FlyingMonkey Posted January 25, 2015 Report Share Posted January 25, 2015 Hey all...in the the last couple of weeks I have noticed that at higher speeds my CTSW has some airframe vibration that feels like fast buffeting. It starts to come on about 125-130 knots indicated, and by 135 knots indicated it's very strong and downright unnerving, shaking the whole airframe. At first I thought the wheelpants might be shaking, so I removed them. The problem did not go away. Afraid this might be some type of flutter, I mounted a GoPro pointing to the tail and did some runs up to 135 KIAS. Below is a video of one of the runs. The video has a little shake to it, but at about 12sec into the video, it looks to me like I'm seeing trim tab flutter developing. The trim tab seems to flap up and down pretty strongly until I reduce speed toward the end of the video, then it settles back down. The Vne of the CTSW is 145kt, so I don't think I should be seeing flutter of any aerodynamic surface below that speed (or actually below Vne + 10% safety margin). Has anybody else see this before? What might cause it, slack in the trim cables or something else? Quote Link to comment Share on other sites More sharing options...
Roger Lee Posted January 25, 2015 Report Share Posted January 25, 2015 Hi Andy, I believe you are right. Time to pull the stab and check the pivot pin nuts for torque. 200 in/lbs. Check the trim tab rods to make sure they have not worn and look for any free play. It is also possible that the bearings for the stab need replacement. Did your plane fall into the FD SB recall group to have the stab stiffened? If so did it get done? Until you get this fixed stay away from those speeds that generate this. You must be in at least a shallow dive to get to 135+ knots. That's a 155 mph and I have had mine to 160 and it was smooth. My 2006 CTSW had the recall stab on it until a mechanic way back when ruined it. He replaced it with one from 2007. It wasn't in the SB recall. Quote Link to comment Share on other sites More sharing options...
FredG Posted January 25, 2015 Report Share Posted January 25, 2015 Andy, Do you feel any vibration being telegraphed through the control stick? fg Quote Link to comment Share on other sites More sharing options...
FlyingMonkey Posted January 25, 2015 Author Report Share Posted January 25, 2015 Hi Andy, I believe you are right. Time to pull the stab and check the pivot pin nuts for torque. 200 in/lbs. Check the trim tab rods to make sure they have not worn and look for any free play. It is also possible that the bearings for the stab need replacement. Did your plane fall into the FD SB recall group to have the stab stiffened? If so did it get done? Until you get this fixed stay away from those speeds that generate this. You must be in at least a shallow dive to get to 135+ knots. That's a 155 mph and I have had mine to 160 and it was smooth. My 2006 CTSW had the recall stab on it until a mechanic way back when ruined it. He replaced it with one from 2007. It wasn't in the SB recall. Mine has had the stiffening tab added under the trim tab per the SB; I think that's the one you are referencing. It's pretty easy to stay under 130kt, so that's my new Vne until this is addressed. It makes me really wish there was a great FD repair shop nearby. This might be worth a trip to Lockwood. Quote Link to comment Share on other sites More sharing options...
FlyingMonkey Posted January 25, 2015 Author Report Share Posted January 25, 2015 Andy, Do you feel any vibration being telegraphed through the control stick? fg It's more through the entire airframe. If you look in the video you notice the stabilizer doesn't really move in response to the trim tab moving, probably because the moves are too rapid. So there's not much to make it through the stick. I might feel it in the trim wheel if I tried to retrim at that time, but I have not done that. Quote Link to comment Share on other sites More sharing options...
Roger Lee Posted January 25, 2015 Report Share Posted January 25, 2015 Is your trim set to neutral at these speeds? Quote Link to comment Share on other sites More sharing options...
FlyingMonkey Posted January 25, 2015 Author Report Share Posted January 25, 2015 Is your trim set to neutral at these speeds? I start off with neutral trim at about 115kt, then I trim in some nose down anticipating the higher speeds, then go to 5500rpm and pitch down about 5°. As the speed builds I come off the throttle to maintain 5500rpm and pitch as necessary to achieve the desired speed (in this case 135kt). It usually does not take more than 5°-7° IIRC down pitch to hit the speed pretty fast. When speed is hit, throttle down and slowly bring wings level. Quote Link to comment Share on other sites More sharing options...
gbigs Posted January 26, 2015 Report Share Posted January 26, 2015 Looking at CTSW POH Section 4.4 Vne is 145kts CAS, not IAS. You may be overspeeding if you show IAS 140... Since your Va is only 98kts CAS (again from the SW manual), anything above that in other than smooth air may result in buffeting, etc. Quote Link to comment Share on other sites More sharing options...
Runtoeat Posted January 26, 2015 Report Share Posted January 26, 2015 Andy, check your messages. Quote Link to comment Share on other sites More sharing options...
FastEddieB Posted January 26, 2015 Report Share Posted January 26, 2015 Andy, I would be hesitant to fly the plane until the matter is resolved. Flutter is one of the most frightening things that can happen to a pilot, rapidly getting out of hand. http://youtu.be/qfkkiDsEXUA Quote Link to comment Share on other sites More sharing options...
dick747 Posted January 26, 2015 Report Share Posted January 26, 2015 I have seen flutter in the wind tunnel and in flight test. It appears you are experiencing limit cycle flutter where the damping, aerodynamic and structural, is sufficient to prevent divergence. Once the damping line is crossed catastrophic failure is just a few cycles away. I agree with Fast Eddie. Do not fly until you figure out the problem and get it fixed. Quote Link to comment Share on other sites More sharing options...
chanik Posted January 26, 2015 Report Share Posted January 26, 2015 Small point, but Vne is defined as true airspeed. Partly because flutter is a function of true airspeed, not indicated. It's a serious risk factor that many pilots with turbo'd engines that can fly very high and fast, often neglect. So how high did you do your 135kt indicated testing? Quote Link to comment Share on other sites More sharing options...
FlyingMonkey Posted January 26, 2015 Author Report Share Posted January 26, 2015 Small point, but Vne is defined as true airspeed. Partly because flutter is a function of true airspeed, not indicated. It's a serious risk factor that many pilots with turbo'd engines that can fly very high and fast, often neglect. So how high did you do your 135kt indicated testing? 3000ft. I did not look at TAS, but onset of the vibration was 130-135 KIAS. OAT was 48F and altimeter was 29.98. Quote Link to comment Share on other sites More sharing options...
FastEddieB Posted January 26, 2015 Report Share Posted January 26, 2015 Kurt, Please point me to a reference for Vne being TAS. I think all POH's give it as either IAS or CAS or both. Quote Link to comment Share on other sites More sharing options...
chanik Posted January 26, 2015 Report Share Posted January 26, 2015 http://www.flyingmag.com/technique/proficiency/technicalities-are-you-feeling-lucky Well flutter is definitely linked to True airspeed. V speed tests are about as messy as the FAA could manage. Start with FAR 23.333 and read the next few sections, but the point is VD is defined at sea level. More specifically, there is no requirement to run testing at multiple altitudes. There is also nothing about flutter testing when setting any of these speeds you will note. FAR 23.629 describes flutter testing. But also not at different altitudes. Quote Link to comment Share on other sites More sharing options...
ctfarmer Posted January 26, 2015 Report Share Posted January 26, 2015 In relation to flutter, there does appear to be good reason that its occurrence is related to TAS and not IAS. I understand Vne is normally stated as IAS and normally has a margin to account for such variances. Flutter is not typically caused by excessive forces (ie high IAS) but the synchronizing of often small forces over the air frame to coincide with natural frequencies. In which case your TAS is the relevant measure which produces the effect. Therefore, in a case such as yours Andy, it might be reasonable to expect that this problem will present itself at lower IAS at higher DA. Ie constant TAS. Quote Link to comment Share on other sites More sharing options...
Ed Cesnalis Posted January 26, 2015 Report Share Posted January 26, 2015 When we cross the Sierra in the Europa (914), and descending from 17,000' it is very easy to overspeed (TAS) while IAS is still in range. Quote Link to comment Share on other sites More sharing options...
FastEddieB Posted January 26, 2015 Report Share Posted January 26, 2015 Kurt, Thanks so much for pointing out the TAS nature of something normally given as IAS/CAS. I may have known that at some point, but if so I had forgotten it. Only a finite amount of brain cells, after all! That's the beauty of forums such as this one, and the depth of knowledge the members bring to the table. Thanks again. Quote Link to comment Share on other sites More sharing options...
Runtoeat Posted January 26, 2015 Report Share Posted January 26, 2015 My knowledge, which is not official, is that the stab trim tab on the CTSW is a control surface that is not balanced for C.G. My rudimentary understanding of flutter is that a control surface that is not balanced must have pivots and movement control systems which have little, if no, play to avoid flutter and this only takes one so far regarding Vne. Quote Link to comment Share on other sites More sharing options...
FlyingMonkey Posted January 26, 2015 Author Report Share Posted January 26, 2015 It's pretty rare to come across something new or unique in the CT community, with over 300 flying airframes and a decade and a half of experience. Has anybody else seen or heard of this problem occurring in other CTs, or am I really the first on this? Quote Link to comment Share on other sites More sharing options...
Roger Lee Posted January 26, 2015 Report Share Posted January 26, 2015 Your not the first. FD did have a couple and that's why they came out with the stiffening SB. I would give FD a call. Quote Link to comment Share on other sites More sharing options...
Ed Cesnalis Posted January 26, 2015 Report Share Posted January 26, 2015 I have felt similar but never video'd the stab looking for flutter. I fly in a more turbulent environment and it takes a power on descent for me to see speeds beyond 130kts TAS. When I do get into that speed range I have always thought the vibration was due to descending through gradient, or different layers. I also realized that flutter is a function of TAS and at my altitude's I can easily exceed Vne so when I descend I just throttle back to 500fpm and leave the trim speed where it is for flutter safety margin. My CT didn't qualify for the stab mod, maybe it needs one? Quote Link to comment Share on other sites More sharing options...
Tom Baker Posted January 26, 2015 Report Share Posted January 26, 2015 Andy, the serial number range in the SB for the mod Roger is talking about stopped just the month before your airplane. Quote Link to comment Share on other sites More sharing options...
Ed Cesnalis Posted January 26, 2015 Report Share Posted January 26, 2015 After reading my post I realize that I do feel a vibration that is likely from the same source but I have only felt it when approaching/making 140kts TAS. I was however under the impression that the Vne was lowered for the BRS and otherwise was 160kts. Perhaps along with the Vne getting lowered early on the 'standards' for stab performance lowered as well? Quote Link to comment Share on other sites More sharing options...
ralarcon Posted January 26, 2015 Report Share Posted January 26, 2015 After reading my post I realize that I do feel a vibration that is likely from the same source but I have only felt it when approaching/making 140kts TAS. I was however under the impression that the Vne was lowered for the BRS and otherwise was 160kts. Perhaps along with the Vne getting lowered early on the 'standards' for stab performance lowered as well? Check with Tad, at Lockwood, he is now with Tecnam. However he has an approved tail modification, he did on my CTLS , because some screws, on the Stabilator, were wearing into the carbon composite. My aircraft is a CTLS with a 2010 build date. He made the modification in 2013. Cheers. Quote Link to comment Share on other sites More sharing options...
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