Increase net thrust?

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larry cottrill
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re: Increase net thrust?

Post by larry cottrill » Fri Nov 04, 2005 8:42 pm

Webpilot wrote:PS Larry, is your use of the word contention in the above, the right word to use?
Contention, n. Struggle; quarrel; debate; emulation.
Do you mean to say we, too, are still in some kind of disagreement?
I don't think so. I was just saying that I contended for a particular view. Perhaps I ended up quarreling with myself ;-)

As to thrust and momentum, I should have said momentum per unit time, since it can be expressed as mass flow (a rate) x velocity, which by transposition becomes a rate of change of momentum. A certain momentum gain per second will be sensed (and can be measured) as a certain thrust force, as in the Zucrow rocket equation.

I have gone back to my sloppy post and edited in a corrected version of that paragraph as penance. (I knew what I meant to say ...)

L Cottrill
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Hank
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re: Increase net thrust?

Post by Hank » Fri Nov 04, 2005 8:48 pm

Hello- Forrest , long dead Nazarines have nothing to do with it. We are on our own.

There was mention in one of the above posts that thrust is in one direction and the drawings in others show flow in one direction.
Please see the attached drawing, made by me when I was working on how a pulsed combustion engine operates. These engines breath from the exaust as well as the intake. Remember that we are dealing with engines whose operating cycle is represented by a sine curve that has 0 pressure at some point, with internal conditions moving to the left of 0. Vacuum.
Thrust is a momentary condition; thrust over time is momentum.
Augmentors, aka Ejectors, are effective in increasing thrust. They do this by increasing mass flow. They also add weight and drag.
Hank
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Copy of PULSEJET INTERNAL FLOW.JPG
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thecheat
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re: Increase net thrust?

Post by thecheat » Fri Nov 04, 2005 10:06 pm

now to just read this repeatedly for the next couple of years when I get to calculus and actually undrstand it... :P ok really that's quite a load, but thanks for the info, I'm sure it'll seep in sometime... but for the moment my head has exploded in a giant firey ball visible from space.

The vaccum cycle MUST play a part as the tube is preventing it from being completely null as it's missing the air that comes from that point in...space... heh for the moment I think I'll just agree and make one before I step in the same pile of whatsit. :roll:
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re: Increase net thrust?

Post by Dang911 » Sat Nov 05, 2005 1:37 am

I have a shogun, but with carbon blades (carbon everything). It gave me near stitches on my leg, when a big gust came along and took it. It was going to hit my friend (girl) standing next to me, but I couldn't let that happen, so I kind of stepped into it, to protect her. When the wind grabs the heli, the only chance you have to save it is full throttle. So it hit my leg and lift a nice cut 1" long. It wasn't deep but it bled :(
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re: Increase net thrust?

Post by Hank » Sat Nov 05, 2005 2:30 am

Hell, Cheat- If that one threw you get a load of this.

Hank
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Hank
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re: Increase net thrust?

Post by Hank » Sat Nov 05, 2005 2:32 am

and this......
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Anders Troberg
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re: Increase net thrust?

Post by Anders Troberg » Sat Nov 05, 2005 7:29 am

That depends on the design of the particular engine. On many valveless engine designs, the intake air mass flow is shockingly similar to the tailpipe mass flow.
Sorry, I was vague in what I was saying. What I meant to say was that there is much less air moving in towards the engine. The intake sure can blow as much as (give or take some) the exhaust, but the amount sucked into the engine is relatively small and relatively slow.
That's true, but in terms of thrust forces, it doesn't matter. Once that air is inside the intake tube, it is ALL pulled inward in a single direction - just as the air is ejected from a tailpipe.
Yep, but where is the acceleration of the incoming air taking place? Inside the intake or outside it? Since the intake usually is more or less the same cross section area all the way, I would guess that most of the acceleration is done outside, while the air still is sucked from all directions.
The real benefit of the omnidirectionality of suction out in front of the intake (and behind the tailpipe, too) is that you do NOT pull the ejected hot mass back in. You get a fresh charge of cold air from all sides.
That's true, but then we are on a different effect, we are no longer discussing if the engine creates thrust by sucking itself forwards.
I have a shogun, but with carbon blades (carbon everything). It gave me near stitches on my leg, when a big gust came along and took it. It was going to hit my friend (girl) standing next to me, but I couldn't let that happen, so I kind of stepped into it, to protect her. When the wind grabs the heli, the only chance you have to save it is full throttle. So it hit my leg and lift a nice cut 1" long. It wasn't deep but it bled :(
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WebPilot
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re: Increase net thrust?

Post by WebPilot » Sat Nov 05, 2005 8:40 am

Larry wrote: (I knew what I meant to say ...)
I knew that you knew, but other readers may have gotten the wrong idea
after reading your post as it was originally written.

cheers,

-fde
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serverlan
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re: Increase net thrust?

Post by serverlan » Sat Nov 05, 2005 12:44 pm

Hi Guys,

one of the issues with the Feynman sprinkler concept is in its extrapolation to our engine situation.

The thought experiment implies that the vacuum accelerates mass
that was previously stationary,
and that mass is then stopped, or turned to a perpendicular direction,
so that its momentum in the relevant direction is zero.

The conditions of zero velocity to zero velocity of the mass
is necessary for the experiment to be vaguely comprehensible.

If however we have an engine intake that sucks in gas
and the gas is not entirely stopped, then some thrust could result.
It will all come down to net momentum change.

The point I was originally trying to make is not one of fine nitty-gritty details of any particular engine,
but the notion that if you suck in gas, stop it, and then throw it out agin,
you get thrust. Which is generally counter-intuitive, but important to understand with some forms of pulsejets.

Usually with pulsejets, there is little or no continuous flow. The vioklence of the pulse overrides everything else.

On the other hand (jeez I hate being able to see both sides)
If the incoming gas is stopped not by the engine, but by other gas, then its momentum could be transferred to that other gas, adding to the momentum at the exhaust, thus adding to the thrust.
Arrgh.
My advice, forget about the intake momentum, Much depends on the design, and needs simulation of construction. It will generally be insignificant compared to the pulse thrust. Forget about suction thrust,
or you'll go mad, like me!

Except on Tuesdays, when I become sane and everyone else goes mad.

Don

thecheat
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re: Increase net thrust?

Post by thecheat » Sat Nov 05, 2005 2:42 pm

the problem with showing me those diagrams is that...I can't understand what's happening, at all. I just see what seems to be a pulse jet with some colored lines and wording at the bottom. Though I don't think I'd want to understand it as I'd probably get a kink in the brain.


ah this nessatates a quote from someone: "just when I thought I had hit the bottom you threw me a shovel"

PS: err if it was going striaght towards someone why didn't you pull the move that most beginners are known for: pushing throttle all the way down so if you have negitive pitch there it just smacks into the ground really hard saving the people but maybe not the heli, don't think stepping on it did any worse though.
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Re: re: Increase net thrust?

Post by larry cottrill » Mon Nov 07, 2005 3:42 pm

Anders Troberg wrote:
That's true, but in terms of thrust forces, it doesn't matter. Once that air is inside the intake tube, it is ALL pulled inward in a single direction - just as the air is ejected from a tailpipe.
Yep, but where is the acceleration of the incoming air taking place? Inside the intake or outside it? Since the intake usually is more or less the same cross section area all the way, I would guess that most of the acceleration is done outside, while the air still is sucked from all directions.
Anders -

Well, in thinking this over, I have to mostly agree with you. What I missed was that obviously, the largest part of the acceleration of the intake mass is where we get it moving into the intake, and that is essentially omnidirectional. Eric has noted on his "Advanced FWE" that the flare size and shape are fairly critical, which obviously has to do with optimizing this action. While the Bernoulli effect on the air speed takes place as soon as we get inside just past the flare, there is still some acceleration in the straight pipe during the early part of the inflowing gust, though, due to the gradually decreasing chamber pressure, and that's what I was thinking of.

The combined effect of this is a jetting action into the chamber, which is a negative thrust effect (if the intake pipe is aimed rearward a la Chinese / Thermojet / FWE), and of course, the same thing happens later on at the tailpipe. What we have is a "momentum rectifier" rather than a "flow rectifier" intake. Nothing wrong with that!

It is really amazing that Eric was able to show how this could be powerfully optimized to form a "negative thrust" engine. As I said before, the thing to do is to optimize the negative thrust effect at the front end of a linear form factor engine. Practically speaking, it beats adding a recuperator hands down. The desirable qualities of this would probably disappear at high forward speed, however, unless we provided some variable-geometry damping or some other unsavory complication (but maybe just a streamlined cap would be enough).

L Cottrill

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re: Increase net thrust?

Post by El-Kablooey » Mon Nov 07, 2005 5:27 pm

I guess that would help a linear engine, but still, as with all linear engines the intake's positive thrust would be wasted.
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thecheat
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re: Increase net thrust?

Post by thecheat » Fri Nov 18, 2005 7:59 pm

ok I came up with yet another design. I heard that crossing the mufflers (intesect in an X) increases horse power because exaust helps pull the next cycle by producing suction, which has good effects in pipes unlike what I've been trying to do. OK so I came up with something that could possibly benifit from this.

My thero...hypothesis :roll: is that if done correctly the exaust produced from the intake would run though a very short teslavalve (venturi duct directly behind it) so air can pass through but not back into the engine and comes from the vent on pull cycle. this tesla valve is placed nearly in the END of the long barrel, so while the air is nearly at the intesection from the bottom it the intake vent would be pulling it producing more thrust, correct? It'd also probably make much more of a vaccum, creating better fuel mixing.

just a guess, but tell me if I need to clarify. (and what on) or if I'm still just making uneducated blunders, and therfore being educated.

PS: my shoes are already dirty from this same pile of whatsit, so it can't hurt to step in it again!
Attachments
pulsejet.GIF
well here's the design, yes it looks like a caribiner, and the top is the main barrel. I labeled the venturi duct "duct". I hope this helps you get the drift of what I'm thinking... and maybe JUST maybe, that duct is situated so it would pull fresh air th
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re: Increase net thrust?

Post by Mike Everman » Fri Nov 18, 2005 9:58 pm

Consider the main reasons in my estimation that Tesla's don't work on an intake. That is, arguably the most important part of the cycle is how low a partial vacuum can you get in the CC. The other is the plugging effect of *un-impeded* inward flow in the intake, prior to heat release, which leads to higher pressure due to the heat release.

In short, the easier the pressure pulse can escape, the lower the partial vacuum thereafter, the higher the velocity coming in before the next pop, the greater the pressure developed, the greater the thrust. This is as succinct as it gets for "what's important in a valveless", IMHO, before you start discussing tail geometry/front-rear loading, etc. for maximizing thrust.

I believe this is why The Frenchies didn't just bend the intake to face the rear, but turn the flow with a recouperator where turbulence in the 180 won't impede the exit velocity.

The kernel of your idea has some merit, I think. If there is a place that you can have a big hole (your "y" junction) in your tail, and still have it run is one question. Another is, both upstream pressure pulses are in phase at this point, so they both want to blow into the junction at the same time. How does this fit? I suspect it's a bad deal for the intake, making acoustic problems on the exhaust phase which you want to be substantially unimpeded.

My $.02
Mike
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thecheat
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re: Increase net thrust?

Post by thecheat » Sat Nov 19, 2005 12:54 am

well I'm just going to live and learn, I tried to make that so the cycle wouldn't have reached that point through the main shaft yet and thus enabling it to pull the air though. This may require different legnths then usuall and create some interesting propertys... maybe if someone who has the parts would like to just give it a try, I'd be much obliged, or at least sim it. :roll:


anyone else?
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