth a look http://www.averytools.com/prodinfo.asp?number=9883 or just carve one from balsa wood

[Dave Holl]

I have used a wedge but not on a kitfox but a Jodel DR1050 and very effective it was too!

[HighWing]

Some thoughts. I eventually put a trim tab on my first Model IV. It was the riveted on type and it worked well. I don't recall the dimensions but it conforms to the dimensions mentioned thus far. Previously, I had used a wedge and it also worked OK. It was a trial effort and I made the wedge of formed sheet aluminum and taped it to the trailing edge with 3M vinyl tape. It also worked well, but a permanent attachment was too much of a challenge to me.

This time, I tried something I had been thinking about for the last five years of the earlier Kitfoxes life. Doing something like the wing - creating an airfoil on the side needing the additional lift. I was able to do this with a servo and a set of cams that warped the two lower ribs which were made of carbon fiber arrow blanks. It works fine getting the left turn tendency out. The problem is that once trimmed, I find very few occasions that I want to adjust the trim. If I were to do the same thing over again, I would put in a jack screw accessible with an allen wrench so it could be adjusted on the ground saving the 8 oz. of servo.

[t j]

I know this generates quite a bit of drag which I would like to get rid of.


If you can get your hands on a copy of "Stick and Rudder" by Wolfgang Langewiesche, he has a very enlightening discussion of the function of the rudder in chapter 11, "The Rudder".

The bottom line is the rudder is on the airplane to correct a design flaw. We as pilots though demand that we have it so we can be in control of the torque. That creates drag.

[jamesmil]

hello high country, I in fact did buy the trim tap from avery tools and found that there wedge was not enough to center the ball. we made a thicker wedge out of pvc trim board and taped it on the rudder in different places until we found the best position for our plane.
I posted pic,s of the tab on the thread "rudder trim tab" . sorry I don't know how to move them over to this thread.:o

[DanB]

Lowell,
Your latest trim design sounds interesting. I will have to take a look at the next fly-in. Are you making it up to ID in Sept?

Tom, I actually have a copy of Stick and Rudder on the shelf. I'll have to go back and re-visit that chapter. Still, no getting around the fact that the little tab poking it's head into the slipstream is generating drag. Be it... drag that we as pilots are using to our advantage, but it comes at a price. If the tab is bent to the port side it compensates our need to hold right rudder. The price that is paid is in some right rudder authority... If I'm in left field here, tell me how. I may be barking up an imaginary tree to think of a way to alleviate this, but that is why I put this post up. Wanted to hear from veterans.
Thanks

[t j]

Lowell,
Your latest trim design sounds interesting. I will have to take a look at the next fly-in. Are you making it up to ID in Sept?

Tom, I actually have a copy of Stick and Rudder on the shelf. I'll have to go back and re-visit that chapter. Still, no getting around the fact that the little tab poking it's head into the slipstream is generating drag. Be it... drag that we as pilots are using to our advantage, but it comes at a price. If the tab is bent to the port side it compensates our need to hold right rudder. The price that is paid is in some right rudder authority... If I'm in left field here, tell me how. I may be barking up an imaginary tree to think of a way to alleviate this, but that is why I put this post up. Wanted to hear from veterans.
Thanks

No, I don't think you're barking up the wrong tree. I do think any device or design to counteract the torque or thrust line of the engine and prop will create some drag but there has to be two ends to the spectrum...highest drag on one end and lowest drag on the other.

There are a lot of certified designs that use an offset vertical fin to do it. That may be an efficient one. I have always wondered about just putting a stronger rudder pedal return spring on one side??

[HighWing]

Your latest trim design sounds interesting. I will have to take a look at the next fly-in. Are you making it up to ID in Sept?
Thanks
Dan,
That is the plan. I am hoping to renew some old acquaintances and forge some new ones while there. I am still waiting for my repaired radio and that gives the excuse for spending the time putting the final touches on my external video camera mount.