Warbirds-RC.com / HET-RC ARF Skyray review - RC Groups

Warbirds-RC.com / HET-RC ARF Skyray review

Chris True reviews the Warbirds-RC.com-distributed HET-RC ARF Skyray review...and provides a detailed -- and positive -- comparison to its direct predecessor -- the RBC skyray kit!

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Introduction


Wingspan:25.5"
Wing Area:310 sq. in.
Weight:48 oz. with 10 1950 cells
Length:32"
Wing Loading:22.2 oz/sq. ft.
Servos:Hitec HS-55
Transmitter:Futaba 9C
Receiver:Hitec Super Slim 8 channel
Battery:10 Sanyo 1950, 12 Sanyo 1950, 3S2P Thunder Power 4.2 AH
Motor:Hacker B40-8L
ESC:Hacker Master Series 70 amp Opto
Manufacturer:HET of Holland
Available From:Warbirds-RC

Background

The Douglas F4D Skyray (later redesignated F-6) was the first carrier-based fighter to break the world's absolute speed record and was the first Navy fighter capable of exceeding Mach 1 in level flight. Although it served with the Navy for only a short time and never fired its weapons in anger, it is still fondly remembered as an extremely attractive airplane. The Skyray also has the distinction of being the only Navy interceptor ever to be assigned to the North American Air Defense command, serving with VFAW-3, an all- weather interceptor unit based at NAS North Island, San Diego. The Douglas F4D Skyray was the result of a 1947 Navy design competition for a new aircraft based on delta wing planforms. Analysis of the studies carried out by Dr. Alexander Lippisch on delta winged aircraft in Germany during the war attracted a considerable amount of interest on the part of both the US Army Air Forces and the US Navy, since such aircraft offered particular promise in the design of fast-climbing interceptors.

Several manufacturers submitted proposals for interceptor aircraft to the Navy that embodied a delta-winged format. On June 17, 1947, the US Navy Bureau of Aeronautics (BuAer) selected the Douglas proposal as being the best of the lot, and a contract was awarded for a preliminary design study. Douglas engineers E. H. Heinemann and C. S. Kennedy directed the preliminary design study. Very early on, the pure delta planform was abandoned in favor of a tailless aircraft with a pair of mid-mounted, thin, low aspect-ratio, highly-sweptback wings with round tips. The single turbojet was fed by a set of lateral air intakes located in the wing roots. The deepened fuselage provided additional space for fuel, whereas the wing itself had leading edge slats and trailing-edge elevons. A set of trimmers were installed on the inboard trailing edges of the wing next to the jet engine exhaust. These trimmers were to be locked full-up on takeoff to provide a strong nose-up moment, leaving the larger outboard elevons in their most-effective neutral position. The outer wing panels folded hydraulically for carrier stowage. The tricycle undercarriage had to be supplemented by a small retractable tailwheel mounted under the rear fuselage because of the aircraft's high angle of incidence during takeoff and landing. The tailhook extended from the base of the tailwheel assembly. The armament was to be four 20-mm cannon, the guns being mounted in the wing roots just outboard of the main wheels, their muzzles being located slightly below and behind the wing leading edges. The cockpit was situated well forward in the nose and was equipped with a Douglas- designed ejector seat. Source - http://home.att.net/~jbaugher1/f6_2.html

The HET ARF's Predecessor -- the RBC Skyray Kit

There has been a Mini-fan 480 sized F4D on the market for several years. Produced by RBC in Holland it builds into an excellent representation of the original. The RBC Skyray was one of the very first scale EDF models to have good performance right out of the box. I first saw one fly several years ago, built by Mr. Jim Ryan of Cincinnati Ohio, it was powered by a HP 200-20-6 motor, 10 800AR cells and the Wemotec Mini-fan 480. That power plant is in the 200-250 watt class and is about the minimum required for decent performance, more is always better! These days with brushless motors and Sanyo HR1950 cells or even the latest 10C lithium polymer cells 300 to 350 watts will provide a very comfortable performance level, 400+ will make the Skyray really sparkle. I think quite a few RBC Skyrays have been sold over the years but they do not often show up at flying events. The reason I think is the building complexity. The balsa construction kit is really only suitable for fairly advanced builders. Personally, I picked up a kit a couple years ago and only recently completed it! The instructions provided are minimal and the airframe is quite complex.

Enter the HET Skyray ARF

Enter the HET Skyray ARF, the subject of this review. HET is co-operating with the RBC company to bring us an ARF version of the classic RBC kit. Markos at www.Warbirds-RC.com is importing this almost ready to fly kit, and provided it along with a Mini-fan 480 for this E-Zone review. Since I only recently also built the full balsa construction kit version from RBC I will include pictures of both, commentary on differences if any and comparisons of weight and flying characteristics. Since the ARF is largely based on the construction kit presumably any difference will be minimal.

Kit Contents

The kit arrived nicely packed in a plain brown box with “HET” High End Technology Skyray ARF printed on the box top. Opening the box revealed a two level packing system. The top layer contained two ready to fit covered wing panels, a rudder / fuselage top spine, plywood former to support the thrust cone and 2 wire pushrods. The airframe components were all tightly wrapped in poly bags and each of the parts was securely taped to the cardboard divider. Removing the divider revealed the remainder of the components – the fuselage again pre-covered, the poster board thrust tube already formed into a tube, and the bare balsa canopy frame loosely fitted into the un-cut unpainted canopy. The fuse and canopy assemblies were again wrapped in poly bags and taped in place. Additional hardware included were sheet metal screws, washers, tow hook, control horns, machine screws and clevises. The hardware was strewn throughout the box, sometimes trapped in place by tape securing other items in place. In my opinion the one packaging improvement needed would be to place the hardware in a bag rather than just throwing it in the box.

Construction

Since my ARF kit was one of the first to arrive in the US there were no instructions provided. Since then a construction thread has been posted on the E-Zone which is very detailed, with dozens of pictures and descriptions of each step. In July at the Mid-America Electric Fly event in Michigan one of the vendors had a couple HET Skyrays for sale and I noticed that the richly illustrated internet construction thread is now included in the kit. I will provide a description of the construction sequence that I used.

My first step after removing all the components from the box and unwrapping from the poly bags was to prepare the servos for installation in the wing panel. I used Hitec HS-55 servo’s for elevon control. The wings have thick plastic servo covers with the servo arm clearance slot precut, I secured these plates with a sheet metal screw in each corner. I removed the four screws and, using sand paper or a permagrit tool, roughed up the underside. I also roughed up the side of the servo and secured each servo to the cover plate with CA. Using a screw driver, I knocked the foam out as required for the servo wire to pass from the servo pocket (pre-cut) through the clearance hole in the root rib. Once both servos were installed I attached the elevons with the included Ca hinges then added the control horns and hooked up each surface with the included 2mm wire rod, clevis on the elevon and a “Z” bend on the servo end. When gluing the elevon hinges in place, I spaced the control surface away from the wing by a slight amount to insure enough throw in the down direction – about a .5 mm gap was plenty.

Time to attach the wing panels. The panels were attached with a locating dowel front and back and a 1/8” plywood spar. First I test fit both panels and made sure the incidence was the same for both. My right panel looked good, it lined up with the fuselage section well. The left panel’s trailing edge did not line up well at all, the wing was at a degree or two negative incidence to the other panel! To check whether the error was in the fuselage or the wing I placed the fuselage on a flat surface and tilted it over to one side until the front area of the root airfoil contacted the surface. At that point I measured the distance from the bench top to the top of the airfoil at the rear. Tilting the fuselage over in the other direction and measuring the similar point revealed that the fuselage root ribs were straight and not twisted. So, to fix the problem I opened up the rear dowel hole on the left wing panel until it would sit at the same angle as the right panel. I attached each panel one at a time with 5 minute epoxy. I used a paper towel soaked in rubbing alcohol to clean up any epoxy that oozes out. I used short bits of masking tape to hold things in place until the glue cured.

I chose to install my receiver in the rear of the fuselage. The factory provided pull strings to route servo wires from the back to the front as well as power wires for the ESC. Since my receiver was going in the rear I removed the red strings which went from the wing root area to the cockpit. The white strings in the center section on each side of the fan were used to pull a 13 gauge power wire on each side. When doing this be advised that the holes are not overly large. I taped the power wire to the string with thin scotch tape then clipped the end of the wire at 45 degrees, to help it self guide through each hole as I pulled from the rear.

To install the fin I first carefully removed the ultra coat covering on the underside of the fin / spine assembly to ensure good glue adhesion. I used the cockpit frame to locate where the spine needed to be. After carefully centering it and temporarily taping in place I placed strips of masking tape on the fuselage to mark the cutting edge, remove the spine, cut the covering on the line and removed the covering from the fuselage where the glue needed to go. The fit of the spine to the fuselage was pretty good in all places except at the rear where the plastic molded section is. I used thick Ca on the front 90% then later made a dam from masking tape and poured some 5 minute epoxy into the joint to secure the rear to the molded plastic section.

The one area of the kit which required some actual modeling skill was the fitting of the cockpit frame and canopy. I painted my cockpit frame first with Nelson Hobbies water based paint, added a pilot by Steve's Jets then glued the frame to the cockpit after cutting away most of the excess plastic at the front, rear and bottom. The sides were then trimmed down exactly with the bottom of the frame at the front and the rear area trimmed a bit at a time until it finally drops into place. I used 3M fine line plastic tape to mask the windows, filling in the field with regular paper masking tape.

Finishing The Skyray

The marking scheme I used for my Skyray is the same one I used on the kit version – a few miles down the road from my house is the museum associated with the Patuxent River Naval Air Station. There is an F4D sitting outside which served for many years as a steed for the test pilot school. At the time of the initial release Markos had no markings for the F4D other than a standard set of stars-n-bars by Prism. I had used the Prism sticky back decals on the kit version. To fill out the marking I used a US Navy Long Beach TrueType font to create the lettering and numbers. I printed these on clear Avery label stock, cut out and applied to the model. Of course plain grey was a little plain! I masked the iron on covering again using 3M fine line tape for the color breaks and a combination of paper masking tape and sheets of typing paper for the field area. I painted the boat tail, elevons and rudder area with “Classic White” Model Master Spray. The orange fin and tips were done using “Go Mango”, again by Model Master in a spray can. The wing walks were brushed in testors flat black. Once all that was done I added the orange and black to the nose area. In all cases I first scuffed the covering with medium steel wool then wiped it down lightly with lacquer thinner to ensure good adhesion.

Power System

I used a Hacker B40-8L in my ARF Skyray for power. At various times I have used 3 different battery packs: 10 cell Sanyo 1950, 12 cell Sanyo 1950 and a 3S2P pack of Thunder Power 2100 mah Lipo cells which gives a total capacity of 4.2 amp hours. The 10 cell pack of Sanyo’s gives right around 400 watts and flies the model in bungee launch format extremely well. 12 Sanyo 1950’s gives a little over 600 watts when used hot -- I used this pack when the fixed landing gear is attached. Speed is about the same between the 400 watt bungee launch and 600 watt fixed gear formats due to the extra drag of the 3 wheels. Vertical performance of the fixed gear setup is a bit better than the 10 cell non-wheeled format. Flight times on Nimh’s is in the range of 2.5 – 3.5 minutes depending on throttle usage. On 10 cells you can fly 2.5 minutes at full throttle then line up to land. On 12 cells the current draw is a touch over 50 amps, the make 2.5 minutes requires a few circuits at half throttle somewhere in the flight. Don’t worry, it flies just fine at ½ throttle! In fact, you might want to fly most of the time at half throttle, it’s a bullet at full throttle.

Later I tried the 3S2P Thunder Power 2100 mah cell pack. These cells are the long skinny ones sold for speed 400 usage mainly and are rated at 10-12C. 2.1*2*12 = 50.4 amps so there is plenty of current rating there for EDF usage. A quick wattmeter check showed current in the low 40's to start with watts over 400 then a bit under 400. After 30 seconds or so if I back off the throttle then blip it up to full again I saw 450 fading to 425 then a while later 470 fading to 440ish. As the cells warm up the resistance goes down and they output more amps. Most importantly this pack is very light! 10.5 ounces vs. 14 for a 10 cell pack of 1950’s or 16.8 for a 12 cell pack. This pack will fly the plane in either bungee launch or fixed gear format. With fixed gear the lightness of the pack compared with 12 cells compensates for a lot of the lower watt output. Flight times are very good on this pack, with no particular effort to manage the throttle I got 6 minutes in the air. After the 6 minute flight the pack was at 135-140 degrees and took 3.8 amp hours to recharge.

Throws and CG

I balanced the plane 100mm from the front of the wing at the wing / fuselage joint. This probably will require an ounce or two of lead in the tail in most installations, unless the motor is on the heavy side. I set the aileron throws to 3mm each way on high rate and 2mm (Yes, only 2mm!) on low rate – DEFINITELY test fly on LOW RATES. I set the elevator throw to 6mm each way on high rate and 4mm on low rate. Added 30% expo on both functions. The elevons were trimmed with approximately 1/16” up trim at neutral.

Setup & Flying

The Skyray must be bungee launched since there is nothing to hold on to on the bottom of the fuselage. Using a minimum of 10 pounds of pull, 15 is safer, I set the plane on a launch ramp with a 10 to 15 degree angle upwards. Holding full up elevator, I stepped on the release. Once it was in the air, the up elevon could be released and the model trimmed. I worked on trimming the ailerons first, as the elevon trim changed a bit with speed and I was gaining speed through the first circuit. If the model isn’t accelerating, I knew I was holding too much up!

Talking Through a Basic Flight

In level flight the Skyray is quite a little bullet. Depending on power it will do something over 80 mph, with 400+ watts it’ll go a lot faster than 80! With the 2mm aileron throw the roll rate should be very comfortable. Yes, you can get it to rotate at 3 or more revs per second with more throw but get comfortable with the plane first, it is small and gets smaller in a hurry. After 2 minutes, throttle back to half and fly a wide circuit. Entering the 180 turn to upwind, cut the throttle to zero and pull moderately hard on the elevator which will loose some speed. On final hold a bit of up to keep the nose above the horizon then add a little power as needed to control the decent rate. If you are out of electrons keep the nose level and only bring it up as you cross the fence. The Skyray slides nicely on grass, I haven’t managed to flip or tumble it yet on landing. Overall it is a very solid handling plane with no vices.

Fixed Landing Gear

A couple of times a year I go to events with paved runways and having landing gear on the Skyray would be a big plus. I made my gear from a Dubro antenna mount drilled out to accept a 3/32” wire leg for the nose gear. For the mains I used bits of ply, balsa and maple to make mounting blocks which accept 3/32” bent wire legs. The wheels are a 1.5” Hangar 9 Prolite on the nose and 1.75” on the mains. Nose steering was initially a Hitec HS-50 but the gears in this proved insufficient so it was replaced by an HS-81 metal geared servo.

Differences between the kit and ARF version

So, what do you give up by going ARF instead of doing a full build? Not much it turns out!

The fuselage section including air inlets, cockpit area and fin are straight from the RBC kit – with almost all the work done for you! The wing panels while the same outline and airfoil as the original kit switch construction methods from the RBC kit’s built up balsa and ply format to a balsa sheeted foam core format. All the airframe components are pre-covered at the factory in light grey Ultracoat. The covering job is nearly flawless, my kit had a grand total of 2 small bubbles. For those familiar with the original kit, the curvy back end of the fuselage is still vacuum formed plastic. This boat tail fairing is glued in place at the factory and smoothly painted to match the Ultracoat. I must say, the factory did a better job on that fairing than I managed on my kit, there is no filler and it matches the fuselage contours nicely with a good but not perfect transition from the balsa section to the plastic molding. When I did it on the kit version my match was not nearly as good until I added a bunch of filler.

My kit version is approximately 5 ounces lighter than the ARF version. MOST of this is due to equipment differences and the addition of fixed landing gear hard-points which you may or may not do. The real difference in airframe weight is only about 2 ounces. The kit version is glassed and has a painted finish, the ARF version is film covered with paint added for the non-grey areas. As far as flying characteristics go there is no difference at all, both planes handle exactly the same. Cost wise the kit version is actually more expensive if you add the cost of covering and additional adhesives, sandpaper etc. The ARF Skyray is a real bargain when you look at the cost of the kit it is based on!

Conclusions

The HET ARF Skyray builds into a wonderful representation of the Douglas F4D Skyray. Power options are broad with brushed 200 watt systems usable on the low end and 600 watt brushless systems providing truly exhilarating performance of the high end. Battery choices are equally broad with the inexpensive Sanyo 1950 cell being the low end choice up to the excellent Thunder Power 3S2P generation 2 4.2 AH pack on the high end. Construction is quick and painless. The Skyray has truly neutral handling characteristics which means the pilot should have quite a bit of high performance aileron experience before attempting this plane. The plane is not at all difficult to fly but if you put it in a less than level upright attitude it will hold that all the way to impact with the ground – and it gets there quickly! Build the kit, check the wing incidence carefully, make sure you use a small amount of aileron throw (2mm each way) to start with, and you will have a superb flying scale EDF model.

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Oct 01, 2004, 05:41 AM
luc
luc
I plant balsa sticks too
luc's Avatar
Hi Chris
nice test!!
I noticed on mine that in flight , high speed, it is a bit unstable (wings banking left and right slightly), even it tailerons do not move at all. Did you experience that kind of wing movement, or is something wrong on mine?
Oct 11, 2004, 08:19 PM
Registered User
In calm air mine is pretty stable - not perfect but real close. In bumpy air it tends to dance in yaw/roll more. My kit version is better, very stable in roll in the calm - just the slightest amount of dance in bouncy conditions. The only real difference between the two is the kit version has one of those base loaded fine wire wound around a sraw antenna's glued to the trailing edge of the fin. The ARF has a trailing wire antenna.

I've since read of some 30% Imac gluing round straws or those blastic binding things you use to hold a few sheets of notebook paper in book form to the TE of the fin to stop wiggles in yaw. Also, check that your motor wires are not lined up at an angle in the duct - this is REALLY BAD on a highly swept delta!!! The vectored side thrust will induce yaw which then results in roll due to the big dihedral effect of a lot of sweep. Then the fin pulls it straight again and the cycle repeats...

Of course check servo's for good nuetral and all pivot points / clevises etc. for a free fit but no slop.


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