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Posted July 11th, 1998

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I had hoped for more input on both sides of the coin on this issue. However, after a few requests from a couple different sources with non responce, I just gave up and posted what I have.
When it comes down to it, which do you feel more comfortable with, a 2 cycle internal combustion engine which has it's own set of problems or a surplus gas turbine engine from an APU..... Nether one seems to have been designed to power a man carrying air craft from the get-go. So which one do you put your butt under?
Your comment welcome and may be posted.

Glenn Ryerson wrote:
> Thanks for your eM Reg.
> Can't tell, which side of the fence are you on.... Personally, I not in
> live with eather.... But I have flown my 582 powered Angel 70+ miles
> from my home with out a skip of a beat..........
> Cya, Glennnnnnnn
> Reg Wilcox wrote:
> > Glenn thanks again for all the fun and excitement.  I must make
> > another
> > contribution soon.
> >
> > Now for the tough one, would you rather have an APU turbine or a
> > Ski-Doo
> > snowmobile two-cycle engine in your chopper?  A private confession is
> > OK.
> >
> > The 1989 Formula Mach 1 Ski-Doo sled debuted using a 583 Rotax engine
> > with a 76mm bore and a 64 mm stroke (sound familiar).  The 1992
> > Formula
> > Plus Ski-Doo sled sported a new 582 Rotax engine, simply a toned down
> > 583.  The 582 retained the 76x64 dimensions, and my brother has one in
> >
> > his sled just up the road.  I have often thought that all one needed
> > was
> > a few quick change connectors and presto, a multi purpose power plant
> > to
> > fulfill your every dream!
> >
> > Seriously, the quality of the engineering used to apply the engine to
> > the chopper and not the geneology of the engine is the important thing
> >
> > here.  Your 582 history and the reports on the turbines both sound
> > fine
> > to me.
> >
> > Best Regards, keep smiling, and don't let them get you down for more
> > than a few minutes!  Some folks are just like that.        Reg

Glenn, it has taken me a long time to answer this.

I am an electrican engineer not a mechanical engineer, but here is my
$.02    I know next to nothing about turbines, but go Karts in the 60's,
chan saws, and then snowmobile two-cycle engines have always been my
favorite source of noise and relaxation!

The helo power plant application issues in my opinion should be: initial
cost, operating cost, TBO, failure modes (and the ability to avoid
failure with good prediction capability like EGT, etc.), operation over
altitude ranges expected to be experienced, and impact upon power
train/ariframe.  A good spreadsheet giving accurate DATA in each of
these areas would be interesting.  I have no data on any of these;
however, the transmission temperature ranges reported by Rinke on his
M500 conversion were very interesting.  Heat is the enemy of lubricated
mechanical systems and transmission life could be greatly affected by
the higher temps reported for the Rotax 582.  Airframe cracks is also a
concern which  was reported by Revolution to be linked to engine
vibrations.  It sure would be interesting if Renke could apply stress
gages to his airframe and report the results of that work on the Rotax
582 and the turbine.  The temperature data convinced me that Rinke does
a great job with engineering analysis of the complete system.  I would
hope that the strain guage work would be his next area of interest.

Your promise of a turbine vs two-cycle section on new stuff is another
great service for your intersested readers.  Keep up the good work.  How
about another section on the care and feeding of a two-cycle power
plant, or pointers to material already available.  Your bird is doing
well in this area, what are you doing???

Best Regards,    Reg

 Why you can't use APU's for powered flight.

This is a discussion of the merits of using Turbine engines not designed  for aircraft, or APU type conversions in different  applications. As you can see, there are many good reasons to think seriously before jumping into such a project. What follows is a brief discussion provided from Andrew Russell, as culled from rec.aviation.homebuilt newsgroup


There has been a lot of discussion on the use of modified APU and  turbine starters for use in homebuilts here lately. As an engineer in the turbine engine industry, I feel I need to add my own (humble) opinions to this topic. I don't want to sound like some arrogant SOB by saying "I'm an expert, believe me when I say this is realy dumb idea". I hope by giving some background (and I apologize for this post's length) that the readers of  r.a.h might get an appreciation of what a complicated problem statement this is and why it is not a good idea.

Adapting an APU to be a turboprop is a much more complex problem than simply designing a new gearbox. Turbine engines are designed for very specific duty cycles. The duty cycle for
an APU is very different from a propulsion engine.

Running a turbine to the wrong duty cycles can be a recipe for disaster. Here are just a few examples ( and there are many more ) of what can go wrong (I might add, that during my career I have learned many of these lessons by experience):

1) All of the blades and vanes in an engine have natural vibratory frequencies, which when excited result in very high stresses. These natural frequencies can occur in the normal operating
range of an engine, and when they do, failure due to high cycle fatigue can occur in a very short time. I was running a vibration survey on an experimental engine and had the compressor shed all of it's blade tips in a very spectacular fashion after dwelling on a resonance point for less than two minutes. There are certified engines out there with resonance points in their operating range. For example, an APU may run at two or three fixed rpm's (such as IDLE, No load and Max Load). There could be a resonance point between No load and Max load, but since the
engine never spends any appreciable time at that point, there is not much of a problem. However, if you aren't aware of  these problems, you might just wind up with your cruise power setting operating at that resonance point with disastrous consequences.

2) A small change in the turbine inlet temperature can have a big impact on the life of the turbine. A rough rule of thumb for current engines is that an increase in turbine temperature of 25F will reduce life by half. You could easily mismatch an engine and get this type of temperature increase. Especially if the engine is controlled by exhaust gas temperature (and most are), you could be extracting more that the design amount of horsepower or operating at some off design condition and the engine would run right up to the EGT limit and but the turbine would see a higher than design inlet temperature with the resulting loss of life (the turbine's life that is).

3) The environment in the engine is very hostile. The temperatures in the turbine can exceed the melting points of the metals and parts are routinely operated into their plastic range. As a
result, most of the critical components (disks, blades, vanes, etc.) are life limited due to creep, fatigue or stress rupture. After so many hours or cycles of operation, they must be scrapped. If you don't have any documentation on the engine, you have no idea how much life it has left in it. In some cases, the damage to the parts is not easily detectable, which is one reason the life limits are imposed in the first place.

The energy released by an uncontained disk failure is truly amazing and as I once watched one from the relative safety of a reinforced concrete control room, it still scared the hell out of me.

4) Another thing to consider is that the design and certification requirements for APUs are different that propulsion engines. For example, an APU has a much smaller design "flight
envelope", since it is primarily used for ground operation and is not required to operate in some of the more extreme flight conditions that may be encountered. Imagine flying your homebuilt turboprop at 300+ kts through IFR conditions, rain and ice and realizing that the powerplant was not designed to operate in that environment. If an APU failed in such a situation, it would be merely an inconvenience, while the same failure on a propulsion engine would be a little more serious.

I can sympathize with everyone who wants to build a 300 kt turboprop powered plane, it would make one cool cross country plane. But there aren't that many good choices of powerplants.

If you really want to do it (safely), get a certified turboprop engine.

If the general aviation industry ever booms again, maybe there will be a market develop for a small GA turboprop. As a hobby (!), I've toyed around with laying out some flowpaths and
performance cycles for some small 200 - 400 HP turboprops and have convinced myself that they are feasible. But until there is a big enough market to convince an engine manufacturer to invest the hundreds of millions of dollars to design and certify such an engine, we'll just have to be happy with our recips.

Mark Johnston
Sr Engineering Specialist
Military and Rotary Wing Enterpris
LT101 Engine Project
AlliedSignal Engines
Phoenix AZ

 Let me whole hartedly agree! I am not a turbine engineer, nor do I play one on TV. However, I was the chief engineer in charge of a program to utalize an existing APU turbine in a military
smoke generation system, a job much simpler than a turboprop as the engine would run only at one speed, is not subjected to gyposcopic loading due to aerial manuvering, and operates in a nice shockmounted, filtered engine compartment. With both the engine manufacturer and the development prime contractor working on the program it took 3 years, over a million
dollars and unintentionally destroyed 8 engines before all of the major problems were resolved (several minor but livable ones remained). We had ALL of the mentioned problems and more. We had vibration failures, thermal cyclic creep, combuster can burn thru, nozzle coaking, gearcase lubrication failures, blead air controler instability, cold and hot starting failures, main bearing failures, main shaft fatigue failures, and a hostmore. All in all most APUs are single design point devices that are VERY expensive to re-engineer to broad band


If the Solar T-62 APU turbine is so against common sense then why is BD-Micro Technologies Inc testing one in the BD5-T? Or is there a significant difference between the T-62 and the
T-62-2A1 that I'm missing?

I'm surprised that someone who appears to work with turbines should be so generally against using them in homebuilts. I doubt Turbine Design Inc is a subsidarly of Lycoming so there must be another reason. What is it? .Sometimes I get a similar puzzled response when I mentioned to
clubaviators in the UK that I wanted a homebuilt that can cruise at over 300 mph. "Why on earth would you want to do that?", they say. Do I needa reason?

Douglas Wagner
Mr. Wagner,

Some of the homebuilders (we get calls from them all the time) have wild ideas about what is safe. You can get on a soapbox, and preach to me about the grass roots homebuilders spirit
wright brothers ect. but the fact remains, these engines were designed to run at a constant speed. Controlling the engine makes a big difference when you casually stick it on an airplane. Have
you ever engineered a fuel control???

I have, and I can tell you, that the design and proper testing could take years. Many years.

There seems to be an undercurrent of missunderstanding about these small turbine engines. Just getting the engine to drive a prop is quite a challange. Lubricating the gears in the gearbox properly can take months. Months turn into years, and so much for your project. (How many unfinished homebuilt planes are taking up good garage space)? We have thought long and hard about designing an application for one of these small APU engines. (Turbine applications happens tobe what we do:)

If you want to have a safe plane to fly, and you want to finish the plane in this lifetime, quit dreaming about APU constant speed engines.

Douglas Karlsen

Turbine Design Inc.


 There is also another turbine unit some guys are putting in a KR2.

 It is made by Allied Signal and is called a JFS100.

 This one has twin turbines, one to operate the compressor,  the other drives the gearbox. This way, an adjustable prop is not  needed, (similar in design to the Pratt&Whitney PT6).

Comments? Opinions? Suggestions?

 Sidney Rhodes

LongEZ 19SH

My comment is that putting a JFS100 in a KR2 is not a good idea. The JFS100 is a Jet Fuel Starter and is used to start the engines in an F-15. The unit has a design duty cycle optimized for many short duration start/stop cycles and is not designed for operating for any appreciable
duration. Putting this unit in an airplane as a propulsion engine would subject it to more abuse that it was ever designed for (and it dosen't do all that well in the F-15 as it is). Stick with a converted car engine, it'll be a lot safer.

Mark Johnston
Sr Development Specialist
Military Engine Enterprise
AlliedSignal Engines
Phx AZ


Bart Hull wrote:

Could you provide more info on the Solar turbine. Who makes it, thrust, cost weight etc.

Short answer - don't even think about it. A few individuals have tried to install small gas turbine APUs built in the 60's by Solar Turbines, Inc. (now a manufacturer of large industrial gas
turbines). At least one of these people is dead as a result.

These are shaft power engines designed for ground start and electrical power on military helicopters.

Here's a repeat of a message I posted last March:


How well does a Solar (or any other) APU work for aircraft propulsion?

Poorly, very poorly.

A bit of history. Solar Turbines sold off their APU line to Sundstrand in 1985. What used to be Solar's Radial Engine Division, then Solar Turbomach, then Sundstrand Turbomach, is now
Sundstrand Power Systems. I used to work there in the Development Test department. A few years ago, the rights to build and overhaul a lot of the old APU designs that are no longer in production were sold to a company called Alturdyne.

The most common Solar/Sundstrand APUs that are floating around in private hands are probably the T-2 series (95 hp) for the CH-47 or the T-11 (75 hp) for the CH-46. Both of these are long out of production.

Sundstrand absolutely will not provide any support of any kind for any engine in private hands. If you contact them, all you will get is a letter from their lawyers telling you that. Ditto for
Solar Turbines. All these engines are constant speed gas turbines, with a single radial compressor and a single radial turbine. Speed for most of these old APUs would be about 60,000 rpm. 'Constant speed' is even a bit of a misnomer. These things are really single speed engines. Any attempt to run them off their design speed canresult in overtemperatures, obscenely poor fuel economy, flameouts, and a number of other side effects, all bad.

Trusting your life to one of these engines is not wise. You have a lot of energy stored in the rotor spinning at 60,000+ rpm. A significant overspeed could result in a catastrophic failure (rotor
burst). Safe operation of these even as an APU requires regular overhauls and inspections for turbine cracks, bearings, lubrication, fuel controls, etc.

Any engine likely to be in private hands is also likely to have not been properly maintained or inspected for a long time.

Small gas turbine APUs are neat to work with. They provide high specific power density (small volume, low weight, high power - great for APUs) but with poor fuel economy and high capital
expense. They are not designed as an aircraft powerplant.

Andrew Russell


Small gas turbine APUs are neat to work with. They provide high specific power density (small volume, low weight, high power - great for APUs) but with poor fuel economy and high capital expense. They are not designed as an aircraft powerplant.

We used some AF issue types in Turkey, hooked to A/C units. Cost us $600/day in fuel to run. One ran low on lube oil and cooked expensely. Impressive to start, loud when running.


Michael Brown wrote:

The T-62 is used in the Army's UH-60 BlackHawk helicopter as an APU. Other specs are:

The 'T-62' is a class designation that is on nearly all the older APU's built by Solar/Sundstrand. You need the full model number to identify the APU. Here are a few of the Solar/Sundstrand APU basic specs:

APU Model Application Max Bleed Air PPM Max SHP

T-62T-2A CH-47 0 95

T-62T-11 CH-46 0 75

T-62T-16A2 CH-54 0 75

T-62T-40-1 UH-60 (Blackhawk) 72 90

T-62T-40-8 F-16 Jet Fuel Starter 0 230

T-62T-40LC-2 KC-135R 150 40

Any model numbers that have 'C' alone in the model number are for commercial applications. Thus a T-62T-40C4 is an APU for the Fokker F-27. The T-2 and T-11 have been out of production for over twenty years. Solar (and Sundstrand) will not provide *any* support to old military APUs in private hands, for obvious legal reasons. Solar

Turbines sold off their APU product line to Sundstrand in 1985, all Solar makes today are industrial gas turbines and related products.

Andrew Russell

former Sundstrand test engineer

I hate following-up to my own post, but I realized that I had neglected to include some other info. There is a series of Soviet designed turboshafts of small size, suitable for use in homebuilt turboprop designs.

First, the Isotov GTD-350, manufactured by PZL in Poland. This is normally used to power the MI-2 helicopter in a twin engined installation. 400 Shaft Horsepower, weighing in at 298
pounds dry. Length is 54 inches, diameter 24 inches. Neat engine.

Second would be the Czech Walter M 601B, a bit larger, 690 SHP, 65 inches long, 24 inch diameter, 425 pounds. The power-weight ratio of these is excellent. I'd love to see a high
performance homebuilt done with either one.

These are both available, and are first class designs. They are remarkably cheap, compared to western designs (I sell the complete MI-2 helicopter with TWO of the GTD-350's in it, all freshly overhauled, for about $50K, so you can figure out what engines alone are worth).

How about a 7/8 scale P-51 Mustang type all-metal or composite design with a M 601B in it? What a scream it would be. A mini-Tucano would be a great design, or a 7/8 scale Pilatius PC-7. The possibilities are endless....How about a Lancair with one?

Then there is the Soviet APU which also gives 400 pounds of thrust at  takeoff, Used in some of their turboprop designs, not all that big, and providing mega KW of electrical power too, How about this thing giving thrust coupled with a wicked big electric motor driving a propeller??

So many idea, so little time.

Best regards to all......

Dave Sutton

Red Star Aviation

I wonder what is the output shaft rpm on GD-350and on the Czech M 601B. Is it within the range to drive a prop directly?

Jerzy Krasinski

The M601 B just happens to be in the prop range, as a matter of fact, when we install one on an AG plane, the next thing we do is install the prop. (It comes with the engine). M601 B, $15,000 USD.

691 hp takeoff, and 606 continuous

Douglas Karlsen

Turbine Design Inc.


There are a few 200# class turbojets on the drawingboard/workbench that may  be affordable soon (10-20K), but for a good time, try the Marboro (sp) jet  that is used in the Fouga Magistars. These 800# thrust jets are plentiful, throttleable (they are used in primary jet trainers in use all over NATO),  and most definately designed for jets. The recent jet homebuilt in S.A. used one.  I would contact David Sutton at Red Star Aviation for more information
or  to place an order.

 (Both David and Andrew are frequent posters to RAH, Skeeter will have to be  called on the landline until he gets fully web-ized so I gave his
phone number.)

Thanks for the vote of confidence from Scott, Who I don't pay :-) The Turbomeca Marbore-II is available with about 1200 hours of life on it for the mere sum of about $8-10K. There isn't really anything else available in it's class. Actually, the Contimental J-69 as used in the Cessna T-37 'tweety Bird" is a licensed version of this engine. It is nominally 880 pounds thrust, and developed versions of it are current production for cruise missiles/drones at ratings up to 1700 pounds thrust, albeit at much lower life durations and GREAT cost (they are not available surplus).

These are proven, aircraft application, FAA certifies engines at reasonable cost. Larger than anything you might construct from an APU, but reliable and immediately usable as-is.

Cheers to all,
Dave Sutton

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