Hirobo Falcon Helicopters

The predecessor to the GMP Competitor, Cobra, and Legend

was the Hirobo Falcon line of R/C Helicopters produced from 1977 to 1983

Hirobo is a Japanese RC model company best known for their line of high-quality radio controlled helicopter models, both nitro-powered and electric. The company was founded October, 1949 as textile machine manufacturer Hiroshima Spinner, Limited. In 1957, the company partnered with Nichibo Company to form Hiroshima Synthetic Fiber Spinners Limited, today known as Unitika Limited. The name change to Hirobo Limited came about in 1970; the electronics division was established in April, 1973. Production of radio controlled models began in July that same year. By October, 1977, the company had completely withdrawn from the textile market.

GMP was for many years the exclusive U.S. distributor for Hirobo helicopters, and many of GMP's own helicopters shared parts and borrowed heavily from Hirobo designs.

One of Hirobo's first model helicopters, the original version of the Hirobo Falcon released in 1977 (widely known today as the "Falcon Mk1"), is remembered for its then-revolutionary mechanical layout as being the first "pod and boom" helicopter. Almost all of today's modern aerobatic R/C helicopters share the same basic "pod and boom" arrangement of major mechanical components as the Hirobo Falcon Mk1.

In a nutshell, the Falcon is the slightly less-beefy, slightly more primitive ancestor of the GMP Competitor / Cobra / Legend. Many of the steps in building the Falcon are are the same as those later GMP models. For example, the main frame (except for a few evolutionary details), tail rotor gearbox, horizontal stabilizer, servo trays, and many other parts are essentially the same.

The Mk1 had a primitive main rotor head design that, with only slight hardware improvements, ultimately proved to be an evolutionary dead end with the later Falcon 505 three years later. The Mk1's swashplate's anti-rotation link was actually attached to the top of the fiberglass canopy.

The Falcon MkII débuted approximately two years after the original Mk1. It was marketed as a more advanced stunt helicopter, as compared to the slightly more stable, beginner's Mk1 best suited to basic maneuvers and hovering around. With a slightly revised rotor head, and most significantly in terms of visual differences, a new canopy which would also be featured in the later falcon 505 and 555.

While the Falcon Mk1 suffered from a number of early issues such as chronically broken motor mounts, erratic flight behavior, and hardware issues, the next generation represented an incremental improvement over the Mk1.

Both the Mk1/Mk2 and the 505's were sort of "autostable". When trimmed correctly, they would just sit there hovering and did not like to be disturbed. One of the main fundamental differences between the 505 and all later derivatives of the family line is that the 505 was the last to use the 8mm diameter main rotor shaft and the unique rotor head design that debuted (albeit with a somewhat different control rod linkage setup) in the original Falcon Mk1 (see pictures above). One of the really strange things about this design is that the Hiller paddles (which in the 505 are very heavy cast alloy pieces with hollow jet-engine looking tubes on the outside edges) actually go collective! In other words, when you increase collective pitch on the main blades, the paddles also increase pitch. They do not stay flat and in sync with each other they way they do on all modern designs, and indeed on all later Falcon and GMP helicopters. The 505 therefore, represented the end of the evolutionary line for this rotor head design.

The 505 had a slightly revised rotor head that was finished in a "gun-metal" color (sometimes tinted slightly green), red anodized frames, a more solid engine mount, a larger cabin and a different servo layout than the original Falcon. Fit and finish of the 505 was also much better than the Mk1.

We have obtained what may very well be the last sealed, unopened, complete Hirobo Falcon 505 kit in existence! Considering the limited numbers in which they were produced, and that the kit is now 28 years old, this is an incredible find! Here it is, with the March 31, 2008 USA Today for proof!

Reference photos for posterity of how the kit was originally packaged by the factory in 1980 (click each picture to enlarge)

For power, we located and purchased a perfect, new-in-box, never run, period correct vintage O.S. Max .40 FSR engine with the classic OS Max #743 muffler:

This was of course originally an R/C airplane engine, as they did not make helicopter-specific motors back in those days. What was normally done was to install and airplane engine and either use the specific Hirobo manufactured muffler made for the Falcon 505 / 707, or use a regular airplane muffler such as the OS Max #743 with a 90 degree angle exhaust adaptor so that the muffler points out to the back of the helicopter, rather than straight up or down, as would otherwise be the case:

So far, so good. Next, we got to the point of installing the clutch. Here is where we made our first deviation from the a completely factory specified build. The 505's clutch is made of a black plastic or resin material:

It was reported to us through some old-time helicopter flyers that these clutches were notorious for burning up, and that they would typically last only around 20 flights or so. Upon inspection, we could easily see why. This clutch seems to be made from a relatively low-density plastic, and rotates inside a plain aluminum clutch bell with no friction material! One of our vintage helicopter friends reminisced about how he had hired a machinist to fabricate a metal version of this clutch for his 505 back in the 1980's. That got us to thinking.....

Very easy to do! We simply took an old GMP metal clutch, machined out the center hole to the proper diameter, and press-fit the stock 505 roller bearing assembly in the resulting hole. Next, we epoxied some clutch lining material into the inside of the 505's clutch bell, and bingo! Problem solved, and no visible changes to our Falcon! The metal GMP clutch fits the 505's flywheel perfectly (no surprise there I suppose)!

The next modification we made is another time-honored fix that's also both invisible and improves reliability. The supplied .062" diameter tail drive wire on the Hirobo and GMP helicopters is known to snap from stress, causing tail rotor loss. In building the 505, we did what countless helicopter modelers have done for decade before us: we drilled out both the front and rear tail drive couplers to .081" to accept a .078" diameter tail rotor drive wire (as we had in our Competitor project) and rather than using the stock brass tube and wooden drive wire spacers, we opted for a brand new set of vintage Tech Specialties TS107Z nylon spacers. Problem solved.

Our dipping into our Competitor parts bin did not stop there. While on one hand, we are very concerned with keeping this kit as original as possible, on the other hand, we are also concerned about preservation, given the fact that we intend to fly this machine on an ongoing (albeit infrequent) basis. Upon setting up the mechanical swash mixer assembly, two things because apparent: (i) the main pivot bushings in triangular plates supporting the collective arm had a very sloppy fit…we could actually see the collective shaft wobbling back and forth inside the bushings!, and (ii) many of the white plastic ball links that came with the kit were both brittle and ill-fitting.

Problem #1 was quickly corrected by substituting the bushings with a set of pivot ball bearings from the GMP parts bin. The bearings provided a much better tolerance against the 505 collective shaft, and practically eliminated all play on the pivot shaft.

Problem # 2 however was a really serious issue because should any of the ball links fail or pop off in flight, we could almost be guaranteed of a crash, and that is something that we want to avoid at all costs, because of a lack of spare parts to rebuild this heli. One of our old-time helicopter friends remembers that back in the day, everyone with a Hirobo Falcon he knew had to swap out those ball links for something stronger. Out of necessity, we followed suit in our build, and substituted all of the white Hirobo links for black rocket city brand ball links that were supplied in the later GMP kits. There was actually an early GMP Competitor bulletin urging all owners with the earlier links to upgrade to the bigger rocket city links due to multiple reports of the earlier links failing in flight.

The next issue became apparent once we started setting up the swash servo endpoints and travel adjustments. The primitive mechanical collective / cyclic mixer arrangement causes some strange swash interactions. At mid collective, we set the swash to level. So far, so good, however, when we raised the swash up to full collective, we noticed that the swash does not stay level as it should, but it tilts forward and to the left. Likewise, at full negative collective, the swash tilts rearward and to the right. This is not because the elevator or aileron servos are moving…they are not. It’s a problem inherent with having the elevator bellcrank offset to the left side of the frame, as the 505 was designed. We note that starting with the Falcon 707, a different elevator control mechanism was used that went around the main shaft, and thus tilted the swash from the helicopter's centerline, rather then from the left side, thus avoiding this strange swash interaction. Our solution was to program some custom electronic swash mixing (pitch > elevator, and pitch > aileron) into our DX7 transmitter to keep the swash level throughout its full collective travel, a feature we admit was likely not available to pilots in the 1980’s, but our doing so of course does not in any way alter the originality or appearance of the model.

We got through many of the other twists and turns involved in building this kit without much drama. There were a number of little issues that arose as things started to come together (take a look at that elevator control link for example).

We had always thought that GMP's solution of installing a hose clamp on the end of the tail boom to secure the cast collar for the tail gearbox was an "interesting" engineering decision. In light of what Hirobo recommended before that on the Falcons (namely, grind down the collar, and wrap around epoxy-saturated white thread), the hose clamp idea seemed a great solution!

We set the collective travel to 8 degrees positive and 4 degrees negative. The 505's manual, as poorly written and incomplete as it was, seemed to suggest a max of 7 degrees positive and 2 degrees negative (while idling). We could have obtained a mechanical max of around 10 degrees positive according to our pitch gage, but we did not like the extreme angles of the bell / hiller mixer arms in the rotor head, so we decided to keep the max and min values near what was suggested on page 13 of the instruction manual. Since we were using 2 servos to control throttle and collective, it seemed as though we could deviate from this suggested setup and go with a more modern setup that started out a zero pitch at mid stick, then we could set a v-curve for our throttle curve to prevent overspeed at or near mid-stick, which otherwise would have been an issue using only one servo to control both throttle and collective. The ability to do this is yet another advantage of modern helicopter-specific radio gear, something only dreamed about back in 1980.

Speaking of radio gear, as we did with last year's GMP Competitor project, we opted for our standard radio system package comprising of a Spektrum DX7 radio, Futaba GY401 gyro with S9254 servo for the tail, and Futaba S9252 servos for the rest of the functions. Part of the reason for this is that we want to have a "fair" comparison of how this almost 30-year old helicopter flies against the modern designs of today, and against its later GMP derivatives, so it would only be fair to have the same quality gear (i.e. control response time, etc.) as available today. The other reason, quite frankly is again is safety and preservation of this model. Modern, spread-spectrum radio gear provides a much more robust control link than traditional, single frequency modulated R/C systems.

After several weekends of free time, we were finally glad to announce that our Falcon 505 project was complete, and it was time to move on to flight testing and trimming!

The first order of business was to break-in the engine a bit, get the blades tracked, and sort out any little issues we find once things get spinning, before we commit this model to the air!

Despite being extremely careful balancing and double-checking all rotating components, we got a bad vibration that would not go away. After some trial and error, we solved the problem by slotting the inner main blade mounting holes. That way, the main blades could pivot slightly and find their perfect alignment via centrifugal force, the way that every modern helicopter today does. The 505 was designed to have its blades fixed in position relative to the rotor head, in other words, non-articulating. Just a few mm of slotting of that inboard hole, and loosening up the inboard main blade mounting bolts slightly did the trick! Through trial and error, we set up a nice, smooth throttle curve to that yielded a pretty constant RPM throughout our collective pitch range. Due to the Falcon's lower gear ratio (smaller pinion but same main gear) as opposed to the later GMP models, the Falcon's headspeed RPM seemed to settle in at about 1,300 RPM, very slow by today's standards.

Flight Report: Our very initial first flight impressions was that the Falcon 505 would be very similar to the Competitor in flight. We could feel that our powerplant (a .40 sized airplane engine) gave us only marginal flight power. Once tuned properly, however, the .40 does OK...not great, but OK. This helicopter could certainly use a .50, but we purposely wanted to built it as recommended by the manufacturer in 1980, and use a standard planker .40~.45 size engine, which we did.

Initial trimming and hovering were no problem. The CG was spot on, and we only needed very minor trim. About 76% gain on the GY-401 gyro which is a bit on the high side, but helps with the low tail rotor speed. Cyclic response felt ok just test hovering around. In this mode of flight, the helicopter was very stable and felt very much like a Competitor. However, in transitioning to forward flight, we were disappointed to see how little cyclic authority this helicopter really had. It remained very stable, and controllable, it just really did not want to go too far out of its way in any direction in response to cyclic inputs. We had taken great pains to completely maximize swash travel and set the most aggressive cyclic endpoints we could, but in the end, we really did not have enough authority to even try a basic loop or roll with this helicopter, so we flew around the field for a few circuits before coming in for a nice, smooth landing.

Either because of its primitive rotor head design, or because of its incredibly heavy cast metal Hiller paddles, the Falcon 505 is so stable, that it's really a chore getting it to move in any cyclic direction. In this sense, it was probably a great beginner model for its day, because panicked cyclic inputs would not upset the model so much. On the other hand, we found this "feature" kind of disturbing, because we did not feel the confidence to do any kind of advanced flight maneuvers. Even the Cricket moves around with more cyclic authority than the Falcon, albeit with more delay due to it's Hiller-only control.

The next Falcon model, also released in 1980, was the Falcon 707 (referred to at the time as the "Toaster" due to its appearance) featured a new design rotor head that was later adopted by GMP as their "custom" rotor head, and was also offered in all of the company's scale helicopters at the time (including the Gazelle, Llama, Huey, and Jet Ranger). This "custom" rotor head was later offered by GMP in their Cobra and Legend helicopters.

The final Hirobo Falcon models were more sophisticated again, with a washout unit fitted, a new type of Bell-Hiller rotor head, a larger 10mm diameter main rotor shaft (now the defacto industry standard for .50 sized helis) and other incremental improvements, such as a metal clutch to replace the resin clutch in the earlier Falcons that would tend to burn up.

The 808 featured a new machined aluminum rotor head center yoke, with no "coning angle" (i.e. the blades did not rise upwards from the center yoke). This design was essentially the same as the GMP Competitor "Pro" rotor head, and this model served as the basis of the GMP Competitor which débuted in 1982.

At first glance, other than the canopy and tail fins, GMP's later Competitor, Cobra, and Legend looked just like the Falcon underneath. Even though the GMP models were initially comprised of perhaps 50% or more Hirobo parts, and were for the first several years advertised as a combined "GMP / Hirobo development", there were a number of major differences between the GMP's first derivate model, the Competitor, and the Falcon 808.

While the Competitor used Hirobo main and tail rotor systems as well as the Hirobo main and tail rotor control systems, GMP made their own aluminum side frames, landing gear, clutch, clutch bell, canopies, and servo mounts. The GMP tail booms were shorter than the Falcon, and the rotor span was shorter. GMP added a starter cone above the clutch assembly, while Hirobo maintained a starter belt system. Both the Falcon and the Competitor flew very well, although somewhat differently. The Competitor's main drive pinion had one tooth more than the Falcon's, increasing the final drive gear ratio, and the main rotor head speed. The Competitor was hotter, the Falcon was slower and smoother. Both are considered excellent machines derived from a common evolution.

Special message:

We are finding that Hirobo Falcon parts and documentation are EXTREMELY difficult to come by. If you have any drawings, manuals, literature, or diagrams not posted here, please email us. Also, we are looking to build a Falcon 707 next and feature it here. If you have a partial or complete Falcon 707, or parts, please email us!

falcon@1017.org