Electric Sea Fury problem

[JohnCallentine]

After about 30 flights, the trimming is coming along pretty well but I've got a big problem with the spinner. This plane uses a 3 1/2" spinner and it vibrates so badly that I can't even let it get up to full rpm. I've tried 3 different models, 2 carbon fiber & 1 plastic. It seems like the motor shaft & prop adapter are just too flexy for that much mass hanging out the front. I've tried running it with the motor mount in a vice to test it with the same result. I haven't tried to balance the cones. How would that be done when there's no center hole?
Anyone have any ideas?

John Callentine

[RC Storick]

Tru-turn is what I would use

[Keith Miller]

I've had some smaller motors in which the bearings aren't real sturdy.  On the small EFlite 180 motors, the only thing holding the rotor can (outrunner can) is the magnetic field itself!
Possibly mounting the motor from the front would provide a shorter moment arm to the spinner/prop and would reduce the vibration.

[Randy Cuberly]

Hi John,
I should know the answer to this question, but can't remember.  Is the motor front mounted, or rear mounted?
I assume running the motor with just the prop and not the spinner gives no vibration?

I would suggest that the problem is more related to the motor bearings than the shaft.  That's admittedly just an intuitive statement, but is based on a thourough knowledge of bearings in general.  Typically the bearings in most of the electric motors are not superior quality and any out of roundness in the balls will be exaserbated by higher side loading so that the balls can slide rather than rolling.  This would tend to happen within certain loading and vibration situations than others.

What motor are you running?

Randy Cuberly

[Garf]

This is funny. Most people go to electric to get away from vibrating engines.

[Randy Cuberly]

This is funny. Most people go to electric to get away from vibrating engines.

Well, the fact is that electric motors do vibrate they just do it at higher frequencies and lower amplitudes than IC engines!  Most mechanical things simply don't respond to those higher frequencies! 

Randy Cuberly!!!

[Dan McEntee]

Hi John;
   Can you post a picture of the prop and spinner assembly to show how it all mounts? I think I saw a motor mounted on one of Crist Riggotti's models that had an extra bearing on the opposite end of the shaft, as part of the motor mount I think, to help support the motor shaft. Are you doing anything like that? Maybe you may have to ballance the prop and spinner as an assembly? Probably have to dynamic balance it that way. Very nice looking model but won't quite look right with out that spinner!
   Good luck with it,
    Dan McEntee

[John Rist]

I had the same problem with my scale Brodak LA-5.  It also requires a large spinner.  I started out with a Trunigy motor (cheep).  No matter what I tried it would not handle the large spinner.  I went to an OS Max 3825-750 and the problem went away.  My set up is a rear mounted motor swinging a 12 x 8 EP prop on 4 cell LiPo.  The OS Max is a 3 ball bearing motor and it has a bigger shaft then the Trunigy.  If your motor is rear mounted front mounting will help.  Time I discovered the problem the airplane had been test flown with a spinner nut.  I did not want to rework the noise.  The scale rules call for the flight spinner to be similar is size to scale display spinner.  The number of blades can be different.  Anyway you gets what you pay for.  The OS max cost 4 times what the Trunigy cost but it has been a great motor.  It does have one problem - can't be front mounted.  It the mean time I have had good results with Cobra motors.  They are reasonable priced and are well built and can be front or rear mounted.

[wwwarbird]

 I agree with Sparky, go to TruTurn...


 http://www.truturn.com/cgi-bin/store/agora.cgi?p_id=tt3502b100:16&ppinc=spinners160&exact_match=on


 The slots in the 3-1/2" inch one shown above are probably quite large for what you need but they will custom cut one for you if you tell them what prop(s) you're using. I just got a custom cut Ultimate 2-1/2" one from them a few weeks ago. Not real cheap, but they're nice.
 

[John Rist]

What brand and size motor? What size prop?  If the motor can't handle a large spinner a True Turn may not help.

[Brett Buck]

Well, the fact is that electric motors do vibrate they just do it at higher frequencies and lower amplitudes than IC engines!  Most mechanical things simply don't respond to those higher frequencies! 

Randy Cuberly!!!

   In this case the motor mount is pretty wimpy, the shaft itself is pretty wimpy, and it's got a bunch of mass cantilevered out on the end of a shaft. I expect that the problem is a combination of the shaft whipping and the mount flexing. There is no obvious fix that i can see - eve if it was perfectly balance I would expect that as soon as it gets off-center, it would tend to stay that way.

    Brett

[Kim Doherty]

After about 30 flights, the trimming is coming along pretty well but I've got a big problem with the spinner. This plane uses a 3 1/2" spinner and it vibrates so badly that I can't even let it get up to full rpm. I've tried 3 different models, 2 carbon fiber & 1 plastic. It seems like the motor shaft & prop adapter are just too flexy for that much mass hanging out the front. I've tried running it with the motor mount in a vice to test it with the same result. I haven't tried to balance the cones. How would that be done when there's no center hole?
Anyone have any ideas?

John Callentine

John,

I do not think this is a spinner problem. (I am almost certain)

How far does the motor shaft project from the motor?
How far from the mounting face of the motor is the face of the prop adapter/collet ?
Is the motor mounted directly to plywood? (this could be the source of the problem if no hard mounting face)
Is there a metal or G10 mount? (if so how far from the mounting face of the mount is the mounting face of the motor?)
Does the prop track perfectly?
What diameter is the motor shaft?
Measure the run out of the motor shaft with and without side load if possible. (dial indicator on the bench)
What keeps the square mounting box from twisting/racking?
A picture of what the motor is mounted to would be very helpful.

Initially I would try a well balanced and perfectly tracking hard (carbon) prop and see if condition persists.

Incredibly beautiful model!


Kim.

[JohnCallentine]

 The motor is front mounted. The mounting face is 3/4" to the adapter face. The mount is 3/16" G-10,keyed into the plywood box. When installed,the box has 6 sides - it's rigid. The shaft has only.002 runout. I can deflect the end of the prop shaft .025" with moderate thumb pressure and the motor doesn't move.  The motor is a Cobra 3520, shaft is 5mm. I've been flying it with the backplate on and a 15-6 carbon prop with no vibration at all. I wonder if anyone makes a steel prop adapter?  It might be that I'll have to use a smaller spinner.

[RC Storick]

I have a smaller spinner like that, that does the same thing.

[Dan McEntee]

   If it were mine I would have a great concern about the .002" run out, and the .025" deflection of the shaft. Is this normal for out runner motors? I would expect that is at least a part of the issue. The mount you have built seems rigid enough, but the movement of the shaft isn't affected by that. Have you tried another motor, even on a bench test?
    As food for thought, go to Sparky's thread on his Shoestring in the scale section and find the pictures of the original airplane. That spinner would be giving you nightmares!!!
    Type at you later,
    Dan McEntee

[Steve Helmick]

I was launching an electric model this last Sunday that did something similar. Apparently, the prop/collet was not tightened enough, the collet slipped forward on the motor shaft, started wobbling, and things went very bad, very fast. There were parts everywhere, and the motor itself was about 15'-20' away. Not nice...so I reached up and pushed the button and then pulled the arming plug. Maybe it would be a good idea to have the launcher dude have thumb hovering over the red button as the motor starts to spool up?

DuBro makes this balancer (there was another maker previously that is highly sought after) that I believe will balance spinners, props, prop drivers, spinner backplates, etc. If you wish to get more info, under "Tech Notes", you can email DuBro and ask, maybe send them a picture of your CF spinner, and tell them what's happening. E-Flight may also have an opinion, or at least be interested in your project.  Steve

http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXD712

[Al Rabe]

John,

If you don't have a large balancer, you can borrow this one. It is more than large enough for your spinner and you could probably figure a way to include both the spinner and prop at the same time.  My spinners are MECOA (still available) and are mounted with a screw in the nose of the cone.

Steel prop adapters are no problem.  I use them for nose weight.  MY steel ones are made for me by Dirk Tollenaar custom made to desireed weights.  They are simply pieces of hex stock drilled and taped.  Some have additional machining to adjust weight.

Al

[Dean Pappas]

Hello John,
May I suggest plywood buttresses like this to increase the stiffness of the mount so as to move any wobble resonance up in frequency where nothing will excite it.
All the Best,
  Dean Pappas

[JohnCallentine]

Hello John,
May I suggest plywood buttresses like this to increase the stiffness of the mount so as to move any wobble resonance up in frequency where nothing will excite it.
All the Best,
  Dean Pappas

Thanks for the suggestion Dean but the the vibration occurs the same way even with the motor box clamped directly in a bench vise. I think I may have to use a smaller spinner.

[Howard Rush]

I don't know anything about dynamics of rotating things, but here's a story.  Long ago I was involved with a B-17 stunter.  My involvement was primarily in slowing down development progress.  It had some carbon engine mounts that caused a vibration of about 3/8" amplitude at the prop nut.  I made some elaborate carbon buttresses to stiffen the mounts.  That made the vibration worse.  Paul consulted with a structural dynamicist who worked for him.  His advice was to make the mount more flexible.  Pausing with Dremel in hand as one would just before committing to a sex change operation, Paul whacked off the carbon mounts at the bulkhead and replaced them with plastic radial mount adapters.  Those worked fine, although they still had a resonance at a higher frequency that limited prop choice a little.  This may not apply to your situation. 

[Randy Cuberly]

Vibration resonance can be a strange animal.  It certainly does follow some rules but they often are not very intuitive.  "Stiff" can sometimes be a path for energy that can resonate in a distant path but not in the stiff area.

Isolation is often a better route than stiffness.  Isolation as in iso-elastic mounting.

However in this case, after talking with John last night I'm inclined to agree with Brett, as He stated earlier, that this is a question of flexibility in the shaft and motor itself allowing whip to develop in the spinner/shaft area.  

The answer might be a bigger stiffer motor and shaft.  And again...maybe not!  It is what I would try first if practical!

Hey, How about a PA75 they're very stiff!    Hey, only kidding John!

Randy Cuberly

[Brett Buck]

Vibration resonance can be a strange animal.  It certainly does follow some rules but they often are not very intuitive.  "Stiff" can sometimes be a path for energy that can resonate in a distant path but not in the stiff area.

Isolation is often a better route than stiffness.  Isolation as in iso-elastic mounting.

However in this case, after talking with John last night I'm inclined to agree with Brett, as He stated earlier, that this is a question of flexibility in the shaft and motor itself allowing whip to develop in the spinner/shaft area.  

    The whipping can be the shaft itself, the mount, and/or the firewall. All you need is the natural frequency or a harmonic line up with the RPM and you are going to have a problem. On a Sea Fury, the firewall would be generally very large, with the engine mounted in the middle, which would lower the frequency, and a fair bit of mass cantilevered on a relatively long shaft, all of which would also lower the frequency and also create a lot of deflection when it starts.

     Brett

[Randy Cuberly]

   The whipping can be the shaft itself, the mount, and/or the firewall. All you need is the natural frequency or a harmonic line up with the RPM and you are going to have a problem. On a Sea Fury, the firewall would be generally very large, with the engine mounted in the middle, which would lower the frequency, and a fair bit of mass cantilevered on a relatively long shaft, all of which would also lower the frequency and also create a lot of deflection when it starts.

     Brett

Brett,
Yes I agree completely.  However there is one caveate.  John said that removing the engine mount assembly and mounting the "box" directly in a vise still produced the same resultant vibration.  From this I would assume that the whip was probably contained within the motor mount and the motor itself, and the shaft and spinner, not in the firewall mount!
I have personally seen the "Box" and the mounting arrangement and it appears to be very robust and stiff.  At least as much as visual inspection can insure!

Randy Cuberly

[Chris Wilson]

What are the chances of balancing the armature, prop and spinner as an assembled unit?

Mark the alignments and then reassemble.

Or can you indeed add a reciprocal flywheel mass to the back of the shaft behind the motor to eliminate any overhung problems?

[jfv]

A friend of mine had this problem with an RC plane a while ago.  Problem was solved when he added a rear bearing so the shaft was supported at both the front and back.

[Chris Wilson]

I assume that your prop tips are tracking in the same plane?

[Dean Pappas]

Vibration resonance can be a strange animal.  It certainly does follow some rules but they often are not very intuitive.  "Stiff" can sometimes be a path for energy that can resonate in a distant path but not in the stiff area.

Isolation is often a better route than stiffness.  Isolation as in iso-elastic mounting.

However in this case, after talking with John last night I'm inclined to agree with Brett, as He stated earlier, that this is a question of flexibility in the shaft and motor itself allowing whip to develop in the spinner/shaft area.  

The answer might be a bigger stiffer motor and shaft.  And again...maybe not!  It is what I would try first if practical!

Hey, How about a PA75 they're very stiff!    Hey, only kidding John!

Randy Cuberly

Randy is absolutely right here, but ...
It is my experience watching the front ends of e-powerd ships (both CL and RC) explode, that the mount must be dead stiff in nutation or "like a child's top" wobble. Softness in rotation around the motor shaft axis makes the plane quieter and makes bearings and gearboxes (if applicable) last longer, but wobbles turn into resonances that have been known to tear the noses off airplanes.
Regards,
 Dean P.

[Howard Rush]

The Jive Combat Team's
The best in the nation.
We're soft in the corners, but
Dead stiff in nutation.

[JohnCallentine]

Success! The 3" Tru-Turn works fine.

[Howard Rush]

That's great, John.  I look forward to seeing this wonderful machine in Madera. 

[Randy Cuberly]

That's great, John.  I look forward to seeing this wonderful machine in Madera. 

I finally saw John's Sea Fury fly on Wednesday and it is truly "MAGNIFICENT" both on the ground and especially in the air.

There are very few Stunt airplanes around, if any, that could could challenge it for simple outstanding attraction.  The only thing missing is the sound of the big radial...Hey John!  How about a small speaker with a disk player!   

Randy Cuberly

[wwwarbird]

Success! The 3" Tru-Turn works fine.

 Glad to hear it John, they make some nice stuff.

 Just curious, did you end up with a custom cut one or a stock item?