I once thought assembling a helicopter kit is just by following the instructions manual. However, all it takes is one trip to Hirobo factory to have my self confident wiped out! I remembered once in Hirobo factory I saw a box stock Freya 60, with absolutely no option parts installed. On the bench, I turned its main rotor, and it was the smoothest belt driven tail machine that I have ever see! I checked the whole machine and did not find any "special parts" installed. Later I asked Hirobo people to let me fly this machine, and the Freya flew so good that my megabucks Hirobo Eagle with zillions of upgrade parts installed shies... I was stunned with such good performance, and asked Hirobo people why. Hirobo people says the performance of a helicopter first lays on its design, but in order to have the true colors shown the high skills of assembly and setup is required. They further says most of the times upgrade parts are not needed to make a helicopter flies good, and in fact sometimes upgrade parts would downgrade the performance of a helicopter. This contradicts with my old believes that whenever stock parts does not work good upgrade parts are the solution. Needless to say, for the remaining years I have been the humble student of Hirobo's factory pilots in order to learn the "factory secret" on assembly, and time and time again Hirobo people proved to me that their words are right. Here are some tips that I want to share with you here so that hopefully you could build a better helicopter yourselves.
Plastic main frames have been a main stream design for most helicopter kits with limited budget in mind. Big factories like Hirobo, Kyosho and JR have their entry level helicopters all in plastic frames, because while the investment in molds are phenomenal, the production cost of each component is very economical. The goods of a plastic main frame is that less care is required on the assembly and maintenance, but the downside is the weight and flex that hinders performance. With nowadays evolving plastic technologies plastic used in our plastic main frames becomes more sturdy and light, I would not be surprised to see one day plastic frames helicopters have performance in par with graphite stacked frames helicopters one day. Is it really that with plastic helicopter I could just "bolt and fly"? I'd say if you do not pursue a smooth operating helicopter then yes, but I think most r/c helicopter pilot would have the characteristic to always pursue for mechanical precision. One phrase that I learnt from Hirobo people is that "All mass production things have tolerance, and this tolerance might be too big for your personal acceptance". If you are one of these characters, I will illustrate what's the most common problem as faced with a plastic main frame, and ways to remedy it so they work smoother in the following paragraphs. I would use my latest work on the Hirobo Sceadu Evolution 50 as example here.
Warpage
Think of the production process of the plastic frame pieces, soft molten plastic are being injected into the mold, and released from the mold when formed at the prescribed shape of the mold. Although the mold might be machined in a high precision, the product, when left out of the mold, might deformed a bit when they are cooled. All factories have sort of a quality control system to spot check their plastic products for deformation, but it is impossible for factories to check all parts, so its highly possible that the parts you get might not be 100% conform to the factory tolerance. While this would not prevent your helicopter from operation, this would sure not make your helicopter operating smoothly. So what should we do? Do we have to buy a lot of the same part and check each one to use? That's impossible right? As a result, during assembly we could do something to make our helicopter operates much smoother.
Take a look at the photo below:
This is two halves of a main frame of the Hirobo Evolution, just taken out of its box, stand on each other on a flat piece of padding on a flat piece of glass. Do you see the front and the rear part touch on each other, while the middle, right on the main shaft bearing, have quite a gap? This is due to the deformation of plastic in the cooling process, which is inevitable in most mass production plastic molding process. Hirobo is not the only one to face this problem, but all plastic manufacturers does. I've consult a friend in the business and according to him in order to take out this deformation problem a lot of extra process have to be instilled into the manufacturing process, and hence the cost of the plastic main frame would be multiple. What does this deformation does to our helicopter, you might ask? As the main frame holds most of the drive train, and our helicopter is a precision equipment with acceptable tolerance to the 1/10 of a mm, small imperfections might lead to not smooth in operation. In addition, even though if you got a completely straight main frame without warp, tightening of screws with different pressure on the main frame might warp the whole main frame, thus made operation not smooth.
My way to counter this warpage would be to attach screws to the main frame in a way that would guarantee the trueness and alignment of the drive train. As we know, most of the power from our engine is transferred to the main gear and to the main shaft, the alignment of the main shaft is of the utmost importance to smooth operation. When I assemble plastic main frames, I would first put in all bearings at the main shaft bearing blocks, slot in the main shaft to hold the bearings on a straight line, and secure the screws on these blocks first, then gradually work my way back and forth to both ends of the main frame. While securing the screws, I would hand turn the main shaft to see if its turning smoothly, and would take the main shaft out of the bearings and drop it back onto the main frame to see if the main shaft could drop through all bearing blocks and if so, that means all main shaft bearings on the frames are aligned, By doing this way, I could ensure the alignment of the main drive train. In addition, I would use an electric screw driver with torque control to secure the main frame screws, so that all screws are secured at the same tightness, and I would set the main frame on a flat piece of marble during assembly to check for sqaureness.
|
 |
| Main frame pieces ready for assembly | The screws that holds main shaft bearing blocks |
 |
 |
| Tighten these screws with main shaft | Main frame on marble to check squareness |
Rough edges (Flashes)
Aside from warpage, another problem that plastic main frame might encounter are flashes. Flashes are unwanted, surplus tiny fragments of plastic that protrude on the sides of moldings. Once again no matter how good the mold is, tiny flashes are normal, and is problem that all plastic manufacturers faces. What affect does the flashes affects us? While most flashes would only give a rough edge that would cut into wires, sometimes flashes are evident in sides of bearing blocks. Take a look at the photo below:
Can you see a gap on the lower side of the bearing? It is because flashes on the sides of the bearing block prevent the bearing from seating fit onto the bearing block half. Take a look at this photo:
While the photo is not too clear (Sorry about that....), the arrow point to the side of the bearing block, where tiny fragments of flashes protrudes on the side of the block. These flashes, if not taken care off, would pre-load the bearings or push the bearings on one side so that alignment is not possible. To remedy this, use a razor blade and carefully scratch out the flashes on the sides of the bearing blocks. Do not go overboard or the bearing hole would be deformed. Carefully take out flashes bit by bit, and until the bearing could sit into the bearing block fully. In addition, put a little loctite on the side of the bearing to ensure the outer wall of the bearing would not be slipping inside the bearing block during high rotation. Once again when securing the screws on the bearing block I would first slot the part that is supposed to go into the bearing to make sure squareness.
|
 |
| Take out flashes using razor blade | Bearing sit fit into the block |
Final alignment of drive train
Finally, I wanted the whole drive train to be fitted and align so that operation is smooth. I would hand feel the main gear and all shafts to see if they are smooth, and try loosing and re-tightening some screws to see if operation becomes smoother. In some cases, I would also try to move the parts a little bit to see if operation becomes smoother. In fact, I checked and most screw holes in all main frames have a tiny little bit of free play, just by 1/10 of a mm, and this free play could mean tight or smooth operation. All in all, try to use your hands to feel the shaft rotation and adjust accordingly.
More to come later...