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Old Dec 02, 2014, 07:31 AM
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Build Log
Kit Bash an STOL

So here's the thought process:


After a serious bout of house maintenance, I decided that some profound rehab was in order. So, here I was looking around the shop, taking in the various kits, partial builts, and sundry pieces/parts. I actually have a couple of pattern ships in the cue, but am waiting on materials to show up for those, and as a result I decided to get as much stuff off the shelves as possible and convert a little of this and a little of that into something resembling an airplane.

As it works out, my son asked me to build up a plane or two for him, and, while I'm at it, maybe a couple for his wife. Bit extreme, but I guess I could do one, well, maybe two. I spotted a partial kit of a Goldberg Eagle 2. I don't mind purchasing partial kits. They typically represent a better value than a complete kit in that the collectors and their deep pockets tend to stay away. For not much more than the cost of the plans I end up with a bunch of precut wood for less than I'd need to spend buying just the balsa. So, there I go. I have the plans, instructions, fuse and tail feathers sitting in the box all nicely punched out. No wing though.

Over in the corner I spotted a wing and stab pair quietly laying around. Old wing, span being 56”, silk and dope covered. Stab had elevators glued on with pinked canvas hinges. I remember at the time of purchase that shipping seemed a tad low. I guess I should have asked further, because when the package arrived it was considerably smaller than what I imagined 56” to be. The guy cut the wing in half, splintering wood all over the place in the process. Like I said, the wing was old, and if I ignored the mayhem, rather well made. No ailerons, and judging by the center bevel, now exposed, it originally had a fair amount of dihedral. Let's use this and get it off the shelf.

The Eagle wing has a span of 63”, a chord of 11,12” or so. Like most all trainers, original had a flat bottomed airfoil so assembly would be easy for the novice. Hmmm, my wing is going to be rather sloppy at a chord of 10”. With the smaller area of the donor wing the stall speed is going to be greater, just what I'm not wanting for my son. I do want an aileron ship, not rudder, so I'll need to cut into the wing some. Also, I'll need to slow the plane down, so I'll throw in some flaps as well.

The decision to install flaps laid open the entire project. The wing has both a main and a rear spar. The rear spar is located such that a split flap can be hinged from it. By extending the flap beyond the existing trailing edge, and reforming the top slope of the airfoil to match up with the top slope of the flap, I can increase the effective area of the wing without having to create new ribs. Ailerons can be started from the newly defined trailing edge, and taper in toward rear of wing tip, ala Cessna, again adjusting slopes somewhat. Even more area! OK, one last thing, lets go for STOL, we'll add in some leading edge slats.

So why not just start the wing from scratch? Well, I want the wing gone, and, I don't want to lose all that very light, contest grade balsa. The airfoil is semisymmetric, with high thickness, lots of wood involved. Apart from what I need to sacrifice for repairing the spars and center sheeting most all the wing is salvageable.

Having the wing concept down, I turned my attention to the fuselage. What truly bugs me about this class of airplane is the blatant disregard to scale. I find it offensive to my eyes. If you measure out the span and fuselage length, the plane comes in at just about 1/6 scale. Now, try to sit in it. Ha! The fuselage height and width is closer to 1/12 scale. No way. I will resize both those dimensions so that a real scale person can fit in there. Just to show them (whoever they are) I decided to widen the fuselage to a scale 42” Very comfy. No tandem seating for this guy, no sir. Likewise, I'll need to add height to the cabin.

Having given the fuselage a major workover in the cabin area I then focused on the impact of STOL. Big thing is ground clearance when the plane is rotated to stall. Conventional gear can be used for STOL. I refer you to a number of planes coming out of WWII, the Storch may be the best known of the bunch. Looking at those planes, though, it becomes rather apparent that the main gear is quite long, needed to allow the high alpha all the way to the ground. Have fun getting in the cockpit. Trike gear can be used if the tail of the airplane is lifted high enough to let the plane rotate around the mains and not cause a ground strike at the tail. Using trike gear allows a shorter take off run, as the plane doesn't need to lift the tail first before rotating. You pull the yoke to your belly, hold it there, and apply a ton of power. Simple and quick. Cutting the fuselage sides just behind the main gear and reattaching for appropriate clearance is my choice.

Then there's the stab. To minimize the downwash during high alpha the stab should be positioned high on the vertical fin, away from the most offensive part of the airstream. Mechanically, what helps out here is the thickness of the stab. A large degree of support can be achieved because of this thickness. I abhor a wobbly set of tailfeathers.

The plane will be electric powered, in that I have no choice. The kid speaks, I listen. Thing is, the Eagle has an open engine bay and I just aggravated the condition by making the fuselage taller and wider. Yuck. I'll cowl the thing.

Ah, skin and paint, or whatever. This plane will be the grand experiment. I will be full time rv'ing in about a year, and my travel companion, says no wet fuel (“ok......”) and no stinking paint (“what, I use color crayons?”). I love my wife, I really do. She does have a knack of stretching the envelope.
So being electric minded, she has inadvertently heralded in a whole new way to approach covering. We'll mess around with this a bit. I just won't tell her son!

There it is. My plan and we'll see how well I stick to it.

Barry
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Old Dec 04, 2014, 01:30 PM
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What does the stripped wing reveal?

I removed the silk covering from each of the panels and received some interesting results. Seems that at some point the nearly 5' wing tried to go down a 4' path. A patch was found in the leading edge sheeting on both the port and starboard wing panels. Never saw evidence of those repairs before the silk was removed. Having discovered the patches and finding extensive cracks in the le sheeting I removed all for sheets. The first 2 pics show the results. With the sheeting removed, I can now remove the patch supporting structure as well. A gram, a gram there....Photo 3 and 5 give a closeup of the original repair. Photo 4 is an interesting thing. If you work your way through the shadow, you should see a huge chunk of material removed from the main spar. Pic 2 also shows how close to the inboard section it was. Not a clue! Fortunately it was hidden under the rigid, supporting, center section sheeting.

I do not have any photos of the saw cut and the ensuing damage. Sorry about that.

Barry
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Old Dec 04, 2014, 03:06 PM
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Looking at picture 4 that bay and the one next to it look a lot like a mouse has been there for lunch! Don't ask how I know.

L.
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Old Dec 04, 2014, 03:35 PM
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Ya, you think? Thing is absolutely no indication that M. Mouse was there. Must have been a "fast food" pit stop. Bee Dub's maybe? (Buffalo Wild Wings).

Got my replacement wood today. Let's see how lucky I was.
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Old Dec 04, 2014, 05:14 PM
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Maybe he just needed a little fiber.

L.
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Old Dec 04, 2014, 06:17 PM
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The fuselage

I spliced the 2 spars on the wing and while waiting for the guessets to dry so I can remove the clamps, thought I'd get started on the fuselage.

The first photo is one I downloaded from the internet. Google Goldberg Eagle 2 and you should find it in photos. Nicely done, I thought. The strap mains looks much nicer than the wire that is stock. The transparent fin is a nice touch.

Pics 2 and 4 show the cut just behind the mains and Pic 4 does a decent job of displaying the additional angle that I put on the tail. Compare that with picture #1. You can also see the rise I gave to the cabin height. Lines drawn on the side are for removal of material to help lighten things up a bit. I will be coloring in the glass, not using transparent clear.

Pics 3 and 5 show where I'm going with the cabin width. Bracing is 1/4" stock. Formers are from lite ply.
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Old Dec 05, 2014, 08:03 AM
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Found 2 pennies being used as balance in the wing. The dates on the pennies were 1957, both of them. Shiny.

Barry
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Old Dec 05, 2014, 11:40 AM
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Full fuse side view

This shot shows very clearly the angle that the tail section has been given. The cabin is sitting just about as it will be on its gear.
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Old Dec 07, 2014, 05:54 AM
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Shot of spar splices

These pics show splices on main and rear spars. If the splices were to repair a broken spar say toward the center of the wing, gussets would typically end twice the spar thickness away from the outer edge of the splice. In this case the gussets continue to the end of the bay at which the center sheeting terminates. I'm doing this to pick up loads due to wing flex wing struts.
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Old Dec 07, 2014, 06:29 AM
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New ribs in center of wing

Couple of shots showing the ribs, spar gussets, mouse munch. The pencil lines beneath the wing represents the width of the cabin. The clamps are holding an aluminum L extrusion in place to make sure the leading edge is straight. A couple of half ribs need to go in yet to support the le sheeting. The 2 ribs at the cabin walls will need to be positioned to facilitate structuring of fastening.

Couldn't help myself. Had to take another shot on the mouse munch!
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Old Dec 07, 2014, 07:20 AM
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I would just plate those two spots on the spar. 3/32" Bulsa both sides. Might be lucky and that be the side with the two coins.

L.
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Old Dec 07, 2014, 08:04 AM
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Guess I better fix that munch spot pretty soon, hm? I took the pennies out. I kept them, though. Might need to glue them back in again, who knows. You what, the pennies were on the side that has the mouse munch. Now that's interesting!
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Old Dec 07, 2014, 08:23 AM
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I'm now in the process of arguing with myself on flaps. Down to 2 finalists. Easiest way out is the split flap. I have yet to do the math, but it appears that the most efficient size for split flap will not require any additional chord, in fact it looks like I loose effectivness if I extend the chord. I sure do like simple.
Other side of coin is the Fowler flap. Quasi, actually. Sub arguement is a 3 position flap : closed, partial, and open. I can get some good dCl from a Fowler, but hinges and activation is perhaps more than I care to do for this plane. Thus the 3 position possibility. Not sure that the minimal reduction in airspeed during landing is any big deal to me, though the dCl may be. I really need to consider the dCd as well. The drag helps with approach angle so whichever configuration has the higher drag will certainly gain points.

Guess its time for a little calculating, and maybe I'll start cleaning up the rough edges on the wing and fuse so that it photographs better.
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Old Dec 08, 2014, 02:36 PM
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Split flaps it is

After considering my options, I have chosen to go with a split flap arrangement, If I extend the trailing edge by 1/2" (giving me new chord of 10.5"), I will gain about 30 sq in of surface area, but I will also reduce my AR to a bit closer to 6. New chord will give me a 4" flap chord, with the hinge line of the rear of the rear spar. Nice place to be. The thickness of the airfoil is just about 1.7".

Given this data then, lambda1(Cf/C) is approx .75 and lambda2(beta) is approx 2.0 at 80deg.

If I use a full span flap this will give me a delta Cl at nominal 10deg of an additional Cl=1.5.

So, do I use a standard configuration for aileron, or do I go to a spoileron?> Hmmmm.
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Old Dec 08, 2014, 05:58 PM
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What radio are you going to use?

L.
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