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Model Forum / Radio Controlled / Helicopters / February 2005Vibration problem with training gear
Hi all, As gloomily predicted by some of the people helping me with the heli in real life, my R50 V2 SE gets a shake after a few seconds in the air with the training gear on. (This particular version of training gear is 4 sticks with balls screwed on the end, each plugged into a small white central puck. The sticks are 21" long, the balls about 2 1/2" in diameter.) The resonant frequency of the lot seems to be a few Hertz, and once it starts (a few seconds after lift-off) the only way to control it is to drop the heli to the ground. I've turned the gyro gain right down and I don't think this is a tail-wagging thing due to excessive gyro gain, although there is a tail-wagging (yaw) component to the vibration. Interestingly this is the first time the heli has vibrated with the gear on. On previous occasions I attached the gear using cable ties; this time I used rubber-bands. I shudder to think what extra stresses this has all put on the servos, linkages, main shaft, bearings, etc. (but, I'll admit, I didn't stop right away! Had to practice, right?!) The aluminium landing gear and the training gear now each have complementary wear marks from less than a couple of tanks of flying. I bravely did a take-off without landing gear but I admit that I was too timid to attempt an adequate hover to assess the effect of "no gear". Unfortunately there were no other mode 2 flyers at the club that afternoon. Theoretically I can increase the resonant frequency by shortening the sticks. Are there other tricks known to help? TIA, Gerard > Hi all, > [quoted text clipped - 40 lines] > > Gerard Hi Gerard, One thing you can try is to adjust the main needle on your engine's carb. I can't remember off the top of my head which way to go, but try 2 clicks richer first. If that doesn't help or makes it worse, reverse the change and try 2 clicks leaner. It happens to my heli m8s which are still learning and happened to me while i was learning. HTH Andy Changing the lenght of the 4 sticks will help (longer or shorter ) Changing the type of stick, aliminiun or hard wood dowel even taping a wood dowel to the aluminiun stick double up > Hi all, > [quoted text clipped - 40 lines] > > Gerard > Hi all, > [quoted text clipped - 36 lines] > by shortening the sticks. Are there other tricks known > to help? You could also try increasing/decreasing the head speed si the hover point is not the same as the point of resonance. You could also try attaching some weight to the legs again to change the resonant frequency.....  SignatureAlan alandotrait@btopenworlddot.com Remove the dots to reply http://heliweb.users.btopenworld.com/ I flew a raptor 30 this weekend with a training aid that sounds exactly the same as the one you are using & guess what ..... no shakes at all. I suggest that you try balancing the rotor blades with a bit of electrical tape...... no don't laugh try this. pick a blade put 2 turns of tape around the blade about halfway along the blade. hover the heli ... does it shake more or less ? more ..... take it off & try the other blade. less ...... add some more. repeat until you get it as smooth as possible then do the same thing with the paddles. it is possible to get a very smooth running heli this way honest. (I have tuned out a bent mainshaft this way) give it a go ..... nothing to lose ..... except vibration of course :-) CHEERS Dave P > Hi all, > [quoted text clipped - 40 lines] > > Gerard | Theoretically I can increase the resonant frequency | by shortening the sticks. Are there other tricks known | to help? You're on the right track, I suspect. If the driving frequency is close to the resonant frequency, and there's not much dampening, the amplitide will go up and up and up ... Changing the resonant frequency is probably the easiest thing to do. Things that will do so include : - making the balls weigh less or more - making the shafts stiffer or less stiff - making the sticks longer or shorter Of course, the problem is that your driving frequency is going to depend on your rotor speed I imagine, and this varies, so doing these things may just change when you see the shaking. Adding dampening would be sort of tricky ... Another idea might be to connect the balls to each other rather than just in the middle -- so instead of having an X with a ball at the end of each point, you've got your X and your balls, but then a stick between the balls too, giving you a diamond shape. How effective this is will depend on exactly how it's vibrating ... SignatureDoug McLaren, dougmc@frenzy.com Marge, it's uter-us not uter-you. --Homer Simpson Thanks to all, Unfortunately (as is the way of these things!) I cannot be certain as to what I changed between having a nice non-vibrating heli and a vibrating one. There are two principal candidates (aside from the intrinsic perversity of the universe) - I changed the brew I was feeding to the engine, and I also changed the method of attaching the training gear to the heli. Perhaps the fuel changed the rotor speed for hover (more nitromethane) and the other reduced the damping. Thanks to all of the responders for your various suggestions. Gerard You've hit the nail on the head yourself! Happened to me back in the day as well. DO NOT USE rubber bands, pull some cable ties up nice and tight and the problem should go away. The rubber bands allow some pretty bad resonance - to the point that it can destroy lift. You can get releasable cable ties from your LHS. Cheers Rubber bands can certainly be a problem for the reasons stated. I'm working with a friend, buddy boxing him and the training hear we bought uses carbon fiber rods with the wiffle balls glued to the ends etc. We found that if we let the main rotor rpm on the Raptor 30 get a little to the high side, the machine will enter a harminic resonance that has it shaking like a proverbial leaf. NOT fun! And this is "with" the tie straps instead of the rubber bands. We could use wooden dowls instead of the carbon fiber rods but that would add weight. There's no free lunch. Anytime you add something to the model, you run the risk of changing the vibration frequencies that might cause problems. Changing the mounting method may help or simply reprogram to keep the model out of the rpm range that causes the problem. That's what we're doing so far. Fly Safe, Steve R. > You've hit the nail on the head yourself! Happened to me back in the day > as well. DO NOT USE rubber bands, pull some cable ties up nice and tight [quoted text clipped - 3 lines] > > Cheers |
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