Minuture Jet

What about the other direction? Is there a limit on the maximum size of a single jet engine (other than those imposed by the materials of construction)?

You have any Ideal how many parts are in a jet Eng.

http://www.quicktechhobby.com/Engines/QTH%20JET%20ENGINES.htm

There have been miniature jet engines flying model planes since the early 90's. In fact, a scale B-52 has been built and crashed. The builder of that now has a C-19.

This all started with a German Model Engineer who published a book on making your own jet engine.

They also have turboprops now. But nobody has done an afterburner as far as I know.

In fact, there have been miniature "jet" engines since the 1950's, specifically the Dynajet. $35.00, which was alot then, got you one. Today you can search the net and find a handful of miniature jet turbines that will produce 25-35 lbs. of thrust. 1/4 scale planes fly at 200mph+ (realtime speed).

These are really Pulse Jets, they have more to do with the V1 pilotless flying Bomb engine than a modern jet engine. They are as far as I am aware still made by people and companies.....here are a few links that you should find interesting, especially the YouTube film of a 230MPH twin Pulse jetted model aircraft, the cameraman had problems keeping the plane in view!!!

The sound shows the differences immediately and the almost complete lack of moving parts means that reliability is high and building costs are low.....fuel consumption is not low though, but as this is not a problem for most model makers anyway....who cares!!

http://aardvark.co.nz/pjet/gokart.htm

http://en.wikipedia.org/wiki/Pulse_jet_engine

http://www.youtube.com/watch?v=1e-Ar7gb4tQ

"Crappy Isthmus" to all readers.....

http://www.burgerman.info/jet.htm

http://www.quicktechhobby.com/Engines/QTH%20JET%20ENGINES.htm

the best example for last at 1/6^th scale

http://cpl.usc.edu/eschuste/

joshua

Another thing to take into account: the limits on a jet are heat and centripetal or centrifugal forces. As the diameter shrinks the rpm can increase proportionately.

Car turbos run at speeds that would instantly destruct a commercial jet engine.

Don't know about some of the military non-bypass jets made of unobtainum.

One jet that fits in your hand makes something like 17lbs thrust.

That is a little much for a model.

Brad

Hi U V.

I can buy a small turbo jet engine at my local model aircraft shop, they utilise the spindel from a diesel powerd lorries turbo.

Spencer.

As the diameter shrinks the rpm can increase proportionately.

It's interesting you bring that up. Many, many years ago, I knew a man who was in business performing micro-machining operations for the semiconductor tool industry. He reported (and I did not confirm) that he had either recognized himself or been informed of an equation that allowed him to decrease op times while maintaining tool life by increasing machining RPM's by a factor of 10 along with an increase in cutting speed and a reduction (slight) in d.o.c.

I don't know first hand of the truths or exact formulations he derived but he (a rather young man at the time) was well known in the area for his work.

These mini jets are so COOL, if only I had a use for one. It has opened my eyes to something I was not aware existed.

However, the original question was about scaling down a commercial jet engine. In the same vein, I'm sure scaling these up to those sizes would not work well either.

I think it is apparent, that whether the jet fits in the palm of your hand, or powers a 747, it must be designed for that purpose, and not mearly scaled up or down from another design.

Hey, I just remembered seeing some nutters that race bar stools, wonder if I could put a jet engine on one of those?

Just a mildly related thought ...

Scaled down versions of anything are not truly scaled down. One way to obtain a truly scaled down version of a jet engine would be to use a machine like the one in Hunny I Shrunk the Kids. Unfortunately, anything that was shrunk by that method would not work. Material thickness seems to never be taken into account in those types of scenarios. For instance, if there was a component made of 1/8" steel. To shrink that down to 1/64 scale would result in a material thickness of less than 2 thousandths of an inch, which is not much thicker than some types of gold leaf. Obviously no components of a mini jet engine could be so thin. So that's just a warning to anyone considering shrinking a machine by such a method.

The other alternative is to shrink component size, but not shrink material thickness by the same ratio. Doing this would throw off all relative sizes between individual parts, as they are specified in the original design. Things would not work the same way.

So therein lies the dilemma. Is a true scaling down, to that degree, of any mechanical components actually possible ... while still remaining functional?

Sir,

The definition I read regarding gold leaf states its thickness is 4-5 millionths of an inch, or 0.000005" thick. That is a factor of 400 thinner than your shrunk steel thickness. I suspect that material strength would allow wall thicknesses to be shrunk at the same scale as other dimensions.

--JMM

...but the strength is gone completely even if it were possible......furthermore, steel and other metals are not as "ductable" (which i believe is the correct word!!) as gold, none are.........

Gold can be (and is!) made so thin that aircraft windshields are "see-through", but the gold can carry enough electrical current to stop icing!!!

I think the word is "ductile" and you are correct, gold is the most.

One thing the previous posted has neglected is that structural strength is not just a function of the base material strength, but also a function of component size and geometry.

Simplest case is a beam. The rigidity varies with cube of the depth of section (bd³/12), so if you double the depth of the beam, it becomes eight times as stiff (all other things being equal).

In the same manner, if you half the depth of section (using a 1/2 scale component) it is eight times more floppy (to use the technical term!)

Consider a sheet of paper held at the edge, if will bend under its own weight (so the creases run away from you) and see if it flops!

Hope this helps explain why whether gold, steel or titanium, material strength alone is not the whole answer!

IPG

Beautifully well put, many, many thanks!

Oops

Reconsider the piece of paper!

It should have said

"Consider the piece of paper held at the edge, it will bend under its own weight. Now fold it into a "Z" shape, hold it by the middle of the "Z" (so the creases run away from you) and see if it flops!

A little too much Christmas cheer on my end I think!

Happy holiday to all.

IPG

......and from me a A Very "Crappy Isthmus" to all readers too......

Yes, typical gold leaf is measured in millionths of an inch. But there are many types of gold leaf, and many thicknesses, suited to different purposes. 4-5 millionths of an inch is suited to fancy desserts and consumption. It will wrinkle from a slight breeze. Others are much thicker.

In any case, I believe I will still stand by my original statement that most steel components scaled down in every way including scaling down the thickness, to 1/64 scale ... could not function due to the material being too thin.

Correct!

There are many more problems in scaling real-world engines of almost any type.

Those include the aforementioned structural issues,

Fluid dynamics don't scale well. All sorts of problems with fuel metering.

Electricity, especially high-voltage, doesn't take to scaled-down components either.

Most scaled models are externally to scale (mostly) but internally, if left to scale dimensions, frequently won't operate or operate poorly.