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  #21  
Old 03-08-2019, 05:39 PM
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snopercod snopercod is offline
 
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Here's a short video of my "Turtle Cam", looking at the empennage. I forgot to "flip" the camera in the settings so it's upside down, but you get the idea. It's a Garmin Virb Elite and is very streamlined. I stuck it to the fuselage with double-sticky foam tape, then put pieces of duct tape over on top, just in case.

https://www.dropbox.com/s/mr9a0iag4c...20cam.mp4?dl=0
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  #22  
Old 03-08-2019, 08:28 PM
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is there any chance the trailing edge was installed backwards?
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  #23  
Old 03-08-2019, 11:44 PM
rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by Jared_Solomon View Post
Hi Scott,

I wasn't trying to be harsh or offensive. I apologize if it came off that way. I was only trying to express that I wasn't satisfied with the solution. Software engineering pays the bills and I can empathize with trying to troubleshoot something remotely with limited information. There are a lot of variables at play here, many I've likely not thought of yet. I was hoping the community would stir up some ideas of things I hadn't considered yet so I could continue to troubleshoot.

I will try to explain in a bit more detail exactly what I've observed so far.

Scenario 1:

During climb-out after take off, somewhere in the speed range near Vy (95 - 110kias) I will notice the control stick lightly pulsing back and forth at approximately 1-2 Hz. There is no detectable oscillation in the pitch of the aircraft, and passengers don't notice it. In this scenario I am not applying any pressure to the control stick and I'm allow the elevators to take their natural position based on trim. If make a trim adjustment, either nose up or down the oscillation dampens out and goes away.

Scenario 2:

While maneuvering, typically around 145 - 150kias I can often induce the pulsing. I will enter into a 30 - 40 degree level turn, while applying back pressure on the stick the pulsing will occur at the same 1-2hz. Sometimes applying greater stick back pressure alleviates the pulsing, but sometimes it exacerbates it and makes it more pronounced, but at the same frequency of 1 -2 hz. If I apply some "up" trim and retry the maneuver (at the same IAS) the pulsing usually subsides

Oddly there are days where it's more pronounced and others where it's not so much. I haven't been able to determine a particular load profile (it happens both solo and with a passenger, baggage vs no baggage, etc..) where it's more prevalent. I acknowledge the trim position could also be a "red herring" and just coincidental.



This issue has been present since the first flight, before I machined the SS bushings to remove all slop. All items I mentioned in my first post were things I tried after experiencing the issue

I'll give Vern's idea a shot and try adding some weight to the baggage compartment and see if it has any effect. Weather should clear here Monday or Tuesday. I'll report back my findings with that test.

Thanks,
Ok, so a pitch trim change makes it go away but then the airplane is no longer trimmed for level flight (if I am understanding you correctly).

My initial gut feeling is that it is being caused by a variable flow or shape change.
You previously said that "removing oil canning areas from the elevators has no effect". I take from that to mean that you do have some oil canning in the elevators?

At this point I think what would be the most beneficial would be to mount a camera (gopro or similar) so that it is looking at the top surface of the elevator(s). I would start with the left since that is the one with the trim tab. I have flow quite a few tests on RV-14's with a camera mounted at the outboard end of the horizontal stab., just fwd of the rear spar. The mount was a thin plate of alum., .025-.032 thick, held in place with quality (very sticky) duct tape.

In response to many of the well meaning guesses of the cause, I hope people will keep in mind that there is nearly 100 customer built RV-14's flying now, and this is the first I have heard of anyone noticing this behavior. That in itself is a strong indicator that there is something different about this one example..... the challenge is to figure out what that something is.

It has been mentioned already, and I think it would be hard to build the elevator trailing edges wrong, but can you confirm that the trailing edge wedge on the elevators and trim tab is installed with the thickest portion aft (totally the opposite from what would seem logical).
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  #24  
Old 03-08-2019, 11:46 PM
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Jared_Solomon Jared_Solomon is offline
 
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Quote:
the elevator trailing edge wedge was supposed to reduce this effect but it seems it is still present. the air separating at the trailing edge of the elevator is changing with minute changes in elevator AOA. as an experiment, run a 1/2" wide piece of tape at the elevator trailing edge and let it overhang the elevator trailing edge by 1/4" into the flow stream and see if that helps.
Hey Steve,

What's the reasoning behind placing tape on the trailing edge? I'm a bit cautious about making any changes to control surfaces, as there consequences can be serious if not well understood. I don't want to sound alarmist. Having a better understanding may make me more inclined to experiment.

Quote:
Just something else to throw out there, I noticed you have Garmin G3X displays. There is a function you can turn on and off called ESP. It is configurable and if you exceed set pitch/roll limits it intervenes in the stick, even with the autopilot OFF. Might check this setting.
The Garmin ESP is disabled


A few folks have mentioned potential vortices or turbulence coming off some part of the airframe. I could try progressively taking off my intersection fairings, gear fairings and wheel pants to seeing if it changes. Other then having a Garmin GPS installed on the top deck behind the canopy, my airframe is constructed to plans. All antennas on the belly are in the stock RV-14 locations. The fairings and their alignment are based on how the builder installs them and are a good candidate for something that may be different on my plane vs the other RV-14s currently flying. Anecdotally is "feels" like flow separation or turbulence from the airframe. It feels a lot like how the ailerons pulse on RV-7s and 8s when you give them full deflection during an aileron roll, only the pulsing is fore-aft in the stick instead of side-to-side.
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Powder Springs, GA
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RV-14A (N214WJ) - Flying since Sept 2017
RV-7A (N211WJ) Flying since 2009 (SOLD)
RV-7A (N132RD) bought flying 2014 (SOLD)

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  #25  
Old 03-09-2019, 08:59 AM
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Jared_Solomon Jared_Solomon is offline
 
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Default Oil Canning

Quote:
Originally Posted by rvbuilder2002 View Post
Ok, so a pitch trim change makes it go away but then the airplane is no longer trimmed for level flight (if I am understanding you correctly).

My initial gut feeling is that it is being caused by a variable flow or shape change.
You previously said that "removing oil canning areas from the elevators has no effect". I take from that to mean that you do have some oil canning in the elevators?

At this point I think what would be the most beneficial would be to mount a camera (gopro or similar) so that it is looking at the top surface of the elevator(s). I would start with the left since that is the one with the trim tab. I have flow quite a few tests on RV-14's with a camera mounted at the outboard end of the horizontal stab., just fwd of the rear spar. The mount was a thin plate of alum., .025-.032 thick, held in place with quality (very sticky) duct tape.

In response to many of the well meaning guesses of the cause, I hope people will keep in mind that there is nearly 100 customer built RV-14's flying now, and this is the first I have heard of anyone noticing this behavior. That in itself is a strong indicator that there is something different about this one example..... the challenge is to figure out what that something is.

It has been mentioned already, and I think it would be hard to build the elevator trailing edges wrong, but can you confirm that the trailing edge wedge on the elevators and trim tab is installed with the thickest portion aft (totally the opposite from what would seem logical).
Hi Scott,

I'll try to get a camera setup going. Yes, I've confirmed the trail edges are install per plans. I'll post some pictures next time i'm out at the airport. Regarding the oil canning; yes I have oil canning in two locations on the bottom of the elevators called out in the diagram below:



I duct taped .032 strips onto the bottom of the elevators to remove the oil canning in both locations. The problem still remained.
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Powder Springs, GA
2019 Dues Paid
RV-14A (N214WJ) - Flying since Sept 2017
RV-7A (N211WJ) Flying since 2009 (SOLD)
RV-7A (N132RD) bought flying 2014 (SOLD)

KPUJ (Silver Comet Field)
Youtube channel: http://www.youtube.com/jmsolomon2000

RV-14 Build Log: http://rv14.jaredsolomon.net
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  #26  
Old 03-09-2019, 02:09 PM
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Steve Melton Steve Melton is offline
 
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Quote:
Originally Posted by Jared_Solomon View Post
Hey Steve,

What's the reasoning behind placing tape on the trailing edge? I'm a bit cautious about making any changes to control surfaces, as there consequences can be serious if not well understood. I don't want to sound alarmist. Having a better understanding may make me more inclined to experiment.
1/4" piece of tape off the trailing edge provides a small barrier to the elevator trailing edge flow separation shifting back and forth because of minute elevator angle changes. agree, do this at your own risk. it would only be an experiment, not a final solution.
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  #27  
Old 03-10-2019, 10:33 AM
PilotjohnS PilotjohnS is offline
 
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Default Simple check

Sorry if I missed this in earlier comments, but I would say have another 14 builder flier look over your plane. JMHO
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  #28  
Old 03-13-2019, 08:58 AM
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Jared_Solomon Jared_Solomon is offline
 
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Talking Problem Solved!

Thanks everyone for your comments and suggestions. They inspired me to reevaluate the problem and look at it from a different perspective. I test flew the airplane Monday and paid special attention the elevator position when the pulsing occurred. I noticed that it only happen when the elevators were completely neutral, regardless of trim position. Airspeed also didn't have much of an impact. The problem ended up being caused by the horizontal stabilizer tip fairings.

There naturally molded shape as they arrive in the kit shapes them to be wider and thinner the the elevator fairings profile



Having the smaller profile on the leading edge, when the elevator counterbalance arm was centered within t he fairing, it was inducing the pulsing.

I fabricated a couple of new tips. I used a plywood sheet epoxied in to the tips to force there shape to be the same, and slightly larger then the elevator counterbalance's profile.



Went up and test flew last night. No more pulsing! In retrospect I'm not sure why I didn't consider this sooner. Another variable that may have changed is the gap between the elevator counterbalance are and the tip fairing, although I tried hard to make them the same.





RV-14 Builders beware. The plans are very detailed on how to fabricate, mount and closeout the HS tip fairings (which is appreciated). If you follow the the plans as I did without any improvising you will likely have a similar issue with your HS tips being a smaller profile then your elevator counterbalance profile. Whether that induces the same pulsing as mine did is yet to be seen. Perhaps this may be a good thing to add to the "Plan Gotchas" document.
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Powder Springs, GA
2019 Dues Paid
RV-14A (N214WJ) - Flying since Sept 2017
RV-7A (N211WJ) Flying since 2009 (SOLD)
RV-7A (N132RD) bought flying 2014 (SOLD)

KPUJ (Silver Comet Field)
Youtube channel: http://www.youtube.com/jmsolomon2000

RV-14 Build Log: http://rv14.jaredsolomon.net

Last edited by Jared_Solomon : 03-13-2019 at 09:00 AM. Reason: Added a smiley!
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  #29  
Old 03-13-2019, 10:15 AM
Bavafa Bavafa is offline
 
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Wow, this is an awesome discovery and much appreciate sharing it. I would have never thought this would cause an issue.

During the build, I noticed the different width of each fairing, including that of the VS/rudder. Just purely for the esthetic reason and nothing else, I worked to make those of same profile. I guess now we can add another reason other than esthetic.

I appreciate sharing this.
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  #30  
Old 03-13-2019, 10:39 AM
TimO TimO is offline
 
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I haven't had time to read much on this topic up to this point, but skimmed it through start to finish today quick.

I can say that I know of what you are speaking. I wouldn't be able to call it "pulsing" or anything like that. To me your later descriptions were more on target. It's more of a quick hunting that only happens when nearly perfectly in trim and everything lined up. I have warned (warned is too harsh a word, but tipped off maybe) a couple people during transition training to this feeling.

Basically, if you're just getting settled in to level cruise, all trimmed out, you may get a sudden shift in trim. Nothing fluttering or anything like that, just a quick minor pitch trim change where it hunts to another stable point. Then it's all over. I will have to check to see if the block in front of my elevator counterweights is smaller than the counterweight, but I believe its very close and just a tiny bit smaller than the counterweight also. So if that's the case, then that would explain it as well. The feeling you get makes it feel like your trim tab shifted positions, but as you noted, it may be the wind facing elevator counterweight is just going from perfectly hidden out of the airflow to suddenly catching the airflow as it peeks into the airstream.

For me, it hasn't been very drastic, but it is noticeable when it happens. And, I notice it more when I'm alone than when I'm more loaded down with people and luggage. So what you have diagnosed makes sense to me. I don't notice it every flight, but did a couple days ago when I went out for some pre-spring aerobatics.

Thanks for posting the original topic. It's educational reading through it all. But put me down as someone who has seen this first hand.
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