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One thing to think about is airflow and how smoke/soot comes out of the oven chamber itself. I cant say for sure, but my gut tells me that your hole/slot in your arch might be small.
I have watched my chimney and noticed that it "breathes" in-and-out depending on the wind conditions.
I designed in a "settling chamber" for the smoke to enter before it goes up and out the chimney. This settling chamber acts to provide a staging area for the smoke to gather before it gets pulled up the chimney. If there is no wind, it really doesnt matter. But if there is the slightest wind, then this chamber works great. Attached is a rough picture of what I am talking about.
There are many pictures of ovens on the FB website with soot on the front. Also, you dont want smoke or hot air exiting the front of your oven.
Jeep - that is an interesting concept and I see where it would work. After a thousand more builds - these overs are going to be dialed in. The vent that Bob proposes is pretty much like what I have done, and yes, the wind WILL jack with the smoke.
I made mine like your shorter version also. You might want to consider shaping the inside of the arch (see attached photos) like a few people have done so then your transition of bricks onto the arch is both easy and neat. I was glad that I read through the forums and saw how others had taken that approach. It takes a bit of setting up and cutting but I think it is worth the extra work.
As I'm a fan of casting I would recommend casting the whole entry. That way you can make the entry even shallower and create compound curves easily so you end up with the entry funnelling to the flue pipe for efficient smoke removal. It also has the advantage, if you make the casting thinner than the bricks, of less thermal mass which makes it less of an energy robber. It is quite simple to create a damp sand mould and trowel the castable mix over it. No complex brick cutting required.
I don't follow the "weak" issue. Many ovens have this approach (mine included ) and there is absolutely no problem - what am I missing?
Well if you think about the forces and how they distribute in the structure it seems pretty obvious to me. The unsupported ends on the vent hole will want to move towards the empty space and that will cause a twist that will make the front arch want go away from the oven. See the picture, red arrows are the directions those bricks will want to move (at least how I understand the forces).
But as said it might be a no issue with the weights we are talking about in here. All the same it is a weakness in the design as far as I can reason it. (please do correct me if my reasoning is flawed )
I think (?) there are numerous ovens on the site built with arches exactly like the drawing, and I haven't heard about any arch collapses.
But to your point - the thing to do to avoid the issue you raise would be to construct the inner and outer arches at the same time, and stagger the full bricks across both arches so as not have that potential plane along which the front arch could crack away from the back arch.
To Al's point, a buttress may be a more important thing to think about given the downward pressure on the arch pushing the sides out.
I also think you could tie the two arches together across the top with your chimney transition - creating a more solid connection between the two. That's not yet in the drawing
Well, over the weekend I did a little redesigning, but I guess I should have stayed tuned in to the thread. Anyway, if you would, take a look at the attachments and let me know what you think. I will post additional pics in a separate post.
Some of the things I did are:
Captured the end bricks of the vent opening.
Added a Smoke Box.
Defined planar cuts on the arch bricks that interface with the dome bricks.
The dome base diameter is 42"
The vent opening is 7" x 12" (84 sq in)
The smoke box has a cross section of 9.75" x 16.25" (158 sq in)