N254MR

My good friend David Brick posed a question in a comment about an illusion of discontinuity in the geometric plane presented by the bottoms of the flaperon, fairing box, and main wing section. In one photo, it looked like perhaps the front of the fairing box was hanging down below that horizontal line. In response to his comment, I posted this next photo, showing that the bottom lines of flaperons, fairing box, and wing are all pretty much in the same horizontal plane.

However, if you look closely, this photo raises another issue. When the trailing edge of the flaperon is in this "neutral" position, it is even with the trailing edge of the fairing box (and the wing tip when installed.) But look at the forward section of the flaperon in the photo above. It actually curves up a bit, leaving a "cavity" in the transition from flaperon to wing. Why is this? Looks like it would create some turbulence and drag there -- and it does, particularly at cruise speeds, because drag increases by the square of speed increase.

The answer is that this rounded shape to the front of the flaperon is intentional in this wing, which is designed to have its least amount of drag not in the "neutral" position, but in a "reflex" position, where the trailing edge of the flaperon is actually about 5-8 deg. up from neutral and the plane is flying in level cruise with the wing flying at a relatively low angle of attack. The next photo shows the bottom of the wing with the flaperon in an approx. 5 deg. up, or "reflex" position.

Notice that in the photo above, the forward section of the flaperon curves smoothly into the bottom of the wing, leaving no "cavity" at the front of the flaperon as it does in the "neutral" position. The next photo gives a slightly different view of the this (approx.) 5 deg. reflex position.

The next shot gives yet another view of the wing shape when the flaperon is in the same 5 deg. up "reflex" position.

The benefits of the reflex shape with this particular wing design are substantial. For example, when the plane is flying with flaperons in "neutral" position, trimmed for straight and level flight (low angle of attack) at max. continuous cruise power, and the flaperon is suddenly raised to a 5-8 deg. up "reflex" position, the change in speed is dramatic and felt as a distinct and immediate acceleration. How much speed does it add? In the Murphy Rebel, somewhere between 5-8 knots! (Other airplanes also use a reflex position in cruise -- the 4-place RV-10 being just one example.)

Because the flaperons act on the wing shape along the entire length of the wing, the effects are enhanced -- even though the fairing box and the wing tips are still in neutral position! You'd think that those two discontinuities, causing drag from turbulence, would cancel out any advantage of the reflex positioning, but that's not the case. A thoughtful reader may wonder, "If reflex is so beneficial to this shape of wing, why not just mount the fairing boxes and the wing tips up in the reflex position to start with, so they would assist the increased efficiency of the reflexed flaperons at cruise speed?" A good question, because indeed, if the boxes and tips (or at least the latter) were also aligned upward in reflex position, cruise speed would be a bit faster.

Fortunately, however, the question has a good answer, which is that all wing designs are compromises, based on the intended primary mission of the aircraft. Because the primary mission of the Rebel is to be a sturdy, short takeoff and landing (STOL) airplane, the wing is optimized not for top speed (least drag) but for high lift at takeoff and relatively slow climb speeds. With the fairing boxes and wing tips built fixed in the "neutral" position, the wing is able to produce maximum lift, at takeoff and slower speeds, throughout a wider range of flaperon positions, giving the airplane its greatest overall asset for its intended mission, namely, the ability to takeoff and land in short spaces, at relatively slow speeds, on rough strips (or floats), while carrying substantial loads.

There are several design tradeoffs inherent in design decisons to maximize lift or minimize drag at various speeds and other performances requirements of the aircraft. In short, cruise speed is a nice thing when you can get it, but not the only thing.

Not a lot of time to work today, but I did get deburring, sanding, priming and riveting for the right wing trailing edge fairing box all done, and it's in. Yea! The first photo shows the box all riveted together.

The second photo shows the fully primed inside. You can see the addition of a doubler strip to the inside edge of the vertical part -- to the left in the picture -- where there is no rib flange to support that edge.

The last photo shows the completed box installed on the fuselage. Installation included changing 4-5 rivets to countersunk rivets to better accomodate the rest of the wing fairing strips to come.

It's been a very long day, with a little over 10 hours spent working on the right wing trailing edge fairing box (Rev.2) and related pieces -- but at least it has been completely fitted now, including the inside ST-31 channel, ready for deburring, sanding, priming, and riveting... but not tonight! It's a complex region of the plane, with lots going on, inside and out. Still, it's incredible how many operations are required to get this three dimensional object properly built and mounted in three dimensions... most of them involving small spaces and clearances. The fairing has gone on and off at least a dozen times to get to this stage. =:O Top rear view and bottom up views are below. Hopefully, Rev. 2 of the left fairing box will be easier.

Celebration concerning the fairing box was premature. Due to an unfortunate measuring error, the first one was too wide. I went ahead and made the right side box last night, but in attempting to fit it to the fuselage today, it turns out the the width dimension given in the manual for the attachment flange that lies over the roof skin isn't even really wide enough. So much for the manual. In addition, it turns out that at least on the right side, if there's going to be a constant gap between box and aileron, the width of the box will not be a constant. In any case, I'll be making at least two more of these before they're done. Sigh.

After about 6 hours of work, I got the left fairing box nearly complete. It mounts on the fuselage, filling the space between the fuselage and left flaperon. This component is one of the more difficult metal forming tasks I've done so far -- in part because I have only a small, rudimentary bending brake. The fairing box is made from a pre-formed triangular rib, covered by a skin fabricated from bare sheet metal that has a number of complex bends, some of which had to be done quite literally by hand. In short, there were a dozen places to easily screw this up, but miraculously it turned out well, the first time! There's a separate page of detailed, annotated photos showing how this was done, for those who may be interested.

Here are a couple photos showing the end result of today's fabrication work. The first photo shows the outboard face of the triangular rib that goes next to the flaperon, and the single skin that starts at the lower left corner of the photo and wraps all the way around the triangular rib, finally tucking back under the first part of the skin. The second photo is a rear view of the side that goes onto the fuselage. The 3/4" flange will attach to the fuselage. Of course, it still needs priming of mating surfaces & riveting, and mounting onto the fuselage, but this is a big "whew!" moment. Hopefully, the second one will go as well. It's cross fingers time.

A 2-door!  Well, getting there.

Over three years ago when I first got this project, building the doors was one task that could have been my first to do. Good thing I put it off, because it's best to have some familiarity with the tools and techniques to tackle this job.  :-)  It finally got started today. The top half of the door will be window, with a horizontal hinge in the middle so the window section folds out and down when opened. The first step was measuring and bissecting the vertical dimension of the door jamb, then transferring the vertical center onto the frames, cutting and trimming them.

The first photo shows the two halves of the left door (outside up) clamped to angles to keep the vertical sides of the halves straight, and measurement for the upper and lower center cross frames. It was important to make sure that the layout surface was perfectly flat, using the straight edge on the right. The bricks may have helped a bit, but just a few slight movements on the hangar's concrete floor was what quickly established the requisite über flatitude.

The second photo is a closeup of the horizontal cross beam gusset connections. The large blue clamp in the foreground has only a very light compression. Although the precise vertical separation between the halves will be established with placement of the hinge, I thought I'd put at least a piece of heavy paper between these beams for this stage of construction. Probably pointless but... it seemed like a good idea at the time.  :-)

The third photo (inside surface up) shows all the gussets drilled and clecoed, establishing the complete frame of the door. All that's left are hinges, skins, window, latch & lock, gap seals, air vents and.... In short, lots, but it's a good start, fun to do, and feels like progress.

The last photo shows the frame sitting in the left door jamb. On the inside, the gussets are riveted. There's no middle hinge yet, so the clamps are holding the top and bottom halves together, but the fit is great.

At last, the new heavy duty .040 eyebrow skylight doublers are done. A very slight roll was added to each doubler before installation. The outside circle, upper roof contact edge of the doublers were sanded fairly round to minimize any tendency there might be to bend or chafe the roof skin at the outer edges of the much stiffer doubler. Compared to the .040 doublers, the .020 roof skin is pretty flimsy, so I wanted to take extra caution getting it firmly and evenly riveted down. Consequently, as shown in the first photo, in addition to liberal use of clecos, every hole was double clamped before riveting. The second photo shows the completed doublers, with virtually no bulging distortion of the lower edges now, as there had been before with the previous lightweight .020 doublers. About 11 hours of work in this redo job. Ouch. At least it's done and done right. Onward.

Although the original doublers were used as templates, it still took about 5 hours today to make the two new .040 eyebrow skylight doublers, mostly lots of hand work with the nibblers (pneumatic and manual) and mini-belt sander to cut the center circle. Here's the right side doubler, with countersunk holes and exterior circle all sanded, ready to be clecoed in place for final sanding of the interior circle to match the roof skin hole. I'd hoped to get this done today, but 'tis not to be. Patience is essential in this biz.