Motorjet

[larry cottrill]

Matt -

You need to check out this post in the Tools and Construction Forum:
http://www.pulse-jets.com/phpbb2/viewto ... sc&start=0
- scroll down a ways to where Al Belli talks about a really high temp material that is easy to get hold of. That's the kind of thing you need to make it really work well.

L Cottrill

[Al Belli]

Hi Matt,

Anodizing is for aluminum.
Black oxide is for carbon steel.
There is a black oxide process for stainless steel. ( expensive )

As for protection at pulsejet temperatures, I doubt that it would make much difference, but it might be worth a try.

Al Belli

[Al Belli]

Hi,

I don't know of anything that will withstand the high temperatures.

Al Belli

[larry cottrill]

Larry i know you reserve all the rights to this, and it would be unethical AND illegal for me to commercialize it, but if i make this thing into a good motorjet (or mabye even axial turbine! ill give it a shot...) can i sell it then? i mean i plan on modifying the design a fair bit (injection instead of aspiration, might change the flame can) and seeing as its going to be throttle-able, i need to work out a way to keep mixture at perfect throughout the range (ive got some ideas...).

could i sell it then? of course id give all of you guys credits too :D, i wouldnt stand a chance without your help!

ive always wanted to be an entrepreneur and this could be a potential way to start, but i want your approval first...

thanks -Matt

Matt, as far as I'm concerned, you can run with it. If you become so successful that your conscience starts bothering you, just send me a "royalty" check at the end of each sales year equal to TWO PERCENT OF RETAIL SALES (made out to Cottrill Cyclodyne Corporation). I consider two percent to be a reasonable royalty for any product developed from my original design, and a great value for any successful developer. This design was published years ago - I can't legally do anything to protect it anyway.

The only thing I WOULD object to is someone using my "Maggie Muggs" name (or anything too similar) commercially, as I consider that a trademark for that particular design. Same with my "JetZILLA" trademark, which I always intended to apply to various products that have yet to materialize.

Good luck! (Hint: It's going to be a lot harder than you think to pull this off, but if you're the kind of guy that has the grit to do it, go for it! ;-)

L Cottrill

[larry cottrill]

im not quite sure where to cut the mugs off at, and the intake is slightly smaller than the exhaust (but i can cut it so the ratios are different...)
any best intake/ exhaust diameter ratio? keep in mind this thing is meant for low speed, its powered by an edf so something like the 60mph is about ideal.

I'm no engineer, just an amateur designer ... but here goes: I think having the air inlet just a small percentage smaller than the exhaust outlet is the right way to go, at least for a low-speed design. My reasoning is that the air enters relatively slowly and at normal density - it exits the engine MUCH faster, but the density is of course much lower. Except for the minor addition of the fuel mass, the massflow (mass / second) is the same. The reason the outlet should be a little bigger is that the drag of the shell is a much more important restraint to hot gas than to cold gas! This is also the reason that the chamber/nozzle section should be as smooth a curve as possible (or just a long cone). In the front end (diffuser), the theory is a little different - there you want smoothness because you don't want separation from the side wall to break up the flow into turbulence before you get through the flameholder.

oh, and i keep hearing the thing is a beast on fuel and not designed to be efficient.

It really isn't a matter of design. What can be said is that BOTH theoretically and practically, it is impossible to build a slow-moving jet engine that is high in thermal efficiency (i.e. how effectively it converts chemical energy into useful work). This is why it was so important for things like turboprop and fanjet engines to be developed for the commercial aviation market. The best efficiency for a pure air-breathing jet occurs when its forward speed is HALF its exhaust velocity. With a low-speed jet (as used on a model vehicle), you can't even get close to that.

what fuel consumption figures did you get on yours?

Maggie has only been built and run as cloned by others. As far as I know, no measurements have ever been taken. It is a hog, by all accounts.

oh and, if by difficult you mean getting this engine to work and be good? ya.

In my opinion, here's what you're up against, at least (yes, some of this is arguable, but pay attention anyway!):
- The production engine MUST be liquid fueled by carburetion, without pressurization
- There MUST be IMMEDIATE, absolutely FOOL-PROOF fuel cutoff either if the fan fails outright or weakens significantly (loss of battery power)
- There MUST be IMMEDIATE, absolutely FOOL-PROOF fuel cutoff on crashes (any sudden cessation of forward motion)
- You MUST have an easy starting method worked out
- You MUST somehow provide adequate cooling between engine start time and vehicle launch time
- You MUST prevent lean extinction on sudden throttle-down events

if you mean starting a business... i have some ideas.
for starters (by the way, you'd want to consider this too) advertise on a remote control aircraft forum site. i have a membership for a few, and i know alot of model aircraft flyers who would be all ears if i mention jet engine for 150 (approx).

Well, that's a marketing idea. Marketing is important, but only about ten percent of the skills you need. One of your biggest problems will be balancing manufacturing with order fulfillment. Your second-worst nightmare will be service after the sale - without handling that properly, your reputation will be shot overnight. Your WORST nightmare will be product liability - sooner or later, someone WILL get hurt. And when that happens, you will find out quickly that your best-crafted disclaimers are meaningless compared to what some judge and/or jury think after being influenced by a given attorney on a given day.

its a great market here in north america. your located in the UK right? im not sure how the market is there though...

No, I am located in Iowa, USA. Yes, it should be a great market - for a great product with great service behind it. I can't imagine a powered model flyer anywhere in the world who wouldn't like to try flying a real jet (even if some old die-hards won't admit it). BUT, CONSIDER:

Why isn't everybody flying Dynajets or Tiger Jets? Not because of cost - that's reasonable for most people that are into powered models already. Not because they don't perform well - they do fine. Not because of noise (except where legally restricted). Not because of fuel consumption - to most hobbyists, that's practically a meaningless issue. Here's why, in my opinion: The hassle-to-fun ratio of jet powerplants is just too high. There's just too much effort to building models that will take the engine heat, weight and bulk and too much effort in having a crew that can handle starting and launching. The real trick to making this hobby take off is to reduce these difficulties - at least, the start-and-launch hassle. Once you get it as quick and easy (and safe) as getting a propeller-driven job running and in the air, you've got it.

L Cottrill

[larry cottrill]

so, the optimal thing is to channel the heat back. what substance has a realy good thermal reflectivity? or i could use a realy good insulating material around it, but that would affect cooling.

I think insulation would be counterproductive. What you want is a reflective shell that encourages airflow around the engine in flight. Thin sheet aluminum is almost always used around pulsejets, and works really well, when adequately spaced from the hot engine shell.

firstly i dont quite know how to get it to autoshutoff in a crash situation. any ideas?

You want a simple mechanical system - nothing that depends on electrical power, for instance. The simplest thing I have thought of is an ordinary spring-loaded mousetrap acting as the servo to shut off a miniature valve. The trap would be triggered by a small weight that is held back by a breakable replaceable 'shear pin' (like a piece of stick balsa spanned across the front of it). In a crash the weight keeps going forward, shears the pin and is guided to hit the trigger on the trap. The trap swings closed and instantly shuts off the fuel valve via a pushrod. Something like that.

and secondly i dont know what you mean by "lean extinction on sudden throttle-down events"

Your normal running mode is on "the lean side of the power curve" - a compromise between fuel economy and thrust. In flight at 100 mph, let's say you suddenly cut back on fuel. The fan and the incoming air will not cut back rapidly because of the forward motion of the vehicle, and you will suddenly have a mixture that is way too much air and not enough fuel, and the engine will die with fuel still flowing. The lesson to be learned is that you MUST throttle the incoming air, not just the fuel!

the one thing you brought up, that i havent thought about, was the liability! it never crossed my mind! well... what could i do about that? have you encountered such problem (someone made a pulsejet designed by you and got burned or something).

What did McDonalds do when they had a lady spill hot coffee on herself while trying to carry it between her knees while driving away? They paid, that's what. Then, they printed silly warnings to remind you that hot coffee might actually be hot.

i could potentialy make a trainer aircraft to go along with it, but that makes things harder. it would likely be one of the outboard engine planes

(http://en.wikipedia.org/wiki/Image:PST_ ... rainer.png)

but thats not too stylish. i find the biggest technical challenge will be making it a drop in engine that can be used where a normal ducted fan would be.

I have stated on the pulsejet-related forums that I think the ideal way to market a model jet engine would be to package it with an open-engine ready-to-fly model that would have a hydroplane-like fuselage that could be flown off water or wet grass. Since most of the public views model flying as a fair weather or summer sport, such a craft (if sufficiently compact in size) could have a very wide appeal. As mentioned, very simple start and launch systems would need to be developed and marketed as ancillary products. I would not even try to provide a fully enclosed engine design craft for broad marketing to the public. Of course, the engines would also be offered as standalone units for scale builders and such, who would need to be sophisticated enough to engineer a fully enclosed hot engine in a vehicle of their own design/construction.

L Cottrill

[multispool]

I think Larry covered that very comprehensively!
Regarding Liability, here in the UK and almost certainly in the US as well, it's the manufacturer that's ultimately responsible for anything nasty happening to the operator, regardless of disclaimers. It's their responsibility to enclose written instructions for the care, handling and use of the equipment, fuel types etc. If something blows-up then they will come looking for you!

The emergency shut-down device would also need a system that would shut off the fuel supply in the event of the radio going into failsafe and also a manual shutdown channel as a minimum requirement.

Matt, I think that your biggest marketing problem in the US is getting the engine AMA approved, else, I'm sure it won't be allowed to be flown.

Anyway, let's get back to the engine itself. Have you actually done any calculations? I am certainly not expert in Ramjets and would be happy to be proved wrong, but my calculations show that a Maggie Mugs size engine travelling at 300 mtrs:sec will produce about 28N of thrust. That's about 6Lbs in real money. Not only miserable but that's probably less than than just the fan on it's own running in ducted fan mode, and what air velocity are you expecting the fan to produce anyway?

Another complication which Larry quite rightly described is mixture control with varying fuel input. Gas turbines can be considered as a closed loop system whereas a separate fan stuck in front of an engine is open loop. Ie you have to control both fuel and air instead of the engine doing it automatically...

[Zippiot]

For all of you a little FYI
In subsonic ramjets there shuld be a much greater margin in size difference between the intake and exhaust!!
Also the faster the ramjet is moving the less of a "nozzle" it needs.

Look at pics of a pair of ramjets strapped to the X-15, they are long with little diff in intake and exhaust. There is a cone in the intake of the ramjet which speeds up and squeezes the air. The air then enters an expansion area where it is slowed by the flameholder. This creates more ehat and gets the air flowing at combustion speed (only a few meters per second as it seems). Subsonics donnot have this cone so the change in area of intake and exhaust must be more dramatic.

So in short, supersonics have a cone in the intake to squeeze the air as it enters making less need for a big diff in intake to exhaust ratio. Subsonics do not have this cone so there must be a bigger step between intake and exhaust.

A good high flow ducted fan unit can cost over 150 bux, and it must be linked with the fuel flow on the motorjet so they can be throttled together. No small feat on a tight budget but still possible...

[Zippiot]

Been in the sun too much today, words get all mixed up.

But yes you see my point. And one time I tried to explain to someone how the expanding air is really at a higher pressure, they looked at me dumbfounded (good thing engineering isnt their major!).

I noticed a massive leap in power and stability when I stuck a washer in my intake on the muggs. I will soon make a "real" ramjet, with a smoothe intake and such but the muggs gets the point across. The washer reduced the intake area to 1/3 its original size, maybe even less than that!

[multispool]

superhornet59,
I detect an uninformed arrorgance in your replies, I'm done with this thread!

[Najm]

I noticed a massive leap in power and stability when I stuck a washer in my intake on the muggs. I will soon make a "real" ramjet, with a smoothe intake and such but the muggs gets the point across. The washer reduced the intake area to 1/3 its original size, maybe even less than that!

Please post these leaps in performance (if you want to).
Why do you think this happened?Pressure increase or a decrease in wind velocity?

[Zippiot]

The jet never made more than 4 pounds of thrust, but oce the washer was in place it easily made 3 times the power than it had without it!
It was a big washer (maybe from a car?) and it fit snugly into the intake mugg, then I folded the metyal down around it to help seal it it.

I did notice an increase in "backfiring" when the flame shoots out the front and dissolves your fingers!!

[Zippiot]

It was just the thing to slow the airflow to combusation speed I *think*.

If you look closely at the cups there really isnt a huge area for expansion, and most leaf blowers dont exceed 200 mph (my shop vac can shoot 400 frm a nozzle). The washer decreased the intake and made a huge expansion chamber, also since its somewhat aspiriated it let the fuel air swirl around before hitting the flameholder. Still had backfires but much fewer.

[Zippiot]

I would say it'll make a fine muggs ramjet, but I doubt you will get any real thrust outta it. Also It will prolly blow apart unless somehow sealed.

That basic shape, though, is perfect for a muggs ramjet.

[Zippiot]

For 60 mph? None

For 200 mph, now we are talking. The lowest speed functional ramjet will *usually* be no less than 200 mph, 400 is better and 600 is preferable. Mach 2.7 is ideal for a ramjet, most efficient.

You can design a ramjet the size of a roll of paper towels to make 80 pounds of thrust at 200 mph (it has been done!) but for model plane use a ramjet wont work until you break the 200 barrier. Which isnt that hard as some props near 190 and many jets surpass 250!

But the title of the thread is motorjet, a motorjet will create thrust at 0 airspeed if made properly, and thrust increases along the functional range.