kmrcjets

looking for help. have a krill katana am haveing problems with elevator flutter. first time i had
hitec 7955 servo's. the model was flown with no initial problems, after about fifteen flights i
experienced the flutter problem at full power. i have just replaced the 7955 servo's with 7950
servo's .flew the model today still got the same problem flutter at anything over half throttle.
the model was set up as per krill's instructions. i primarily fly jets the katana was to be my fly
for fun model. can't say i am haveing much fun yet.any ideas or help would be great
ken
kmrc jets australia

speedracerntrixie

My Feedback: (29)

Ken, there could be several reasons for your flutter issue. A picture of your linkage would be a great help. The first thing and easiest is the linkage. You never want to go above a 1:1 ratio between the servo arm and control horn. Being that the Krill has the pocket type hinging, the fiberglass control horns from what I remember will place the ball link about 1.5 to 1.75" away from the hinge line ( Hinge rod ). By using a servo arm that is 1.25" this would give you good mechanical advantage yet plenty of overall throw. Not knowing how you intend to fly the airplane, I would continue to move the linkage in on the servo arm until you get the maximum throw you desire. The way the elevator servos are mounted means that you may have to make the hole bigger in the stab skin as the ball link may now be below the skin.

The second and more difficult is making sure you are feeding the servos enough power. Again not knowing your setup makes this difficult but startting at the servo this is what I like to see. All extentions must be 20 ga wire. I make my own extensions and solder them together so the only connectors are at the RX ( Or powerbox ) and at the servo. If you have multiple servo connectors such as just adding pre made extensions, you are loosing voltage and current through each connector. All battery wires need to be 18 ga wire or better. I prefer 16 ga. This gauge must be maintained from the battery to the RX. If power goes directly from the battery/switch or regulator to the RX, you must have two power connections going into the RX. A single connector even with 20 ga wire is not going to be enough power delivery to feed 6+ 300 oz servos.


Post some pictures of your setup and I can get more detailed.

kmrcjets

hi speedracerntrixie
i am running a 42% power board dual redundancy with 20 ga wire
i only want to fly the model for fun ie blow smoke and general
aerobatics nothing flash my geometry is nothing like you sugest
so i will try what you sugest soon. have next two weeks flying jets
will post results on the katana soon thank you very much for your
time and help.
regards
ken
kmrc jets

Mrs R

You haven't got an aircraft problem you have a 90% chance of having a servo problem.
I have been working on solving the jumping problem for 12 months and am writing a servo repair manual so if your servos are out of warranty you can fix the servo circuit problem.
HITEC have a new repair out and will replace the circuit board free of charge in the USA. The repair is being sent to
MODELENGINES very soon " they sent me servos to destroy and help solve the problems that plagues all 7XXX G2 servo"
Most of the problems showed up after a few flights and the servo Jitter and jump under light loads " see videos on
YOUTUBE "HITECJITTER" and similar names. We only just found out that some servos are Jumping under load and usually
shows up when the servo draws 1amp. Do not use wire larger than 20 AWG " BOOMARC " is getting 20AWG servo wire in
the next couple of weeks. We found that large Dia. wire especially wire with thick insulation makes the problem worse
due to capacitance building up in the wires and discharging into the servo.
We have 3 repairs that can be carried out by the owner 3 because it depends on what system you are using.
An example would be if you are using a SMARTFLY or POWERBOX system you only need to use repairs 1 and if required
repair 1.6 if using just a receiver directly to the servos you would need to do repair 1,1.6 and 2.
send me a PM and I will try to assist you.

KINDREGARDS
Rachael

billdor

My Feedback: (11)

I have the same airplane with JR 8611's all round and have never had a problem. Make sure your linkages are tight and that you have good quality servos and you should be ok. It would also be wise to make sure you have as much mechanical advantage as possible in your linkage. ie. hook up as close as possible at the servo and as far as possible at the elevator keeping the max throw required in mind and using all of the servo throw available (150%)

Bill

Flyer88

Ive seen this once before on a Krill Katana at our field.

Can't remember what the solution was though..........sucks for such a beautful plane.

I think the owner sold the plane.

I would check the mechanical advantage on the servo arms also.

Very short at servo longer at flight surface.

kmrcjets

hi guy's
thank's for the imput on this problem
1 not the servo's (hitec 7950)
2 not geometry. checked with other krill's
going to mass balance stab's
if that don't work going to pull out running
gear an put it all into a wild hare 35% extra
then fill the katana with bag's of petrol and
light it up. let you know how it goes after xmas
hope you and your families have a great xmas
regards
ken
kmrc jets

billdor

My Feedback: (11)

This excerpt was copied from a Composite Arf manual for an extra 300 2.1 which uses the same phenolic horns as used on the Krill Katana.


Do NOT use balllinks
on the servo arms or the control-surface
horns, because they will twist the servo
arm/horn and cause flutter. This is a solid experience
and you should consider it a FACT.


I hope this helps

Bill

speedracerntrixie

My Feedback: (29)

Bill, dosen't the Krill have double control arms with the ball link in the middle? Prretty sure I remember the one that I had built had double arms. Maybe the OP could post some pic we may be able to find something?

billdor

My Feedback: (11)

I agree Speed, a picture is worth a thousand words.

sensei

For about 5 years I owned and flew several 3.3 Comp Arf Yaks. The trailing edge of the control surfaces all came to a thin sharp edge and the surfaces were also very large and I could get them to flutter with enough speed. So the one thing that kept them from entering into flutter mode was adding a 1/4" or 5/16" wide plastic T strip glued to the trailing edges. By adding these strips it increased the trailing edge thickness and removed them from the dead band area that excites flutter mode in the first place.

Bob

kmrcjets

hi all
changed a few things on the katana. placed two ounces of led
in the counter balance of the stabs checked all horns and screws
also checked the h/d ball links all looked good .went out for a fly
this morning. flew the model straight and level flat out . no flutter
did a flat out dive to build up as much speed as possible. no flutter
looks like this problem is solved. had four flights no further problems
i thank all for there imput
regards
ken
kmrc jets
kingtech turbines australia

K-Bob

My Feedback: (1)

Great! Now all that's left is practice, practice, practice.

billdor

My Feedback: (11)

Great to hear Ken:

The Katana is a great flying aeroplane.

Bill