Avid Bart

It says not for highway service. Doesn't say anything about airplanes.

I have had my eye on these as well. Just need to find a cheap aluminum 4" wheel that would fit the tire and we would be set.

That was Brooks that bought an A model Avid and sold it last year. His plane was was a lot closer to flying condition than the basket case I brought home.

Brett

It has been a long time. I'll try to get a new post started to show my progress.

Long story short, the more I tore into the plane, the more I found wrong with it and it ended up needing new wings and a complete rebuild. I don't regret buying the plane as it has been a great learning experience, but a ton of work.

And yes, I did visit Jim a couple years ago and am looking forward to stopping by again after I get the Avid flying.

My searching found a bunch of manufacturers of the polyurethane film but no distributors. A local plastics film company tipped me off to a film distributor and I'm waiting to hear back to see if they will send me a sample and quote.

Your "polyurethane air bladder" search brought me to the kite surfing crowd. Another hobby I know little about but apparently they use polyurethane air bladders on the kites so maybe there are some diy people that have leads to get the film.

Anyway, thanks again for the ideas.

Brett

Thanks Chris. I have not heard of kayak float bags before. That is exactly the information I was looking for.

Now I need to find out where I can buy the material without buying a 1000 yard roll.

-Brett

I bought some older Full Lotus floats this weekend that have six of the rear air bladders needing replacement. Replacement bladders are available from Full Lotus for $124 each, but they seem like they would be easy to make for a lot cheaper. The bladders are just a single sheet 0.010" thick folded over and heat sealed on the three sides. There is also a circular reinforcement where the valve stem goes through the bag.

Does anyone know what type of plastic these are made out of? I was originally thinking window vinyl, but they are stretchier and more rubbery feeling. My guess is they are either polyurethane or polyester TPU plastic film and I have found a few manufacturers of these types of plastic, but no distributors. It would be best if I could find an end user of the film where I could just buy a small quantity.

It would be great if anyone has any info or leads. I'm planning on calling some of the companies that make the plastic to see where it can be purchased.

Thanks,

Brett

Pictures, or it didn't happen  I'm curious how you are building the ribs. I only have one wing to rebuild but if I were doing both I would have made composite ribs. 

You must be just weighing the main part of the rib between the spars without cap strips or tails. I am just finishing gluing my ribs up and they weigh 9.3oz with without the flaperon tail and 10.3oz for the long ribs with the tail. The short ribs behind the gas tank are 3.4oz and 4.1oz. Total weight is 6.35lbs in each wing. I'm using the 5mm 10-ply plywood for the main rib and 3mm 5-ply for the cap strips. I'm thinking composite ribs would easily save 25% in weight and similar cost vs buying the plywood from Aircraft Spruce. 

-Brett

I'm not an expert, but from what I understand, a sharp leading edge can get you into trouble with nasty stall characteristics and it's best not to mess with the leading edge unless you know what you're doing. Extending the trailing edge shouldn't make much of a difference. With that said, the KF4 uses the Ribblett GA30U-612 airfoil and the coordinates are posted on the internet. Why not plot the airfoil with a 2" longer chord and you will have a known airfoil and not be left wondering how your changes will affect the flight characteristics? I can plot it out in CAD if you need. Just get me the chord length.

-Brett

I bought an AHP AlphaTig 200X last year and have had very good luck with it so far. I hate buying Chinese, but a comparable Miller or Lincoln would have cost 5x more and out of my price range.

If you go the Tig route, I would recommend one that is capable of AC and DC. That way you can weld aluminum and any other weldable metal.

Then take some time and watch Jody's videos on his weldingtipsandtricks YouTube channel. He does a great job explaining the settings and proper techniques.

Agreed and I didn't intend to sway this into a "Cub Killer" conversation. I don't think a Kitfox will ever be able to compete with a Cub due to the narrow, high AR wing on the Kitfox. One of the compromises of having a foldable wing.

Lift changes with the AoA at a constant airspeed, but the analysis I did was an AoA a sweep at fixed lift (750 lbs in this case). The variable is airspeed.

What I'm after with the simulations is to compare the different airfoils and see what effect changing from the Avid undercambered airfoil will have while leaving the wing incidence as it is. There was talk a few years ago about how Kitfox increased the angle of incidence when they switched to the Ribblett 612. Would this need to be changed to make an Avid perform well with the 612 airfoil?

It would also be fun to compare the Kitfox STi rib to see where it falls. Does anyone know what airfoil they are using?

I used a refrigerator compressor that i salvaged many years ago. They work great for this and are easy to find at a good price (free).

Wow. I didn't know it was that much.

I ran the analysis through an AOA sweep at a fixed lift. With the washout at 2 degrees, the velocity would be 97mph and at 4.5 degrees washout, the velocity would be 119mph! Also the LE of the wing tips do not start contributing to the lift until 7.5 degrees AOA. Interesting...

-Brett

I thought I would share my method of reinforcing the rib tails on the wings I am rebuilding. I know other people have had good results with aluminum reinforcements, but that seemed like a lot of work and tough to make it look like it is not a band-aid afterthought fix.

I ended up using 2" wide 12k unidirectional carbon fiber tape and epoxy. I originally was going to fold the 2" tape over the top and down the sides of the rib tail, but decided to cut the tape down the middle and laminate the 1" strips to each side. I vacuum bagged the rib tails using and cured the epoxy in a high tech cardboard box oven. Vacuum bagging and elevated temperature curing improves the strength and reduces extra epoxy, but really is not necessary for a part like this.

In the end, I have around $40 in materials with a bunch left over for the other wing and other projects on the to-do list and around an hour of time. It is incredible how much stiffer the rib tails are with just one layer of carbon fiber. I suppose I could maybe break the rib tail over my knee, but I am not able to break the parts bending them with my hands.

-Brett

From what I have read, the USA35B is an old design and there are other more modern airfoils that could perform better on a Cub. I am in the process of modeling the Ribblett GA30U-612 that is used on the newer Kitfoxes in XFLR5 software. I have the wing modeled with an approximation of the standard Avid wingtips. The aerodynamics field is far from my specialty and am finding the learning curve to be steep. My plan is to model and compare the 30U-612, Avid undercambered, and speed ribs to compare the theoretical performance. Maybe I'll add the USA35B to the list as well.

I still have a little ways to go before I get somewhat accurate data, but one thing I found interesting is how much the washout affects the lift toward the wingtips at low angles of attack. The first picture is at 5.5 degree AOA and the second at 1. I have the washout of the wing set at -2 degrees. I'll have to measure the actual washout on Avid wings, but I think I have it close.

What are your thoughts on the Ribblett airfoils? Do you think the USA35B would perform better than the GA30U-612 that is used on the newer Kitfoxes?

I made my own using 5mm 10 ply plywood from Aircraft Spruce. Very easy to do if you have a rib you can use as a pattern. I rough cut them out with a jig saw and then used a flush edge laminate trimmer bit in a router to finish the edges. 

The cap strips are 3mm 5 ply cut into 1" strips.

These are for an Avid but a KF should basically the same.

I don't have part numbers, but you will want to look for Seadoo pistons. They have two rings in the pistons.

Build a "new" plane from parts.

The pictures that Doug posted are inserted and scaled in my cad drawing. The problem though is a picture taken with an uncalibrated camera and lens will have distortion and will affect the accuracy of the measurements taken off the picture. The drawings that Doug posted are very interesting and show the differences between the airfoils but I wouldn't trust them to recreate wing ribs.

Does anyone know which airfoil is used on the Kitfox STi wing? I found this picture posted on another forum where it shows a normal Kitfox rib in front of a STi rib. It would be interesting to see how the STi airfoil compares to the Avid undercambered wing. I'm guessing the undercambered wing would have more lift at slow speeds but much greater drag at high speeds.

I don't remember where I found it, but I downloaded a copy of Harry Riblett's book, GA Airfoils. This book has the coordinate points for the GA30U-612 airfoil that is used on the Kitfox.

Here are the points in an excel doc and a disorganized cad drawing with those points plotted out. I thought I had a drawing with the Avid undercambered and speed airfoils, as well as the riblett airfoil all plotted out in a cad drawing, but I am not able to find it. 

Riblet Airfoil Plot Points.xlsx

NONAME_0.dwg

The robots are getting really advanced but you have to be careful with the AI technology.

As far as i can tell, the tanks are made out of normal polyester resin.  They make fuel resistant polyester resin that should work very well to repair the holes in your tanks.

https://www.expresscomposites.com/iso-aoc-774-fuel-resistant-laminating-resin

It has 2X cooling power. Might be too cold for an Avid