Strut speed fairings.

[inzersv]

Anyone have any photos of the finishing up your speed fairings. In particular, using the aluminum cuffs that come with the kit. I saw what Tom Waid did, aka TropicalTuba, and like it. May go that way. But before I decide, I want to see what others have done. Looking through the Build photos, Tom is the only one I have seen that has documented his so well. Thanks very much gentlemen.

[Av8r3400]

I didn't care for the look of the aluminum sheet wrapped on the bottom of the struts, so I made a small triangle block of wood for in between the struts, sanded it smooth and fabric covered the area. You are putting fabric over the farings anyway, so why not the bottom as well. This allows you to make it look much more finished, IMO.

12000
This is a Kitfox SLSA taken at Airventure. It was the inspiration for mine. I will try and snap a photo or two next time I'm at the airport.

[jiott]

I did not fabric cover my fairings. I thought most people did not. Am I wrong?

I mitred the fairings together at the bottom as best I could, then used Hysol to glue them solidly together and to the steel strut tubes. Then used Superfill to fai it all in. After painting they looked like Av8r3400's.

[inzersv]

Thank you for your input. I played with the aluminum wraps Saturday and didn't like what I was seeing. Av8r3400, I look forward to seeing your photos when you get a chance. I really like the SLSA look. I am with jiott on fabric covering the fairings. I wasn't planning on that. Jim McBean, do you fabric wrap the SLSA's? What are the pros and cons of fabric wrapping?

[HighWing]

My fairings look a lot like Larry's and Jim's but more complicated so I will skip that.12003

Pros: It will help mask the stripes down the extruded fairings. In the old days, guys looked at the stripes and figured they were intentional like the dimples on a golf ball for drag reduction. But if you compare the location of the stripes with the ridges to help center the fairing on the lift strut, it becomes obvious that they are simply areas of shrinkage during the cooling of the PVC after extruding. When I used the factory fairings on my first IV, I just sanded them smooth and did a little micro to get them nice.

Cons: Heavy. The fairings already add about 13 lbs to the empty weight of the airplane then with fabric, and what it takes to finish fabric, another couple of pounds added to that.

Rethinking skipping the complicated mentioned earlier, I put some clear packing tape on the rough cut miter area of the fairings and layed down a couple of layers of fiberglass cloth (clear packing tape makes a very nice separating medium to prevent the glass from sticking).12004
I used the micro to finish the surface of the glass and then using a technique similar to Larry's glued in and faired the surfaces of the joint patch.

[jiott]

You can get all the fairing stripes off, except for one, by a lot of sanding. I used a power sander. I was afraid to sand off the last and deepest shrinkage stripe because the material would end up being quite thin. Fairing in the "V" with Superfill was quite easy. It is great stuff if you haven't used it; lightweight, very easy to sand, sticks good to most any surface (if prepped good and scuffed up), easily fairs to a feather edge.

Regarding fabric wrapping the struts, I see no point in it unless you want the surface texture of fabric; it will add several pounds of unneeded weight. I am pretty sure the factory SLSA's don't fabric the struts.

[inzersv]

I sanded all of the stripes off of the fairings using 220 grit on an ocillating sander. Took awhile, but I am very happy with the results. Definitely not using the fabric. I probably already have enough weight as it is. I am going for your look Av8r3400. I will try to post some pictures as I go.

[Floog]

I made a channel out of the aluminum wrap material. That was riveted to the trailing edge. I've heard some builders fill the trailing edge gap with Hysol or some sort of filler. That's a lot of filler and weight. I smoothed it all with Superfil.

[Geowitz]

Friendly reminder!!! The way you fair the end can affect whether or how far you can fold your wings!!

[jiott]

My method of filling the trailing edge where a lot of the fairing is cut back to allow wing folding, was to use some of that lightweight foam that comes with the kit to 90% fill that gap, then finish the filling and smoothing with the SuperFill. Many ways to skin the cat, but I think most of these alternate methods come out looking much better than the aluminum cuff supplied in the kit.

[Av8r3400]

Just to set the record straight, the strut in my previous post is not mine. It is a photo of a Kitfox SLSA parked in their booth at Airventure.

Below is a top and bottom view of my struts.

This is from the top:
12010


This is from below. (Please ignore the mud...)
12011


I filled in the trailing edge gap again with a fitted piece of wood, hysol'ed into place then covered the whole thing with fabric. I went with the fabric because I have never had any luck getting paint to stick to plastic like this. So now they are fabric covered and match the rest of the plane.

[SkySteve]

(Please ignore the mud...)

AV8r3400,
No, we will not ignore the mud. Mud is definitely a badge of accomplishment!! Mud on, dude.

[jabkwab]

Today I made a test flight with my new strut fairings fashioned from insulation foam and Oratex (the rc plane stuff). Picking up about 9 miles of cruise, impressive!

One thing i noticed is the orientation of the fairings. I made them so that they can rotate, the theory being that they will align in the most aerodynamic way possible during flight.
What surprised me is how far down they seem to go during flight!

https://scontent.xx.fbcdn.net/v/t1.0-9/15727166_731248647025760_6872152847939416706_n.jpg ?oh=6861dba5f28efa7cd9a7c595dcbc9f85&oe=58D77A26

On the ground the fairings align perfectly with the small connecting tubes between the struts.

Would this make my Kitfox weird? Or is this the way everybody is aligning their fairings, just nobody told me :eek:


Greetings from the Low Countries, Jan

[kitfox2009]

Try doing the jury and elevator struts as well. Probably gain some more.
I think that's why the Vixens fly faster.
Have fun
Don

[jiott]

That's really interesting! I wonder if the rest of us who have our fairings in a fixed position could gain some more speed by angling them down. From the photo it looks so extreme, I wouldn't have believed it. Does both sides of the airplane do the same? Wonder if it is a spiraling propwash effect?

[av8rps]

I'd be careful about having the wing struts move too easily, as like any flying surface that is not dampened some way or other could allow flutter.

I have my horiz stab struts faired and I one day switched them out with a standard (un faired - just round tube) set of struts to learn streamlined horizontal struts give me 2 to 3 mph. Next mission is to fair out my jury struts. I have some faired from a Speedster, but I think they are not wide enough in chord, so I'll probably make up my own fairings on those. I'm guessing those will be another couple mph.

It all adds up!

[SkySteve]

Does anyone have an idea what size the fairings should be and what material they should/could be made of for jury and stabilizer struts?

[av8rps]

General rule is the chord of the fairing should be aporoximately 4.25 times the thickness of the tube you want streamlined.

While Kitfoxes plastic snap on wing struts are a bit heavier than I'd like, they are sized just a bit over 4 inches (as I recall) and the struts on the 4-1200 are 1 inch in diameter, so they are about perfect. I believe thats why you see 9 to 12 mph cruise speed increases on average when they are used.

Unfortunately most readily available (and expensive) streamlined 4130 tubing has much too narrow of a chord to be truly effective, so for jury and tail struts I would just make my own. I believe my tail struts are about 1.75 inches in chord. They were some leftover material my Rans buddy had, so I just used that.

[Floog]

Just curious. Has anyone done a retractable gear conversion on a KF? It would be nice to get those 29" wheels in the wells after I streamline my jury struts :)

[HighWing]

I did fairings on the Jury Struts and the Elevator Struts. The ratio I used was 1:3.5 which is very close to the ratio on the extruded lift strut fairings. I prepared an album series of pictures with the technique I followed. As a side note, during the Lancair build there was talk of the drag created when a right angle is produced by large surfaces. Specifically the horizontal wing butting against the vertical fuselage. On high performance airplanes, these are always faired. I would suspect that the junction between the doors and lower wing surface on our design is extremely draggy. Fix that, and I suspect performance would improve about as much as adding the lift strut fairings. To see the way I did my strut fairings, go to my Albums "Techniques Page" http://www.teamkitfox.com/Forums/album.php?albumid=474

[Danzer1]

General rule is the chord of the fairing should be aporoximately 4.25 times the thickness of the tube you want streamlined.

I'm going expound on this statement. The only one I know of that put a "number" to it was Hoerner in one of his aerodynamics books. For struts that number was a c/t ratio of 3.7 or t/c of 27%. That is based on fairing thickness NOT tube diameter or member being faired thickness.

I thouroghly respect Mr. Hoerner and all he has done for aviation. But, some of his 1960's experiments and modelling have been vastly expanded upon by the likes of NASA (as a major example).

It has been found that more elongated shapes are more effective (better laninar flow) - the widest point more in the range of 40-50% from the leading edge (think more like a cigar). And streamlined symetrical airfoils with a length of up to t/c 9% are also more effective (diminishing returns beyond that). 9% probably not very practical and elongated streamlined shapes more difficult to build and heavier, so may not be worth the effort for likely minimal gain. So as a guide, I would say min. 27% to max. 9% t/c, keeping weight to a minimum.

I do agree with Lowell that the fuselage (door) to wing intersection could benefit a lot from streamlining that intersection.

[av8rps]

Danzer1,

Thank you for expounding on that. Excellent job I may add. Yes, there is a lot of unproven theory out there, and then there is just a bunch of guys experimenting who sometimes luck out, and other times just plain give up. I guess that's probably why many of us enjoy playing around with these experimentals.

Lowell,

I love your horizontal stabilizer strut fairings. Mine don't cover up the rod ends, nor do they fit flush against the fuselage skin like yours. But yet they work well. I really do think if we could clean up all the small areas (like all the rod ends as just one example) it would provide us with a nice drag reduction when they are all added up.

Oh, and btw, your latest Model 4 looks really nice in those pics, with an exceptional attention to final fit and finish. I liked it so much I just finished looking through all your photos in your photo albums.

Here's the million dollar question? Did you think your speed mods were worth the effort?

Oh, and I agree if we could fair out the wing to fuselage area I agree it could be a good thing to try. But for what its worth, many Lake amphib owners spent a lot of $$ to install a really slick set of leading edge wing root fairings, but unfortunately no one but the company selling the fairings saw any improvements. I like to look what the old air racers did to their airplanes for drag reduction ideas. I think they figured a lot of that out through trial and error.

Paul

[airlina]

Lowell, Great job on those fairings , I suspect more pictures of your plane would reveal some more of your nice workmanship, lets keep them coming! Bruce N199CL

[PapuaPilot]

I have been thinking about this area too. It's too bad there isn't a carbon fiber airfoil extrusion that could go over the wing lift struts and horizontal stab support.

Even better yet would be someone who could design some CAD cuffs for these fairings. If someone has the ability to draw these they could be made with a 3D printer out of nylon, ABS or even carbon fiber material. Is there anybody in the KF group that is good with CAD (I don't have that skill)?

[rosslr]

That's really interesting! I wonder if the rest of us who have our fairings in a fixed position could gain some more speed by angling them down. From the photo it looks so extreme, I wouldn't have believed it. Does both sides of the airplane do the same? Wonder if it is a spiraling propwash effect?

I would like to pick up on Jim's comment and Jan's original comment about the strut fairing. I too was surprised by that photo Jan. If the fairings have given you about +9mph and they naturally adopt that angle, then that would appear to be telling us something about the airflow in that region. I suspect that the follow-on discussion about the area between the doors and the wings may relate to this phenomenon? However, if Jan is getting +9mph with the fairings at that angle, are those of us who have aligned the fairings 'straight' actually not gaining the streamlining we assumed? I have only flown with them on so have no comparison. I know some here have added them after flying with the exposed struts so would be interesting to hear their results. If I remember rightly, most people who add them after report similar gains to those of Jan, with them fixed. If this is the case I struggle to understand what is going on from an aerodynamic viewpoint??

thoughts??

r

[Danzer1]

The optimum angle will vary somewhat depending on attitude (aircraft not yours), prop direction of rotation and what axis' you trim and how much. So it might vary a bit between flight phases and between aircraft.

Assuming the longest duration of your type of flight is straight and level - I'd put tuft streamers along the length of the struts, video tape them and see if the angles match your fairings. You'll probably never get them perfect anyway as I'd bet the angle changes between the top at the wing and the bottom at the fuselage anyway. Likely will be a compromise somewhere.

[jrevens]

I believe that builders have reported similar increases with the struts installed "Conventionally". Even if it did give you, say, an additional 1 or 2 mph, I'd opt for installing them so that they looked good... I think they'd look strange to me drooped like that. That's just me.

[Av8r3400]

+1 to that, John.

[av8rps]

Mine are not set down like that and yet my Kitfox is pretty fast, even on floats. I agree with doing some tuft testing to know where they really should be set.

Is it possible that the heavy weight of the plastic fairings pulled them down in the photo rather than being moved by the airstream?

[jiott]

Yes, I also agree that Greg's suggestion of tuft testing off the trailing edges of the fairings (both sides and fore and aft fairings) would answer a lot of questions. It would still leave the "why" unanswered. If its spiraling airstream from the prop then the right hand side of the plane should see the tufts deflected opposite. In Jan's photo they are deflected down, which indicates a LH turning engine maybe?

[HighWing]

Here's the million dollar question? Did you think your speed mods were worth the effort?

Paul

Paul,
Thinking about how to respond to the comments, the above quote sort of leads into my thoughts. I have an addiction, I must confess. I am a garage rat, tinkerer, tool guy, or whatever other moniker can be used for a guy who is always in the Hangar - Shop - or if not there, trying to think of one more thing to do. The pic is my first real project other than the push carts as a kid. Yes it works. It is .25 Cal. as that was the largest drill bit my dad had in his toolbox.

12129

So in that sense, yes they were worth the effort. But in a practical sense, I don't really know. Sadly, my current Model IV hasn't been flown very much since Phase I. My initial intent for the mods was to hopefully keep up with the buddies who mostly went to the ULS while I stuck to the 80 HP UL in the Kitfox that was to replace the one destroyed in 2007 with my emergency landing. During the time I was building N338S One of the guys got married and quit flying, another sold his IV, another went to a Glastar which sort of paced him out of the group. This resulting in the group sort of vanishing. The side by side flights were gone.

I have not played with prop pitch since first flight. My airspeed at 5500 rpm at about 2000 ft. is about 105 mph. I can climb from our 1300 ft. strip here at 1500 fpm. so I could play with the prop and likely get a few more mph without compromising too much in climb. I do have cabane gear with fairings on the gear legs and have wanted to fair in the cabane and spring struts. The plan has been to have them rotate as was suggested in a previous post.

Being the tinkerer, I have been thinking for a long time about wheel pants. I have the Nankang tires and have thought of something like what we see on the Ercoupe - a fairly tight fitting cone behind the wheel. I still need to use the tire surface as my step to get in and out of the airplane and this would work to keep the tire surface uncovered. With that would be hubcaps similar to what I had on the first IV,but an integral part of the Pants on this one. With the pants and a faired cabane, I might get a bit more speed.

I also took a look at the wing to fuselage junction and am going to play with that as well. When I decided to build in 1992 it just opened so many doors.

[jabkwab]

My fairings are extremely light, just insulation foam board cut with a hot wire and covered in model aircraft fabric. I don't think that weight is playing a big role in the position of the fairings.

At the inboard ends the front and rear fairing are attached to each other in order to create a nice finish.
Because of this, and because of the jury struts sticking out halfway the struts only the outboard half of the fairing is able to rotate to some degree in flight.

Both the left and right fairings have the same position in flight, and do not really change angle in relation to the aircraft during climb cruise and descent. After landing they assume their "logical" position again.

Anyway, I don't think the position of these fairings matters that much. I just think that on a draggy airframe large gains can be made with simple streamlining.

Jan

[av8rps]

Lowell,

Thanks for taking the time to explain all that. I admire your tinkering trait as I like to do the same, but I always seem engaged in other more domestic tasks, so my tinkering on my plane typically is limited.

I am really impressed with what you've done with yours, and even though you don't have specifics, just by seeing what you've done I'm sure it is gonna be a lot "cleaner" than a stock Kitfox. So keep tinkering :)

Paul

[jrevens]

Lowell,

Thanks for taking the time to explain all that. I admire your tinkering trait as I like to do the same, but I always seem engaged in other more domestic tasks, so my tinkering on my plane typically is limited.

I am really impressed with what you've done with yours, and even though you don't have specifics, just by seeing what you've done I'm sure it is gonna be a lot "cleaner" than a stock Kitfox. So keep tinkering :)

Paul

I agree with you, Paul. The more you see and learn about Lowell, the more impressive it is... he is truly an innovator and craftsman of the first order. I suspect that he (like others on this forum) has created and accomplished a lot in his life, aside from his beautiful Kitfox creations. The little flint lock Derringer (his first "real" project) was an early indicator of that, I'm sure.

[av8rps]

Yeah. It's just too bad he is so modest that we have to basically pry all his great ideas out of him. ;)

[inzersv]

Well, I am scheduled to begin my tailwheel training with Stick & Rudder the 18th of January. Sure hope the weather clears up a bunch. I will stop by the factory to see how they do the SLSA fairings. Thanks for all the input. You people are awesome. Purged my 912 oil today and found a small leak at the oil cooler. Ran out of time so hopefully tomorrow I will fix that and will be ready to do my first start. Getting butterfly's in my stomach..so close.:D:D:D

[DesertFox4]

That's great Stan. First start is BIG!! Wish I was there too. A while before I get to first start up.
Have fun with Stick & Rudder. Fun folks. Hope weather is on your side.

[jiott]

Here are some shots of the factory SLSA's several years ago.