The Great Ramjet Challenge!!!

[Irvine.J]

Larry wrote:

Note that the air speed implies a weird tradeoff: All drag values will increase with the SQUARE of speed, i.e. twice the speed = 4 x the drag; on
the other side of the problem, more speed means more pressure development (with a perfect diffuser design), which means more thrust.

That explains a few things... for instance, why as my engine moved foward against the blower (closer to the nozzle) the amount of weight needed to
push it foward on the scale increased from 150 grams through to nearly 300 grams at maximum extention. (I've been playing with the setup to try and make initial and end values nice and linear) As it got closer airspeed and drag were increasing, however, as it got closer to the nozzle it accellerated much faster. When the weight was attached, I found it had a much easier time lifting it closer to the blower nozzle. So I've made a little graph here after some dubious 1:45 am thinking (I have to stay up to wake my father up for his fishing trip in an hour)...

I cut the movie to only and as precisely as I could, the time taken for the slide to move 10 inches was 1.13 seconds. Using that, I could deduct a 1/2 way point on the slide (as if you look at the two pylons the engine is attached to, the front of the slide starts and finishes exactly behind each pylon...(see I thought about it :D lol!) I wrote down all of the thrust points with my ruler, whilst measuring it at each 2 inch marker. Anyway, notice as the pressure increases as it moves closer to nozzle a net thrust is gained exponentially. (Though pressure is much greater acelleration/thrust is higher)
Great stuff Larry...this correllates preciesely to what you were saying.
Next I need to get the blower nozzle about 4 inches away and try and lift a little more the 100grms on the weight. Right now I'm always starting at 12 inches.

Larry wrote:

A good way to accomplish what we want would be to have the carriage (stretching a spring) push a "marker" that would stay at the maximum extension obtained. Then, the fuel would be shut off with the air still running, and an accurate spring scale used to pull the carriage forward until it just touches the marker. The value measured on the scale would be the net thrust at speed for that particular airflow and engine.

EDIT:Yep gotchya :D


I got a little fuel system in the works too larry, infact I'll just go ahead and say i'm going to be using a liquid propane setup out of those campmaster stoves, cut my own notches so they work upside down in the liquid position. So I will mount that on the sliding rig too, so it has to push it too, which should make for some interesting results.(And I'll make a quick streamlined little pylon for it should only take me 10 minutes or so, like an end cap. Anyway heres the graph. Its more just a visual refrence then an accurate one for the acelleration. Though the line for the dry resistance is pretty accurate.

[redneck]

well dang guys, thanks. thats a real compliment, especially coming from you guys. now i just need to figure out what the opening area of the inlet and exhaust should be. do yall reckon i could get much thrust from this sucker?

[Zippiot]

Ramjet thrust is a tricky thing...

Irvine is the onlyone here who has a working ramjet that makes thrust

Time for me to get working on repairing my jet and building a new one, sad sad day :(

[Jim Berquist]

Don't Feel bad!

Ive been off work for the holidays and can't use the shop at work. It's cold as hell out side and I can just do what can be done in the kitchen.

That's why I am working on the Can Man Thing!!!! I started the little beast on the BB-Q on the front porch to check out one section of it. The Cats the Dogs and My son was outside.......My Wife said dam,,,,,That that thing makes noise.......Cabin Fever or what!!!!?????

Don't worry Eric: Fire apperatus at hand and the little beast sparked up first try.......

Jim

[thecheat]

well... my design is still coming along... The flameholder... well... let's just say that stainless is really hard to bend, so my "origami" holder doesn't quite resemble the paper one... but no matter, it's starting to shape up, and if I want to allow more airflow, I'll just shave off some material, it's pretty blasted strong... hehe, an explosion would amount to this: WHOMPH! gee... I think it burped.... I'd probably get the most thrust of any of us...

I'll post pictures of it later. I'm still adjusting the flameholder so it'll fit. lots of grinding as I don't have any cutting wheels... and a hacksaw won't do. Once I get it mounted, I can weld the rear up, weld the diffuser up, and it should be ready for action!

PS: oh... and besides being excessively strong, it's also excessively heavy... might have to call it bulky instead of boxy...

[Irvine.J]

Redneck wrote:

now i just need to figure out what the opening area of the inlet and exhaust should be.

No problem redneck. Now working out your diffuser and exit nozzle sections are a little tricky but no too hard. If you know the airspeed, and know the amount of thrust you want, it should be quite easy. (As you would design it for a particular speed.)
For instance, if you look at the sticky at the top in the ramjet forum and click on the second link to the decker page, you will see that if you wanted to design a ramjet that produced 1 lbs of thrust at 293fps or 200mph, you would need to work how many lbs of thrust you would need to achieve per square inch of your entrance area and make your inlet diffuser specifically for those conditions. If you were getting the same results and used the same design as described on the decker page, you would need 2 square inches to make 1lb at 300fps.

For instance on my ramjet, my max thrust was measured (at a distance) to 310 grms, not including the weight of the slide etc (prolly would get more if i put the blower closer but for now we will use this example.) My diffuser entrance diameter is 2 inches, or area of 20.25sq/cm. So if I were to divide 310grms net thrust by the diffuser entrance area, I am getting 15.3 grms of thrust per square cm of entrance area with this particular design. I have no idea how fast my blower is pushing however, so i can't be sure what operating velocity I'm getting.

I have a "little wonder power blower" (hand held version, not the walk behind, but if your serious about getting into it then I'd recommend something like this. I went looking for stats and I found these, I really think they are good quality, maybe look around on ebay for a good price. If your lucky they might be willing to provide you one...who knows "Powerful enought for your Ramjet Engine!" would be a good selling point! :D
For instance
http://www.mayberrys.com/little_wonder/blowers.htm

[redneck]

im going to have to study everything you just told me. thank you for the info. i just tried my first liquid fuel runs on this jet today. i have a problem with my injector nozel shooting too much fuel in and having it run out of the tail pipe. but once that cleared up it looked like it was running much better, though i had very yellow flames.

VIEW PICS FROM BOTTOM TO TOP TO SEE ORDER OF EVENTS.

now i aint claiming anything just yet but that engine was bouncing up and down alot on that stand, alot more than when it is just the blower on it. maybe 2 inches up and down. caint wait to get a good fuel nozel designed and built so i can get some good mixing. then to tweek the pipes and measure the thrust.

[Irvine.J]

Way to go redneck, looking very nice. If you use different fuels you will get different flame colours. The propane burning blue like that just shows your getting good mixing. Try the same nozzle with your liquid fuels perhaps?

I took my roller stand off the runners about 5 minutes ago, after a bit of cutting some templates... The rollers, and engine, weigh in at almost precisely 1kg! GREAT SCOTT!

I couldn't believe it, but its true! So what does this mean???
I believe the engine (though I have not taken it off the stand yet) is about .25kg. The rest of the weight in the runners and pylon.

So I ask the question: (Remember I'm asking)
If we were tally it up in terms of overally power generation (Not net thrust) and incorperating the weight of the engine and stand, (as thrust is determined by the weight of the object force is acting upon,) would we be looking at somewhere along the lines of,

Oops had to edit this...
250grms - engine
750grms - stand
50grms - kinetic (to get it moving)
175grms - as the engine slid foward till weight/airflow stabilised with 100ml water bucket? (check the graph above) had to overcome airflow/drag.
100grms - the water.

So I ask, if one was to encorperate all of these things as an overall thrust measure, would we be talking about 1325grms thrust?
Fully aware that Net thrust would be somewhere much much less.

If we were to subtract that from the engine weight, we are talking,
1075grms?
thrust if it were mounted on an aircraft, completely free of anything but very thin pylons? Or a Thrust to weight ratio of 5.3 DRY?
PLEASE TELL ME THIS IS TRUE!!! (Though I think its probably not, hence why i'm ASKING!) It would mean about 65grams sq/cm. Which doesn't seem too unlikely *cringe*.

Looking foward to larry blowing me out of the water here LOL!
Your sheets patterns are cut and ready for the metal by the way, I'll post something shortly about it.

I'm only speculating here, as i'm not totally sure, if this is all wrong let me know, but It sounds very cool right now.

[redneck]

dangit if you beet me im gonna be mad! you only win cuz you know more.... lol just kidding! those numbers sound awsome to me! i hope you got what you are hoping fore. if so.... then here is a question we all hate.... now what?

[Irvine.J]

Its far from over yet my friend, besides, as soon as Larry see's this he is gonna freak out and shatter my dreams into a thousand tiny pieces hahahaha! I really don't think there was ever going to be a "Winner" so to speak, just a bunch of inspired people having a go, having a laugh, and most importantly a good time whilst learning something fun, hell, I've learnt so much, and still know practically nothing, the guys are constantly pulling me up for going way off tangent :D Hehehe.

[redneck]

reckon i could get much out of my lil jet?

here is a retarded math question.... can the area of a circle be less than the diameter of it? reason i ask is what is the area of a 1'' diameter circle? then compare that to a 2.15'' diameter circle.

[Irvine.J]

I'd say so, your propane run looks really good, if you post all the dimentions + the airspeed and the net thrust and weights we could tell you for sure. I think it looks great though, I think you "win a prize" simply through design, your the only guy with a ramjet that has an easily changeable flameholder, you should be damn proud of that right there :D
I'm still cringing at what the guys are going to say about my post!

EDIT: you changed your question above, and I don't know what it was earlier, it now a question about a circle. If you get confused working in inches, use google to convert it to millimeters and try that way. The area of a circle is ALWAYS PiR2. So if the diameter is 1 inch, then its Pi(0.5sq) :D

If you change your question again let us know :D

[redneck]

well since you asked her ya go. the intake difuser intrance diameter is 2.15 inches, the intake exit is 2.9. the intake difuser is 4.18 inches long, down the side. the burner is i inch long and 3 inches around. the exit difuser is 2.8 inches long and the exit is 2.3 inches in diameter. the bullet in the nose cone is 1 inch in diameter. i have 230 mph windsbut id say 200 cause of the distance away the blower is right now. that is all i know about it so far. i dont know the agles of my cones yet.

and naw i aint an aussie, im american, though i have been there once. nice place, and nice women too ;)

does it mater how long the difuser is? so long as it is the right diamter in the intake and in the exit diameter? i just did some math and if i were to cut my intake to the opening size it seems i need, it would be much shorter than the exhaust cone. it will be 2.5 in diameter and 2.5 long. the exhaust is only 2.8 inches long. will this be a problem?

[Irvine.J]

Redneck wrote:

i just did some math and if i were to cut my intake to the opening size it seems i need, it would be much shorter than the exhaust cone. it will be 2.5 in diameter and 2.5 long. the exhaust is only 2.8 inches long. will this be a problem?

Yes, unfortunately. The diffuser should be longer then the exit nozzle. However, you need to calculate what length you need, by the area ratio for the INLET of the DIFFUSER and the EXIT of the DIFFUSER. For example

The ratio of diffuser entrance area to diffuser exit area varies from three to four for most subsonic ram jet designs. The most widely used value being three and one-half.

So If your using no insert the diffuser length at 10* should be calculated to reach whatever distance required to get that ratio. (You'll be surprised its not as long as it seems)

The aerospike ramjets are kind of funny from what I have seen in regards to there area, but start longer and a slightly greater angle then the the "Default" and play with it from there.

Ok, so your airspeed is (roughly) 300fps, what you need to do now is get some sort of thrust reading on your propane atleast. Your operating velocity (300fps) mean that you should aim for maximum thrust a that airspeed, configure your ramjet for the airflow you can get immediately on the diffuser entrance and slightly around the outside of the jet. Effectively, your tuning your ramjet to the conditions presented to it. Until you start measuring the thrust there is no definitive answer as to how much thrust one can gain from a particular design. Alot of it is trial and error, try and get a net thrust reading as Larry mentioned, for me, i'm happy with 310/325 grams, which makes sence after taking away all the stand, airflow etc etc, pretty much works out for the net thrust I was seeing earlier.

[redneck]

well i know that the exhaust exit should be about 1 1/4 lager than the intake intrance, so instead of making one longer or shorter, what if i were to play with spikes in thew intake AND in the exhaust???

by the way, my intake has an intrance area of 2.85 inches square. the exit for the intake is 6.9 inches square. shouldnt this be pretty close? that is about a 2.5 to 1 ratio.