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Start Solenoid Orientation

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JLuckey(at)pacbell.net
Guest

Posted: Sun Sep 23, 2012 9:03 am Post subject: Start Solenoid Orientation

Group,  

   

I’m preparing some electrical hardware for my RV7 & this question came up:  

   

How should I orient the start solenoid, terminals up or down?  I’m curious how G loading will affect the plunger.  

   

I’m planning to use this solenoid http://www.autopartswarehouse.com/sku/Standard/Starter_Solenoid/SISS581.html (a generic automotive type) because it is inexpensive & should be easily found in the future.  

   

In the picture at the above link, the solenoid appears “terminals down”.  Is that how it should be mounted in the airframe?  In that orientation, any positive G loading will try to keep the contacts apart or open which is what I want. (don’t want starter to inadvertently engage at the bottom of a hard pull).  

   

I think that the return spring in this device is pretty strong therefore it would take more than 6 Gs to get it to move but I thought I would pose the question to the group.  

   

Also, this solenoid has an “I” terminal which gets B+ when the solenoid is energized. (I think it is used to bypass an ignition ballast resistor in older cars) Does anyone have a schematic of the wiring inside this device?  I’m curious how this “I” terminal is wired internally.  ( I googled for about 20 minutes but could not find any info on this, which I though was odd )  

   

So, I’m looking specifically for feedback on the following 2 questions:  

     applicability of this device for cranking a      Lycoming  
mounting orientation  
internal schematic   

   

TIA,  

   

Jeff Luckey  

   

       [quote][b]

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nuckolls.bob(at)aeroelect
Guest

Posted: Sun Sep 23, 2012 12:31 pm Post subject: Start Solenoid Orientation

At 12:02 PM 9/23/2012, you wrote:

Group,



I'm preparing some electrical hardware for my RV7 & this question came up:



How should I orient the start solenoid, terminals up or down?  I'm 

curious how G loading will affect the plunger.



I'm planning to use this solenoid 

http://www.autopartswarehouse.com/sku/Standard/Starter_Solenoid/SISS581.html 

(a generic automotive type) because it is inexpensive & should be 

easily found in the future.



In the picture at the above link, the solenoid appears "terminals 

down".  Is that how it should be mounted in the airframe?  In that 

orientation, any positive G loading will try to keep the contacts 

apart or open which is what I want. (don't want starter to 

inadvertently engage at the bottom of a hard pull).



I think that the return spring in this device is pretty strong 

therefore it would take more than 6 Gs to get it to move but I 

thought I would pose the question to the group.



         Exactly. The g-load closing of starter contactors

         is one of those hanagar myths that has been around

         for a very long time . . . and repeated often enough

         it morphs into fact.



         That contactor mounted on the firewall orients the

         the terminals forward. A study of these pieces and

         parts http://tinyurl.com/96wy5mq shows that the

         contact bar is pushed against the insulated lid

         which retains the terminals. G-forces tending to

         close the contacts in this orientation would be

         achieved only if you flew into a mountainside.



         When oriented on a horizontal surface, you'd have

         to be in a wing-stripping push-over to close

         the contacts.



         I was working the B&C booth at OSH when one of the

         air-show famous showed up to buy a new starter and

         a starter contactor. After completing a performance

         it was discovered that his starter ring gear was

         trashed and the pinion on the starter damaged. The

         starter contactor was "stuck shut" which common

         wisdom of excited moment suggest was victim of

         daring-do g-loads.



         I didn't get to see the as-installed carcasses

         nor were we asked to do autopsies on the damaged

         parts.



         After some reflection in following months I

         concluded that he most likely suffered a stuck

         contactor at engine start up. But given the cockpit

         noise of these hi-performance machines and attention

         to upcoming demands on pilotage, the event went

         unnoticed. He probably flew his routine with a

         starter engaged and running.



Also, this solenoid has an "I" terminal which gets B+ when the 

solenoid is energized. (I think it is used to bypass an ignition 

ballast resistor in older cars) Does anyone have a schematic of the 

wiring inside this device?  I'm curious how this "I" terminal is 

wired internally.  ( I googled for about 20 minutes but could not 

find any info on this, which I though was odd )



         You're correct. This feature isn't used in cars

         with electronic ignition but would still be useful

         on a '57 Chevy. Few cognizant enthusiasts today

         are even aware of this feature or its significance.



         There are two popular form factors for this contactor.



http://tinyurl.com/8my46tv



        There's a number of I-terminal references in this

         document



http://tinyurl.com/9looejm

So, I'm looking specifically for feedback on the following 2 questions:



applicability of this device for cranking a Lycoming



         Works good, lasts a long time



mounting orientation



         Doesn't really matter much but you'd have to

         go out of your way to mount it such that

         flight g-loads tended to close contacts. This

         is unlike popular starter contactors of

         yesteryear



http://tinyurl.com/8fwygp4



         which featured large area, low pressure contacts

         and massive solenoid armatures. Even in the

         "continuous duty" versions were not well suited

         to high-inrush duty of spinning up cranking motors.



         That's the best we had in 1960 but have not been

         seen in that service for many decades. But even

         these parts are exceedingly unlike to get closed

         under g-loads of flight. Now, there WAS a good

         reason for mounting them upside down on the firewall

         but that was for moisture reasons . . . that's

         another story of a CFD and hat-dance carried out

         at the Cessna Single Engine facility about 1980.



internal schematic



         See figure 11-21 in the 'Connection. Functionally

         accurate. The "I" terminal in modern incarnations

         of a starter contactor is bridged in one stroke with

         the main terminals of the device. In this

         picture http://tinyurl.com/96wy5mq the "thin and

         light moving contacts" are propped up on a spring

         strut at right angles to the main current pathway.

         The end of this strut contacts the "I" terminal

         simultaneously with closure of the main terminals.

   Bob . . .

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JLuckey(at)pacbell.net
Guest

Posted: Sun Sep 23, 2012 1:54 pm Post subject: Start Solenoid Orientation

Thanks Bob for your usual thorough mini-dissertation (I don't know how you

find the time to deliver such thoughtful answers to the group but it is much

appreciated! - If you are ever in Southern California, the cervezas are on

me)

--

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