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Author Topic: Airframe vibration influencing flight characteristics?  (Read 13525 times)

Offline Peter Germann

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Airframe vibration influencing flight characteristics?
« on: October 03, 2016, 06:00:34 AM »
Problem
Flying electric F2B In competition since 6 years, I have repeatedly noticed a phenomenon previously unknown to me when I flew all kinds of IC motors since the 1960’s:
In level and inverted flight the electric airplane does not hold the initial altitude and it requires constant effort to maintain the 1.5m height. I have owned models, and observed others, suffering from the same, being virtually unable to keep level within the tolerance given by the rules. Possibly misidentified as “hunting”, all my efforts to eliminate the cause by shifting the c.g, building-in (or out) stabiliser/motor incidence or adding turbulators on wing and/or empennage have failed to fully eliminate this very annoying difficulty.

Thesis
As the above problem quite often surfaces when a previously “tracking like a freight train” airplane is converted to electric becomes a beast to fly level and inverted, it could actually be that the cause is not the airframe but the motor and/or its controller. An interesting experiment done by a scandinavian flyer supports this:
His take apartable “Shark” was modified to be able to fly back-to-back with both, an IC fuselage nose and alternatively with an electric nose section.The unquestionably state-of-the-art IC version did as expected: Very solid level and inverted flight tracking, combined with highly sensitive manoeuvring ability. Not so the electrified same airplane which was very difficult to keep level.

Questions:
From the above I would like to have comments on these questions:

•   Could it be that the (IC Motor induced) vibration of the airframe influences the flight characteristics of our airplanes (boundary layer on wing and empennage)?

•   Could it be that the pounding of the ST.60 or the Saito 4-cycle motors kind of masked aerodynamic problem areas now being unveiled by electrics?

•   Could it be that the governor’s failure to maintain motor speed within a tolerance of less than +/- 50 RPM influences altitude in level flight?
Peter Germann

Offline Tim Wescott

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Re: Airframe vibration influencing flight characteristics?
« Reply #1 on: October 03, 2016, 10:00:35 AM »
There's been lots of discussion about this over the years, which I have read but never tested (I still fly slime).  So going over past posts may be fruitful.

Some high points that I recall, and some opinions:

The prevailing theory is that this behavior is due to a combination of the control system needing to be a lot freer than it does in an IC-engined plane, and aerodynamic effects.  Control systems have stiction, and one of the well-known methods of overcoming stiction in machines is to vibrate the @#$% out of things; IC engines provide that for free where electrics don't.  I've also seen theories being flung around by some pretty solid people of boundary layers being separated by vibration, although I'm not sure I sign up to that one.

Certainly the first thing I'd do would be to see if Paul Walker or one of the other top 'lectric flyers around here would let me fondle their airplanes at a meet, and feel what the level of friction (or lack thereof) is on their machines.

Finally, from opinion-land -- what is the level of governing that you see in your setup?  Do you have a way to record RPM variations, and have you actually seen them?  I have a plot of recorded IC engine RPM variations (46LA on a profile), and it shows about 100RPM peak-peak in level flight.  I'm not sure how much of that is real and how much is measurement error, but can you be sure that IC engines don't vary in speed?
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Offline Dave_Trible

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Re: Airframe vibration influencing flight characteristics?
« Reply #2 on: October 03, 2016, 11:03:18 AM »
There's been lots of discussion about this over the years, which I have read but never tested (I still fly slime).  So going over past posts may be fruitful.

Some high points that I recall, and some opinions:

The prevailing theory is that this behavior is due to a combination of the control system needing to be a lot freer than it does in an IC-engined plane, and aerodynamic effects.  Control systems have stiction, and one of the well-known methods of overcoming stiction in machines is to vibrate the @#$% out of things; IC engines provide that for free where electrics don't.  I've also seen theories being flung around by some pretty solid people of boundary layers being separated by vibration, although I'm not sure I sign up to that one.

Certainly the first thing I'd do would be to see if Paul Walker or one of the other top 'lectric flyers around here would let me fondle their airplanes at a meet, and feel what the level of friction (or lack thereof) is on their machines.

Finally, from opinion-land -- what is the level of governing that you see in your setup?  Do you have a way to record RPM variations, and have you actually seen them?  I have a plot of recorded IC engine RPM variations (46LA on a profile), and it shows about 100RPM peak-peak in level flight.  I'm not sure how much of that is real and how much is measurement error, but can you be sure that IC engines don't vary in speed?
+1.....going back to the old fashioned pushrod through a hole controls would solve most of this issue.  Yes you give up a little after-the-fact control adjustability but get the airplane set up carefully the first time with alignments and known control throws and it's not much issue.  A lot can be adjusted at the handle end rather than in the airplane as long as there weren't built in problems.  Tight controls aren't much fun.

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Offline Tim Wescott

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Re: Airframe vibration influencing flight characteristics?
« Reply #3 on: October 03, 2016, 12:17:35 PM »
+1.....going back to the old fashioned pushrod through a hole controls would solve most of this issue.  Yes you give up a little after-the-fact control adjustability but get the airplane set up carefully the first time with alignments and known control throws and it's not much issue.  A lot can be adjusted at the handle end rather than in the airplane as long as there weren't built in problems.  Tight controls aren't much fun.

Dave

Or hand-select the ball links so that they're loose.

Yes, tight controls aren't fun.
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Offline Peter Germann

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Re: Airframe vibration influencing flight characteristics?
« Reply #4 on: October 04, 2016, 12:49:03 AM »
Finally, from opinion-land -- what is the level of governing that you see in your setup? 
Thank you, Tim. I'll recheck the controls and give it a try with carfully cleaned lines, too.
In level flight, the governor keeps speed within +/- 50 RPM. In a tight pullup, it drops approx 120 RPM to recover within 0.3 sec. Attached is a graph showing RPM in level flight.
Peter Germann

Offline Tim Wescott

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Re: Airframe vibration influencing flight characteristics?
« Reply #5 on: October 04, 2016, 10:50:18 AM »
From the looks of that graph it only records things in 50RPM increments.  In more formal settings, the rule of thumb is to not trust a measurement that's down at the level of the quantization, at least not without deep investigation into the measuring instrument.  It could be that you're just seeing measurement noise and not actual variations.

In aerospace there would be a couple of guys out in the lab right now with a strobe light, running the thing on the ground and comparing the strobe-light results with the log.
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Offline Peter Germann

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Re: Airframe vibration influencing flight characteristics?
« Reply #6 on: October 05, 2016, 04:41:38 AM »
From the looks of that graph it only records things in 50RPM increments.  In more formal settings, the rule of thumb is to not trust a measurement that's down at the level of the quantization, at least not without deep investigation into the measuring instrument.  It could be that you're just seeing measurement noise and not actual variations.
In aerospace there would be a couple of guys out in the lab right now with a strobe light, running the thing on the ground and comparing the strobe-light results with the log.

RPM was logged at 2 samples per second  (Castle Edge 75 A)
Static run speed was tached manually and compared to logged RPM. Tach shows slightly (-100) more than log and no variation
Peter Germann

Offline Mike Alimov

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Re: Airframe vibration influencing flight characteristics?
« Reply #7 on: October 07, 2016, 03:14:59 PM »
Peter, not exactly sure what causes it, but could this be the reason why people report their electric setups requiring more nose-heavy trim?  I see you indicated that CG movement has been tried - maybe not far enogh?

Offline Peter Germann

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Re: Airframe vibration influencing flight characteristics?
« Reply #8 on: October 08, 2016, 03:07:39 AM »
Peter, not exactly sure what causes it, but could this be the reason why people report their electric setups requiring more nose-heavy trim?  I see you indicated that CG movement has been tried - maybe not far enogh?

From "very poor corner at all" to "really twitchy". C.G. range was +/-  6 mm (1/4 in)

Adding, by means of  double sided foam adhesive tape, a bit of lead (6 x 6 x 1.2 mm or 1/4 sq. x 0.047.  i.e. 0.6 Grams) to the inside of a 2in spinner and running it at 10'000 RPM leads to a mild vibration of the airframe. Holding it in static run, it now feels like a tuned pipe airplane. Weather permitting, I will communicate flight test results soon.
Peter Germann

Offline Wolfgang Nieuwkamp

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Re: Airframe vibration influencing flight characteristics?
« Reply #9 on: October 08, 2016, 12:01:34 PM »
Peter, could there be any slip/stick friction at the lead outs? My new canard has two pulleys as lead out guide, absolutely no hunting.
Maybe sapphire bearings would be an alternative ;-)

Offline Peter Germann

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Re: Airframe vibration influencing flight characteristics?
« Reply #10 on: October 09, 2016, 04:11:00 AM »
Peter, could there be any slip/stick friction at the lead outs? My new canard has two pulleys as lead out guide, absolutely no hunting.
Maybe sapphire bearings would be an alternative ;-)

As putting a bit of grease on the lead-out cables, running in oversize brass tubings, reduces (but not solves) the problem, friction is undoubtedly part of the cause. "Vibrating" will kind of "unstick" elements...
Peter Germann

Offline Mike Alimov

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Re: Airframe vibration influencing flight characteristics?
« Reply #11 on: October 09, 2016, 02:04:19 PM »
I have a test airframe that easily accept a change in powerplants.  Originally built for a Saito 56/62, I can also install an electric setup (Cobra C2826-12, 5S 2700 mAh) in minutes.  The electric version has the same CG as the 4-stroke.
I have flown both setups today in the same weather (winds 10-12 mph, gusting to 15+ mph, unfortunately), and have observed the very effect you are reporting here.  It is very real.
There must be both an explanation and a solution to this, as electrics have been flown (and won) at the highest levels of competition...

Online Brett Buck

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Re: Airframe vibration influencing flight characteristics?
« Reply #12 on: October 09, 2016, 06:02:14 PM »
HI Peter. I am not an engineer, but a Diesel Mech. I do know that electric motors put out tremendous amounts of torque compared to IC engines.  Thus Diesel Electric locomotives. I wonder if the torque applied to the prop is inducing gyroscopic precession even in level flight, thus causing the airframe to "hunt" as the nose tries to remain level? If all things being equal, then the different engine/motor inputs are the cause. D>K

   The torque in level flight is *exactly the same* as an IC engine with the same prop. In fact, most electric props are much lighter than typical IC props, and therefore have less nose-up (or nose down, if it goes backwards) precessional torques.

  The nose-up precession present at all times with conventional rotation is almost certainly a huge part of the reason that downthrust, elevator down-rigging, and positive tail incidence works, whereas upthrust, up-rigging, or negative incidence can sometimes result in wild instability at low load factors.

      Brett

Offline Peter Germann

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Re: Airframe vibration influencing flight characteristics?
« Reply #13 on: October 10, 2016, 03:58:17 AM »
Thanks Brett, Makes sense.  Just curious Peter, how is the motor mounted? Front or back? I wonder if it is in fact moving just enough to cause the hunting? I have had one literally fly out of the model. I was still in the learning curve with that one.  LL~ LL~

I have observed the problem with several airplanes all of them with front mounted AXI 2826/12's driving 12 or 13 in pushers in reasonably stiff nose sections.
Peter Germann

Offline Wolfgang Nieuwkamp

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Re: Airframe vibration influencing flight characteristics?
« Reply #14 on: October 10, 2016, 09:53:40 AM »
Peter, is there any difference in hunting between pusher and tractor props?

Online Lauri Malila

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Re: Airframe vibration influencing flight characteristics?
« Reply #15 on: October 10, 2016, 10:32:39 AM »
Peter,

As I don't know how stiff your "reasonably stiff" is, I ask if it would be possible to support the motor or shaft from behind of it too. I could make you a nice bearing holder if you want.
Brett says that gyroscopic torque is same in ic and electric, but has anybody calculated it? There must be a difference between gyroscopic action of outrunner and a spinning crankshaft.
I think it was Orestes who said that he found a location for c.g. where hunting is minimal. You move c.g. to either direction and hunting gets worse.

Lauri

Offline Mike Alimov

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Re: Airframe vibration influencing flight characteristics?
« Reply #16 on: October 10, 2016, 08:17:54 PM »
I had the same  concern as Lauri: if vibration does indeed have to be re-introduced (the most likely culprit, it seems), the motor would have to be supported on both ends, or else the bearings will have a pretty short life...

Offline Mark Scarborough

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Re: Airframe vibration influencing flight characteristics?
« Reply #17 on: October 10, 2016, 09:42:17 PM »
while I think there is some validity to this idea, there are some pretty darn succsesful airplanes that are electric and dont have any artificial vibration induced
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Online Brett Buck

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Re: Airframe vibration influencing flight characteristics?
« Reply #18 on: October 10, 2016, 10:26:37 PM »
while I think there is some validity to this idea, there are some pretty darn succsesful airplanes that are electric and dont have any artificial vibration induced

  Right. Thats a function of how much stiction is built into the system. You have to take what most would consider extreme measures to ensure that the controls are free enough. The two really severely hunting electric airplanes I have flown both had clearly locked up controls at small deflections. Neither had previously hunted to any great degree with IC engines, and one of them flew fine once the control binding was reduced. To me, adding vibration is just an experiment to attempt to prove the theory, not a real solution.

     In fact, just throwing it out of balance by an eccentric weight may not be enough. The torque pulses from a real engine are remarkably high, something like 20x the average torque, and very "sharp". A smooth 200 hz sine wave is a lot more benign and will result in much lower peak loading on the moving parts.

     Brett

Offline Mark Scarborough

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Re: Airframe vibration influencing flight characteristics?
« Reply #19 on: October 11, 2016, 08:14:07 AM »
I have to agree with you on this Brett,, I know on my Impact that one time I taped the controls and the thing would hunt like a golden retriever,, then I retaped the controls and it pretty much went back to stable and locked in,, I had not sufficiently lubricated the hinge barrels to keep the tape from sticking and they felt free, but they had just a slight drag.... enough apparantly,,

Just to be clear, the tape wasnt really stuck to the barrel, it just drug on it,, It could have also been partly that the tape was stuck to the wing in the wrong place, regarless, it was ever so slightly sticky,, and the plane refused to track,, it was fine before and fine after,,

I know my new build I will be a lot more carefull about picking the best ball joints,, at its best my green impact still wasnt really locked down,,
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Offline Chris Wilson

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Re: Airframe vibration influencing flight characteristics?
« Reply #20 on: October 11, 2016, 02:04:05 PM »
Interesting thread but lets polarise the question even further, is there any difference between flight characteristics whether ANY engine is running or not?

1. When power cuts normally and the model enters a flat glide, is it any different to control - as in more difficult?
2. When the power cuts prematurely during a schedule does it become more difficult to maneuver to safety?
3. When the power cuts and you want to have some further fun in the wind by whipping and doing some down wind loops, does the model become harder to control?

In my limited experience I would say that the model handles just fine minus any vibratory input.
(And perhaps the 'experience' of silent electric flight colors the perception of what is going on.)

What I am getting at here is that if there is an airframe difference between the vibrations of power plants and that all disappears when the model is not under power then perhaps the electric run characteristics are derogatory to the system rather than neutral.

In other words, perhaps the I.C. solved nothing and its more that the electric that is causing an issue?

Just food for thought.
« Last Edit: October 11, 2016, 02:45:06 PM by Chris Wilson »
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Online Brett Buck

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Re: Airframe vibration influencing flight characteristics?
« Reply #21 on: October 11, 2016, 03:18:17 PM »
Interesting thread but lets polarise the question even further, is there any difference between flight characteristics whether ANY engine is running or not?

1. When power cuts normally and the model enters a flat glide, is it any different to control - as in more difficult?
2. When the power cuts prematurely during a schedule does it become more difficult to maneuver to safety?
3. When the power cuts and you want to have some further fun in the wind by whipping and doing some down wind loops, does the model become harder to control?

In my limited experience I would say that the model handles just fine minus any vibratory input.
(And perhaps the 'experience' of silent electric flight colors the perception of what is going on.)

What I am getting at here is that if there is an airframe difference between the vibrations of power plants and that all disappears when the model is not under power then perhaps the electric run characteristics are derogatory to the system rather than neutral.

In other words, perhaps the I.C. solved nothing and its more that the electric that is causing an issue?

    I don't think so. You don't get hunting on landing because you are continually changing the control input as the airplane slows down. Its a well-known phenomenon that landing with twists in the lines is much more difficult than when flying when the engine is running. That's why you unwind them at the end of the flight with outside loops.

     Brett

Offline Steve Thompson

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Re: Airframe vibration influencing flight characteristics?
« Reply #22 on: October 11, 2016, 06:44:07 PM »
IC and electric are subject to or produce different kinds of vibration.

An electric motor with 14 poles would "see" 7 torsional pulses per rotation of propeller (I think).  For the same power output and RPM of a single cylinder IC engine, the motor would produce seven pulses of one seventh the torque, where the IC would put out one big bang torque pulse.  Motors seem smooth, but there is still a lot happening there.  Both IC and electric have "torsional vibration" but at different magnitudes and frequencies.

The IC engine also has reciprocating (from the piston) and radial (from the counter-weight) forces that cannot be completely eliminated by the counter-weight in a single cylinder model engine.  The vibration that would result is due to the unrestrained plane nose wanting to do a small elliptical "orbit" about the centerline of the fuselage.  (restrained only by the mass of the nose)

A bunch of stating the obvious, but...

Do you think the "stiction" is reduced from torsional or radial vibration.  Granted the IC engine has a larger magnitude of both, but at much lower frequency.

Would using an intentionally slightly out-of-balance prop to simulate IC vibration help the electrics rattle loose the stiction (and maybe the wing)?

Do twin motor electrics also have a tendency to hunt?  If not, would a low frequency "beat" from slightly different RPM increase the magnitude enough to reduce stiction?


  

Offline Joe Yau

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Re: Airframe vibration influencing flight characteristics?
« Reply #23 on: October 12, 2016, 12:12:53 PM »
I have observed the problem with several airplanes all of them with front mounted AXI 2826/12's driving 12 or 13 in pushers in reasonably stiff nose sections.

Hi Peter,  I would try a tractor prop first before going any further.. better yet try a 11x5C 3-blade from Igor or something similar.  I have put together 3 Electric set ups in the last year or so, and the level flight tracking has been pretty much the same as it was with IC .  

1/ My first Electric was a Brodak Legacy, it consist of E-flite 32 (front mount), Igors timer, Jeti 66 ESC, and a 11x5C 3-blade (tractor).  It tracks Loc-on in level flights both ways.. even in rough conditions.  I have no reference to IC on this one, as it was built for Electrics.

2/ I converted an old plane I had which has some stability issues on inverted level flights,  and it had a 4-stroke in it.  It consisted of a Cobra 3520-12 (rear mount), Castle ICE-lite-50 ESC, Hubin fm-9 timer and Igors 11x5C prop.  and it actually flew better then the 4-stroke, but it still has some stability issues on level flight at times.. but not as bad as when it was with the 4-stroke.

3/  I converted a Pathfinder which has an OS .46LA  set up.  It consist of a Cobra 2826-12 (front mount), Castles Phoenix 60 ESC, Hubin fm-9 timer, and also a 11x5 3-blade prop.  This plane always tracks like it was on rails.. and it is still very solid on the level flights (both ways) with this set up.

« Last Edit: October 12, 2016, 09:22:19 PM by Joe Yau »

Offline Air Ministry .

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Re: Airframe vibration influencing flight characteristics?
« Reply #24 on: October 16, 2016, 07:18:09 PM »
[youtube=425,350]https://www.youtube.com/watch?v=OhwLojNerMU[/youtube]


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Re: Airframe vibration influencing flight characteristics?
« Reply #25 on: October 16, 2016, 07:23:16 PM »



Online Brett Buck

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Re: Airframe vibration influencing flight characteristics?
« Reply #26 on: October 16, 2016, 10:09:35 PM »
crash video



    Entertaining as it was, it doesn't look like this has anything to do with vibration, the fin was just not strong enough for the flight loads.

     Brett

Offline Tim Wescott

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Re: Airframe vibration influencing flight characteristics?
« Reply #27 on: October 17, 2016, 09:26:53 AM »
Matt:

Not sure what your point is with the flutter videos.  Yes, it's vibration, and yes, it's influencing flight characteristics, but it's not the kind of vibration we're talking of here.
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Offline Tim Wescott

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Re: Airframe vibration influencing flight characteristics?
« Reply #28 on: October 17, 2016, 09:29:47 AM »
An electric motor with 14 poles would "see" 7 torsional pulses per rotation of propeller (I think).

21, probably (there are different schemes).  And they'd be somewhat more overlapped than pulses from an IC engine.  The torsional vibration would definitely be much less than with an IC engine.
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Offline Brent Williams

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Re: Airframe vibration influencing flight characteristics?
« Reply #29 on: October 17, 2016, 09:57:39 AM »
If vibration is the secret sauce to fixing control system stiction, then would the installation of a small buzzer motor like what's used in a cell phone be a possible solution?

These inexpensive little motors can be programmed for different patterns and are certainly small enough to fit inside the airframe.  There are disc and cylindrical versions of these motors.  One could position a few of them throughout the airframe and program an appropriate vibration pattern.  Perhaps Tim's TUT, or some other small board could perform this function.  Most function on lower voltage, so a step down and logic circuit would need to be devised if these were to run off of the flight battery.  

Perhaps some flight attitude intelligence software that can sense and engage the vibration only in level and inverted level segments of the pattern?

Maybe one of these near/on the leadout guide and another near/on the bellcrank post or position one near the back of the battery area to simulate engine vibration coming from the front of the plane?

Digi-Key, Mouser, Newegg, Amazon, ect are easy sources of these vibration motors.

Any thoughts on this?

https://www.google.com/search?sa=X&espvd=2&biw=1377&bih=879&tbm=shop&q=cell+phone+buzzer+motor&source=univ&tbo=u&ved=0ahUKEwjvkKLqlOLPAhXKrFQKHZSoBXwQ1TUINg&dpr=1#q=cell+phone+buzzer+motor





« Last Edit: October 18, 2016, 08:46:49 AM by Brent Williams »
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Offline Tim Wescott

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Re: Airframe vibration influencing flight characteristics?
« Reply #30 on: October 17, 2016, 10:03:55 AM »
If vibration is the secret sauce to fixing control system stiction, then would the installation of a small buzzer motor like what's used in a cell phone be a possible solution?

<snip>

Any thoughts on this?

I think that the real secret sauce to fixing control system stiction is to build your control system right.  Yes, this can be trying if you've got a plane that's all built and otherwise working, but intentionally introducing vibration should only be done as a diagnostic measure, if at all.  (I can't remember if I was the one to suggest it -- if so, it was tongue in cheek).  Lots of people have clearly solved this problem, without introducing intentional vibration, so why do something so undesirable?
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Offline Brent Williams

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Re: Airframe vibration influencing flight characteristics?
« Reply #31 on: October 17, 2016, 10:25:46 AM »
I think that the real secret sauce to fixing control system stiction is to build your control system right.  Yes, this can be trying if you've got a plane that's all built and otherwise working, but intentionally introducing vibration should only be done as a diagnostic measure, if at all.  (I can't remember if I was the one to suggest it -- if so, it was tongue in cheek).  Lots of people have clearly solved this problem, without introducing intentional vibration, so why do something so undesirable?

Just a thought exercise.  Brett Buck has mentioned that he has experienced hunting on his experiments with e-power on a plane that heretofore, was rock solid with the greasy pulse inducer bolted to the front.
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Offline Chris Wilson

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Re: Airframe vibration influencing flight characteristics?
« Reply #32 on: October 18, 2016, 02:23:39 PM »
A maxim that I have heard here in Oz is that the flaps should fall under there own weight and that sounds like a good point to aim for initially.
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