Flutter discussion
05-03-2015, 07:58 AM
#1
Thread Starter
Flutter discussion
I recently had my first experience with it and luckily for me it only destroyed my 250.00 servo. What is confusing to me was the lack of what I thought were symptoms. I did not have the typical noise or anything flight noticeable my spotter did say he saw my controls were fluttering but I think the servo had already stripped. I landed to be safe and once on the ground the rudder servo was only working halfway. Upon inspection of the servo I had two gears with one tooth on each gear broke.
This is a 500+ oz servo and the jet had no other signs, the servo motor isn't working right either so after replacing the gears I found the servo bad. I think my issue is the setup, the servo arm was longer than the control horn on the rudder and I had cut down the endpoints killing the resolution and torque. I know better as long time builder and I am correcting this for next time.
My worry is even with the changing of the mechanical setup, if the control horns may be to short? This is also a worry on my other project since it does not have a traditional style control horn. I think as long as I keep the link as close to servo output shaft and the distance is less than the distance of the Hinge to the control horn pivot point I should be okay. Both have hinge gaps that are covered so other than adding longer control horns is there anything else that can be done? Other than take off I don't remember even applying rudder inputs during the flight and other than gear play there is no slop.
Thoughts, tips, hints and tricks are welcome! Cheers
This is a 500+ oz servo and the jet had no other signs, the servo motor isn't working right either so after replacing the gears I found the servo bad. I think my issue is the setup, the servo arm was longer than the control horn on the rudder and I had cut down the endpoints killing the resolution and torque. I know better as long time builder and I am correcting this for next time.
My worry is even with the changing of the mechanical setup, if the control horns may be to short? This is also a worry on my other project since it does not have a traditional style control horn. I think as long as I keep the link as close to servo output shaft and the distance is less than the distance of the Hinge to the control horn pivot point I should be okay. Both have hinge gaps that are covered so other than adding longer control horns is there anything else that can be done? Other than take off I don't remember even applying rudder inputs during the flight and other than gear play there is no slop.
Thoughts, tips, hints and tricks are welcome! Cheers
05-03-2015, 08:42 AM
#2
In my experience, the vertical stabilizer is the weak link in many designs. If the vertical is not stiff enough the rudder/fin coupling can cause flutter. The servo and linkage can only do so much. Maybe one of the aero engineers can chime in.
I have fluttered the fin on 2 different planes. Not just the rudder, the whole fin moves with the rudder.
I have fluttered the fin on 2 different planes. Not just the rudder, the whole fin moves with the rudder.
05-03-2015, 09:13 AM
#3
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F-16s of all scales have this problem. The whole fin and rudder assembly flutters. a lot of the time it is because the top of the fuselage doesn't have enough strength to keep from flexing at the base of the fin.
05-03-2015, 09:27 AM
#4
On the Eurofighter with the fin plugged in I could easily flex the whole spine of the fuse with one finger. We made a nice laser cut bulkhead to distribute the twist load into the fuse better. That could be all thats needed on the F-16 instead of relying on the shape of the fuse as a structural shape without support and load distribution.
05-03-2015, 09:27 AM
#5
Thread Starter
Well this particular model they did add carbon to stiffen the rudder up. Mine does have the carbon but could need more because if the flutter came from flexing then I'm really glad I didn't try any knife edge stuff. Being a jet that only had four flights on it it I always keep the first handful of flights to basic stuff so if things do show up hopefully I can catch it before it goes really bad.
I forgot to mention this was a sm avanti xxl with p200. I didn't get the jet very fast. I think the thrust vector pipe is killing the push. I didn't active it since it was just the first few flights.
I forgot to mention this was a sm avanti xxl with p200. I didn't get the jet very fast. I think the thrust vector pipe is killing the push. I didn't active it since it was just the first few flights.
Last edited by FenderBean; 05-03-2015 at 09:30 AM.
05-03-2015, 03:22 PM
#7
Thread Starter
Yeah its a BLS 172 they are 513 oz at 7.4 volts, Its a big jet and the rudder is beefy.... I normally only use these on my stabs but this being a 3D jet figured why not, futaba doesnt have a nice good 350-400oz Sbus servo.
The jet is right 50lbs wet so using something in the 300oz range didnt seem right.
I was more worried about ripping the control horns out of the control surface not stripping gears.....thats a costly servo to loose.
The jet is right 50lbs wet so using something in the 300oz range didnt seem right.
I was more worried about ripping the control horns out of the control surface not stripping gears.....thats a costly servo to loose.
05-03-2015, 03:30 PM
#8
Exactly bro, let me put it like this.....the rudder structure can't take that much force.
Anyway, discarding that the rigging was done incorrectly (I'm sure you did it right), it comes down to two possibilities: bad labor or bad design.
Knowing that there are other Avantis without this problem.....it comes down to one: bad labor!!!
Anyway, discarding that the rigging was done incorrectly (I'm sure you did it right), it comes down to two possibilities: bad labor or bad design.
Knowing that there are other Avantis without this problem.....it comes down to one: bad labor!!!
05-03-2015, 11:02 PM
#9
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In my view airframes are simply getting way to big and the methods used to build at this larger scale are not different enough from the smaller models. Forces multiply in a non linear way as you get larger planes
05-04-2015, 09:56 AM
#11
Another thing, I was speaking to a major importer and he confirmed some problems I am seeing with some ARFs, mainly improperly or incompletely dried plywood.
When you get into certain climatic (humidity) changes this moist plywood expands and contracts and you end up with loose glue joints.
I doubt knife edge would have much to do with this, the issue is speed, if you were not going too fast thank your lucky stars, speed kills with fin flutter.
When you get into certain climatic (humidity) changes this moist plywood expands and contracts and you end up with loose glue joints.
I doubt knife edge would have much to do with this, the issue is speed, if you were not going too fast thank your lucky stars, speed kills with fin flutter.
05-04-2015, 01:01 PM
#17
Thread Starter
Yeah after this pick I moved down to the next hole, the problem I ran into was the lower hole and linkage would need to go inside the rudder and then the rod hits the side of the rudder where the ply box is for the carbon stud. This pick was taken before I moved down on the arm but I am going down one more hole. This is as low as I can go unless I add much longer control horns because of the rod hitting.
So "A" is the location on the servo arm where the flutter happened, the "B" is where im moving to because any lower on the arm and you when the servo arm is like position "C" the rod hits "D" which cannot be cut.
So "A" is the location on the servo arm where the flutter happened, the "B" is where im moving to because any lower on the arm and you when the servo arm is like position "C" the rod hits "D" which cannot be cut.
Last edited by FenderBean; 05-04-2015 at 01:08 PM.
05-04-2015, 01:10 PM
#19
Thread Starter
I have thought about this as well, it will help some but it only gets my a just a hair closer and I will need to get a longer rod. The down side to this is less mechanical advantage.
05-04-2015, 02:04 PM
#22
Thread Starter
Now thats a thought, dang should have thought about that before I cut a bigger hole for the ball end. Well when I move the rod end to the inside hole I am less than an inch from center so I should be okay there. Damn never crossed my mind to move the servo towards the outside wall, man what a super idea. Well once I get the new servo and hook everything up if it hits the wall I will then move the servo out some. Everyone think less than one inch on the servo arm should be good?
