V-1 Video

[Bruno Ogorelec]

I'll peruse my Reynst files. I have his analysis of the Schmidt tube somewhere. A kind Dutchman made an incredible job of scanning the book and sending it to me a long time ago. It went on for days, a few pages at a time (BIG files!).

[Mike Everman]

do you have a pdf distiller? (adobe acrobat is pretty cheap, it's become worth it's weight in gold, you just select it as the printer, so anything can be put in that format. old news, I'm sure, I just love the thing.) or can convert to jpg, of course that is if you want to share! :-)

[Graham C. Williams]

Dear Bruce. As expansion waves tend to fan out I had my doubts about the expansion shock, but real gas is stranger than perfect.

The second law of thermodynamics and the perfect gas relations imply that entropy, pressure, density, temperature and the speed of sound all increase as fluid passes through a shock. Also, for a steady moving shock, the second law of thermodynamics requires that the Mach number M=u/a must decrease from greater than one to less than one in a coordinate system moving with the shock.
Real gases may reverse the shock relationships. For example, in a real gas, shocks may decrease rather than increase pressure and density – this is called an expansion shock.
Computational Gasdynamics. Culbert B. Laney. 3.6

[Bruce]

On page 2 of this thread, Bruno said:

>They could not even agree, really,
> on such a fundamental issue as
> whether the engine vibrated or
> not. Bruce, this will interest
> you in particular. You were among
> the people on the forum who
> warned most strongly about the
> problem of vibration. Curiously,
> the thing does not really
> vibrate!

Hey Bruno, did you read the notes associated with the pictures of the Me 328 that was highlighted in another thread here today?

Look at the reason they canned the craft -- and look how the engine's taipipes extend well beyond the back of the airframe -- so it can't have been down to accoustics as you suggested.

http://www.luftarchiv.de/flugzeuge/mess ... /me328.htm

[Bruno Ogorelec]

Bruce,

A good point. I'll think about it.

By the way, the tailpipes are not too distant from each other. How did they avoid bad interference?

[Mike Everman]

Here's the translation for what it's worth:

"Beginning 1943 an einsitziger hunter was projected in a partnership working between the Messerschmitt CORP. and the German research institute for sail flight, who should carry out without characteristic drive in the Mistelschlepp at the hostile bomber association here proposition and then in the Gleitflug its attacks. This airplane recovered the label Me 328 A. The variant driven by two Argus aces 014 obstruction ray engine recovered the label Me 328 B. The desired production was cancelled through the strong vibrations, that evoked the Argus-tubes, again. "

You guys are arguing semantics in my opinion, there is no acoustics without vibrations and vice-versa! The poor empenage on this plane must have shaken right off from the vibration excited by the pressure waves. It does not matter how well you shock mount the engine to the airframe.

[Bruce]

[quote="brunoogorelec"]Bruce,

A good point. I'll think about it.

By the way, the tailpipes are not too distant from each other. How did they avoid bad interference?[/quote]

Just look at how it was launched for the answer to that one :-)

In the presence of a very fast slipstream, the coupling between the engines (despite their close proximity) would be greatly reduced. An air-launch means that the engines would always be operating in such a slipstream so coupling issues would be of little relevance.

Likewise the effect of pressure waves from the tailpipe affecting the airframe structure. A pressure wave leaving a pulsejet will (remember) only ever travel at the speed of sound or less (otherwise it's a shockwave).

Now, if that thing flew at the speeds indicated, there would be very little power contained in any pressure wave that made it to the aircraft structure itself because the "effective" distance of those tailpipes in a 0.6-0.8m airlflow is much greater than the physical distance.

[Bruce]

Mike said:

> You guys are arguing semantics in
> my opinion, there is no acoustics
> without vibrations and
> vice-versa! The poor empenage on
> this plane must have shaken right
> off from the vibration excited by
> the pressure waves. It does not
> matter how well you shock mount
> the engine to the airframe

Actually I think there is a difference - if you read the german documents that were uploaded here a while back.

One of the German groups quite rightly noted that the pressure waves (accoustic) from the exhaust created tremendous damage. I fully agree with that -- having been on the exhaust side of a large pulsejet too often to even suggest that those waves aren't incredibly powerful.

What was also suggested however, and a point that Bruno picked up on, was that these engines don't vibrate much -- and that's the point I disagree with.

Having sat in a kart with even a little 55lbs valveless and a more powerful 100lbs conventions engine mounted to it, I can tell you that a goodly deal of vibration *is* produced by the engine.

The mere fact that when used on gliders, pulsejets were cleverly affixed by a shock-absorbing mount and spring system indicates that I'm not alone in my observations that these things vibrate like hell.

Just because the acoustic waves are extremely powerful doesn't mean that the engine itself isn't contributing significantly to the vibration transferred to whatever it's mounted to.

So it's not really semantics :-)

[Mark]

I was thinking about the Gluhareff and its coil of tubing that collects heat inside the engine and how a lot of vibration would probably cause fatigue to the thin walled tubing. I can't remember who brought it up, but a significant lose of thrust came about by using too thick of tubing and the propane not getting enough kick on its way out. The thin tubing transfered the heat faster and more effectively to the propane, it was essential for the highest thrust.
Then I was thinking about Bruno's EP, with the sizable jacket surrounding the twin exhausts. Think how flexing and torsion would wrestle the thing apart eventually, unless it was extremely well mounted.
It just occurs to me that any pulsejet with extra bells and whistles is going to be more prone to fatigue than a simpler design. I guess that's obvious, but it's worth keeping in mind.
Mark

[Mike Kirney]

Anybody who thinks acoustic vibrations do not carry enough power to ruin a machine is way off the mark. I played bass in a metal band a few years ago and I was in the habit of putting my beer on top of my amp for easy sipping between songs. That didn't last too long, because by the fourth song, that bottle had been rattled right over the edge of my equipment and onto the floor. Sometimes the beer would just foam up and spill all over, nearly frying my poor little transistors. Even on my stereo speakers at home, a few high volume AC/DC tunes puts all the books, ashtray, pile of junk mail, whatever that I put up there onto the floor. Acoustic vibrations cause extreme metal fatigue and are easily capable of moving objects weighing at least a couple of pounds by several inches or more, given enough time. Any use of the pulsejet in an aircraft would require some sort of vibration isolation, just like the engine in your car or truck. It would seem those Germans ignored this basic fact of nature. If they weren't such cheapskates, I'm pretty sure they woulda won the war.

[Mike Kirney]

Maybe some jets vibrate sinusoidally, but this one does not. A little arithmetic will tell you that the pressure bottoms out at about 9 psi and peaks at about 27 psi absolute. Let us all remain silent for a moment to remember the accumulated commentary of Mr. Forrest D. Eckstein, who once posted on the forums quite frequently, but remains absent from the roll today.

[Hank]

Hello- The Argus as used for Fi 103 propulsion was air started as mentioned in several of the above posts. The fuel was rich until the secondary (flight) fuel circuit kicked in at a speed of around 120 mph. The control was a pneumatic switch operated by air intake that was located on the starboard lower side of the fuselage.
The Fi 103 was kicked off its launch ramp (when ground launched) by a hydrogen peroxide generator pushing a steel piston. A 6mm pin held the bomb in place on the ramp until enough force was generated by the expanding H2O2 to shear it.
Some enterprising slave laborer (still alive) figured out that the leads could be switched between the fuel relay and the explosive gain (detonator). When the Germans went to air launching the Fi-103 several (77) aircraft were lost due to the bomb going off after the planes hauling them reached the speed required to close the normaly open switch. The circuit appears to have been hot whether the engine was running or not.
There is a pot-boiler of an action film named "Operation Crossbow" that has some good V-1 footage in it. George Peppard at his best.
Later, Hank

[Bruce]

They may not vibrate sinsoudally, but they do vibrate.

just do the math:

Take an engine like the V1 with a 20-inch tailpipe. That's a cross-sectional area of 314 square inches.

Now assume that we have peak combustion pressures (as you quote) of 23psi and minimum pressures (as you quote) of 9psi.

Let's ignore the comparitively small effect that pressures below 14.7psi (ambient atmospheric) represents so we'll just consider the 8.3psi of over-pressure created.

Simple math shows that at the time of peak combustion pressure, a net force of some 2,607.5lbs is applied to the engine -- albeit for a very brief period.

Then, once the pressure drops to ambient (or below), zero force is applied to the front of the engine.

Now if you repeatedly (at a rate of 40-50Hz) apply and remove a force of over a ton to any object weighing just a few hundred pounds, you're going to get one hell of a lot of vibration!

[Bruno Ogorelec]

I seem to have been misunderstood in the matter of engine vibration and acoustical damage it wrought.

First, I am not trying to propose anything -- I am just quoting results of other people. I am offering findings of the Argus team and of Paul Schmidt, who disagreed on the matter even though they cooperated (to some degree) on the same project.

It would be insane to propose that V-1 (or other pulsejets) didn't vibrate. I certainly never meant to say that. Every single engine in the Universe vibrates. I think I even said that, early in the course of the debate. My point was that the damage wrought to the craft the Argus engine was attached to was primarily due to acoustic waves, rather than 'conventional' engine vibration.

Namely, the vibration caused by the trembling of the engine itself was effectively contained by its mounting. Schmidt did it in one way and Argus did it in another way, but both ways worked, making vibration a non-issue. That was my point.

A Harley shakes some 150 lbs of its engine at about 60 Hz at 3800 rpm – would you think of mounting that engine rigidly in a motor cycle frame? Of course not. Well, perhaps for drag races. You mount it flexibly and vibration becomes something you can handle without much ado.

Mount your pulsejets flexibly (in a proper way!) and you will experience the same thing Schmidt did – the mounts will depress to the extent the thrust forces them to, but actual vibration after that will be minute and easily absorbed.

[Mike Everman]

Mount your pulsejets flexibly (in a proper way!) and you will experience the same thing Schmidt did – the mounts will depress to the extent the thrust forces them to, but actual vibration after that will be minute and easily absorbed.

I can see no good way to mount a lockwood, but it can be done. A valve type or n-intake (bent to the rear) valveless is a single degree of freedom absorbtion, in the thrust direction. I imagine that the u-shaped Lockwood has huge torsions going on in the plane of the engine. Makes a sticky wicket, eh what?
Exceedingly cool thing #5 about that "low drag" Ecrevisse.