[Courtesy of Dick Williamson, williamson "at" ll.mit.edu September 2001]
Improving the Great Planes Spirit
A Great Planes 2-m Spirit was my first RC plane. With it, I learned to fly, but not without mishap. My Spirit is about 10 years old and has been repaired about 20 times. In the process, a number of design deficiencies were corrected and failure points strengthened. A lot of flyers say that the Spirit is a lousy plane. While I did learn to fly successfully, I learned enough about the Spirit to agree with many of the detractors' comments. Built according to the plans, the plane seems to plow through the sky, wobble in both yaw and pitch, and tip stall rather easily. However with the modifications outlined in this summary, my Spirit has become a joy to fly.
Center of Gravity, CG
The standard instructions and design of a Spirit are aimed at beginners. Accordingly, the initial recommendation for the CG (CG right on the middle of the spar, 3 1/16 in. behind the leading edge) is conservatively far forward. This is supposed to make the plane fly more stably. However, the Spirit flies quite sluggishly at this CG. An additional consequence of this forward CG is that the Spirit has a tendency to go into pitch oscillations where the nose points up, the plane stalls, the nose points down and accelerates, the nose rapidly rotates up and the cycle continues. The relatively short distance between the wing and tail feathers as compared to other 2-m sailplanes exacerbates the problem. A better choice, even for beginners, is to put the CG about 1/4 in. behind the middle of the spar (3 5/16 in. behind the leading edge). I have been very happy with the CG at this position. The plans suggest that more experienced fliers may want to put the CG about 3/8 in. behind the middle of the spar. If you put the CG too far back, the plane will become unstable and impossible to handle. Find a CG that you are comfortable with for your flying style.
Incidence and Decalage
When a Spirit is balanced with the "beginner's" CG on the middle of the spar and trimmed out for a shallow glide, you will find that the neutral position of the elevator is tilted down with respect to the stabilizer. This stabilizer angle is probably another conservative choice by the designer. If you remove weight from the nose and move the CG back, the elevator will be tilted down even more when the plane is trimmed out for a shallow glide. This angle between the elevator and the stabilizer adds some drag, but does not make a major change in the flying characteristics of the plane. If you change the "decalage" (the angle between the stabilizer and the wing), the angle between the stabilizer and the elevator, when trimmed out, will be reduced thus reducing drag. To change decalage, lower the front of the wing in combination with raising the rear of the wing for a total of 1/8 to 1/4 in. (maybe more). When you do this, the elevator and stabilizer should be close to parallel at the proper trim. An alternative is to cut some material off the rear of the top of the fuselage so that the rear of the stabilizer is lowered about 1/8 in.
Size of Fin and Rudder
Quoting Mark Drela, "The Spirit has a somewhat marginal vertical tail area. With sharp rudder inputs it really tends to wobble, and it also doesn't have a lot of yaw power at low speeds. Enlarging the vertical tail by at least 50% would largely eliminate these shortcomings." Following Mark's suggestion, I made the area of the fin and rudder about 50% larger than the size indicated in the plans. Keep the fin-rudder hinge line in the same position and angle as on the plans, but make all linear dimensions (except thickness) of the fin and rudder about 1.25 times larger than on the plans. (1.25 squared = 1.56, a 56 % increase in area.) If this is done, some material must be cut off at about 45 degrees on the lower rear of the rudder so that the rudder clears an up-deflected elevator.
Sheeting the Outer Wing Panels
Quoting Mark Drela, "The Spirit is also much more prone to tip stalling than its wide-tip planform would indicate. This is surely due to the lack of leading-edge sheeting in the tip panels. Extending the leading-edge sheeting all the way out will allow more aggressive thermal turns and will give more docile behavior in general." The plans call for sheeting the top front portions of the inner wing panels back to the spar. This sheeting helps maintain the airfoil shape. The plans show no sheeting in the outer panels. The result is a lot of sag between the ribs when the covering material (e.g., Monokote) is shrunk over the outer panels. This screws up the airfoil. To sheet the outer panels, I used 1/32-in. balsa sheeting. To accommodate the sheeting, 1/32 in. was trimmed off the top of the ribs back to the spar. Because there are two angular breaks in the leading edge of a Spirit in the outer panels, three separate pieces of sheeting are required for each outer panel. With the outer panels sheeted, the Spirit flies fine and needs no washout to reduce tip stalling.
You may want to beef up the wing. I have seen six spar failures of 2-m Spirit and Spirit 100 gliders. These failures occurred during winch launches and in crashes. In each case, the spar broke right at the end of the wing joiner. Others have commented to me that this is a well-know failure point for the Spirits. Unfortunately, I conducted two of these unintended experiments. In the post-mortem examinations, it looked as if the top spars failed in compression right where they met the outer end of the joiner. This is exactly what you might expect.
The easiest way to ward off a compression failure of the top spar is to make the joiner longer by running it outward one or two bays (A bay being the distance between the ribs). One approach is to use the supplied wing joiners and then add balsa (width and height matching the space between the spars) between the spars for the distance of two bays beyond where the supplied joiner ends. For best results, the balsa should be oriented with the grain vertical to best resist compression. Shear webs should be used on the sides as is done normally. Another approach is to make your own longer wing joiner out of plywood or aluminum. The end-grain balsa is a lightweight solution and seems to work well. Adding carbon fiber to the top and bottom of the spars should help. Wrapping the joiners with strong thread or kevlar string also helps. If you wrap the joiner boxes, put the carbon fiber on first. With these wing modifications, my Spirit can now do a very respectable zoom launch off a winch.
One of the best ways to reduce the damage in a crash is to reinforce the front of the fuselage with fiberglass. The nose tends to break at one of the bulkheads forward of the wing. To counteract this, about 1-in.-wide strips of 2-oz. cloth were wrapped circumferentially around the outside of the nose astride each bulkhead joint. After these strips were in place, the entire front portion of the fuselage from the nose block back to the front of the wing was covered with 2-oz.cloth. Fiberglass strips can also be added to the interior along the bulkhead joints to further beef up the nose. Spirits tend to come out tail heavy so try to keep tail weight low and move batteries, receiver, servos, etc. forward as much as possible. You will still need lead in the nose to balance the plane, so adding fiberglass in the nose doesn't increase the weight of the plane. An article on how to do fiberglassing is on our web site:
The front hold-down for the canopy is a small dowel. This tends to break in a crash. I use a piece of a nail in place of the dowel. This nail replaces some lead, so there is no net weight increase.
The fuselage of a Spirit has three positions for a tow hook. Placing the tow hook in the forward position seems to work best. A tow hook in the middle position tends to lead to pop-offs on launch. The rear position leads to a very unstable launch. With a strengthened wing, I have bent the supplied tow hook on a winch launch. I replaced it with a stronger (larger diameter) tow hook.