I found that many certified LSA aircrafts and, also, some standard FAR 23 certified aircrafts don't have mass-balanced surfaces. I'm interested in how they are designed to show freedom from flutter, especially control surface flutter and if these aircrafts can handle a disconnected freefloating surface emergency (due to cable failure, for example).
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Not mass balanced & cable failure
- Thread starter SteveJeff
- Start date
Mike S
Senior Curmudgeon
Yes, the factory. While there are aero engineers here on VAF, I doubt any of them were involved in the actual design of these planes, unless they happen to be on staff at Vans Aircraft. There are a couple of Vans Aircraft employees who frequent this site.
Here is the link to the factory. https://www.vansaircraft.com/
N546RV, you are right.
az_gila
Well Known Member
I believe freedom from flutter is certified by a flight test (the design speed + 20%), and either a design calculation or engineering requirement...
Details here -
http://www.flightsimaviation.com/data/FARS/part_23-629.html
Details here -
http://www.flightsimaviation.com/data/FARS/part_23-629.html
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Okay, this is my guess, I am not an aero engineer.
1. LSA's are, by law, low speed aircraft. Most (but not necessarily all) flutter problems get worse with increasing airspeed. So they may be able to get away with less sophisticated solutions, or not have problems within their flight envelope.
2. As far as I know there are no requirements to test for flutter with controls disconnected. They do test for it with hands off the controls. In particular I do not believe the elevator trim tab is tested for flutter without the actuating mechanism. Although not a certified airplane last year's Reno crash was blamed on trim tab flutter due to loose screws on the tab hinge.
1. LSA's are, by law, low speed aircraft. Most (but not necessarily all) flutter problems get worse with increasing airspeed. So they may be able to get away with less sophisticated solutions, or not have problems within their flight envelope.
2. As far as I know there are no requirements to test for flutter with controls disconnected. They do test for it with hands off the controls. In particular I do not believe the elevator trim tab is tested for flutter without the actuating mechanism. Although not a certified airplane last year's Reno crash was blamed on trim tab flutter due to loose screws on the tab hinge.
Already read it. As I stated in my first post, talking about freefloating surface scenario in not mass balanced LSA (ASTM standards, not FAR 23) or FAR 23 aircrafts. In that FAR 23 text there is no specific info on this issue.
Like what? I hope not cable tension.
(f) Freedom from flutter, control reversal and divergence up to VD/MD must be shown as follows:
(1) For aeroplanes that meet the criteria of sub-paragraphs (d) (1) to (d) (3) of this paragraph, after the
failure, malfunction, or disconnection of any single element in any tab control system.
EDIT: I don't think they would take care about flutter regarding a small tab cable failure and won't for primary surfaces, especially ailerons. Usually the other surfaces are mass-balanced, that's why I'm asking how it comes that some under this FAR23 are not, and many, if not most LSA the same.
Pilot unconscious due to high G effect at very high speed, and it seems there was a trim tab surface itself failure, it was not just cable failure or freefloating.
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Okay, so I stand corrected, there is a requirement on the trim tab with a hardware failure, for certified planes.
When I owned a 182 there was a spec on control cable tension. I do not know how critical it was.
Order at Reno was: loose screws on trim tab hinge, tab fluttered, tab physically failed and broke off, tab was holding elevator in nose down position so when it failed elevator went to nose up, plane pulled 17 g, pilot passed out...
When I owned a 182 there was a spec on control cable tension. I do not know how critical it was.
Order at Reno was: loose screws on trim tab hinge, tab fluttered, tab physically failed and broke off, tab was holding elevator in nose down position so when it failed elevator went to nose up, plane pulled 17 g, pilot passed out...
David-aviator
Well Known Member
Seems like the question is baiting - with some 7953 RV's flying and no discernible evidence of control surface flutter - what is the point of it?
Except for the rudder, there are no cables here. Improperly tensioned control cables can lead to surface flutter. That is not an issue with any RV. Torque tubes do not lose tension.
The question is valid from a broad aerodynamic perspective but it is not so far as RV's are concerned.
Except for the rudder, there are no cables here. Improperly tensioned control cables can lead to surface flutter. That is not an issue with any RV. Torque tubes do not lose tension.
The question is valid from a broad aerodynamic perspective but it is not so far as RV's are concerned.
az_gila
Well Known Member
It's not even required by FAR 23 if the plane cofiguration meets (d)(1) to (d)(3) {less conplex, slower planes} and the flight test to 1.2 VD and the engineering requirements of Engineering Report 45 (stifness and mass balancing) are met...
az_gila
Well Known Member
Seems like the question is baiting - with some 7953 RV's flying and no discernible evidence of control surface flutter - what is the point of it?
Except for the rudder, there are no cables here. Improperly tensioned control cables can lead to surface flutter. That is not an issue with any RV. Torque tubes do not lose tension.
The question is valid from a broad aerodynamic perspective but it is not so far as RV's are concerned.
Actually it is applicable to RVs. The FAR portion for certified planes - that the OP referenced - says "failure, malfunction or disconnect" of any control surface. Lots of nylock nuts/bolt connections could be part of a failure under that reading. "Cables" are not mentioned...
I konw, doesn't apply to our RVs....
Flutter
It has been stated azd restated that this was a generic question that would not apply to any RV except possibly the 12.
I am very skeptical about how many certified as well as homebuilt airplanes would survive a trim tab disconnect.
The original Cessna Conquest was lost because of an elevator tab disconnect.
A elevator tab disconnect WILL cause flutter on a Pitts S1. It is surviveable perhaps because of the relatively small size of the elevators and the strength of the entire system. Many homebuilt airplanes get by with control system designs that could not meet certification standards. In many cases this is because of relatively small control surfaces. The Cassutt is a good example. Tom Cassutt originally balanced the elevators, but later removed the balance. On his second airplane he did not use a balance and none is shown on the plans. The rudder on a standard Cassutt is not balanced. The ailerons of course are balanced. I have flown two different Cassutts to 270 statute indicated, probably 320 true.
It has been stated azd restated that this was a generic question that would not apply to any RV except possibly the 12.
I am very skeptical about how many certified as well as homebuilt airplanes would survive a trim tab disconnect.
The original Cessna Conquest was lost because of an elevator tab disconnect.
A elevator tab disconnect WILL cause flutter on a Pitts S1. It is surviveable perhaps because of the relatively small size of the elevators and the strength of the entire system. Many homebuilt airplanes get by with control system designs that could not meet certification standards. In many cases this is because of relatively small control surfaces. The Cassutt is a good example. Tom Cassutt originally balanced the elevators, but later removed the balance. On his second airplane he did not use a balance and none is shown on the plans. The rudder on a standard Cassutt is not balanced. The ailerons of course are balanced. I have flown two different Cassutts to 270 statute indicated, probably 320 true.
Not even required what? As you said, the engineering requirements of Engineering Report 45 (stifness and mass balancing) are met.... If Report 45 is met, I figure out the aircraft has mass-balanced surfaces. I'm talking about aircrafts which don't have mass-balanced surfaces.
"(1) For aeroplanes that meet the criteria of sub-paragraphs (d) (1) to (d) (3) of this paragraph ..."
And however, after a close analysis of that phrase, I figure out it might not be mandatory to meet Engineering Report 45, it seems they took care for not using it for aircrafts which large mass concentrations, that's why they included (d) (2) and it is said "sub-paragraphs (d) (1) to (d) (3)", including (2) which states: "(2) The wing and aileron flutter prevention criteria, as represented by the wing torsional stiffness and
aileron balance criteria, are limited to use to aeroplanes without large mass concentrations (such as engines,
floats, or fuel tanks in outer wing panels) along the wing span;".
Otherwise, if mandatory to meet Report 45, they would simply say "For aeroplanes that meet the criteria of paragraph (d) (Compliance with the rigidity and mass balance criteria...)", including the paragraph itself which requires the compliance with Report 45.
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az_gila
Well Known Member
The report is here -
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA955270&Location=U2&doc=GetTRDoc.pdf
..and does not require all surfaces to be balanced....
Meeting the report is optional if a detailed flutter analysis is done. The flight test is not optional...
Ok, so we agreed that even some of FAR 23 airplanes might have unbalanced surfaces.
The question is, how a not mass-balanced aileron would behave if it's freefloating (assuming a linkage failure) and how likely is to flutter in this scenario considering in normal operation the airplane was tested for flutter and also there were no flutter occurences during normal operation?
The question is, how a not mass-balanced aileron would behave if it's freefloating (assuming a linkage failure) and how likely is to flutter in this scenario considering in normal operation the airplane was tested for flutter and also there were no flutter occurences during normal operation?
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David Paule
Well Known Member
Don't overlook that if the control system is redundant, one control system disconnection should still leave the surfaces fastened to the control system. The airplane then would need to be shown flutter-free with either of the remaining connections.
Dave
Dave
David Paule
Well Known Member
For example, hypothetically assume that a conventional trim tab has two actuators. They are connected to two trim tab horns that are close to each other. If one actuator was disconnected, the other would still be attached. In this case, the trim tab would not be free to flutter. It would work the same as an ordinary trim tab that has no parts fail.
Dave
Dave
az_gila
Well Known Member
A dumb question to the aerodynamicists -
Wouldn't a disconnected surface be less likely to flutter (at our speeds) than one that is connected. The control surface wouldn't have anything to excite any elasicity in the fixed surface. It seems to me a disconnected surface would just trail in the breeze...
Wouldn't a disconnected surface be less likely to flutter (at our speeds) than one that is connected. The control surface wouldn't have anything to excite any elasicity in the fixed surface. It seems to me a disconnected surface would just trail in the breeze...
It isn't that simple. Imagine a disconnected left aileron. In perfectly smooth air it would just trail. But suppose a tiny bit of turbulence causes it to go slightly up. That up aileron will cause a down force on the wing, and it will bend down, overshoot, then spring back up. Meanwhile the air will be forcing the aileron back down. But it too will overshoot, down, and so push the wing up. If this happens just as the wing was already springing up, it will push the wing higher, leaving the aileron behind (remember the cg of the aileron is aft of the hinge), and the whole cycle repeats, with the wing bending more and more with each oscillation. The key is whether or not the wing bending frequency and the aileron flapping frequency can coincide. RV's tend to have very stiff wings. But some gliders, with long, thin wings, have wings that almost flap.
az_gila
Well Known Member
That was my point (and question), the stiff wings we have in our metal planes won't get into resonance like the very flexible - but equally strong - fibreglas wings of a glider. Your description of "springing" isn't really applicable to stiff, rigid wings....
I wouldn't expect it to flutter in this case. I figure out the trim tab won't be freefloating considering it is still atached to the other actuator, otherwise it would be pointless. Or, am I missing smth?
Even if so, I would bet there are also fibreglas, composite and fabric covered wings with not mass balanced surfaces.
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David Paule
Well Known Member
SteveJeff wrote -- I wouldn't expect it to flutter in this case. I figure out the trim tab won't be freefloating considering it is still atached to the other actuator, otherwise it would be pointless. Or, am I missing smth?
That's the whole point of redundancy.
It's an approach that designers may use to prevent a single failure from becoming a hazard.
Also, even our relatively stiff wings have a certain amount of flexibility, and for things like flutter it needs to be properly assessed.
Dave
That's the whole point of redundancy.
It's an approach that designers may use to prevent a single failure from becoming a hazard.
Also, even our relatively stiff wings have a certain amount of flexibility, and for things like flutter it needs to be properly assessed.
Dave
The way you said it made me think you mean even double linked systems may suffer from flutter, so they must show not able to flutter even they are redundant and just one of the two connection failed. I think it was a misunderstanding because I figure out as long as one connection is still atached the tab can't flutter.
David Paule
Well Known Member
Flutter is kind of a funny thing. It depends on so many factors that it's entirely possible that a surface could flutter with no trim tab on it, or with a fully connected trim tab, or with a redundantly connected trim tab that has one connection missing.
It's up to the designer to evaluate the stiffness and mass distribution of the aircraft and assess the flutter probability, and the test pilot to verify that in fact the airplane is flutter-free.
If you feel that you need to show that a trim tab can survive a single disconnect without fluttering, then you need to see what design tools you have that would help you with that goal. You can't make a blanket determination that a certain configuration won't flutter arbitrarily. You have to work through the process.
I suggested that redundant controls might be a possible design tool. Another might be that you could, if you wanted to, make the tab itself mass-balanced. Would that help? It might - but it might not; it's additional mass, so far aft on the control surface and needing more mass to balance the main surface (e.g., an elevator and trim tab), might lower the natural frequency of the elevator enough that now it's more likely, instead of less likely, to flutter.
As a personal example, I'm building an RV-3B, which does not have mass-balanced elevators. It has a standard type of trim tab that's not redundantly controlled; a single disconnection will set it free. I worked out how to mass-balance the elevator and decided not to, since so many are flying without flutter.
The tab itself only has one connection to its push-pull cable. That control connection will have self-locking nut that's also got a cotter pin. A disconnection would be unlikely, and the connection is external and easily examined on pre-flight.
As a U.S. experimental amateur-built airplane, this is ample to meet the very limited requirements set by the government for this kind of airplane. But someone somewhere else might need to satisfy different, more stringent, requirements and might choose a different approach to the details of this part of the airplane.
Dave
It's up to the designer to evaluate the stiffness and mass distribution of the aircraft and assess the flutter probability, and the test pilot to verify that in fact the airplane is flutter-free.
If you feel that you need to show that a trim tab can survive a single disconnect without fluttering, then you need to see what design tools you have that would help you with that goal. You can't make a blanket determination that a certain configuration won't flutter arbitrarily. You have to work through the process.
I suggested that redundant controls might be a possible design tool. Another might be that you could, if you wanted to, make the tab itself mass-balanced. Would that help? It might - but it might not; it's additional mass, so far aft on the control surface and needing more mass to balance the main surface (e.g., an elevator and trim tab), might lower the natural frequency of the elevator enough that now it's more likely, instead of less likely, to flutter.
As a personal example, I'm building an RV-3B, which does not have mass-balanced elevators. It has a standard type of trim tab that's not redundantly controlled; a single disconnection will set it free. I worked out how to mass-balance the elevator and decided not to, since so many are flying without flutter.
The tab itself only has one connection to its push-pull cable. That control connection will have self-locking nut that's also got a cotter pin. A disconnection would be unlikely, and the connection is external and easily examined on pre-flight.
As a U.S. experimental amateur-built airplane, this is ample to meet the very limited requirements set by the government for this kind of airplane. But someone somewhere else might need to satisfy different, more stringent, requirements and might choose a different approach to the details of this part of the airplane.
Dave
TRIM
About 12 years ago I had an elevator trim tab cable sheath slip about 1/4 " that allowed the trim tab perhaps 3/8" of "Free play" at the trailing edge of the trim tab.
The aircraft had a very stiff +6G -3G airframe used by at least one country for over 25 years as a basic airforce trainer.
I was well below VNE (20 knots under) when all he11 broke loose. The resulting elevator flapping only lasted 2-3 seconds before I got the speed bled off, however my shoulders were beat black and blue from the harness and my tail bone was sore for several days, from the pounding between seat and harness from the pitch porpoise that the trim tab set up when it was free to swing in the breeze. It nearly wrenched the stick from my hand and it really beat up the back end of the AC. The stick and elevator were pounding lock to lock and only my death grip on the stick and arm were dampening the pounding.
VANS trim tab wisely has the hinge at the top edge and way off center. This design has proven to tolerate some free play in the cable / linkage.
However many other designs do not tolerate any such free play.
Be careful if you stray from a PROVEN design like the VANS and don't mess with any certified AC by modifying the design of any trim system.
If you build a VANS AC to plans and keep it below VNE and well maintained, you have nothing to worry about.
Best of Luck!
About 12 years ago I had an elevator trim tab cable sheath slip about 1/4 " that allowed the trim tab perhaps 3/8" of "Free play" at the trailing edge of the trim tab.
The aircraft had a very stiff +6G -3G airframe used by at least one country for over 25 years as a basic airforce trainer.
I was well below VNE (20 knots under) when all he11 broke loose. The resulting elevator flapping only lasted 2-3 seconds before I got the speed bled off, however my shoulders were beat black and blue from the harness and my tail bone was sore for several days, from the pounding between seat and harness from the pitch porpoise that the trim tab set up when it was free to swing in the breeze. It nearly wrenched the stick from my hand and it really beat up the back end of the AC. The stick and elevator were pounding lock to lock and only my death grip on the stick and arm were dampening the pounding.
VANS trim tab wisely has the hinge at the top edge and way off center. This design has proven to tolerate some free play in the cable / linkage.
However many other designs do not tolerate any such free play.
Be careful if you stray from a PROVEN design like the VANS and don't mess with any certified AC by modifying the design of any trim system.
If you build a VANS AC to plans and keep it below VNE and well maintained, you have nothing to worry about.
Best of Luck!
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You could take a 200 MPH leaf blower and stand in front of your H stab with a free pivoting trim tab on the back of your elevator to see what might happen... Just don't try this with an AC you want to fly any time soon after the experiment. As stated in an earlier thread on flutter, trim tab and or control surface flutter is not a linear or totally predictable event. A flag flapping in the wind might be a useful thing to visualize what a free pivoting trim tab on the back of an elevator is like. Much less trouble and destruction than the leaf blower too!
balance
I have seen a few Luscombes with mass balanced elevator tabs. I think they were the fairly rare pre WWII models. Most Luscombes do not have the balance.
The Pitts S1 elevator trim tab has a continuous piano hinge across the top. The Pitts tab WILL flutter in the event of actuator disconnect.
I have seen a few Luscombes with mass balanced elevator tabs. I think they were the fairly rare pre WWII models. Most Luscombes do not have the balance.
The Pitts S1 elevator trim tab has a continuous piano hinge across the top. The Pitts tab WILL flutter in the event of actuator disconnect.
LATER EDIT:
What makes it so prone to flutter? You state that based on past occurences or it has a particular design which makes it prone?
So, basically you can't figure out if the critical flutter speed might be in the normal operating range in a disconnecting event? What I wonder is that speed might change and come below Vno and in that case it would be really a problem.
What makes it so prone to flutter? You state that based on past occurences or it has a particular design which makes it prone?
So, basically you can't figure out if the critical flutter speed might be in the normal operating range in a disconnecting event? What I wonder is that speed might change and come below Vno and in that case it would be really a problem.
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Ray Allen Servo Disclaimer
This discussion reminds me of a question I've had since reading the paperwork on for the Ray Allen server than Van's sold me to control my trim tab.
"Ray Allen Company, Inc. does not recommend that servos be installed in an aircraft to operate any apparatus or control surface that, if failure were to occur, could result in mechanical damage or control malfunction."
This seems to me that they're saying that I should not install their servo unless I'm prepared for any sort of failure of that servo ... stuck full up, stuck full down, etc.
Is this the sort of testing that normally occurs during phase I?
This discussion reminds me of a question I've had since reading the paperwork on for the Ray Allen server than Van's sold me to control my trim tab.
"Ray Allen Company, Inc. does not recommend that servos be installed in an aircraft to operate any apparatus or control surface that, if failure were to occur, could result in mechanical damage or control malfunction."
This seems to me that they're saying that I should not install their servo unless I'm prepared for any sort of failure of that servo ... stuck full up, stuck full down, etc.
Is this the sort of testing that normally occurs during phase I?
Also, have a look here at the C172 flight control systems. Page 210-211. There are two cables which run from trim wheel to trim tab pushrod. Only if the pushrod would detach from the tab surface you'll have a freefloating tab which is extremely unlikely cause we can consider it as strong as the tab itself. This kind of fail-safe I was talking about when I referred FAR 23 requirements.
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