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Click on the image to get the page size GIF (112 kb). You may want to print a copy of the small plan for reference while reading the construction notes. |
Those of you who have read my Early Experiences in Soaring page, know that the Legionair sailplane holds a special place in my memories, and I always thought that it was ahead of it's time for performance. The flapped versions that were built may have launched a little higher, but otherwise the standard configuration was quite acceptable. When I started looking for a RES subject for potential contest use, there really was only one other possible choice. I built and flew several Aquilas, and they can be built very light, but the airfoil is too slow. I don't see too much being written about the old Legionairs either, so this is a chance to convince the rest of the world that it is a great glider.
The addition of composite construction will easily beef the plane up to the winch launches of today, but what really intrigued me is the thin airfoil with Phillips entry. How would this perform at higher wing loadings against today's planes? I might as well make the plane Nostalgia legal, just make it the best engineering for strength to weight. When this plane breaks it will explode into a shower of confetti, since everything will fail at once. I have been thinking about the best methods for fabricating this thing, trying only of staying within the rules and with no concern for time and effort. So here we go...
OK, John Hipwell was nice enough to lend me a set of plans and instructions for the Legionair 132, and the manual includes a letter size drawing of the Legionair 140 so I can get the right dimensions. A full size Xerox has been made so the plans will remain available for other users. If someone has obtained the rights to this model and wishes the small plan to be removed from this web page, please E-mail me and it will be removed pronto.
It looks like the Legionair 140 simply has another bay added to the tip panels to add another 8" of span, the inner panels are identical. Let's consider what the rules will allow for the plane to be nostalgia legal, and increase the strength for a modern launch.
I calculate that the old wing design is good for about 43 ft-lbs. of bending moment at the end of the joiner rod. Using the Oliver Wilson method of estimating bending moment, this results in a winch line tension of only 50 lbs. when the wing will snap at the weak point. Using a winch line tension of 100 lbs. means that the wing will have to be good for 110 ft-lbs. of bending moment. Let's use a 5/8" joiner rod that is 7" long per side. The bending moment of the wing needs to be 77 ft-lbs. at the end of the joiner rod. Now if we use Carbon Fiber (CF) for the strength, the balsa will carry none of the load due to the low modulus, so the spar will have to take it all. A CF spar that has a top cap of 0.034" by 0.5" will take this load, and can taper to nothing at the poly joint. So the inner panels will have a 33" long spar of CF instead of the aluminum tube, and we will retain the open bay structure as required by the rules.
Now I look at the torsional strength to prevent wing twisting and flutter. The front D tube of the structure will be perfect to resist torsion, if we put a layer of bias ply Fiberglas under the wing skins. To make these skins, I will cut foam blanks to vac bag the 1/16" balsa to the Fiberglas, then the skins will be pre-shaped for attachment to the rib assembly. Fiberglas cloth will help prevent buckling failure of the balsa skins which was a big problem on the originals. The same core beds can be used as jigs to build stress free rib structures. To prevent trailing edge flex, the aft rib sections will have to be made stronger in fore/aft bending. I've calculated that a 0.014 CF cap strip with laminated grain ribs will take the load, similar to free flight rib construction (see Stamnov method in the Aug/97 issue of Model Aviation). We will need 28 of the ribs for the main panels so they will be made in a big block and stripped with a bandsaw. (NOTE: After a halfhearted attempt at this, I decided to change the method of building the inner panels as described below). This thing is going to be a pain to construct and still stay within the rules. I may make modifications to this construction method to build lightweight outer wing panels for the composite planes.
I don't need to have four piece wings, so the wing will be built in two halves, with the poly joint being much stronger as a result.
Let's examine the tip panels and do the calcs. The bending moment at the poly break will be 24 ft lbs. Conventional built up construction with a spruce spar cap will be sufficient, and a 3/8" diameter CF transition rod imbedded across the joint will transfer the load. Twisting forces on the tips will be high, so I will add CF strip bracing across the open bays for the first seven ribs, and use the underlay of bias Fiberglas for the sheeting.
This structural modification appears to push the limits of the nostalgia rules, but will retain all of the necessary elements to look the same. Is this in the spirit of the rules? The CD will have to break open the wing to discover the mods, will all contestant's planes be X-ray'd for compliance? If disqualified, I can always use the plane in RES contests.
The tail surfaces of the original are too weak, so 0.007 CF facing will be added to all of the ribs and frame elements. Otherwise, the pieces must remain the same as the original.
A fuselage pod blank will be made from foam, and then hollowed out. Since the aluminum tube was heavy and over built, I will build a rolled CF/kevlar tailboom of the right length. I remember adding about 8 ounces of lead shot to the short nose of the old Legionairs, so lightening the tail should help a lot.
Spoiler servos will be mounted in the wings, and we should be set.
So that's the plan for making the new Legionair 140 with "improvements" for today's launch equipment. The goal is to get the plane in at under 65 ounces so it will float, and the thin airfoil should allow it to penetrate reasonably well.
December 24, 1999
The fuselage has been started and the components assembled below:
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The pod has been cut from a Dow 60 block on the bandsaw, and the wing fairing will be cut and fitted into the notch. This will insure a good fit for the wing airfoil profile. The 3/4" diameter tail boom has been made using the O'Sullivan Roll Fuse technique, with a layup of 6 ounce Fiberglas, Kevlar tape, 25k CF tow (top and bottom), and an outer layer of bias 3 ounce FG. The resulting boom weighs 1.3 ounces and is as stiff as the aluminum tube. Fit the boom and sand the pod to shape and it's ready for layup.
January 18, 2000.
The design of the wing layup (top secret, and will be concealed with opaque Monokote) is done, simplifying the construction process and it will look just like built up balsa construction. It took 8 hours(!) to fabricate, lay up and vacbag the left wing tip panel, not including staring at the plans and calculating the loads. I could have built the balsa version from sticks in under three hours. The panel weight is an amazing 4.8 ounces, can take a 120 lb. winch launch and 70 mph zoom. We've got a carbon spar, carbon/balsa/carbon ribs, bias FG for torsion and a carbon trailing edge. I could have used kevlar to save about 1/2 ounce, maybe the next one. I can guarantee that this construction method will never make into the mass production market. As the panel was built, I could not help thinking about the airfoil accuracy achievable on the balsa structures of yore. This wing will be built to the standards of current foam technology, will it make a hoot of difference?
Here is the tip panel being slotted on the table saw for the balsa ribs. Lucky for me, the 10" diameter saw blade is precisely 1/8" wide for the balsa ribs. The CF spar has been grouted flush with the core surface using lightweight spackle. I find the lightweight spackle to be the right compressibility for bagging, although heavier than microballoon mixture. In the background you can see the stockpile of different foams for wing cores, when inspiration strikes, the right core material better be there. Although not standard for hobby construction tools, the table saw is very useful for cutting styrofoam and other materials at precise angles and lengths. With the tilting blade it is easy to cut precise root and poly breaks in the composite wings.
The center panel spars have tapered CF cap strips and are built using the standard vac bag method. I've made new molds and can now build tapered CF spar caps from tow. The taper can be now be sized exactly for the wing planform loads. Above, the main panel is ready to be vac bagged. The full depth spar is glued and grouted, a plywood root rib is glued to the inboard (medial?) face, balsa ribs inserted into the slots, CF cap strips on the ribs, the spoiler sub-spar routed in, and wing pin incidence block inserted. As mentioned earlier, the spar takes the entire bending load, the skin will take the torsional loads.
The plans call for 7.5 degrees of dihedral at all of the breaks. Experimenting with the dihedral angles on the originals did not improve the handling, so this plane will use stock dihedral angles. Nobody makes CF joiner rods in a 7.5 degree angle and a steel rod in 5/8" dia will be heavy. The nostalgia rules will only allow a 25% deviation on the dihedral angles so a 5 degree rod would be illegal anyway. Dihedral can't be built into the wing spars because the joiner tube is almost full thickness of the airfoil. So the only solution is to build a spliced carbon rod with the right angle. The design calcs say that 1/8" X 1/2" carbon steel flanges are enough, look at this thing:
The CF rods are encased in reinforced epoxy using a 3/16" plywood box with the mild steel braces on top and bottom. A braided CF sleeve wraps the box and kevlar end wraps prevent the ends of the box from bursting under load. For rigorous scientific testing, I jumped up and down on the box with the ends of the joiner rods supported, and it did not break. The rod is good for a design load of 117 ft-lbs. Just like the original, the joiner rod will be built into the fuselage.
Feb 8, 2000 Progress report.
Work has severely cut into my building time, and I want to get this project done for springtime.
Here is a picture of the fuselage pod attached to the boom, and ready for wrapping and layup. The nose cone relief is barely visible, Nostalgia rules do not require that the canopy remain. Note the airfoil shoulder fairings, and the fit of the joiner box. After dissolving out the foam, the joiner box attachment will be reinforced with milled glass epoxy.
The wings are joined and only require spoiler installation. Then covering material will be used to cover the open bays. Although the inner panels are painted, the CF layup underneath is still visible. A full length tapered spar runs from the root to the tip. . Each wing panel weighs 16 ounces at this point, we are on target for the final flying weight of 65 ounces. Remember, the Legionair 140 has 1325 square inches of wing area, the wing loading works out to 7.06 ozs/ft2. The span loading will be 5.57oz/ft. In the picture below, there is a balsa rib with a CF capstrip in each of the bays. Two layers of Uni-CF brace the polyhedral joint.
Spoiler bays were routed out and the servo is set into a well accessed through the spoiler bay. It took a while to decide on the best layout for the spoiler actuation, the system uses a pushrod less than an inch long so the layout was critical. Thank goodness for adjustable servo throws!
The tail parts are exactly to the original plan, except that the ribs are all made from 1/16" balsa laminated to 0.014" CF. The trailing edge also has a CF laminate, and the elevator will be joined with a 3/16" CF rod. Even with all of the reinforcement, the weight of the covered stab is 2.3 ounces and the rudder/fin is 2.2 ounces. Total weight before trim and balance is:
| Fuselage with radio | 23 |
| Wings with spoiler servo | 34 oz |
| Tail | 4.5 oz |
| Total | 61.5 oz |
This is first lost foam fuselage I have done and it did not turn out very well. It's kind of lumpy and the general shape is OK, using 6 oz. fiberglass cloth with a very open weave required too much filling and I just got tired of patching and sanding. The pod is plenty stong enough, and the joiner rod is really solidly attached. Total time to this point is estimated at over 90 hours. No, I will not be building a backup. The areas and moments of the plane look really strange in comparison the the planes of today, really short in the fore/aft direction. The aspect ratio looks very good There's about 5 hours of work to go, then I will be tele-transported back to 1977.
March 4, 2000 - It's done, final weight is 65.5 ounces due to the requirement of 4.9 ounces of nose weight! What a waste, I'll probably add some reinforcement to the nose instead of the lead, and a nose tooth. The orange strips are to hold down the edge of the Monokote, since it is just ironed on to the glass cloth. The center panel bays have not been cut open, I'll see it the plane needs to have an additional 3 ounces shaved off by making the cutouts. My feeling is that the plane will be pretty slow so it will need more weight if there is any wind at all. I'm happy with the rigidity of the wing, it is much stronger than the original construction, and only about 4 or 5 ounces heavier.
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Copyright 2000, W. Man-Son-Hing