Blade CP crash, mod, fly, crash, mod, fly

Well as you may or may not know, I have a. Eflite Blade CP. Worse yet, I learned to fly radio controlled helicopters with my CP. As you also may or may not know, the ground beats the helicopter every time they fight. So, as an unfortunte result, I've become somewhat versed in the repair, assembly, and setup of these machines. Presented here is my odyssey of what I like to call: fly, crash, mod, repair, fly for a few more seconds, rinse and repeat.

The weak points of the stock CP, mechanically speaking, are the rotor head and tail boom. The tail boom is mostly weak because, well, I'm weak. You see, I have this AWFUL instinct to just chop the throttle when anything bad happens. I'm usually close to the ground, so my logic must be to bleed off rotor speed in order to minimize impact damage with the ground. This results in the chopper slamming dow, causing the blades to flex down and strike the boom. Sounds like 'BOOM'. :-)

When a boom strike occurs, one of two things happen. The carbon tube either breaks or splinters. Or the back end of the main frame breaks where the cabon tube inserts. The former is the most desireable situation. Having the frame break sucks, and is a pain to replace. Boom strikes can be minimized by not chopping the throttle dummy! Duh. I've been workin' on it.

I've read some posts by users at RC Universe and Watt Flyer about using a wooden tail boom. This seems like a logical solution to me, however very low tech. The nerdy engineer in me yearned for a more sophisticated answer. Enter the static safe chip packing tubes. I've got a bunch of these from old projects and I just knew they would come in handy one of these days! A 2 foot long tube weighs about 20 grams. Not bad. I cut up some CF tube to retain the stock tail motor assembly as well as attaching to the stock CP frame without modifications.

I also decided to make the tail slightly longer. This did two things. First, it gives the tail motor more leverage against the helicoper chassis. This allws greater rudder authority. More control = never a bad thing. It also served to put some weight back and far from the center of gravity. As a result, I had to move the battery forward slightly to compensate. The combination of these two things helped to increase the polar moment of inertia of the helicopter. Essentially, the craft will resist changes in direction of the yaw axis slightly better than it did previously. The change is noticeable, with more predictable rudder reactions and more graceful movement. This change in PMI was incidental but proved to be beneficial. To take credit for planning that would be, well, obviously greedy. :-)

I flew again today, and wow. The helicopter reacts much more predictably to rudder inputs. I had to adjust the gyro a bit, but man, this makes a world of difference.

The other weak area is the rotor head. The head is subjected to tremendous forces. It has to deal with the inertial forces of the blades wanting to fly out of their grips. Most of this force is cancelled out by the opposing however there is still a great deal of tensile force between the blade grips, rather than a side loading of the shaft. The other forces come from the mass of the helicopter hanging on the rotor head, as well as forces generated by changes to the cyclic control. The standard head is very strong, enough so that it stays together even during aggressive 3D manuvers. What it doesn't stand up to is blade strikes or crashes. This is OK, because the head and it's constituent parts are quite inexpensive. Replacing this with an aluminum head will only cause other things downstream in the drive train to fail. Hopefully you stop crashing eventually.

I've found that the balls tend to break off of the rotor head frame as well as the paddle control frame. I've seen a couple of local guys drill holes where these balls once resided and install threaded balls with retaining nuts. This may work with proper washers to distribute the load, although I would be wary of cracks propigating from the holes.

While I haven't flown a CP yet with an aluminum head, I'm reading good things about it online. Many flyers rave about the quality of the CNC parts and talk about the lack of slop in the components. Slop or play is the #1 reason to upgrade to an aluminum head in my opinion. The factory pieces work just fine, albeit a bit weak during crashes as discussed. The tighter tolerances make the helicopter more predictable with less wandering. You also get a true Bell-Hiller control system with the CNC aluminum heads that I've seen.

The Bell-Hiller control upgrade is making waves too. This control strategy is essentially a combination of two systems, developed by Bell and Hiller. Bell is an obvious name in helicopter history, but Hiller is a bit more obscure. Hiller was started in 1942 by Stanley Hiller. He wanted to develop helicopters, and develop he did. Hiller was involved in a variety of prototype and military projects, evantually developing proprietary control systems. Enough on Hiller, on to the Bell-Hiller rotor head.

The Bell-Hiller rotor head quicknes cyclic responce by imparting control forces directly into the blades. The stock rotor head delivers control movements to the flybar, which moves the rotor blades via linkages. The Bell-Hiller system commands both the flybar and the blades. The Bell-Hiller mixing ratio is the ratio of flybar input vs direct blade input. What this allows is almost instantaneous cyclic responce. If you thought the old Hiller (flybar) system was twitchy, stand back! For experience pilots, this results in better stability, the ability to make precision movements in close proxcimity to objects, and confidence in the craft. Knowing that the helicopter is going to do exactly what you tell it to do when you tell it to do so can boost a pilot's confidence enough to allow special things to happen.

Another potential disaster is the 4 in 1 unit. Specifically, the electronic speed control unit inside the 4-1 box. The ESC is just barely heavy enough to cope with the demands placed on it, most likley due to space, weight, and cost targets during the design phase. During normal operation, the stock motors dray less than 7 amps. Closer to 5 or 6. When a blade strike occurs, the motor stalls and the current increases exponentially. If not off throttle when the blades toush an object, chances are you will let the smoke out of the 4-1 unit. And we all know that electronic devices run on magical smoke. Once the smoke has been let out, the electrons won't do their thing.

In order to prevent ESC meltdown, I've added a standard automotive fuse in the main motor power wire. I used a 7.5 amp fuse. You should use the new style mini blade fuses. I used a standard ATO fuse because it's what I had on hand. The mini style will be easier to package. See a picture of the fuse mod at the bottom of the page.

There has been some talk of overheating problems with the 4 in 1 unit. I believe this to be either older units, or an old wive's tale. Ive experienced no heat related issues with my unit. I've seen people drilling holes in the cases of these things! I simply added a small NACA duct to the body of my Blade CP in order to allow some cool air to circulate. Even while hovering, the blade wash enters the duct, during forward flight there is plenty of air movement in there to keep things cool.

Heat may be an issue when using high powered LiPo batteries or perhaps different motors or pinions. In this case, I would consider a 3 in 1 unit and a stand alone ESC. Getting the ESC out of the battery case makes a lot of sense. This is a noisy, hot, and potentially interference producing device directly inside the receiver case. It works, but isn't ideal. A stand alone ESC will give you programmable throttle curves and such as well as allowing greater motor and battery capacity.