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Mountain Models Switchback Senior Review

The Mountain Models Switchback Sport now has a big brother that's 150% bigger! Bernard Cawley gives us an in-depth look at it with two power systems and even some in-flight data logging.

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Introduction – The genesis of the SwitchBack Senior

 The SwitchBack Senior being held by my photographer/videographer, daughter Barbara
The SwitchBack Senior being held by my photographer/videographer, daughter Barbara
Wingspan:55.4 inches (140.7 cm)
Wing Area:600 sq. in. (38.7 sq. dm)
Weight:1 lb., 15.4 ounces plus battery with the first power system (890g)
Wing Loading:9.0 oz/sq. ft. as flown (27.8 g/sq. dm)
Length:43.3 inches (110 cm) with Hyperion 52 mm diameter standard spinner
Servos:Four required – I used GWS Naro Max BBs
Transmitter:Multiplex Evo-9 synthesizer
Receiver: Castle Creations Berg 7P
Battery:various 3s Li-polys – see text and sidebars
Motor: Scorpion 3008-32 , then AXi 2814/20
ESC: Castle Creations Thunderbird 36
Manufacturer:Mountain Models in Colorado Springs, Colorado, USA
Available From:the manufacturer and These dealers around the world.
List Price: $116 US from the manufacturer

The Original SwitchBack

Since its introduction in early 2002 as the third kit from Colorado Springs, Colorado-based Mountain Models, the SwitchBack has been the favorite low-wing park sport flier of just about everyone who has tried one, including me. It’s one of those rare airplanes that looks good, flies better than it looks, yet is easy to build and easy to fly and has no bad habits. It’s also very versatile. There is a version with no-dihedral wing and a symmetrical airfoil, called the “3D” AKA “GT” (though it’s not a “3D” airplane in the sense we now use the term). There is also a version with a slightly larger wing with moderate dihedral and a true Clark Y wing section, called the Sport which is my favorite. (Note that the picture of the Sport on the MM site is really the prototype “3D”). There are lots of good SwitchBack pictures of both variants in this recent thread. Both versions have been flown on wheels, floats and skis and even, when built lightly and powered moderately, indoors.

For those who would like to read the whole story and see how the original SwitchBack came to be, look HERE.

Doug Binder, the SwitchBack’s designer and founder of Mountain Models, overhauled the SwitchBack structurally in mid 2003, making it easier to build, easier to service and giving it a more rugged (and better placed) landing gear.

For a time there was even a retractable gear option available for either wing, for added cool factor. I’d like to think I had something to do with that being available since I fitted GWS pico retracts to my first SwitchBack Sport back in the spring of 2002 (details here) and shared how I did it with Doug.

The SwitchBack was originally designed around a GWS EPS-100 or -300 gear drive and 8 AAA NiMh cells – from which it got truly astounding performance. Nowadays, a brushless/lithium system of anything above about 60W makes it sparkle, and with a decent 100W power system one has a 14-ounce sport/aerobat that can accelerate going straight up, yet is comfortable to fly within the confines of a Little League baseball field.

Then Came the Senior

A while ago Doug passed the Mountain Models torch to Brian Eberwein who was already the proprietor of Laser Arts in Colorado Springs (what is it about Colorado Springs and airplane designers, anyway?). In the meantime, Mountain Models also offered Mike Molt’s Tyro cabin trainer/utility plane . Somewhere along the way, Mike and Brian got the idea (with some help from the peanut gallery on the RCGroups Parkflyers forum) to enlarge the Tyro by 150%, and the Tyro 150 was born. This has proved to be a marvelous flier by all accounts, and before long the suggestion was floated that perhaps the SwitchBack should get the 150% treatment, too.

This idea was well received by Brian and Doug and in the spring of 2007 the two of them started work on what was to become the SwitchBack Senior. The airplane that resulted from that idea is more than just a simple one and one half times scale-up of the SwitchBack Sport. While that was the starting point, many aspects of the airplane were revisited and refined, yielding a 55.4 inch span, 600 square inch, just-over-two-pound sport plane that carries on for and in a number of ways improves upon the original.

The general configuration of the airplane is the same as its smaller SwitchBack Sport predecessor: low “flat-bottomed” wing with moderate dihedral and full-span ailerons, a big bubble canopy on the fuselage suggesting a late golden age or early WWII single-seat fighter plane, and generous tail surfaces with plenty of control surface area for sport and aerobatic flying (but probably not enough for what we now call “3D”). Differences from the original besides the size include:

  • A bolt-on rather than rubber-band-retained wing with less dihedral than the SwitchBack Sport
  • Leading edge sheeting back to the main spar, preserving the airfoil much better than on the little ones
  • Nice stiff built-up constant chord ailerons, each driven by a separate servo which is mounted well outboard of the wheels
  • A motor mounting arrangement set up for a rear-mounted direct drive outrunner (rather than a motor/gearbox on a stick)
  • A curvy plastic cowling rather than the boxier all-wood SwitchBack fuselage nose
  • A torsion-bar type main landing gear with a more rugged approach to mounting
  • A predominantly plywood crutch-style forward fuselage construction that is quite rugged and yields a useful battery/equipment tray above the wing
  • A completely removable canopy that is still retained by magnets for very easy interior access

Once I heard about this one it didn’t take long for me to see if I could get my hands on a review sample. The kit I received is from the first production batch, which was released shortly after the 2007 NEAT Fair. Yes, I know, it’s now well after NEAT 2008. Apologies for being so slow, but I do try to be thorough - it's the engineer in me.

Kit Contents

The SwitchBack Senior comes as a typically complete Mountain Models kit that supplies everything you need to build the airframe except adhesives and covering. Mine arrived well packed in foam peanuts via the US Postal Service.

Inside the box (besides the foam peanuts) are:

  • Twenty-seven (!) sheets of laser cut parts in four thicknesses of balsa and two thicknesses of plywood (plus one separate 1/8 inch ply part)
  • Pre-bent 1/8 inch diameter wire landing gear legs (yay!)
  • Quality DuBro control system bits, main and tail wheels
  • Pushrod housings and wire for all control linkages
  • Various fasteners, CA hinges, magnets for the canopy mount and even the sticky-backed Velcro for mounting the battery
  • A vacuum-formed canopy and cowling
  • A nice 33 page illustrated instruction manual.

Significantly absent from the package is any sort of drawing or plan other than a small two-view in the back of the manual suitable for planning a color scheme. I have to admit that building an airplane like this without plans was strange to me, but with care in looking at the pictures in the manual I can now happily say it does work. Of course the fact that just about everything interlocks and indexes together with only a few places where you can put in a part the wrong way around, makes this not only possible but pretty easy to do.

To complete the SwitchBack Senior, in addition to basic tools and a flat surface on which to work, you need to have:

  • A minimum of a four-channel receiver – I used a Berg 7P
  • Four servos – rudder, elevator and one for each aileron – I used GWS Naro Max BBs (and got amazingly low in-flight servo current draw). Three Hitec HS-81s (ailerons and elevator) and one HS-85 (rudder) are recommended.
  • A servo Y-connector for the ailerons (if you’re not going to use a separate channel for each one)
  • Two servo extensions for the ailerons. How long they need to be depends on the lead length of your particular servos (and your Y-lead if used). Twelve inch (5 cm) extensions were adequate for me.
  • A 150-300W power system (though I’ve heard of folks going to 450W or more!) Choose something that can swing at least a 10-inch prop at that power level. More diameter (as long as you don't go too flat on pitch) is better. See the sidebars below for my choices.
  • Adhesives – primarily thin and thick CA. There are a couple of spots where 10-15 minute epoxy is appropriate, and a clear-drying “canopy glue” will be handy for attaching the clear canopy to its base
  • Two rolls of lightweight covering (or more if you color scheme needs it)

Some additional tools I found really helped with the build included:

  • A small razor plane – it makes the work of shaping the leading edges of the wings and the tail surfaces go so much faster than trying to do it all with sandpaper
  • Several nice flat sanding blocks of various sizes
  • Small pipette-type CA applicators so you can reach into areas and somewhat control the application of thin CA
  • Some weights for holding assembled but not yet glued assemblies aligned while you apply thin CA. The instructions suggest some 4 inch squares of ¼ inch steel plate. There are other ways to weight things down but having some way to do it is very helpful for both the fuselage and the wing. I have an array of weights made from melted-down tire balance weights and some ½ inch square steel key stock strips for this purpose.

Note that the SwitchBack Senior is a BIG airplane compared to most of Mountain Models’ other products – the wing spans four and a half feet. And since the wing is built all in one piece from the very first assembly step (the main spar web) you have to have room to move around and a flat surface that’s big enough. This is a “heads up” for those of us who have fairly congested work areas.

Building the SwitchBack Senior

It is my custom as I build a review kit to make notes in the margins of the instructions as I go. These notes serve as the basis of detailed feedback to the kit maker (in this case Brian Eberwein) and jogs my memory about what to put in the build section of the review. What follows is based on those notes and some of the pictures I took along the way. By the way, a current version of the manual with many very good color pictures of the build in it is available by picking the “manual” tab from the SwitchBack Senior page on the Mountain Models web site. In the sections that follow I refer to build steps by the numbers they have in the manual, so it would be handy for you to have it open in another window for reference.

Before I get into specifics, a couple of general reminders: First, as always with a laser cut kit, gently sand off the little bumps left behind by the retention gaps – the little uncut sections that are left so the parts don't fall ouf of the sheets - wherever the edge of a part butts up against another to form a glued joint. One critical place to do this is on the edges of the wing main spar web since they butt up against the spar caps. The integrity of the spar depends on a good joint between the caps and the web. A couple of light swipes with a sanding block to bring the bump down flush with the rest of the part is all that is needed.

I don't bother to remove the browning of the part edges where the laser has cut them – to me they're almost a badge of honor for a laser-cut kit. I know some builders prefer to remove the browning. One way you "picky" builders can remove the charring from the parts without sanding is to use rubbing alcohol on a paper towel or rag, before the parts are glued. Before gluing, it comes off easily. After gluing, some of it will end up trapped by the thin CA that invariably gets everywhere.

Second, don't force fit parts. The engineering of this airplane is such that if you have to work to get the part to fit, the first thing to check (at least twice) is whether or not you have the correct part. Chances are very high that if you have to force it, you have the wrong part.

The only major exception has to do with parts fit into slots. One thing no kit maker has control over is the actual thickness of the wood from which they cut the parts – and often there is some real variation. If the slots are designed to have good tight joints when the mating parts are cut from stock towards the thinner range for that nominal wood thickness, then the same slots will be really tight when the mating part is thicker. Personally I'd rather have the fit be snug rather than too loose.

I've noted below a couple of other exceptions to this that I encountered building my early kit.

Fuselage

The fuselage is first, and starts with installing blind nuts in the two ply parts that will later be used to attach the motor mount assembly and hold the wing mounting bolts at the trailing edge of the wing.

The first place I had to stop and scratch my head for a bit was step 5 of the fuselage build – this is the aft longeron assembly. The thing to notice is that the longerons themselves are made from 1/8 inch thick balsa yet the bits that fit at the aft end are 1/16 inch thick. I must’ve looked three or four times through all the 1/8 inch thick sheets before I reread the description of that step. By the way, let me reiterate what it says in the instructions here: be sure you make one RIGHT and one LEFT side. Pay close attention to the part orientation in the picture in the manual for this step.

At step 16 of the fuselage build I made a minor modification. The SwitchBack Senior uses one of my favorite forms of landing gear – separate torsion bar legs for each main wheel. In step 16 you install the ply parts that have the slots into which the upturned leg of the torsion bar gear legs is installed later on. Years ago I had a Goldberg Mirage 550 which also had a slot rather than a pocket for this upturned leg and I occasionally had the gear collapse on a less-than-graceful landing when one leg popped out of its slot. Because of that experience, I took some scrap 1/8 balsa from the kit and made the slots into pockets on the SB Sr.

I mentioned that I'd done this to Brian and he advised me that a mod like this is NOT necessary. However, I opted not to take it back out again before finishing the airplane, so I can’t really say one way or the other. What I can say is that so far, through two dozen flights and MANY more landings, the gear is holding up quite well in smooth and not-so-smooth landings (as you will see on the video).

At the next step I had a minor part fitup problem – the first of the two light-ply plates which support the main landing gear would not go in until I narrowed it a bit on the aft half and widened the keying slots along the fuselage sides.

At step 19 you install little ply tabs with washers on them that match up with the magnets on the canopy frame. I’d suggest laying a straightedge across the fuselage and use it to align the tabs so that they are neither tilted up nor down. The aft tabs are just cantilevered out from the fuselage side and I think they’d benefit from a little gusset or brace underneath them. That said, so far they are holding up just fine.

At step 20 you install ply tabs that will later be used to mount the plastic cowling. Sheet metal screws are supplied in the kit to attach the cowl and they go in the laser-piloted holes in the tabs. I prefer to use pan-head nylon screws for jobs like this (less likely to pull through the cowl) so I tapped the holes in the tabs for 4-40 threads before installing them. I also used portions of a wooden coffee stir stick (from one of our Seattle area coffee companies) on the outboard sides to better support the cowl and so that it wasn't squeezed down when installed.

There is a bolt-on motor mount box that supports the motor in front of the “firewall”. Assembling the plywood parts of the mount box (which comes rather later in the manual – at section 11) is the only place where I used epoxy – NHP 12 minute epoxy in my case.

The motor mount box is set up with a mounting pattern that exactly matches the rear mount of the recommended Scorpion motor. Most similar sized rear-mounted outrunners should also bolt right up. If not, you might have to elongate the holes in the X-shaped mount just a little. I had to do that for the AXi motor I now have in the airplane. One small item here: I had to do a little sanding in order to get the rear of the motor mount box to fit in between the upper cowl mounting tabs. Mr. Eberwein tells me that this will be addressed if it hasn't been already since my kit was made.

One other thing: be sure to trim the molding flange off the base of the cowl’s rear piece before fitting it up to the fuselage (and sanding the fuselage as needed to get it to fit). I didn’t do this and wound up sanding far more off the fuselage at the forward lower end than was necessary, making my cowl’s fit kind of loose. You’ll need to do this fitting before covering the fuselage even if you choose not to join the cowl halves until later.

Also, when fitting the cowl, be sure to have the motor you're going to use on hand and installed on the mount. Not every suitable motor/prop adapter combination is the same length from the mounting surface to the plane of the prop driver. Interestingly, the recommended Scorpion is the longest of the three that I considered for my SB Senior, though a little of that length is due to the extra long Hyperion prop adapter I used so that I'd be sure to have enough threads for the prop and a spinner backplate.

As an aside, this cowling is quite thin. That means it is very light, but it also means that one botched landing that hits on the nose and it'll be history (though the two-piece cowl has a sort of reinforced band where the parts overlap). Personally, I've never had much luck repairing thin ABS plastic parts. Mountain Models has spares available for $7. An alternative that will be available by the time you read this is a fiberglass molded version of the cowl. The folks at Fiberglass Specialties will be making it available for $20. It's a bit heavier – about 0.3 ounce or 9g plus any paint - but MUCH tougher than the stock unit.

Power System Number One

Scorpion 3008-32 , Castle Creations Thunderbird-36 electronic speed control, 3s XCaliber 2200 mAh battery, APC 11x7E

This is the motor recommended by Mountain Models for the SwitchBack Senior except I'm running it with a bit more prop – and it’s plenty of power for just about everything you'd expect to do with the Senior except “unlimited vertical”. You CAN get that by going to the APC 12x6E though that’s pushing the motor fairly hard In flight logging of current and winding temperatures (see below) indicate that it IS doable, however.

The Scorpion motor is appears to be very well made and is rather elaborately packaged in a foam-lined tin. It runs smoothly and quietly yet with a fairly distinctive sound. I very much appreciate that it comes with a complete set of hardware for mounting either from the front or the rear, as well as mating connectors for the pre-installed 3.5mm bullets that are installed on the fairly short, fairly stiff motor leads. An adequate length of 4mm diameter shaft projects from each end of the motor, so you use the same prop adapter with either the rear or front mount. In order to have plenty of threaded shaft length to use a spinner I did not use the supplied prop adapter (which is a good quality collet type) but an extra-long one from Hyperion. Depending on your choice of prop and spinner this may or may not be necessary.

Startup of the Scorpion motor with the Thunderbird speed control was sometimes a little ragged. These starts often appeared on my data logs as spikes of both high current and high RPM. The best technique I came up with was to give a little jab of throttle and if the motor didn't start properly just back off and then advance the throttle again while the prop was still moving. Once it was running, and in the air (brake off) there were no issues. I have always liked the in-air “feel” of Castle Creations' reverse exponential throttle curve (which gives pretty much linear RPM with throttle stick position rather than linear power) and the Thunderbird is no exception. I also like it's “don't forget I'm still powered up” reminder tone every 30 seconds when the motor was not running.

The low-voltage cutoff of the Thunderbird is automatically set at 3V per cell. In-flight logging showed that if you insist on running well into the soft low-voltage cutoff (you can hear the motor pulsing when the LVC starts to engage) you can pull the voltage down to about 8.5V (3s battery) for very brief periods.

After static testing with an APC 10x7E prop, I decided to fly with an 11x7E and did one brief sortie with a 12x6E. The 11x7E is fine but the 12x6E is pushing it with peak currents of nearly 40A and a motor winding temperature after a 9 minute flight peaking at 170 degrees F.

The X-Caliber 20C batteries hold voltage well at this power level and don't get more than a few degrees above ambient temperature - a good sign that they're not being overloaded. My typical flight times are over 15 minutes with a little bit left before the LVC cuts in with this motor, prop and battery combination. That's quite comfortable and allows me to shoot lots of landings. I see these X-Caliber batteries as a good value. They're not the least expensive batteries out there but they're amongst the best overall values. The balance plug is a JST XH type – similar to many batteries today. Use the ElectriFly adapter for your Cellpro to charge these.

This is a good combination for powering the SwitchBack Senior and should satisfy most sport fliers. I'd be tempted to try the 12x6 on this motor at Brian's altitude in Colorado Springs or maybe, just maybe, even a 12x8.

Based on my experience with the other motor (see the other motor sidebar) I might suggest the Scorpion 3014-22 on 3s starting with the 12x6E or even the 13x6.5E as an even better match to the airplane than the 3008-32. I've not tried this combination, but the numbers look very good to this engineer who likes the added efficiency of more prop diameter on his planes.

Wing

At wing step 9 and again at step 17 – I recommend using some scraps of 1/16 balsa from the kit to support the spar web off the building surface when installing the ribs and the top sheeting. This will help assure that the relationship between the ribs and the spar web is correct (and the spar web isn’t too low in the rib slots). I don’t have a picture because I didn’t do it this way – and had one rib I had to crack free of the web and redo in order to get it right.

For wing steps 15 and 16 – note that it is possible to put the LE parts on upside down. The only difference is the angle at the inboard ends of the outboard parts to account for the dihedral angle of the wing. By the time you get all three sets of LE parts on it will be apparent if one or more is upside down, but since they are thin and take a little finesse to get into place, getting it right the first time is a good thing.

The ailerons on the Senior are built up and nice and stiff. They also have a clever pocket for the aileron horn built up out of three 1/16 inch thick sheet parts.

NOTE: The manual at least twice admonishes you to be careful to make a left and a right aileron. That predates a change Brian made so that the ailerons are identical. There is no "right" or "left" aileron.

The Tail Surfaces

I have no notes about them – they just go right together. Here are a couple of close-up shots showing how the tail surface parts interlock. You just dry-assemble them and then put a touch of thin CA on the joints to lock 'em in.

Here are a couple of pictures comparing the Senior's tail surfaces with those of a regular-sized SwitchBack.

Final Assembly and Equipment Installation

Brian suggests putting a string in the wing between the aileron servo mounts and the center section to help pull the aileron servo leads once the wing is covered. This is a GOOD idea.

I had to widen the servo cutouts slightly for the GWS Naro Max servos I used. I expect that if you stick with Brian’s Hitec recommendations the servos would fit perfectly as is.

I covered my SwitchBack Senior with transparent orange and cream Solarfilm (sold as AeroFilm). As usual I had Greg Judy of Vinyl Graphics By Greg cut the graphics I added to the wing.

The fussiest bits of building the Senior are the fitting of the molded plastic canopy and cowl. I mentioned the key bit about the cowl up in the fuselage section. The main challenge with the canopy is getting a snug fit between the built-up base (again some sanding finesse and a bit of trial-and-error is required) while leaving a bit of a flange around the base of the canopy to locate it on the fuselage. I used glue from an ancient (over 20 years old!) bottle of Wilhold RC-56 to glue the canopy to the base while the base was also installed on the fuselage. I think there are other more current versions of “canopy glue” on the market that are suitable. Use some plastic wrap between the canopy base and the fuselage itself to prevent gluing the canopy to the fuselage as well as to the base.

Under that canopy I fashioned a mount for my Bob the Tomato finger puppet. I'd purchased him a long time ago (something like 10 years now) with the idea that someday I'd use him as a pilot figure. Now I have :-).

The rudder and elevator servos mount into the equipment tray at the back end of the over wing compartment. There’s plenty of room forward of the wing to stuff in the speed control and between them any lithium battery you’d reasonably use in the airplane will fit easily. I mounted the Berg 7P receiver on one side of the fuselage with Velcro and provided a similar mount for my EagleTree MicroPower V3 logger on the other side of the fuselage. Access to all of this through the magnetically retained removable canopy is about as easy as it gets.

One departure from familiar practice – at least for me – is the use of DuBro EZ connectors on the control horns and Z-bends at the servo arms for the rudder and elevator. I'm used to doing it the other way around. I've found that doing it this way works just fine. The ailerons are done the more traditional way with the EZ connector on the servo arms.

That’s about it – take your time and follow the manual and you’ll be rewarded with a straight strong airframe.

My SwitchBack Senior initially ended up just a fraction of an ounce below two pounds without a battery, but with Bob aboard. This gives a floaty 9 ounces per square foot or 27.5 g per square decimeter wing loading. This is in the upper range of “park flyer” territory, but for a plane this size is quite low. Note that the flying weight above two pounds WITH a battery aboard just pushes it out of the the “park flyer” category as defined by the AMA for their Park Pilot program, though based by in-flight logged data (more on that below) it does meet the “no faster than 60 mph” criterion as mine has only touched 60 miles per hour in a dive. My airplane cruises comfortably at a little less than half that (as reported by in-flight logging).

Power System Number Two

AXi 2814/20, Castle Creations Thunderbird-36, various 3s batteries including an Xtreme Lite 3s 2500 mAh battery, 12x6E, 13x6.5E and 13x8E APC propellers.

This is a slightly heavier and somewhat more expensive motor than the recommended one, especially when you take into account the rear mount kit is an optional extra. With the rear mount kit installed, it's only a few mm longer from rear mount face to prop driver face so fits easily in the cowl. I did have to elongate the mounting holes in the X-mount to allow bolts to line up with the holes in the SB Sr's motor mount box. I guess not all motors of this general size have exactly the same bolt spacing on their mounts.

The AXi's greater mass and lower Kv allows it to swing larger diameter props that are a better match for the airplane. I started out with an APC 12x6E and got generally better airplane performance with lower input power than with the prior power system. I then tried a 13x6.5E, which draws about the same power as the Scorpion with the 11x7E on it. The difference, especially in vertical performance, is very noticeable. With this prop I now have “unlimited vertical” albeit not at anything much below full throttle, until well into the battery charge. Top speed is down a little bit as you'd expect with a lower theoretical pitch speed, but not hugely so.

All other aspects of the airplane's performance that I care about are as good or better. The AXi runs quite a bit cooler at this power level, which told me there is some “headroom” yet to load it down.

I now have one flight with a 13x8E on the airplane. This resulted in another jump in performance in most areas. Not surprisingly, straight up vertical is a little worse, but top speed in level flight is up to about 45 mph again, as with the 11x7E on the other motor (though it takes about 10% more power with this setup than with the Scorpion/11x7E to reach that speed). But otherwise the feel is much more responsive and maneuvering is more crisp with the added power. I was even able to do sustained knife-edge flight with this power system (and rudder throw of 1.5 inches each way). I'm still learning how to do this, but it was quite a thrill to be able to do it with the SB Sr.

Motor winding temperatures with the 13x8E got into the same territory as the Scorpion did with the 12x6E - hot but not into the "burnout imminent" realm at all.

AXi motors have always been well made and if anything the standard of workmanship has improved since the early ones. They have always been smooth and exceedingly quiet and this particular motor is no exception. It is true that you have to pay more for the motor and the rear mount kit is an optional additional expense, but this is one airplane that deserves it.

The Thunderbird controller has an easier time starting the AXi, though it still is a little rough some of the time. As with the Scorpion, once the motor is started, all is well. The AXi runs almost silently but with a touch of turbine sound.

I like this power setup and will be leaving it in the airplane for the foreseeable future.

In The Air

As you can see from the sidebars, I tried two different power systems built around 3s lithium-polymer batteries. I’ll get into the details of the different power systems in the sidebars.

Let's get the basics out of the way right now. For anyone who has flown the original SwitchBack Sport the Senior will feel very familiar, but more solid. That said, if anything it will seem “lighter on it's feet”. This is to be expected since it's half again larger yet is flying at rather less than half again the wing loading. Or to look at it another way, it has a slightly lower Wing Cube Loading, at 4.5, than its smaller sibling (4.7 for mine as it is currently being flown). By the way, for more on Wing Cube Loading, also see Scott Stoops' Primary Training column in the February 2009 issue of Fly R/C magazine. WCL is a very useful tool for comparing and predicting handling qualities of airplanes of different sizes.

On the other hand, at least with the recommended power system, it doesn't have the outrageous performance of a standard SwitchBack with 80 or 90W of power in it. This again is not surprising simply because the airplane is half again the size, over twice the weight, and yet with the recommended power system it is turning a prop that's only a little larger in diameter. That is not to say that the SB Sr. feels underpowered that way – far from it. Just don't expect to go straight up until it's out of sight with 250W into an 11 inch prop. For that you need more like 300W+ and a 13 inch prop (as I later found out).

The Senior even has the same ability to get nose-high and on the back side of the power curve without getting nasty about it that the little one has. The wing's sheeted leading edge allows it to get to a much higher angle of attack without stalling.

But I suppose I ought to describe it for those of you who have never been fortunate enough to own a SwitchBack in the original size. So – how do I describe the way it flies without reusing a bunch of cliches from model airplane reviews? I'm not sure I can.

In the air the SwitchBack Senior is obedient and well-mannered when using the recommended starting balance point and “low rate” control throws (half an inch each way on the ailerons and elevator, one inch each way on the rudder). Rolls are moderately fast and nearly axial with only a tiny touch of forward stick needed just as the airplane is inverted. In fact, I was quite surprised to find that little to no forward pressure is needed to maintain inverted flight at anything above about half-throttle. This is one big difference from the Senior's smaller sibling. When using those throws, the ailerons easily overpower the rudder even at very slow speeds. I'd suggest starting out with a little more rudder travel. There is adequate authority for most “normal” maneuvers and moderate crosswinds, however.

On the other hand, the elevator is VERY effective and I wound up putting in quite a bit of exponential. Responsiveness in pitch is a trait of the little SwitchBack, too.

My current high rate settings are about 7/8 inch each way on the ailerons, about an inch each way on the elevator and 1.5 inches each way on the rudder and the airplane is nicely responsive there. That's enough rudder throw to have enough authority to keep it straight in vertical climbs and to make nice stall turns. Rolls are quicker and almost the same rate either direction. As before there is plenty of pitch authority, so I have availed myself of the "crutch" of exponential rates.

The airplane is positively stable in all three axes – but not to the point where it's a distraction. It's just enough to make it comfortable to fly even if you're a low-time pilot and relaxing, not demanding, for those of us who have been at this game for a while. It is not a stays-where-you-point-it type airplane as you'd want for all-out precision aerobatics. But that's not the point of the SB Sr. anyway. It is instead a good sized and agile sport flier. My 14-year-old daughter referred to it as a “gentle giant” and that's not a bad description (with apologies to those of you who fly giant scale models). From the perspective of someone who flies mostly small, light airplanes, this is exactly what it is.

I know the smaller SwitchBack Sport (and its first cousin the Dandy Sport) have often been recommended as aileron trainers, and I see no reason why that recommendation wouldn't enthusiastically extend to the SwitchBack Senior as well.

Ground handling is pleasant with little tendency to ground loop or nose over (except on really dense grass). The steerable tailwheel does its job well. With the recommended Scorpion motor, an 11x7 APC-E prop and a healthy 3s battery takeoff is almost immediate, even from grass. Once off the ground the climb is strong and you can go straight up for a hundred feet or so before running out of thrust. Loops are as large or small as you wish.

As I alluded to above, its behavior near (and through) the stall is benign, with no tendency to snap off either way. When the stall does come it is not sharp or sudden (this is in contrast to the smaller SwitchBack Sport which breaks pretty sharply when it does finally stall). If you insist on holding full back stick with the power off it just mushes or nods its nose up and down as long as you stay off the rudder. It CAN be spun (power off, full up elevator, then full rudder at the stall). Mine spins a little more aggressively to the right. The spin stops almost immediate upon releasing the sticks. Then just pull out of the dive. I haven't figured out how to drive it into a flat spin yet though I have tried. I don't recall ever getting my Switchback Sport into a flat spin either (but the Dandy Sport - which has the same wing - will flatten out dramatically if you do it right).

I really haven't explored the high-alpha post-stall flight regime both because I don't have much skill in this area and because, again, that's not really the point of the airplane. If you insisted on going there with a reasonable amount of power/weight, I think you'd want a power system that swings a larger diameter prop (say at least 14 inches) such as the AXi-based system in the second sidebar.

The SwitchBack Senior handles breezes reasonably well but it isn't my first choice as a windy-day flier – it is, after all, large and lightly loaded and isn't really designed with low drag (which is a real advantage in the wind) as a primary objective. However, as I've noted before, that really isn't the point of the SwitchBack, large or small. Even so, benign and predictable handling and the robust landing gear both make handling a bit of gusty wind pretty easy. Just be prepared for the airplane to be tossed around a bit in the gusts.

Speaking of landings, shooting touch and goes with the Senior is a real pleasure. The gear so far has stood up to my predilection to shoot many landings with little complaint, and the general low-speed in-air handling as well as ground handling mean there are no nasty shocks or surprises. Do pay attention not to let it get too slow and too nose high or it'll sink fairly fast. This is the “behind the power curve” situation I mentioned before. If you get into that mode you'll need to add some power and climb out then try the landing approach again. The good news is that while you get that sorted there's no danger of the airplane snap-rolling on you. More good news is that if you get it to that slow, nose-high point just before touchdown you get gorgeous three-point landings.

Flight Gallery

About the video

This was done with power system number one on a damp November afternoon. You can see that I was still learning to get the right amount of rudder compensation on takeoff. Also, few rolls are shown because I was still overusing the elevator most of the time.

I thought it amusing that an airliner climbing out from Sea Tac went over just as I was doing the slow flying (though one can be heard later in the video as well).

The video is a Quicktime file. I have found that VLC (at least on the PC platform) also plays is nicely.

Downloads

One battery I've been flying behind both power systems is from the Canadian firm Absolute RC. It is their Xtreme Lite 2500 mAh 18C pack – and it is impressive both in how well it holds voltage under this sort of load and how cool it runs. I have been more than impressed by its performance. I can really tell when this battery is in the airplane. There are also some nice details, such as a balance lead that is longer than most and made with flexible silicone-jacketed wire. The balance plug is a JST XH type – similar to many batteries today. Use the ElectriFly adapter for your Cellpro to charge these.

It comes with a genuine Deans Ultra Plug installed – which I had to remove to make the battery compatible with my power systems (I use Anderson Powerpoles on everything that's rated for more than 15A or so) but if you use the Deans, that's one less thing you have to worry about.

Anecdotes and info from in-flight data logging

One of the real surprises I got from flying my Eagle Tree MicroPower V3 logger in the big SwitchBack was how little current those GWS Naro Max BB servos draw. There are four of these servos in the airplane, one on each aileron, one on the rudder and one on the elevator. The peak servo draw over two logged flights was only 380 mA! So have no worries about the battery eliminator circuit load when using these.

The maximum level flight speed of my SB Sr. is a little over 40 MPH with either power system and minimum sustainable speed is about 13 MPH. Brian told me he calculated a stall speed of 12-15 MPH depending on wing loading. It's nice to see measurements validate the calculations from time to time. I used both the new V3 airspeed sensor and the GPS data logger to cross-check these values. The indicated airspeed reads noticeably low when the pitot tube, which is mounted on the left wingtip of my airplane and parallel to the wing chord plane, is pointing up while the plane is flying at a high angle of attack. Here the GPS (in calm air) gives more accurate results. The Senior likes to cruise in the 25-30 MPH range. I recorded top speeds of a bit under 70 MPH in dives.

I also learned that, at least with the 11x7E on the Scorpion motor, it took less than 20W to keep the airplane in the air at minimum flying speed. This validates an old Bob Kopski rule of thumb that said that the minimum power required to maintain flight was (wing loading in oz/sq ft) Watts/lb. That rule would predict a minimum of about 21W (9 Watts/lb. times 2.4 lbs.). So the SB Sr. with this power system is better than the prediction here.

Credit Where Credit is Due

Thanks are due to my wife of 30 years, Avis, for putting up with my hobby pox for all this time and everything that goes with it. Thanks to our youngest offspring, daughter Barbara, for both still and video camera work.

Thanks also to the various suppliers including Mountain Models for providing the airplane itself, Hobby Lobby for providing the AXi motor and mount, and Innov8tive Designs for giving me a really good deal on the Scorpion motor and two X-Caliber batteries.

Is This for a Beginner?

Well....not a first-time flier for sure – at least not without quite a bit of simulator time and an instructor. It could certainly be one's introduction to full three-axis controls, though.

Someone who is careful and understands the principle of "following the directions" could build it as his or her first wood kit, I think. That said, Mountain Models has more suitable first kit builds – that's what the EZ series is for (though that's not all they are good for, believe me, as they are fun little airplanes).

Conclusion

This is a good looking, trustworthy and capable sport plane that is the size of a 25-40 sized glow powered airplane but at less than 2 ½ lbs it can be flown more slowly and in smaller areas than would make any sense at all for that similarly-sized glow sport plane. It is a straightforward build with all the ease we've come to expect of well-engineered laser cut kits.

It is fun yet relaxing to fly – which is exactly the way I like my airplanes. I think it has enough inherent stability that a low-time pilot who is comfortable with a moderately fast three channel trainer/sport plane could make the transition to ailerons on it. It is at least as suitable for the "aileron trainer" role as it's smaller relatives. Even so, it is not so stable as to feel as if you have to argue with it to do aerobatics.

It achieves its low weight through good engineering of the structure, well-selected and well utilized materials. Even though it is light it is quite rugged. So far over 20-odd flights totaling just short of five hours of flight time and probably well over 150 landings I have had essentially zero maintenance.

Access to all of the electronic bits is easy thanks mostly to that large, completely removable but magnetically retained canopy. There is plenty of room for equipment and basically any lithium battery that you would reasonably expect to put in it. I haven't tried to see if a 10-cell sub-C sized nickel pack would fit, but it might. The additional ¾ of a pound of battery is probably not a good idea, though. There is no need to turn the airplane over to change batteries – an inherent advantage to the low wing, top hatch configuration.

I don't think it's really a "park flyer" though – there are no parks that I fly in where I would feel comfortable flying the Senior for long. Because of its light loading and handling qualities it could be flown in some of those parks, but it would be turning all the time and not fun for long. Now if your park has a clear space that's perhaps at least three soccer fields long and at least two wide (with the prevailing winds down the long axis) then it could be a "park flyer" for you. As I noted above it is just a bit too heavy to be classified as eligible for the AMA Park Pilot program. I think it might be possible to build one with a light power system and light covering (SoLite rather than Solarfilm) and come in right at the two lb. limit. Fortunately for me, the meadow I use as my prime flying site most of the year is plenty large enough, so I'll be flying mine a good deal there.

In short, Mountain Models has another winner for sport and casual aerobatic fliers here. If you're looking for a good looking sport flier that has much more presence than a "park flyer", the Switchback Senior could well be your airplane. Congratulations to Brian Eberwein and Doug Binder for a great kit and a very good flying, solid airplane.

Pluses:

* Builds easily with virtually no fitting * Light and strong construction * Solid, trustworthy and relaxing flying qualities * Easy internal access to the battery and all electronics * Even better looks than the original SwitchBack

Minuses:

* Sensitive elevator

Last edited by Angela H; Jan 08, 2009 at 06:36 AM..

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Old Jan 08, 2009, 11:05 AM
Sink? Circling now to confirm.
JONBOYLEMON's Avatar
United States, UT, American Fork
Joined May 2002
17,091 Posts
Great review. I NEED one of these.
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Old Jan 08, 2009, 11:08 AM
did a reversed Immelman..once.
SKYPILEIT's Avatar
Novi, Michigan
Joined Nov 2002
2,251 Posts
Switchback Senior..

Great review BEC,you did a very detailed build report & your tips will help a new builder avoid building mistakes.It's interesting to learn that those servos worked well in this application with such little current draw.I may use those if I build another SB Senior.
I certainly had alot of fun with mine last year,I logged in 48 flights altogether without any problems,many people remarked how smoothly it flew...the Senior can perform most aerobatic manuvers & will still slow down nicely for touch 'n go's...it's an all around nice 'n easy sport flyer.
I may be fitting my Senior with skis' for some winter fun,when the temps moderate in the next several weeks.If I can fly it with ski's I'll post pics & details.
Thanks from all SB fans to you for a great build/flight report & your daughter Barbara for the nice camera work.

If I may..a pic of my SB Senior..
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Old Jan 08, 2009, 11:35 AM
BEC
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Of course you may! Thanks for the good words.

Yes, Jon, you need one .
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Old Jan 08, 2009, 01:39 PM
Motor Maniac
Orange County, CA
Joined Feb 2002
2,875 Posts
Hi Bernard, good review and good pics. Once correction to make. You listed a 12-inch servo extension as 5 cm. Last time I checked, 5 cm is about 2 inches. 30 cm is about 12 inches.
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Old Jan 08, 2009, 04:54 PM
Obviously I'm a "Minus Member"
buzzltyr's Avatar
USA, MA, Longmeadow
Joined Jun 2003
1,832 Posts
BEC, very thoughtful and complete review of one of my favorite MM airplanes. I appreciate all the extensive logging and data collection you did and published. I'm sure it will help those who might want to also experiment with power systems, etc.

Thank you for doing the review in such an extensive and thorough fashion.


Mark
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Old Jan 08, 2009, 07:11 PM
BEC
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Quote:
Originally Posted by 10Thumbs
Hi Bernard, good review and good pics. Once correction to make. You listed a 12-inch servo extension as 5 cm. Last time I checked, 5 cm is about 2 inches. 30 cm is about 12 inches.
You know, I read that so many times and at least once in awhile a little bell would go off in my head that said "something's not right about that....." You're right of course.

Mark, Thanks for the kind words.
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Old Jan 08, 2009, 07:12 PM
BEC
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.... (mysterious double post)
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Old Jan 08, 2009, 09:08 PM
God is my pilot
JimTMich's Avatar
United States, MI, Kalamazoo Township
Joined Aug 2006
2,414 Posts
Wonderful review BEC!

Everything said is true. I can add from experience that it is also very tough! I've landed in a tree, the only damage was a few tears in the so lite and one little broken piece of balsa tn the tail. I've flown it through a tree at a high rate of speed, almost came to a stop...fell 20+ feet landing square on the motor, broke the firewall, and a small piece of leading edge of the wing. I enjoy my Switchback Senior.

Jim
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Old Jan 08, 2009, 09:33 PM
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Joined Nov 2007
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Great review, BEC ! Lots of good tips and info for when I get started on mine. I especially appreciate the in depth powering data and advice, as that is still a part of this hobby that I struggle with
Gotta love that BOB
fly well,
M
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Old Jan 08, 2009, 10:03 PM
God is my pilot
JimTMich's Avatar
United States, MI, Kalamazoo Township
Joined Aug 2006
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Here is a picture of my parkflying Switchback Senior.
I run a e-flite Park 480 with an APC 12x6e and HS-81 servos. I have my ailerons and rudder maxed out and elevator toned down with some expo.
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Old Jan 08, 2009, 11:19 PM
WAA-08 Survivor
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Joined May 2004
2,644 Posts
Bernard,
It's a pleasure to read such a well written review. Nice work!

Even when you review a plane that I am not immediately planning to build, I still come away learning a thing or two.

Happy Switchback Flying,

-Steve
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Old Jan 09, 2009, 04:17 PM
Wanted for breaking OHM's law
Dennis Sumner's Avatar
United States, MI, Canton
Joined Sep 2002
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Quote:
Originally Posted by SKYPILEIT
Great review BEC,you did a very detailed build report & your tips will help a new builder avoid building mistakes.It's interesting to learn that those servos worked well in this application with such little current draw.I may use those if I build another SB Senior.
I certainly had alot of fun with mine last year,I logged in 48 flights altogether without any problems,many people remarked how smoothly it flew...the Senior can perform most aerobatic manuvers & will still slow down nicely for touch 'n go's...it's an all around nice 'n easy sport flyer.
I may be fitting my Senior with skis' for some winter fun,when the temps moderate in the next several weeks.If I can fly it with ski's I'll post pics & details.
Thanks from all SB fans to you for a great build/flight report & your daughter Barbara for the nice camera work.

If I may..a pic of my SB Senior..

Jim,

That's a familiar looking ship!

Denny
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Old Jan 09, 2009, 04:44 PM
BEC
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Auburn, Washington USA
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Thanks, all.

mararra - that's one of the things takes me awhile - I really like to do multiple power systems and then put that info out there.

I'm actually feeling a little conflicted about that because the next airplane in my queue is a ready-to-fly and changing the motor is probably out of scope. But I am going to show different batteries and perhaps a couple of alternative prop choices besides the one that comes in the box.

Steve - much appreciated.
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Old Jan 09, 2009, 10:16 PM
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I'll be watchin.
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