The full-size RV-4 is one of the most popular high performance homebuilt aircraft in history. It is an all-metal aircraft capable of very short takeoff and landing, high cruise speed, and excellent aerobatic performance. For more information on the RV-4 and other RV aircraft visit the Van's Aircraft website at- http://www.vansaircraft.com/public/tp-welco.htm
Another wonderful site with as much information on RV's as anyone could want is- http://www.metronet.com/~dreeves/vaf.htm
This is an excellent place to find photos of completed planes if you want to do a scale color scheme. The "RV of the Week" section is highly recommended.
An electric conversion of a popular sport scale glow kit with minimal modifications using moderately priced equipment. The power system chosen was an Aveox 1409/2Y on 14-16 cells. Why this particular motor? Well, I already had it, it seemed to match the plane pretty well, and it falls in a common power range where many other motor choices are available. It seemed quite possible to build the plane with this power system and still come in just a little over the high end of the manufacturer's recommended weight range. My actual weight goal was 6 3/4 lbs. or less. I also hoped to use some of the many standard sized servos I have collected over the years.
The Great Planes RV-4 is a very traditional built-up wood kit with some unusual innovations. It is described as a "semi-scale" model, but is generally quite accurate in outline. The low aspect ratio of the wing provides a large wing area in a compact package. Construction is mostly balsa with some lite-ply and aircraft ply. Cowling, wingtips, wheel pants and canopy are vacuformed plastic. There are two large sheets of full-sized plans and a complete hardware package. The builder must supply wheels, 2 1/4" spinner, and pilot figure.
Some pleasant surprises waited inside the box- first was the instruction manual. 52 pages long, it has very clear photo illustrations of every significant construction step, and lots of useful background information. A very nice feature is a reduced size plan bound into the center of the booklet. It is meant to be removed from the instructions and used as a building reference. This turned out to be an incredibly good idea- it is much easier to find details on the small plan. Another nice touch was a simple two-view drawing to use as an aid in designing color schemes.
All the wood parts are die-cut. This may seem backward in these days of laser and CNC cutting, but it turned out to be another pleasant surprise. Nearly all the die-cut parts all pushed cleanly out of their sheet, only a few small parts needed a bit of trimming before popping loose. None of the wood was crushed, and all the ribs matched perfectly. Parts required much less sanding than any laser or CNC kit I have built. Wood quality was only fair. The sheeting and sticks supplied varied wildly in weight. Some was very light, some was very heavy C-grain wood. I decided to replace about 1/2 of the supplied sheeting and 1/3 of the sticks, which saved about five ounces. Some of the wing ribs were also quite heavy. The vacuformed parts are nicely shaped, but have indistinct or totally invisible cutting lines which makes them difficult to trim.
Construction begins with the tail surfaces and right away it is obvious that this is not a typical glow kit- instead of huge slabs of wood, the tail surfaces are built mostly of 1/4" sticks. One minor change made to the kit design was to reshape the tips of the horizontal stabilizer. The plans show them as a simple straight line, but they are actually curved like the top of the vertical fin. A few minutes work with a sanding block made the stab much more attractive.
Balsa built-up tail feathers
Wing construction is next, and it features some unusual building techniques. All the ribs have large holes, but they are not for weight reduction. The ribs are slid onto the die cut shear web, then twisted to lock onto the web. The basic wing structure- ribs, shear web, spars, and trailing edge spar are assembled completely without gluing. Once everything is perfectly aligned and straight, the structure is locked together with CA glue. The trailing edge spar is oversized and extends below the wing forming an alignment jig.
The wing prior to the application of sheeting.
At this point the structure is so strong that I was able to weight it down with bricks while installing the LE sheeting. Once the sheeting is installed the trailing edge spar is trimmed off flush with the wing surface. The entire wing assembly was very straightforward with no difficulties encountered. The trailing edge, ailerons, and flaps are built up from sheet balsa and small ribs. This method is much lighter than the solid trailing edge stock found on many kits. Flaps are provided for in the standard construction, no extra parts are needed. If you chose not to install them, the flaps are simply glued in place on the trailing edge. The two wing halves are joined by a plywood part that twists into place like the wing ribs. The correct dihedral is set automatically by the joiner and the wings are glued together with epoxy. The wing design is completely self-aligning- if you follow the directions it is impossible to build a twisted or warped wing- very clever! The final step of the wing construction is to install servo extension wires from the center section to servo locations.
Using bricks to hold the wing in place while sheeting is applied.
Only one cosmetic change was made to the wing construction- the plastic wingtips were replaced with carved balsa blocks. The wood tips simply look nicer, and are very much easier to cover. I laminated blanks from light 1/2" balsa and cut them to the airfoil shape with a bandsaw. They were roughly carved using the plastic part as a guide, then sanded smooth. The wood tips were also an ounce lighter than the plastic parts.
The fuselage sides are built over the plans from die-cut liteply parts and balsa sticks. The fuselage top crutch is built on the plans, formers are glued to it, and then the sides are added while the top is still pinned down. This ensures a straight fuselage with minimal effort. One change was made to the fuselage design to accommodate electric motors with gearboxes that tend to be longer than a .40 size glow engine. I simply moved the firewall back 1/4" from the standard position by trimming all of the front fuselage parts by that amount. This gives a little extra room to wrangle those wires into place on long motors. I was careful when cutting the parts not to change the built-in right thrust angle. When the basic fuselage box is finished it is lifted off the plans and turned right side up. The firewall, wing mount and stab reinforcement are added, then the upper formers and longeron are glued to the top side of the fuselage. The graceful shape of the RV-4 fuselage is built by wrapping sheet balsa around the curved formers. I saved the lightest, most flexible sheeting for this job and was able to install it without wetting to aid the bend. I had hoped to modify the fuselage with a top hatch for battery changing, but eventually decided not to do this, since it would have meant removing the entire, very large, canopy section. The completed fuselage is obviously very strong, and surprisingly large.
Die-cut fuselage parts and framed up fuselage.
Cowl and plastic parts
Next, one of those jobs no one likes, dealing with the plastic parts. The parts must first be trimmed. As mentioned earlier, the trim lines are very hard to see on most parts, so work slowly and carefully. I marked the lines on the cowl with trim tape to be sure I made the correct cuts. Most of the cutting was done with small scissors sold at hobby shops for trimming R/C car bodies. The wheel pants and large cowling are molded in halves that must be glued together. I used masking tape to hold the parts together while flowing thin CA into the joint from the inside. The parts fit together all right, but there are large gaps where they join, particularly on the cowling.
Using trim tape to highlight the lines prior to cutting
With all the major parts completed, I could assemble the entire airplane for the first time. Seeing the complete framework with the cowling mounted really inspired me- this is an excellent looking airplane. It also worried me a little, because it seemed a bit heavier than I expected. So I decided to do a weigh-in. Gathering up all the items that would go into the plane- radio, wheels, motor, battery, etc. I came up with a total weight of just under six pounds. This seemed quite good, the major construction was completed, and I had been careful to include the little items like pushrods, prop, and spinner. It seemed like there was plenty of leeway to cover the plane, add a few details, and still hit the weight goal of 6 3/4 pounds.
My goals began to change a little after seeing the RV-4 assembled for the first time. It already looked so good that I wanted to do an excellent job on the covering and painting. The first task was to fill the ugly seams on the cowl and wheel pants. The instructions recommend a Bondo type filler which worked quite well, although it took two applications and lots of sanding to get a smooth surface. The cowl is quite large, and I was worried that the glued seams might split, especially around the front openings. I reinforced it with fiberglass tape along the side seams. A balsa ring, a little epoxy filler, and plastic doublers around the openings helped stiffen the front.
interior shot of the cowl after cutting and gluing
The cowl is attached to the plane with three small sheet-metal screws; each of these holes was backed with a small piece of 2 oz. fiberglass CA'd inside the cowling. The wheel pants needed filler on the seam, but no additional reinforcement. After filling the parts were painted with spray can white lacquer primer from the local auto parts store. This is as far as I go with painting until extensive flight testing is completed.
While working on the plastic parts I began to do the equipment installation in the fuselage. The motor was mounted on a standard .40 size glow engine mount with a CF reinforcing bar screwed across the arms. A small pipe clamp was tightened around the motor and mount to lock them together. This type of clamp will stretch or tear in a crash, often preventing damage to the motor or gearbox.
Motor and mount. I used a modified .40 size glow motor mount.
The drive system is an Aveox 1409/2y sensored brushless motor with a Robbe Planeta Pro 3.7:1 ratio gearbox. This gearbox might seem like an odd choice, since it is a planetary type more typically seen in gliders. I actually think it is excellent choice for this size model- the planetary arrangement makes mounting and alignment with the cowl much simpler. It is a very strong box with a hardened 5mm shaft, and replacement shafts are available. Any drive system capable of at least 500 continuous watts would be a candidate for this model.
Speed controller is an older Aveox F5LV, a very reliable sensored controller with smooth throttle response and low idle speed. After considering a lot of possibilities for mounting the controller I finally decided to put it alongside the motor where it would receive direct air flow from the cowl inlets. Two holes were drilled into the firewall for small dowels running parallel to the motor mount. The controller was placed between the dowels and held in place with nylon tie-wraps. The kit provides a removable panel just behind the firewall for access to the fuel tank. I simply left this hatch off to run the controller leads back into the fuselage. This also provides airflow into the fuselage. Since this model has six servos and large control surfaces, I decided to run a standard receiver battery.
Fuselage servos were mounted in the stock location, but shifted toward the sides of the fuselage to leave the center clear for a battery box. I made a modification to the elevator control rods- the kits uses semi-flexible nylon rods and runs separate rods to each elevator half. This seemed overly complicated, and I don't like plastic control rods anyway so the elevator halves were joined with a music wire "U", and the elevator and rudder are operated by 3/16" carbon fiber rods (actually tubing) with 4/40 hardware. These made a light and very stiff connection to the servos. Both the servos in the fuselage are old Airtronics that came with a transmitter bought many years ago. They're quite fast, with ball bearings and a little smaller than the standard JR servos I had. Servos and control rods were installed before covering the fuselage.
A little stand-off scale detail
Another cosmetic change was replacing the kit's bent wire tailwheel strut with a nice scale unit from SIG mfg. It is the smallest of their scale tailwheel units. To install it I had to add plywood reinforcement at the rear of the fuselage and the bottom of the rudder. Tiller arms on the rudder drive the tailwheel via springs. This system actually prevents shocks from damaging the rudder and looks great, although it is an ounce heavier than the stock system.
I used a Sig scale tailwheel to enhance the scale appearance of my RV-4.
I covered my RV-4 in Monokote except for the plastic parts which are painted. The tailwheel and main landing gear were removed so the fuselage could be sanded before covering. The entire airframe was block sanded with 150, then 220 grit paper. After sanding each part was thoroughly vacuumed, then further cleaned with a tack cloth. This cleaning is an essential step- dust on the parts will keep the covering from sticking tightly and may cause unsightly bumps.
The color scheme I chose was reasonably simple but did have a few interesting problem areas. The fin is two colors, but the color line has no structure under it. There is also a difficult curved color line on the fairing at the base of the fin. The stripes on the fuselage also have a gradual taper which is surprisingly difficult to lay out. The instructions are a bit vague in covering suggestions, especially on how to handle the fin and stabilizer. I decided to cover them before assembly, being careful not to cover the areas that would be glued to the fuselage.
For the two-color fin I used a technique I had read about but never tried. I cut pieces of red and white Monokote, then laid them on a piece of glass. The backing of the red Monokote was loosened along one straight edge, then this edge was placed on top of the white piece with a 1/4" overlap, with the backing underneath both pieces. The seam was ironed together with a trim iron on medium heat. Try to iron only on the overlap area, but be sure that every bit of the overlap is sealed together. Medium heat is better, because we only want them to adhere, not shrink. When the seam is done, all the backing is removed and the two-color sheet is applied just as usual. Of course you must be careful to get the color line in the proper location. I was careful not to overheat the seam area during shrinking, and had no problems getting all the wrinkles out. The same technique was used on the top of the rudder.
The wing has three colors, but is a fairly simple covering job. First the white covering is applied to both the top and bottom surfaces, except for the tip. Then the red covering is added to the leading edges. I covered the tips separately, since this seemed like the most difficult area. This way if I screwed up, I wouldn't have to re-do an entire wing panel. The lower center section was covered last in dark blue to match the fuselage bottom. The wing covering was shrunk with a hot air gun, and then the gray trim strip was applied with a sock-covered iron at medium heat.
The fuselage covering is much more complicated. To lay out the tapered stripes I first used the small drawing supplied with the plans, then transferred that layout to the full size fuselage plan. The fuselage sides were first covered with white, then the dark blue applied to the bottom and lower sides. The stabilizer and fin were temporarily mounted on the fuselage, and a mark made on the fuselage fairing to indicate the color line on the fin. They were removed, and the fairings were covered with small pieces of white Monokote. I made a template for the curved line from a business card. The fuselage top was covered with two pieces with a seam along the top. Before starting the covering, two layers of masking tape were stuck to the top of the fuselage at the centerline.
Each piece was cut with a steel ruler for the long straight edge that would adjoin the white. Then the pattern from the business card was traced onto the Monokote with a Sharpie pen and the curve was trimmed. The straight line was also marked on the fuselage side with a ruler. The first place attached is the curved line at the fin, since it must match the line on the fin. Then the straight line down the fuselage side is tacked in several places taking care to keep it from curving. Once the straight line is confirmed OK it is completely ironed down. Then the covering is ironed around the curved sheeting until it gets to the masking tape. Using a steel ruler the covering is trimmed just over the edge of the tape for a perfectly straight line. The tape is removed, and the edge ironed down. Before doing the other side, another piece of masking tape is stuck down lightly, exactly aligned with the edge of the Monokote, about 1/16" inside the edge. When the second side is covered, the Monokote is trimmed just inside the edge of the tape. The tape is removed, and when the edge is ironed down it will have a perfectly straight seam with a small overlap. At this point the fuselage covering is completely shrunk using a hot air gun and a sock covered iron.
Alignment marks are drawn on the fuselage side for the gray stripe which is ironed over the white covering. It is tacked down to maintain alignment, then fully ironed down at medium heat to minimize bubbles. Any large bubbles are pricked with the tip of a knife and ironed again. Once the stripes are down, all the alignment marks are cleaned off with some rubbing alcohol on a paper towel.
With the covering complete final equipment installation and some detailing can begin. All the wing servos are mounted on the bottom surface. The covering is cut away over the mounts, and short wire pushrods connect directly to the control surfaces. The instructions show the servo bays remaining uncovered which is fairly ugly, and has to cause some drag. Once my servos were mounted, I cut small pieces of Monokote to fit the bay and used a hole punch to make an opening for the servo shaft. The bays were covered, then the servo arm mounted. All the servo wires were brought together at a single six-pin plug so only one connection had to be made when installing the wing. The fin and stabilizer were now glued in place, the control surfaces hinged, and control horns installed.
Mounting the wing servos.
The RV-4 has a huge canopy so at the very least a little detailing and a decent pilot figure are needed to fill that space. I painted the interior dark gray, then airbrushed some black on the cockpit floor to make it less visible. After a long search I found a decent looking pilot that didn't weigh a ton. It is a 1/5 scale civilian pilot made of hollow cast vinyl sold by Vailly Aviation. The pilot was painted with acrylic enamels for plastic models.
Pilot and canopy
The kit comes with an instrument panel sticker, but I was able to find an excellent color photo of an RV-4 panel on the web. I corrected the perceptive and retouched the photo using PaintShop Pro, then printed it out and glued it to front panel. The pilot's shoulder harness is made from strips of fabric tape. The canopy is temporarily held on with 1/4" wide 3M red vinyl masking tape. After the initial flight tests some additional cockpit detailing may be added, then the canopy will be permanently glued in place.
Time for a weight check
Construction was now complete except for the battery box and final radio installation. It was time for another weigh-in to determine the actual ready-to-fly weight. The result was a horrifying 71/2 pounds!! I was shocked and confused, unable to imagine where all this extra weight came from. I had gained more than 3/4 pound over my estimated weight.
I ran an Ecalc simulation at the actual weight and was somewhat reassured- the RV-4 still had a reasonable power loading of 83 watts/pound. Wingloading went from 25 ounces/sq.ft. to 27.5 ounces/sq.ft. A few years ago I would have considered this dangerously high, but I was currently flying a small plane (200 sq.in.) at a similar loading with no problems. My initial panic faded away, but I was still baffled at how badly I had estimated the final weight. When I weighed the uncovered model I had been very careful, or so I thought, to include everything except the covering. After doing some quick calculations, I realized there was probably six to eight ounces of Monokote on the plane, much more than I would have thought. A scale model like this has a lot surface area than a typical sport model of the same size. The rest of the excess weight probably came from the accumulation of many tiny parts not accounted for. Things like: 21 hinges with epoxy, over eight feet of servo extension wire, at least 50 small wood screws, six clevises and keepers, etc, etc. It was a valuable lesson- I generally build smaller models, and can estimate their weight quite accurately. But on a much larger scale model, my normal estimations simply will not work as well.
The battery box.
The battery box is a simple design with a 1/16" plywood base and 3/32" hard balsa sides. It is screwed and glued to the servo mounts and an additional support behind former F4. Three velcro straps hold the cells into the box, there is also a block of EPP foam glued to the wing which hold the batteries in place even if the straps should fail. The battery pack ended up right at the CG, which is nice, since it means I can experiment with other packs without changing the balance. There is enough room for a 20 cell pack.
Specifications as flown. Weight: 7 1/2 pounds Wing loading: 27.5 oz./sq.ft. Battery: 16 x 1900scr (will be replaced with 2400's soon) RPM: 7600 (7300 at 45 seconds) at 40A Prop: 12 x 10 APC-E Power loading: 83 watts/pound
Before the first flight I took the RV-4 out to our glider field for a range check. No trouble was noted and the model had excellent antenna down range even with the motor at part throttle. I decided to do some taxi runs to see how the plane handled on the ground. At low speeds it handled fine and turned smoothly. But when I tried some full throttle starts, it wanted to swing strongly to the right, and if not immediately straightened out with the rudder would do a partial ground loop. The design has three degrees of built-in right thrust which seemed like the obvious culprit, since a typical model will want to swing left, not right. I phoned the technical support number at Great Planes to ask if they had any advice or comments. They were quite interested in the electric conversion, and after some discussion we all agreed that the larger, deeper pitch prop I was using might exaggerate the effect of the thrust offset. I put a wedge behind the motor mount to remove about 1/3 the original right thrust, and was ready to fly!
Ready to fly.
Since this plane was built for a review article, it was essential to get photographs of the first flight. I chose to let a good friend and excellent pilot, Johnny Westbrook do the flying while I took pictures. We met at the glow club field where Johnny flies, assembled the model, and started charging the batteries. The field is fairly new and a little rough, so I had removed the wheel pants and installed slightly larger wheels. We agreed to do a photo pass immediately after the plane was trimmed and set a three minute timer so there would be enough power for a go-around if needed. We also decided not to use the flaps on the first flight.
Johnny throttled up and the RV-4 accelerated very quickly and swung to the right again. He over-corrected a little, then straightened out and lifted off after less than a 30 foot run. He held it level for just few seconds, then pulled just a bit of up elevator and it zoomed upward at about a 30 degree angle, accelerating at the same time. Clearly, it had plenty of power! By the time he made the first turn it was about 200 feet up. He throttled back and did a quick stall test- it got ridiculously slow, and finally did a slow-motion break straight ahead. No problems there! After a low photo pass he throttled up again and gradually pulled the nose up- the RV-4 climbed strongly, then held a good vertical line, and did a beautiful stall turn. At this point it obvious that Johnny was completely comfortable with the plane. He did two slow rolls, a slow four-point roll, a large loop, another graceful stall turn, and flew around inverted for a while. He was setting up for another photo pass when he commented that the power was starting to drop. At the same moment the timer went off, so he set up for landing which was totally uneventful, and quite slow for a seven + pound airplane. There was plenty of power left for a long taxi back to the pits. We let the motor run and decided that there would have been enough power for four minutes of moderately aerobatic flight with a bit of a safety margin. Ecalc says the plane could fly at cruise speed for over seven minutes.
Johnny with the RV-4
Johnny has flown a couple of glow RV-4's and actually thought that the electric version performed better! The only problem he noted was that the rudder was overly sensitive on takeoff at the recommended setting. Although the built-in right thrust does cause a right swing on takeoff, it seemed to be exactly correct once in the air. He was particularly impressed by how well it tracked when pulled vertical, and by the lovely stall turns. The stall is very mild, and ailerons remain effective right down to the point where it finally breaks. Recovery occurs quickly and automatically just by neutralizing the elevator. It has no apparent vices.
This is a good kit of a very attractive aircraft. The instructions are excellent, and the design is reasonably easy to build. For an electric conversion you should plan to replace any heavy balsa sheet. A better conversion could be done by replacing most of the lite-ply parts with balsa. This would be big job, since some of these parts have complex shapes, but it would probably bring the weight down by over eight ounces. It has excellent flight characteristics including moderately advanced aerobatics, just like the full-size RV-4. The first flight was so impressive that I have begun final detailing and painting of the plastic parts. I plan to enter this plane in some local scale contests and at the DEAF fly-in this fall.
Overall, a simple and highly recommended conversion project!
|Dec 01, 2008, 10:04 PM|
USA, NY, Buffalo
Joined Jan 2003
Thanks for the video on your RV-4. What is the flight time on the 6s 5000 pack? Is it 6s-1p (no parallel pack) configuration you are using? How much throttle do you need to keep her in the air for a nice cruise around? I'm interested in the 1/4 arf for electric but there is not much discussion on this one.
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