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Author Topic: Motor pads or plates  (Read 1934 times)

Offline Dave Moritz

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Motor pads or plates
« on: July 31, 2017, 02:26:54 PM »
I see on most profile models a pair of motor pads or plates located between the engine lugs and the hardwood motor mounts (bearers). Most are made of aluminum (1/8" or so). A few questions:

1) Why are they used? More strength in a crash? Spacers because you want the motor located farther outboard? Adaptors for different motors on the same plane? I see that Stunt Hangar Hobbyshop makes them available in angled configurations for motor offset but flat ones (no offset) are also offered.

2) Does the added weight justify their use?

3) Any advantage of aluminum over phenolic or other materials?

A past thread on this topic likely exists, but I'd no luck finding it.

Thanks.

Dave Mo...
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Offline Paul Smith

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Re: Motor pads or plates
« Reply #1 on: July 31, 2017, 07:38:34 PM »
I am a believer that best reason is to prevent the small engine mounts from sinking into the wood.  Experience has shown me that aluminum less that 1/8" thick gets bent by the stress and sinks in anyway. 

With double or triple the area you have a half or a third of the psi.
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Offline john e. holliday

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Re: Motor pads or plates
« Reply #2 on: July 31, 2017, 07:43:45 PM »
I have used the plates/pads under engine mounting lugs to spread the surface over the so called hardwood/plywood.   Lately I am using them to be able to use different engines  as the some are wider than others and some time the bolt holes don't line up.
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Offline Tim Wescott

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Re: Motor pads or plates
« Reply #3 on: August 02, 2017, 09:37:08 AM »
On profiles, I cut away the plywood doubler around the motor adapter plates, bolt the plates to the bearers, and drill & tap the plates for the engines.  This is because:

  • Plywood crushes.  It crushes more when the load is concentrated (as with a motor lug), less when it's spread out (as with a T-nut on the back side) and not at all when it's not there
  • Maple crushes, too, just not as bad
  • The local profile rules we fly under say the engine needs to be exposed "lug to plug".  They're not strictly enforced, but I like following them anyway.  Plates that are a bit thicker than the doubler satisfies that
  • If you use a decent alloy then drilling & tapping a 3/16" thick adapter plate works just fine
  • Keeping a separate adapter plate means that I can swap engines without making a bazzilion holes in my engine bearers
  • Keeping a separate adapter plate means that the threaded holes that can wear out are really easy to replace, and -- should you bury your T-nuts -- the threaded holes that are hard to replace are buried

I've only been doing this fully on my last two planes, which are quite happy, well-wearing profiles that have been crashed a number of times (my wife's learning to fly).  I haven't gotten any mileage on a full-fuselage with this, but there's one on my bench that's getting built this way, because about half of my bullet-points apply.

On the down side -- yes, it's a bit more weight.  And yes, it's more work.  So pick your poison.
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Offline Dave Moritz

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Re: Motor pads or plates
« Reply #4 on: August 03, 2017, 09:32:55 AM »
Good responses here. Testimony by Paul and John convinces me to go ahead and use them.

Tim: I like your approach and its flexibility, that is if I understand it correctly. A followup question: Is each set of aluminum pads "married" to an engine or to an airframe? Or, maybe there is another way of doing this that I'm not understanding (e.g., one set of pads for all engines and planes)?

Thanks a bunch!

Dave Mo...
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Offline Tim Wescott

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Re: Motor pads or plates
« Reply #5 on: August 03, 2017, 04:18:49 PM »
Quote from: Dave Moritz on August 03, 2017, 09:32:55 AM
Good responses here. Testimony by Paul and John convinces me to go ahead and use them.

Tim: I like your approach and its flexibility, that is if I understand it correctly. A followup question: Is each set of aluminum pads "married" to an engine or to an airframe? Or, maybe there is another way of doing this that I'm not understanding (e.g., one set of pads for all engines and planes)?

Thanks a bunch!

Dave Mo...

If I were a really good machinist there would be one set of pads that took one engine but went into any of my airplanes.  I'm not -- so if I want to put Engine A into airplane C, then I have to make a set of pads for THAT COMBINATION.  But Metal Supermarket sold me a bunch of 3/16 x 1/2" 6061, from which it takes little time to whack out pads.  So I'm happy.
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Offline Dennis Toth

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Re: Motor pads or plates
« Reply #6 on: August 03, 2017, 08:03:47 PM »
Dave,
The pads started to be used in full stunters to keep the motor from crushing down into the mount and changing the spinner alignment. On profiles its not to critical unless you have the nosed shaped into a spinner (like on the OTS Secret Weapon). You can use them for mounting different engines but if you are using one engine its not a big deal. However if you are using a FOX 29/35 (likely all FX motors) and you use the pads make sure you put a piece of 1/32" plywood crush pad between the AL pad and the engine mount lugs. The reason is the mount lugs have a slight mold relief angle and if you bolt them to a hard pad it can distort the case and cause binding in the piston/cylinder and result in very poor engine runs. If you are a machinist you could mill them flat then you don't need the crush pads.

Best,    DennisT
« Last Edit: August 04, 2017, 04:37:06 PM by Dennis Toth »
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Offline Dave Moritz

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Re: Motor pads or plates
« Reply #7 on: August 04, 2017, 10:38:03 AM »
Dennis:

Great to have your bit of history recounted here. Within the context of spinner alignment, the use of motor pads finally makes sense to me. Add in the possibility for interchangeable motors per Tim, and I end up with a complete answer to my original question(s).

No Fox engines in my stable (yet). Thanks a bunch for your response.

Dave Mo...
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