Fred, yes, it would

Fred, yes, it would absolutely be better for air flow to shorten that duct.  A lot of poor air flow problems in ducts are on the return side so anything you can do there to fix it should result in noticeable improvement.  Two possible reasons your return duct is so much longer than it should be:  (1) The installer did it to reduce the transmission of noise from the blower into the living space; or (2) the installer was too lazy to cut the duct.

“The more you reduce the size of that tube or make it longer or turn the air with it, the more static pressure builds up. And the more the air flow is reduced.”
and
“Grilles and registers, filters, and balancing dampers also add resistance. That resistance results in decreases in the static pressure, or pressure drops.”

does resistance increase or decrease static pressure?

The reason why I am looking for something besides sheet metal pipe is 1) plastic is a much better insulator and therefore less heat is loss through convection of the duct itself. Sure, you can insulate a sheet metal duct, but why not buy a pipe where that is not necessary?

Another reason is the smoothness of a plastic pipe is better than even galvanized steel, which would then be better to reduce the turbulence mentioned in the article. And because it is a better insulator, there is less sound produced within the pipe than there is in typical sheet metal duct.

A third reason is that plastic pipe can be made stronger than sheet metal. Much lower rates of dents that occur in transit or installation.

A fourth reason is that proper installation requires the pipe to be either fused together or joined by some sort of adhesive. This makes it air and water tight. No additional sealing required (think duct tape that is used with sheet metal duct), which would make for easier and quicker installation.

I understand the two prevailing thoughts against using plastic pipes are the flammability and smoke specs. However, plastic pipes have been made to follow these standards. And by code, I can use different material that burns (like wood) as long as I have 1/2 inch of gypsum board covering it. So, I don’t see these as insurmountable obstacles.

Plastic pipe is also used in water systems, even hot water boiling systems…where the water temp is hotter than any residential plenum should ever get.

While installing cable in a duct and manhole system, one has to worry about the friction built up around corners as well as the increased pulling tension it takes pull the cable through the pipe due to the corners (gotta make sure that one does not stretch the limits of tensile strength on the conductor or insulation). Obviously, with HVAC, we are not worried so much about tensile strength of air, but the amount of friction around the corners, I think could be a concern. I understand the lingo in the HVAC industry is “Equivalent length” and that the author addresses it in another article. However, one way to help combat this is with larger radius sweeps as opposed to 90 degree elbows. It allows a smoother transition to the air moving in a different direction. Similar to a car on a road….the bigger the curve, the faster you can take it. A 90 degree intersection will require the car to slow down or flip to make the turn.

Another issue in the electrical sector is the idea of skin effect. Where most of the current flowing through a conductor flows at the edges of the conductor, rather than the the center. Water pipes show the same phenomenon. Turn on your hose and notice that the water sprays from the sides and not the center. What we find in the electrical world, is that a 4/0 stranded conductor can carry a bit more current than a 4/0 solid conductor. So, I wonder if multiple, smaller pipes would lessen the pressure requirements at the blower motor? The idea is less turbulence in the center of the pipe and more consistent air flow across the entire pipe diameter.

Also, in the electrical world, if we have 2 separate, but equally sized conductors, starting and ending in the same spot, but tied together at both ends, the resistance drops by half. I have noticed similar behavior with water. And I wonder, since ducts all have the same starting pressure and ending pressure, if you were to run parallel groupings of smaller pipe (multiple outlets in the same room), if the amount of resistance from blower motor to room would drop? That would also lower the amount of pressure the blower would have to surpass to move the air.

D’oh! Yes, Lee, you’re right.

D’oh! Yes, Lee, you’re right. Actually, we’re both right, but you’re more right. The number I gave in the article (which I’ll change in a moment) was 0.0807 pound per cubic foot. That is correct for dry air, but real air does have water vapor in it. Since water vapor is lighter than dry air, it brings down the average density. At 75° F and 50% relative humidity, it’s about 0.75 pound per cubic foot, as you say.