The SR Batteries X250 - Aileron Version

Paul Bradley reviews SR's little laser-cut aerobat, and likes what he finds.

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SR X250

Specs:
  • Type: Speed 400 aerobatic model
  • Wingspan: 36 inches
  • Wing Area: 266 Sq. In.
  • Length: 28.5 Inches
  • Recommended Power: 7.2v Speed 400 with a Graupner 2.33:1 gear drive and 10 500 mAh cells
  • Weight: 24 oz typical - Review model weighed 24 oz. with an SR 10 cell 500 mAh pack
  • Controls: Throttle, Aileron, Elevator, rudder
  • Construction: Balsa with some lite-ply featuring laser cut parts, and  carbon fiber wing spars
  • Suggested Price: $79.95 
  • Manufacturer:SR Batteries http://srbatteries.com

I don't think I can wait to the end of this review to give you my bottom line ... this is a great model that delivers in performance, building ease, and all at a great price. You NEED an SR X250. Whew, with that said I guess I can start filling in the blanks.

It is interesting to look at what is happening in the kit business these days. With the arrival of more and more quality ARF’s, a long time builder like myself might begin to wonder if there would be any future for people who like to build from kits. Fortunately, the marriage of computer aided design, laser cutting (computer aided manufacturing), and rapid setting adhesives seems to have dramatically changed the traditional world of kits. Not only have the quality of the kits improved significantly, but perhaps even more importantly the time to build them has been drastically reduced. Having parts that fit precisely, parts that require almost no clean up, the ability to dry fit parts with self aligning designs and the use of quick setting adhesives that fully penetrate the joint makes for very quick and accurate building. While not quite as fast as doing the final assembly of an ARF, it is surprising how quickly the basic process of building a modern kit can be accomplished.

The SR X250 really sets a mark in terms of kit building time. Almost immediately after opening the box you know that building this model is going to be a different experience. The detailed and fully illustrated 53 page assembly manual gives you a nice window into the obvious amount of homework that has gone into this kit by its designer, Larry Sribnick . The SR X250 comes in two versions. One is a rudder/elevator set-up, and the other is a full-house configuration with ailerons. The review model is the aileron version.

I found the X250 to be an excellent kit that does not require the usual commitment of time to transform a pile of parts in to a great flying electric powered model aircraft. Right from the beginning let me say this project was nothing but grins from the assembly of the first parts through the first flights.

 

Kit Contents

X250kit contents  X250 Manual Picture  Laser Cut Parts Guide
         Kit Contents           Assembly Manual      Laser Cut Parts Guide

It is always interesting for me when I first open a new kit box and place the contents on my building board. Each new kit always offers something of interest. When I opened the X250 box and placed its contents on the bench, I could tell this was going to be a great building experience. The 53 page assembly manual has each step of the process broken out and are fully illustrated with great photos. A nice reference guide is also provided to make identifying the laser cut parts a snap. All the hardware needed to complete the model is included. Not only is all the hardware included, but it is from name brand suppliers such as DuBro and Sig. The kit even includes custom made light wheels from Dave Brown Products. Some very nice touches. 

Being a long term scratch builder, one thing I find myself doing with a new kit is weighing the sheets that contain the parts. I like to know if I will be able to use the supplied parts, and if they are likely to be an appropriate density for the intended application. I guess I’m a victim of too many kits from the past era of kit manufacturing. I find the kits being offered today generally provide decent, if not good, quality wood. I found the wood supplied with the Z250 to be very good to excellent. Every sheet and the associated laser cut parts were very useable. The specifics for the wood in the review kit are included in the following table. Of the wood in the kit, I might have used different sheets for different parts, but given the difficulty of controlling this in a production process, I certainly have no complaints. The laser cutting service was obviously presented with good wood to use, and the results are very satisfactory.

Components Avg. Density (lb/ft3)
Wing parts 6.5
Fuselage parts 7.2
Tail surfaces 6.4

Construction

Fuselage

The building sequence begins with the fuselage. You could start with any of the major airframe components, but this is where the construction manual starts, so that is where I started. It became apparent very quickly that building this models was going to be fun, and not real time consuming. You are able to dry fit the key components before gluing, and the process really goes quickly when using a fast setting joint penetrating adhesive such as CyA. Basically, the construction sequence is add balsa doublers to the fuselage sides, fit the motor mount/bulkheads, and glue every thing together. A nice laser cut plywood alignment jig is provided, to help keep things square. Once the bulkheads are in place on one fuselage side, the other side is put into position and glued when the alignment is verified.

         
Laser cut assembly guide and self aligning parts makes things go fast and straight.
Lite Ply plates are included for future mounting of floats.

Once the sides are glued together, you come across one of the many nice features of this kit. To allow for later fitting of floats, should you have the opportunity to fly off water, two plywood plates are installed just ahead and behind the wing mount location. They add almost no weight, and provide for future enhancements to the completed model. The laser cut top and bottom decking is glued in place next, and you suddenly have a completed fuselage. The removable top hatch is also fitted during the addition of the top planking. It all fits perfectly and almost seems to put itself together. A little sanding to round off the edges and smooth things out and the fuselage is ready to cover.

 
Laser cut top and bottom planking and top hatch
makes it easy to keep the fuselage straight.

Tail Surfaces

Not much to discuss here. The tail surfaces are made up from several laser cut 1/8" balsa pieces. As you would expect, everything fits perfectly. The only real work in putting the tail surfaces together is the sanding, and that’s not much work. Simply round off the edges, provide a bevel on the movable surfaces, and make sure every thing is smooth.


About 3 hours work to this point.

At this point in the project I noticed that I had two major portions of the model ready to cover, and it seemed that I had just begun to hit my stride for the building session. This type of building is very satisfying.

Wing

For some reason, the wing often seems to be the airframe component that intimidates most builders. We are aware of the need to have every thing straight, and the sight of a pile of ribs and sticks tends to conjure up visions of a lot of fussing and fitting. In preparing to build the X250 wing, it became apparent that Larry did his home work in terms of developing a structure that would be robust while also being easy to build accurately without taking a bunch of time.


The basic parts.

The X250 wing is based on two 1/4" carbon fiber spars. One of these spars also serves as the leading edge. The first step in the process is to simply string ribs on to the two carbon spars. The laser cut holes in the ribs make this easy and the fit is again perfect. Once the ribs are in their approximate position, you clamp the two spars between a pair of balsa blocks near each wing tip. The jig blocks get their clamping force from two screws in each pair of blocks. The jig blocks allow the spars and ribs to be supported above the work surface in the proper alignment.


The jig blocks make assembly very easy.

Once the jig blocks are in place and the emerging wing structure is properly aligned over the plan, the next step is to get each rib into its final location. Once again a nice laser cut set of tools is provided to make this step easy and accurate. In what seems like no time at all, I was able to have the basic wing structure sitting on the plan in proper alignment and ready to apply the adhesive. I used thin CyA, as called out in the assembly manual, to lock each rib to the two spars. If you are not able to use CyA adhesives, I’m sure something like Superphatic would work equally well. The only difference would be the time needed for the glue to set.


Rotating the wing on the jig blocks keeps things
straight for adding the trailing edge pieces.

Once the ribs are glued to the spars, the next step is to add the trailing edge. The X250 uses the two-piece sheet trailing edge design. This style of trailing edge is quite light, and stays straight when built straight. The method used to add the trailing edge to the wing structure is a departure from other methods I have used or seen. It is apparent that Larry had developed a construction method that will be used for a variety of additional kits to be offered from SR Batteries. This method calls for the wing structure to be rotated 90 degrees on the building board to rest on the leading edge side of the jig blocks. When this is done you can quickly sight down the ribs to insure that the trailing edge of each rib is in perfect alignment. The trailing edge stock is then placed over the rib trailing edges. The stock is held together with a few pieces of masking tape. You get the feeling of covering the trailing edge of the wing with a balsa tent. You are able to easily line everything up at this point. A few drops of strategically placed CyA, and the trailing edge material is locked into place. You are then able to pick up the assembly and complete the gluing process. You end up with a very straight and strong trailing edge. The process could also be done easily with an air dry adhesive. You would just have the adhesive on the ribs before doing the "balsa tent" application. There would be plenty of time to make any alignment adjustments.


Just add the center sheeting, tips and landing
gear supports and the wing is finished.

The remaining steps basically are to remove the jig blocks, add the landing gear mounts, apply the center sheeting, add the tips, and then do the final sanding. The entire process takes a surprisingly small amount of time.

Final steps before covering

At this point the model is almost ready for covering. All we need to do is prepare the ailerons, and landing gear. Preparing the laser cut ailerons is really a mater of sanding the edges round, adding a bevel to the hinge line, and fitting some laser cut plywood torque rod supports. The laser cut plywood aileron torque rod supports are something I had not seen used in other models. They are a nice touch, and make setting up the top hinged ailerons a real breeze. The fitting process is really just a matter of creating a clearance notch in the balsa aileron for the actuation end of the torque rod.

The landing gear preparation is also a very minor process. You need to cut a relief slot in the lower wing sheeting to accept the 1/16" landing gear wire, and you need to run the mounting screws into their holes to cut the threads. Not much to do since the pre-bent wire for each of the three legs fits just right in the holes that were included in the laser cut parts.

While I did not carefully note the time I had taken at each stage of the total airframe assembly, I did note that a total of only 6 hours had elapsed since I removed the first laser cut part from its sheet. I was really quite shocked that so much could be accomplished in such a limited amount of time. I used two extended building sessions to complete the airframe (had some weekend time). Based on my 6 total elapsed time, I would guess that three or four evening sessions for many people would be enough to frame up an SR X250. I would consider this a very acceptable level of building time commitment.


Time invested to this point - 6 1/2 hours.

Covering and Finish

As all electric modelers know, you can never have too much light weight. This is certainly true for a Speed 400 powered model that is to also be capable of a fairly full set of aerobatics. With this in mind, the assembly manual strongly suggest the use of a transparent covering such as transparent Ultra Coat or its cousin, transparent Oracover. I would certainly concur with that recommendation.

In my case I happened to have a good supply of Litespan on hand. This would certainly be light enough, and meant that I would not have to wait to finish the model until I could get to a source for a transparent plastic covering (my local shops don’t carry it). So the die was cast and I used Litespan. When I do use Litespan, I don’t leave it in it’s native form. I usually add two coats of nitrate clear dope that has been thinned 50% with thinner, followed by one spray coat of Krylon Fixatif. I use the first coat of dope to act as an adhesive base for the application of trim, and the second coat to seal the applied trim. The dope has an added advantage of giving the Litespan more of a drum like feel on open bays. I just happen to like that more than plain Litespan that feels soft to the touch. The coat of Krylon Fixatif offers a dull gloss and some protection from UV rays. I have measured the weight of sample of Lightspan finished in this manner with samples of transparent Oracover, and find them equivalent in weight. The downside of a covering material like Litespan is the time required to do the covering job. You have to use an adhesive like Balsarite or Balsaloc to attach the covering. This adds time to the process. For me, the time is about doubled from that required for a material like transparent Oracover. I do like the look of Litespan, especially when dope and Krylon are added, so I guess all things have a way of balancing out.

I should note that the pilot figure you see in the photos did not come with the kit. While on a business trip in the Detroit area, I was able to visit one of my favorite hobby shops (the Prop Shop). That shop had a nice assortment of Williams Brothers pilot busts and I decided the 1" version of the sport pilot would look nice in the X250. Other than adhesives and covering material, that is the only item I added to the very complete kit.


Getting close ... just final assembly left.

Equipment Installation

The power package used for the review model was the performance package offered by SR Batteries. This consists of a 7.2 volt Speed 400 mated to a Graupner 2.33:1 gear drive, and a Jeti 350 ESC.  This is a pre-wired package that is ready to simply drop into the model. Also used was the SR recommended Graupner 9x5 CAM folding prop and associated prop adapter. The only problem I encountered was the prop assembly. Graupner included the wrong collet in the package. The prop hub requires the hex style collet and the package included a cylindrical collet. Fortunately I happened to have the correct collet in my collection, so no time was lost having to get a replacement part. This problem was certainly not SR’s, but rather one created by Graupner when they packaged the prop assembly.


The SR Performance power package.
Comes just as you see it ... ready to drop in.

The SR recommended performance power set-up uses a 10 cell 500 mAh battery pack. SR offers ready made packs, and their packs were used in the review model. If you buy the recommend performance power package and associated battery packs, you will be rewarded with a true plug and play set up. Every thing comes wired with good connectors ready to simply drop into the model. A nice feature of the SR performance set-up is the use of an ESC that has a BEC that will allow a full complement of servos while also using higher cell count battery packs. The Jeti 350 specification indicates that up to four servos can be used with as many as twelve cells. The X250 calls for three servos and ten cells. Well within the specified limits of the BEC for the Jeti 350.

An interesting feature of the X250 is the installation of the aileron servo. Typically, the servo is mounted on the top of the wing so all of the control linkages are inside the fuselage. On the smaller Speed 400 powered models, this can lead to some real gyrations in terms of getting everything to fit while also being able to position the components to get the proper center of gravity. To make the internal space management an easier proposition, Larry set up the X250 with the aileron servo mounted on the bottom of the wing. Since the model features landing gear, the bottom mounted servo works out really well. It frees up a lot of internal space that can be used to adjust the  position of the battery pack. It also makes it possible to adjust the linkages without having to remove the wing. 

My set-up in the review X250 included two FMA S-90 servos (rudder and elevator), one Volz servo (ailerons), and an FMA Micro 2000 8 channel receiver. I know the FMA S-90 servos can be a problem in terms of current draw for some BEC equipped ESCs, but I felt the specifications of the Jeti ESC would support this combination of equipment. So far this has proven to be the case. There have been no problems running the servo complement on the 10 cell pack. The total weight of the review model with a charged battery (you know, allowing for the weight of electrons) and ready to fly is right on 24 ounces.

   
Bottom mounted aileron servo makes internal set-up real easy. 
Ready to fly weight of the finished model is 24 ounces.

Flying

This is really where the rubber hits the road ... or I guess more appropriately where the wing hits the air. The SR X250 with the aileron package is intended to be an aerobatic Speed 400 model. The finished review model hit the specified typical weight of 24 ounces exactly, so unless something drastic had happened, I expected to have a high grin factor when flying this model. That is exactly what I got, lots of grins on my face. I also noted a bunch of grins on the faces of several gathered glow pilots who are beginning to try electric powered models.

The day the maiden flights were performed was far from ideal. The wind was blowing pretty steady at about 12 mph with gusts closer to 20 mph. I really wanted to fly, and would not have another opportunity for several weeks, so I decided to go for it. The first flight was off the ground in less than 10 feet. Although the CG was in the specified location, the model immediately felt tail heavy to me. This may have been a function of the somewhat stiff breeze that was blowing. I dialed in a bit of down trim, and away we went. The model handled the turbulence from the wind very well. The benefit of self aligning parts and the included construction jigs was evident as the model required no rudder or aileron trim. Power was excellent for a Speed 400 model. and I spent the first flight getting the feel of the model and did not do much in the way of aerobatics. I cut the first flight short because I wanted  to move the CG forward, and I was using a new pack with no previous cycles.

In subsequent flights I made gradual adjustment to the battery pack location to incrementally move the CG forward. I finally ended up with the pack moved about inch forward from the initial position. There is a nice range of pack location available in the fuselage when using 500 mAh cells. During these flights I was able to really test the model’s aerobatic capability. At least within my skill level. Using the SR performance power set-up the model has excellent power. Take off runs are short, and the model climbs quickly. Duration is very good with throttle management. Throttle management is really easy for this bird. If you don’t throttle back, it will simply keep climbing. I found myself constantly pulling back on the throttle stick to keep the model at the altitude I wanted to maintain. The X250 cruses nicely on about half throttle. When you pour on the coal, she responds accordingly. Nice wide loops are easy from level flight. Rolls are clean when using coordinated controls, inverted flight is easy with just a slight amount of down elevator, and she will do inside loops from level inverted flight. Other than sustained vertical maneuvers, I can’t think of much the model won’t do in the hands of a capable pilot. I fully expect to significantly improve my flying skills with this model.

Landing the X250 turned out to be a real pleasure. As you reduce the throttle setting, she really slows down nicely. There is plenty of control authority at the reduced speed. With a little throttle adjustment to maintain the glide slope, all you have to do is just let the model settle on to the ground or runway.

After building the model, I was mildly concerned that the landing gear might be a little on the light side. One of the challenges to a designer is getting the right balance between gear flexibility and stiffness. If too stiff, you run the risk of transferring larger shock loads to the airframe that can result in damage. Based on my observations during each landing made, I think Larry hit it just right with the landing gear on the X250. The 1/16" wire legs really help absorb landing loads, but seem to be more than adequate to handle a wide range of "arrivals" without being bent.

Another factor regarding the landing gear is the fixed nose gear. You might think this would result in poor ground handling. Larry again has taken this into consideration in his layout of the X250. The main gear have been placed to keep the nose light. This means that with a surprisingly little added throttle, you get good rudder authority. You may not win a slalom contest, but you sure don’t have any problem controlling the model during a take off run.

In summary, the configuration as reviewed is a great package. The specification for the X250 with the SR performance package is 7-9 minutes of aerobatics. This appears to be very valid based on my flights. I held my flights to 5 minutes due to the wind and new battery packs, but based on the battery capacity I had left at the end of the flights, 7 minutes would have been easy. Going longer was very doable. The model is capable of a very wide range of aerobatic maneuvers while not being twitchy or difficult to fly. I feel the price for such a complete kit is very reasonable. When you also consider the minimal time required to build this model I will be real surprised if we don’t see the SR X250 in large numbers where electric powered aircraft fliers gather to commit model aviation. This is a great model that really delivers a lot of flying enjoyment.

Recommendation

My recommendation is real easy for this model.  If you can invest less than 10 hours in building time, and want a great aerobatic small field model ... get an SR Batteries X250. 

If you have any question feel free to contact me BradleyP(at)ix.netcom.com

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