Hey Guys, When I plug my flaps in on the King Cat they jitter really bad and chatter quite a bit.
I know that this subject has been addressed here in the forms. I have done a search and can't seem to find it. Anyone have a link to this topic.

Thanks

Paul

I don't know of any links for discussion, but you may try to remove the chokes from the flap channel. Also make sure you are running JR heavy duty extension wires, make sure they are making good contact. As always perform a valid range check after modifying you plane's electrical system.

i had the same issue on the elevator and rudders. Replacing 2 extensions solved the problem. Sometimes the connectors are just not in correct contact.


Wojtek

I had a similar problem but only experienced it with my Duralites off a fresh charge. changing servo extensions etc. did help but always had some jitter until after the first flight of the day.

If you are using JR-2721s then they are probably the problem. Sooner or later, every 2721 on the flight surfaces of my airplanes have begun chattering. Not only that, if you put an amp meter on them you will find that they are drawing over 1 amp each when doing this. I ended up replacing all of the 2721s in my Bandit, Sabre and Kingcat with 8411s. Overall, I have replaced eleven (11) JR-2721 servos with JR-8411s because of these jitters and all of the vibrations immediately disappeared. I did leave the 2721s on my nose wheel steering in one of the airplanes and it seems to be ok there. Its something about the load pressures of a flight surface that causes the 2721 to start rapidly recentering itself unsuccesfully that causes the problem.

Gordon

Gordon do you know if 4721s have the same problem? that's what I was running on the flaps/ rudders

hmm ... i'll watch out for all my 2721 servos ... i have them on most of my flaps, all nose wheels, and rudders on the king Cat .. no problems yet ..


Wojtek

I've never used a 4721 so I don't know if they exhibit the same problem but I do know that they are the exact same servo except the 4721 has nylon gear whereas the 2721 has metal gear.

The 2721s are sorta wierd in that they always seem to work fine when you first put them in an airplane and for a while thereafter. Then, one by one, they start this chattering/vibrating phenomona and once replaced with 8411s all of the problems go away.

Thanks Gordon

YIKES I just mocked up 2.

Thanks for the tip, Gordon!

Hey Guys

Thanks for all the suggestions, I will try with removing the chokes first, then new extensions and then change the servo's fo 8411's as Gordon suggested.

Thanks again guys!

Paul

I've had 4721's do the same thing . They get into a funky kind of harmonic .

Marc

Keep in mind that when the 2721s/4721s are shake-rattle-and-rolling that each servo is drawing over 1 amp of current which ain't good. I would urge anyone that is observing this phenoma to replace the offending servos ASAP.

Gordon

Gordon,

How many amps have you seen on the 8411's? One of the big warnings from BV and others about using digitals on the flaps was potentially high amp draw. Have you seen any evidence of this with the 8411's?

JS

Any servo can draw a large amount of current when it is stalled. I like to measure the current draw from the servos in my airplanes and adjust the flight surfaces accordingly to assure that none of the servos are able to approach a stall situation. I do not remember the exact current draw for my 8411's other than to say that it is close to their idle current. In any event, a properly installed 8411 will draw significantly less current than a 2721 that has gotten the jitters.

Gordon

I have stopped using 2721's on anything but nosewheel steering on the KingCat where they seem to hold up well on heavy takeoffs on our grass field......All the other control surface servos are 8411's....

I have about 140 flights on my KC with zero problems......my total current draw on those 8-8411's is always no more than about 400 maH/10 minute flight.....I use a SR 2200 maH pack 5-cell Nicad pack with no regulator.....

A certain percentage of 2721's and 4721's have had that jitters problem dating back into the mid-ninety's when they were the JR servo of choice on Big Bird Prop jobs....always blamed on vibration-induced deterioration, but obviously not the cause with jets...I noticed at the Minnesota Jet Rally that one of my Bandit 4721 flap servos started jittering, bad enough to replace again.....that is 4721 servo #5 in about 150 flights on the Bandit....one of these days I am going to go to 8411's....

With 8411's, you just must be careful setting them in the full up position on flaps so they are not stalled......I use an in-line ammeters.....takes a couple of minutes.....advantage then is that you have a digital servo with all it's improvements holding very tight around center which I consider important for rudders, which in my experience has been a common surface to experience high speed flutter on our model jets.....plus, it definitely prevents blowback of the flaps if you happen to deploy them at a higher airspeed, within the limits of the play and geometry of the linkage, of course....

Had the 8611's been out when I built the KC, I would have used them....I have exercised them and compared with 8411's....they draw just a bit more average current than the 8411 during normal motion, but do draw somewhat more when stalled....bottom line, if you are using digitals, just be sure they never stall.....

Tom