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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 John Burns on
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I am looking forward to your detailed instructions before I make my purchase.
I am interested in the fairing for the horizontal stabilizer to fuselage. Do you have that one too?
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Permalink Reply by John Austin on
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I am interested in the fairing for the horizontal stabilizer to fuselage. Do you have that one too?
I had previously made this fairing from aluminum and think it is much better!
John
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Permalink Reply by Scott Ahrens on
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Beautiful work. How did you fabricate the curved area (hump?) in the fairing?
I am not sure I want to slow down my building progress. I am finishing attaching the gear / strut weldment to the fuselage. Now would be a good time to initially fit the fairing.
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Thanks, Scott! I simply templated the fairing with a piece of thin poster board and transferred it to some ?0.020? aluminum. The curves of the "hump" were formed by rolling the aluminum over an appropriate diameter tube to make the correct curves - just takes patience and a lot of trial fitting! Tip: to get the fairing edges to lay down flush against the underlying skin, use an edge roller to slightly turn-down the edges. This eliminates a lot of "waviness" in the skin edges and stiffens the edges considerably. The black vinyl edging is used where there is a possibility of fretting. I elected to rivet this on and left an inspection hole (which I now have a matching green plug to close it off with) where I can insert a borescope to check the torque paint on the bolts attaching the HT to the fuselage.
John
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If no one else has installed this (successfully!) yet, I've thought of a new approach to fitting it so the location where the struts penetrate the fairing is a close-fit and also in proper relationship to the landing gear leg penetration:
My new idea is to initially cut the fairing horizontally across the "chord" of the airfoil-shaped area where the struts go through and cut-out the opening for the gear leg:
The leg opening would then need to be cut on the underside to allow the resultant flaps to be deflected so the lower-half piece can be slipped onto the gear leg. Then, slide the lower-half piece up the gear leg till it hits the underside of the struts and start cutting-in the fairing around the underside of the struts until the lower-half piece fits against the fuselage.
Next, bring the upper-half of the fairing down from above the struts, cutting-in as you go until it conforms to the top side of the struts and matches-up with the lower half.
Once all this is done, one could then secure the fairing in several places to the fuselage and glue tabs to the fore and aft portions of the fairing to allow screws or rivnuts to hold the two halves together. This would also facilitate inspections as you would only have to remove the upper fairing fuselage screws and the screws joining the two halves to remove the upper half and inspect the strut attach bolts.
I like to "think out loud" and post this so if anyone sees a flaw or better approach they can point this out and maybe save me a co$tly mi$take! LOL!
John
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Joining in on thinking out loud...Does the portion under the gear leg need to be there at all? For that matter what if it was cut horizontally between the strut and gear leg. The top portion could be slid up the strut for inspection and maintenance but the down side is disconnecting the strut to get it on. I don't how much drag reduction the fairing gives you by covering that little bit of gear leg. The rest of the gear leg is still hanging in the breeze. Although it might look better with the gear leg portion still attach...don't really know about that part.
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Thanks for your ideas, but the big problem is there's not one strut, there's two of them! And, they angle away from each other. So, even if you disconnected the struts to install the fairing, you couldn't slide it away from the fuselage for inspection (or even the installation) because the struts are getting farther away from each other due to the angling-out. You'd still have to have a relief cut for the struts to allow the fairing to open up. Also, I really think you need that horizontal cut I indicated if you want to scribe-in/cut-in the struts for a really accurate, close fit.
John
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Sorry I spaced out on the two strut issue. On a different airplane I had issues with the wind cracking a similar fairing that was split in the same fashion. I ended up having to fiberglass a lip on one side that slid over the edge of the other side to direct the wind over the split area.
In this case maybe a lip on the upper half that would fit over the lower half. Help hold the lower half in place?
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Usually what you do on these fairings is glue a tab to the underside of one edge that slides beneath the underside of the mating edge and either use screws or put rivnuts in the tab to receive machine screws to hold them together - ideally, nothing will be visible except the thin cut line and the screws.
John
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BTW, Scott, I do agree there's probably not a whole lot of benefit to the portion of the fairing that runs under the gear leg since the leg is flat and parallel to the airflow in that area. I'd probably initially make a cut perpendicular to the leg opening cut-out about 3/4"s of the way aft and slip the lower half of the fairing over the lower portion of the leg and slide it up. If it turns out to be too complex trying to cut-in everything, I'd probably just cut-out completely the underside of the leg opening, but I'd try to retain where the fairing wraps around the edges fore and aft of the leg (to keep some stiffness in the fairing and also would look better).
... I just wish someone has "been there, done that!" LOL!
John
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I took a deep breath today and made the first cut on the fairing, cutting the gear leg opening:
The resultant oblong hole looks like it will easily fit around the gear leg (I'll have to make a cut on the bottom-side, perpendicular to the opening, and about 3/4 the distance aft so the resultant flap can be bent back and the fairing put around the gear leg). It won't be precision-fit, but that's probably desirable as I anticipate there'll be some flex of the gear leg and it'll need some room to move - I could line the opening just inside with some foam weatherstrip if I want the opening sealed, but compliant.
I then started looking at the area where the struts will pass through. I decided the horizontal cut will be the way to go where the opening is actually blocked-off as this will allow me to scribe around the struts and cut-in for a very close fit. However, it doesn't look like a good idea to just continue the cut perfectly straight and horizontal all the way through the fairing as this will result in unnecessarily long cuts (with more potential problems for realignment of the resultant gap) and also cut through some fairly complex compound curves, making it harder to attach tabs to hold screws or rivnuts to secure the mating surfaces.
To me, it seems better to angle the cuts so they are much shorter and where the fairing's curved surfaces are less complex, allowing tabs to be adhered easier on the inside. Here's some proposed cut lines:
This will allow the resultant two pieces to maintain a good bit of their shape and rigidity and still allow the blocked-off area on the top piece to be scribed and cut-in, similar to the bottom piece.
Any better ideas out there???
John
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Update: Well, no one popped up with a better idea, so I made the proposed cuts and began fitting the lower portion of the fairing to the left strut/gear/fuselage fitting. I made the cut on the bottom to allow it to snap around the gear leg and then advanced it onto the fitting - scribing with a Sharpie and then cutting-in either with scissors or a sanding drum. Here's some pics so you can see where I'm going with this:
One thing has become obvious to me - when I cut in the top portion, it will be absolutely necessary to orient the top piece with the bottom piece so they close together correctly! The plastic is so flexible I could easily see the bottom piece flexed out of shape so they don't mate accurately. Also, when attaching to the fuselage, it'll be again necessary to have the two pieces joined so everything lines up!
John
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