Had a friend who is knowledgeable fly my 7A today it has 4 hrs and a heavy wing (left) he tells me that he experienced some flutter @135knots and after (slaping) the stick the controls didnt come to netural properly He says the bottom of the right aileron has some severe oil canning about 2 feet that could be causing the problim, and that the aileron should be re riveted on the bottom side with heat to stiffen the skin. I have no Idea what to do. I didnt slap the stick when I flew it, but I didnt notice this at all Any sugestions????? Marc
Van's Air Force
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control surface flutter
- Thread starter mknight
- Start date
Kyle Boatright
Well Known Member
Is your friend familiar with the RV's odd characteristic of aileron stall (and burble) that causes a stick shake at large aileron deflection angles and certain speeds? It may be that this is what he experienced, not flutter.
As far as re-riveting with heat? No idea. I especially do not know why heat would have any impact other than to change the heat-treat status of the aileron and I'd stay far, far away from that...
If all else fails, talk to the tech support people at Van's or get someone who is an experienced RV "stick" to take a good look at the aileron in question. If he doesn't see a problem and is willing to test fly the airplane, I'd consider that, at his discretion.
Fly it and forget it
It sounds like a jerk blowing up his knowledge well beyond reallity and you should find a new friend. He's trying to keep himself one up in the aeronautical pecking order. I'm surprised he didn't suggest bending the wings up to increase the dihedral. Oh, you should probably try to fly the airplane normally and not apply roll control inputs by slapping the stick. You have a good thing in your RV-7A, don't screw it up.
Bob Axsom
It sounds like a jerk blowing up his knowledge well beyond reallity and you should find a new friend. He's trying to keep himself one up in the aeronautical pecking order. I'm surprised he didn't suggest bending the wings up to increase the dihedral. Oh, you should probably try to fly the airplane normally and not apply roll control inputs by slapping the stick. You have a good thing in your RV-7A, don't screw it up.
Bob Axsom
az_gila
Well Known Member
Not quite...
...Bob...
....my old RV-6A Construction Manual has a whole section called "Limit Testing" which gives the "sharp input" (aka "slap the stick") procedure mentioned above that you make fun of. The manual mentions that the stick should return to neutral, which is not the reported case here.
This testing should be done up to 20 mph above the Never Exceed speed - building up speed gradually in small increments - to ensure no flutter at operational speeds.
If some flutter (or even lack of stability) is being detected at 135 kts IAS, then it certainly needs to be investigated - it is even below the yellow arc speed. That is what the Phase I testing is all about.
The "fly on" recommendations may be dangerous in this case....
Did Van remove this section in the later RV-7 manuals?
...Bob...
....my old RV-6A Construction Manual has a whole section called "Limit Testing" which gives the "sharp input" (aka "slap the stick") procedure mentioned above that you make fun of. The manual mentions that the stick should return to neutral, which is not the reported case here.
This testing should be done up to 20 mph above the Never Exceed speed - building up speed gradually in small increments - to ensure no flutter at operational speeds.
If some flutter (or even lack of stability) is being detected at 135 kts IAS, then it certainly needs to be investigated - it is even below the yellow arc speed. That is what the Phase I testing is all about.
The "fly on" recommendations may be dangerous in this case....
Did Van remove this section in the later RV-7 manuals?
Slap the stick is briefly mentioned. Without a past RV manual, I don't know how it compares.
From my preview plans:
Is your neutral point offset? How's the lateral stability? Could it be aileron snatch? (I'm an armchair quarterback, tell me if I'm way off and I'll sit down and be quiet).
From my preview plans:
Is your neutral point offset? How's the lateral stability? Could it be aileron snatch? (I'm an armchair quarterback, tell me if I'm way off and I'll sit down and be quiet
).
Bottom of the aileron
I'm not sure how someone can see the bottom of the aileron in flight unless they are in a Cessna. I guess he was pushing on the skin and got some oilcanning while on the ground.
Regarding flutter on ailerons - I looked this up on the net and thought you might want to check your linkages and hinges to make sure there was not slop. Although a new plane, you shouldn't have wear but something could be loose. You might be getting a warning sign.
FLUTTER:
A phenomenon whereby the elevator or aileron control surface begins to oscillate violently in flight. This can sometimes cause the surface to break away from the aircraft and cause a crash. There are many reasons for this, but the most common are excessive hinge gap or excessive "slop" in the pushrod connections and control horns.
One of the most common causes of flutter is loose linkage between the servo and control surface. This is easily corrected but often overlooked or ignored. It only takes a moment to jiggle your control surfaces by hand to see how much movement is there and determine its origin. Servo gears get worn, clevises get worn, and control horns get worn. Not if, buy when your surface flutters you will likely crash if immediate action hasn't taken place. If you ever hear a low-pitched buzzing sound, reduce throttle and land immediately, if you haven't already crashed...
Flutter can occur at any time on any control surface if the conditions are correct. All of your controls are subject to flutter but the typical pecking order is: AILERONS, ELEVATOR, then RUDDER. This isn't always the case though! If you lose a control surface because of flutter here is the likelyhood of crashing and here is your percentages.
CONRTOL SURFACE LIKELY to CRASH % NOTES
Single AILERON dismemberment: 30-50% You can fly on one aileron, it's mushy to control but very flyable
Double AILERON dismemberment: 50-80% You can fly with no ailerons, but you must use your rudder in place of ailerons
Elevator dismemberment: 100% Crash is immenent.
Aileron single stripped servo gear 80-100% depends where the damaged aileron neutrals at, and IF it doesn't move
Aileron double stripped servo gear 100% Crash is immenent.
Elevator stripped gear 100% Crash is immenent.
Rudder stripped gear 80-100% it will likely crash but there is a slim chance of getting down
These percentages are based on "experienced" pilot skills and on "common" flutter conditions which will vary dramatically in flight. As you can see, the odds are stacked against you. So check those control surfaces.
Other causes of flutter are; Airgaps, Loose or broken hinges, and speed.
Sealing the Airgaps on your control surfaces not only minimize flutter, they also improve the efficiency of the controls. This is a good thing.
Loose or broken hinges must be repaired, as this gets perpetually worse with every flight.
Speed. If you fly your airplane beyond it's intended envelope you are rolling the dice every time. Sooner or later flutter will occur unless preventative measures were employed.
One thing about flutter is, it makes LOTS of NOISE prior to blowing up! If you react quick enough there is a chance you can save your airplane. Immediately cut your throttle and land.
I'm not sure how someone can see the bottom of the aileron in flight unless they are in a Cessna. I guess he was pushing on the skin and got some oilcanning while on the ground.
Regarding flutter on ailerons - I looked this up on the net and thought you might want to check your linkages and hinges to make sure there was not slop. Although a new plane, you shouldn't have wear but something could be loose. You might be getting a warning sign.
FLUTTER:
A phenomenon whereby the elevator or aileron control surface begins to oscillate violently in flight. This can sometimes cause the surface to break away from the aircraft and cause a crash. There are many reasons for this, but the most common are excessive hinge gap or excessive "slop" in the pushrod connections and control horns.
One of the most common causes of flutter is loose linkage between the servo and control surface. This is easily corrected but often overlooked or ignored. It only takes a moment to jiggle your control surfaces by hand to see how much movement is there and determine its origin. Servo gears get worn, clevises get worn, and control horns get worn. Not if, buy when your surface flutters you will likely crash if immediate action hasn't taken place. If you ever hear a low-pitched buzzing sound, reduce throttle and land immediately, if you haven't already crashed...
Flutter can occur at any time on any control surface if the conditions are correct. All of your controls are subject to flutter but the typical pecking order is: AILERONS, ELEVATOR, then RUDDER. This isn't always the case though! If you lose a control surface because of flutter here is the likelyhood of crashing and here is your percentages.
CONRTOL SURFACE LIKELY to CRASH % NOTES
Single AILERON dismemberment: 30-50% You can fly on one aileron, it's mushy to control but very flyable
Double AILERON dismemberment: 50-80% You can fly with no ailerons, but you must use your rudder in place of ailerons
Elevator dismemberment: 100% Crash is immenent.
Aileron single stripped servo gear 80-100% depends where the damaged aileron neutrals at, and IF it doesn't move
Aileron double stripped servo gear 100% Crash is immenent.
Elevator stripped gear 100% Crash is immenent.
Rudder stripped gear 80-100% it will likely crash but there is a slim chance of getting down
These percentages are based on "experienced" pilot skills and on "common" flutter conditions which will vary dramatically in flight. As you can see, the odds are stacked against you. So check those control surfaces.
Other causes of flutter are; Airgaps, Loose or broken hinges, and speed.
Sealing the Airgaps on your control surfaces not only minimize flutter, they also improve the efficiency of the controls. This is a good thing.
Loose or broken hinges must be repaired, as this gets perpetually worse with every flight.
Speed. If you fly your airplane beyond it's intended envelope you are rolling the dice every time. Sooner or later flutter will occur unless preventative measures were employed.
One thing about flutter is, it makes LOTS of NOISE prior to blowing up! If you react quick enough there is a chance you can save your airplane. Immediately cut your throttle and land.
Wes Hays
Well Known Member
I am not sure this would have helped at all. On the old kits, I have heard of folks who would heat up the skins while they were DRILLING them to make them tighter when they were riveted on. However, once drilled, they are where they are. I just don't see how heating the skins during the riveting process is going to stiffen them and take out any oil canning. Just my thought.
FredMagare
Well Known Member
The idea of applying a heat source to the skin is to expand the aluminum beyond what it would normally encounter prior to riveting the aluminum. Supposedly when it cools the skin will remain tight and not oilcan. Personally, if I had an aileron with an oilcanned portion, I'd likely just build a new aileron.
Flying Scotsman
Well Known Member
I'm also curious as to how someone can detect "severe oil canning" on a control surface that isn't visible in flight. There *will* be some flex of the aileron skin...I'm not an ME, but the ones I know tell me that without at least a small amount of flexing available, the surface would crack at various places. Is he flexing it by hand and making this assessment?
gmcjetpilot
Well Known Member
Flutter/Snatch burble different things / "oil can" normal
Marc I am not going to attack your friend but is he a trained test pilot? Has he ever flown a RV before? He sounds like he may be getting neutrally stable and flutter mixed up.
The TEST pilot slap is not looking for the control stick to not center but to actually GO further and/or snap back and forth in an oscillation which eventually left could go full-stop to stop.
At high speed full deflection would damage the structure almost assuredly, whether pilot input or flutter induced. Your test pilot buddy saw the stick stay over..........THAT IS NEUTRAL STABILITY NOT FLUTTER.
If he thinks it will react like a piper or cessna it will not. RV's are not built to FAR's, which is why they have light and enjoyable controls. There is no LEVEL-O-MATIC super positive roll stability. There is plenty of roll stability for safe flight while maintaining a great feel.
Doing a slap test is a serious thing and should be done with a parachute. If your buddy was not ready to bail-out, than that tells me he does not know what he is doing.
#2, 135kts is not fast but is over (Va). The ailerons are fully mass balanced. You balanced the ailerons before you installed them and did RIG them per the manual? It all has been figured out. There are 5000 RV's flying. Van is an engineer and has over built this thing in every aspect.
COMMON CONSTRUCTION ISSUE
Not saying this is your problem, but some RV'ers miss build their control surfaces and don't bend down the radius flat and leave a bulge at the trailing edge. That can cause all kinds of weird feel and heavy wing. In fact just pinching one aileron just ever so slightly will cause your heavy wing to go away, totally like magic.
First check your ailerons are in rig. Than make sure your control surface trailing edge radius is NOT to large.
Last I don't know if he tried to push the stick full deflection at 135kts. Va is 123 kts I recall.
COMMON IDIOSYNCRASY - Aileron burble
FYI - There is a known idiosyncrasy of RV's with full stick aileron deflection for a roll (below Va of course); at high deflection (high roll rate, large or full roll input) you can feel "snatch" or burble in the stick, at full deflection is normal for RV's. This is not flutter, just air separation. Some planes near stall, turbulent air off the wing hits the tail and it can be felt through the elevator. It is normal. This is not what you describe, but just FYI
Flutter is a different thing. NOT RETURNING TO CENTER IS NOT FLUTTER NECESSARILY. I JUST MEANS NEUTRAL STABILITY. RV's ARE NOT CRUDDY CESSNA'S THAT HAVE LEVEL-O-MATIC. You can put a RV into a turn and it will stay there. It is a fighter plane not a truck. RV's can NOT be certified by normal part 23 FAR because they handle to well. The FAR's call for positive stability. Positive stability means you have to fight that to make it maneuver. RV's are mildly positive but not so much you feel it. Control deflections will stay put when you let go, verses returning to center. This is good and why they FEEL so good to fly.
Flutter tends to be a buzz more than the ailerons or elevator not returning to neutral. However flutter left too long or severe could result in a large uncontrolled (by pilot) deflections (eventually full stop to stop deflection) and destruction of the airframe. But putting the aileron over with a slap and leaving it, where it stays there is NORMAL.
It is possible that ailerons are not "rigged" properly, causing them to deflect or hunt and no return to neutral.
"Oil Can" is normal normal normal if built to plans. I have seen and heard it on factory planes. As a former structural engineer oil can and diagonal tension are normal. You can make oil can free structure. Guess what? It's very heavy. The amount of oil can does not affect the controllability.
Webb Willmott "Aileron Dismemberment"; That is a nice failure theory and it may be true, but severe structural damage can occur to the wing spar, failing the entire wing structure, not to mention a jam and loss of control.
Flutter is not good and can be fatal. RV'ers have had flutter and survived (they where WAY OVER Vne), which is a testament to the design. The pilot reports a buzz. If it got so bad that the control stick went full deflection at high speed, you will likely fail the structure.
The famous and late Steve Wittman died with his wife in a plane of one of his famous designs. He used fabric techniques incompatible with the fabric system he used. The wing fabric started to de-laminate. Due to separation of the fabric, flutter occurred and loss of airframe, Steve and his wife Paula. (A small oil can on an aluminum control surface is not a fabric surface blowing out.)
http://www.ntsb.gov/ntsb/GenPDF.asp?id=ATL95FA092&rpt=fa
http://www.ntsb.gov/ntsb/GenPDF.asp?id=ATL95FA092&rpt=fi
Bottom line if you build your RV to plans it is VERY safe; if flown with in the limitations of the airframe, which are well established, you are safe. RV's fly more hours every year (what 5000 RV's have flown?). There is no issue with flutter. Most if not all airframe failures I have seen in 20 years with RV's has been due to gross overload (too fast pulling too hard) or builder construction deficiency.
Marc I am not going to attack your friend but is he a trained test pilot? Has he ever flown a RV before? He sounds like he may be getting neutrally stable and flutter mixed up.
The TEST pilot slap is not looking for the control stick to not center but to actually GO further and/or snap back and forth in an oscillation which eventually left could go full-stop to stop.
At high speed full deflection would damage the structure almost assuredly, whether pilot input or flutter induced. Your test pilot buddy saw the stick stay over..........THAT IS NEUTRAL STABILITY NOT FLUTTER.
If he thinks it will react like a piper or cessna it will not. RV's are not built to FAR's, which is why they have light and enjoyable controls. There is no LEVEL-O-MATIC super positive roll stability. There is plenty of roll stability for safe flight while maintaining a great feel.
Doing a slap test is a serious thing and should be done with a parachute. If your buddy was not ready to bail-out, than that tells me he does not know what he is doing.
#2, 135kts is not fast but is over (Va). The ailerons are fully mass balanced. You balanced the ailerons before you installed them and did RIG them per the manual? It all has been figured out. There are 5000 RV's flying. Van is an engineer and has over built this thing in every aspect.
COMMON CONSTRUCTION ISSUE
Not saying this is your problem, but some RV'ers miss build their control surfaces and don't bend down the radius flat and leave a bulge at the trailing edge. That can cause all kinds of weird feel and heavy wing. In fact just pinching one aileron just ever so slightly will cause your heavy wing to go away, totally like magic.
First check your ailerons are in rig. Than make sure your control surface trailing edge radius is NOT to large.
Last I don't know if he tried to push the stick full deflection at 135kts. Va is 123 kts I recall.
COMMON IDIOSYNCRASY - Aileron burble
FYI - There is a known idiosyncrasy of RV's with full stick aileron deflection for a roll (below Va of course); at high deflection (high roll rate, large or full roll input) you can feel "snatch" or burble in the stick, at full deflection is normal for RV's. This is not flutter, just air separation. Some planes near stall, turbulent air off the wing hits the tail and it can be felt through the elevator. It is normal. This is not what you describe, but just FYI
Flutter is a different thing. NOT RETURNING TO CENTER IS NOT FLUTTER NECESSARILY. I JUST MEANS NEUTRAL STABILITY. RV's ARE NOT CRUDDY CESSNA'S THAT HAVE LEVEL-O-MATIC. You can put a RV into a turn and it will stay there. It is a fighter plane not a truck. RV's can NOT be certified by normal part 23 FAR because they handle to well. The FAR's call for positive stability. Positive stability means you have to fight that to make it maneuver. RV's are mildly positive but not so much you feel it. Control deflections will stay put when you let go, verses returning to center. This is good and why they FEEL so good to fly.
Flutter tends to be a buzz more than the ailerons or elevator not returning to neutral. However flutter left too long or severe could result in a large uncontrolled (by pilot) deflections (eventually full stop to stop deflection) and destruction of the airframe. But putting the aileron over with a slap and leaving it, where it stays there is NORMAL.
It is possible that ailerons are not "rigged" properly, causing them to deflect or hunt and no return to neutral.
"Oil Can" is normal normal normal if built to plans. I have seen and heard it on factory planes. As a former structural engineer oil can and diagonal tension are normal. You can make oil can free structure. Guess what? It's very heavy. The amount of oil can does not affect the controllability.
Webb Willmott "Aileron Dismemberment"; That is a nice failure theory and it may be true, but severe structural damage can occur to the wing spar, failing the entire wing structure, not to mention a jam and loss of control.
Flutter is not good and can be fatal. RV'ers have had flutter and survived (they where WAY OVER Vne), which is a testament to the design. The pilot reports a buzz. If it got so bad that the control stick went full deflection at high speed, you will likely fail the structure.
The famous and late Steve Wittman died with his wife in a plane of one of his famous designs. He used fabric techniques incompatible with the fabric system he used. The wing fabric started to de-laminate. Due to separation of the fabric, flutter occurred and loss of airframe, Steve and his wife Paula. (A small oil can on an aluminum control surface is not a fabric surface blowing out.)
http://www.ntsb.gov/ntsb/GenPDF.asp?id=ATL95FA092&rpt=fa
http://www.ntsb.gov/ntsb/GenPDF.asp?id=ATL95FA092&rpt=fi
Bottom line if you build your RV to plans it is VERY safe; if flown with in the limitations of the airframe, which are well established, you are safe. RV's fly more hours every year (what 5000 RV's have flown?). There is no issue with flutter. Most if not all airframe failures I have seen in 20 years with RV's has been due to gross overload (too fast pulling too hard) or builder construction deficiency.
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Hmm, flutter is what happens when you hang tape on your lip and blow. It flaps up and down. It is dependant on the c.g. (and probably weight) of the control surface, speed and the stiffness of the control linkages. If you have very sloppy control rods or cables that flex alot, or your aileron is not balanced, you can get flutter. If it is truly flutter, the deflections of the aileron (and the stick) usually diverge and get so large that the aileron will tear off the wing. So it is very serious.
If you have an aerodynamically poor surface on your aileron, this could affect it, but I don't imagine a bit of oil canning would make a difference.
Get your friend to write down exactly what he did and felt and then speak to Vans. I suspect he doesn't know the right terms for what he felt. Also check all your control lines (links, rods, brackets, cables etc) between the stick and the aileron and also the aileron hinges. You would be looking for any sloppyness or flexing. It should all be very stiff -also try moving the stick while the aileron is restrained. Look into the mass balancing too. I wouldn't fly again until I'd done all this. You have nothing to loose by going through this process, but a lot to lose if it really is flutter and you don't fix it.
If you have an aerodynamically poor surface on your aileron, this could affect it, but I don't imagine a bit of oil canning would make a difference.
Get your friend to write down exactly what he did and felt and then speak to Vans. I suspect he doesn't know the right terms for what he felt. Also check all your control lines (links, rods, brackets, cables etc) between the stick and the aileron and also the aileron hinges. You would be looking for any sloppyness or flexing. It should all be very stiff -also try moving the stick while the aileron is restrained. Look into the mass balancing too. I wouldn't fly again until I'd done all this. You have nothing to loose by going through this process, but a lot to lose if it really is flutter and you don't fix it.
