Intercostals question

[TJay]

Hey guys still building my Kitfox 1 here, was getting ready to shape the intercostals for the tail. Now I see my horizontal and elevator get them but it does not say anything for the vertical. Well I think I would like to make some for it. I believe the leading edge and trailing edge of the vertical fin is 5/8th tubing, so that needs to be the with of the plywood front and rear, now my question is in the middle how wide should I make it. Keep in mind the leading edge on my rudder is also 5/8ths and the trailing edge is 3/8ths, should I just lay a strait edge on the rudder and onto the vertical and see how far it sticks out and go with that. And one more question, if I am to do this how far back from the leading edge on the vertical do I come back before starting to taper back to the trailing edge? Hope this all makes sense.

[Av8r3400]

My IV-1200 project didn't have any ribs in the tail.

Using my IV-1050 as an example, I made symmetrical ribs for the fixed fins on both surfaces, but nothing for the rudder and elevator. I am going for looks more than any real speed function.

Your 1 will hit an aerodynamic brick wall around 90-100 miles per hour due to the under camber wings and no amount of horsepower will drag you through it. Just keep in mind that Piper didn't use any ribs in the tails of their fabric planes and they work pretty well...

Here are my root ribs, tail ribs and side stringers getting some varnish.

[av8rps]

I agree with Larry. And not that it wouldn't help to minimally reduce drag, but on the early airplane, I'd do everything I could to keep it light. That way it will perform like it was designed (and the early very light airplanes were incredible!) Plus you won't be diminishing your useful load. AND the bonus, you can get your Kitfox flying sooner as you don't need to do all that extra work :)

I've flown about every model of a Kitfox and Avid, and have flown with and without tail intercoastals, and honestly I don't think it does all that much. My current model 4 has them in the horizontal stab and elevator, but none in the vertical or rudder, and it flies faster than most. So if it were me, on an early airplane I would eliminate them altogether in favor of saving weight. On a 4 and later I'd do the horizontal stab and elevator, but not the vertical or rudder. Early airplanes have a lot of adverse yaw, but that's because they have no built in aileron differential like the 4 and up have. Intercoastals in the vertical could maybe help that slightly, but I'd be willing to bet an expert would have a hard time telling the difference in flight.

Just my two cents...

[Av8r3400]

(My apologies, I didn't realize how crappy that photo was. :eek:)

[TJay]

its ok I get the idea I will probably do something like that too, I don't like adding the weight to the tail, but I think I should have something in there. Might never be an issue but I worry when I shrink the fabric I would pull my hinges all out of whack. maybe not, but I do want you to know I really appreciate the comments thanks. Here is a little shot of my kitfox 1 ha

[HighWing]

Good comments. I would like to add my experience, though it was on a Model IV.

First to the question, what I did was take two strips of fairly stiff though flexible aluminum long enough to reach from trailing edge of the rudder to the leading edge of the vertical stabilizer. I clamped them opposite each other at the trailing edge of the rudder, then to the rudder post, then to the leading edge. This resulted in a near straight line from trailing edge of the rudder to the rudder post, with a slight bowed out curve mid vertical stab then tapering to the leading edge. This then became my rib profile. By inserting a trimmed piece of rib material under the aluminum I could trace onto it the rib profile. Though to be strictly accurate on what I did on my project, I added width at the post for gap seals similar to the 7s.

Then to mitigate the concerns Paul brings to the discussion, I made the ribs with quarter inch foam laminated on each side with a single ply of light weight fiberglass for stiffness. On my most recent project I did the same and weighing the finished ribs, the before and after resin mixes for the installation and added that to the weights of the weldments for the trim tabs, the trim tabs, servos, electrical wiring, hardware and inspection covers for both rudder and elevator. This amounted to about 6 lbs. My guess is that you could do rudder and elevator for much less than a pound adding only ribs. Keep in mind, though, that that pound will be at something like an eleven foot arm from the datum for weight and balance purposes.

My take on the mods: My first Model IV proved to be very clean aerodynamically as measured by fuel burn. There were six of us from here that flew a regular annual Idaho Back Country trip. On one such flight - the one they still kid me about, we flew from Rancho Murietta near Sacramento to Winnemucca, NV a flight of about three hours for a fuel stop. As we all flew at the same speed, as was our custom, fuel burn was always a curiosity at the fuel stops. The most fuel pumped that day went into the Rans S-6 in our group at 13+ gallons. My fuel burn was 7.2 gallons and the next closest Kitfox was about 10 gallons. One other thing I should mention is that that airplane weighed 704 lbs. empty - exactly 100 lbs. heavier than my buddies IV - PVC lift strut fairings, fancy upholstery, kick panels, carpet, you name it I had it.

Sorry, the photo doesn't show the vertical stabilizer and rudder profiles well.

Edit: I just read your note. Even with the foam ribs, I found no deformation of the tubing after shrinking, however, I used the recommended shrink temps in the builders manual rather than the Polyfiber cover manual - debated a lot back in the day, but Kitfox was the winner of that argument for me.

[t j]

Tjay, I attended a Polyfibre seminar about 20 years ago right after I bought my kitfox kit. He was from the polyfibre factory. The first thing he said when he began was, You Kitfox guys. You can bend your airframe if you shrink to 300...maybe it was 350 degrees, whatever the maximum is.

His instructions were to start low 200 or 250 and shrink the whole thing. Then increase by about 25 degree increments with a helper holding a straight edge on the tubing and shrink it all again. If you see a tube start to pull away from the straight edge, stop right there.


Mine is a Classic 4. My kit included intercostals for the horizontal stab only. I saw all these fancy Kitfox tails on the internet so called the factory and asked why my kit didn't have intercostals for the rest of the tail. He said it's just for looks, Your plane will fly just fine without them.

[jiott]

On my SS7 I shrunk to 350 on everything and had no structural deformation. In some areas where I was concerned I used the straight edge method to check and make sure.
The earlier models may be an entirely different story.

[TJay]

I decided to spend my Saturday building them. I wasn't going to install any but I really wanted the look of the newer planes. I think it should look nice,I still have to do the elevator but that's for a different day. Im not to worried about the weight since I have a heavy E gearbox way up front. Thanks for the input guys.

[jiott]

Make sure you stiffen any of those ribs that are not glued directly to a steel member, especially on the elevator. Otherwise they will deform into an "S" shape when you shrink. A 1/2" wide piece of ply glued perpendicular to the rib (as the build manual says) is required.

[TJay]

Thanks for the tip I will do that.