The above is quoted from close to the tail end of my "For a Close Shave ..." thread. This is extremely interesting stuff, because it is an observation by a careful builder who has had considerable success with small engines, Bill Hinote. I dubbed these two factors, L/3 for the intake pipe port station and L/5 for the intake pipe length, the 'Hinote Criteria". I have not built enough engines to validate the criteria, but they are observed from successful valveless examples as empirical data, so I don't have any reason to fault them.hinote wrote:I've done some basic research on this engine type (Chinese/Thermojet valveless); I believe it obeys rules somewhat similar to the rest of the operating valveless engines--a combination of resonant (acoustic) and gas-dynamic effects. The real trick appears to be in the intake tube, and how to relate its length and location to the rest of the engine.
If you look at the "standard" Chinese dimensions (currently being re-posted on the thread, "Nord 1500 Project"), you'll find the distance from the front of the engine to the mouth of the intake is very close to 1/3 the length of the engine (all calcs should be made to include open-end correction factor of .6R); this figure is reinforced by the Thermojet drawing that has appeared from time to time on this Forum.
The length of the intake tube itself is subject to some variation, but (again using the "standard" Chinese as a benchmark) the measurement appears to be L/5 (use an end-correction factor for both ends on this one). This figure is at odds with the L/4 spec used on the Thermojet drawing, but I would tend to "copy the Chinese" because of its greater success.
An engine that I did not mention in my "What Makes the Great Engines Great?" thread was the Logan, for two reasons: the most obvious being that I have no direct experience with it, the other being that I have never heard of it achieving very high thrust or propelling anything. It is, however, fascinating in itself for a short handful of very good reasons: (a) All parameters considered, it is arguably the simplest valveless engine to put together; (b) It is graphically simple -- a sort of what you see is what you get kind of design; and (c) It still seems to carry an aura of mystery with it that never seems to be abated.
Part of the mystery seems to be that the Logan appears to be more capable than most valveless types of running continuously while "breathing through a straw"; that is, it disagrees with my observation of the "great engines" having big, low impedance intakes! The Logan intake always seems to be kind of a long, narrow pipe protruding exactly sideways [transverse flow path], with the fuel simply introduced at some point along the way.
I believe that, speculatively building on the empirical 'Hinote Criteria' we can derive a proper theory for the Logan and turn it into a simple, effective propulsive device at almost any scale desired. The following is 100% hypothesis, that is yet to be proven:
Let's assume that Bill's observations of points along the acoustic length are essentially correct. I believe that they are, but that the most fundamentally important point has yet to be expressed -- I believe that the absolutely most critical point on these kinds of engines is the point at which the intake pressure wave merges with [or diverges from] the main pipe pressure wave. Why? Because that is the only point where the two have any direct influence on each other! Particularly, it is the only point at which the intake directly "senses the feedback" of the low pressure wave from the main body of the engine. And, the Hinote Criteria lead us to the exact point where that sensing will be optimized: the L/8 point [measured from the closed end of the pipe]. That station, I believe, turns out to be the "Logan Point" of a closed pipe engine.
The classic picture of the Logan shows it as a smooth teardrop with a stretched tailpipe, plus the absolutely transverse skinny pipe. I believe [and Mark's threaded pipe experiments tend to bear this out] that none of this is really important to whether the engine will cycle. What IS important [and never shown in the classic drawing] is WHERE to put that tiny intake pipe.
Set the Logan aside for a moment and imagine the classic Thermojet layout -- a chamber with two absolutely straight and parallel pipes, usually of somewhat different diameters. Lay this out according to the Hinote Criteria: the intake pipe's external end is at L/3 and the intake pipe length is at L/5. Where does that put the inside end of the intake pipe in relation to the front end plate? Well, it has to be at 33% MINUS 20% = 13% of the overall length L. My hypothesis is that this distance is actually ideally 12.5%, or L/8 for perfect synchronization at resonance.
Now, here is the secret to how the whole mess works together: The pressure wave is a STATIC pressure wave! The same holds for the low-pressure wave that returns after the blast wave leaves the pipe. That means that, at any given instant in time, it is pushing [or pulling] in all directions equally, including, but not limited to, the transverse direction in the pipe. If this speculation is true, consider what this means: If the inside opening of the intake pipe is at the ideal Logan Point, it doesn't matter what direction the intake pipe is aimed from that point, for static operation! It can be a classic Logan intake, ending at the side wall of the chamber; it can be a Rossco intake penetrating from the front end right down the center; it can be a Chinese intake with a diffuser stopping at the wall of the rear cone; it can be an NRL design with the intake pipe practically penetrating clear through the space; it can be an Elektra intake poised in front of the exhaust; it can be a Fo Mi Chin intake cutting into the tailpipe. It doesn't matter whether the engine is flashlight shaped, straight or multi-coned -- if you're in the right place, you're in the right place! All you need to know is that the wave is moving through an essentially same-temperature environment from one end of the pipe to the other [not strictly true, but close enough] and the point can be predicted as the 12.5% point.
Once you have this, you can experiment with everything else. The length of the intake will probably need to be about Bill's L/5, but probably ideally a little more or less. The L/3 wave path length will be about right, but will vary with the average temp you actually achieve in the pipe [after all, in all these engines, that air column will be far cooler than the gas condition at the engine interior!]. But the point where the two wave paths intersect is the absolute magic that must be satisfied very closely for things to start and keep running. I'd bet that if Mark and Steve would measure their fully operational pipe Logans, they will find a reasonably close fit to the mark.
Note that in wavelength terms, this defines the Logan Point as the 1/32 wavelength station, with the intake pipe corrected path length = 1/20 wave, since we are relating to a 1/4 wave pipe [closed pipe]. The total wave path length from end plate to intake entrance [with end correction] would be 1/12 wave.
This will be tested, as close as I can cut and weld it, in the Elektra II engine design. I hope that others will try to validate and/or refute this hypothesis. If it is true, it would mean that the crudest valveless pipe designs imaginable will at least run if the Logan Point is used as the starting point for the rest of the design. I also believe that a classic Logan design would run with a much larger intake than originally shown, providing a simple design that is fit for actual propulsion use. The intake could be bent back, "Chinese" style, for full recovery of the pressure wave component of thrust, of course. None of this should be taken to mean that other design parameters are unimportant, just that closely hitting the Logan Point will be crucial to success.