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Author Topic: Engine vs Motor Vibration  (Read 1174 times)

Online Ken Culbertson

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Engine vs Motor Vibration
« on: March 18, 2019, 02:37:46 PM »
I had an unfortunate incident Sunday.  My first real crash in nearly 40 years. (Broken props and skinned rudders don't count!)  When I build this ship (it is for Classic even though it is of PA size and I was going to use it there too) I left out the motor mounts entirely since it was to be electric.  I had been schooled way back in the early 60's that, other than giving you a place to put the motor, the motor mounts extended way back to help spread the engine's vibration to the whole plane.  Since electrics don't have the vibration other than from an unbalanced prop I didn't put in the mounts.

So, when the controls locked on takeoff for my LAST pre-finish trim flight and it went in FAI style vertical from 10' under full power with enough force to bend my tail wheel wire straight and leave a 1 1/2" deep crater in the asphalt.  I was expecting it to be dumpster time.  What happened still has me puzzled.  The nose crushed from the front back.  Not a piece over 1/2" left for the first 1/2 of the nose.  Zero damage to the motor/esc/timer.  The battery went from 5" to 3" and didn't catch fire.  The rest of the plane suffered zero damage which is probably because it did hit straight in.

100 hours of sanding just got written off of my balance sheet but that is nothing compared to the cost had the final finish been applied!  Enough background:

#1 Have any of you had a Morris bellcrank with the copper tubing line connectors lock up?  I have a theory but I tried to duplicate it and couldn't.  I use a floating arrangement and I leave gobs of clearance around it.  On a new plane everybody likes to grab the flaps or elevator and move them up and down to see how free the controls are.  I do it all the time, especially in building.  There was lots of that going on.  I think what happened is that the slack in the leadouts let the connector flip to the back when the flaps were moved without pulling on the leadout wire.  It is very possible that my giving it too much clearance and too long a leadout let it do that.

#2 Is it reasonable to not use motor mount beams for electric.  Is a collapsible (not weak) nose good or bad?

If Brett is peeking in, YES I did have another plane with me and after that crash I put in 4 of the best patterns I have flown since I came back.  If this is what I have to do to fly better then we have to have a talk....

Ken




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Offline Curare

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Re: Engine vs Motor Vibration
« Reply #1 on: March 18, 2019, 05:11:04 PM »
Ken I'd say the slightly 'softer' nose saved the plane, it's acted as a crumple zone. I'm sure if you'd done that with decent beam mounts in the front you'd be looking at more structural damage, and maybe a throwaway.

I always found it odd that beam mounts are still soup du jour for stunters, considering in almost every other discipline it's been superceeded, but it is light, keeps the engine straight and works. (till they go soggy anyway). I think that there's still some refining to go on with electric mounts for the most part to keep the thrust line stable. Who knows whether your mount isn't twisting when you bang it through a corner, or worse yet, the whole nose?


Also I've never encountered a bellcrank lockup with copper sleeves, but I have had line connectors turn 90 degrees at the airplane end, giving me either a big dose of up or down. I've found this to be very prevalent if you use eyelets, so i tend to use copper yokes instead.
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Greg Kowalski
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Offline john e. holliday

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Re: Engine vs Motor Vibration
« Reply #2 on: March 19, 2019, 12:15:02 PM »
The only bell crank I had lock up on me was in my Sheeks Spitfire.  The post had worked lose and moved up out of the plywood plate.  It warned me but I didn't listen when I was flying.  I think the idea of eliminating hard wood beams like we use on engines is a good idea.   If I ever go electric I will try to remember that.  If you do a search on here you will see how I do terminations at the bell crank. D>K

Okay I found a picture.  If you look close you will see the bell crank terminations. H^^
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Online Ken Culbertson

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Re: Engine vs Motor Vibration
« Reply #3 on: March 19, 2019, 01:34:25 PM »
Quote from: Curare on March 18, 2019, 05:11:04 PM
Ken I'd say the slightly 'softer' nose saved the plane, it's acted as a crumple zone. I'm sure if you'd done that with decent beam mounts in the front you'd be looking at more structural damage, and maybe a throwaway.

I always found it odd that beam mounts are still soup du jour for stunters, considering in almost every other discipline it's been superceeded, but it is light, keeps the engine straight and works. (till they go soggy anyway). I think that there's still some refining to go on with electric mounts for the most part to keep the thrust line stable. Who knows whether your mount isn't twisting when you bang it through a corner, or worse yet, the whole nose?


Also I've never encountered a bellcrank lockup with copper sleeves, but I have had line connectors turn 90 degrees at the airplane end, giving me either a big dose of up or down. I've found this to be very prevalent if you use eyelets, so i tend to use copper yokes instead.
As a follow up I found the problem and it was not the copper sleeves. #^

The bellcrank over-rotated on full up and locked if there was no tension on the down line. ~^

This time I am putting in stops AND one of Morris's new 4" while I have it opened up.  I accidentally put a 3" in when I was building it.  (Never do anything critical on a new ship after Midnight and never cover your controls up until you have to)  I broke both rules and didn't discover my error until the ship was in primer and I went looking for my 3" and found a 4" in the drawer instead.  The three had been drilled closer to center for another project.  It never occurred to me that that closer hole hooked to horns that were expecting a 4" would let it over-center.   I let it slide and adjusted the handle.  Ironically, I had just read every post here I could find on bellcrank stops.  The consensus was that you definitely did or did not need them so I didn't put them in.  That crash was a cheap lesson in not letting things slide.  It was also a lesson that somewhere in the control system there needs to be something, a stop, horn spacing, anything to prevent the bellcrank from over-centering even if it is impossible for it to happen in flight as mine was.

I didn't do any of the normal things you do on takeoff.  I didn't run the lines out, I didn't check the controls when I picked up the handle and I didn't put enough tension on the lines when the motor engaged.  Any one of those would have popped the over-center but it was just a stupid 1 minute trim flight!

From now on if I suspect ANYTHING is not right with the controls I break out the #11 and 60 years doing things right does not give you a free pass when you ignore everything and do it wrong. y1

Ken
« Last Edit: March 19, 2019, 02:03:22 PM by Ken Culbertson »
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Offline Curare

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Re: Engine vs Motor Vibration
« Reply #4 on: March 19, 2019, 08:02:43 PM »
I found on my last ship there were physical constraints that stopped me from having a free swinging bellcrank so I ended up having mechanical stops at about 45 degrees from centre.

I haven't hit them in actuality, and I do tend to bang models through corners!
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Greg Kowalski
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