Model Research Labs

 

Detailed finishing instructions for hand launch gliders using MRL high solids Sanding Sealer

All finishing must be done on a clean glass top work table. It is also a real good idea to do all your construction on a glass surface to avoid problems later.

Get your wing to the point where it is sanded smooth and ready for finishing. Cut some fresh new 400 wet or dry sandpaper into pieces about 2 x 4 inches. Place the wing upside down on the glass surface and sand the bottom of the wing until perfectly smooth, flip the wing over and do the same for the top surface. Clean up the fuzz around the leading edge by placing the leading edge near the edge of the glass and doing the detail shaping and sanding on the leading edge.

For a low ceiling glider repeat this process with 600 or 800 grit sandpaper. Do not apply any sanding sealer on any glider that will be flown in a site with a ceiling less that about 40 feet. Avoid getting fingerprints on the bare wood.

For medium and high ceiling gliders and all outdoor gliders, you should now apply a very sparse coat of sanding sealer and spread it out as much as you can. This is sort of a seal coat to prevent moisture and later coats of sealer from penetrating the soft light wood. Allow this coat to dry for at least 4 hours. Once again, on the glass surface, sand the bottom of the wing with fresh 400 paper and then use the same piece of paper on the top surface of the wing. At this point the wing should be noticeably smoother with little evidence of sanding sealer left on the surface.

This is a good finish for gliders intended for ceilings in the range of 50 to 70 feet. Polish the bottom surface of the wing with 600 or 800 grit and you are ready to go.

For the higher ceilings and outdoor gliders additional coat(s)) of sanding sealer will improve the flight times up to a point. That critical point is controlled by two factors.

(1) Weight, you can easily exceed the ideal flying weight for that model under any conditions, or for that model in that flying site. The site limits are usually ceiling height and room to make that very important first turn.

(2) Too slick. This one is a bit hard to believe, but any reasonably clean low drag design glider can have a finish on the top surface of the wing that is so slick that the airflow can never become property turbulated. The only slick finished glider that will fly well is the one that is also about 20% under the normal weight. An all around unlikely combination, don't mess with slick gliders. My definition of "too slick" is when the wood pores are all filed in, or the surface area between most of the pores is polished and shinny. This is bad news as the glider will fly just like it is far too heavy. Climb, stability, recovery, first turn diameter, and sink rate all suffer. The only solution is to take some 400 sandpaper and remove some of the finish.

Additional coats of sanding sealer should be applied with full flowing coats that are well spread out on the surface. Always paint the bottom surface first. Allow each coat to dry for at least 12 hours so it will not gum up the sandpaper too much. Scrape your glass table surface clean with a razor blade and wipe it down clean. Place the wing on the smoothest part of the glass. Always sand the bottom surface of the wing first using a fresh piece of 400 wet or dry sandpaper. Sand in a circular motion for best results. Carefully sand the top surface. watch out that you do not sand holes in the sealer, hold the wing so that a light reflects off the surface and you can see the fuzzy spots if you do sand through. Pulling the wing up to the edge of the glass will allow you to do a good job around the edges of the wing. You must sand until all the shine is gone from the sanding sealer. Do not even think of more sanding sealer until the shine is really gone. After the final coat you should sand the wing top and bottom with 600 wet or dry sandpaper. A slight sheen will become apparent when held up to a reflecting light.

Two coats of sanding sealer is generally sufficient for most gliders intended for ceilings of around 100 feet. You should strive to keep the total flying weight between 0.5 and 0.7 oz. With a clear ceiling of 100 feet and a decent glider with a weight of 0.6 oz., flight times of 1: I 0 to 1:1 5 are relatively easy and can be expected in any serious contest. Be prepared for it and you can win because, "The bullshit always stops when the clock starts".'

Three coats and sometimes four coats of sanding sealer are necessary for high ceiling gliders and outdoor gliders. With ceilings of 135 to 165 feet. the very best gliders weighing between 0.73oz and 0.79oz are capable of flights of 1:20 to 1:25. The typical glider with a weight of 0.85oz should do about 1:18. Increase the weigh to 1.0 oz and the max flight time drops to about 1:14. 'The bullshit stopped when the clock started. "

When you are completely finished with the finish on the finished wing, stand the wing on its trailing edge on the clean sheet of glass. Rest the edge of a plastic drafting triangle on the glass surface and press it up against the bottom of the wing. Slide the triangle along the wing to the one of the dihedral breaks. Hold the triangle tight against the wing and place the wing flat on the glass surface, now with a sharp new razor blade, make a nice clean cut into the wing. Do not cut all the way through.. Repeat this process for the other two dihedral breaks. It is extremely important that all three dihedral breaks be absolutely parallel. Any error at this point means that you are building a crooked airplane with reduced performance and you are also going to need a crooked stopwatch.

Carefully cut the rest of the way through for the center dihedral break and sand in a bevel for the required dihedral. Repeat this process for the two polyhedral breaks.

 

To assemble the wing panels you need to place a bit of wax paper on the glass surface, (only wax paper works). Then while pressing a dihedral joint together with two hands, wick a thin bead of glue into the joint with you other hand. This glue must be a water thin CA glue, the best CA we have found is MRL's Premium CA. When this joint has set a bit you can then lift it up and go all the way around the joint with a nice bead of CA. Any CA that does not wick into the joint must be wiped off before it sets up. Repeat this process for the other two joints, I prefer to do the tip joints first and then the center joint last after I am warmed up to the task. Note that you are using good glue and gluing the wing together only after all the painting and sanding are completed.

Sand your fuselage very smooth with 400 sandpaper and cut the shallow "v' in the top of the fuselage to fit the wing dihedral. Hold the wing tightly in place and once again wick CA glue into the joint. Give this joint a second application of CA and you can then add the finger rest. Never, never ever glue the fuselage on crooked. There is no such thing as gluing the wing on crooked, the wing is the part that does the flying. What you would be doing is gluing the fuselage on crooked and dragging it sideways thru the air forever after. The circle needs to be the largest that will fit in the site. To obtain the best flight times with a 4" wing chord glider, your glide circle must be well over 100 feet in dia. Trig it out, and the wing goes on straight. (For some reason wing chord seems to affect circle diameter more the wingspan.)

Using the same processes apply the sanding sealer finish to the fuselage and finger rest. You will not be able to add additional sealer so do a good job now.

The stabilizer should be finished in the same manner as the wing, always doing the bottom first. However there is one big difference: Under no condition should you ever apply more than two (2) coats of sanding sealer on any of the tail feathers. Another "never do" is never use quarter grained wood in the tail feathers. About 40 years ago we discovered quarter grained wood was just too brittle for tails. There is no advantage, other that appearance, to using quarter grained wood. Still 40 years later the old wife's tail and rumors still persist. Remember the new digital stopwatch is king. There is no room for bullshit... Quarter grained wood is still great for the fuselage. Quarter grained wood is not very good for wings either but wing quality wood is so hard to find that I will use it if it is otherwise a good piece of wood. Models using such wood on the flying surfaces tend to shatter upon impact. A properly designed model does not need quarter grained wood.

The rudder should be finished just like the stabilizer. The trick here is to cut the outline of the rudder on the end of a balsa sheet and completely finish the rudder before parting it off the balsa sheet. You should layout the rudder so the leading edge is parallel to the grain of the balsa sheet. There is no reason to ever have a rudder that is more than .015 inch thick at the thickest part.

When building tail feathers, keep them light, cut the wood out. Thick surfaces, oversized surfaces, heavy paint are all time killers.Remember this; One unit of weight eliminated from the tail end, eliminates 3 units of weight from the nose. No where else on the model can you reduce weight so efficiently. One of the main reasons for my success was that I always flew strong light weight models with thin light weight tails. My models were usually .05 to .1 oz lighter than the competition. Incidentally, tail feathers with a reasonably low aspect ratio, and most importantly, 15 degrees of sweepback on the leading edges, will not flutter and blow off. Not even with launches of over 100 MPH. I make stabs from the lightest 1/16 sheet I can find or in a pinch I will use medium weight 1/32 sheet. Rudders can be as thin as .010 thick

Back to the fuselage, by now it should be about done warping.. At this time you need to trim the fuselage to fit the stabilizer and to have absolutely no angle relative to the wing, (O-O) setting. There is no magic, you have to build good straight airplanes. Those who have to warp up or down into the stabilizer are just sloppy, crummy builders. In order to achieve this 0-0 setting I use a simple tool that is nothing but a piece of .025 thick aluminum about 1-1/2 inches wide and 18 inches long. I have scribed a series of parallel lines about 4 inches long near one end. These lines are spaced every .050 inch and they start about 1/2 inch from the edge and continue for about ten lines or roughly another 1/2 inch like this: except the lines are real close together and there is a lot more of them.

My word processor does not like to draw lines real close together and I still have not figured out how to import drawings from the CAD program. So you pretend the lines in the above drawing are every .050 inch.

Hold this tool against the fuselage and against the bottom surface of the wing in a position so that the little lines are where the stab needs to be glued on. Lo and Behold. Notice that the area you need to glue the stab is not parallel to the bottom of the wing. You need to take a sharp knife and some coarse sandpaper and trim the stab mounting area so that it fits the airfoil of the stab and is also parallel and 0-0 to the wing. Five minutes of effort and you know the glider will fly. I like to wait a week or so and check this again before gluing the stab on. To glue the stab on you place it flat on the glass and place the fuselage in the correct position on the stab. If you want to use stab tilt, allow for only one half as much as you expect to need. Too much is a duration killer. Wick a small amount of CA into the joint. Lift and wick a bit more CA into the joint and you are done. Cut the rudder off the sheet and glue the rudder on by the same procedure.

Many modelers have ask me about dethermalizers. I must be the wrong person to ask, I have never ever built a truly satisfactory DT. A good DT must not detract from the performance of the model and still get the model down where you can find it. There is no way you can afford the weight, drag and flexibility of a pop up stab. With a pop up stab you will seldom need one, ... and you will get beat at every contest by the flyer who is willing to lose a few models. The best DTs I have seen all worked by popping up the leading edge of the wing. Some of these are quite rigid but I would like something better. There is a tremendous load on the joint at launch and that leads to heavy construction. Generally speaking, outdoor gliders should have a flying weight of about 1.0 oz. Adding this type DT will make the models as difficult to build as good indoor gliders. After another year of losing or busting up gliders I may have a better solution but don't wait for me, do it yourself.


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