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Erratic elevator control...please help

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Erratic elevator control...please help

Old 11-28-2015, 08:28 PM
  #1  
zemanski99
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Default Erratic elevator control...please help

Hi guys,

Well I really have a problem which I can't solve and all our club members are scratching their heads, so if anyone can help it will be much appreciated.

I have 9 different models ranging from small foam park flyers to large 2.2m planes and over the last couple of years I crashed all of them due to erratic elevator control, or should I say lack of control. I would fly the model maybe 40-50 times and all would be well. Then normally during simple straight and level flight I seem to loose elevator control and the model will start pitching violently up and down much like a porpoise and it will end up in the crash.

I had second hand radio transmitter and I thought that it might have a intermittent fault so I bought brand new radio and guess what I had my first crash today, same problem as before, so it is not the radio.

The new radio is JR XG14, all my receivers are JR brand and servos are either JR or Futaba, no cheap Chinese gear.

It is happening on all of my models and models would fly nicely for long time and then bang, up and down like a yo yo and down I go.

If any one of you would have any suggestion please let me know as I am almost ready to give up, thanks Jan
Old 11-28-2015, 10:34 PM
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pimmnz
 
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Tricky one, could be that you havent done maintenance, allowing linkages or horns to loosen, elevator is always the one that has the biggest reaction to such stuff, or maybe you are running the balance a bit to far rearward, allied with wearing pushrod holes, that too can allow flutter to develop slowly, until...but other than that...no ideas...
Evan.
Old 11-29-2015, 04:45 AM
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flyinwalenda
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Hard to imagine the same issue is present on 9 different planes and the cause is not the transmitter.
I think the cause was the used transmitter;if it was a DX6I then I'll guarantee it. The single issue with the new transmitter could be anything as described above(especially if the plane was repaired) to pilot error.
Old 11-30-2015, 01:17 PM
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Jennifer Curtis
 
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Do you have control on other channels?
What is the throttle setting you use when
it starts, and during the remaining time?

Jenny
Old 11-30-2015, 01:31 PM
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zemanski99
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Hi Jenny,

The other channels are OK and it would happened during level flight at half throttle. Anyhow I am moving CG forward on all of my models just in case they might be slightly tail heavy. I usually balance them in the mid range, if they say 90-100mm I would balance at 95mm. I am hoping that it will help. But then again if the CG would be the problem, one would think that every flight would be effected, cheers Jan
Old 11-30-2015, 03:19 PM
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Jennifer Curtis
 
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The symptoms of tail heavy are mostly that
the plane suddenly goes the direction you
apply elevator. For example you are flying
along but the plane wants to climb, so you give
it a little down elevator. It still climbs so you
give it a little more down, and suddenly it is
diving sharply. You pull back in shock and
it is suddenly climbing hard. You try to level off
and as soon as you think you can get close to
level it is suddenly diving again.

You continue to do this while also trying to
keep it in sight range. Eventually you loose
enough altitude that the next dive puts it
in the ground.

If this sounds like what you are experiencing
nose weight is the solution.

If what is occurring is more like flying along
and suddenly the plane pitches up, slows down
and then abruptly points down, speeds up,
levels off, climbs up to a point then drops off
again. This is a symptom of the elevator being
stuck full up. It could be caused by binding
of the linkage, electronics failure or by not knowing
that the elevator is NOT what makes the airplane
climb. (It is the wing that does that with the
help of the engine.) Giving "up" elevator only
increases the angle of attack.

As the plane climbs it loses speed and when it
slows down enough it dives regardless of
how you hold the elevator stick. If this is
what you mean by not having elevator control,
you need to read a little more about flight
controls and stalls, or have someone demonstrate
it to you.

Jenny
Old 11-30-2015, 06:12 PM
  #7  
zemanski99
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Thank you Jenny, I do understand all of your points, been flying for few years. My problem is that the plane, and it's happening to all of my models at least once in a while, is that during level flight without any elevator input, the model start pitching up and down. The oscillations are very fast and violent and usually end up in a crash. Two of my models ended up in tall grass with no damage and checks did not show any problems at all. The problem is that it happened at least once to all of my different models during last few years. I had suggestions from our club members ranging from my fingers twitching on the sticks to CG being out, but no one can point it out. The only common thing if i really think about it would be that it would happen during gusty wind condition but that would only be flying during sea breeze which is about 15 - 18km/h which is still nice flying weather, best Jan
Old 11-30-2015, 08:56 PM
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That reads like flutter. Check hinges and linkages.
Evan.
Old 12-01-2015, 03:18 AM
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Jennifer Curtis
 
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Usually flutter, particularly elevator flutter,
is a faster oscillation, many cycles per
second. The airplane doesn't usually
react this quickly so mostly you don't
see drastic changes in attitude.

Spotty contacts in the servo potentiometer
can make it do wild swings that would
look like the symptoms I'm reading. I
would not expect this on multiple servos
only installed on the elevator over several
years.

I could understand such symptoms if the
flying was done near the edge of a cliff with
wind blowing towards the face of it. Weird
eddy currents would occur downwind of
the cliff. This would affect all flight attitudes,
but could really exacerbate a tail heavy
condition.

Other things like houses, barns, and large
rocks could also cause such eddy currents.
Sand dunes could cause them as well, but
mostly closer to the ground. When there
are multiple obstacles to the wind in close
proximity to each other their interference
can cause some really strange wind
patterns.

Jenny
Old 12-01-2015, 08:50 AM
  #10  
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Are these fuel airplanes , gas or nitro or electric ? What is the level of vibration from the motors ? If vibration , could be spots worn on the servo pots ( as mentioned) as it sounds like the issue is more near the servo neutral point. Another issue if higher vibration levels , could be some intermittent connection created in wiring or switches by the vibration.

anything loose in the fuse that would cause the COfG to shift ?

Last edited by stegl; 12-01-2015 at 12:54 PM.
Old 12-01-2015, 11:39 AM
  #11  
rgburrill
 
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Does it happen at the same place? Do other fliers see the same thing? I had something similar happening and found it was due to multipath - changing the altitude I flew one particular path at solved the problem.
Old 12-01-2015, 01:16 PM
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Zemanski99, Everybody brings up some valid points that need to be followed through on. Like to know if anybody else at your field has had the same problem to one degree or another. We all fly in RF enrich environment. A new cell tower in your area that your not aware of could cause that, but don't understand why it effects only your elevator channel. Frustrating I'm sure. I start with a new good quality servo, tight conections from servo to receiver. If others in your club have experience the same or similar problem, look into the environment.
Old 12-01-2015, 03:24 PM
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Possible cg to rearward,
Also try new receiver with no metal to metal connections inside aircraft,
Old 12-01-2015, 03:32 PM
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Originally Posted by FMRR
Zemanski99, Everybody brings up some valid points that need to be followed through on. Like to know if anybody else at your field has had the same problem to one degree or another. We all fly in RF enrich environment. A new cell tower in your area that your not aware of could cause that, but don't understand why it effects only your elevator channel. Frustrating I'm sure. I start with a new good quality servo, tight conections from servo to receiver. If others in your club have experience the same or similar problem, look into the environment.
Poster using a 72 MHz with analogue signal and shouldn't have 2.4 issues from cell phone towers as I believe that is all digital signals. I wouldn't think . No disclosure on radio brand yet. But I do agree that it could be the servo type/brand and a good quality servo is not out of reach ,pricewise.

In fact the brand I use ; only now, put out two analogues and all the rest are (26) all digital with the lowest starting at $11.99 all the way up to $199.99 with brushless motors. Another thing is they are starting to come out with on the higher end servos are detachable servos wires which look like a male to male servo extension wire and as fare as I know this is a first.

Last edited by stegl; 12-01-2015 at 03:41 PM. Reason: spelling
Old 12-02-2015, 06:38 PM
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combatpigg
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As a general practice it is good to set the mechanical linkage so that the servo is allowed to move in it's full range of travel instead of using just a fraction of the servo's full "Travel Volume".
This not only gives the servo maximum mechanical advantage [highest torque rating] but it also prevents you from prematurely wearing out just a few gear teeth that are doing all of the work. You will also enjoy more precise control...but at the expense of slower control surface response.
If you are flying 3D..this tip isn't for you.
I have done range checks to rule out mysterious problems by turning on the system, leave the TX in the pits and then go for a walk with my plane and the engine running. I'm looking for any area of the field where the controls start twitching.
Old 12-03-2015, 10:04 AM
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BerndBrunner
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Hi,

are these gas or electric planes? do you use balanced props?What do you change after an incident? Or do you take the gear and put it into the next(new) model? What about the battery - how old, which size? Do you use a switch: how old? Servos: which type do you use on the elevator? Where is it located? No slope in the servogear and the linkages? Loose covering? Loose hinges? broken cabels? Are the cables fixed in the plane? Corrosion due to salty air? May be some pictures would help...

Bernd
Old 12-04-2015, 09:33 AM
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So this has happened with different planes, different radios, different servos and always after several flights with no issues.

The only thing I can think of is that you are having an abrupt CG change to aft during flight making the airplane too sensitive in pitch.

Is it possible that you have balanced several of your planes, for example, by adding weight on the face of the firewall or motor mount somewhere such that your routine attachment method is eventually failing after a number of flights (tiny screws that are backing out due to vibration?) Do you have airborne packs installed for balance and they are shifting in flight (ie. velcro adhesive breaking free or strapping etc.?)

Again, I can't think of anything other than a CG shift that would cause sudden and abrupt in-flight changes to pitch sensitivity; and since it is ending in a crash, the shift would most likely be to the tail.

I'm so sorry to hear about your experience with this and I'm interested to understand the resolution. I hope you resolve it!

Merry Christmas RCU buddies!

Tom
Old 12-06-2015, 02:32 PM
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zemanski99
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Thank you all for your help. I think you are right Tom that it is the CG problem. Not that something is shifting during flight, but that I balance my planes on my fingers rather then putting them on the jig. That way I can't see the side of the plane and it could be balanced with tail slightly down. I also use the mid setting for my CG, if I the plan say 90-100mm I go for the mid point which is 95mm. So I build the balancing jig and will go and rebalance my entire fleet. That's the only thing I can find at the moment, so thank you all again.
Old 12-06-2015, 02:40 PM
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zemanski99
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Hi Stegl,
The radio is JR XG14 and all the receivers are also genuine JR, either RG631B or eight channels equivalent. As I've said I thought problem was my old JR radio which was JR XG7 so I bought new one. All was OK for about three months and then, bang, plane down with the same problem again. But as I said in a later post it might be me with balancing my plane on my fingers, so the CG could be slightly out towards the rear. I will let you know when I rebalance my entire fleet on the jig which I am building right now, thanks for your help, much appreciated, Jan
Old 12-07-2015, 09:50 AM
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Just as captain sully did on the Hudson river, nose down.
I balance at an proxy 30% nose down angle,always
Old 12-07-2015, 09:58 AM
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Balancing on your fingers in the middle of the suggested range is a common and sensible way to get a suitable starting point. The idea that this gave you a tail-heavy condition on all your planes (but one you didn't notice for the first 40 or 50 flights!) seems fantastic. Post no. 3 seems right to me. But let us know how your rebalancing goes.
Old 12-07-2015, 12:51 PM
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zemanski99
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This is the first time I heard that you balance with nose down angle. I have all my models balanced with fuse level. Could this be my problem then ??? best Jan
Old 12-07-2015, 01:22 PM
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Top_Gunn
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Originally Posted by zemanski99
This is the first time I heard that you balance with nose down angle. I have all my models balanced with fuse level. Could this be my problem then ??? best Jan
Level is fine.
Old 12-07-2015, 02:27 PM
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Jennifer Curtis
 
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When you balance nose down, you don't know
where the CG is. You only know it is some
unknown distance forward of where it rests.

When you balance it level you know exactly
where the cg is located.

Jenny

ps. if it is nose down it is by definition
NOT balanced.
Old 12-07-2015, 05:52 PM
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ahicks
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To help eliminate one potential source of trouble discussed above, this isn't about CG if the plane is controllable when it leaves the ground. If it's about CG at all (and I have serious doubts about that), it's about the CG moving far enough to make the plane uncontrollable while in flight. Like a battery free to move the entire length of the fuse or something...

If the plane left the ground with a fixed CG that could potentially make the plane that unstable, that fact would be apparent from the time it leaves the ground. It would not happen out of nowhere, mid flight, or after 40 or 50 flights.

I'm left wondering about combatpigg's line of thought. Has the elevator been set up in a manner making very sensitive elevator inputs required for normal flight?

More info on the planes themselves (e.g.ARF or stick built), type of power, etc. might generate some useful ideas.

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