profiled tail

[Eric]

I have a KF 3 with the flat(non profiled tail)
Does it make a big difference if I make the stabilizer and tailfin profiled like the newer ones?
It is than also easyer to reduce the gaps between the fin and rudder>
If it is does anyone have the drawings for the ribs/

[jtpitkin06]

Eric,


Contact the Kitfox factory. They may be able to supply you with a tail speed kit and set of ribs and the fairings for your model 3.


John Pitkin
Grreenville, Texas

[P Morel]

Just bought the rib pattern from Kitfox and they were pretty cheap. The rib kit was not available however, John was able to have a copy of the rib pattern sent to me for around $15 plus shipping. I also bought the fiberglass rudder leading edge and the vertical trailing edge from them. The ribs can be cut from from thin plywood from a hobby shop. I cut the patterns out and used 3M spray contact cement to temporarily stick them to the plywood and cut them out on a band saw. The pattern includes the horizontal stab and elevator ribs too. I finished laying everything up today and it really makes a world of difference. It was well worth the the time and $$$.

[cap01]

if i were going to do all the work of installing ribs , i would enlarge the fin and rudder and make a speedster or model IV 1200 tail out of it .

[Dave S]

Eric & Jean-Francois,

I will offer my opinions on your questions - others may have different opinions.

1) Opinion # 1 - as far as the effect on speed/drag - I don't have any empirical data that would suggest there is a major difference. That would take flight testing of aircraft identical in all respects except for the tail. Theoretically, a control surface which provides the smoothest airflow over the break of the leading edge all the way to the trailing edge so as to minimize any eddy currents should decrease drag - how much - no clue here.

2) Opinion # 2 - in my opinion - after flying series 7 kitfoxes of each design (airfoiled and non-airfoiled vert.stab. & rudder) the rudder with the non-airfoiled tail feels like it has a slight dead spot when using the rudder - that is - the plane doesn't want to quite center itself with the world without a some opposite rudder following a roll out of a turn. The two do feel different and I prefer the airfoiled tail with it's response to control inputs- I just don't know if I am explaining it well enough because it is a subjective thing. I am not certain what to attribute this to - thinking it is either a matter of how the air breaks and flows around the tail or else the slab sided tail is a little more prone to fabric deformation from aerodynamic pressure while the airfoiled tail pretty much means the fabric will hold its shape more like a rigid hard surface tail. Analogous situation - the extra false ribs on the wing improve the wing by maintaining the correct profile - possible the airfoiled tail helps in the same way by keeping the aerodynamic pressures from deforming the surface as much when the rudder is used.

Hope that helps some - just a couple ideas - maybe others have some experiences/opinions they can share.

Sincerely,

Dave S

KF7 Trigear
912ULS Warp
Airfoiled Tail





Sincerely,

Dave

[Jfquebec]

Thank's Dave ,it's help for sure,i have a kitfox 4 1200 project with normal crontrol surface ,so i think i will check for speedster tail if i can modiffy it....



Jf

[rogerh12]

I am not an Aerodynamics expert of course, but installing the profiled kit to the tail just didn’t seem to make much sense to me, from a practical standpoint. Looking at examples of aerodynamic improvements to other planes, it seems to me that for the Kitfox, getting rid of the round tubes out in the wind is the big improvement to be had. Applying an airfoil shape to a round tube reduces the drag by up to 9 times. WOW ! This is where my efforts should go, I figured.

Though the contoured tail adds some aerodynamic improvement I bet, it also adds weight, cost and complexity to the build, not to mention a bunch of fragile wood parts added to my already strong steel tube tail (the wood is just something else to break under the fabric).
For me, I have turned my time and money toward making the wing struts and stabilizer struts airfoil shaped, and have left the tail light simple and strong (and save lots of extra build hours).

[P Morel]

Though the contoured tail adds some aerodynamic improvement I bet, it also adds weight, cost and complexity to the build, not to mention a bunch of fragile wood parts added to my already strong steel tube tail (the wood is just something else to break under the fabric).
For me, I have turned my time and money toward making the wing struts and stabilizer struts airfoil shaped, and have left the tail light simple and strong (and save lots of extra build hours).
Just my opinions of course.
Roger

I don't see how the wood would break on the rudder or verticle unless it was struck. And, Yes, you are correct about adding a little weight and adding the build hours. But for me, building is another exciting part of this journey. This is my 2nd build and this 1200 classic didn't come with the profiled tail. My Speedster did and I liked it so much, I decided to add the tail kit to my classic.

[HighWing]

Quote from Dave S.

2) Opinion # 2 - in my opinion - after flying series 7 kitfoxes of each design (airfoiled and non-airfoiled vert.stab. & rudder) the rudder with the non-airfoiled tail feels like it has a slight dead spot when using the rudder - that is - the plane doesn't want to quite center itself with the world without a some opposite rudder following a roll out of a turn.

I have heard about the dead spot with a flat control surface before so I think it is a valid observation.

On my first Model IV I wanted gap seal cuffs and being the cheapo that I am and always wanting to reinvent the wheel, I made them my self from scratch. Weight considerations led me to quarter inch urethane foam. For the airfoil, I first determined what width I would need at the hinge line to make the gap seal cuffs then taped appropriate width strips of wood about midway between leading and trailing edges of the horizontal stabilizer and tweeked until I had the approximate gap seal dimensions I wanted. Then using a piece of steel strip and at the predetermined rib locations, simply clamped it to the leading edge and trailing edges and scored the foam for the rib profile. I did the same thing for the horizontal, but that was a much bigger challenge due to the different hinge design.
My current project has the same basic design, but the gap seals are made up much like the factory did with vacuum formed fiberglass shapes for further weight reduction. This time, including both horizontal and vertical trim tabs with servos, I figure I added about 6 lbs back there.

I don't think rib weakness is an issue. I put 900 hours on the foam ones and the wood ribs from the factory are much stronger than the wood strip rib designs we see on plans builty airplanes.

A note to Roger12 if you are stll planning on the VW, a little weight in the tail that will not simply be dead weight might be a good thing.

I am attaching two photos.
Lowell

[Geowitz]

I would definitely do the profiled tail. It's a little extra work, but the benefits are worth it. The airfoil is more efficient, thus faster, and looks better as well. It also closes the gap on the rudder making it more effective.

Never have I heard of anyone mentioning one of their ribs being broken short of a crash. It's the same reason most of us have false ribs. The sum of all of these refinements give more speed and efficiency overall. I don't consider them as "extras", just an update to the design.

Edit - If my horizontal stab hadn't been covered before I got my kit I would have done something similar to what Lowell did to remove the elevator gap.