"The Millenium Lance" Project Build

[Zippiot]

Does anyone know if a long, sloping exhaust (15 degrees) is better than a short (30 degrees) one?

I see both, I think the faster you go the more slope you need but I am not sure.

[heada]

I thought it was $550 as well but once you add it to the cart, it shows $350. For slightly less air (750CFM) I could pick up the 8 inch model for $210.

If I wanted to rig something up, I'd take the 2 HP motor from my table saw and connect that to a fan. With a good pully system (stolen from my drill press or my wood lathe) I could get the RPMs up (or down as needed) Sometimes its easier to just buy the item though...and with one of these types of fans, I'm not sure which would be better (but I'm leaning toward buying one)

-Aaron

[Irvine.J]

Cheapest thing i can think of is a 1hp or so electric motor from a large washing machine (which I have), A radiator fan and its mount from the wreckers, then simply make a cage/cowl, and gear it right up for huge rpm...duct it...that should do nicely...

[Bill Lubarsky]

A few comments and a dash of cold water: Many air movers are great when it comes to moving volumes of air. most aren't very good at producing pressure differentials. Most blowers I have seen and all inexpensive fans can only produce static pressures measured in inches of water. I was looking at a surplus blower listing from one dealer that can give you maybe 350 cfm @10" water. $500, 3 phase, 400 Hz (aircraft ventilation).

Now take a look at peak pressures developed in pulse jet engines and the continuous pressure in homebuilt turbines, the pressures go up to at least 40 psig in the turbos ( I don't have numbers for pulse jets).

One more point: I think you need at least 27 psig combustion chamber pressure what ever type of jet you are working on to make a CD nozzle worth using.

WEL

[Irvine.J]

http://cgi.ebay.com.au/Fan-Blower-out-o ... dZViewItem

http://cgi.ebay.com.au/Air-blower-unit- ... dZViewItem

both cheap for Au standards

[tufty]

A few comments and a dash of cold water: Many air movers are great when it comes to moving volumes of air. most aren't very good at producing pressure differentials. Most blowers I have seen and all inexpensive fans can only produce static pressures measured in inches of water.

Pressure's not what we're after in this case, though, pressure gain comes from the jet itself, more particularly the inlet; what we're after for a ramjet is high volumes over a small area.

Turbines are a whole different kettle of fish, of course.

simon

[Bill Lubarsky]

If you are trying to build a wind tunnel using a garden variety fan to test a ramjet in it won't be enough. The way I read the above comments it seemed that that was what was being proposed. I think that all ramjet tests discussed in this forum have been direct-coupled tests with the output of a blower gong right into the ramjet inlet. Getting the same condition with a jet in a wind tunnel would require more than an automotive radiator fan as was suggested.

Very few ramjets have been built with CD nozzles simply because the combustor pressure is too low to produce supersonic exhaust. A simple
convergent nozzle will give you an increased exhaust velocity if you have enough static pressure in the combustor. Otherwise you are looking choked flow and maybe flame coming back out of the inlet.

WEL

[Irvine.J]

Well, the new modifications worked well, slight flare on the intake, slightly reduced intake area, and small flameholder, my injectors worked well to give a nice well rounded and even flame... However this is really a monster engine, its physical size makes it difficult for me to push enough air for a big tailflame and all the bells and whistles. It produces a nice conical blue tailflame, is rediculously loud, but from now on will make them much smaller, but keep this on for one day when I get my giant centrifugal blower I can give it a proper power test. I'll be entering something much smaller in the challenge, I'll post a small video of todays test with the new inlet, there was no flame creeping foward.

In addition, I don't know if at such low speeds I was getting much benifit out of the lance, perhaps its area should have been smaller. I will enter a jet of much smaller proportions and a similar aerospike but tweak it to try to get the most out of my blower, and run a single gas line. Its now 11:21pm, I should be finished the new design in a an hour or two so stay posted.

I guess in the end its been enlightening to build this aerospike engine, it was very exciting to run, and am glad it was performing both initially as a velocity jet (which is quite funny really) and now as a ramjet. My kingdom for a MASSIVE AIRFLOW!

[Zippiot]

It wroks great!! But yes a smaller engine will get slightly better performance out of the blower. My problem is with liquid fuels, there seems to be a minimum size you can make.

[Bill Lubarsky]

This is one of the better things I found on the internet for providing air.
I have been playing around with the idea of building a motorjet using
an RC electric fanjet. The prices of the motors alone made this look like a deal!

http://www.surplussales.com/Fans-Blower ... low-4.html

Not beyond my budget but low on the priority list.

WEL

[larry cottrill]

James -

The video definitely shows a very nice run - Congratulations!

But, yes, I think it is way underpowered with the air supply you have - sort of running "at idle" if you will. As Bill and I said, it's a matter of power, not just CFM.

You can calculate the CFM requirement of your engine at a particular speed. Take the MPH you think you can get, multiply by 5280 ft and divide by 3600 seconds. Multiply that result by the net inlet area IN SQUARE FEET, and you have the cfm at that velocity. But remember, this is ONLY the CFM that actually powers the engine - the blower you use would have to supply considerably more so you can more than cover the intake, to get good diffuser performance, so say multiply by 2. Now, the result is the CFM an air mover would need AT THAT SPEED to supply your engine the way you want it to, with a decent margin for losses around the outside.

Calculating on that basis, and then going to the equipment catalogs, you will be astonished at the HP it takes to deliver sufficient CFM and the desired outlet speed.

L Cottrill

[Zippiot]

some wind tunnel expamples, I doubt they get to 200 mph but a sleeker design might...

http://www.lfst.com/student_project_pictures.html

[larry cottrill]

This is one of the better things I found on the internet for providing air.
I have been playing around with the idea of building a motorjet using
an RC electric fanjet. The prices of the motors alone made this look like a deal!

http://www.surplussales.com/Fans-Blower ... low-4.html

Not beyond my budget but low on the priority list.

WEL

Gentlemen of the jury -

It might be instructive to see just what one of these babies will do. Note the model that will give 330CFM at 12 inches H2O - a really good fan for handling back pressure from whatever you're driving.

They say 6 inches diameter x 10.5 inches long, so I'm assuming the 6 inches is OD - let's say the flow stream is effectively 5 inches in diameter, and make the (ridiculous) assumption that you get the same airspeed over that whole width. In sq inches, that's 25 x pi / 4 = 19.63 sq inches. In sq ft it would be 19.63 / 144 = 0.136 sqft. 330 CFM would be 330 / 60 = 5.5 cuft/sec. The length of that 5.5 cuft for a column of that width would be 5.5 / 0.136 = 40.44 ft, meaning that the velocity is 40.44 ft/sec. Expressing that in MPH, we would have 40.44 x 3600 / 5280 = a whopping 27.57 MPH !!! Incidentally, that astounding 12-inch H2O back pressure amounts to 14.7 PSI / 33 (since 33 ft of water = 1 atm) or a heart-stopping 0.445 PSIG !!!

And all that for only $595 US and enough electricity to run a 1.5 HP motor - wow! In all fairness, that pressure WOULD be enough to drive a pretty restrictive accelerating cone and flow straightener without overheating the motor - but then, of course, the output stream will be much smaller in area. Also, that CFM rating is AT that back pressure - free running, the fan would do significantly better at air moving, of course.

This begins to show what we're up against here, and what HP really means in terms of air in motion. Thus ends the lesson for this day.

L Cottrill

[Zippiot]

So fan bad, me so confused...


I know it would get hit by much heat and prolly not have as much airspeed, but the fans placed behind the jet "might" give more uniforn airsped in the windtunnel...Is uniform airspeed that important?

[larry cottrill]

So fan bad, me so confused...
I know it would get hit by much heat and prolly not have as much airspeed, but the fans placed behind the jet "might" give more uniforn airsped in the windtunnel...Is uniform airspeed that important?

Well, it's not that such a fan is bad ... it's just that you have to have a realistic sense of scale. A high-speed wind tunnel that can handle a good-sized model requires HUGE power expenditure. Similarly, testing a ramjet model of significant size implies moving a LOT of air at pretty high speed - again a lot of power. Keep in mind that to double the speed of a given air column, you need about FOUR TIMES the power!

As to the need for uniform airspeed across the air column: That's the only way you'll KNOW that your inlet and diffuser are really doing what they're supposed to do. Any roughness or inconsistency going in will degrade the performance of the diffuser section to some extent. Yes, good wind tunnels have been built with the air mover behind the test model - what this mainly accomplishes, I think, is it simplifies 'flow straightening' because you don't have air flow that is torqued by the fan. So, it is a good plan, I think.

My point is this: If we do this big, we can't afford to do it right. If we want to do it right, we will have to work small. Of course, if you're independently wealthy, you can do whatever you want ...

Actually, a wind tunnel of decent size is something that could be handled by a club of physics students or some such. Could be an ideal project if a bunch of guys were all interested in aeronautics and/or fluid dynamics, and all had a little expendable income to chip in.

Other approaches that have occurred to me: Air moved by a pulsejet and compound (meaning, multi-stage) augmentor and other smoothing equipment; A friend with a private plane at a small airfield could provide lots of air behind a propeller, which you could capture in a long smoothing duct (it would be pretty turbulent right behind the prop, of course, and would also be "pulsing" air) and you would buy the aviation fuel. Both problematic solutions, esp. in regard to noise. I have suggested before coupling a truck turbo compressor section to a small used auto engine, but this was greeted with disdain.

L Cottrill