|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
While the Cricket represents the first chapter of Gorham Model Products, the Viper represents the last.
The Cricket was a simple, .28 sized, fixed-pitch, beginner's model, incapable of advanced aerobatic maneuvers. Finding the Cricket impractical to modify into a competition-bred machine, GMP decided instead to design a significantly larger .60 size fully-aerobatic helicopter based on the successful Hirobo Falcon helicopter line. During the course of the 1980’s, the wildly successful GMP Competitor gave rise to the GMP Cobra, and then the GMP Legend.
As the 1980’s came to a close, GMP found its longstanding relationship with Hirobo beginning to sour. GMP’s line of bellwether .60 size Hirobo-derived helicopters were slowly evolving away from their Japanese roots, using an ever-increasing American parts count. Most significantly, by 1990, the GMP Legend Elite sported an entirely new GMP-designed rotor head, belt-driven tail rotor, and mechanics. What's more, GMP had by this time developed what was essentially a larger version of the Cricket, the non-aerobatic, fixed-pitch Rebel.
What GMP still lacked was their own small, fully aerobatic competition model. Hence, the GMP Viper was born in the spring of 1990. The Viper was a radical, completely new .30 sized collective-pitch helicopter with a belt driven tail rotor and a shrunken-down miniature version of the top of the line Legend Elite rotor head.
John Gorham's original initial GMP Viper rendering
February 3, 1990 click to enlarge
Unfortunately, GMP went out of business before the Viper could be put into mass production. Only a very small handful of Viper prototypes were ever built. There are only two Vipers still known to exist as of 2009. The first is a test mule painted brown over cream with a blue windscreen.
an early prototype test mule Viper in flight, circa 1990
The second surviving Viper, closer to the planned production model, featured a unique, more fully-enclosed two-part canopy painted white with red trim, and was used for trade show display and flight demonstrations.
the "show model" Viper prototype, circa 1990
Needless to say, the Viper is today the absolute rarest, most unique GMP helicopter in existence! The Viper sports a very unique two-part enclosed canopy. The bottom half is made of molded fiberglass, while the top half is made of high-impact vacuum formed plastic. The top portion is split lengthwise from behind the mainshaft opening, allowing for easy removal and access to the mechanics.
an exposed Viper prototype on display at a 1990 hobby trade show
photo courtesy of Mark Ewert
Greg Milosevich at the 1990 Merced Fun Fly
Narrated by John Gorham
Video Courtesy of JC Zankl
We were lucky enough to be able to acquire the original show model Viper directly from the Gorham estate. It was not in the best condition, and needed restoration. First impressions of the mechanical layout are that this model was quite advanced and ahead of it's time. Power comes from a .30 size nitro engine (the now obsolete O.S. Max .28 helicopter engine was the same power plant recommended for the Cricket). Unlike the Viper's high-performance contemporaries, the engine's cylinder head faces forward. This design allows for the fuel tank to be located directly under the mainshaft, eliminating the change in CG experienced in most every other fuel powered RC helicopter as the fuel load is burned.
Another clever innovation is how the engine's heatsink is exposed to the outside airflow along the bottom of the model.
The Viper uses a unique, shrunken-down version of GMP's top of the line Elite rotor head. Dubbed the "Mini Elite", this rotor head is not found on any other helicopter.
The cognoscenti will recognize the prodigious use of off-the-shelf GMP parts: Cricket clutch assembly, pulleys, starter cone, and cooling fan; Rebel swash plate and mainshaft; Legend collective pitch, cyclic levers, and swash guide; and the ubiquitous Hirobo-derived black plastic washout assembly. The large Cricket pulley and intermediate shaft is topped with an eight tooth Legend tail belt output pulley!
The restoration process went pretty much as expected. As with almost any nitro helicopter of this age, there was quite a bit of castor-oil based nitromethane fuel residue and grit binding the mechanics. Two of the main frame bearings were pretty badly binding. We replaced them with a pair of the standard 61900-2Z 10x22x6 main frame bearings from our .60 sized GMP heli parts bin. An interesting tidbit we noticed was that unlike the Competitor / Cobra / Legend main frame bearings which were made in Japan by NTN bearing company, the same size bearings we originally found in the Viper's main frame bearing blocks were a made in Thailand equivalent. Was this change in GMP's bearing suppliers a cause, or a symptom of the Hirobo -GMP row of the early 1990's? In any case, the Viper represented a clear attempt on GMP's part to create a helicopter with as much "made in the USA" content as possible, or at least, as it seems apparent, reduce the helicopter's Japanese parts content.
The clutch to intermediate shaft belt is a Dayco 120XL037 480DF (essentially, a longer version of the Cricket drive belt) which cross-references to Gates 9257-1306 (60 teeth, 12.00"). The tail rotor drive belt is labeled 512XL 11 V9, which, as it turns out, is essentially a shorter version of the Legend tail drive belt belt, which perhaps coincidently, cross-references to Hirobo part # 2504-020 used in the discontinued Hirobo MH-10 helicopter.
In rebuilding the Viper, we made our selection of accessories and required flight components based on what's currently popular for .30 sized helicopters as of 2009. We choose an O.S. Max 37SZ-H Ringed Heli Engine , SAB 425mm carbon fiber main blades, and a Spektrum AR-6200 2.4 GHz spread-spectrum mini receiver. All of the Viper's servo openings were designed for full-size (standard size) servos. In light of the19 years of electronics evolution since the Viper was designed, we opted for modern high-speed, high-torque digital servos. We were able to save weight by installing carbon fiber servo adaptor plates and Futaba S9650 servos for cyclic, a Futaba S9257 for the Tail, and a Futaba S3154 micro servo for the throttle. We only used only one full-size servo where we felt was the only place where one was actually needed- to run the large collective pitch lever (which does take a fair amount of effort to move up and down), where we used an Align DS610 high-torque digital servo.
In order to help avoid any damage to the Viper's fuselage and landing gear, we quickly fabricated a new, temporary wooden base plate in the basic shape of the Viper's bottom fuselage half, and borrowed a set of Trex skids and struts. This not only made the build-up easier, but it also helped us in making set-up adjustments as well as being better able to observe the mechanics in operation during the Viper's first few test flights.
the Viper after it's first flight in nearly 20 years
After getting the blades tracked, making some adjustments to the main rotor phasing, setting the gyro, setting the throttle and pitch curves, not to mention expo and trim settings, we felt we had a nicely trimmed and set-up helicopter, so it was time to put it all together and do some test flying. We got the Viper up and hovering around in the back yard just fine! Fine trimming and a few adjustments, and she seems like a really nice flying .30 size nitro heli, and could pretty much almost pass for a modern model. However, before taking the Viper out to the field for some real flying, we continued to harbor one very real concern about what may happen.
This concern was specifically regarding the Viper's autorotation ability......or lack thereof. Being a prototype, the Viper did not yet have autorotation capability. This means that should for any reason if the engine die in flight, the Viper will turn into a rock and there goes the rarest, most unique vintage model helicopter in existence!
Although the production Viper almost certainly would have featured autorotation, the prototypes used two simple 32 pitch, 80-tooth plastic spur gears bolted together, leaving an 18mm total height including bottom hub, with a straight 8mm mainshaft bore. We toyed around with the the idea of having a new custom delrin main gear made that would accommodate a modern autorotation housing, such as this:
But ultimately, we decided to keep the original compound maingear, and simply retrofit an off-the-shelf autorotation unit to it, comprised of an Align T-Rex 500 H50003 main gear case and one-way bearing, together with a shortened and modified Hirobo Lepton 0304023 autorotation sleeve:
It turned out that it was a very good thing that we decided to play it safe and do all our our initial flight testing without the Viper's canopy. Not only did this give us a chance to break in our brand new engine purchased specifically for this project, but it also provided a bit of "stress test" to see how all of the Viper's components would hold up in flight, and what maintenance issues would could expect.
Sure enough, we did wind up having a component failure. Nothing to do with the autorotation, and nothing that caused a crash or any damage, but nonetheless, the failure of an important component. During the Viper's third full test flight, the little plastic Legend tail belt output gear that sits above the top of the large Cricket pulley became oblong and off-center. This caused wild tail oscillations as the tail drive belt would slip from the pulley. We immediately landed the Viper and investigated.
A disassembly showed that the pulley was off-center and partially melted! Further investigation showed that it was simply pressed and glued onto the intermediate shaft! Yikes! It should also be noted that in order to remove the intermediate shaft assembly from the Viper without doing another complete teardown of the helicopter, not only did we have to remove the mainshaft, but we also had to remove the starter cone and clutch assembly from the engine! We did have to remind ourselves at this point that the Viper was, after all, only a prototype, so these types of issues should be expected.
In any case, it was clear that we needed to replace the tail drive output pulley....but how to find one? The original Legend pulley is not only extremely hard to find these days, but has also proved to be prone to failure. Further research uncovered some very interesting facts. The Legend pulley was a copy of the earlier Hirobo Shuttle pulley. What's more, some years later, a Taiwanese company by the name of Thunder Tiger produced an extremely popular line of .30 and .50 size helicopters with belt-driven tails, called the Raptor. Perhaps not surprisingly, Thunder Tiger adopted the established Shuttle / Legend belt design for the Raptors.
Even better, due to the extreme popularity of the Raptor line, there were a number of third-party companies offering improved aftermarket versions of various Raptor components. Most significantly for this project, we were able to locate and obtain a metal version of our output pulley that we could use as a direct upgrade replacement for the failed plastic GMP output pulley.
Red stock plastic Raptor tail rotor output pulley, and 2 aftermarket aluminum versions of same
One of the important improvements we were able to realize was the addition of a pin through the bottom of the pulley and the intermediate shaft, following the Raptor's design improvement to this part. We had to drill a new hole in the intermediate shaft to make this work, as the failed original GMP plastic pulley was simply glued onto the intermediate shaft and large white Cricket pulley. Our completed intermediate shaft assembly looked like this:
The upgraded intermediate shaft assembly
With all of the mechanical gremlins taken care of, it was time to head back to the flying field, only this time, with the complete Viper....canopy and all. Flying the Viper is extremely nerve-racking! She is not as locked in and stable in a hover, and at the same time, not as agile as some of today's modern .30 sized helicopters.
When we acquired the unrestored Viper, we noticed that the large GMP flybar weights were set as close as possible to the rotor head. We went back to do some further tests to see if we could improve the Viper's flying stability in hover. Moving the flybar weights all the way out to the Hiller paddles improved our hovering stability, but when we went up to altitude to try a loop, it took a long, long, long time to complete! Lucky we had a lot of altitude!
Even with the flybar weights all the way out, the hover still seemed to not be as stable as we are used to with our more modern .30 size helis. The Viper is still quite agile close in and hovering around, but in fast forward flight, it really is not "3D" capable. In all fairness, GMP marketed the full size Elite rotor head for FAI and precision maneuvers, not the wild 3D aerobatic maneuvers popular today. At the end of the day, we really did not want to take any risks with the Viper trying to "push the envelope" to see what she is capable of doing. It's pretty clear however, that if the Viper had been mass produced in 1990, it likely would have been the very successful, and and would have advanced the state of the hobby.
The Viper makes for an appropriate final addition to our GMP helicopter collection!
This was one of our most challenging projects to date. Not only did we have no owner's manual or other technical information, but we were dealing with a unique mechanical arrangement, and and number of handmade and extremely difficult, if not essentially impossible to duplicate components.
It's been a lot of fun learning about and restoring this rarest (almost mythical) vintage GMP helicopter. This helicopter represents the closing chapter of Gorham Model Products, the most significant U.S. model helicopter manufacturer. The demise of the company represented the end of a golden era in the hobby. It is difficult to imagine where we would be today if the radical and revolutionary Viper had been mass-produced and the company had continued in business!
In the end, this model serves as a reminder of what could have been, but ultimately was not meant to be.
GMP Viper Resources
Mini Elite Rotor Head Drawings
this website created in consultation with
the late John A. Gorham and the Gorham Estate
You will need the free Adobe Reader
to view and print all GMP Documents
Connect with GMP on Facebook
