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750 STOL Fuselage/Gear/Struts Fairing
For a while now, fairing kits for the 750 STOL have been available from Zenith, Mark Pensenstadler (kitplaneenthusiast.com) , and North American Aerospace Holdings (the manufacturer). I initially purchased from Zenith the upper fairings for the struts and jury struts - they all fit quite nicely except the forward strut fairing needed a revision and Mark Seaver at NAA quickly came up with a new version (similar to the Cruzer's) which fit great. I should point out that although a couple of knots extra cruise speed is nice, I had no expectations of turning the 750 STOL into a speed demon, but the fairings do give the plane a more "finished" look.
Obviously, the junction of the fuselage, struts, and landing gear is the "draggiest" area on the plane (talk about "flat plate drag" - this junction literally has a flat plate!) that could definitely benefit from a fairing:
So, I purchased from NAA the fairings for this area:
If you think about it, this fairing requires an extremely complex fitting installation - you've got two struts entering the fairing at angles to each other, and, where the strut openings in the fairing need to be located is dependent on the distance the opening is from the fuselage! On top of this, the struts' opening will need to be in the correct location fore-and-aft in relation to the gear leg opening.
I'm thinking one work-around as far as the strut openings will be to cut out the flat area where the struts pass through completely off, cut a flat piece of ABS to accommodate the struts, and once the fairing is located on the gear leg, glue the strut flat plate to the fairing. Clear as mud, correct? LOL!
Another question I have is how much does the gear leg flex? Can you just run the gear leg through the opening (once you've cut the end off), or, does the entire underside of the gear leg portion of the fairing need to be cut away to accommodate downward flexion of the gear leg?
Finally, where would be best to make a cut to allow the fairing to be flexed open to install around the struts and gear leg? Cut it in half vertically (maybe between the two strut openings?), cut a slit horizontally at the aft end of the strut opening (and a slit between the two strut openings) and either at the aft end of the gear leg opening or the underside of the gear leg opening?
As you can see, this is really a quite complex 3-D problem, to put it mildly! Has anyone successfully installed one of these fairings on a 750 STOL??? (Cruzers don't count as they only have one strut which greatly simplifies the problem!) Pictures of a successful installation would be worth a 1,000 words!
My gut feeling is that it might take a few mangled attempt$ before one gets this right! :(
John
N750A
Replies to This Discussion
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Permalink Reply by Scott Ahrens on
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John, I like where you are going with this project. I was thinking about splitting it vertically between the struts. But your plan looks like it will be easier to match the two halves. I’m very interested in your next steps.
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Permalink Reply by John Austin on
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Thanks for the reassurance! It'll be 2-3 wks before I get any more done as I will be traveling. Stay tuned!
John
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P.S.: It has just occurred to me that there might be an advantage to having made the cuts where I did on the upper portion of the fairing: if substantial tabs are attached to the small top piece forwards, aft, and perhaps between the struts from the upper piece to the lower piece, I "might" be able to get away with not having to secure the upper fairing piece to the fuselage. That would eliminate at least 2-3 securing screws that would have to be removed for inspection!
John
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Update: Finally had time this a.m. to get back to the fairing project. I started scribing and cutting-in the top piece and as I got closer to it fitting, I also did additional scribing and cutting-in of the bottom piece. One frustration is that since the bottom piece is so complex, as you cut-in and move closer in to final position, you'll encounter interference somewhere else, such as the gear bolts or the leading edge of the landing gear leg. I'll soon need to adhere some tabs for connecting the upper piece to the lower piece as this alignment must be permanent before attaching the fairing to the fuselage. If you did it the other way round, the fairing could be distorted when installed and the top piece wouldn't fit properly. I do think I won't have to attach the top piece to the fuselage and that'll make inspection of the strut bolts simple since all I'll have to do is unscrew the top piece from the bottom piece and leave the bottom piece attached to the fuselage.
Here's some pics of where I'm at - close, but not yet achieving a tension-free final fit:
Has anyone else installed these fairings on a 750 STOL and cut them differently?
John
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Thank you for leading the way on this one. I do not have any better ideas.
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Getting a little closer - still need to work on those gaps, but after paint and installation I'll probably tape any remaining gap with white vinyl tape - that's what we used to do on sailplanes. In this pic, the fairing is just sitting on the structure and no fasteners whatsoever, which is meeting my goal of not having any tension or distortion on the fairings.
John
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A little more progress today - the installation/repair kit comes with some pieces of much thicker ABS. I epoxied small pieces of these to the back side of the fairing to accept rivnuts. I then pilot-drilled holes and then enlarged them to accept the #8 aluminum rivnuts included in the installation/repair kit. These aluminum rivnuts are set very easily and you want to set them with your rivnut tool just-snug and no more (DEFINITELY practice on some scrap first!). Any excessive pressure will distort the ABS and/or possibly split it. Fortunately, none that I set spun on me, but I used very light torque on the screwdriver when screwing-in the #8 screws. If the screws loosen too easily, I'll likely use some pink "weak" Loctite as is used on instrument panel screws, etc, which breaks loose fairly easily. I also plan to put a fillet of epoxy around the base of each rivnut to further ensure it doesn't spin in the ABS.
In the picture above, the odd-shaped cutouts above and below the forward strut hole are for clearance on a strut bolt.
Next will be to re-mount the fairing and do final trimming and locate holes for mounting the lower part of the fairing to the fuselage. It looks like my plan is going to work to leave the lower part of the fairing attached at all times and easily remove the top portion at annual for inspection of the strut bolts.
John
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BTW, if anyone is following this and about to cut the fairing, it has occurred to me there is a better way to make the cuts for the top portion of the fairing. I am going to do this on the other (right) fairing, but it'll probably be some time before I get to it.
Instead of cutting the horizontal cut completely across the area where the struts intersect, stop the cut at about where the leading edge of the forward strut and the trailing edge of the rear strut would intersect the cut. Then make a single straight cut at each end of the horizontal cut that angles up towards the edge of the upper portion of the fairing. This avoids cutting through the very leading and trailing edges of the fairing, which likely don't mate the upper and lower portions of the fairing as cleanly as the above described cut would.
Probably difficult to follow with words only, but I'll try to get the cuts made on the other fairing soon and get some pics posted.
John
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I am following along, I am just not that far along to start working on fairings. I like your approach to the right side. Thanks for posting all of your information as you work on this.
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I'm almost through fitting the right side fairing. Here's a pic that shows how I moved the cut on the upper portion away from the trailing edge, allowing one straight cut. The fairing is not yet attached to the fuselage. BTW, the rivnuts attaching the upper portion of the fairing to the lower portion had to be ground down slightly - they were contacting the fuselage and not letting the fairing seat fully against the fuselage. I'm using 3/8" #8 machine screws in these locations for the same reason.
John
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That looks great John. I like this approach more and I plan on copying it for my SD.
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Finally got both fairings installed after final fitting and painting. Before painting, I epoxied tabs to the upper portion of the fairings so as to fit under the lower portion of the fairings fore-and-aft of the struts to help align the fit in those areas:
The mounted fairings:
(Both fairings are the same color - GM white - but the right fairing was beginning to get in the shadows and looks yellowish!)
This is what it looks on the underside:
I thought the gaps were too wide on the underside of the struts, so I used silicone weatherstrip adhered to the underside of the struts to close the gaps:
For now, the fairings are attached to the fuselage by just two #4 stainless sheet metal screws for each fairing and seem quite secure. I went this route because this only requires small holes to be drilled and didn't want large holes or rivnuts until I had everything finalized. I'll stay with them for now and probably replace with rivnuts later.
John
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