Lift Strut Fairings

[jiott]

What is the best way to remove the molded in ridges that run the length of the PVC fairings that Kitfox sells for the SS7? Do I just have to sand them out, or use SuperFill and then sand?

Also do I need to use structural adhesive on the inside where they meet the round steel struts or just snap them on and call it good? They seem to snap on quite tight.

Jim

[Dorsal]

Mine are just snapped on and un-sanded as I plan on making my own light weight ones. I put them on "temporarily" 200 hrs ago and they are still there.

[t j]

For the smooth look, sanding...lots and lots of sanding. The factory recomends using construction adhesive to attach to the strut and to bond the trailing edge shut.

Like mentioned above some people do not attach with adhesive, just snap on and go.

[cubtractor]

I sanded the ridges, painted and snapped on

[Wheels]

I used a palm sander with a heavy grit, then I used a fine grit to make it smooth. It was faster than sanding by hand. I'm almost done.:D

[HighWing]

I like the snap on idea at least for the first few hours. I look at my aluminum glued on fairings and wonder if I got them lined up the way I really want them. Every set of PVC fairings I have seen have been warped longitudinally so I suspect friction will allow for small adjustments to minimize drag and adjust for roll tendency. In past discussions, there has been lots of heavy winged Kitfoxes and I am wondering if this can, to some degree, be attributed to mis aligned fairings. I am not flying yet, but will be particularly interested in the roll tendency of my airplane and what I find will be what I will always have. I think what I would do if I were using the PVC fairings today would be to snap them on and fly some hours - making adjustments. Then mark them for a basic allignment and then remove them and apply some slow curing adhesive and fly again, twisting them between flights till I got it right then letting the adhesive cure. then doing the closeouts etc.
Lowell

[Grady]

I built a jig for the strut and they work perfect.

[jiott]

Thanks for the comments. I am sanding the ridges out with a palm sander and heavy then lighter grit as was suggested. It is working quite well except for one thing: there is one groove that runs full length that is fairly deep and I am afraid to sand it all out for fear of getting the surrounding material too thin. Can I fill it with SuperFill? Will it adhere to the roughed up PVC? If not, I am going to sand it down as much as I dare and then leave it there and call it an accent strip!

Jim

[jiott]

Still looking for replies on whether SuperFill adheres to the PVC or not? Is there a better filler to use?

Jim

[HighWing]

Jim
Can't speak for Super Fil, but I did what you are contemplating on my first Model IV. I was then pretty heavy into Epoxy so used Epoxy with microballoons as a filler to make up the paste. Down side, it sets pretty slow, but it stuck fine. My plan back then was to prep one of the fairings and use it as a plug mold for fiberglass fairings. I got some bad advice that scared me away from that idea and put the PVC on with the grooves sanded off and any sanding glitches were prepped with the micro. I think Super Fil or Featherfil are more like Bondo using polyester resin, but with the light weight filler. I would think they would work especially the prep sanding.
Lowell

[cubtractor]

I used Super Fill on mine. I have about 30 hrs on it.

[Meyertheflyer]

I just test flew my super sport with the snap on strut fairings. I honestly got an increase of 8 mph and 100 fpm increase in climb rate. It would be better if I added wheel fairings but I do not plan on it. Thanks Papa!

[av8rps]

Somewhere on this forum there is a discussion about the ridges being intentionally put into those plastic struts for aerodynamic reasons. I would have never thought the ridges could be less drag than smooth, as was explained, but I have to say my 80 hp Kitfox 4 has them and it really scoots. I too appreciate the look and lightweight features of struts made from different materials, but I can testify that those extruded plastic struts work very well, ridges, excess weight and all. :)

[Meyertheflyer]

I thought these PVC strut fairings were very easy to install. The aluminum shield provided was perfect to line up the angle cut for trimming the aft fairing. I fold my wings to save hangar space.
They grip very tight and do not budge even at 130 mph. I am using temporary tape on the ends to seal the air gaps till I use super fill and paint them.
The ridges may provide a little strength to the underlying grip so I'm going to leave them. I'm not a speed chaser but this will save two or more hours when I fly to Airventure in July.

[HighWing]

Somewhere on this forum there is a discussion about the ridges being intentionally put into those plastic struts for aerodynamic reasons.

I doubt this for two reasons. The first and most plausible to me is that the slight depressions on the fairing exteriors exactly mirror the location of the internal ridges that capture the lift strut tube and the snap fitting on the trailing edge. Plastic has a very high coefficient of thermal expansion and it is my belief that it is a function of the plastic cooling after the extrusion process and the relative thickness of the plastic at the internal projections results in greater dimensional change at those locations. The second has to do with the effort that would be needed to purposely design this into the fairing. As Paul suggests, the dimples have a very insignificant effect on the aerodynamics and I doubt the factory would have spent that effort.

[Dave S]

I tend to hold the same opinion as Lowell regarding the ridges.....I just can't see that the manufacturer would put that much thought into a plastic part to consider the ridges are intended for aerodynamic reasons..

BTW - Lowell, on your lift struts, did you just sand the plastic to a smooth contour or did you use a different process. I left mine but I am considering going back at it so they look nicer.

Sincerely,

Dave S

[HighWing]

In the first one, I sanded and filled with Microballoons and epoxy. I had originally planned and had made fiberglass fairings using the PVC ones, sanded filled and primed, as a mold. I was then told by someone who dropped by the hangar one day that the 2 part Urethane foam I had injected inside the fiberglass and against the lift struts was corrosive. I decided to use the PVC as I was pretty new to all this and didn't want the wings falling off. Just for curiosity, I made a mock-up of the design using a piece of 4130 tubing and a section of the fiberglass skin with the foam. I masked and primed half of the tubing longitudinally with the epoxy "chromate" primer available then - since then the Chromate has been pronounced ikky by the EPA and the green color we now see in the primer is die rather than an anti corrosive chemical. I still have the experimental sample and will periodically unpeel a bit to expose a little more of the tubing. and no corrosion whatsoever even on the unprimed surface - 18 years. So much for unsolicited advice.

My current airplane has aluminum fairings with fiberglass and the can't live without microballoons at the V. Also, I did use the 2 part Urethane foam as a filler this time - with no misgivings - to secure the thin walled fairings to the lift strut and to give some additional strength to the aluminum.

[Esser]

The expanding foam can be corrosive as I have seen it first hand but I believe it come from moisture trapped in cells of the foam that are against the metal. My unsolicited opinion.

[HighWing]

Josh,
Curious. Was your experience with the expanding foam and corrosion the two part Urethane foam that must be mixed or the aerosol variety that is applied from a pressurized can?

[Esser]

Two part urethane professionally sprayed directly to part of a steel roof. The non sprayed part was in perfect shape and the sprayed part was corroded to the point you could poke your finger through in places in 10 years

Maybe there was more going on than met the eye but just what I have seen.

[HighWing]

Thanks for the explanation. Your comments prompted me to dig out the sample I had made and check it once again. I suspect the two experiences might be akin to the old apples and oranges idea. In my application the use of the foam was primarily as an adhesive with some to added structural stiffness. The foam would have been encapsulated in fiberglass, microballoon filler, primer and top coat. My current set-up omits the fiberglass and replaces it with aluminum sheet with the primer and top coat. The whole object being an aerodynamic reshaping of the round lift strut.

I did a little research on the sprayed on roofing. UV is a big issue as the foam is very susceptible to UV damage and the top coating is critical to protect the foam. It is sprayed on and has to be able to adapt to the daily movement of the foam as it cycles from the heat of full sun to the cool of the night. Then the object is as a weather resistant roof with a thermal barrier added in. The typical lift strut is typically in the shade of the wing and those cyclic temperature swings shouldn't be as critical an issue.

The final thought, though is the advice that I got that it was the foam that was corrosive. My test piece fully dispels that idea as the area in question has been in full contact with untreated steel for almost 18 years with no hint of corrosion. One thing I did notice this time was that the foam does not stick as an adhesive to the unprimed steel, but does adhere quite strongly to the portion that was primed. I am able to scrape the foam residue from the steel with a fingernail. If the foam roof coating was sprayed onto bare metal without a primer, it could allow it to lift due to the cyclic daily temperature shifts and possibly allow capillary water flow between the steel roof and the foam - really bad.

[Dave S]

Interesting observations.....what I get from this is the occurance of corrosion does not necessarily mean the material under which corrosion occurs is "corrosive". More a matter of delamination between the substrate and coating (like on the roof deal) allowing oxygen and moisture to work its magic to turn steel back into iron ore and aluminum back into bauxite.

Much of corrosion prevention is sealing a surface with a coating that will not delaminate (meaning the correct coating plus correct surface prep) to prevent moisture/oxygen from coming in contact with the substrate.

I recall working an a certified aircraft where a novel covering (razorback) was laid directly over a bare aluminum sheet with no surface prep - we had a lot of white dust & corrosion craters under that as the razorback works like a wick and did not exclude either moisture or air and there was no other coating to prevent nature from taking its course.

Some things aren't corrosive, they just don't prevent corrosion.

Sincerely,

Dave S

[Jerrytex]

I know it's an older thread.....anyone use anything different to glue the lift strut fairings on besides hysol? It appears that snapping them together will hold them in place so I am thinking that using expensive Hysol may not be necessary. Anyone ever use Gorilla glue adhesive or a quality construction adhesive from Home Depot?

[Denalifox]

I know it's an older thread.....anyone use anything different to glue the lift strut fairings on besides hysol? It appears that snapping them together will hold them in place so I am thinking that using expensive Hysol may not be necessary. Anyone ever use Gorilla glue adhesive or a quality construction adhesive from Home Depot?


I used Gorilla construction adhesive. Haven't flown it yet but those things are on there solid.