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-   -   Flutter and air balancing (http://www.rcuniverse.com/forum/aerodynamics-76/11199538-flutter-air-balancing.html)

Franco2fly 08-20-2012 05:47 AM

Flutter and air balancing
 
I had a crash this weekend that still has me mumbling. I'm not the only one either. The problem is with the Hobby King Zlin-42 which is a .60 size model. In the build log, another flyer reported that the stabilizer and the fin assembly started "Fluttering" in flight and ultimately resulted in a crash.

For me, there were a couple problems with the kit, mostly dealing with poor joints in the motor mount area, and the amount of surface throw. But I had all of these solved when this weekend the Flutter bug bit me. On exiting a loop, someone standing next to me hollered, "Hey your rudder is fluttering!". I think what he actually saw was the whole horizontal stab wobbling. I was able to stabilize and slow, but ultimately stalled into some tall corn stalks.

Here's my question. Can too much Air Balance surface cause this? Here's a picture.

http://i96.photobucket.com/albums/l1...breakphoto.jpg

If you look at the amount of airbalance on both the stab and the rudder its pretty significant. I seem to recall some WWII aircraft that had weights installed ahead of the hinge line. Was this to dampen or prevent flutter?



KKKKFL

speedracerntrixie 08-20-2012 06:25 AM

RE: Flutter and air balancing
 
To me it looks like the whole tail is just 6mm sheet stock and would not be strong enough for this size airplane. A quick fix would be to add flying wires. Yes it would be ugly but would keef this from happening. The other option would be to build up a new tail surface. For a .60 size airplane one would use 1/8X1/4 frame with 1/16 balsa sheeting. This would be much stronger then slabs of balsa. I doubt that the size of the counterbalances had any effect. Other things to look for are flex in the pushrod, size of hinge gaps and servo power/linkage geommetry to make sure you are getting enough power to the surface. I still think it's just a poor design structurally

Franco2fly 08-20-2012 08:26 AM

RE: Flutter and air balancing
 
There's no slop in the linkages, and the hinges are tight..  the stabilizer in front of the elevator doesn't have any flex despite being 6mm thick.  I could install a hardwood leading edge fairly easy, but I don't believe that's the part that is fluttering.  I believe that the large amount of airbalance is the culprit as it creates forces both up and down no matter which way the elevator moves.  The plane was flying fine when straight and level, but it was pulling out at the bottom of the loop, (medium deflection) at high speed where there was "Pull" on the control horn and the surface started to flutter so bad that it ripped off the entire Elevator/stabilizer.


I'm wondering if adding mass to the LE of the tips might help.

KKKKFL

speedracerntrixie 08-20-2012 08:40 AM

RE: Flutter and air balancing
 

Quote:

ORIGINAL: Franco2fly

There's no slop in the linkages, and the hinges are tight.. the stabilizer in front of the elevator doesn't have any flex despite being 6mm thick. I could install a hardwood leading edge fairly easy, but I don't believe that's the part that is fluttering. I believe that the large amount of airbalance is the culprit as it creates forces both up and down no matter which way the elevator moves. The plane was flying fine when straight and level, but it was pulling out at the bottom of the loop, (medium deflection) at <span style="color: rgb(255, 0, 0);">high speed </span>where there was "Pull" on the control horn and the surface started to flutter so bad that it ripped off the entire Elevator/stabilizer.


I'm wondering if adding mass to the LE of the tips might help.

KKKKFL
IMO this pretty much says it all. At speed with some deflection there is a turbulance wake that wants to induce flutter. The counterbalances are there to reduce the workload on the servo/pilot. I have flown 40% Extras with and without counterbalances and could tell no difference. I doubt the countebalances are the culprit here and still think the fixes I mentioned earlier are the way to go. 6mm flat stock on a .60 size model that looks to have 50% of total surface area a control surface just seems like poor design to me.


Franco2fly 08-20-2012 08:42 AM

RE: Flutter and air balancing
 
What if I sheeted the stab with 32 inch plywood, top and bottom?

KKKKFL

speedracerntrixie 08-20-2012 08:57 AM

RE: Flutter and air balancing
 
1 Attachment(s)
That would work provided you removed the stab and did one peice of 1/64 top and bottom. By trying to do it in two peices ( Left and right ) while still on the airplane you would be creating a stress riser at the stab/fuse junction. At that point it wouldn't be much more work to build a whole new stab. The quick and easy fix would be flying wires. Like what I have on this airplane.



Franco2fly 08-20-2012 09:18 AM

RE: Flutter and air balancing
 
Its pretty easy to remove the entire stabilizer, I just tacked in in place at the field with a spot of CA just for transportation.  I would like to avoid the wire approach.  I would run the stab through a band saw about a third the way back and then re-glue together with a Hardwood spar about 3/16ths wide.


KKKKFL

BMatthews 08-20-2012 11:36 AM

RE: Flutter and air balancing
 
In my experience 6mm (1/4 inch) sheet surfaces of this size are more than stiff enough on their own for a model of this size. Adding plywood skins to the top and bottom would be overkill.

Flutter is not something that is related to stiffness by itself. Instead it's got lots of structural and aerodynamic factors that add up to produce the resonance which we know of as flutter.

The aerodynamic balances on the surfaces could be part of the trouble. But if so then likely it's not because they are too big but because they are there at all.

But before you cut them off and make the balances part of the fixed portion of the surfaces I'd try punching a small opening and putting something like 1/4 oz lead sinker weights or similar into the balance horns as far forward on each of them as you can. In fact 10mm long slugs of roll lead from a fishing tackle shop inset into the leading edges of the horns would be a fine mass balance. With these mass balances I suspect that your flutter issues will go away.

Becuase you're adding that much tail weight you may also need to add some nose weight to restore the balance point. It just depends on how pitch stable the model is and if you would like it better with the slightly aft CG location.

Franco2fly 08-20-2012 11:50 AM

RE: Flutter and air balancing
 
Good to hear my estimates confirmed. The plane in question is nose heavy<div>At the moment so no concerns there. I like the idea of opening a pocket in the</div><div>Front of the air balance portion forward of the hinge and adding lead</div><div>Until the elevator stays slightly in an up ele position</div>

mr_matt 08-20-2012 12:26 PM

RE: Flutter and air balancing
 
What is "air balancing"?

da Rock 08-20-2012 01:17 PM

RE: Flutter and air balancing
 

Quote:

ORIGINAL: Franco2fly

What if I sheeted the stab with 32 inch plywood, top and bottom?

KKKKFL
Massive overkill.

Sheeting works great, but use the lightest available. Balsa works far better than necessary.

rmh 08-20-2012 05:56 PM

RE: Flutter and air balancing
 
My own experience on THIS problem is twofold
ONE- proper guy wires (4) to stab.
TWO- run no more than 20% aerodynamic counterbalance but add weight such that the elevator balances fairly close to hinge line
quick n positive.

Franco2fly 08-20-2012 06:12 PM

RE: Flutter and air balancing
 
Mr. Matt,
If you look at the pictures I posted both the rudder and elevator have portions that are in front of the hinge line.  This means that as the rear part of the elevator moves to provide "Up" elevator the small portions on each side move down.  This causes the airflow to assist the larger  elevator surface.  Check the pictures out and you can see the same Airbalance on Rudder.  There's surface in front of the hinge line here too.


KKKKFL

pkoury 08-20-2012 07:56 PM

RE: Flutter and air balancing
 
Airbalance = Aerodynamic balance. I am still trying to figure out the OP post where he says he only tack glues the stab on. Its pretty easy to remove the entire stabilizer, I just tacked in in place at the field with a spot of CA just for transportation". Maybe that is why it flutters.

Franco2fly 08-21-2012 02:28 AM

RE: Flutter and air balancing
 
I'll try and be a little more descriptive so that all can understand.

When I received the kit,  the elevator and stab had the hinges in place, just not glued.  There was a slot at the back of the fuselage, where the stab would normally slide in. Glue was applied and covering on the bottom cut to allow balsa to balsa contact, but there admittedly was not the usual large flat surface area on the fuselage side where (top where it contacts the bottom of the stab).  The top part of the slot extends from just before the rudder hinge back about 2 inches  back.  If you look at the photo you can see where I have pulled the covering back to expose where
the break occurred.  In flight, this balsa snapped such that the whole elevator and stab were totally loose, and eventually the entire stab structure was hanging on by the elevator clevis, waggling like a rag in the wind behind the rest of the plane.  It had snapped loose of the fuselage and was blown completely out of the slot.

Uncontrollable, as it was, I managed to keep it fairly level until it wound up in corn stalks.  When we retreived it the stab assembly (elevator plus stab) was fully intact, just hanging off the back of the plane.  I was able to twist it around (clevis still attached to control horn) and slide it under the rudder mateing the broken balsa from the fuselage with the remaining balsa piece still glued to the top of the stab.  I put a drop of CA in the fracture to "Tack" it in place. 

Just recall your last kit where you slide a stab into the fuselage slot...  now imagine rocking it from side to side violently enough to break the slot apart.  The poor design is in the fact that the stab isn't stiff enough, and the glue joint to the fuselage is too thin.  Taking a 1/2 wide piece of fiberglass cloth, and removing a 1/4 of covering film from the fuselage side below the stab and the bottom of the stab will provide a better "More solid" bond between fuselage and stab, ( now I will have a right angle of fiberglass providing ridigidity plus a better bond between stab and fuselage) and changing the portion of stab where the hinges go in by cutting it width wise and then adding a hardwood spar will eliminate any flex.

  Wires would also accomplish this but would destroy the scale appearance of the Zlin-42.  Adding weight to the portion of elevator that extends in front of the hinge line places a mass (as in weight) that counter balances the elevator making it less likely to flutter, which is most likely the force that caused the balsa slot to break apart in the first place.

Hope that's a better explanation.

KKKKFL


lopflyers 08-21-2012 04:19 AM

RE: Flutter and air balancing
 
Guys I know where the problem is. Is it not flutter, not air balance, not aerdodynamics. IT IS HOBBY KING[:'(]

Franco2fly 08-21-2012 04:55 AM

RE: Flutter and air balancing
 
hahahahaha

Thanks for that insight lopflyers.

Actually, the foundations of the kit were not bad.   However, I found numerous flaws.  Here's the short list.

Nose gear strut too short  - Had to purchase a Dubro replacement
Motor Mount not nearly built strong enough for Nitro.   -  I used fiberglass tape reinforcement along all seams
Internal rudder horn  - required Dremel notching and Locktite on Allen screw
Nose Wheel control Horn  constant loosening, too small an Allen screw  repaired with Dubro replacement
Fuel tank broken at neck seam (leaked fuel)  -  Replaced tank with Dubro tank of slightly smaller width, slightly longer length.
Poor wing design, not really built to be modified  -  modified to accept Flap configureation albeit they are a little too small
Weak horizontal and vertical stabilizer components, poor design  - Will use fiberglass to improve strength
Nose heavy when built to specifications  -  Design could shorten Nose motor mount box by at least an inch
Poor construction manual

Now having said all that, once deficiencies are corrected, the Zlin-42 is a good looking and well behaved (once proper throws are established)
model.  However, I would NOT recommend it to a friend.  The problem is that I cannot find a Low wing intermediate skill Tricycly landing gear model in the .40 to .60 range.  Initially, I thought the plane could be flown on a Saito .40 four stroke, thinking the 42 referred to wingspan.  Once disabused of this notion, I ordered a NEW Super Tigre .60 ABC which took a couple weekends to tweek to perfection.

If anyone has some good recommendation meeting my requirements as set above, send them along...

KKKKFL

rgburrill 08-21-2012 08:21 AM

RE: Flutter and air balancing
 
Perhaps the problem is the stiffness of the rear of the fuse. It could be twisting - after all your friend initially said the rudder was fluttering, not the horizontal stab. You alsosaid the horizontal and vertical stab components were weak so you could try to fiberglass reinforce the fuse in that area.

Regarding low-wing intermediate skill I think the 4-star 40and Tiger II are the best low wing, aerobatic starters around. Despite what HK may say I wouldn't put nitro on a Zlin-42 - electric all the way.

Ed_Moorman 08-21-2012 08:56 AM

RE: Flutter and air balancing
 
No telling what can be twisting, but you already have the answers. To sum up:
1. Add some flying wires. For the lightest weight, use lightweight weed eater list.
2. Drill or cut a big hold in your counterbalances and add some weight forward of the hinge line. As has been mentioned, disconnect the elevator from the pushrod and add weight until it doesn't droop.

Franco2fly 08-21-2012 12:39 PM

RE: Flutter and air balancing
 
As I look at it here in my garage I think I'm gonna do something like the<div>Struts you see on an ME-109. They will be airfoil shaped aluminum a solid</div><div>Piece that  ends up on each side attaching 3/4 out on each side</div><div>
</div><div>Wires would destroy the looks. For the fin in front of the rudder I'm thinking</div><div>Glass the LE about a 1/2 inch back both sides</div><div>
</div><div>Will provide pictures when done</div><div>Kkkkfl</div>

kenh3497 10-12-2012 05:58 AM

RE: Flutter and air balancing
 
Are the elevators just flat plates or do they taper from the hinge (full thickness) to a sharp trailing edge? My CAP had flat plate elevators and I changed them to a tapered design and my flutter went away.

You mileage may vary.

Ken

da Rock 10-12-2012 06:08 AM

RE: Flutter and air balancing
 

Quote:

ORIGINAL: Franco2fly

What if I sheeted the stab with 32 inch plywood, top and bottom?

KKKKFL

woah.... plywood is major overkill

Sheeting would work perfectly if you used balsa. Drill some holes in anything you sheet however to cut down on the area of wood that'd soak up glue. It's also not needed for strength either.


jester_s1 10-12-2012 06:16 AM

RE: Flutter and air balancing
 
It sounds like you're too far into this project to walk away, although I hope you'll buy a plane from a reputable company next time. With all the problems you listed, I'm not a bit surprised they didn't bother to design a tail that would stay attached in flight. For that matter the design may be barely good enough and they just didn't bother to use the right grade of balsa for those parts. If it were mine and I just had to try and make a Hobby King sold product work (at least until another design or materials flaw causes it to crash again) I'd add a piece of tri stock on the underside of the stab on both sides instead of glassing. From what you've described, a better bond isn't going to help much because the problem is that the gluing area is too small. Two pieces of 1/4 inch tri stock gives you another 1/2 inch of support for the stabilizer, hopefully making the joint strong enough that the stab will rip in half next time instead of fall off.

da Rock 10-12-2012 06:40 AM

RE: Flutter and air balancing
 
1 Attachment(s)
Sheet stabilizers usually are strong enough, if they are large enough. Both the horizontal and vertical stabs of the plane in the picture are pretty small things. In fact, if you look at the wrinkle on the LE of the fin, that usually is the result of a fin that has broken it's LE. There really is no reason for covering to wrinkle on a flat surface like that. Even a poorly skilled factory worker will cover flat plates better than that. But back to size........

The stabs on that tail (all 3 of them) are small compared to the overall size of the whole areas. They've got to deal with the forces the movable area can generate. They are usually the majority of the area, yet on that design look like they're less than 1/3 the area. Not a lot of wood or attachment area to deal with significantly more moving area than is on 99% of our models.

You mention the plane is nose heavy. And you've mentioned sheeting with ply. OK, try this... cut out the ply plates before touching the covering. After noticing the small size of the area that'd be sheeted, there really won't be a lot of ply going on. But check to see what effect the weight will have. Once you've got the pieces cut out, simply lay them on the present model and see what it does to the balance. If they are going to affect the balance, then cut some holes in the ply. The stabs are going to be covered after sheeting, right. Holes in the sheeting are a good idea even if the sheeting isn't too heavy.

If you sheet those stabs, make sure you butt the sheeting up against the fuselage. That might be the best support the sheeting supplies. In fact, consider using triangular stock as fillets. Small ones still increase the support much more. And look closely at the wood in the fuselage where those stabs are. Soft wood isn't much support. Soaking with CA can do wonders for the wood around those stabs.

About the plane being nose heavy. That introduces increased load on the horizontal tail. It causes more force needed from the elevator. That puts more load on that very small stab. etc

BMatthews advice about adding balance weight is spot on. The mass aft of the hinge lines is something that decides the flutter frequency. Change or balance the mass and you change the frequency. However, I'd try your first idea of strengthening before trying that.

Oh yeah......... about that wrinkle that might be where the LE broke.... here's the picture

Iron Bottom 10-12-2012 07:00 AM

RE: Flutter and air balancing
 
I have seen an aileron control horn work loose in soft balsa. The aileron was ripped completely off when it fluttered. Any movement of the control horn?


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