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Horizontal stab flutter!!

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Horizontal stab flutter!!

Old 08-29-2023, 04:52 AM
  #1  
MrRover75
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Default Horizontal stab flutter!!

Hi all,

I have for a while battled with a VQ F6F Hellcat. Nice looking model, but has not impressed me much for the time I have had this. Powered by a Saito FA82B. The main issue now, is that the horizontal stab flutters a lot during straight flight at speed. Hinge gap i tight with CA hinges. I have even tried to stiffen up the stab by installing a 5mm carbon tube through its whole length, but without any luck. Seems to me that its basic construction is just to weak. Large stab, conventional built up by 8mm balsa stringers. Any suggestions how to deal with this?

Any advice greatly appreciated

Good looking .40 size ARF model, but it stops there....
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Stab stiffened up by installing a carbon tube through its whole lenght:

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Old 08-29-2023, 08:07 AM
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mgnostic
 
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Sometimes flutter is an aerodynamic process, not a structural one. What I mean is that even very strong structures can flutter under the right circumstances. You mention flutter "at speed", is it high speed in general or above a specific speed? What is the frequency of the flutter? That is, do you hear a loud buzz or can you see the horizontal tail flapping in the breeze? Does the airplane have stability augmentation? I have seem planes that had the gain on the gyros turned up too high and the tail wiggled like a tadpole. It looks like you are using stock hardware. Is there any give in the linkage at all? You don't mention what the elevator servo is. It's possible that it isn't quite up to the task. The FA 82 may just be a bit much for the airframe. I realize I'm tossing out a lot of possibilities and not much in the way of answers but often it is a process of elimination rather than having a answer right away.
Old 08-29-2023, 04:53 PM
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speedracerntrixie
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I would pull the covering off the stab and elevators and then sheet then with 1/16 balsa.
Old 08-30-2023, 02:48 AM
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MrRover75
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Hi, thanks for your feedback

Flutter can be heard and seen during fly-by at approx. 3/4 throttle. The airplane is quite heavy, as most warbirds tends to be, so it does not feel much overpowered despite the engine size. This model tends to be tail heavy, so I know some has flown this one with .91 and 1.20 four strokes to avoid putting lead in the nose, but none has reported any flutter. I have checked elevator linkage and eliminated as much play as possible. Feels pretty firm and nice, but there will always be some play unless using high end servos. No gyro here, just going old school
I am a bit clueless. Think the manufacturer must have used a batch of soft balsa for the stab or something. Pulling the covering and sheet as mentioned would be an alternative ,but will add weight as more lead to the nose will be needed to maintain CG.
I am thinking about adding wires between the stab tips and around to stiffen it up??
Old 08-30-2023, 05:39 AM
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jaka54
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How big is the plane? 140-150cm span? And weight? If you press hard on the elevator...does it flex? Flex in the elevator could induce flutter in the stab.
As speed suggested...I also would cover the stab with 1,5mm balsa on both top and bottom.
I have many times seen flutter occur in both sport and pylon racers and every time it has been due to a too weak stab or if on a wing, having the ailerons go out all the way to the tip of the wing and being to flimsy.
A too short elevator arm could also be the culprit in conjunction with flex in the elevator push rod or linkages. I do recommend using either wires or using a carbon rod for elevator control.
On my airplanes both sport and pylon racers I strive for having as short servo arm as possible and as long elevator arm mas possible and using 100% or more throw in the transmitter, this to reduce play in the gear train in the servos.


Old 08-30-2023, 05:52 AM
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A. J. Clark
 
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How do you have the two elevator halves connected together? Flutter is more likely to be initiated by the moving surface.
Old 08-31-2023, 07:31 AM
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invertmast
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By how the carbon tube seems to be unsupported minus the ribs, its not surprising that it didn't help the problem. It really needs to be part of the rest of the structure by tie-ing the top/bottom skins together. Using some contest grade balsa to fully sheet the top/bottom of the stab as well as placing some 3/16" balsa sheer webbing ontop/bottom of the CF tube to join the tube to the new skins will likely fix the problem and have minimal significant impact to the weight of the model.
Old 08-31-2023, 07:39 AM
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Thinking of things unsupported, how well is the pushrod anchored in the fuselage? Usually anchoring the sheath at either end and once in the middle is enough but it is one more thing that is easy to check.
Old 09-04-2023, 03:58 AM
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MrRover75
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Hi all,

Thank you very much for your advice. Much good input here Seems like everything is shortened down to:
1. Elevator linkage
2. Stab stiffness.

As the first one is easiest to chase first, I will try to take this one first. There was some play in the elevator which I traced down to the servo. Standard Futaba servos was mounted (new) but had some play in the bush/bearing. Replaced these with some more costly servos with ball bearings and metal gears. This instantly helped a lot. There is still a minor play in the linkage. Seems to be from the flex in the pushrod. Red type of Sullivan gold-n-rods have been used, and it seems that there is some clearance between the tubes inner diameter and the pushrod itself! Would the blue ones fit tighter in its sleeve? The rest is tight and good. I think I will do a testflight sooner or later and see if the servo swap help. Some pictures from the linkage:

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Old 09-04-2023, 10:21 AM
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Given that swapping out pushrod can be a real pita, this may be info for the next plane. The red Sullivan rods are usually labeled as "flexible". The general intention is that they are useful when you have to snake a rod around other structures or go around a curve. I generally use them for throttle linkages and small airplanes up to about .25 engine size. The blue "standard" rods are much stiffer. The stiffness comes more from the sleeve then the inner rod. I would have to go out to the shop and check but I'm pretty sure that the inner rod from a blue sleeve wont fit in a red sleeve.
Old 09-04-2023, 11:27 PM
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MrRover75
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Originally Posted by mgnostic
Given that swapping out pushrod can be a real pita, this may be info for the next plane. The red Sullivan rods are usually labeled as "flexible". The general intention is that they are useful when you have to snake a rod around other structures or go around a curve. I generally use them for throttle linkages and small airplanes up to about .25 engine size. The blue "standard" rods are much stiffer. The stiffness comes more from the sleeve then the inner rod. I would have to go out to the shop and check but I'm pretty sure that the inner rod from a blue sleeve wont fit in a red sleeve.
Hi Matt,
You are correct about this. From my memory, the inner rod from the blue will not fit inside the red sleeve. The red version was used due to availability.
It was much easier to get good hardware back in the -90`s then it is today, especially here in Norway. ARF`s has basically killed the market for hardware....

What I can do, is to replace the inner rod with a 2mm steel pushrod and slide pieces of Sullivan inner rod onto it, evenly spaced along its length. This configuration is commonly used on most Great Planes models I have had, and seems to be pretty stiff and slop free.
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Old 09-05-2023, 06:31 AM
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jaka54
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When I look at the photos I see some things that I know is not good.
1. you are using red Sullivan flex rods and they flex! They are only good for small slow flying 0,8cc-3,5cc powered airplanes.
Instead use either pull-pull, thin steel fishing lines or carbon fiber tubing.
2. You do not have contra nuts on the treads on linkages! This makes more play in the clevis due to play in the treads.
3. A rule of thumb is to always use as short servo arm as possible and as large elevator arm as possible.This is to avoid excessive play in the servo gear.
In your photos I can see that the elevator linkage is mounted way to much out on the servo arm and that the elevator arm is way to short.


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