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Minuture Jet

12/18/2007 12:43 AM

dumb Question but what if you sized down say a Pratt and Whitney or R&R jet engine say 1/64th or so and what kind of thrust would you have equally less? just a thought instead of a 15000 ft lb of thrust only 1/64th of or more or less?

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#1

Re: minuture jet

12/18/2007 4:14 AM

Scaling down linear dimensions does not give an proportional scaling down of performance. At 1/64th scale, one wouldn't expect a steam locomotive boiler containing 135 steam tubes and 27 superheater elements to behave the same way as the full size one. After all, acceleration due to gravity isn't scaled to 1/64 just because the linear dimensions are!

In general terms, every time one reduces the scale by a factor of three, one introduces another major compromise to a model. At 1/64th scale linear dimensions, the jet engine might just melt into a blob, and burn up.

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#2

Re: Minuture Jet

12/18/2007 10:23 AM

Consider for a moment the simplifying assumption that the jet engine is a tube. If you halve the diameter, you now only have a quarter of the cross sectional area, so it seems apparent that you will only have a quarter of the thrust, not half. At 1/64th you will have approximately 1/1000 the area of the original......

Other factors that affect it are things like the viscosity of the air. As you scale things down, the air becomes relatively thicker (hence the use of dimensionless reynolds numbers when describing fluid flows).

Using Reynolds numbers you could scale down the engine to produce 1/64 the power, but it would be considerably larger than 1/64 the size.

As mentioned by others, you also have other issues: Most jet engine blades have air channels in them for cooling, if you scale these down they may not flow enough air to adequately cool the blades (the air is now relatively thicker, and the hole much smaller) and you get the melted blob mentioned by others.

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#15
In reply to #2

Re: Minuture Jet

12/19/2007 4:14 PM

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)?

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#3

Re: Minuture Jet

12/18/2007 11:27 PM

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

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#4

Re: Minuture Jet

12/18/2007 11:55 PM

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.

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#9
In reply to #4

Re: Minuture Jet

12/19/2007 8:34 AM

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).

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#18
In reply to #9

Re: Minuture Jet

12/20/2007 7:52 AM

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.....

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#5

Re: Minuture Jet

12/18/2007 11:57 PM
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#6

Re: Minuture Jet

12/19/2007 1:10 AM

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

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#7
In reply to #6

Re: Minuture Jet

12/19/2007 5:47 AM

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.

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#13
In reply to #6

Re: Minuture Jet

12/19/2007 11:47 AM

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.

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#8

Re: Minuture Jet

12/19/2007 8:08 AM

I saw the Coors Silver Bullet Jet at and airshow many years ago, got to talk to the pilot, (Corky Forman If I remember right) it was a small jet that produced 76 lb thrust. Corky had 1" clearence sides and head with the flight stick on the side, not room between his knees. wing span 9ft. full areobatic, flat speed 350mph.

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#11
In reply to #8

Re: Minuture Jet

12/19/2007 10:52 AM

Corkey Fornof - see http://www.youtube.com/watch?v=WDTgITsJA4U. It is a BD5J, originally designed by Jim Bede (hence "BD" in the model name, with "J" for jet). Another source of information is http://www.aerospaceweb.org/question/planes/q0256.shtml. Wing span is quite a bit more, and the thrust higher.

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#17
In reply to #11

Re: Minuture Jet

12/19/2007 7:06 PM

I have to stand corrected. Should not try to rely on old memory. However, ahemmm I am tring to figure the dates of my encounter, I was flying at the time, I think and the last I flew was 1967, Maybe a little after that possibly pre BD5J as it now stands. The low thrust figure has been stuck in my mind as trivia for all that time maybe I just heard it wrong. Got wing 9ft not wingspan maybe. I am pretty sure that it was not the Coors silver bullet at that time, that came later. Guess I'm off on a reasearch run. So much to do, so little time, darn, Anyway thanks for the links.

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#10

Re: Minuture Jet

12/19/2007 10:30 AM

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?

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#16
In reply to #10

Re: Minuture Jet

12/19/2007 6:03 PM

A jet powered bar stool....What fun!! I wonder what the coefficent of drag would be for the whole assembly (you and the stool).

Someone mentioned 17 pounds of thrust. Assuming you and the stool represented about three square feet of flat plate frontal area, a SWAG says 17 pounds of thrust ought to get you up to around 43 feet per second or about 30 miles per hour with the inclusion of an arbitrary rolling resistance figure.

How does one hang on to their beer and negotiate this terrifying vehicle wihile at speed LOL?

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#12

Re: Miniature Jet

12/19/2007 10:53 AM

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?

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#19
In reply to #12

Re: Miniature Jet

12/20/2007 10:54 PM

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

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#20
In reply to #19

Re: Miniature Jet

12/21/2007 2:06 AM

...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!!!

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#21
In reply to #20

Re: Miniature Jet

12/21/2007 10:13 AM

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

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#23
In reply to #21

Re: Miniature Jet

12/21/2007 2:14 PM

Beautifully well put, many, many thanks!

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#25
In reply to #21

Re: Miniature Jet

12/21/2007 3:10 PM

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

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#26
In reply to #25

Re: Miniature Jet

12/21/2007 8:56 PM

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

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#22
In reply to #19

Re: Miniature Jet

12/21/2007 10:26 AM

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.

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#24
In reply to #22

Re: Miniature Jet

12/21/2007 2:14 PM

Correct!

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#14

Re: Miniature Jet

12/19/2007 1:16 PM

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.

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