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This item is discontinued. GREAT PLANES PROFILE 38 KIT
Aircraft Type: Sport Scale Profile Twin RCM PROTOTYPE SUMMARY
There might be no more recognizable aircraft than the Lockheed P-38 Lightning. Fast and heavily armed, this fighter was designed as an Interceptor and saw limited success in Europe as a long range escort fighter and fighter-bomber. In the Pacific theatre the Lightning owned the skies. Its long range enabled it to cover vast expanses of ocean. Its high altitude performance and speed allowed it to dictate the terms of battle. No wonder that the two highest ranking American Aces, Major Richard Bong with 40 victories and Major Thomas McGuire with 38 victories, flew Lightning's. R/C Twins are rare because they are intimidating; it's an old axiom that two engines yields four times as much trouble. The Great Planes Profile 38 is an on-target attempt to simplify the twin and offer it up as a fun-to-fly sport model. The Profile 38 was my second twin, and then my third. That's right, I built two kits for this review. However by the time the review was finished NEITHER had survived. So how can I be so enthusiastic about a model that has TWICE beaten me down? Read-on ...
Construction: The Profile 38 arrived neatly packed in a full-color printed 4.5" x 7.25" x 38" box with numerous pictures of the model. You can also view a short video of the Profile 38 at the Great Planes website. The full-size plans consisted of two 36" x 41" sheets, plus an excel-lent 48-page instruction manual, liberally illustrated with photos and sketches. The instruction manual includes a centerfold reduced-sized plan, which serves as a guide to help locate a particular detail on the full-size plan sheets. Another nice feature is that the manual includes pictorials of all of the die-cut sheets; showing where to look for the parts called out in each step of construction. The hardware was all suitably separated and bagged. None of the die-cut sheets are thicker than 1/8" which helps keep the die-cutting crisp and clear. For assembly, I used Great Planes Thin and Medium CA's and Accelerator, Great Planes 6-minute and 30-minute epoxy, their aliphatic wood glue, along with some Carpenters Goop from the hardware store.
Construction starts with the tail surfaces. All the curved pieces are built-up from the laminated die-cuts with various 1/4" thick sticks forming the straight sections and the ribs. After assembling, take time to sand everything flush and smooth, and round off all the edges together. I went ahead and rounded the edges of the fins and rudders too while they were still small and easy to handle, before they got built into the booms. The wing structure exhibits ex-ceptional prefabrication to help assure alignment and is engineered with incredible strength. The sub-leading edge (3-ply light plywood), main spar web (3-ply light plywood) and trailing edge (balsa) are all notched to space and align the wing ribs. The spars are 1/8" by 1/4" basswood. The wing leading edge is sheeted forward of the spar and from boom to boom behind the spar. Finally, there is a double thickness plywood dihedral brace. This is one strong wing! Even the wingtips are plywood and keyed into the main spar and trailing edge for perfect alignment. The 3/8" thick leading edge is installed after the wing sheeting, then shaped to suit. The ailerons are preformed from solid balsa and need to be final sanded to fit the airfoil. As a personal preference, I deviated by inlaying a piece of plywood for mounting the control horns rather than screwing them into CA-hardened balsa per the instructions.
The Profile 38 uses a single throttle servo driving two throttle pushrods through a tiller bar installed prior to sheeting the top of the wing. The throttle servo, mounted in the center pod, drives the tiller bar, transferring the motion to the ends of the tiller bar, with normal pushrods to the engines. It is simpler than it sounds and works fine. The booms are built-up with 3-ply light plywood cores, 1/4" balsa strips, the balsa fins at the back and a couple of cross braces. The plywood is actually slightly thinner than 1/8" (probably 3 mm) so when laminated it is slightly thinner than the balsa around it - the fuselage frames need a good sanding before application of the 1/16" balsa skins. With a P-38; after you do all of the above there is still one boom to go so you get to back up 17 steps in the manual and build the other one. One unique feature of the Profile 38: the RH engine gets 4° right thrust, the LH engine gets 4° left thrust. This offset is intended to improve controllability if you suffer an engine-out. This is more offset than can be dependably set using a stack of washers under the engine, Great Planes designed the mount as a unit, separate from the boom and is installed at the correct thrust angle. The builder must select the correct set of mounting plates (either the 15 or the 25-sized) then laminate two layers of the 1/8" 3-ply light plywood with an outer layer of 1/16" aircraft ply on each side. The mount is then fit to the boom and installed with epoxy, making sure one mount is offset for right thrust and the other for left thrust. Great Planes supplies a guide for setting the angle but I did not like it because the entire offset came at one end of the mount. I installed the mounts with the offsets coming out both sides of the fuselage; that is with the mounts stuck out from the fuselage by 3/32" at the back and by the same distance from the other side at the front of the mount. The booms are keyed into recesses in wing rib #3; guaranteeing critical alignment. I cannot stress this enough, the wing provides stiffness to the forward boom so a well-glued wing to boom joint is imperative. The center pod consists of vertical and horizontal plywood keels with a couple of bulkheads. The pod has an ABS top shell that is permanently installed using Carpenter's Goop. The removable bottom shell provides access to the receiver, battery, throttle servo, steering servo, and the nosegear steering. The shells were painted and installed after the rest of the airframe had been covered. Covering: There are quite a few choices for P-38's squadron markings. I combined several, resulting in schemes that did not match any full-size bird but which borrowed from several. The first model used MonoKote Olive Drab and Dove Gray with Missile Red for the tail, wingtips and the noses. The second model was all silver with the red tails and wing tips. The silver bird mimicked the 99th Squadron colors, but for the record, it does not look like the Tuskegee airman flew P-38's in combat. The matching LustreKote colors were used to paint the plastic fuselage center pod and the radiator covers. Engines: The Profile 38 is designed to use two .15-.25-size engines; the review model used a pair of O.S. 25 LA's. My limited experience with twins has taught me to opt for consistency and ease of handling first, and the LA engines were right on target. Each engine has its own 4-ounce tank mounted on the opposite side of the boom from the engine. The tank location is mandatory as there is not enough room behind the engine. 9 x 5 Top Flite Power Point props tucked inside 2" Great Planes spinners complete the package. Radio: The Profile 38 was equipped with a Futaba SkySport 6 radio, a Hitec RCD 555 mini-receiver, four Hobbico CS-61 standard servos and two Hobbico CS-67 standard BB servos. One unusual feature of the Profile 38 is that the elevator and rudder servos, mounted in each boom, are hidden from view by the radiator pods. The rudders are connected with a wire link. One weak area of the instructions was that the builder is left to figure out how to route the servo leads from the boom mounted servos to the receiver up in the wing. The rudder servo needs a Y-harness to connect with the steering servo in the nose. The aileron servos are connected via another Y-harness. I offset the aileron servo arms forward two splines to get some differential movement in the ailerons. The low rate aileron settings are fine, but I used the high rate settings for the elevator and rudders. The Profile 38 builds light in the tail, and the LA 25's are at the heavy end of the recommended size (and weight) engines. It took three ounces of lead on the tailbooms just below the stab to nail the recommended C.G. The Profile 38 was now ready for the test flight.
Flying: Test flight day was a little gusty but certainly flyable besides, I wanted the extra boost to help loft the 30+ oz wing loading. Per the instruction manual, I ran one engine then the other. Modern engines do not need much break-in time but on a twin it is vital to get each engine running reliably at top-end speed, not necessarily on maximum peak rpm, and a synchronized idle, not necessarily at the lowest possible. I also wanted to make sure I could shut off both engines with low throttle/low trim. With both engines running I tweaked the needle on the RH engine and was rewarded with the unmistakable sound of the engines running in sync. I was also rewarded a few minutes later with a dead right engine on take-off. It's probably not a good idea to tweak the needles after you have both engines running their best. Taxiing out, the 2" nosewheel was dragging through the grass. I lined up, hit the throttle and started accelerating with me riding the up elevator to try to "unstick" the nosewheel, I finally yanked it off the ground. The right wing dropped but I got it back. But the bird was not accelerating or climbing as it should and the right wing kept dropping. It finally dawned on me that I had lost an engine. You NEVER turn a twin into the dead engine, but I could not get it to turn into the live engine, and I was out of field. The best course seemed to be to throttle back and take my chances with an off-field landing. I found it upside down with the twin booms "parallelogramed," each broken immediately behind the wing and ahead of the fins not bad considering. I spent a day repairing the booms. And, changing from a 2" to a 2.5" nosewheel to help with the grass field take-offs.
Upon taxiing out it was obvious that the larger nose wheel was a good idea. I hit the throttles and it accelerated much more quickly and was not jostling around as badly as before. The larger nosewheel also gave the Profile 38 a more nose-up attitude; it was easier to rotate and break ground, the twin 25's racing the bird into the air. I did a lazy procedure turn and headed back down wind still gently climbing to get at least a couple mistakes high and scooting along at an impressive clip. Sneaking up on it, I tried a loop, then a roll; looked great, possibly the fastest roll rate I have ever encountered. Tried some Cuban eights; ailerons still a little light but okay, elevator fine, a bit heavier than the ailerons. Turnaround with a stall turn this time and it demonstrated better than expected rudder response, but no knife-edge capability. More rolls and turnaround this time with a Split S holding the nose down to see how she looks moving fffast awesome, but it sure gets small in a hurry! Now some BIG loops. This is an easy enough bird to get used to, the engines sound great when you can hear them the LA 25's have very effective mufflers. Circled around, throttled back to about a quarter and descended and flared to a nice boring landing. Taxi back, both engines still running, not quite a half tank of fuel left in each. Hit low throttle and trim, and the test flight is over. What a rush flying a twin brings! Refueled, restarted and did it all again. Aside from having to fill two fuel tanks and start two engines, the Profile 38 is a viable sport-fun flier that will NEVER be mistaken for just another airplane! I was getting more in tune with the bird and started exploring some lower speed regimes, never quite got a snap roll out of it. I scarcely needed half throttle for cruising around, but with the 30+ oz wing loading on less than 3 square feet of wing, it is not a floater. Ultimately it also proved it is still a twin. On the last landing, it was a little high and fast and I hit the throttles to go around. The bird just kind of pirouetted in mid-air, lost ALL its forward speed. Sensing what happened I cut the throttle, but that was really too late to matter as the bird fell like a stone. The left engine had died and the combination of asymmetric thrust, low airspeed, and high wing loading all checked in at the same time. This time the bird was a write-off. More bad news: I discovered that NONE of the in-air shots were usable. I had to build another one.
I was intrigued with the possibility of tricking out Profile 38 #2 to reduce the weight. I was also interested in enhancing the forward boom structure. I was able to pull 12 ounces off of it, improving the wing loading by some 4 ounces per square foot. Other changes in model #2: The structural changes made the engine mounting a lot more secure, and the vibration levels were substantially reduced on the second model. I wish I could tell you that the weight reduc- tion improved the Profile 38's already very good flight performance, but unfortunately, #2 endured a PBF (Pilot Brain Fade) crash on its test flight before I got a chance to discover what it could do. I'm going to build a third one soon, but magazine deadlines wait for no man. Conclusion: Great Planes Profile 38 is a great concept. Its simplicity allows those of us who might otherwise shy away, a chance to try a twin with reasonably priced engines and a reasonably simple kit. The kit exhibits Great Planes legendary quality in engineering and execution, it is no more difficult to build then a "normal" profile, albeit one with three fuselages and six servos. The O.S. Max LA 25's are a perfect match to the Profile 38 offering great power and dependability, and contributing to the spirited per-formance of the Profile 38. However I think LA15's would also work well while reducing the weight and wing loading. I cannot imagine a better choice for an entry-twin, and I hope Great Planes has plans to bring out more twins like the Profile 38. Photos by Dennis Adamisin. Reprinted with permission.December, 2004 R/C Modeler Magazine Editor: Patricia Crews
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