What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Elevator Pulsing/Hunting

Jared_Solomon

Well Known Member
Hi All,

Looking for some thoughts from the VAF brain trust. I have put my 14A through it's paces and have about 230 hrs on it now . I'm puzzled by a "problem" I keep experiencing in flight. It appears that when the trim tab is in the neutral position, or close to neutral, the control stick "pulses" back and forth at about 1-2hz. I've spoken to Van's about it in the past and they believe the elevator is "hunting" when the elevator and the trim tab align just right. There is advice is to "trim it out", which does work, just seems like a bit of a cop out. For the past year I have been testing various theories as to why this might be happening, some theories related to the trim tab and others not. Below is a summary of what I've tried so far

  • Checked all linkages (no slop found)
  • Removed all slop from the trim tab (machined bushings)
  • Taped on reinforcement plates anywhere I found oil canning (removing oil canning areas from the elevators has no effect)
  • Elevators balanced (no effect)
  • Applied speed tape to the outboard lightening holes in the HS

Couple of questions:
  • Other RV-14 drivers, have your experienced anything similar?
  • If it is the trim tab / elevator alignment, any suggestions on how to fix it?

Thanks,
 
Does the pitch change with the hunting, or does it stay stable and the elevator assembly is "floating" as in moving but not making any pitch change? I know back in my R/C days we would make the trailing edges thick like the 14 has to make the sensitivity less so at neutral like you describe. I never noticed this on my 14A but will try to pay close attention to replicate what you describe.
 
Interesting to hear this.

Obviously I don't have nearly as many hours on my 14A as you do but so far, twice I have noticed a very short "vibration/pulsing" in my stick just right after rotating (during take off) that last about 1sec to 1.5 second. I have not noticed it in the air at all. I do believe the pulsing during rotation is related to where exactly my trim tab is which I set it to neutral position but It is not a precise setting. I have, likewise, checked for any 'play' in the control surfaces and can say there is about 1/32" (as best as I can measure it) of play in my trim tab but zero on my elevators in relation to one another and I did balance my elevators each individually.
 
Jared, could you fasten something like a rubber strap, temporarily with tape, to keep the trim tab always against one side of whatever small slop might be in the system? Some way to preload it to one direction. Seems it could be related to that. Putting a small amount of trim in and the putting a small pressure on the stick does a similar thing.
 
Here's an experiment: Try putting a 30-40 lb weight in the baggage compartment. See if you get an improvement when you fly.

If so, you may have to adjust the incidence of the HS.

V
 
No pitch change

Does the pitch change with the hunting, or does it stay stable and the elevator assembly is "floating" as in moving but not making any pitch change? I know back in my R/C days we would make the trailing edges thick like the 14 has to make the sensitivity less so at neutral like you describe. I never noticed this on my 14A but will try to pay close attention to replicate what you describe.

Hi Chris,

No perceptible pitch change. I've asked passengers while it's happening and they can't feel it. It would appear to be "floating".

Thanks,
 
Jared, could you fasten something like a rubber strap, temporarily with tape, to keep the trim tab always against one side of whatever small slop might be in the system? Some way to preload it to one direction. Seems it could be related to that. Putting a small amount of trim in and the putting a small pressure on the stick does a similar thing.

Hi Alex,

The trim tab has 0 slop. I machined SS bushing sleeves that go over the clevis pins to remove all slop.

Thanks,
 
Here's an experiment: Try putting a 30-40 lb weight in the baggage compartment. See if you get an improvement when you fly.

If so, you may have to adjust the incidence of the HS.

V

Hi Vern,

Good idea. Easy to do. I'll give it a shot!

Thanks,
 
I've spoken to Van's about it in the past and they believe the elevator is "hunting" when the elevator and the trim tab align just right. There is advice is to "trim it out", which does work, just seems like a bit of a cop out.

Cop out seems just a bit harsh considering the circumstances. Troubleshooting problems such as this via e-mail or phone call is about the same as using a phone consult with a Dr for an unusual medical condition. It could be a lot of things......

This is not something that has been common with RV-14's so there is no "yea we have heard that a bunch of times.... just do this...."

Regardless, in the interest of trying to help you out, please explain what you mean by "trimming it out does work". (to stop the pulsing?)
 
Hi Alex,

The trim tab has 0 slop. I machined SS bushing sleeves that go over the clevis pins to remove all slop.

Thanks,

Seems like you have identified the one thing that is different from all of the others and possibly why yours is doing what all of the others are not. Sometimes "improving" something doesn't always deliver the results you were hoping for. I can think of numerous mechanical systems that have "slop" designed in, escpecially in dynamic applications. Not saying the slop was designed in in this case, but it still may account for the performance difference. In cases of unique performance problems, you usually start with what might be different from the norm.

Think of the poor guy that said "look at that ugly rivet for a static port; I can make something much better" and proceeded to make a nice flat one that looks just like the expensive certified planes then wondered why his airspeed readings were so far off.

Larry
 
Last edited:
Here's an experiment: Try putting a 30-40 lb weight in the baggage compartment. See if you get an improvement when you fly.
+1 on Vern's idea...forcing the elevator trim position away from neutral by changing CG is a good test. It may be difficult to tell with more than light pressure applied to the stick, but can you feel any "hunting" when you force the elevator away from its trim position, i.e., in a climb or descent?

I'm not an aerodynamicist by education, but based on practical flight test experience I wonder if there's some unexpected unsteady airflow around that reverse wedge in the RV-14's elevator when it's exactly neutral. Perhaps one of the smarter aero folks can chime in here.

Dave
 
Last edited:
I'm currently in the planning/researching stage of autopilot trim servos so this is where I'm coming from. Any chance the autopilot pitch trim servo is "modulating" causing the feel you describe?
 
more context

Cop out seems just a bit harsh considering the circumstances. Troubleshooting problems such as this via e-mail or phone call is about the same as using a phone consult with a Dr for an unusual medical condition. It could be a lot of things......

This is not something that has been common with RV-14's so there is no "yea we have heard that a bunch of times.... just do this...."

Regardless, in the interest of trying to help you out, please explain what you mean by "trimming it out does work". (to stop the pulsing?)

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.

Seems like you have identified the one thing that is different from all of the others and possibly why yours is doing what all of the others are not. Sometimes "improving" something doesn't always deliver the results you were hoping for. I can think of numerous mechanical systems that have "slop" designed in, escpecially in dynamic applications. Not saying the slop was designed in in this case, but it still may account for the performance difference. In cases of unique performance problems, you usually start with what might be different from the norm.

Think of the poor guy that said "look at that ugly rivet for a static port; I can make something much better" and proceeded to make a nice flat one that looks just like the expensive certified planes then wondered why his airspeed readings were so far off.

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,
 
Your description sounds AOA related. I wonder if you are feeling turbulence off the fuselage, leading edge, landing gear intersection that some have installed strakes to counter on the RV 8
 
This is probably totally unrelated but, in a C-172, you can get elevator pulsations like that if you slip with full flaps.

One thing you can do is affix a video camera to the aft fuselage, looking backwards. I did this recently to verify my elevator travel during landing.
 
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

Thanks,

Interesting. I don't know the 14 and how it deals with elevator weights. If it is like the 10, they give you stock weights with no balancing required. This ends up creating a range of balance depending upon paint weight. They say that their testing shows no issues anywhere in that range. Maybe you are at the fringe of that range or something else on your plane is causing sensitivity to balance weight. Might want to ask Van's for a method of static balancing the elevators to get a bit tighter in the range.

Larry
 
Get video if able

I can't claim to be an expert on this type of issue, but I have had discussions on the topic during test pilot school. What your describing sounds like the incipient stages of flutter (which is a really bad word in the aerodynamics community, but don't anyone freak out on me).

However, because this is such a well known and proven design I would find it unlikely to be the problem unless you have some significant changes to your elevator from the normal build configuration. It may be helpful if you post some pictures of your elevator, specifically the fiberglass tip fairings, trim tab alignment, and trailing edge as these are the most likely areas of the build that could be different from the norm. Also, was your elevator balanced with the lead counterweights that were installed, or were the weights trimmed significantly?

What I would caution you from doing is trying to induce the issue. If this were the incipient stages of a flutter problem, it could become drastically worse with a slight change. I say that because many test pilots have lost aircraft and lives due to this phenomenon while trying to troubleshoot an "odd" feeling in the flight controls. If you are truly concerned then the best way to troubleshoot the issue would be to strap on a camera to watch the elevator and timestamp anytime you may feel the pulsation in the flight controls for a video review. Unfortunately without high resolution video of the tail I am not sure you are going to pin this one down. But as stated earlier I am not an expert, just happen to have some practical knowledge from previous flight test.
 
In addition to the video idea, I'd suggest adding tufts to the horizontal stabilizer, elevator and trim tab. If it is flutter onset (hope not!), the video should show this. If it is vortex shedding (and vortex oscillation) the tufts should show this.

The trailing edge of the RV-14 elevator as designed was sharp, and my recollection of its flight testing there was some problem with the elevator. Don't know if it was a dead on center feel until breakout, or vibration (rvbuilder2002 would know), but the solution was to change the trailing edge to be blunt.
 
Garmin G3X with ESP (Electronic Stability Protection)

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.

Excerpt: Should the pilot become inattentive, Garmin ESP-X provides pilots with an added measure of protection and correction to enhance in-flight safety. When a pilot exceeds user-selected pitch, roll or airspeed limitations, Garmin ESP-X provides gentle nudges on the flight controls to lessen the aircraft?s pitch attitude or bank angle.

https://garmin.hqprod.businesswire.com/press-release/garminr-brings-safety-enhancing-electronic-stability-and-protection-g3xtm-and-g3x
 
elevator trailing edge wedge

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.
 
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).
 
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.

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.
 
Oil Canning

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:

elevator_oil_canning.png


I duct taped .032 strips onto the bottom of the elevators to remove the oil canning in both locations. The problem still remained.
 
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.
 
Simple check

Sorry if I missed this in earlier comments, but I would say have another 14 builder flier look over your plane. JMHO
 
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

IMG_0755.jpg


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.

IMG_0763.jpg


Went up and test flew last night. No more pulsing! :D 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.

IMG_0757.jpg


IMG_0764.jpg


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.
 
Last edited:
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.
 
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.
 
I noticed the difference and like Mehrdad, for aesthetic purposes only, I inserted a stiff backing for the filled in area that pushed out the top and bottom to match the elevator tips.

Glad I did.
 
Good to hear

Good to hear you found the issue and were able to solve it. I had found the same mis-match issue while doing the HS tip fairings and ran this thread on it if anyone is interested in some detailed photos.

http://www.vansairforce.com/community/showthread.php?t=168356

Of note, the foam insulation worked great to get the right shape. I then removed the insulation after attaching the fiberglass backing to the tip cap which has continued to hold the shape without the insulation inside the tip cap. Hopefully those who haven't gotten to this point yet are reading this post.
 
good find

I'm glad you found the solution. I remember using balsa to spread my hor stab tip fairings to match the elevator horn. Thx for posting this info.
 
Nice detective work. I wonder if rounding off the leading edge of the elevator horn would have the same effect. The only reason I ask is that the leading edge of the horns on my Lancair has about a 1/2" radius top and bottom. That may be for ease of molding rather than aerodynamics, I don't know... One other thing to think about: Lancair came out with a Service Bulletin instructing builders to leave at least a 1/4" gap between the horn and the fairing because if ice is encountered, it could jam the elevator. I didn't incorporate that on my VFR-only Lancair. Anyway, FWIW, here is what mine looks like:

06XGJd.jpg
 
Back
Top