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Fuselage as ground conductor

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Eric M. Jones

Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts

Posted: Fri Feb 07, 2014 5:04 pm Post subject: Fuselage as ground conductor

Bob N., et al.:



When one builds an airplane with the starter and the battery some distance apart, using the fuselage as a ground conductor is usually (?) done. What is the resistance of a typical fuselage used in this way? Can it carry 200A or so for the starter?



What are your thoughts?

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Eric M. Jones
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nuckolls.bob(at)aeroelect
Guest

Posted: Sat Feb 08, 2014 5:48 am Post subject: Fuselage as ground conductor

At 07:04 PM 2/7/2014, you wrote:

 	  Quote: 	
		  



Bob N., et al.:



When one builds an airplane with the starter and the battery some 

distance apart, using the fuselage as a ground conductor is usually 

(?) done. What is the resistance of a typical fuselage used in this 

way? Can it carry 200A or so for the starter?



What are your thoughts?


	


   The fuselage of a metal airplane taken as a whole

   is entirely capable of carrying and/all electrical system

   ground loads. As a rule of thumb, we used to consider

   the tip-to-tail resistance of an airplane like the Beechjet

   to be on the order of 0.001 ohms.



   How about attaching a high current conductor to the

   airframe? Imagine a 1" diameter disk of .032 aluminum

   being approached radially from all directions by an

   array of 0.032" square aluminum wires (roughly 20AWG

   equivalent). You can attach 3.14/.032 numbers of

   these wires to the edge of the disc for a total of

   100 such wires. What's the current carrying capacity/

   resistance of 100 strands of 20AWG in either aluminum or copper?



   The point of this exercise is to support the notion

   that the sheet resistance of just about any part

   of an airframe is very low; assuming all the pieces

   are well attached to each other, point-to-point

   resistance between any two locations on the airframe

   is also very low.



   Airplanes that get into conductor troubles with

   grounding over time are instances where a particular

   contiguous piece of aluminum has experienced the

   effects of moisture and high density electron

   flow at the joints where gas-tightness is lost.



   You can ground batteries at the tail to about any

   mechanically robust feature that is protected from

   the weather and stays dry.



http://tinyurl.com/mcv3c9j



   I would ground a crankcase to the firewall

   sheet at a centralize grounding location like the

   B&C ground bus and not depend on engine mount

   for ground.



   Other than that, the only caveats for airframe

   grounding are the potential for injecting

   electrical system noise into vulnerable appliances

   by injudicious choices for grounding the

   victim . . . ground loops.



   Bob . . .

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BobsV35B(at)aol.com
Guest

Posted: Sat Feb 08, 2014 11:09 am Post subject: Fuselage as ground conductor

Good Afternoon Bob, 

  

 Obviously, I am getting into this a bit late, but what about airplanes like  the newer Bonanzas where all skins and stringers were painted prior to assembly?  No doubt that the rivets when driven would spread to contact each joined skin,  but would that be sufficient area to carry the current?

  

 Happy Skies, 

  

 Old Bob

  

  In a message dated 2/8/2014 8:17:22 A.M. Central Standard Time,  nuckolls.bob(at)aeroelectric.com writes:

  	  Quote: 	
		  -->    AeroElectric-List message posted by: "Robert L. Nuckolls, III"    <nuckolls.bob(at)aeroelectric.com>



At 07:04 PM 2/7/2014, you    wrote:

 	  Quote: 	
		  --> AeroElectric-List message posted by: "Eric M. Jones"    <emjones(at)charter.net>



Bob N., et al.:



When    one builds an airplane with the starter and the battery some 

distance    apart, using the fuselage as a ground conductor is usually 

(?) done.    What is the resistance of a typical fuselage used in this 

way? Can it    carry 200A or so for the starter?



What are your    thoughts?


	


   The fuselage of a metal airplane taken as a    whole

   is entirely capable of carrying and/all electrical    system

   ground loads. As a rule of thumb, we used to    consider

   the tip-to-tail resistance of an airplane like the    Beechjet

   to be on the order of 0.001 ohms.



      How about attaching a high current conductor to the

   airframe?    Imagine a 1" diameter disk of .032 aluminum

   being approached    radially from all directions by an

   array of 0.032" square    aluminum wires (roughly 20AWG

   equivalent). You can attach    3.14/.032 numbers of

   these wires to the edge of the disc for a    total of

   100 such wires. What's the current carrying    capacity/

   resistance of 100 strands of 20AWG in either    aluminum or copper?



   The point of this exercise is to    support the notion

   that the sheet resistance of just about any    part

   of an airframe is very low; assuming all the    pieces

   are well attached to each other,    point-to-point

   resistance between any two locations on the    airframe

   is also very low.



   Airplanes that    get into conductor troubles with

   grounding over time are    instances where a particular

   contiguous piece of aluminum has    experienced the

   effects of moisture and high density    electron

   flow at the joints where gas-tightness is    lost.



   You can ground batteries at the tail to about    any

   mechanically robust feature that is protected    from

   the weather and stays    dry.



http://tinyurl.com/mcv3c9j



   I would ground a    crankcase to the firewall

   sheet at a centralize grounding    location like the

   B&C ground bus and not depend on engine    mount

   for ground.



   Other than that, the    only caveats for airframe

   grounding are the potential for    injecting

   electrical system noise into vulnerable    appliances

   by injudicious choices for grounding    the

   victim . . . ground loops.



   Bob    . . .  = Use  ilities  ay                 - MATRONICS WEB FORUMS               - List Contribution Web Site  p;                             



	


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