Calculating reluctance of air / iron solenoid

[Qmavam]

The URL below adds iron to an air core solenoid to reduce
the reluctance and increase force.
The author calculates the reluctance of an air core as R1.
He then calculates the reluctance of two iron end plate washers as R2 and R4.
Then the iron tube around the air core as R3.
He then adds R1+R2+R3+R4 for total reluctance.

It seems to me that R1 (the air core) is eliminated because it is replaced (shunted) by the iron.
Can someone explain what I am missing here.
(I understand at some point there will be an air gap,
but it will much smaller, I'll deal with that later.)
Now maybe when he calculates the air core it is just
the internal diameter of my bobbin, but I don't see that
described.
I'd like to compare the reluctance of an air core solenoid
with and without an iron shell
I also need a formula to find the reluctance
of an air core solenoid.

http://www.oz.net/~coilgun/theory/externaliron.htm

Mike

 

[israelkk]

Elecromagnetuc Devices, Roters, 1941

 

[dgallup]

The flux has to make a complete loop. You still have an air core. Only the outside of the circuit has been replaced with iron.

 

[Qmavam]

Yes, there is still an air core but the formula is the same as that used for an air core only.
Reluctance=(length/cross-section)x (1/mu)

It seems for an air core only the length should be
2 times the length plus 2 times the distance at each end. But I don't see this anyplace.

Mike

 

[dgallup]

He does not calculate the total reluctance of the air only condition. All he does is compare the reluctance of the iron return path to the air core.

 

[Qmavam]

Hi dgallup,
The formula used is the same as that used for
a simple air core solenoid.
Reluctance=(length/cross-section)x (1/mu)
Look it up!
Mike