Yaw Stability

[Micro Mong Bldr]

I've heard a lot about Model 1 and 2 neutral yaw stability and the rudder on my model 2 does seem strange compared to Cub, Taylorcraft, etc. Are there any mods to make it handle like a model 3? Just thinking, maybe it's more trouble than it's worth. :)
Cheers,
L

[Av8r3400]

The I and II are not UNSTABLE in yaw. I believe the term was Yaw-Neutral. This means you need to be actively inputing control into the rudder to keep in coordination. I don't believe the rudder and vertical fin were appreciably changed until the IV-1200 was produced.

Compared to a Cessna or a Cub, this just means you really need to learn to use your feet at all times.

You can enlarge your rudder and fin if you want, but I know of no easy way to do it without welding in tubing and recovering.

[SkySteve]

Here's my model I, adding 10" height to the vertical to match the added model IV rudder, along with a few other mods.
http://i237.photobucket.com/albums/ff12/KitfoxSteve/85DD/85DDNewMainTundratires110-3-2007.jpg (http://s237.photobucket.com/user/KitfoxSteve/media/85DD/85DDNewMainTundratires110-3-2007.jpg.html)

[War Eagle]

Whoa! You are in fact deeply engaged in some modifications to your bird. With those new tail feathers you could add a 914 upfront and have a rocketship on your hands. Good luck with your project and hope you get back in the air soon.

[SkySteve]

War Eagle,
These mods were made to my model I a few years back. I made the mods to the tail feathers to change the yaw issues. The mods corrected it and made the plane a joy to fly. I was just throwing the photo out there to show that the yaw factor of the early models can be addressed. But you are right, at the time, I did make several major modifications.

[DesertFox4]

As you can see from Steve's photo it is an extensive modification but certainly doable. Steve did an outstanding job and I can testify first hand that it flies as good as it looks. :)

[Paul Z]

There is a Kitfoxer here in Dallas that did the same mod.

[Av8r3400]

I'm doing both my horizontal and vertical tail:

http://www.avidfoxflyers.com/uploads/monthly_03_2013/post-36-13625277685749.jpg

[DesertFox4]

Looks great Larry. Love the shape and counter balanced controls.

[SkySteve]

Larry,
Looks really nice! I've wondered, what different effect does the rudder have with the top over the vertical stabilizer? Same question regarding the horizontal stabilizer and elevator?

[Av8r3400]

The "balance" or sections of control surface reaching forward over the fixed surface, add a lightening effect to a larger control surface. Meaning it makes it easier to deflect a larger control surface with the same effort applied. There is a balance [sic] to be struck in order to prevent flutter, however. I did do the math and compared to other designs in the same speed range. Flight tests will show my results.

Realistically hoping to fly it summer of '15. Full reports to follow.

Once it warms a little I can get back to working on it. I don't have much for "hard parts" work any more, some plumbing and FWF. Then wings and covering. It may fly this year, but that's not a realistic goal.

[JimS]

Larry,

Looks a bit too much "RANS-ish"!:eek::D to me.

As an aside. Beng a RC flier also,I modified an Extra 300 model by doubling the surface area of the rudder and elevators by increasing cord and also adding aerodynamic counter balance as you are doing. Thought the controls were going to be super sensitive after that but turned out that was not the case at all. I bet yours will handle real nice!

JimS

[Av8r3400]

When I saw the S20 display at Airventure last year, all I could think of was how did they see my project in order to copy it?? :p

[jeffcoaj]

I'm new to the forum - and I have a similar set up on my empennage with rudder and elevator balance For a KF2. I was wondering if weights would need to be added to the front sides of the elevator. I didn't think it would but since you mentioned flutter, it got me to thinkin about the aileron with the mass balance. Also, with similar setup, I've seen guy wires from horizontal stabilizer to vertical stabilizer (Aviat Husky). Will this also be required? - Thanks

[Kurt.A]

Looks very nice Larry, I will be doing something similar to my Model 2 project before to long. Hang on to any pics and dimensions that you may be willing to share down the road.

[av8rps]

Nice job on those tail feathers Larry. I like the shape. My Highlander has a tail much like that. But honestly, I don't notice much difference between that and my Kitfox 4 for effectiveness. But it does look cool :)

Going back to the original question posted on this thread, early Avid and Kitfoxes have much more adverse yaw than later Kitfoxes because by the time Kitfox made the model 4, they completely changed the control system (which is why you have to disconnect flaperons on the 4 and newer when folding wings, but you don't on the model 1-3 ), which for the 1st time on a Kitfox gave the flaperons differential like most GA aircraft have.

Avid never changed theirs like kitfox did, mostly because the Avid's weren't as bad, probaby due to having a wing 2 ft shorter than the Kitfox. Although, Avid later offered a kit that added a bell crank to the existing control system, and that did improve their adverse yaw issues.

I'm actually surprised a taller vertical and rudder alone fixes the adverse yaw issue, as adding differential to the control system would be a way better fix in my opinion. (Avid proved that with their bellcrank kit, as they never did increase their vertical fin height, and yet their latest airplanes flew really nice). BUT, adding differential AND increasing fin height would be best, like the 4 and newer Kitfox have. And they do fly nice...

Funny thing is once you get used to flying the early models with all the yaw, it almost makes them more fun in my opinion. Using that yaw to your advantage, you can turn them on a dime. Plus, if you learn to fly one well, you can pretty much fly anything :cool:

[av8rps]

To further explain the early model adverse yaw:

In an early Kitfox, if you push the stick one direction, the aircraft will yaw in the other direction. Here's why: When you push the controls for full left aileron, the aileron on the right wing is deflected down lifting that panel up, but all the drag from the right aileron wants to pull the wing back. At the same time the left wing aileron is defected up (the same amount the rt aileron was deflected down), making the left wing go down (so you now have the roll of the aircraft), but because the left wing aileron is going up into somewhat disturbed air (being that it is right behind the wing trailing edge), the drag that should pull the left wing back isn't there. So the result is, you push the stick left and the airplane yaws to the right. The only way to make the aircraft yaw left is with the rudder. So whenever applying aileron you need to first apply rudder in the direction you want to go. Yup, thats an early Kitfox...

So to now explain the 2:1 flaperon differential change that was done with the model 4 and newer Kitfoxes:

A differential aileron system means that an upward moving aileron moves twice as far up as the opposite aileron moves down (so the drag from the top aileron "pulls" that wing back, rather than you having to use so much rudder to yaw the airplane) so the result is considerably less adverse yaw, or less rudder input required to keep the aircraft in coordinated flight.

The flaperon concept the Kitfox uses is great, but due to having such a long aileron that flies under the wing in clean air, when it is deflected it can cause serious drag on a wing that a more conventional aileron mounted behind the trailing edge of the wing doesn't have. That's partly why the Cub and Taylorcraft have less adverse yaw.

In fact, my Highlander, which in concept is really just a big Avid Flyer with standard ailerons and flaps (vs flaperons) has no issues with adverse yaw, and yet has no differential in the ailerons. But the ailerons are like the Cub and Taylorcraft, being mounted behind the trailing edge of the wing. So I would attribute its lack of adverse yaw more to having standard ailerons than I would the size of its vertical and rudder.

Hopefully this helps to explain the whole Kitfox adverse yaw issue...

[jeffcoaj]

Thanks for the replies. Will the extended surface area for the elevator or rudder cause a flutter in the tail?

[Av8r3400]

At the speeds it will fly, I believe flutter to be unlikely with the mods I have done.

However, before I share any dimensions of these mods I've made, I want to thoroughly test it to be sure.