The 90-size helicopter is becoming more popular these days. Because of this many 60 machines have simply been stretched into a 90 specification through the addition of a few extra parts. Basically economics and fast marketing are the major benefits. Often the initial idea of installing a 90 engine into a 60 airframe starts through a grass roots movement in someone’s workshop. Some helicopters will adapt better than others while some may have a full redesign based upon the parts that are directly subjected to the added stress. The raptor 90SE fits this manner of thinking, as we shall see in this review.
Some of key areas of interest are mast, main rotor head, blade bolts, gear train including ratios and sideframe structure. I picked this specific machine due to the fact that all the above considerations have been very well implemented by design.
Starting at the top we have an aluminum main rotor head sporting 5mm long shank blade bolts and an 8mm spindle. The blade grips have been bushed at the blade bolt bores for added strength. They are a two-part design allowing for cheaper crash repairs when compared to a one-piece grip. The head comes with two types of flap restraints. The rubber restraints are suitable for easy flying or 3-D pounding. The hard rubbers are indeed very hard. This is good for preventing in flight boom strikes and for increasing the cyclic acceleration rate. The spindle teeter point is ball raced for smoothness and a long service life. By looking more closely at the head block that mounts to the 12mm mast, several other improvements can be seen. Besides being held to the mast with a 5mm bolt the unit also has a clamp up collar serving triple duty as a phase adjuster and washout driver.The mast bolt threads into the head block with loctite and while certainly being strong enough it could have had an unthreaded shank section. The rotor head is so nice I really feel it deserves the small extra effort required. With the rotor hub compressed around the mast by the clamping action of the washout driver, hammering forces are better dealt with as applied to the mast. In other words the hub retention bolt and the hub clamp up to the mast and share the working load. The hub cannot stretch or rock with age. Main rotor hub improvement does not stop here. The TT engineering team wisely moved the rotor head hand brake off the flybar carrier by extending the head block upwards. Besides being easier in the hands by not teetering it also helps for dynamic balance. Think about what happens to the head button when it teeters, the head button mass moves off to one side with no mass compensation on the other side. There is ample clearance for any plastic mixer arm deflection and at full flybar tilt. Granted it is a small issue but nonetheless one which has been addressed. It should not go without mention that the head is fully ball raced employing a stainless steel flybar. In a nutshell this rotor head is far more than structurally sound.
The head design and quality again follows that of the swashplate. Removable blade grip pitch horns can make for a cheaper crash repair cost.
The main rotor has slight negative delta using leading pitch horns, which I feel is non-effective due to the tight rotor head and stiff flapping. Basically if it had any effect at all it would input negative blade pitch angles with flapping occurring in the direction of rotor thrust. This form has been handy in combating blade flutter in a loose or flexible head design. Some people confuse the positive and negative definition but so long as one understands what is happening is what really counts. Too much delta through very soft flapping may cause a phasing change, but remember we have the phase adjusting ring to fine tune. The ball raced flybar carrier has two mixer mounting selections to alter the flybar mixing ratio, however a different flybar control arm is needed to allow for mechanical clearance. For 3-D flying I’d also expect paddle selection to of a personal nature, much like rotor blades. The kit does not include main rotor blades for this reason.
Moving down the 12mm thick mast to the precision washout it can be seen to have a hard-anodized hub to extend service life. It is designed as long as possible to reduce rocking on the mast caused by normal wear. I expect and hope the hard coating will stay slop free longer than a brass-bushed unit would. It also has maximum design drive pin engagement. All rotating controls are perfectly level by design at half collective control travel. This guarantees an even control feel at symmetrical maximum available blade pitch throws. It’s great for 3-D and precision flying.
The heart of the helicopter. This unit has ample throw and is of a top quality and design.
Under the washout we find the heart of the control system. The SE all-metal slop free swashplate is very high grade. It has a slotted adjustment bolt for compressing the bearing should slop develop between the rotating and non-rotating portions. The spherical bearing is aerospace quality. Like all Raptors this is a high tilt, high throw design requiring no extra ball stand-offs or a replacement swashplate to achieve a greedy throw. This is done by selecting a bearing with a large inner race while designing the swashplate and helicopter as a unit. This is not a “hand me down component”.
The SE has metal bearing blocks supporting the 12mm mast bearings. Some 90 conversions use a 10mm mast along with smaller bearings. While the bearings are a tight fit in the bearing blocks they are additionally retained by button head screws underneath as a safety measure. This means there is no chance for the bearing outer race to ever spin. The lower bearing block has vertical adjusting slots at the sideframe attachment. This is to allow for a correct mast bearing positioning because the top-bearing collar is screwed into the mast having no allowance for height adjustment. This method means that the mast will not get marked from setscrews and it will never slip out of position.
Located between the two mast bearings is the major portion of the gear train. Because of the added 90-engine torque the auto unit has been beefed up. It incorporates an adjustable steel-retaining ring surrounding the aluminum hub to prevent torsional distortion while under maximum stress. Do not clamp it down too tightly or you will distort the aluminum housing it is intended to protect. The driven gear for the tail drive resides under the main gear and is bolted directly to the mast. The clutch and bell are larger that the 60 and have 3 pinions available. The main gear now comes in four sizes. Together the system has 12 possible engine to main rotor gear ratios from 7.58-9.51 making it very tunable. The SE kit comes with a ballpark 11 and 12 tooth pinion along with a 93 tooth main gear. The provisional horizontal slots for the clutch bell-start shaft bearing block and engine mount attachment allow for a proper gear mesh. The clutch pinion bearing is retained with a nut threaded onto the end so the bearing cannot chatter on the shaft. This can be tightened as the pinion is screwed into the clutch bell. Advanced modelers may also use sleeve retainer for extra measure. Clutch clearance is supplied at 17″ thousandths per shoe which to me seems excessive based upon past R-50 helicopter experience. It has a bigger swing radius for the shoes so that most likely is the reasoning. I shouldn’t really complain as there have been no reports of failures and so far it works fine as is. The clutch bolts to the high quality machined “squirrel cage” aluminum fan.
The squirrel cage metal fan offers precision and cooling air volume.
The helicopter is quite modular in build. One self-contained unit is the forward or intermediate tail drive gearbox. The non-adjustable 4.65:1 tail rotor drive system takes its power off the red mast spur gear and shifts the power direction by 90 degrees through two bevel gears. The spur pinion and first bevel gear are pinned to the shafts and secured with setscrews in the hollow threaded metal gearshaft. The hollow plastic integrated output pinion gear/gearshaft is designed to mate with the male drive adaptor end of the long drive shaft located inside the tail boom. This gearbox required no shimming even though shims are provided.
The long carbon fiber driveshaft has two tail boom support bearings while the drive adaptors are glued and bolted to the shaft. These bolts need to be shortened with a Dremel tool in order to clear the inside of the tail boom. They must be cut off flush at the nut with no exposed threads showing through. The shaft end interface is strong enough to consider it as a one piece shaft. With a few other 90 helicopter conversions using only glue in this area there have been failures.
The drive shaft bearings are glued to the carbon fiber shaft with CA and are housed in plastic supports meant to interface with the tail boom. The supports are sprung plastic and are very difficult to install. The manual suggests gluing these into the carbon tail boom but I assure you they will not move or spin as is. Once started into the boom it still takes substantial force to slide the shaft assembly into its location. It is not an especially hard job once you figure out that the two plastic bearing supports need to be heated with a hair dryer to become flexible enough for normal finger strength to compress them. It is a very nice design other than for the initial installation. The easiest way to find the shaft positioning in the tail boom is by using the aft tail rotor gearbox half with pinion as a jig leaving a tiny bit of space between the drive adapter and the bottom of the input pinion gear shaft. The forward shaft clearance is set by tailboom placement depth into the sideframe holder.
The aft tail rotor gearbox is very easy to build and while supplied with shims, it needed none. Gear retention to the output shaft is again by pin and setscrew design. I decided to lubricate the gearbox with grease even though plastic gears generally survive without it. The manual however does not mention lubrication. The tail rotor head is the same as on the R50 and R60 with one change you need to be aware of.
The long 3mm setscrews, which lock the hub and bearings to the output shaft, have a different cone shaped end since the output shaft now has a deep conical indent. This is intended to hold the hub more securely however it has created a problem. The 18mm setscrew now seats deeper in the hub with the hollow wrenching portion now fully engaged into the bearing retention nut. The wrenching hole in the long 18mm grub screw is also deeper by .050″ than with the R50 situation. This means the material thickness at the shear line is critical. If not built exactly it may fail. A longer, “cone point” setscrew (grubscrew) should be used, 20mm in length. Alternate methods of compliance are possible using 19-20mm hex bolts and washers. The setscrew reconfiguration to the cone point over the previous flat end is intended to prevent the hub from becoming slightly loose after many flights, which has happened in the past. This did not mean that the loose hub would depart but only that it would eventually make aluminum paste by chattering on the shaft. I tend to also bond the hub with sleeve retainer to the shaft even though heat is needed for future removal. You have been forewarned.
Since we are already at the tail section we shall work back to the sideframes. The SE tail rotor feathering mechanism is typically raptor but employs a metal pitch fork. The brass slider that came in the kit did not match the threads in the fork exactly and was impossible for me to start. I had a spare R50 slider, which threaded in easily and is now retained with proper thread locker. If you come across this issue remember the fork is too nice of a part to bugger up with force, and I suspect my experience is a one off. A steel push pull control rod running from the aft gearbox belcrank attaches to a rear-mounted servo located at the intermediate gearbox/tailboom holder. I had a minor issue installing the control rod into the guides. The metal threads appear to be rolled and have an OD bigger than the rod. I had to forcefully screw the guides over the rod before installing the guides onto the tail boom. This required holding the rod in a machinist vice. It is best to rap tape around the rod so you don’t mar it with the vice jaws, or use soft aluminum vice jaws. Don’t be tempted to take the easy route drill the guides out, as it will create slop. Finally, the servo arm and belcrank are set at 90 degrees with zero subtrim.
The SE collective arm is made of carbon fiber and aluminum spacers making for a very rigid assembly. The fore/aft mechanism that rides with the arm is also largely aluminum. The plastic “A” links have removable rod ends and it took notable force to fit the threaded rod into them. It might be an idea to carefully relieve the holes with a precision drill set so you don’t have to force matters. Once the mechanism was completely built as per the book I found that it could be improved ever so slightly by using some shim washers included in the kit. The roll belcrank lower control rod connecting to the swashplate was rubbing on the fore/aft pivot arm. While it would not jam it did add resistance so 1 shim was installed under the roll belcrank to move it outboard. In the name of geometric symmetry the other belcrank received the same treatment. The carbon end pieces of the collective arm initially made slight contact with the cap head screws retaining the start-shaft bearing block. After everything is torqued down it should guarantee enough clearance. I was temped at first to implement thinner washers under the hex screw heads since it utilizes very thick washers. The mechanism is very free running, stiff and interaction free. This is something many high-end mechanical mixing control systems cannot lay claim to.
By following the book for the specified length of 113mm at the long mixer arm control rods attaching to the swashplate I ran into serious interference. This occurred with the swashplate in the lower position close to the mast bearing block and with cyclic applied. At this control configuration the flybar could not be teetered as the mixer rods contacted the head block. The dimension that worked best for me is 117.5mm center to center. This means the blade grip pitch links need to be adjusted to 41.0mm for a 3-D setup yielding -10 and +13 degrees collective with half travel at 0. Maximum cyclic is available at up to +/-7 degrees. This is an industry standard for available blade pitch angles.
The last flight control area needing a small tweak is the flybar and control arm balls. The threaded machine screws holding the balls seemed to thread stiffly into the arm. I managed to get it to work-in correctly, but wished I had a tap on hand the clean up these threads in a more professional manner. The sleeves that the flybar teetering bearings ride on are slightly too long. This gave .010″ side play. The sleeves may be carefully bonded with sleeve retainer (Permatex 620) into the bearing inner races with the flybar accurately centered with feeler gages before things set up. Alternately one can carefully grind equal amounts off the length of both bushings to remove the slop as I did. I also tend to use sleeve retainer on all bearing sleeves. While you don’t have to be this critical with the build, the machine certainly deserves such treatment.
The built up structure is extremely robust. Extra sideframe spacers are used surrounding the fuel tank front and rear. Every module interface uses metal spacers inside the plastic structures. Thick washers are installed under all load-bearing bolts contacting the carbon sideframes.
A rather massive aluminum lower sideframe doubler is engaged for the purpose of spreading engine vibration and torque loads over a very wide area. The engineering manner in which all the modules tie together makes for an extremely rigid structure with no need for any additional internal metal structures to connect the mast and clutch bearing blocks. The fuel cell is rubber isolated from the structure to reduce fuel foaming and the boom supports are very stiff.
Regarding the carbon fiber boom supports. We have in the past experienced failed glue joints at the end fittings with various brands of helicopters. Since that time it has been my procedure to drill a hole through each fitting and the tube and then install a screw for extra security. The carbon tail boom is very strong but so is the aluminum horizontal fin and boom support fitting. If the clamp up is too tight it will crush the tail boom leading to in-flight failure. The book makes clear mention of this fact and cautions about excessive bolt torque. At the end of the build we had some small shims left so we carefully fitted the correct number between the top and bottom fittings which now prevents any crushing of the tail boom.
Shown are how all the main frame modules efficiently attach together.
A lower carbon fiber plate is supplied to join the 4 landing gear hard points. While this does little in flight it does make the sideframe attachment areas less susceptible to crash damage.
All and all this is an easy and accurate build but as with any high-end machine TLC must be brought into play to get the best from it. I am very impressed with the overall close tolerance of the kit. The instructions are rudimentary at times, but the experienced builder should have more than enough information. He has the skill to know how to jump from page to page and from section to section to find build information. The parts bags and their numbers follow no exact pattern. You cannot always build an assembly module from one bag as some pieces are scattered throughout the kit. However, since the parts in assembly pictures are numbered correctly and because the legend at the top of the page offers thorough dimensions, I found this to be a simple inconvenience. If you ask me, instead of trying to go the numbered bag route continuously, all similar fastener parts could be put in separate bags. Examples come to mind such as 3 x 15mm socket head bolts etc in their own bag and special washers, circlips etc also segregated. It would be for the assembly technician but a simple matter of verifying kit contents both from a kit quantity and installation location point of view. It is something I preferred to do during the build, and also something that could make packaging easier at the factory. The kit is generous in hardware as I found extra shims, tiny circlips, cap-head washers, and 4mm blade bolts etc remaining after a very critical build. To some this could suggest that the machine is not built correctly, but that is not the case. A nice finishing touch is that hex wrenches are supplied in the SE kit. I expect from a structural standpoint that this airframe could in the future handle more engine, it is that robust.
The 90 engine, radio installation, and setup tips will follow in the next issue and shall include full flight-testing.