MX400 Bad Horizontal Tail Vibration - Need Help

[kcgraves]

I have an MX400 pro that I am trying to put the 345mm carbon tail on, but with the stock wood blades. With the stock boom I have had this this problem but was usually able to get rid of it. The horizontal tail fin vibrates about an 1 1/4" on each end starting at about 1/2 throttle. I have adjusted the belt, replaced the tail drive shaft, checked the blades and main shaft, etc. No amount of adjustment seems to get rid of the vibration. It seems better if I put the stock boom back on. Any ideas would be greatly appreciated.

Thanks.

[chopper 3]

Ok i had this prob with my T rex and it was the feathering shaft that holds the tail blade grips mabye this will help. it could be bent slightly and not running on a true plane

[AaronS]

try going through the machine without blades, main or tail, see if you get the same vibrations, keep taking parts off untill you are able to see where the vibes are coming from. rebalance everything. Make sure not to overspeed the mechanics when trying this, you only want to spin it up to look for parts that are vibrating or spinning out of true. this method is best used on small models like the MX-400, this may be unsafe with a larger machine.

[kcgraves]

Thanks to both of you for the suggestions. I will try them but probably not until next week. I do not think it is the rotor head/blades. I did take those off and spin it up and it still vibrates, so it looks more like something in the tail. I will have to order a new spindle shaft as that is one piece I do not have and have never replaced.

Looks like a nice fall weekend here in upstate NY which means raking leaves, putting the porch furniture away, mowing, weed wacking, etc. Probably not may more left. Has rained the last two weekends. Will let you know.

[cbflys]

I actually had this problem on an MX-400 Pro I built for a buddy - brand new out of the box and never flown. I began removing parts. When I got to the main blades the vib stopped. HOWEVER - when I looked at the spinning main blade holders - they werent tracking together (and I mean the nuts holding them on). Turns out the feathering shaft was slightly bent. Also - the main gear was slightly warped. Replaced both and machine is now rock solid.

[Chickenhawk]

This sounds suspiciously like a tail vibration problem. Have you replaced your tail blade grips recently? When I upgraded to metal tail blade grips, I suddenly had vibration so bad it shook every screw loose in the tail case in under a minute. All I could find was one of the new bearings weren't quite seated all the way in and the one tail blade was VERY slightly farther in on the shaft than the other one. This was enough to cause the serious vibration because of the tremendous speed that the tail blades are flying.

Take your tail blade grip assembly off the tail shaft by loosening the grub screw and popping off the ball link. If you hold it up to the light, you should see an equal amount of light on either side of the feathering shaft. If there is little to no clearance between the feathering shaft and the tail blade grip, then the bearing isn't seated all the way.

This is also a good time to check the tail shaft fro straightness, as I found my was slightly bent. (Maybe from running out of ballance??)

[kcgraves]

Chickenhawk,

I have the metal blade grips, have had since about week #2 after a bad crash. But I just put them on and never gave it a thought. I will have to pull it off and check it. With the stock metal boom on I have a little vibration in the horizontal fin, about 1/16". With the longer carbon boom the vibration appears to be well over 1" on each side. I have had carbon fiber fins since last spring as I was breaking the stock plastic ones on an average of 2 of month. The carbon can take the vibration much better.

I replaced the tail pully shaft, main shaft, and main blade feathering shaft just for my own sanity and it was no help. I did have a bent tail shaft a few months ago that caused a similiar vibration in the fin, took me a few days to find it as I did not suspect it. Never hit the tail on anything, must have just been its time to bend. I will look to the other pieces of the tail and also start removing parts AaronS mentioned.

Thanks all, will let you know what I eventually find out.
Keith

[Chickenhawk]

Yes, check your metal tail blade brips for bearings not seated all the way. If you hold them up to the light, there should be an equal amount of clearance on either side of the feathering shaft. Mine had some clearance on one side but very little clearance on the other. (The tighter side didn't have the bearing seated all the way.) I honestly thought that tightening up the blade screw would seat the bearings, but I guess the metal blade grips cannot seat them this way. All it took was a proper size punch and some GENTLE tapping though, and now they are perfect.

Or at least they are perfect on my balancer. I am still waiting for a new tail case because the vibration shook the whole thing apart.

[kcgraves]

Chickenhawk,

It definitely is something in the tail blade grips and adding the blades makes it worse. I remove the blades, head, etc and it was still there. Removed the tail blade grips and it is gone. Just put the blade grips on and the vibration is about 1/4" put the blades on and we're back to an 1" or more.

Checking the space between the grips and the spindle hub the gap appears the same. About 2 sheets of paper can fit between each grip and the hub.

Now a couple of questions.

1. How far on the tail spindle do you have your spindle hub? Flush or some shaft sticking out? I have always had mine flush but thinking about it the farther in it is the less torque is placed on the outside of the shaft.
2. Looking at the MX400 vs the MX450 tail parts list they have the trailing edge of the blades on each grip different. The MX400 has the leading edge closer to the ball and the MX450 has the trailing edge closer to the ball. Does it matter?

Getting frustrated but at least I have it narrowed down, now just how to fix it?

Thanks again.
Keith

[Chickenhawk]

Sounds like you are getting closer. I would suggest taking the blade grips off the feathering shaft and seeing if you can't gently tap the bearings further into the grip. Use locktite when re-installing. (You do NOT want the grips flying off in flight!) Be careful that locktite only gets on the ends of the threads, not on the outside of the feathering shaft, nor on the inside of the balde grips.

I have my blade grips mounted so that there is about 2 to 3mm of shaft sticking out. This gives me more travel throw than with the shaft flush. But the distance down the tail shaft you place the blade grips will also affect the distance to the tail servo, so you may need to adjust the servo rod length at the ball link. (You should aim for almost neutral tail blade pitch when the servo horn is 90 degrees from the servo, or a VERY slight amount of right rudder pitch. This compensates for the torque effect of the main blades, but you only need a slight amount of pitch because the tail blades fly VERY fast.)

Some people have the tail blades with the ball at the leading edge. The factory has them at the trailing edge. Apparently, reversing them with the ball on the leading edge gives more authority in right rudder. It is easy to do. Take the feathering shaft off, flip the balls around, take the tail blades off and flip them around and be sure you reverse your gyro. (The tail blade grip screw heads should now be on the INSIDE of the blades instead of the outside.)

First, test your rudder direction with the motor disconnected and your gyro in non-heading hold ("rate") mode. The leading edge of the blade should move toward the tail on a right rudder input and vice versa. (Look at the tail blades as "screws" that are "screwing through" the air in the appropriate direction.) Then switch your gyro to heading hold mode (Futaba calls it AVCS.) Now with the motor still disconnected, watch what direction the tail rod moves with a right rudder input. Hold the heli in your hand and quickly swivel it to the left. If you watch the rod, it should now move the same direction.

Remember, when the heli rotates to the left, the gyro is trying to get it to stay straight by adding RIGHT rudder input.

Gosh ... I hope I have this right!

[kcgraves]

Thanks Chickenhawk and everyone else that replied. It appears to be "fixed" although it was only 1 and 1/2 batteries worth.

I took the blade grips off, cleaned the inside and tapped the bearing in to really seat them. Then put the spindle shaft about 3mm in on the shaft. At one point during a spin up the horizontal fin vibrates but then smoothes out by the time I lift off. With the spindle that far in my servo is now centered and my control rod is no longer "too short". And yes, I did use locktite on the screws.

Thanks again for all your help
Keith Graves

[Chickenhawk]

Nice to hear. The momentary vibration is normal and is simply the tail blades finding their correct lead/lag positions. Are your tail blade grips too tight? The tail blades should be very slightly looser in their grips than the main rotor blades. (I know my main rotor blades are the right tension when I can hold the heli vertically and shake it up and down a bit and not have the main rotor blades drop, but finger pressure will cause them to pivot in the grips properly. The tip I learned in RC Heli is to lay the heli on the floor and put a finger against the ends of both rotor blades and then pull them toward me. If the tension is equal, they should both pivot in the grips equally. If one is too tight, all the movement will be in the other blade.)

I also use this technique to check my tail blade grip tension. If I tug gently on the tip of both tail blades, they should pivot equally in the grips.

Maybe you had the tail blades too tight in their grips?

[kcgraves]

I figured the momentary vibration was the tail blades centering.

The tail blades were not the cause of the original vibration, I had them and the main rotor blades adjusted as you describe. The tail blades are always a little loose without flopping and the main blades are shake proof but not tap proof. I too read the article in RC Heli and now test the even pull on the blades. Thinking of getting a small oz. scale, similiar to a fish scale and using that for a finer adjustment.

The original problem was a combination of the tail blade grip bearings not being fully seated and the position of the tail blade spindle assembly on the shaft. Moving it in, as you suggested, to about 3mm got rid of the final vibration and gave me better tail authority.

[Chickenhawk]

Nice! Have you tried moving your tail blade grips around so the ball is on the leading edge?

I did that but never even tried it the stock way, so I can't judge if it made a difference or not. I am using a Futaba 3110 on the tail servo and a Futaba 240 gyro. I am setting everything up using a Spektrum DX6 radio at the S-75 servos on the main rotor.

Can you tell me how long your tail servo horn length is? This is a debate on another forum. I always thought you want as long as possible because speed is important on the tail, but someone is telling me I should make the horn as SHORT as possible for better authority. (I have my 240 gain turend up to 100% with no tail wag.)

[kcgraves]

I was reaching my frustration point with the vibration so I did not try turning the grips around yet. Wanted to deal with one problem at a time. It is working nice now so I might not until I have to take it apart for something (crash) else.

I also have the 240 Gyro and I have it at about 75% which is where it has been all along. My tail servo is a Futaba 3103 with the stock horn, I am using the 2 hole leg and it is on the outer hole. It is between 9 and 10mm from the center. I do not have it in front of me at the moment to get the exact measurement. I have seen several discussions on the distance and it boils down to the further out the more travel but the slower it is. The closer in the less travel but the response is faster. My other servos are JR DS281s and JR PCM 770 receiver. I have a JR6102 which I also use on my hacked Blade CP. I put a JR610UL receiver on it.

I just fly around my backyard, which is a horse pasture without horses so I have plenty of room and a barn for when it is too windy. Found a flying club about 35 miles from me and am joining next year to learn some 3D. Also saving up for a Raptor 50 after seeing Curtis Youngblood perform there in August at a Heli Jamboree.