Isthmus Model's Zephyr 2 Built Up Kit Review - RC Groups

Isthmus Model's Zephyr 2 Built Up Kit Review

Classic 7.5 ounce 1.5 meter rudder/elevator sailplane kit with excellent performance either hand-launched, off a light high-start, or as a light wind sloper.
*12/11 updated with more extensive flight video!

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Wing Area:350 sq. in.
Weight:7.5 oz.
Wing Loading:3.04 oz/sq. ft.
Servos:GWS Pico
Transmitter:Futaba 9Z
Receiver:Hitec 555
Battery:4c 110 mAH NiMH
Manufacturer:Isthmus Models

The first time I saw the Zephyr 2 flying two years ago, the modern prototype spiraled up in a thermal on a day when thermals for me were scarce. The javelin toss that sent this agile little craft on its way up was by the hand, arm, and shoulder of Mark Miller – owner of Isthmus Models.

The Zephyr 2 is a go-anywhere sailplane, small enough to keep assembled on your back seat or in your trunk, ready to take out for a few flights at the local ballpark during lunch, between meetings, or after work – anytime the mood strikes. Its low wingloading and efficient airfoil will satisfy pilots of all abilities. When the opportunity came to build, fly, and review this exciting little classic HLG, I jumped at the chance, remembering the exceptional performance of this little craft in Mark’s skilled hands. And, I found it filled an essential “hole” in my quiver of sailplanes between the diminutive Bug I currently fly and the contest-proven SuperGee I’m building.

Whom This Kit is Meant For

This kit is for the pilot who wants a high performance javelin style HLG at a fraction of the cash and resources needed to build a modern discus-launch glider. The builder will be rewarded with the satisfaction of crafting and owning a little sailplane that is unique to his or her building style, and will stand out from the crowd with its’ classic good looks, convenient size, and excellent performance. I prefer the lift of the slope above all, and will use the Zephyr 2 as my ultra-lightwind sloper. With a quick toss I’ll be able to clear the slopeside trees and get out there to hunt the elusive updrafts and thermals that are too subtle for heavier gliders to exploit. At the thermal field, I’ll trade the exercise of chasing the ‘chutes of the winches for the thrill of coring a thermal from my low altitude chucks.

Quality kits are becoming a rarity these days. I recently searched through RCGroup’s extensive archive of product reviews and found very few kits featured, but a lot of ARF’s. The scarcity of kit reviews echoes the hobby market today. I feel some of my fellow flyers are truly missing out on an important aspect of modeling if they’ve never built a kit. The skills learned are never regretted. As a kit builder I’ve learned skills that have transferred to scratch building, repairing models, and evaluating quality when purchasing ARFs and used models.


The original Zephyr was a classic 1.5 meter polyhedral 2-channel rudder/elevator hand launch sailplane designed for tight efficient “thermal” turns, good float, and effortless launches. Mark updated Bob Owens’s original 1981 Zephyr design. Now it’s the Zephyr 2. He lengthened the nose to accommodate today’s lighter radio equipment and stretched the wings to a 1.5 meter standard. If you’re lucky, you’ll have a copy of the May 1981 Model Aviation magazine that featured the construction of the Zephyr that month to compare. I did.

Kit Contents

The 20” long box arrived in perfect condition, with rolled plans, balsa parts, quality hardware and an instruction manual. The small parts were sealed in plastic bags, sticks bound together (straight!), and the sheets of laser-cut wood packed neatly. Everything was included in the kit to complete the model except adhesives, covering, and electronics. The full-size plan sheet was drawn precisely and clearly reproduced. Isthmus Models knows their plans, and an experienced builder would likely have no problem building this plane from the plans alone. The plans did not include rib profiles, an omission that might make future repairs more difficult, but I traced the outlines from the punched-out laser sheets onto a white area of the plans – just in case.

I like to have instructions too, and was not let down with those included. The instructions were written clearly, with detailed construction pictures on the last two pages. I notified Isthmus of a few places where I felt the instructions were vague, and the enhancements have already been made. I would also prefer pictures and diagrams to be in-line with the text (like articles here!) and have made that recommendation.

Building Tools


  • an Exacto knife with about a dozen #11 blades,
  • a razor saw,
  • medium and fine grit sanding blocks,
  • t-pins (and a board to push them into),
  • various rulers and squares.

I built the model with aliphatic glue, a small amount of 5-minute epoxy, medium CA and thin CA.

I have a number of CA applicators, which I’ve fashioned from a sewing needle. I stuck the sharp end into a short wooden dowel, and cut or file off the very top of the threaded eye. The tool looked very much like a tiny cocktail fork. This tool picked up a precisely metered drop of CA from a little puddle I kept at the edge of my bench. When touched to any surface, the drop of CA wicked away instantly into the workpiece. When the needle became fouled, I just held it in the flame of a lighter to burn off the crusted CA, watching out for the little wisp of smoke.

I found a Dremal Moto-Tool helpful for carving out the interior of the balsa canopy with a large bur, and Dremel’s cutting disc was the best choice for slicing hardened steel music wire.


A flat six foot table had more than enough room for the building board, sorted kit parts, and tools. I cleared the bench space for building, set down a 2x4 ceiling tile, taped down the plans, covered with wax paper for protection. Then I read entirely through the instructions, and studied the plans. The instructions began on building the fuselage, then the tail group, and finally the wing. Though I usually begin with the wing (the wing’s the thing!), for this project, I decided to build the model entirely stock and followed the directions exactly.

The Build

Fuselage Construction

The fuselage was laser-cut and rapidly assembled directly on the plans. I started with the bottom, gluing on 1/8” sq. balsa along each side of fuselage bottom, then I tacked on the solid balsa nose plug. I set that assembly aside, and built the two fuse sides. Each side had a back half and a front half. I butt glued the front side piece to the rear side piece, glued in ply reinforcements over the joints and ply reinforcements along the top for the wing saddle. I did the same for the opposite side. The fuselage top was tacked in place much later.

A builder can never be too careful when joining the fuselage sides together at the tail. Any alignment error at all will be difficult to correct and will adversely affect the model’s trim in almost all phases of flight. This was a good time for a break. After a day’s rest, I returned to the building table fresh and alert and successfully completed this critical step.

Fuselage shaping

The final shape.

Now the fun part, shaping and sanding! Before shaping, the assembled fuselage was blocky, the nose almost square, and the canopy just a balsa block. Mark’s instructions reassured me that carving this chunky thing into an aerodynamic shape would take only a few minutes and required but modest care and minimum technique. I tacked the canopy on with a few drops of medium CA and began shaping with a small plane. As the profile began to emerge, I switched to a medium grit sanding block to reveal the final shape. There’s plenty of balsa in the corners of the fuse, and the model benefited from aggressive sanding, especially in the tail. Finally, I finished off with a medium-fine grit sanding block lightly touching up and rounding all the corners. Then I used CA debonder to help break free the carved canopy cover. I hollowed out the inside of the canopy with a few strokes of a handy Dremel and a teardrop-shaped bit.

I was very satisfied with my fuselage!

Tail Group


The polyhedral wing was built up in four panels that were later cemented together. It was a D-tube design with 1/32” sheeting forward of the upper and lower spars on top and bottom. Pre-cut shear webs filled in the gaps. Lasers cut ribs provided the support for the spars and the covering.

I gradually learned throughout the construction of the wing, to handle it gently – especially when shaping the LE. The 1/32” sheeting was very delicate until it was covered with film. It seemed a thoughtless caress might damage it. I punched my thumb through the sheeting in several places while shaping and sanding the leading edge but it only took a few moments to repair my punctures with spare sheeting from the kit.


I chose Ultracote light transparent white and light transparent purple for the wings and tail feathers. To save weight, Mark recommended finishing the fuselage with two or three coats of water based urethane on the fuselage, leaving it uncovered, which I did. I tinted the balsa canopy with a quick shot of Krylon purple spray paint. Covering the model, a clean task, took about three hours of relaxed pleasant work upstairs on the den table with the family.


Once all the parts were manufactured, assembling the model was very straightforward with no ‘head-scratching’ at all. The kit came with Dubro micro hardware, so installation of the control horns involved drilling a single tiny hole, pushing the horn into the surface and setting it with a drop of CA. I centered the wing, and drilled the front wing bolt hole, rechecked the wings’ alignment and drilled the rear wing bolt. Now I had a complete model!

Placing Components

I chose a hitec 555 receiver, two GWS Pico servos, and a four cell 110 MaH NiMH battery in the nose. I was very pleased that the model balanced at the 3.25”, which was just past the tail end of the recommended 2.75 to 3 inch CG range.

Flight Report

When I finish an aircraft, I’m driven to fly it immediately. Unfortunately our Minnesota Winter is beginning, and with it the cold, the snow, and the wind. This stretch of bad weather is forecast to last ‘til May, but many of us fly anyway. I prefer to do my test flights at a local slope. I like a test flight with plenty of air between the plane and the ground. The most dangerous thing a new plane can be near, on its first flight, is the ground. I’ve seen many models damaged on their first test glide, which is completely unnecessary. Keep it away from the ground! At the first break of snow and rain, I bundled up, put the transmitter in a fleece radio mitt, and headed for a hill.

First flight – a quick trim on a windy hill

The first test flight was at a local high school athletic field that has an elevated soccer-field with 30 foot slopes on three sides and large expanses of football grids and practice areas beyond. The 15 - 18 mph. west wind blew a little too stiff for comfort, but I was eager to get in a flight. Just by holding the Zephyr 2 aloft in my left hand while working the controls with my right, I could tell that the model needed just one click of down trim to feel just right. It felt like it wanted to fly. Before launching, though, I set the model down and took a picture – just in case. Then I launched it with a gentle toss. The Zephyr ballooned up in the lift, so I pointed the nose down to work it forward over the slope. The model tracked well, but in this wind had trouble penetrating. Not a problem, I thought while flying it. The Zephyr 2 was designed for the light winds of a good thermal day, not a gusty winter blow. I crabbed back and forth, gaining and losing altitude with the wind gusts. There was no time to test Zephyr’s thermal turns. I was really at the mercy of the wind. After a half-dozen passes, during which I added a couple more clicks of down, I set up for landing. I did a momentary downwind turn to get the model just behind me, banked the model over on it’s wingtip and pointed it into the wind, and it just hovered down. I picked up the glider, thankful to have it back, and grateful that it rewarded my building efforts with a satisfactory test flight.

Because the controls felt oversensitive, and the model wanted to porpoise, I agreed with the manufacturer on CG range. I felt my model was slightly tail heavy during this quick flight.

Second Flight – Smooth air on a big slope

Two days later brought south wind and warmer temperatures – about 40 degrees -- so I brought the Zephyr 2 to my “home” slope where the lift frequently is seemingly endless. I did a control check and found that the elevator servo (with just that one brief flight) was flaky. It drifted from center, and was sometimes unresponsive. I hurried to the hobby store and bought a larger Hitec HS55 servo (they were out of the GWS Picos), and in my car broke out the servo tray, pried the gooped-in GWS servo out, enlarged the hole in the tray to accommodate the larger servo, and reassembled the plane. I also added ¼ ounce of lead in the nose, putting it into the manufacturer's recommended range.

Back at the slope, I gave the Zephyr 2 its first real feel of flight. I shoved it gently out and it flew well. In fact, the 10 mph wind created more than enough lift. I had to point the nose down to maintain level flight. I'll bet the nose was two inches lower than the tail in the strongest band of lift.

It is truly a light-air ship on the slope. Today's 10 mph was about the upper limit for this plane. The wind was no problem for penetration, but it was a challenge to keep the little glider low for photographs. At 7.75 ounces (including the needed .25 oz. of lead in the nose) it's a real floater. It indicated lift by raising a wing. When flying directly into lift, the nose rose slightly, and then the whole airframe responded instantly - going up, up, up! The Zephyr 2 didn't noticeably raise its nose in lift, it just moved up within the parcel of air. I then stalled the plane by flying level and adding up elevator. In a stall, the model just dropped the nose, losing just a few feet of altitude before continuing on straight ahead. I then tried unsuccessfully to tip-stall the model.

I kept the rudder at full rate, but hit the switch for elevator, reducing the throw to 60%. I still had all the control I needed, but the Zephyr flew much smoother. I flew the plane several hundred feet out from the slope where the lift was less intense, and trimmed the elevator for level flight. With the model trimmed for level flight in that smooth air, I needed to push a bit of down elevator in the stronger lift to fly level.

I performed the classic dive test out in the smooth air to see if that down elevator in strong lift really was a result of a nose-heavy condition. I climbed about 150' or 200' up and gently pushed the Zephyr 2 into a moderately steep descent. It picked up speed, tracking straight with no hint of flutter. Then I let go of the elevator for the real test. It continued down with no rise or tuck that indicated to me the CG at 3” back from the leading edge was perfect for me.

It's a rudder/elevator sailplane, so it traded the instant response and sensitivity of ailerons for the smoothness and stability of a polyhedral design. Point the nose down, and the Zephyr 2 stepped on the gas. Level it out, and it became a floater. It could even do a carving "bank and yank" style turn, but I had to be careful near the trees as it resisted going from level to a turn as much as it resisted stopping a turn.

Day Three – Still too cold and cloudy for flatland thermals. Out to an east wind mini slope.

This third flight was a good test for the Zephyr 2. I met a friend at a very steep slope with a 60’ face with just a notch in the trees to launch through. A nearby flag hung limp about half the time. The Zephyr proved it’s worth by efficiently harnessing the weak updrafts and making it an effortless flight for me. I concentrated on keeping the glider low for videos. I landed only because I chose to, certainly not from lack of lift.

Then I flew my Weasel, and my buddy his Combat Wing. Neither plane could stay up more than a minute or so in the same lift that was endless for the Zephyr 2.

Day Four – Javelin Toss Thermal Hunting.

You're waiting for this part of the flight report, right? The Zephyr is waiting. I’m waiting for this day too. Hopefully we'll get a good enough day weather-wise soon, and I'll update this article with this added information!

CG and Throws


The model balanced naturally at 3.35” back from the leading edge. On the first short flight, it was difficult for me to maintain smooth level flight, especially in the turns. I added ¼ oz. of lead to the nose, which moved the CG forward to a very responsive 3” back of LE. The plane weighs 7-¾ oz., which is still very respectable for a 60” sailplane.


The manual and plans said nothing of throws, so I set high rates at 100% and low rates at 60%. I was satisfied with high rate for rudder, and flew the elevator at 60%.


This is really pilot’s choice. I have -30% on the rudder and -40% elevator to fit my style.

Flight Video



The graceful Zephyr 2 is a kit for kit builders; the instructions assume some experience, at least in assembling an ARF. There are many folks in my local soaring club who are better builders than me, but I found the clear plans, excellent laser cut parts, carefully chosen wood, and concise directions allowed me to build a model not only tolerant of my little errors, but a pleasure to behold. Where the Zephyr 2 really shines is in flight. This model is truly a lift-hunter, and a pleasure to fly.


  • Excellent performance for a rudder/elevator HLG
  • Robust but light construction
  • Classic Design
  • Hand-selected, interlocking laser cut parts
  • Clear, detailed plans and instructions
  • All components provided


  • Construction photos not adjacent to building instructions.
  • A few vague points in the manual.
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Jan 02, 2005, 10:56 AM
Registered User
Aio_1's Avatar
Nice to see a built up kit review. Can't remember when I last read a review that mentioned carving and sanding wing tips and leading edges.
Has the airfoil been changed from the original? I'm assuming the flying speed will have dropped significantly along with the weight using modern RC gear.

Jan 24, 2005, 08:00 AM
In the (inclined) loop
Originally Posted by Aio_1
Nice to see a built up kit review. Can't remember when I last read a review that mentioned carving and sanding wing tips and leading edges.
Has the airfoil been changed from the original? I'm assuming the flying speed will have dropped significantly along with the weight using modern RC gear.

Go to You'll find an email link to Mark Miller who knows this model better than anyone on the planet.
Feb 03, 2005, 04:00 PM
Registered User
Mark Miller's Avatar
The airfoil was changed from original to a propriatary airfoil based in an S3014 airfoil. I don't recall how much the original weighed but it was also small er in just about all dimensions. There was probably a trade off of weight savings from smaller radio gear to additional size of materials. Still, 7.5 AUW is pretty good from stock wood from a kit.

Mark Miller
Isthmus Models
Feb 18, 2017, 11:14 PM
MorE t0Yz iz m0Re gO0dER
betterrcbuilder's Avatar
Just found a Zephyr 2 kit today and am excited to build and fly it .
Any pointers as far as what to use for a receiver pack . i think i have a new pair of HS 55s i will use.
Feb 18, 2017, 11:38 PM
Sagitta Fanboy
A 1s pack in the 500-650mAh range should work well, pretty common in the DLG side of things.

I use leftover E-Flite 500mAh 1s packs from a 120SR in my 60" HLG.
Feb 19, 2017, 05:31 PM
Registered User
craigrrr's Avatar
Originally Posted by betterrcbuilder
Just found a Zephyr 2 kit today and am excited to build and fly it .
Any pointers as far as what to use for a receiver pack . i think i have a new pair of HS 55s i will use.
And you can electrify this pretty little thermal machine with AUW at 10.0 oz.


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