For those who have questioned whether a valveless pulsejet can be successfully designed with the intake attached to the tailpipe alone, the answer is: yes. Today for the first time, I got continuous sustained operation from the ancient and honorable Fo Mi Chin. The engine started easily and sustained on propane fed at the center of the intake flare through 1/8-inch copper tubing. I was able to get two runs of 3-4 minutes in order to get still shots of the hot engine. This was at about 40 degF, and it's obvious to me that the device would have sustained operation indefinitely. Altogether I got about five good runs before my battery ran down and I was unable to get spark for starting.
The bad news is that this success was attained at an overall engine length of 47 [count 'em - FORTY-SEVEN] inches! That's the longest length needed for any of my designs. I think this is because the intake layout of Fo Mi Chin is very poor in terms of acoustic resonance. Naturally, the frequency was about half that of the "standard" FWE, possibly even less.
FUEL and AIR SETUP -
Propane vapor was set up at 12 PSIG on the regulator to the needle valve, then fed through 6 ft of rubber hose and about 18 inches of 1/8-inch copper tube wound around the tailpipe and aimed into the intake flare at dead center, straight in. Starting was secured with almost a trickle of fuel and the open end of the shop vac hose about 10-12 inches back from the flare and slightly to one side [high-speed nozzle not used]. Once sustaining, a wide range of fuel flows could be used to keep the engine running, though I was never able to set it too rich to run [probably due to the restriction of the small copper tubing].
I have to agree with Steve, Eric et al that for these rear-facing intakes that's almost the ideal fueling spot. There was absolutely no flame visible from the tail or the intake flare during sustained running, although there was a lot of flame during starting [pops and bangs], especially when the overall length was just wrong.
I can try only a few lengths, by swapping out sections of tubing, i.e. I cannot experiment with fine increments. Lengths of 33 and 37 inches would produce good roars with air, but never would sustain. One experiment with the ludicrous overall length of 61 inches gave me one brief sustained run that gradually weakened and died over about ten seconds, and I interpreted that as being too long. The next thing I tried was coupling two sections to the tailpipe for an overall length of 47 inches, which must be nearly perfect, for it started easily and caught hold running immediately. So, that was basically a three-piece tailpipe with two fairly leaky couplings - the end section did happen to have a decent flare, however.
The length ratios will be interesting to some who followed the 'Hinote Criteria' related threads. The overall length L of 47 inches = 1194 mm. The total length of the bent intake from flare to spout is about 122 mm, only .10L - only half the size dictated by the criteria! The 'Reynst Point' - the station at which the intake air spouts into the engine - is almost at the desired L/8: 163 mm from the apex of the front dome. The actual value is about .14L. This coupled with the short intake length puts the flare at a theoretically pitiful wave path length of 285 mm from the apex = .24L, far short of the "ideal" L/3 point - yet the engine runs! All this suggests that the 'Reynst Point' might be the most critical of all the criteria [but, maybe for poorly proportioned engines like this, there are other factors involved and this is just coincidence - who knows?]. Anyway, the point is that as far as the criteria are concerned, only the Reynst Point comes close, on this engine build.
The chamber heat is unusual for me in that it seems astonishingly uniform all over, including the front dome. I just couldn't see much variation. Perhaps when I get the photos processed some detail will show up more prominently. There was no observable heating of the tailpipe, but I was using the Bukowsky Quik-Mount [i.e. vise] right behind the nozzle point, so there was a lot of heat sinking right there. The copper eventually became discolored where it wrapped around the tailpipe, aft of the vise jaws. My fancy white paint on the dome and the nozzle zone, smoked vehemently for a while and basically turned to chalk-like powder with no lasting discoloration [probably Titanium oxide].
The strangest heat observation came from looking into the tailpipe from a few feet back. There was the usual lovely purple that comes from seeing red hot steel through a slug of blue flame, but the INSIDE surface of the intake spout [way back at the front end of the tailpipe] was BRIGHT orange, while the outside surface visible just above it was relatively dark!!! [Recall that the spout is cut back at a raked angle so a little of the interior is visible from directly behind.] I swear I am not making this up. One of the strangest things I've ever seen. Note that the spark plug on this engine is mounted low on the front dome, and is not visible through the tailpipe.
That's about it. All comments welcome. Pictures, maybe next week - I have half a roll of film to work through before they can be processed.