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I have an older 601 with the stiff legs.
I am getting significant air into the area under the seat and rear fuselage.
I installed a small scoop at the back under the tail and built covers for the gear box.
All the holes under the belly have been scooped with a punch so they draw air out.
There is enough air pressure that makes the stick boot puff up like a balloon.
Any suggestions where all the cold air is coming in?
An sealed rear fuse access hatch? Assuming you have one. I put a thin strip of weather seal around the opening.
Steering rod holes/boots
I found I had a bit of air coming in through the Front HT Frame and rear bulkhead in the tail. Could feel air coming in through the lightening hole in the center spar, around the center stick/boot and around the seats which I eventually tracked to the two circled holes in the picture.
Insignificant during the summer months, but in the dead of winter makes it so the heater isn't doing much. Not sure how to stop it at this point. Thinking some fairlead material, with a slot in it to allow cable to pass through, riveted over the lightening hole.
When you say you have on older 601 with the stiff legs, I assume that you mean a 601HD or HDS. In that case, you probably do not have the bottom access door to contend with. Most of the air infiltration in the 601HD that I had come in through the wing center section and poured in through the lightening holes in the ribs. Cover all the lightening holes that enter into the cockpit with monokote or anything similar, even packing tape will do the trick. Also cover the spar lightening holes. Seal off whatever you are using as the seat backs below the baggage shelf. As Dave mentioned, the steering rod holes are another source, but they weren't near as significant as the rib lightening holes.
Don't forget to look at where the air might be exiting. You won't have much inflow if there is no where for it to get out.