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Grounding a Super Cub

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Tundra10

Joined: 14 Jun 2010
Posts: 102
Location: Scarborough, Ontario

Posted: Mon Jan 28, 2013 10:27 pm Post subject: Grounding a Super Cub

A friend of mine is building a tube and fabric aircraft with an O360.   

The tubes are thin wall 4130.  The battery is in the rear.

My understanding is that it is normally preferable to ground strap the  

engine directly to the firewall, rather than to the engine mount,  

since this avoids putting starter currents through the engine mount.

If that is correct, then I wonder whether putting starter currents  

through the frame is also a bad idea, and running a heavy wire from  

the battery to the firewall is desirable ?



Jeff Page

Dream Aircraft Tundra #10

peter(at)sportingaero.com
Guest

Posted: Tue Jan 29, 2013 2:18 pm Post subject: Grounding a Super Cub

Jeff,



I have tube & fabric aircraft with an IO-360 and the battery in the 

back. I have a tab welded on on the engine mount and a tab welded on the 

frame near to the battery both used for earth strap connection. What is 

the down side of putting very short duration starter current through the 

engine mount, and equally through the fuselage frame? Why would the 

(very thin) firewall be any better? Can you provide some more details 

where you obtained your understanding from? I think you are 

mis-informed. Most people I know have saved the weight of a 2nd heavy 

gauge wire by using the fuselage frame as the ground return.



Peter



On 29/01/2013 06:25, Jeff Page wrote:

 	  Quote: 	
		   



 A friend of mine is building a tube and fabric aircraft with an O360.  

 The tubes are thin wall 4130.  The battery is in the rear.

 My understanding is that it is normally preferable to ground strap the 

 engine directly to the firewall, rather than to the engine mount, 

 since this avoids putting starter currents through the engine mount.

 If that is correct, then I wonder whether putting starter currents 

 through the frame is also a bad idea, and running a heavy wire from 

 the battery to the firewall is desirable ?



 Jeff Page

 Dream Aircraft Tundra #10

Tundra10

Joined: 14 Jun 2010
Posts: 102
Location: Scarborough, Ontario

Posted: Tue Jan 29, 2013 2:59 pm Post subject: Re: Grounding a Super Cub

Peter,



I looked back into Bob's book trying to find it, but although he says connect the ground strap from engine to firewall (better than using the P-leads!), it doesn't say there that that using the engine mount is bad.



Searching this list, I found the "rules" by which Bob Nuckolls would wire his own airplane:

Rule 9 includes this bit:

    Any ground straps provided around the rubber biscuits

    of an engine mount will be removed. Engine mounts are

    for holding engines on airplanes and not used for any

    part of the electrical system.



I have extrapolated that to assume that starter current through the engine mount is undesirable.  My imagination sees a similarity between a 200A arc welder and 200A of starter current and wonders what happens to the welds as the current passes through ?



The firewall is just the convenient place for a single point ground.



If there is no problem with using the frame, we would wire it as battery to local tab on frame at the rear, plus firewall to local tab on frame at firewall, plus firewall to engine.



If there is a problem, then it would be 2WG from battery to firewall, firewall to local tab on frame and firewall to engine.  This is heavier by the 2AWG cable from the battery to the firewall, but one less connection.



Jeff Page

Dream Aircraft Tundra #10

ceengland7(at)gmail.com
Guest

Posted: Tue Jan 29, 2013 4:37 pm Post subject: Grounding a Super Cub

1. Steel has a much higher resistance than copper.

2. One way to make a magnet is to pass a DC current through a ferrous 

metal (steel).

FWIW.....

Charlie

On 01/29/2013 04:17 PM, Peter Pengilly wrote:

 	  Quote: 	

 <peter(at)sportingaero.com>

 Jeff,

 I have tube & fabric aircraft with an IO-360 and the battery in the 

 back. I have a tab welded on on the engine mount and a tab welded on 

 the frame near to the battery both used for earth strap connection. 

 What is the down side of putting very short duration starter current 

 through the engine mount, and equally through the fuselage frame? Why 

 would the (very thin) firewall be any better? Can you provide some 

 more details where you obtained your understanding from? I think you 

 are mis-informed. Most people I know have saved the weight of a 2nd 

 heavy gauge wire by using the fuselage frame as the ground return.

 Peter

 On 29/01/2013 06:25, Jeff Page wrote:

> 

>

> A friend of mine is building a tube and fabric aircraft with an 

> O360.  The tubes are thin wall 4130.  The battery is in the rear.

> My understanding is that it is normally preferable to ground strap 

> the engine directly to the firewall, rather than to the engine mount, 

> since this avoids putting starter currents through the engine mount.

> If that is correct, then I wonder whether putting starter currents 

> through the frame is also a bad idea, and running a heavy wire from 

> the battery to the firewall is desirable ?

>

> Jeff Page

> Dream Aircraft Tundra #10

Tundra10

Joined: 14 Jun 2010
Posts: 102
Location: Scarborough, Ontario

Posted: Tue Jan 29, 2013 8:34 pm Post subject: Re: Grounding a Super Cub

Yes, a magnetized frame would make compass swings pretty difficult.

When Bob discussed using the airframe as ground in the Book, he seemed to be referring to all metal aircraft, which would be aluminum.

nuckolls.bob(at)aeroelect
Guest

Posted: Tue Jan 29, 2013 9:18 pm Post subject: Grounding a Super Cub

 	  Quote: 	
		  I have tube & fabric aircraft with an IO-360 and the battery in the 

back. I have a tab welded on on the engine mount and a tab welded on 

the frame near to the battery both used for earth strap connection. 

What is the down side of putting very short duration starter current 

through the engine mount, and equally through the fuselage frame?


	


    They're magnetic materials . . . steel . . .

    with higher resistance than copper. While

    carrying high currents, the steel conductors generate

    strong local magnetic fields which do not go

    completely away after the current goes to zero.



    Remote battery grounding on aluminum airplanes is

    less problematic . . . but many builders still run

    starter currents through the engine mount and/or

    fire wall sheet . . . for the most part they don't

    perceive any problems. Performance degradation is

    most likely to present during cold weather cranking.



    So MANY things pile on top of each other in cold

    weather that one is unlikely to see much difference

    fore wired vs. airframe battery ground. But a combination of

    design and preventative maintenance efforts can

    add up to a significant difference. Those would

    include selection of starter, keeping the battery

    well maintained, minimizing joints in the cranking

    path, and in extreme conditions, warming the engine

    and battery, etc.



 	  Quote: 	
		    Why would the (very thin) firewall be any better?


	


    It's not. I try NEVER to depend on fire wall sheet for anything

    other than grounding of light draw accessories like case-ground

    regulators, facet fuel pumps, starter contactors, etc.



 	  Quote: 	
		    Most people I know have saved the weight of a 2nd heavy gauge wire 

 by using the fuselage frame as the ground return.


	


    Which is what's been done on thousands of airplanes

    for 100+ years . . . but I'm aware of no study

    that quantifies performance or maintenance issues

    for having done so.



    My personal design goals call for a minimum number

    of made up joints between battery (-) and crankcase

    and avoid using airframe ground for all but the few

    outlying appliances depicted in Figure Z-15. The ideal

    battery ground is an engine-cranking-sized-wire from

    battery(-) to fire wall single-point ground . . . and

    a second conductor from single-point ground stud

    to the crankcase.



    But if one wishes to embrace another philosophy,

    it will probably function.

   Bob . . .

Tundra10

Joined: 14 Jun 2010
Posts: 102
Location: Scarborough, Ontario

Posted: Tue Jan 29, 2013 10:09 pm Post subject: Re: Grounding a Super Cub

Bob,

I am going to suggest he go with your recommendations.  He is keenly interested in being able to start in remote locations.  What you say makes sense.  Thanks (again) for your very helpful advice.



Jeff Page

Dream Aircraft Tundra #10

peter(at)sportingaero.com
Guest

Posted: Wed Jan 30, 2013 12:13 pm Post subject: Grounding a Super Cub

Not trying to cause any trouble here!!!



My airplane is a single seat aerobatic One Design - I was very keen to 

keep weight to a minimum, and I wasn't too concerned about the compass 

(definitely VFR only). To ensure good starting I decided to 'spend' 2lb 

on a Sky-tec NL starter - which draws much less current than a permanent 

magnet type (also achieves better engagement on starter ring, but that 

may have been a peculiarity of my engine). With a PC680, starter 

solenoid on top of battery and once length of wire from solenoid to 

starter so far starting has been good, even in damp, cold weather after 

several weeks of inactivity. I could have gone with Eric's copper 

covered aluminium wire, but I had the copper wire to hand ... If I was 

going to use heavy gauge both ways I would definitely use his Super-CCA 

wire.



Thinking about it I guess my design goals might not be that similar to a 

Tundra or Super Cub builder!



Peter

On 30/01/2013 05:17, Robert L. Nuckolls, III wrote:

 	  Quote: 	
		   

 <nuckolls.bob(at)aeroelectric.com>

> I have tube & fabric aircraft with an IO-360 and the battery in the 

> back. I have a tab welded on on the engine mount and a tab welded on 

> the frame near to the battery both used for earth strap connection. 

> What is the down side of putting very short duration starter current 

> through the engine mount, and equally through the fuselage frame?



    They're magnetic materials . . . steel . . .

    with higher resistance than copper. While

    carrying high currents, the steel conductors generate

    strong local magnetic fields which do not go

    completely away after the current goes to zero.



    Remote battery grounding on aluminum airplanes is

    less problematic . . . but many builders still run

    starter currents through the engine mount and/or

    fire wall sheet . . . for the most part they don't

    perceive any problems. Performance degradation is

    most likely to present during cold weather cranking.



    So MANY things pile on top of each other in cold

    weather that one is unlikely to see much difference

    fore wired vs. airframe battery ground. But a combination of

    design and preventative maintenance efforts can

    add up to a significant difference. Those would

    include selection of starter, keeping the battery

    well maintained, minimizing joints in the cranking

    path, and in extreme conditions, warming the engine

    and battery, etc.



>  Why would the (very thin) firewall be any better?



    It's not. I try NEVER to depend on fire wall sheet for anything

    other than grounding of light draw accessories like case-ground

    regulators, facet fuel pumps, starter contactors, etc.



>  Most people I know have saved the weight of a 2nd heavy gauge wire 

> by using the fuselage frame as the ground return.



    Which is what's been done on thousands of airplanes

    for 100+ years . . . but I'm aware of no study

    that quantifies performance or maintenance issues

    for having done so.



    My personal design goals call for a minimum number

    of made up joints between battery (-) and crankcase

    and avoid using airframe ground for all but the few

    outlying appliances depicted in Figure Z-15. The ideal

    battery ground is an engine-cranking-sized-wire from

    battery(-) to fire wall single-point ground . . . and

    a second conductor from single-point ground stud

    to the crankcase.



    But if one wishes to embrace another philosophy,

    it will probably function.

   Bob . . .

Tundra10

Joined: 14 Jun 2010
Posts: 102
Location: Scarborough, Ontario

Posted: Wed Jan 30, 2013 6:19 pm Post subject: Re: Grounding a Super Cub

My friend has some 2AWG available from another project, so he will be using that.



However, I tried to do a comparison between 2AWG and 4AWG, based on Bob's discussions of voltage at the starter motor.

It seems that battery internal resistance and connection resistance are the most significant factors.



I calculated that for his airplane it will cost 2.2 additional pounds to gain 4% more voltage at the starter.  For my airplane, with the battery on the firewall, it would cost 0.8 pounds to gain 1.5% more voltage at the starter.



I think longer cable runs are required before the lower resistance cable makes a significant difference.



A battery with lower internal resistance makes the biggest difference of all.



Jeff Page

Dream Aircraft Tundra #10

bbradburry(at)bellsouth.n
Guest

Posted: Wed Jan 30, 2013 6:41 pm Post subject: Grounding a Super Cub

Tell him to try some of this lightweight wire.



http://www.periheliondesign.com/fatwires.htm



Bill B



--

nuckolls.bob(at)aeroelect
Guest

Posted: Thu Jan 31, 2013 8:44 am Post subject: Grounding a Super Cub

 	  Quote: 	
		  A battery with lower internal resistance makes the biggest difference of all.


	


    RIiighhttt ONnnnnn!



   Bob . . .

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