Increase net thrust?

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thecheat
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Increase net thrust?

Post by thecheat » Wed Nov 02, 2005 11:39 pm

well I did some thinking, and just wondered about thrust on these things. I am correct that when it pushes air out it moves directly in the pointed direction, and on the "return" stroke it pulls air from all over the place making it not pull as hard as it pushed in essence making thrust instead of the thing just vibrating on the ground right?

if so I came up with this pic


the blue is fresh air, the red is hot, and Tesla's valve is turned around so air is refused entry so it must pull the jet forward to continue with the cycle making both refreshing the air in the chamber and outtake create thrust and possibly lower the vibrations. (that thing is an air scoop by the way) Plus at faster speeds it's should force air in it by catching the passing air and smashing it into the chamber. It would make it more effecent at higher speeds, as well as increase the power. I was thinking of putting something similar on the main barrel (what's the correct term for that) but I though the scoop might be detremental, as you don't want to push air into the main barrel, so you would not include it on the front making it just pull from the sides.
Attachments
crappy pic.GIF
horible I know but the bulge is the expanion chamber, and the pipe you see is the intake
crappy pic.GIF (1.55 KiB) Viewed 7738 times
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serverlan
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re: Increase net thrust?

Post by serverlan » Thu Nov 03, 2005 1:39 pm

You are right to an extent,

and if the air being sucked in from the sides flows across a surface (say a plate at right angles to the tube)
then you will get reduced pressure there, which can be used to add to thrust. (I think this is the effect of the rolled lip on the lockwood augmenters)

but, there is something you really need to understand first.
Blowing and sucking out of the same hoke (pardon)
will gennerate a net thrust. The blow creates thrust, and the suck has no net effect.
I know this sounds wierd, but the theory comes from the great demi-god of physics - Feynman, and is superbly explained in the article to be found at
http://www.nmia.com/~vrbass/pop-pop/aapt/crane.htm

Read that and then see if you still need your theory to explain things.

Don

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re: Increase net thrust?

Post by thecheat » Thu Nov 03, 2005 1:52 pm

ok I see when the air is sucked in it slams togheter its inertia pushing the jet forward nullifying the effect of the pull, so the jet just coasts on the pull stroke. Then the rest continues with out question...interesting...ok well now that I know that, I do still think that it would create a bit of extra thrust, because it makes the jet move while pulling and once it slams into the spare flame that creates the next cycle. I'd like to try it sometime but I have no way to make tesla valve... still gotta make the jet first anyhow. :roll:
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Anders Troberg
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re: Increase net thrust?

Post by Anders Troberg » Thu Nov 03, 2005 1:58 pm

I may be way off here, but I have a strong feeling that, while perhaps theoretically sound, this solution will reduce thrust.

The main reasons behind this are:

Sucking in air does not provide significant thrust, neither does it hurt much if done in the wrong direction.

The Tesla valve, if I understand your idea correctly, is there to make sure most of the exhaust move away from the engine instead of getting sucked back. The first issue with this is the same as above, suction does not impact thrust. Secondly, since the exhausts have already left the engine, the thrust has already been been generated by accelerating the exhaust gas. If it is later slowed down or even reversed does not impact the engin much. Third, the Tesla valve needs to be attached to the engine, and it will provide some resistance for the exhaust gas, which will actively push the valve backwards, reducing thrust.

All these extras will impact the aerodynamics of the engine, causing more drag. Sometimes a more crude but sleek design will be more effecient than an advanced but messy design.

But don't take my word for it. I'm often wrong. It should be easy enough to attach these extras to several engines known to work and see if there is a difference, so don't be afraid to try it.

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re: Increase net thrust?

Post by thecheat » Thu Nov 03, 2005 3:52 pm

If I had the equipment I would, but seeing I don't... is there any way to CHEAPLY make the tesla valve from common materials? the only things I have wing shaped are blades for my heli, but they aren't metal and would probably burn/melt I do still find it hard to believe that sucking (like a vaccum) wouldn't produce any thrust, I guess the only way to test that is to make a bell shape for a electric turbine, so the exaust is pushed out creating no thrust and see if it does anything, maybe by setting it up on stilts then put it on a gram measurer and see if it does a thing.

ok another pic: I think why it usually creates no thrust is because it's sucked from all directions nullifying any thrust created, maybe creating negligible negitive thrust

the next one is what I think would happen with my valve mod, I think it would suck more from just back, maybe nullified by the other side still... I do know sucking creates thrust, believe me, ever stick your hand above the blades of a helicopter? If you could find a way to make the pull of the air come in a current like the push of the air, then it would make a difference, do you get what I'm saying?
Attachments
regular tube.GIF
ok this is without the valve
regular tube.GIF (1.22 KiB) Viewed 7690 times
regular tube2.GIF
with valve
regular tube2.GIF (1.42 KiB) Viewed 7689 times
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re: Increase net thrust?

Post by Dang911 » Thu Nov 03, 2005 5:28 pm

Your second set up in my opinioun will decently increase static thrust. Its practically an augmentor, sucking in cold air, and heating it. This has been proven to work.

The problem with your new design is as the thing moves though the air, it will create a tremendous low pressure area right at the point of intake for that cold air. Instead of cold are being sucked into that passage way. The low pressure area will create its own suction, sucking exhaust out during that phase, and not allowing air to be sucked in during intake phase. This will render it quite useless, and probably hinder thrust.
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regular_tube2_852[1].JPG
regular_tube2_852[1].JPG (16.82 KiB) Viewed 7676 times
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re: Increase net thrust?

Post by Anders Troberg » Thu Nov 03, 2005 6:23 pm

I do still find it hard to believe that sucking (like a vaccum) wouldn't produce any thrust
Ther reason you get a decent amount of thrust from the exhaust is that the air is moving fast and in one useful direction. On the intake side, there is much less air moving, it is moving slower and it is being sucked from almost all directions. Even if you can direct the intake to point forwards, it will still suck from the sides as much as from the front.

In fact, once you get moving, the intake will no longer suck air, it will be force fed through ram pressure, even at a relatively low speed. At that point, it is rediculous to say that suctuin creates thrust.
I do know sucking creates thrust, believe me, ever stick your hand above the blades of a helicopter?
No, but I've been under quite a few helicoptes and I very much advice against trying to get your hand above the rotor if you are attached to it and wish to remain so.

I think you are looking at another phenomenon here. What you feel on your hand is not suction, it is more like a sail in the wind. The question is not what your hand feels, it is if the suction on top of the rotor disc provids lift to the rotor. Even so, it is not a fair comparison, as the rotor disc is a way of accellerating air downwards, and it really does not matter where that accelleration starts. A pulse jet sucks in a lot less air than it blows out so the icoming air does not matter as much, it is more a requirement for the operation than a part of the propulsion.

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re: Increase net thrust?

Post by thecheat » Thu Nov 03, 2005 11:51 pm

ok I see, just to tell you that "second model" is the same as the first one, it's just simplified.

the heli is my RC one, so it wouldn't cut off my hand, just give a bruse and maybe cut me.

Ok I guess I'm slowly getting the point, and it seems that it wouldn't help that much if any, except when ram pressure is applied. Ok well if there's any other opinions on it, glad to hear it and thanks for helping me on this, gotta say I'm already smarter than when I started! :)
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re: Increase net thrust?

Post by Dang911 » Fri Nov 04, 2005 12:23 am

I hope we are talking a micro electric R/C helicopter like the humming bird. About 2 years ago while out at the field flying helis, I witnessed the worst r/c related injury. Now granted I have seen fingers lost in maybe 4 incidents before with a propeller, but this was far, FAR worse.

To properly tune any engine, it really needs to be at full throttle. With airplanes, one holds the plane at full throttle and tunes it. With a helicopter, you make an adjustment, step back, test fly it, and go from there.

This guy flying a 45cc gasoline powered heli, decided that took to much time. Instead he asked a partner to hold the skids on a table. These tables were 4 feet off the ground. So the guy holding it down got under the table, and with his hands sticking up around the table held the heli's skids down. Soon after throttling up to full throttle, the other guy ducked down and went after the needle valve. He wasn't protected by a table top.

Many people watched in amazement that someone was so stupid. In a moments notice the guy holding the heli yelled, "I can't hold it anymore" (45cc gasoline powered heli's are BIG and POWERFUL)!!! The helicopter slipped slowly out of his fingers, and in slow motion, we all watched as the heli went tail down. First to hit the guy tuning, was the tail boom, which on a heli is spinning 5X faster than the main rotor. That took off his thumb, pointer, and middle finder, and a large portion of the side of his wrist. Next came the long main rotors, hitting the guys arm, halfway to his elbow. Huge gash there.

The man looked at his injuries in shock, and began to run to the car to get a towel (like that will does a lot of good). In his 50 foot journey, he collapsed halfway there, in total shock.

Once the ambulance arrived, the paramedics were missing 3 fingers. They asked everyone out there to form a line, and walk slowly finding the fingers. The the person standing next to me, found the thumb, which flew over 30 feet from being cut off. Luckily he got all of his fingers back and they were sewn on.........

A lesson to be learned!!!
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re: Increase net thrust?

Post by Anders Troberg » Fri Nov 04, 2005 8:43 am

Sorry, I didn't read properly, didn't notice that it wasn't a full size helicopter. Still, there is a lot of momentum in those rotor blades, probably not that far from a small lawn mower, so take care. It's not mainly the engine size that hurts, it's the spinning mass of the rotors.

There was a contestant in Robot Wars called Hypnodisc (which I think placed first or second), which basically was a big flywheel with some bits of steel sticking out of the edges. It took a while for the small engine to rev it up, but when it crashed into the other robots (is it really correct to call a remote controlled model a robot?) the flywheel just tore them to shreds. It hit them like a burst of machinegun bullets.

Anything with mass and speed has potential to be dangerous. A real helicopter may have a powerful engine, but if you hold the rotor before it starts spinning, you can keep it still with one hand. Put that hand up there once it is up to speed and you'll quite literally be unarmed.

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re: Increase net thrust?

Post by thecheat » Fri Nov 04, 2005 4:02 pm

heh yes this is hummingbird size, well, slightly bigger (shogun/TRex) size with a good brushless engine otherwise I'd never be able to fly in my back yard or neighborhood. I've been hit by the blades at lower speeds and it hurts, (I'd not be that stupid with a gas, ever) and I ALWAYS secure it if I'm doing something like that. It'll lift 200g at max so it's not that hard to do. Anyways the blade tips on my ABS ones will slice you up(after much concrete skimming they sharpen very nicely), I got nicked by it once didn't feel much (due to high speeds) but it cut me very easily.

oh and you watched that? that sounds very horrible, uhng I can't imagine "hey go in a line to find the fingers" I'm not quite sure what I'd do if I found one. probably barf.

and though this is off topic, why don't they (in robot wars) make one that has a very high powered welding torch with the nozzle imbedded in the middle of a phumatic hammer. First you kill the wheels then you heat up a spot and smash! that would be cool, or have one that flys.
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Re: re: Increase net thrust?

Post by larry cottrill » Fri Nov 04, 2005 5:05 pm

Anders -

Forgive me - just nit-picking here:
Anders Troberg wrote:
I do still find it hard to believe that sucking (like a vaccum) wouldn't produce any thrust
Ther reason you get a decent amount of thrust from the exhaust is that the air is moving fast and in one useful direction.
Exactly so. It is all a matter of stream momentum.
On the intake side, there is much less air moving,
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.
... it is moving slower
Again, that depends on the engine under consideration. In a Lockwood, for example, the gas velocity through the intake is much greater than through the rear end of the tailpipe (according to UFLOW1D analysis, at least).
... and it is being sucked from almost all directions. Even if you can direct the intake to point forwards, it will still suck from the sides as much as from the front.
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. This requires the same force as if it had been pulled straight in. 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.

THRUST IS MOMENTUM. The net force is mass flow TIMES velocity, integrated over some time interval. The difference between positive and negative thrust forces can always be expressed as the difference in positive and negative stream momentum, whether viewed at some particular instant or taken over the whole cycle.

EDIT - Forrest has forced me to be precise in my terminology. The paragraph above should be re-written as:

THRUST IS INCREASED MOMENTUM per unit time. The net force is mass flow TIMES velocity at any instant in time. The difference between positive and negative thrust forces can always be expressed as the difference in positive and negative changes in stream momentum per second, whether viewed at some particular instant or taken over the whole cycle.

L Cottrill
Last edited by larry cottrill on Fri Nov 04, 2005 8:53 pm, edited 2 times in total.

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re: Increase net thrust?

Post by Mike Everman » Fri Nov 04, 2005 6:03 pm

No matter where you point the inlet, the air comes from everywhere with nearly immeasureable net force imparted to the duct. Look up Richard Feynman's "lawn sprinkler" experiment.
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Re: re: Increase net thrust?

Post by larry cottrill » Fri Nov 04, 2005 6:52 pm

Mike Everman wrote:No matter where you point the inlet, the air comes from everywhere with nearly immeasureable net force imparted to the duct. Look up Richard Feynman's "lawn sprinkler" experiment.
OK, I read his own description of the experiment, quoted from his book. In the simple rig he set up, the center "pivot" simply consisted of a long hose, so any motion of the S-shaped duct would twist the hose. Before the experiment blew up (literally!), he says the hose DID show some twist.

Well, OK, of course, he didn't say in which direction ... ;-)

It isn't the momentum of the air OUTSIDE the duct that gives propulsive force, whether you're blowing or sucking. It's what it does in contact with the machine that counts. This is exactly like my old "front plate" argument with Bruno. Also like my contention with Forrest that the forward force on a ramjet is basically exerted on the diffuser wall.

You could design a "reverse lawn sprinkler" with spacious arms and inward pointing deLaval nozzles, and it would spin nicely in the "reverse" direction in normal air, once adequately pumped out. This would take a heck of a vacuum pump, though, unless the thing were VERY much miniaturized.

The main reason the suction force is small is that the pressure difference is small, and hence, the velocities attained are low. Remember that Eric has built an engine with a net thrust in the suction direction. How would that be possible if what you claim about the intake is correct? It can only work that way because of the momentum attained by the air after it gets INSIDE the duct.

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re: Increase net thrust?

Post by WebPilot » Fri Nov 04, 2005 8:20 pm

dear sweet jesus,
THRUST IS MOMENTUM. The net force is mass flow TIMES velocity, integrated over some time interval.
Thrust is NOT momentum. Check out the units of each if you need
some proof. Better still, start over again with F=ma=dP/dt
and redefine. I think you're trying to talk about a term called impulse,
not thrust.

Check out one of my logos and its derivation for thrust is
somewhere else on this forum. Remember, that derivation is for
somewhat, steady flows.

I went to http://www.nmia.com/~vrbass/pop-pop/aapt/crane.htm and the most
interesting thing I read was:
You may question very logically how, if water is expelled and sucked in alternately, there can be a net force forward to make the boat go. In the past I spent time puzzling a similar problem. It started with a question posed by Richard Feynman in 1985 about the familiar whirling lawn sprinkler. His question: if the sprinkler is submerged in water, and water is sucked into it instead of being squirted out, will it rotate in the same or opposite sense? (Evidently he assumed it would rotate.) I did the experiment, and couldn't get rotation either way. Feynman's question started a firestorm of letters and articles that didn't peter out until the early '90's. As I remember, the end was quiet, with no agreement.
I have the utmost respect for Dr. R. P. Feynman. Was his topic ever resolved?

-fde

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?
Last edited by WebPilot on Fri Nov 04, 2005 8:51 pm, edited 3 times in total.
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