Vibration while spooling up then calm

[Birdmanpa]

After rebuilding my 300X using the Microheli CF frame, I now have vibration int the Heli when spooling up. From 0 to about 60% collective or when the Heli is about two feet off the ground there is a vibration that is visible in the vertical tail stabilizer, which shakes until there is a change in the pitch of the motor above this point soon after lift off. You can hear the change in the sound of the motor, it becomes smooth and you can see the Heli smooth out and the vertical stabilizer becomes rock solid. It remains smooth throughout flight and landing. This happens every time I lift off then smooth flight. Everything is tight and all adjustments including beast x setup has been gone over and I can't figure out what is causing this. Any ideas will be appreciated.

[vtown nick]

Take the main blades off and spool it up, watch the tail fin as it spools up and note the point at which it has the worst vibe. Then dynamically balance your tail till it no longer does this. If that is what's causing it that is. Are you still running the stock tail box setup?

[Leyline]

Yes balance the tail. Not just the tail blades. Take the whole tail assembly off the axle and balance it like a propeller

Then also dynamically balance the main blades

Check to make sure the bx is not up against the frame and getting extra vibes

[Birdmanpa]

Thank you, I will do that today. The answer to whether or not I am running the stock tail case is yes, but I have not balanced the tail blades. They took a hit but still look okay, when I had the crash that split the case and took out the main shaft and spindle shaft, which is why I decided to go to the Microheli CF frame. I will also just change out the tail rotors as well. Thanks again for the advice.

[Murph_d]

Just a thought, but easy to check. I've had spool-up vibes just because the the main blades were too snug or too loose (or tightened unevenly). Probably not your case since your vibe doesn't go away until you reach a certain collective. Interesting though, that you are smooth the rest of the flight.

[andycrazy]

Are you letting your motor reach full speed before taking off, as you say "until there is a change of pitch in the motor". Is this a change in pitch of the sound of the motor which indicates the motor is not at full speed before take off?

I get a bit of a shake just as the heli is reaching my governed head speed and gets light on the skids, lasts a couple of secs, and disappears as soon as its off the ground. I've never balanced a set of tail blades let alone dynamically balanced a tail on any of my helis yet they are all vibe free.

[DoubleCH]

Originally Posted by Birdmanpa

but I have not balanced the tail blades. They took a hit but still look okay

"Look okay" is nowhere near good enough. Even if you have perfectly weigh-balanced tail blades, a 0.05mm distance difference (thickness of a piece of aluminum tape) between each blade grip to the tail shaft center is enough to cause vibration like what you experience. This is why never trust weigh-balancing the blades alone, especially when you're in the middle of diagnosing a vibration problem. You need to dynamic balance the entire rotor assembly. This tiny imbalance may be amplified by the tail fin when the bolts to the fin and the tail case are too tight. This will cause vibration at certain frequencies and may go away at others. Find the HS/throttle that gives you the most vibration and dynamic balance tail blades at this HS/throttle by adding small piece of clear tape to 1 tail blade thru' trial-and-error method.

[Leyline]

You can greatly narrow the trial and error of you balance the whole tail like you balance a plane prop. Then check it again spoiled up with no mains on.

[DoubleCH]

Prop-balancing works as good as dynamic balancing which I still prefers because it only takes less than a couple minutes, much shorter time than to remove the assembly to put on the prop-balancer and re-install afterwards and re-test after prop-balancing. Trial-and-error sounds too much guess work but in reality if you know what you're doing it really takes less than a couple of minutes, for the ENTIRE process. Don't forget that prop-balancing needs to remove the hub from the tail shaft and re-install, possibly giving you a "surprise" result after re-install.

[Leyline]

Hrmmm I am new to this so I haven't developed a whole lot of tricks but I just loosened one screw and popped the links to separate the tail grip assembly. Then I put it on a spare main shaft and then just checked for balance. Adding tape to the light side until it matched.

Is tail "dynamic balance" the same as main blades?
Because on this 300x the tails are so small you can have just as much or more problems from the whole assembly

Only took me about 3 minutes and the. I spooled up. Butter smooth.

Then I dynamically balanced the mains.

Then I flew 3 batteries of some sweet ff circuits and then put her into a tree. So time to start again. Lol

[Unsafeatanyspeed]

For the record when I had some tail vibes someone suggested that a slightly loosen the tail case screws that hold it to the boom. I did ever so slightly and it's been smoother than Al Green after a fifth of coniac. Still is.

[DoubleCH]

Originally Posted by Leyline

Hrmmm I am new to this so I haven't developed a whole lot of tricks but I just loosened one screw and popped the links to separate the tail grip assembly. Then I put it on a spare main shaft and then just checked for balance. Adding tape to the light side until it matched.

See that's the problem. If your tail shaft is ever so slightly bent (reportedly it can happen out of the box), then balancing the tail assembly on the hub without tail shaft or pitch linkages may still have different results once you put it on. Why not just dynamic balance the whole tail assembly? No need to disassemble/reassemble tail assembly. I know some people think hey I have the super duper prop balancer why not use it and it's easy to see when the heavy side drops to the bottom no guess work. But in reality it's just as easy to add tape on dynamic balancing. You only need to add tape to random 1 blade the 1st try. If it's worse you know it's the wrong blade. If it's better you know it's the right blade. From then on there's no difference then prop balancing to find the right amount of tape for perfect balance.

Is tail "dynamic balance" the same as main blades?

Yes, but with main blades removed so that you can sense all the vibration coming from the tail.

Because on this 300x the tails are so small you can have just as much or more problems from the whole assembly

Exactly! If you prop-balance just the tail assembly without tail shaft or pitch linkages, you're doing half the job. Like I said, heli this small the culprit can be the tail shaft even if it's ever so slightly bent and maybe even right out of the box.

[Leyline]

I meant when I google dynamic balance. It talks about taking the main blades and finding the cg of the heavier one. Then adding tape to the lighter one. Inside or outside the cg of the lighter one depending on the position of its cg to the heavier blade. This is next to useless on tail blades this short and light. So I think by "dynamic balance". You mean something different than I do.

[DoubleCH]

That's not dynamic balance. I call that CG-balance. By dynamic balance I mean spin-balance, like when I take my race car engine to the shop to spin-balance. Install the entire assembly with all moving parts. Run it up and balance it by adding/removing weights at strategic points. Weight-balance or CG-balance each individual parts without spin-balancing the whole assembly in a race car engine is useless. Similarly for the 300X main/tail rotor assemblies. For bigger helis you may away without dynamic balancing because the tolerance is tighter in the bigger parts like tail shaft, bearings, etc. That said, I still dynamic balance my stretched Protos (470mm main blades) main/tail rotor blades AFTER weight- and CG-balancing the main blades and still find a little bit of tape is required to make it perfect.

[Leyline]

Oh yes. Well that is why I said to balance it like a prop. To narrow down which side is heavier and get it close. Then spool up the tail with no mains on. Fine tune it from there