I still remembered years ago when I had a model shop, always people will come in and ask for a starter package. The first thing that comes to their minds are electric helicopters. A lot of people conceived electric helicopters as silent, easy to handle, clean and small footprint. Back then, model electric have to lug a battery pack that is almost two times the weight of the airframe, and with the not so powerful brushed motors the performance of these electrics left much to desire. In the recent years, the introduction of brushless motors in r/c application, and most importantly Lithium battery packs with their very favorable power to weight ratio makes r/c electric aircrafts no more the hog as in the past. In the helicopter field, a lot of cheap, small electrics are introduced due to the brushless motors and Lithium batteries, and while some of these airframes are constructed of high quality design, a lot other cheaper introductions have very poor quality and requires lots of upgrades parts to perform. Hirobo, without missing a beat to the competition, after some years of R&D, introduce the Lepton, which is their version of high performance electric helicopter.
The Lepton line of helicopter basically is based on the so call "15 engine" size helicopter, which according to Hirobo staffs would achieve the best performance at the most compact package deemed necessary. It is targeted to 3D flying, with power comparable to a 50 class nitro engine, in a small package. The Lepton line will have different packages, and the first one to come out of the market is the Lepton EX kit, which is a full option version of the Lepton airframe minus the electronics. Below are some basic dimension of the Lepton EX:
Length: 937mm
Height: 281mm
Main rotor diameter: 955mm
Weight: 1.6Kg
Gear ratio: 3.92:1:4 or 3.61:1:4
Compatible motor: 890KV Continuous current 45A Max 60A
Compatible ESC: Max 60A
Compatible battery: Lithium polymer battery 3,200mah 4S or 3S
The Lepton EX also features the following designs:
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Magnesium main frame rail |
Magnesium main frame rail with carbon side frame |
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Simple inner construction of main frame |
Machined Delrin main gear with one way hub |
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Metal rotor head and controls |
Continuous driven tail |
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Metal tail gear box assembly |
3D flybar paddles |
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Carbon side frames |
Dual ball bearing tail rotor |
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Aerodynamic canopy |
CG located weight distribution |
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Hirobo LEX 4s 3,200mah Lipo battery |
Hirobo LEX charger |
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Hirobo LEX battery connector |
Hirobo LEX motor with cooling fan |
The list price of the Lepton EX is at 81,900 yen, which is about US$770-. At this price range, after discount from hobby shops, the Lepton EX kit pose itself at the same level of the competition's full option offering, but with others what you could not get is Hirobo's quality and technical prowness in design.
To make this Lepton EX kit flyable, I would have to get a set of brushless motor, speed control, lithium polymer battery, small servos, gyro, and a mini-receiver. Hirobo later would make available a set of brushless motor with ESC that has BEC function, and their own lithium battery pack and charger. According to Hirobo, the LEX power pack (Hirobo# 0304-002) includes a 10 turns, 890Kv, brushless, out runner style motor with cooling fan, operating at 45A and a ESC that could operate max at 60A with BEC function. The Hirobo LEX battery pack (Hirobo# 0304-001) includes a 4s 3,200mah lithium polymer battery pack and a compatible DC charger that includes automatic cell balancing functions. The list price of the LEX power pack is 51,450 yen (About US$480-) and the LEX battery pack is 44,100 yen (About US$400-). All the components of the power and battery packs are separately available.
Besides using Hirobo offering, the Lepton could accept any motor that have physical can size on not bigger than 37mm diameter and 50mm length, and a recommended KV of around 850-1,000 with the standard gear ratio. With battery pack a 3s2p or better yet a 4s2p pack is recommended, with the maximum physical size of not bigger than 160L X 50W X 37H (in mm).
For servos the Lepton requires mini servos for swashplate control, and maximum medium size servo for tail control is at 34L X 17W (in mm). Below is a chart for suitable servo usage on the Lepton EX:
| ¡@ | Futaba | JR | Sanwa |
| For swashplate control |
S3102 S3150 S3101 |
DS362 DS361 NES-341 |
SDX-762 |
| For rudder control |
S9650 S9602 S3102 S3150 S3101 |
NES-3125 DS362 DS361 NES-341 |
SRM-141HRZ SDX-762 |
At the moment of writing this webpage, the Hirobo power package is still not available, so I would only write about the assembly of the air frame for this moment and wait until I got the power package to finish up the flight performance report. Now let's take a look at the assembly of my kit.
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| Lepton EX kit box | Sticker shows what's included and what's not |
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| Sticker shows dimension | ¡@ |
The one kit that I received from Hirobo is a pre-production beta kit, but according to Hirobo it is pretty much the same as the final version that will be introduced to the market. Upon opening of the box, I see all parts are being laid out in a orderly fashion. Inside the box the first thing is the blow molded plastic canopy, with a package of decals and instructions manual, underneath it is the carbon frame and the magnesium frame rail, main blade, then on the side is a small partition that houses bags of small parts. The packaging of the Lepton EX kit is in line with other Hirobo helicopter kits, with all parts packed nicely into different bags and according to the steps of instructions. All bags are packaged in a way that when I arrived to a step of instruction I open one bag that is tagged with the step number in relation to the instruction and all the parts and screws required for that step of assembly would be included inside that bag. Due to my kit is pre-production beta version, I noticed during assembly that some bags have the step number wrong, but I am sure Hirobo would fix this minor thing in their official release kit. The instruction manual of the Lepton EX is at par with other latest Hirobo offering, not only does it have the usual highly detailed drawings on assembly, but it has a lot of in depth tips on how to setup, flying and maintaining the helicopter kit for beginners.
The first package that I open is the magnesium frame rail. This is what makes the Lepton EX so special. The rail looks like regular aluminum, but with dark gray surface treatment that has little sparkle on it shows this is not the run of the mill aluminum alloy. Holding it on the hand, try bending it, I could not help but appreciate how hard and light this frame rail is. I would imagine after the Lepton kit comes out, there will be copycats around from Asia, but while they might be able to copy the look, what they could not copy exactly is this magnesium alloy rail. According to Hirobo, this magnesium alloy rail is being produced in Japan by a big corporation that specialized in manufacturing of rare exotic metal parts. According to the design of the Lepton this rail is the foundation of the whole air frame, where the carbon side frames would be mounted on to build up the whole structure of the main frame. As a result, due to the high power of the motors/battery, if this rail is not constructed with good material that is strong enough the whole air frame would flex and even collapse.
While rummage through the various package I saw the 425mm FRP symmetrical main blade. Its airfoil is very thin, and the overall finishing is very good. On minor point, Hirobo even goes to small details to include the foam receiver pack protection and a main blade holder that is made of foam.
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| Lepton EX kit box | Inside of the box |
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| All parts neatly packed in small bags | Magnesium alloy frame rail |
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| Blade holder | 425mm FRP main blade |
The first major part of the assembly dealt with assembly of the main frame.
Since the Lepton uses ECCPM control, the mixing assembly are quite simple compared to the regular HPM control. The side point mixing arms are in plastic T shape, while the middle point is in metal. All control swashplate control points are connected to servo at push-pull to ensure precision. While assembling the two T-shape ECCPM control lever pay attention to the use of different control balls.
After the assembly of the mixing arms, then comes the construction of the whole main frame. The Lepton EX design is quite simple in that everything builds around the magnesium frame rail, with most attachments on the carbon side frames. Before assembly, like all other helicopters with carbon stack frames, I would first prepare the frames before assembly. (For a detailed description on how I assemble a carbon stack main frame, please refer to my page about "Tips on assembling a helicopter ".) Basically I would round of the sides of the carbon boards, flat off the holes, and seal the side with CA glue.
After preparing the side frames, the assembly is to have the carbon side frames encasing the magnesium rail, two main shaft bearing blocks, one block with two roller guides holding the tail belt, the shaft for front ECCPM control, and the rear plastic swash plate guide. Everything is hold together with 2.6mm hex screws. During the assembly of the main frame, I use the main shaft to align the bearing blocks.
Assembly of the main frame is a pleasant experience, cause the magnesium alloy rail provides a very good base for the squareness of the whole main frame structure. In fact, during assembly of the main frame, I just finger tight the magnesium rails and all bearing blocks onto the side frame, and placed the whole main frame on marble to check for squareness, and even though the screws are not completely tightened the whole frame is squared! I then re-tighten each screws with force and loctite.
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| ECCPM mixing arms | Main frame components |
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| Magnesium alloy rail in between the side frames | Main shaft used for alignment of bearing blocks |
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| Preliminary assembly of the main frame | ¡@ |
After the basic frame structure is erected, further parts like the tail boom holder, battery holders and the ECCPM servo holders are added into the frame. The ECCPM servo holders are two "H" shaped" parts machined with Delrin, with holes drilled to take different servos. I did not fast these servo mounts for later fitting of servo and adjustment. The tail boom holders are machined aluminum, further provides structure of the main frame. Four control balls normally used in swashplate are now used on the side for mounting of canopy. Then on, some plastic platforms are mounted onto the main frame for gyro, receiver and battery support. The Landing gear is a one piece design where construction is superb. A lot of metal parts are used in the main frame for strength of structure. Overall the structure of main frame give a very solid impression, unlike the other electric helicopters that gives a flimsy, weak feeling.
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| ECCPM servo holders | Erected main frame |
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| Erected main frame | One piece landing gear |
After assembling the main frame, the instruction calls for assembly of tail parts, but I skip steps to the installation of the servos and main rotor parts before coming back to tail assembly. The design of the Lepton EX requires the use of mini servos. The design of the Lepton servo frame enables the use of mini servos from Futaba, JR and Sanwa(Airtronics). I fetched some Futaba 3101s from my old stock to fit into the frame temporary. Mounting of these servos are mostly through screws going through plastic shims that is molded in different size and shape to hold the servos to the frame. I checked and these spacers are being molded with similar to the carbon plastic that is used in Freya and Sceadu Evolution, which is very hard. The front three ECCPM control servos are to be seated straight, with the center of the servo disk align with the center of the ECCPM control level, and the servo frame allows a little sliding adjustment for exact alignment. This is a clever design that ensures direct, differential free control. The tail servo is located at the rear part of the main frame, and could take a medium size car steering servo for tail control.
Frankly if I would contest the design of the Lepton EX, the use of servo is what I would contest most. Hirobo recommend for Futaba radio to use 3 pieces of Futaba S3150 servos for swashplate control, and 1 piece of S9650 servo for tail control. I checked the Futaba catalog, the total weight of Hirobo recommend setup for Futaba radio is 95 grams. Compare that to using 3 pieces S9255 servo for swashplate control and one S9254 servo for tail control, the total weight is 215 grams. The two setup have a weight difference of 125 grams. However, with this 125 grams added weight, I would get high precision servos that are high torque and high speed, not those flimsy mini servos that only churns out 3.7 kg/cm at 0.24 seconds! More baffling, I measure the opening for the tail servo, and it could barely fit in a regular size servo, and with a little change in design mounting of a regular tail servo is possible, but Hirobo opt to use those medium servos for car steering, that is 0.14 seconds, instead making the opening a little bit bigger to take "real" tail control servo like the S9254 that is 0.06 seconds? The Lepton boast its power similar to a 50 class nitro engine, so what's good in using those flimsy, no precision, slow mini servo? With the power to weight ratio as the Lepton, I would want to use the fastest and most precise servo available, as even minor imperfection would get magnify due to the enormous power. Still it leaves me baffled, but I will think of a way to change that....
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| ECCPM servo mount grommet | ECCPM servo placement |
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| ECCPM servo placement top view | Tail servo placement |
Since I am still waiting for the power and motor packages to arrive, I could not perform radio setup as stated by the manual and I skipped steps to fixing the main drive train. The Lepton EX features constant tail drive system, and its the job of the one way assembly. Once again Hirobo use a two stage split gears to drive the main shaft and the tail, but this time instead of the DTDS type gear, Hirobo choose to use a big belt pulley to directly drive the tail belt from the one way. This design looks very similar to the ones used in Thunder Tiger Raptor. The one way assembly consists of a a machines aluminum one way hub that encase a one-way bearing, a harden steel sleeve goes through the hub, with the tail pulley fastened to the sleeve and the main shaft to provide constant tail drive, while the main gear is connected to the hub for spinning free during autorotation. I checked and the one way hub and the one way bearing are very beefy. The plastic tail pulley, although molded, is very well made with the carbon plastic. The main gear, 94T, 0.6 pitch, that is machined in Delrin to ensure precision. The whole one way drive assembly is to secured to a 8mm main shaft which is made by 8mm hardened duraluminum that is strong yet light. The main shaft has 3 holes drilled that is supposed to provide different head height for personal tuning, but I used the middle hole as recommended by Hirobo. The components of the Lepton's drive train is no expense spared for strength.
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| Components of one way hub assembly | 8mm hardened Duraluminum main shaft |
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| Main gear assembly | Main frame assembled with main gear |
After drive train installation, I proceed to installation of rotor head. Another gem of the Lepton EX is the SZM rotor head, which is nothing more than a shrunken SSZ-III rotor head of the Freya. The rotor head is mostly made of machined aluminum, anodized in the usual Hirobo blue. The main rotor head parts comes completely assembled as with other Hirobo metal rotor heads. However, I completely disassembled the rotor head, which originally come pre-assembled by the factory, to take a good look at the rotor head. Looking at the components of the production SZM, I notice the basic design of the hub and the whole flybar control system comes from the SSZ-III rotor head. Only the dampers are in square, not round. The sprindle is being held by the dampers inside the yoke. Each blade grip has two bearings and one thrust bearing. The flybar control system, similar to the SSZ-III, provides a near 1:1 Bell/Hiller mixing ratio. The flybar is 2mm diameter, couple with a set of new paddles with a very aggressive airfoil. The construction of this rotor head is a leaque of its own. On re-assembly of the rotor head, I noticed that while the bearings and thrust bearing have been applied ample grease, the dampers are not. As a result, I applied grease liberally on the dampers to ensure smooth operation. I also pay high attention into measuring the flybar and paddles to make them level and balance.
Next comes the washout mixers and the swash plate, they are in fact the same design as with the Freya EX, but smaller. The swashplate and the washout slider assembly are completely made in aluminum. No phasing setup is possible as the washout guide pins are fixed to the rotor hub. Once the whole rotor head is assembled, I put it onto the main shaft, and think about how good this rotor head would perform.....
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| SZ-M rotor head component | SZ-M rotor square damper |
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| Lepton EX swashplate | Lepton EX swashplate and washout assembly |
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| Lepton EX swashplate and washout assembly | Lepton EX main control |
Next comes the assembly of the tail unit. The tail rotor once again is a miniature of Hirobo's other offering, where each tail grip supported by two bearings that make tail pitch change very smooth. The tail gear box is machined in metal, that encase the tail gear and fit together alright. The tail pitch change mechanism is the same as the original Sceadu, just smaller, and in fact the whole tail of the Lepton EX is much the same as the Sceadu. Tail belt was fine pitch type. The whole tail is secured to the main frame through the machined aluminum boom holders, and the tail boom seemed very secure after tightening these screws. There were two tail boom supports to further strengthen the tail boom, and these supports are secured to the frame using 2mm ball links. The tail fins are shaped like the original Sceadu, just smaller.
The tail control of the Lepton EX is the second thing that I would contest with original design. A 2mm metal wire is used for tail control, but the original design had one end using ball links, with the other end using a chevis like the airplanes. Chevis are not a secure connection in my opinion, so I also change that to a 2mm ball joint. If you are going to use the original chevis, remember to cut a small section of silicone fuel tube and slip it over the chevis so it won't come out during flight easily. Also, the tail wire just suspend on the air without any guides in between, and with this thin wire I afraid there would be flex. I plan to convert that to a carbon tail pipe that is rigid enough for operation even without any tail guides, and I would look for a suitable guide for the tail control rod.
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| Tail rotor component | Tail rotor |
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| Tail rotor and control slider | Metal tail boom and gear box |
The canopy of the Lepton is the made of "plastic bottle" material. The canopy comes with mounting holes predrilled, so alignment of the canopy to the mounting post is not an issue. The glass part of the canopy have to cut to shape, and is to be secured onto the canopy by small screws. The decals of the Lepton EX are very similar to the Sceadu Evolution. The decals are very thin and sticky, and adheres to the canopy very well. Holding the canopy in place are the four linkage balls previously fixed onto the main frames. Mounting of the canopy to the main frame is by slotting the canopy to four mounting post. Rubber grommets are used on the canopy to hold it in place.
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| Plain canopy | Decal sheet |
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| Finished canopy | Finished canopy mounted |
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| Right side view | Left side view |
At the moment of writing this webpage, the Hirobo power package is still not available, so I would only write about the assembly of the air frame for this moment and wait until I got the power package to finish up the flight performance report.
To be continued.....