Flap 'vibration' Problem ?

[Robin_Holden]

Greeting guys from the Charente in S.W.France.

Advice needed please for a problem I've never encountere before  !

Went out to fly today . First test / trimming flight for a new model. Profile fus' , 58 in. span , 700sq. ins , Saito 56 [the one with the factory c/l venturi]  and a 12.5 x 5 cyclone prop' .

Took off with around 8,400 on the tach' , circulated at 5 secs but ....Around half way through the flight I felt a 'vibration' through the lines .
Spooky !

I thought the silencer was vibrating loose or worse.

At the end of the flight , my pal said he'd seen the flaps 'vibrating' .... shock , horror  !

So , could this be an engine vibration at the low RPM  perhaps ? But I only noticed it half way through the flight.

Any contributions really appreciated.

Robin [ ex-pat Brit' in the Charente full of ex-pat Brits ' ] . 

[John Stiles]

Sometimes hinges are so free, they allow extra movement that wouldn't be there with the non-pinned types of hinges...such as CA hinges. The ones with pins have no resistance.....sometimes good, sometimes not so good. Maybe that's it...........maybe not, just sayin.

[john e. holliday]

Tape the flap hinge line.

[Dick Pacini]

Sometimes hinges are so free, they allow extra movement that wouldn't be there with the non-pinned types of hinges...such as CA hinges. The ones with pins have no resistance.....sometimes good, sometimes not so good. Maybe that's it...........maybe not, just sayin.

My P40 barrel hinges bound up from ca glue didn't allow the flaps to vibrate.  In fact, they didn't allow them to move much either.  However, I felt the earth tremble when I stuffed it in.

[John Miller]

I had a plane develop slop in the flap horn. It showed up as "flutter". It was soo bad that I could see the oscilating waves standing out on the flying lines. The plane was trying to destroy itself.

I cut into it, and resolderd the horn, and bushed the flap pushrod hole. Problem solved.

There are several likely causes, most of which were previously mentioned.

[Wynn Robins]

check that it isn't the wing doing it - I had a profile cardinal a few years ago that when it got to a certain speed, the wings vibrated like you wouldn't believe .....the outboard wing ravelled up and down about 3 inches from what the guys outside the circle told me

[jim ivey]

I'm sure many will disagree , but th truth is elevator and flaps arent zeroed. it sets up a resonsnce between the elevator and flaps. nothard to fix on a profile.also hold the flaps firm and see if there is any play. alignment is one slop in the controls is 2 that is harder to fix .    jim

[Steve Helmick]

Is the Saito .56 side mounted? That could be the root problem...vibration hitting the pushrods, flaps, and even lines.  Steve

[Brett Buck]

I'm sure many will disagree , but th truth is elevator and flaps arent zeroed. it sets up a resonsnce between the elevator and flaps.

I guess will have to take back the 30 or so Open championship trophies won with a bunch of down elevator rigged in at neutral flap.

 

    Brett

[Michael Massey]

I just got onto the forums to ask about...guess what?  Vibration!  What a coincidence.  I am certainly going to look at all of the points raised here but would like to tell my "symptoms" to see if anything else pops up.

My airplane is relatively new.  It is a "modified" Pathfinder that I made into a built up fuse as opposed to a profile.  Unfortunately it came out too heavy.  I weighs in at 66 oz with an LA 46.  Seems to fly straight and ok without any real dramatic trim issues.  But the control lines "vibrate", much like a resonant vibration, when I fly towards the wind.  Going away from the wind, the vibration goes away.  I was flying with .018 lines but tried a set of .015 lines to see if that made any difference.  Although the wind was much more calm during that flight, the characteristic was still there, just more subdued.

I was flying the plane at about 5.3 laps, 62 foot lines, with a 12.25x3.75 APC turning about 9,900.

I would certainly appreciate any additional ideas to what has already been posted.

[Bob Reeves]

It's called flutter and has nothing to do with using a 4 stroke. Could be your flaps were not built stiff enough or who knows what. I agree with Doc, first tape the hinge lines and see if it helps. Only airplane I ever flew that had a flutter was a Super Clown ARF, I just started cutting down the flaps till it quit.

[Avaiojet]

Robin,

I know this suggestion will sound crazy, however, there's a couple of dozen guys that will, explain how to, and will get you through it.

Try wind flying the model and see if that makes a difference. Pay close attention while in the overhead maneuvers.

CB

[keith varley]

I have never had this problem on Stunt planes but it is quite prevelant on R.C.Pattern planes (F3A).  The solution is to counterbalance the offending item (usually the flaps) in much the same way that full scale aerobatic planes are counterbalanced .You add a weight , equal to the weight of the offending surface, attached ahead of the hinge line so that when the flap goes down the weight goes up.You can see this item all the time on the bottom of the flaps of full scale aerobatic planes like the Extra 300.Look under the wing of aerobatic planes flying in competition. Keith
Probably Ted Fancher could tell it better than I can.   Keith

[Robin_Holden]

Much obliged for all the advice fellas.

I've changed the flap horn as I noticed the hole was a tad too big , allowing some 'slop' in the connection.

I've also taped the top of the flap/wing along the gap with some brilliant clear , surgical tape.

Waiting for a calm day , thanks agin all ....

Robin.

[Igor Burger]

Yes, I had ... that problem can happen (if all is well built, stiff, no slopes etc) also when several parts of the frame has the same resonantion frequency (or harmonic), it was the case of my model. Solution was instaling small weight on TE of flap tips to change its of resonantion frequency. It was 1/8 oz on both sides. It solved all problems.

[Howard Rush]

There are a couple of phenomena that it could be.  One is a resonance excited by another vibration on the airplane.  Another is flutter, which is self-exciting.  Robin's sounds like a resonance.  Mike's sounds like flutter.  Igor's was certainly a resonance: adding weight behind the hinge line would have made flutter worse.  Flutter is an interaction between aerodynamics and structure.  Flutter is a function of airspeed.  As an airplane approaches the flutter speed of a wing or control surface, it can go from quiet to the surface blowing off in a few knots.  Pilots doing flutter testing get extra pay and are very careful approaching the predicted speed.  I haven't seen it much in stunt planes, but a combination of speed, control linkage flexibility, low hinge friction, hinge line too far aft on the surface, and weight behind the control surface hinge can cause it. Fixing any of these can cure it.  Putting a balance weight forward of the hinge is standard practice on full-scale planes, and on a stunt plane would be preferable to increasing friction.  Stiff pushrods should do the trick.  

[Dan Bregar]

Very well & clearly stated Howard ! 

[Dave Hull]

A comment on Michael's symptoms:

A possibility is that you are experiencing what might be called drag instability of the lines. This is common on faster racing planes. When I first started to encounter it, it was quite unsettling, thinking that there was something wrong with the plane or the controls. I have felt it on everything from F2Cs, F2CNs, Goodyear racers, etc. You can definitely feel the difference between going upwind and downwind. If the effect on a particular plane is fairly minor, you will only feel it only on one side of the circle.

Here is what I believe is happening:  the lines are changing in their relationship with one another, such that one will fall behind the other line, drafting behind the leading line and then it will pop out. The drag on the lines is therefore constantly changing at the oscillation frequency, and you can certainly feel it and you can see it if it is bad enough and your eyes are good enough.

Why do I believe this is what is happening, at least on my racers, and perhaps on your stunter? First, I could make the problem go away on some airplanes by getting rid of the line grouper I was using at the handle. Second, I could change the characteristic by changing to lines with different drag values. For example, changing from solid lines to stranded lines or vice versa.  Third, it was speed and line-tension dependent. Finally, it changed if you put twists in the lines.

The reason you may be experiencing this on your plane, when it doesn't appear to be common on stunters (in my experience), is that you have a heavier plane, and you may be flying fast, giving lots of line tension. Without significant line tension (at least relative to the line size) I haven't seen this effect. Just a related comment--at the weight you stated, I sure wouldn't be using .015 stranded lines!

There are other possible causes that others have already listed such as control slop, control lack of stiffness of the controls, structural softness of the control surface, and even lack of stiffness of the fixed surfaces. However, if each of these seems ok, then the above might give you ideas on how to check for line instability and perhaps how to reduce or eliminate your problem. For example, if you are lucky enough to have independently adjustable line guides, move them apart, while keeping the average location the same. Check to see if it goes away with no/some/lots of twists in the lines. And flying on longer lines at the same speed should also reduce the problem.

Let us know what you find out!

Dave "McSlow" Hull

[Howard Rush]

Wow, that's interesting.  I wonder if there's a way to get one line to stabilize behind another (Figure 1).  This calls for a handle-mounted video camera. 

  oo
Figure 1

[Michael Massey]

Thanks for all of the input.  I taped the hinge lines and checked for too much free play in the control system.  The elevator does have a lot of play.

Weather here is wind and rain so not sure when I can get out and try it again.  The plane was flying slow so I will pick up the lap times and see if that makes a difference.  The lap time was slow enough that the lines would get slack on some maneuvers. 

At 66 oz I only tried the .015 lines once to see if that was the problem.  Since the problem still existed, just not quite as pronounced, I went right back to the .018 lines.  The line spacing is not adjustable since the leadout adjustment moves both lines at once.  I can, however, try a handle with wider spacing to see if that makes a difference.

More to follow as weather permits.  Have a Happy Thanksgiving y'all.

[Dan Bregar]

Howard

Yes there have been experiments done to try and do just what you describe by at least one speed flier I know of back in the mid 70's.  Bottom line is the inboard wing kept getting longer to streamline more of the lines outboard going at increased velocity compared to the lines at the handle. 

[John Leidle]

 I had more than one plane have the flap's outer hinge vibrate apart.In other words the outer flap had the outer hinge break .  I felt there is too much force on the concentrated area so I put 2    1/4 scale hinges on the outer of the outboard flap   , ,the breaking of hinges went away ...  worked for me.

[Michael Massey]

I finally got out to fly.  I taped the hinge lines and did little else except pick up the speed.  I went from 5.3 laps to 4.8.  Not planned but that was the change.  I will now go back and digest all of the above comments but the result of my experiment was this.

The plane vibrated/fluttered/shook worse than you could imagine.  It truly would not take too many flights at that "frequency" before coming apart.  I could see the entire tail section vibrating.  The vibration would stop if I flew well over 50 degrees and remained there.  As soon as I came back towards level, it started again.  If I let it vibrate, the vibration "transmitted" to the handle so I could feel the handle want to move the control up, then down.  It did that at a very slow rate, I would guess less than 10 or 15 "variations in one lap.  As I would come into the wind (which was mild), the plane would climb slightly and would then would drop slightly going away from the wind.

I did another flight and put 1/4 oz weight on the very trailing edge of the elevator, seeing if I could dampen the vibration.  That worked and the plane flew fairly well although you could feel a very slight vibration occasionally. 

Not wanting to vibrate the plane to pieces, I quit flying it and plan on taking the tail off and eliminating the excessive play in the elevator.  I think that is the problem and solution.  I think...I think...

[Avaiojet]

Do you have photos of this model?

CB

[Michael Massey]

I'll take one tomorrow and post it. 

The wing is a "stock" profile Pathfinder wing.  The tail surfaces are simply "plank" balsa, I'll have to measure the thickness as I do not remember off hand.  The fuselage is a built up fuse following the profile measurements except for the width (thickness) of course.

[REX1945]

.....  I weighs in at 66 oz with an LA 46.  Seems to fly straight and ok without any real dramatic trim issues.  But the control lines "vibrate", much like a resonant vibration, when I fly towards the wind.  Going away from the wind, the vibration goes away. ..........

I was flying the plane at about 5.3 laps, 62 foot lines, with a 12.25x3.75 APC turning about 9,900.

   Mike,
 
    If you covered with silk, it could be "silk roar" which not vibrates the handle, but make an audible noise
when a certain critical airspeed is reached.

Rex

[John Stiles]

   Mike,
 
    If you covered with silk, it could be "silk roar" which not vibrates the handle, but make an audible noise
when a certain critical airspeed is reached.

Rex

That's what I call goin "sonic" 

[Michael Massey]

I'll try this again.  Here are the pictures.  I tried to take the angles I thought would be relative.  As far as silk, it is covered with silk but the control surfaces are all solid.

Tx All

[rob biddle]

 Hey Michael, what guage is your horn wire?, they look fairly light.

 The 1st .35 size stunter I built was a strathmoor, that I roughly scaled up from the mini plan in Ron Moultons "control line manual".
It used a then new fp.40 running 4-1 fai fuel (I knew no different then lol).

 I busted my butt to get it light using contest grade wood and had it ready to go with a minimal finish @ 37oz.

 I scrimped on the wire guage for the horns (1/16th ) and from the 1st outing it had really (I mean pathetically) soggy controls. Within a couple of flights I realised I had no control over the engine (runaway) and around the 10th flight during a super lean high speed run the outboard flap fluttered insanely for about 5 seconds. The vibration was so strong It foamed the fuel to the point of cutting the engine and before I knew it the outboard wing had sheared off at the root and was floating back to earth whilst I landed the model in total disbelief.

 My flying buddy Tigga thought it was funny as hell, it was a valuable lesson

 Cheers, Rob..

[Avaiojet]

Mike,

Bring both surfaces to neutral, then take a photo.

Have you checked to see if both control surfaces are centered, when centered?

Nice looking model.

CB

[Michael Massey]

Hi Rob.  I am using 3/32 wire for the control horn.  I use that size pretty consistently and have not had, until now, a problem.  I will say, however, I made this control horn from piano wire while I usually use a commercial control horn.  I did go back and check my soldering job and it is fine.

Hi Aviojet.  I have checked "neutral" on the control surfaces.  I do have some "down" elevator from neutral that I did put in intentionally.  I usually have a little in all of my planes.

The thing that makes this most puzzling is that when I initially flew the plane about 9 months ago, it seemed to fly fine.  I then hung it up while I was doing other things and flying other planes.  I recently took it down and immediately began to experiment with props.  The ONLY change (as far as I can see) is that the early flights all had the spinner in place and while I was experimenting with props the spinner was off.  I should also note that it is cold and damp here now and when I last flew it I flew in much warmer and dryer weather.

[Avaiojet]

MM,

I'll be honest with you, as you would want me to be. And I should have mentioned this before.

I was actually surprised as to the look of your model.

Great job! In fact it looks really good. Looks square and straight plus nice color choices.

It's almost my favorite color. The hue is charming, but I'm a bit more towards the pastel with teal. That is teal? The accent color works well also off white or cream? Good combo. Your choice or someone else's?

The "empty" area on the elevator, could add material and close it up? Would look prettier in IMHO.

Certainly that can't be the issue?

Do you balance your props?

Your model, does it have a name?

CB

[jim ivey]

gee howard I had an intresting thought you might appreciate. maybe the viberation was gaused by his plane  noticing another airplane on the ground 

[John Stiles]

MM,



The "empty" area on the elevator, could add material and close it up? Would look prettier in IMHO.

Certainly that can't be the issue?


CB

I started to mention that, and the fact that the elevator looked "heavy" ........just couldn't think of a tactful way to say it, without sounding like I thought it was unsightly.......but I would certainly try something different next time.

[Avaiojet]

I started to mention that, and the fact that the elevator looked "heavy" ........just couldn't think of a tactful way to say it, without sounding like I thought it was unsightly.......but I would certainly try something different next time.

John,

Was I offensive about my observation? Didn't mean to be.

See what MM says.

CB

[Howard Rush]

gee howard I had an intresting thought you might appreciate. maybe the viberation was gaused by his plane  noticing another airplane on the ground 

Goosed?  You may be onto something.  I suspect the kitty, though.  Looks like he's up to no good.

[John Stiles]

John,

Was I offensive about my observation? Didn't mean to be.

See what MM says.

CB

Nope, just tryin to sneak in under yer wing.

[Michael Massey]

First to Avaiojet.  No offense taken.  I wasn't sure what "empty" meant.  You mean "built up" or different shape?  The shape is that of the Pathfinder.  I should have built it up and will if I do another.  I have been thinking of doing another and "lightening the load" and adding a rib or two extra to the wing since the fuse takes up some wing area.  And of course, build up the tail.  Any other suggestions on how to make the next "prettier" please let me know.  This was an experiment.

The color is Montego Green from Randolph.  Bought it through Aircraft Spruce.  As far as what I call it, I have been loosely calling it "Crooked Path."  I do balance my props.  Now doubt that question arose from seeing the Zinger on the plane.

The cat is a culprit.  I call her Lilly because she showed up at our door on Easter 2004.  Skin and bones, terrible ear mites and a BB in her side.  Now she is too fat and lazy.  She still has the BB in her side but it does not bother her.  She also has, what the vet describes, as the worst case of back arthritis in a cat she has ever seen.  As a consequence, there are mini stairs and ramps throughout my garage so she can get around and get into EVERYTHING!

She is one of my 2 garage kitties.  We have 3 more house kitties.  In all, 2 on purpose and 3 "volunteers."

[Brian Massey]

First to Avaiojet.  No offense taken.  I wasn't sure what "empty" meant.  You mean "built up" or different shape?  The shape is that of the Pathfinder.  I should have built it up and will if I do another.  I have been thinking of doing another and "lightening the load" and adding a rib or two extra to the wing since the fuse takes up some wing area.  And of course, build up the tail.  Any other suggestions on how to make the next "prettier" please let me know.  This was an experiment.

The color is Montego Green from Randolph.  Bought it through Aircraft Spruce.  As far as what I call it, I have been loosely calling it "Crooked Path."  I do balance my props.  Now doubt that question arose from seeing the Zinger on the plane.

The cat is a culprit.  I call her Lilly because she showed up at our door on Easter 2004.  Skin and bones, terrible ear mites and a BB in her side.  Now she is too fat and lazy.  She still has the BB in her side but it does not bother her.  She also has, what the vet describes, as the worst case of back arthritis in a cat she has ever seen.  As a consequence, there are mini stairs and ramps throughout my garage so she can get around and get into EVERYTHING!

She is one of my 2 garage kitties.  We have 3 more house kitties.  In all, 2 on purpose and 3 "volunteers."

Trust me folks, Lilly is more than "fat and lazy". She is a real Blobarama! Just try to work on anything on that bench and see how long it takes her to come over and plop down in the way. (And when she "plops", she takes up a lot of room.)

Brother Brian

[bob branch]

Flutter of control surfaces is not an uncommon situation in rc models nor in 100% scale aircraft. It is generally caused by inadequately strong push rods for the size of the control surface in rc, and unbalanced control surfaces in 100% aircraft.  Airspeed is also a component. Prior to RC 3D designs planes generally had small control surfaces. The planes flew quite fast and had small control throws. When 3D planes started to be designed the control surfaces became huge by comparison and the throws went from 12 degrees or so to from 45 to 70 degrees now. With large control surface and large throw planes you do not fly fast. Almost every builder's instructions for 3D planes warn to use conservative "throttle management" or risk the possibility of flutter. Stiffer pushrods from the servo to the control horn became the norm. No flex in the system can be accepted. Stronger servos provide greater strength in holding the control surfaces still. When flutter occurs you can hear it as a buzzing sound from the plane. In RC you must pull the throttle back quickly or risk the control surface vibrating off. I have personally had this happen on two airplanes and have seen it on a number of others. On my two occasions an aileron departed the aircraft before I could pull the throttle back. Both planes were landed without damage but required re engineering of the control systems. Of note, both the failures were with kleet type barrel hinges. One set epoxied in and one set with gorilla glue. I have never had a failure with CA hinges. A member of our club did however when he bought a 3D plane from a member of the club that was an experienced 3D pilot and used "conservative throttle management". The new owner never flew a 3D plane and only knew one speed, wide open. On the first flight the elevator departed the aircraft after flying the length of the field making a loud buzzing sound.

I would recommend looking at your entire control system and beefing up the horn wires and pushrods. Something is flexing under load. A harmonic is forming at flight speeds and before long something will seperate from the plane and it will crash. May be horns, may be slop between pushrod and horn... check it all and beef it up. Regardless what you have used, there is a flight load exceeding the limits of the control system. The early flights without the problem probably were due to the surfaces not having worked very much yet. The wires could be getting cold worked or some hinges may have already pulled loose. Whatever, the underlying vibrations seem to be taking their toll. Going faster generally makes the condition worse.

In 100% planes we lost a world class aerobatic pilot locally on first flight of his new original design aerobatic plane when aileron flutter led to an aileron departure on his first pass over the field. Both plane and pilot were lost. During the last couple of years a very popular home built light sport class plane has expereinced a significant enough number of these aileron separations due to flutter to the point that the FAA grounded the entire fleet  until it was solved. The value of the planes after the "fix" suggests pilots do not accept that it has been fixed.

bob branch

[Steve Helmick]

First to Avaiojet.  No offense taken.  I wasn't sure what "empty" meant.  You mean "built up" or different shape?  The shape is that of the Pathfinder.  I should have built it up and will if I do another.  I have been thinking of doing another and "lightening the load" and adding a rib or two extra to the wing since the fuse takes up some wing area.  And of course, build up the tail.  Any other suggestions on how to make the next "prettier" please let me know.  This was an experiment.

I also noticed the gap at the root of the elevators (and flaps).  The elevator horn would be much stiffer if shorter, which would then be possible. I'm not a big fan of flap/elevator root fairings. Especially since Gary Letsinger mentioned that no fairing and a close fit to the fuselage is a more efficient, lower drag design. Gary is a retired Boeing aeronautical engineer. If you look at the Impact (if built per plans), there are no flap or elevator root fairings.  Steve

[Michael Massey]

After reading all of the great input, I decided to start with the most obvious, that being the excessive "slop" in the elevator.  So, I cut the entire tail off, and "bushed" the elevator control horn.  Put the whole thing back together and ...YES!!!...filled in the "empty space...gap...whatever else you want to call it" at the elevator/fuse.  Put a couple of coats of dope and test flew it today.  Last weekend was way too cold but today it warmed all the way up to 42 by 1:30pm, but of course, I flew at 11am.  So with stinging hands and a lap speed of 4.6, I tested the fix. 

WORKED GREAT!  No more flutter.  Still working on the prop/RPM but with a Xoar 11x5 launched at 9900, I got 5.1 laps.  That works OK but as I mentioned before, it is heavy so probably need to get to at least 5 or maybe even the high side of 4.9.

Anyway, thanks to all for the input.

[john e. holliday]

What length line and size?   Once you find the speed the plane like to fly at, you can control lap speed by line length.  I have been told several times by another person a happy stunt flies at about 60-65 MPH.

[Michael Massey]

Right now I am flying 62' (eye to eye) on .018.  I will try a little longer but I have to be careful because this plane is "pudgy" and the overheads suffer with slower speeds.

[Brett Buck]

Right now I am flying 62' (eye to eye) on .018.  I will try a little longer but I have to be careful because this plane is "pudgy" and the overheads suffer with slower speeds.

    Longer lines will permit more airspeed (helping with the weight) without decreasing the lap times, so longer is definitely worth a try.

    Brett