Here is an eye opener for any of you entertaining the idea of building and flying pulse jets. Take a look at the regulations AMA Document 510-Q (I have included with this post) that the AMA came up with for us flyers in the US.

According to this document, the cross section of the smallest diameter of the tailpipe, can not exceed 1.25 square inches. This regulation would eliminate the use of all but the smallest pulse jet engines available to us today (Dynajet, HK Jet, OS Jet, Bailey Jet) to name a few.
You would not be able to operate the Inferno, Scout, or Virus type of pulse jet planes in the US, because of the tailpipe size restriction the AMA puts on pulse jet engines in the US. I got into a disagreement with another RCU member in another post, that said as an AMA member you were forbidden to operate a plane powered by any engine with a tailpipe exceeding 1.25 sq. in. in the US, even if you were on private land. He cited that this was because of insurance issues. I'm not sure if this is correct, and am contacting AMA on this issue.
The 510-Q pulse jet document leaves a lot open to interpretation. Another example is the declaration requirement in 9A of 510-Q. It says a high speed aircraft designed to fly 150 mph or faster, but doesn't actually say that you have to fly the plane at that speed, only that you fly a plane designed for this speed. The 510-Q document was approved in 2002, but before that I think the only requirement for RC pulse jet flying was to obtain a turbine waiver, which only requires a declaration of 50 hours with a plane designed for 100 mph or faster. At 100 mph there are a lot of planes that could be used for the speed declaration purpose, but at 150 mph you almost have to get into yet another area of RC modeling, just to have a plane to qualify you to for the 150 mph declaration. I highly doubt that many planes with a 1.25 sq. in. tail pipe jet will get anywhere near 150 mph, so where did they come up with that number?
As it states at the bottom of 510-Q, any infractions of the rules and you could loose your AMA membership.
I almost seems if you want to get serious about pulse jets in the US, you can't be a member of AMA.

Greg