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Introduction
In the realm of model aircraft, there are a small group of electric sailplanes that are designed to push the limits of pilot skill, driveline strength, and aerodynamic efficiency to their absolute limits. These sailplanes are usually known as F5X models, as a nod to the fact that most are designed to conform to the FAI (International Aviation Federation) rulesets for F5B (16 cell for 2006, 7 in a provisional class) or F5F (10 cell, provisional) competition. F5X competition models must be light, strong, and clean in order to be competitive, and as such they make fantastic additions to any sport or competitive flier’s hangar. Prospective owners need only be aware that these models will do only what is asked of them - no more, no less (no self-correction, for example) - so such persons would be well-advised to have a number of models under their belt before owning such a model. For the experienced pilot, however, they are fantastic tools - smooth, stable, and fast. The Zoom F5B is the latest release from Baudis Model Products, who are already well-known for their Banana (slope/electric) and Trinity F3B/F models. Distributed in North America by the wonderful folks at SoaringUSA.com, the Zoom is a 1.8m hollow-molded model. It incorporates a thinned version of the MG06 airfoil – a proven choice, in that the F3F slope racing speed record is currently held by a model sporting a version of this airfoil. The wing has generously-sized live-hinged flaps and ailerons which represent 30% of the wing chord (dictated by the design of the airfoil). The fuselage will accept any current F5B or F5F drive system, which most commonly consist of a geared brushless motor and 7-27 cells. The wing incorporates a molded recess to allow for easy placement of large speed controls or “hump-style” battery packs, which is a thoughtful and appreciated touch. It is offered in two versions, a kevlar and carbon “Race” layup, and a carbon and fibreglass “Sport” variation. The Sport version, which is the subject of this review, has a fibreglass fuselage and tail group with generous carbon fibre reinforcement in all the critical areas. The wing and elevator are hollow-molded, and both use balsa as the core material. The wing spar uses carbon rovings as the cap material, and a carbon/balsa shear web in between. No firewall or hardware (other than metric wing and tail bolts) is provided with the model, as the manufacturer feels that each pilot likely has their own methods for installing their own gear.
In order to power the Zoom, Aveox kindly provided their latest F10-series motor – the F10CS. Matched with an Aveox SH96 speed controller (100A continuous, 125 peak), this system is perfectly suited for the Zoom. The motor incorporates an integral 3.7:1 gearbox as part of the case, keeping driveline weight to the absolute minimum. This motor is most suited to operation on 10 cells, and so power was provided from 10 2400mah sub-C Ni-Cad cells, and thrust from a Graupner 16x13 folding propeller.
Kit Contents
After ordering the Zoom, I was pleasantly surprised to get both a follow-up call AND email from SoaringUSA.com to confirm that the model had shipped out successfully. While most of the time companies will acknowledge that they have received an order, in my experience it is fairly rare to get unsolicited follow-ups like this. Big thumbs up to SoaringUSA – thanks folks! When the model arrived, I was relieved to find that the box had suffered no damage in transit (always an issue coming across the Canada/US border...plus this is a big box), and that the contents were in perfect shape. The model had been packed very nicely in bubble wrap, secured with strapping tape, and had generous amounts of bunched paper added to the box to prevent the contents from moving around.
After eagerly unpacking the box, I was impressed by the build quality that was clearly evident. The surface finish was very good, with no warps or bows evident on quick inspection. The model came largely pre-finished, with pre-finished wing and tail mounts. Since this was one of the first two examples to reach North America, so instructions were available when the model was sent, so I was on my own to complete build sequence and model setup. Metric metal wing bolts were provided to secure the wing and elevator, identical in both locations. While the metal bolts should be used in the wing, some weight savings would be possible by switching the elevator bolts to nylon, or at least trimming them slightly with a cut-off wheel. As mentioned previously, no firewall was installed or supplied with the model. The model was supplied without any hardware, save the aforementioned bolts. Light fibreglass wing servo covers were provided, but they were flat rather than the more-typical bulged style of cover to conceal control surface linkages. These covers were workable with a small cutout for the servo control horn to pass through, but did not offer the aerodynamic efficiency or landing protection that a bulged cover would. More about this concern in the build section. One unusual feature of the model was that the trailing edge of the vertical fin was left open (i.e. the edges were not bonded). While I was initially annoyed at the prospect of having to ensure that this edge was secured properly, it proved very useful for properly installing the elevator servo and linkage.
After assembling the model using the provided bolts and allen key (a thoughtful addition), I was dismayed to notice that the elevator didn’t seem to want to sit at the proper 90 degree angle to the fin. The vertical fin seemed to be square to the wing, but the stab platform was clearly misaligned, and would have to be fixed during construction. The following photo shows the misalignment.
Equipment Installation and Finishing
The steps required to finish the Zoom are relatively few in number, but they do require some careful work to ensure that the finished model is strong and light. Since this version of the Zoom is marketed as the sport version, I chose to approach the install in a manner that most people would – a solid install, with no off-the-wall attempts to save a gram here or there.
Wing
I chose to start with the wing first, since servo installation and making up a wiring harness for the wing represented the majority of the work ahead.
Servo MountingSince the wing is very thin even at the root, only the smallest servos will work without bulging out appreciably. While Dymond D60 servos are very popular with a lot of European F5X pilots, I chose to install Volz WingMaxx HP servos in all four pockets in the Zoom. These servos are the finest quality units I have ever worked with, and provide solid torque and quick transit times in a very thin package. (Beware of using them on 5 cells however – I would not recommend that practice, having melted a processor board in a Volz servo in my F3B sailplane while running 5 cells. Four cell operation is not noticeably slower, and current draw and life expectancy are both increased on 4 cells.) Finally, these servos have more than enough torque to drive the surfaces on the Zoom, and center with the utmost precision. Contact Icare Sailplanes if you are interested in purchasing Volz servos. Even given the compact nature of the Volz units, installation of these servos in the wing was not going to be easy. There aws barely enough depth at the flap location to conceal the servos, and it was impossible to do a completely flush install at the aileron due to the reduced thickness. Even tucking the servo under the wing skin would not completely conceal the aileron servo, so I was resigned to molding a custom cover or living with a slight bulge with the stock cover. My preferred method of servo installation in molded aircraft involves wrapping servos in masking tape or battery heat-shrink material, and then directly epoxying them to the upper wing skins. Since there was no local carbon reinforcement of the servo cut-out from the factory, my first task was to install a patch on the upper skin that would prevent air loads from puckering or bending the skin in flight. The material I chose to use was 4.7 oz. uni-web, which is made from carbon tow which has been loosely stitched together with a bonding web. The material comes in 12” wide rolls from a local supplier (Alberta Plastics and Paint), and is also available from Aerospace Composites Products – abeit at a substantially increased cost. After cutting patches that were about a 1/2" wider all around than the servo opening, I proceeded to apply masking tape to the lower wing skin to protect it from any stray epoxy. I then mixed up a small batch of West Systems 205/106 laminating epoxy, wet out the patches, squeegeed out the excess resin, and lightly pressed them into place. I chose the West System resin due to the quality of the product, but also because the slow cure (16-24 hours at room temp) would not inadvertently distort the wing skin while curing. 30 min hobby epoxy would also work, but I always stay away from the quicker epoxies due to their exothermic (heat generating) curing cycles.
Wiring HarnessThe next task that was required is my least-favorite of all – soldering together the wiring harness for the wing. In order to save a bit of weight, I elected to go with standard servo wire instead of the heavy-duty wiring that I usually use in four-servo wings. (I made sure to remember to give the leads at least one twist per inch to prevent radio problems). While SoaringUSA was nice enough to include a 6-pin green Multiplex connector for use in the wing, that would have required me to splice all four positive and negative power leads onto a single pin for each polarity, which didn’t seem like a safe thing to do. Instead, I chose to use a 9 pin DIN connector, which only required a common positive and negative for each wing half. Therefore, I made up two leads for the wing servos – one per half – with individual signal leads, and a common positive and negative for each pair of wing servos. That would leave a single remaining pin for if I wanted to run my antenna into the wing – a possible option for the Sport version given that the only carbon in this wing is around the saddle and in the spar. The next problem was deciding where to lead the servo wires out of the molded wing saddle. There was a recess built into the wing to allow for a hump-style battery pack to sit up into the wing, which was basically in the way of where I wanted to fit my servo connector. Since I didn’t plan to use a hump pack, I decide to simply lead the wires out into this opening, and drilled a hole in the side of the inside skin to allow the wires to pass out of the wing. After threading the harness appropriately and connecting the small white Volz WMS micro-plugs to their respective servos, I wrapped the WingMaxx’s in a single layer of masking tape (joint in the wrap under the servo!). If you’re using different servos and have to clip the leads to solder them in place, be sure to test them beforehand. Next, I mixed up some fifteen minute epoxy, coated the bottom of the servos with a thin layer, and tucked them into place. When epoxying the servos in place, I made sure that the servo horn was mounted at 90 degrees to the control surface hingeline.
The last wiring tasks that were required were to solder the DB9 plug into place in the wing, and to make up an appropriate extension for the part which would remain in the fuselage. Ultimately, the fuselage harness was terminated in four standard JR-type servo plugs.
LinkagesAfter finishing the wiring, it was time to make up some linkages for the model. For horns I used some cut-down carbon plate horns from Whyte Wings Products, and for linkages I used 2-56 all-threaded rod and a set of metal clevises. On each linkage, one clevis was CA'ed or soldered in place, and the other used for adjustment of the linkage length. This isn’t the ultimate in lightness, but it’s very strong, and nicely adjustable. After drilling some new holes and trimming the Whyte Wings horns to size with a cut-off wheel (wear a dust mask!), I applied some masking tape to the lower wing surface and marked where the cut-out would need to be in the lower control surface wing skin. The skin was cut with a new, sharp Exacto blade, but I had to carefully grind out the aileron and flap closeout web. The epoxy/flox “splooey” that was used to bond the web in place aws tough to remove, but this had to be done in order to bond the horn to both the upper and lower wing surfaces. Don’t worry about removing the web – the horn will easily replace the missing material if you keep the tolerances of your cutout tight.
After cutting the channels for the horns, I mixed up a small batch of 30 min epoxy. I added a strip of tape around the perimeter of the wing skin to catch excess epoxy, and used a thin stick to add some epoxy to the channel. I then grabbed my heat gun, and carefully heated the epoxy to get it to flow into the channel (don’t inadvertently warp the wing doing this!). I then added a bit more epoxy on the horns themselves, and pressed them into the wing – making sure that they stood perpendicular to the surface. Before the epoxy set up, I towelled off the excess, and removed the masking tape. If you don’t do this before the glue cures, a thin strip will be trapped under the fillet of resin, which will be more annoying than harmful. While the glue was curing fully, I made up the four linkages. With all-threaded rod, it was a simple matter of four cuts with a cut-off wheel, and a quick bit of soldering with a hot iron.
I cut small slots in the supplied flat servo covers in order to clear the linkages. While I normally secure such covers with a very thin coat of Goop, the slight bulge induced by the servos made this difficult to do. Instead, I chose to hold the covers on with thin strips of colour-matched Monokote trim sheet, which worked nicely.
Drive System
After completing the wing assembly and testing the servos, the next task was to hook up the driveline and install a firewall in the fuselage. Curiously, the Aveox 96SH speed control was supplied without motor leads attached, which lead to some initial head scratching on my part. There were six solder pads on the end of the controller opposite to the battery leads, and since they were joined into two sets of three by the tails of some electronic components, it seemed logical that this was where I was supposed to solder the motor leads. No reference to doing so was made anywhere in the manual, however, and the pads were not easy to get to with the tails sticking up in the way. In the end, I took a leap of faith and used them anyway. I soldered the motor leads to the pads horizontally to gain the biggest contact patch, and used a hot iron to allow the tinned motor leads to “melt” into the pads without putting too much heat onto the circuit board. Even after being fairly confident that what I was doing was correct, I was much relieved when the motor fired to life properly!
I soldered some 4mm gold plugs on the battery side of things. These seem to be the standard for high-current applications, and have served me well in the past. I was tempted to try a dual-plug system (2 positive and 2 negative wires to the battery, with each side carrying half the total current) for the decreased electrical resistance, but in the end decided against it since the anticipated current load (120A) wasn’t quite high enough to justify the effort. Anyone running a true competition setup would be well advised to double-wire their packs, however. I initially left the wires long, since I didn’t know where the motor pack would sit to balance the model.
Fuselage
Since no firewall was supplied with the kit, I decided to make my own out of some old circuit board material. After measuring the required bolt pattern for the Aveox F10CS motor, I drafted a quick sketch of the required firewall in AutoCAD and printed the pattern at full-size. I then tacked the pattern to the circuit board, drilled the motor mounting holes, rough-cut the firewall on my bandsaw, and finished with a small belt sander.
Not having much of a reference otherwise, I guessed that the thrust line was likely to be parallel to the long axis of the fuselage (ie, no down or side thrust) like most other high-speed aircraft. Since the nose of the fuselage seemed to be cut to the correct angle, I decided to use the nose to set the thrust line by holding the firewall in place with the spinner. That was accomplished by screwing the motor to the firewall, lightly sliding the assembly in place in the fuselage, and then tightening the spinner on the motor shaft. Since the provided 36mm “turbo-style” spinner was collet based, tightening the collet actually pulled it very slightly backwards towards the motor, which pulled the firewall into place. Friction then held the firewall in position, and I carefully removed the motor and tacked the firewall in place using some light drops of medium CA. (It was important to not get glue in the gearbox, and to scuff the edges of the firewall and the inside of the fuselage with sandpaper first).
After tacking the firewall in place, I mixed up a batch of 30 minute epoxy and cotton flox. After protecting the motor and mounting screw holes with masking tape, I applied a generous fillet of the epoxy mixture to the inside of the nose. When the epoxy had initially set-up, I removed the masking tape, and cleaned out the area around the mounting bolts (this task was much easier while the epoxy is still “green”). I bolted the motor back into place, tightened the spinner assembly back onto the motor shaft, and set it aside to cure fully.
While the firewall glue was curing, I made up a small receiver battery pack. Since 99% of all high-current speed controls are optically coupled rather than having a BEC (battery eliminator circuit), your choices to power the radio system are basically a small receiver battery, or a dedicated BEC unit like the “Ultimate BEC”. I personally like the redundancy that comes with a separate receiver battery, and so chose to make up my pack from GP650 Ni-mh cells. These are small and reasonably light cells, and provide enough capacity to last throughout the course of an average flying session. Competition packs would more likely be made up of GP370 cells or a small 2S li-poly pack with regulator. After tacking the cells side-by-side with a light coat of Goop adhesive, I soldered the cells into a circuit using small pieces of copper de-soldering braid. I then added a male servo lead to the new pack, which was strain-relieved using another small dab of Goop on the wires. I set my Infinity 2 charger to cycle the pack three times at +0.2A/-0.3A @ a 3.4V cut-off (still one of the nicest units for doing this), and went back to working on the fuselage. Actuating the elevator was the next task to be addressed in the assembly process for the Zoom. Given that balancing the model was going to be easy with a big 10 cell sub-C pack to move around, I decided to mount the elevator servo directly in the vertical fin. That would require a small servo to avoid a draggy bulge, since the fin was not particularly thick even at its base. My choice for this task was the JR 241 sub-micro servo. Although not the thinnest servo available, I chose this servo for its reputation for precision and durability. While it is also available in a digital version (the JR281), I went with the 241 for reduced current draw and price. I clipped the servo lead in half, extended the wire to reach the receiver (mounted just behind the wing), twisted the wire, and added a purple MPX toroid ring to cut down on possible radio problems (belt and suspenders I admit, but I like to be safe). Since I wanted the elevator to be removable, I decided to use a captive-tube arrangement and an L-shaped pushrod. This arrangement required a slot to be cut in the elevator, into one of half of which a short length of aluminium tube was embedded. The L-bend slid into the tube before the elevator was bolted in place, and the rod pivoted inside the tube. It’s probably harder to explain than it is to do.
After clipping the servo lugs off the 241 with a set of diagonal cutters, I marked out a small circle near the base of the fin that would just clear the servo case. After rough-cutting the light fibreglass with an Exacto blade (only difficult around the carbon reinforcement), I checked to make sure the servo would fit, then cleaned up the hole with a Dremel sanding drum (not shown yet in the photo). The elevator linkage was simply a piece of piano wire (slightly smaller than 2-56), supported by the servo on one end and a small hole in the top of the fin on the other. Given that very minimal elevator throws were required, I decided to use the smallest servo arm that came with the 241, and then even drilled a hole closer to the center than was provided on the stock arm. After clipping the other three arms off of a 4 arm cross-type horn, I cleaned up the cuts with the drum sander. After checking to make sure that the new hole I had drilled in the arm would properly clear the wire without play, I put a tight z-bend in one end of the wire using two sets of vice-grips. While I personally hate z-bends in most applications, there was no room for a clevis in the fin, and the linkage did not need to be adjustable. While z-bend pliers are commercially available, I find that they often make overly large bends (which lead to sloppy linkages), and tend to put gouges in the wire at the bend point. As a result I prefer to make them by hand. In order to prevent the exposed tail of the bend from sticking out of the fin, I bent it in a slight arc around the servo arm, so the end wrapped back into the fin. After some minor adjustments, I had a linkage that was slop-free.
I then drilled a small hole in the top of the fin where the elevator linkage would pass through, directly through the fuselage seam which ran down the center of the top of the fin. After trying in vain to get the z-bend on the servo and the L-bend on the elevator side of the linkage though the hole at the top of the fin, I ultimately decided to split the top seam from the linkage hole to the TE (remember, the TE of the fin is open). By carefully doing this with a thin blade, I was able to get the linkage in place, with no further damage to the fin. After checking to make sure the lengths were correct, I installed the elevator, wrapped the servo in tape, and epoxied it into the fin. A patch of self-adhesive Monokote trim sheet covers the servoopenining in the fin. The next task at the rear of the fuselage was to seal the trailing edge. This task must be done carefully to avoid inducing a warp, but was a small price to pay for having that much extra freedom when installing the elevator pushrod. My solution was to use my workbench as a straightedge, and tape to fin to the bench while it was curing (ensuring a straight TE). Since the TE is angled the fuse sits at a funny angle, but it was easy to support it properly with a bit of thought. So, after protecting the outside of the fuselage with a couple of strips of masking tape, I mixed up another batch of laminating resin. After scuffing the inside of the TE with sandpaper as best I could and applying some wax to the elevator pushrod, I lightly coated the inside of the TE with resin. I then taped the fuselage to my bench, and propped up the nose at the correct angle. After triple-checking that everything was still straight, I set it aside to cure.
After the trailing edge had cured, I lightly sanded the finished seam, and checked one last time that the fin was still straight (it was).
Horizontal Stabilizer
Since the fuselage was largely complete at this point, the last construction task that remained was to true the stabilizer saddle so that the stab sat at the proper angle laterally. While I initially tried to shim the low side with strips of tape, ultimately it proved easier to lightly scrape the high side of the saddle with a knife blade. It took about half an hour of careful measurement to square the elevator saddle, which I checked by securing the wing and measuring to a flat surface (pool table). Checking these angles by eye can be deceiving, since there are a number of compound curves on these models. After the receiver pack had finished its conditioning cycles, I tacked it to the fuselage side just behind the wing saddle opening using a small dab of Goop. While Velcro might have worked to hold it in place, I wanted to make sure that the pack couldn’t move. Using a light dab of the adhesive meant that the pack wasn’t going anywhere, but still remained removable if required (not a field-servicable item at that point, mind you). Being quite a bit lighter, the receiver was secured opposite the RX pack using Velcro. While I will typically use switch-jack (combination charge plug/switch in a phono-style plug) on tight installations like this, I decided to simply use a female/male servo plug connection this time instead. While I'm now required to take the wing off to turn the radio system on or off, I didn’t think it was a good idea to leave a 1000W F5B sailplane sitting on the ground with the motor pack connected but the RX turned off! Therefore, I made up a short lead with a male and female servo connector on either end (sort of a short extension cable), and lightly Gooped the female plug to the top of the fuselage at the rear of the saddle. To turn the model on, I simply plugged the male connector from the RX pack into this female plug - nice and easy.
I chose to let the receiver antenna exit out the back of the fuselage, with the excess trailing behind. If I had been in competition I might have run the antenna into the wing or used a guitar string for reduced drag, but on a sport model the trailing antenna is tolerable. I taped the antenna to the bottom of rear of the fuse as it exited as a bit of a strain relief, lest I do something silly like step on the antenna (been there I’m afraid).
Motor BatteryWith the radio installation complete, all that remained was to install the motor pack to balance the model longitudinally (no wingtip weight was needed to laterally balance the model). I initially chose to use a pack of 10 GMVis 2400 Ni-Cads with the Zoom, which I had retired from another model earlier in the year. While GP3300 Ni-Mh cells are the more popular choice for these models due to their higher capacity and lower internal resistance at high-temperatures, I went with the Ni-Cads in order to keep the current levels under control for the first flights. These 2400’s are still wonderful cells, and the pack had relatively few cycles on it. The 2400’s are available from Icare Sailplanes, and are quite a bit less expensive than the GP3300’s at the moment. In order to provide some measure of shock-absorbancy, the front of the battery pack was held in place with bricks of EPP foam. This foam is resilient and fairly temperature stable, and will help to prevent damage to the motor in the event of a hard landing (in a crash, all bets are off). One block went between the motor endbell and the pack, and another on top of the cells to prevent them from moving upwards. The rear of the pack was held in place by a small spruce tab, which I securely epoxied to the bottom of the fuselage. While a slight c/g shift forward isn’t the end of the world, having the pack slide backwards would be disasterous – hence the solid battery restraint there.
Having no control throws or c/g to work with, I balanced the model on the wing bolts (about 30% MAC), and made sure everthing was secure by violently shaking the model. After bolting the prop in place and putting a couple of cycles on the motor pack to make sure it was still up to snuff, I was ready to head to the field.
Flying
Initial Flights
The initial flights on the Zoom were done with the center of gravity well forward of where I guessed it should go, mainly to ensure that I had some margin of safety in case my guess was wrong. That put the initial balance point under the wing bolts. After running some numbers on the handy virtual motor test stand on the Aveox website, I determined that the F10CS would likely draw around 120A on 10 cells with a 16x13 propeller. This was on the upper end of the recommended burst range for the controller, but should still have been safe. When I tried the motor on a pack that had been charged the night before, however, I was getting readings of between 130-140A! When the controller seemed fine after the tests, I decided to try to fly the model anyway, even with the current well above the recommended levels. I range checked the model at the field from a variety of angles, and everything checked out fine. After topping off the motor pack, I enlisted the help of fellow pilot Ryan van Beurden for launching honours, just so I could have my hands on the sticks in case my c/g guess or control throws were off. After a nice level throw, the model accelerated strongly, and I proceeded to get a feel for how it was flying. Right away I could tell that the c/g was way too far forward, since fast level flight required a healthy amount of down elevator compared to thermal speeds. Additionally, the elevator response felt sluggish even with moderate throws, which is usually the best indication of a forward c/g. Stall tests were tricky to do, since I didn’t have enough elevator throw to get the model to stall in slow flight – it simply mushed ahead without dropping a wing. I didn’t need to touch the trims at all, which confirmed that the model didn’t have any warps in the structure. My biggest problem for the first flights was visibility. The test day was fairly cloudy, and not being used to the model’s silhouette, I had trouble tracking it when the top of the aircraft was showing. The white upper surfaces of the wing meant that I usually saw the “Zoom” logo on the right wing, the prop, and the LE of the tail (for whatever reason). After I established a bank angle or heading, I had to trust that the model would stay where it was pointed until I saw the underside again (which was easy to see). I went home thinking that stripes on the top of the wing would be a good addition, but decided to try the model against a blue sky before changing anything. Landing was very easy, which was a nice plus. The combination of flap and spoileron was very effective in controlling the decent rate, though with the forward c/g the flaps didn’t affect the model’s speed as much as I’d hoped. I landed well short of where I wanted to, but safely on the field – and in one piece. The drive components were warm but not hot, and the speed control didn’t seem to be affected in any negative way. I like it when the first flight of a new model goes so well! Setting the c/g is always the first trimming task that should be done, and it took a considerable amount of time. My highly technical method involved taping 1/4 oz lead tire balancing weights to the tail in order to bump the c/g back, which were added on the upper spine of the fuselage just in front of the t-tail. The aerodynamics of the model didn't seem to be particularly affected by this procedure, since at one point I was flying with 2 oz (eight 1/4 oz sections!) of weight back there! My final c/g ended up at about 46% of the root chord (way off of the initial 30% mark), at which point the model was neutral in pitch at all airspeeds, and inverted flight required minimal elevator trim. The elevator was noticably more responsive at this setting, so I reduced the elevator throw accordingly. The stabilizer required shimming at this c/g setting, since noticable down-trim was present in the elevator now. Note that all control settings can be found in a summary table at the bottom of this article.
Flying the Zoom
Having had a few months to get used to the model now, I feel I have a better idea of its performance potential. It is a wonderful aircraft to fly -- fast, stable, and responsive to all the flight controls. The stall is really quite benign on the Zoom, even with the c/g moved back to the neutral point. If the wings are level at stall speed, the break is straight ahead, with a moderate loss of altitude (it does take a while to start flying again - mostly due to wing loading). When slowed up too much in a banked thermal turn, the Zoom will fall into the inboard tip, but the stall does not develop into a spin. Again, a fair amount of altitude will be lost when this occurs. As a tie-in to the above, the Zoom is happiest when flown fast. The airfoil will slow down quite nicely (especially with camber), but the glide ratio is best when the airspeed is kept to a fast cruise. Large swaths of sky can then be covered looking for lift, or while performing distance runs. If slowed down too much (coming off power too soon after a climb being the best example), the sink rate of the model skyrockets, but all control surfaces remain effective right up to the stall. While I have since become used to tracking the profile of the model, I did modify the standard colour scheme slightly by adding a few red stripes to the left wing (Monokote trim sheet matches the colour exactly, funnily enough). I found that the extra bit of red helped to see the left wingtip more clearly against the clouds, where before only the red "Zoom" logo on the right wing was visible.
LaunchWhile having someone else launch the model is definitely safer than launching yourself for the first few flights (assuming you have competent help), I quickly became used to launching the model on my own. Launching with the 16x13 prop is simply a matter of a firm level push, after which the model is flying immediately. Throw it appreciably nose-up and you invite a stall. Switching to a square (16x16) or over-square (16x17) prop would likely have made the launch a little more tricky. Even then, however, a firm level toss is always the correct way to launch these models. While I never used such a prop with this model, since doing so would have dramatically exceeded the current limits of the speed control, I have a couple of years experience launching Ryan’s Ariane V6 (about the same size). Most square props are stalled at launch, which means that the model takes longer to accelerate before the prop starts to “bite”. The firm throw helps there. The 16x13 prop on the Zoom is not stalled at launch, which really makes the throw a bit of a non-issue.
High-Speed FlightFlying at high speed is what the model was designed to do, and what it does best! Even while getting used to the model, I was immediately comfortable with the high speed performance of the Zoom. The ailerons were responsive, the elevator smooth but very effective, and the overall structure of the model was nice and stiff (no flutter was ever encountered). On-power, the Zoom would climb at any angled requested of it. Very little time was required to take the model out of sight, so the available run-time was always used in short bursts of power (the throttle is linked to a momentary switch as a result). No thrust-line adjustments have been neccessary, since there is no power-on trim change. Since the elevator is more effective when running in the prop wash, I found it much easier to push or pull out of a vertical climb under power. That way, the model came into gliding flight with extra energy, and nothing was wasted getting the model back up to speed (the pitch speed of the 16x13 prop isn’t as high as a true F5B “square” prop). In gliding flight, significant gains in speed can be achieved by reflexing (raising) the trailing edge of the wing slightly. The MG06 carries a fair amount of camber in “clean” trim, and so the drag coefficient of the airfoil benefits from reflex. Reflex also drops the pitching moment of the airfoil quite a bit, meaning that the tail isn’t loaded quite as much, and allowing the drag of even the tail section to drop according. As a result, there is a fair amount of elevator compensation needed in reflex. Turn performance in high speed flight was shown to benefit from “snap flap” coupling, where the ailerons and flaps drop slightly in response to up elevator, and rise slightly with down elevator. Too much coupling would likely make the model hard to control and might lead to a high-speed stall, but moderate coupling allowed for nice tight turns with reduced speed loss through the turn. I personally chose to have this mixing active only when the camber slider was in reflex mode, though it could be left active all the time based on pilot preference.
Thermal Performance
The Zoom is surprisingly adept at working lift, especially for a model with this sort of wing loading. The designer can be complimented for keeping the model clean and efficient, and their efforts show up nicely here. Thermal performance will be greatly enhanced by having a neutral c/g setting, since the elevator sees less load in level flight. I personally found that changes in the pitch angle (angle of the fuselage relative to the flight path) were the easiest thermal signs to cue off of, since the tail will rise slightly upon encountering lift. Since the model has no rudder (an awkward point for me, being a long-time thermal pilot), the proper way to keep the Zoom’s speed up in thermal turns is to establish a fairly generous bank angle – 35–40 degrees wouldn’t be unreasonable. Lacking rudder, the aileron differential (more up than down) required for efficient thermal turns will be far different from that which results in a pure roll, so the Zoom is best flown with a radio having variable differential settings or flight modes. Since the MG06 was so responsive to camber changes, I found that climb performance in lift could be greatly enhanced by adding 3-4 degrees of camber after coring the thermal - more if the situation warranted. Having camber on a slider allowed me to play with this setting to account for the changes in wind speed or thermal strength, and I would recommend that Zoom owners not use a fixed amount of camber linked to a switch. Slight elevator trim was required to compensate for the increased camber (see the table at the end of the article). As in all phases of flight, it was important to remember to keep the speed up to avoid slowing the inside wingtip too far. Stalls when thermaling tend to have the model perform one turn of a spiral dive into the inside tip, which can lose a fair amount of whatever altitude was being gained. They are easy to recover from, however, since the model will recover on its own if allowed to dive to gain some speed.
Landing
While my initial flights were done with a traditional CROW or “butterfly” landing mode (ailerons up, flaps down), I soon discontinued my use of spoileron (up aileron) entirely. I found that it was too difficult to judge decent rate using both aileron and flap combined, and flaps alone were much more intuitive to use. Additionally, not raising the ailerons as brakes makes them more effective at controlling roll, since the roll rate suffers with the ailerons up. Nowadays I can usually land the model within 10 feet of myself (off to the side), which is as close as I’d care to get to an F5B model on landing. When the c/g is set properly, the Zoom will land quite slowly for a heavily loaded model. That said, it is much safer to land a little quicker, since that way you maintain extra energy to cope with sudden thermals or wind gusts. Also, especially if you fly at grass field, remember to fly the model all the way onto the ground, since the model has minimal wingtip clearance. My usual landing pattern is to set up a long approach, usually carrying about 20 percent flap. That way, you can always reduce flap to decrease the decent rate (less drag outweighs less lift), or add flap to increase the decent rate. Try not to use the flaps to control speed too much – if you find yourself consistently landing long, just setup your base leg a little further out. If you get the model too slow monkeying around with the flaps, you’re likely to suffer from a low-altitude stall (read: crash – these models need quite a bit of altitude to recover). In competition it’s also more difficult to land at the correct time if you play with flap a lot on landing, since the time to complete your landing pattern is constantly changing. Fly a consistent approach and you’ll learn to judge the correct time to turn onto final. After rolling onto final approach about 300 feet out I never touch the ailerons except to correct for gusts. Excessive use of aileron on landing invites a nasty tipstall (though I can’t say I ever experienced one with this model). Letting the model “have its own head” is also advisable since the front profile is so hard to see. If you set things up correctly, the model should basically land itself. After that point, landing on the spot at the correct time is simply a matter of practice.
Video
Conclusion
Having about 40 flights on the model to date, I am quite comfortable in providing the following control settings. These resulted in a neutrally stable model that was responsive without being twitchy, and one that was optimized to fly over a wide speed range.
From a construction standpoint, the only things that I may still go back and do are to make some proper gap seals for the wing control surfaces, to make some custom servo covers for the wing and tail servos, and to secure the DB9 plug to the wing with a small bracket of some description. Note that bugled covers are available from a variety of commercial sources, including Icare Sailplanes and Hobby Lobby. The Zoom has no bad habits, and the model is very strong. The molded airframe has held up well under normal use, and no stress cracks have developed anywhere on the model. While the spar has proven to be more than capable of supporting the 10 cell Aveox driveline, for those wanting to truly push the limits aerobatically or to use bigger 18-24 cell drive systems, the Zoom is also available in a carbon and kevlar Race version. Given the strength of my fibreglass and carbon version, the Race model should take anything anyone cares to throw at it. While the speed of the model is probably a couple of percent shy of true F5B competition numbers (the manufacturer is developing another version with a faster airfoil), the price tag is far less than that of any F5B sailplane. As well, the thermal performance is quite a bit better than a competition model, since the airfoil is biased more towards generating ample lift. Building and flying the Zoom was an enjoyable experience, and I can happily recommend the model to anyone looking for a high-performance sport sailplane. |
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Sep 10, 2004, 02:08 AM
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#2 |
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Registered User
Join Date: May 2001
Location: Laurel, MD
Posts: 12,163
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Nice review...
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Sep 10, 2004, 02:28 AM
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#3 |
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earth belowus drifting falling
Join Date: Feb 2003
Location: Victoria B.C. Canada
Posts: 1,409
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Yup, very nice review. I'm stoked to buy one now.
Well done Adam. Ron. |
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Feb 02, 2005, 06:41 PM
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#4 |
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Registered User
Join Date: Jun 2003
Location: Twin Cities, MN
Posts: 376
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Very nice review!
Great to see a hotliner build on the zone. As a hi-perf newby it was nice to see some of the depth and detail of the techniques you used. Thanks, PR |
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