FAQ

0. Origin of the name "JetBeetle" ?

The name Jet Beetle is in fact inspired by a British scientific project which aimed on re-ignite failed jet engines after take-off by mimicking the defense mechanism of the African Bombardier Beetle.

The tiny yellow-and-black creature blasts a jet of boiling liquid at its enemies at 300 explosive pulses per second. The fiery potion, which reaches 100C, is made in a heart-shaped chamber less than a millimeter long and tipped with a swiveling nozzle on the beetle's backside.

No matter what might out come for this project, we shouldn't de-associate the words "Jet" and "Beetle", because they do relate, let alone a new company's name is called JetBeetle.

1. I haven't seen any RC or manned aircrafts use one of your engines as powerplant ?

Actually our engines are not in the RC category, as our engines are too big, nor in the manned aircraft category, as our engines are too small. Instead we are in the UAV category, applications like target drones. One famous target drone called the Banshee (originally designed by a British target drone company), originally design it was powered by two 40kg turbojet engines, they are replaced by a single H150, 70kg of thrust. Yes the newest Banshee version uses just one H150. And it flew very well.

Our turbofan engines are designed for manned applications, they will become reality very soon. RC applications ? no, thanks, our engines are too big. :-)

2. I have to say that I dispute the term "micro" as used in this particular case...

150lb engine is not of micro class, I admit it, take a look at this table (from "Revolutionary Propulsion Systems for 21st Century Aviation", by Arun K. and Jaiwon Shin, NASA/TM, 2003-212615) : Micro : < 10lb Mini : 10 - 100 Small : 100 - 1000 Medium : 1000 - 10000 Large : > 10000 According to this definition, MW54, KJ66 cannot be called "Micro" either, they are of "Mini" for sure. I think nowadays people call something micro is because it is relatively compact in size, not necessarily according to strict scientific definitions.

3. 2.5 Liters/Min fuel consumption pretty much rules it out for anything but bench testing...

Thanks for your comment, yes 2.5 Liters/Min shouldn't be a goal, it should be something like 2.0 liters/min or something more ambitious. If I am not mistaken, SFC for KJ66 is 0.154 Kg/N/H, mine is much bigger and have higher pressure ratio, there is no reason X-150 shall perform worse than that. OK, This is my new goal : SFC < 0.12 Kg/N/H, which is equivalent to 1.5 liters/min at max throttle.

Most engines operate at 70-80 % throttle condition or even lesser, which will consume much less fuel and save more money spent on fuel. During normal use the fuel consumption will be around 1.0 liters/min I believe.

4. Can't get to your web site ?

Yes, many people have reported to me that they couldn't get to my temporary hopersoft web site. A friend in UK has found where the fault lies for me; it's the domain name server (DNS) we should blame. I have discarded that DNS and adopted a good one, and our jetbeetle permanent web site has been online since 28 Dec '04, let's get to the new web site by using this link:

http://jetbeetle.com

Old links will be redirected to this new web site ! Sorry for the inconvenience caused. My crappy web site has been improved a little bit; I hope you can find something beneficial from it.

5. Alu material for the shaft tube in a design like this is wrong, it should be stainless, particularly with the critical ngv/shaft tube interface area ?

One of the many reasons I put the very detail of X-150 under the sunlight for public's inspection is that I wish I can have some input to my engine regarding the design or material choice aspects. I am glad my thinking starts to work. I have said that I am taking a risk of engineering change to this component, I certainly don't want it to happen. Let's take a look at some physical properties and mechanical properties of steel and aluminums first :

Material	Steel	Cast Al	Wrought Al
Alloy	Galvanized	A356-T6	6061-T6
Process	Sheet	P/M cast	Extrusion
Density (g/cm3)	7.8	2.76	2.70
Young's modulus (Gpa)	210	72	69
Yield strength (Mpa)	200	186	275
Tensile strength (Mpa)	320	262	310
Elongation (%)	40	5	12
Fatique strength(MPa)	-	90	95
Coef. of heat conductivity(W/m.K)	46	159	167
Coef. of thermal expansion (mm/m.deg K)	11.7	21.5	23.6
Melting point (deg C)	1515	615	652

In my rough calculation, the weight of shaft tunnel would add approximately 0.5kg, which is about 5% in total weight if stainless is the choice. So In most micro turbojet engines like KJ66, and MW54, they all use aluminum shaft tunnels for this reason. Certainly I want to keep the total weight of my engine as light as possible, so aluminum will be my first choice. However, there does exist the hazard of overheating of our tunnel at where the NGV/tunnel contacts. I am in fact thinking to add heat barrier between them, so that the hazard can be eliminated. We'll have stainless tunnels as an option for customer's choice.

6. What is the difference between Taiwan and Thailand ?

I know there must have some people can't tell the difference between them. Thailand is a nice place for tourists to have some fun, while Taiwan is an industrialized island for manufacturing some decent products. We Taiwan have nice industrial infrastructure, plus a complete metal/mechanical industry system; this is why I can build my engine at acceptable quality and low cost. People buy things made in China for one reason that is the extremely cheap price, but they couldn't get recognized in quality, while things made in Taiwan have long been recognized as something cheap and nice, Integrated circuit (IC) industry for example, Taiwan design/manufacture most IC chips in the world. I have used many machine tools (CNC lathe, CNC mill) made in Taiwan and had never got regretted.

7. Who are you and why you know the manufacturing method regarding turbines ?

Let me introduce myself. My name is Horace, T.L.Ho, B.S. in mechanical engineering from National Taiwan University (NTU), Taipei, In my career as a mechanical engineer, I have many years of experience in CAD/CAM, and manufacturing processes such as mold tooling, investment casting, sheet metal forming, welding, CNC machining, electric discharge machining etc.
After I started my jet engine business in 2005, I went back to school for learning theory of gas turbines and gas dynamics, rocket propulsion etc. I am member of the Phi Tau Phi Scholastic Honor Society of Taiwan.
In early stages I design/manufacture key engine components for other engine companies, such as Wasp H20's turbine wheels, NGVs and other parts. Through these years in developing my own successful jet engines (such as H25, H85, H150) now I can say I am 100% a micro/mini/small jet engine designer.

8. You clearly have an excellent grasp of English, the grammar on the web pages lets them down.

Well, Hehe. English isn't my first language, in fact, it's not my second language either. I as a kid spoke Taiwanese first, after went to school I learned to speak Mandarin. They are so vocally different so when two parties meet they cannot communicate with each other if not through written Chinese. The best English teacher should be English speaking people, but I regret that I didn't choose to go abroad for my undergraduate study, neither for graduate study. As a result I am speaking "Tailish" (a dialect combining Taiwanese and English), instead of pure English :-) Yes I will improve my English grammar, please give me some time, before I have grasp of pure English, please tolerate my Tailish for a while.

9. The next move in twin spool technology will no doubt be towards the two stage centrifugal compressors to get the compression ratio up and therefore the fuel consumption down which I think would be the most desirable result for any model flyer in particular.

Very appreciated your input, good points, except one thing, that is, the compression ratio.

I am thinking of what compression ratio would cause a TIT higher than which inconel 713 could withstand, and end up homebuilders have to seek one or more of these expensive technologies :

1. Directionally solidified structure,
2. Single crystal,
3. Thermal barrier coating,
4. Turbine internal cooling,
...

I do think item 1-4 are exclusive games for those multi-million dollar aviation industrial giants only. I don't know, maybe I am wrong, but a compression ratio of 4.0-5.0 could very possibly be the redline
homebuilders should never trespass. I wish someone could correct me or confirm for me. Any input would be very appreciated.

10. Jet engine basics ?

1. A jet engine is a machine whose purpose is to increase the momentum of the air passing through it.
2. Increase mass or increase velocity or both, to get large momentum.
3. Both C/R and TIT are crucial factors to thermal efficiency, so higher C/R and higher TIT are preferred.
4. Experience tells us, for an engine with single staged compressor/single staged turbine, always select a compressor with exducer about the same size to turbine.
5. For centrifugal compressors of specific exducer size, bigger trim size, higher air flow but lower C/R; smaller trim size, higher C/R but lower air flow.
6. Once the compressor is selected, for a specific pressure ratio, to keep the compressor operating at optimum condition, there is only narrow range of air flow value allowed, check the compressor map
please.

11. Raising TIT is the path to higher thrust, not higher efficiency.

Thermal efficiency of a practical turbojet engine is a function of PR, TIT, CIT (compressor inlet temp.), and several other factors, please check any engine text books. People tends to get confused on why this is true, since the thermal efficiency for a Brayton cycle is a function of PR and K (specific heat ratio) only.

The reason is, Brayton cycle is so idealized, in a practical engine the assumption of isentropic compression and isentropic expansion never occurs, instead, entropy increases for both processes. This introduces TIT and CIT into the formulation of the thermal efficiency.

If we examine the complex formula, we'll find that higher PR results in higher thermal efficiency, no doubt. In addition, we'll find that lower CIT and higher TIT tend to improve the thermal efficiency too.

12. Could you show me the picture of X Ray examination for your wheel?

Yes of course, please check the article named "turbine".

13. If you compare 713C and 718, what are the pro's and con's of these materials?

There is nothing special for nickel based super alloy in room temperature (specific strength a little bit higher than steel though). What we need is its excellent performance on strength and creeping resistance in high temperature. Turbine made of Inconel 713LC can withstand TIT up to 927 deg C, while for 718 it is only 704 deg C. 713LC is more expensive than 718. Use 718 unless TIT is higher than 700 deg C.

14. Balancing calculations ?

According to ISO 1940, turbine is of grade G2.5, under the speed of 75000 rpm(1250hz), the eccentricity of mass center allowed is

e = G/ω = 2.5 / (2 x 3.14 x 1250) = 0.000318mm = 0.318 μm

Mass of my turbine wheel = 650g, the unbalance amount allowed is

M x e = 650 x 0.000318 = 0.2 g-mm

m = M x e / r = 0.2 / 33 = 0.006g

where r is the radius of sacrificing ring.

In other words, to be in accordance with ISO 1940, the residual unbalanced mass at sacrificing ring must be less than 0.006g. Following is an aluminum chip, 0.25mm of thickness, 0.006gram of weight. Imagine the case of inconel, this chip will be of 1/3 of its size only!

15. Static balancing vs dynamic balancing of a rotor ?

If carefully done static balancing could obtain smooth running shafts as well. Thomas Kamps' book (Model Jet Engines) demonstrated a static method you can take of look at it. It needs only simple tools so the cost is very cheap to have a rotor balanced. However it needs practice to obtain repeatable results. Dynamic balancing requires expensive equipments (mainly accelerometer and analyzer) and you can have reading of how much residual imbalance mass is there in the rotor, and the repeatability is very nice. Jetbeetle is developing a dynamic balancer wishing it could be shipped soon (at low price of course).

16. What's with the P2?

A quote from Bruno Ogorelec's article (http://www.pulse-jets.com/):

"A jet engine is a machine whose purpose is to increase the momentum of air stream passing through it".

So increase the mass or velocity or both. Increasing CDP is the best method, as higher the pressure, denser the air, thus higher mass flow is delivered. If you read any text book of jet engine, you'll find that the Brayton cycle is the idealized jet engine cycle, through the analysis the thermal efficiency is directly related to pressure ratio p2/p1. Higher PR, better fuel economy. If we remove the assumption of isentropic compression/expansion which Brayton cycle used to simplify the formulation, T1 and T3 will also be factors affecting the thermal efficiency. Lower T1 and higher T3 will help improving the performance of a jet engine.

17. Is preload in the bearings used to limit the turbine speed ?

No.

Preload is used to remove radial clearance which has bad influences on bearing life as radial clearance will cause vibration and noise.

So it is preload that made the bearings to last longer in high rpm.

Radial clearance of course refers to clearance in the ball races. The existance of radial clearance is a big problem in high speed, especially when we are using the ball race bearings to run far beyond its rating RPM. We are saying to run at 150-170K RPM.

The axial force on the bearings is not used to limit the RPM, on the contrary, axial force must be applied adequately or bearing cannot go up to such high speed. If axial force is too big of course it will act like a brake so a lot of heat will be generated, and too much heat will cause the bearing to fail.

Use throttling to the fuel pump instead of using axial force in the ball races to limit the turbine speed.

18. What's the composition of steel En24T, which is commonly used in shaft application ?

BS970 (1955) En24 is a 1.5% Ni-CrMo steel. C 0.35/0.45, Si 0.10/0.35, Mn0.45/0.70, Ni 1.30/1.80, Cr 0.90/1.40, Mo 0.35% max, S & P 0.050 ,max.
Condition T (with a limiting ruling section of 6") gives a min yield of 44 Tsi and a min UTS of 55 tsi.

19. I eagerly follow the progress of your jet engine development activity, but very recently I have seen that the website jetbeetle.com links to some “zoological” trade activity, instead of your more familiar “jet site”.

That problem was due to a bad reputed domain name reseller registrar, when the name was still under their management. Now the problem is gone as we have found the original registrar and moved the name to them. jetbeetle.com is our permanent name and will always be !

20. I hope you and you family are well, nobody has hurt in the southern Taiwan earthquake 26 Dec. 2006 !

Yeah quite scary but we are still kicking. It was 8:26 PM and 8:35PM we had two big earthquake strikes. It was of magnitude 7.2 and 7.1 on the Richter scale respectively. The intensity of shaking was grade 4 (25-80 gal), we experienced a small flood on the floor of my living room due to water spilt out (25%) from my fish tank. My pet fishes were scared but they are ok too. I am very proud of the DIY fish tank, it has been proven to be able to withstand earthquake of intensity grade 4. Maybe I should open a new business to produce earthquake resistance fish tank. :0)

21. What's the part number of matching compressor for your 55mm turbine wheel ?

You can use Garrett 446335-09 T25. We will try to sell this part in year 2007.

IF you are interested, the compressor for X-150 will no longer be ST-50, it will be out of our own design, pressure ratio up to 5.0.

22. Does anyone use a small (mini) turbine engine to turn a generator for power?

Yes there are small gas turbines used in distributed power generation applications. Our small turbine engines are made specifically for propulsion applications, if it is used for generator, the exhaust gas is still very hot (around 500-600 deg C), the energy must be recycled or the total efficiency will be very poor. This is why generator systems based on gas turbines generally also produce hot water.

23. Why the turbojet thrust curves going up for higher speed but turbofan's going down?

Please check Thrust/FC Curves, to understand this question is all about.

That's a good question. if you look carefully, both engines curves have a minimum, turbojet at around 0.4Mach, and turbofan at 0.7Mach, after which points the both curves tried to rise, just that the turbojet rise much higher than the turbofan. So it is bad idea to fly at 0.4Mach for turbojet and 0.7Mach for turbofan.

When the aircraft is moving forward, the forward speed reduces the thrust due to the exhaust speed is now smaller relative to ambient air. This is so called ram drag effect, but at the same time, forward speed of air causes more air to get into the engine, which helps the engine to get more thrust, this is called ram recovery. These two effects are fighting with each other.

For both engines, before the lowest point, the ram drag effect dominates, after the lowest point, ram recovery effect dominates, just that for turbojet the ram recovery is very obvious, but for turbofan the ram recovery is not so obvious. This is why turbojet is used at high speed applications, even supersonic, more suitable for high speed flights, yet turbofan isn't.

Turbofan design is a trade off between amount of air moved and exhaust speed, that is to move larger amount of air for more thrust with reduced exhaust speed, and turbojet moves smaller amount of air at higher exhaust speed for more thrust.

24. What is the typical flight altitudes for the experimental jet ?

Turbine engines burn much less fuel at altitude, please check the FAQ#23 for charts. So to fly high is the goal, but for higher altitudes the air is thin, you must fly fast in order to get enough lift. The practical cruise altitudes for experimental aircraft ranges between 8,000 ft to 12,000 ft without oxygen. duration Flights up to 17,000 ft requires Oxygen. Flight up to 25,000 ft requires different set up like pressurization. We are looking at Flights such as 16,500 and 15,500. these altitudes will require an Oxygen bottle.

25. For a pilot what's the main factors affecting the turbofan engine's SFC ?

I would say it's the air speed. Different altitudes do affecting the SFCs, but only in minor ways (see figure below). Air speed on the other hand, SFC increase as air speed increase ! I think the reason of this is due to what was explained in FAQ#23. But you can't fly very slow such as below 0.2Mach for low SFCs, you have to reach your destinations as fast as you can, so we have to optimize an air speed. I would suggest 0.4-0.5Mach for HGF500. So the optimized air speed should be in the range of 265-330knots.

26. The story of ARX dynamic balancer

Many years ago I was in need of a dynamic balancer, then I enquired Schenck, the well known Germany balancer supplier, they offered me a balancer with very small rotor stand, yet at over 50KUSD. My engine is very big, can't fit in their machine.

Then I bought a Chinese balancer instead, at 6KUSD, but it was a junk, not working. And I bought from a Hungarian supplier, sound card based sensor, it is also a junk. These frustrations gave me the motive to develop my own balancer. And now our ARX2 has became standard equipment in our shop. We simply can't live w/o it.