I did some research a while back on the Innodyn engine, and I wasn't pleased with what I found.
Innodyn was previously known as Affordable Turbine Power (ATP). It would appear their reputation was
somewhat less than spotless.
To anyone who is remotely interested in purchasing an engine from ATP -
Affordable Turbine Power - LISTEN UP!
What follows may save you $10,000 (or more)!
ATP is selling "snake oil" as far as my opinion goes.
Several ********* customers have placed orders and deposits for ATP engines -
and have been the victims of broken promises and outright fraud.
One ******* customer placed a $12,000 deposit with ATP towards a +300hp
version of their experimental turbine - waited to the promised delivery
date - called for the delivery - and was told by the "good" folks at ATP
that:
a. They discontinued development of the 300 hp version of the engine
b. They had no immediate plans to renew development of the 300 hp engine
c. Despite the written order form that guaranteed all deposits would be
escrowed - ATP nevertheless spent his deposit, and
d. They have no plans to refund his deposit.
To add insult to injury, this ******* builder had purchased a constant
speed propeller that works (for all intent and purposes) on a turbine
engine - so he is also "out" the money he invested in the propeller.
This is not an isolated case. Another ******* customer that I spoke with
ALSO lost a substantial amount of money due to the fact that ATP could not
provide an engine and would not return his deposit.
In the last edition of AOPA's ePilot, it had an announcement piece promoting
ATP. I wrote to the editor of ePilot and told him about the fraud this
company has committed and gave him the names of two ******* customers who
were cheated by ATP. The editor followed through - and attempted to reach
ATP for comment. They (ATP) refuse to talk with him. The customers did
talk with AOPA and told them what happened.
One year ago ATP had a booth at Sun n Fun showing their ATP powered RV.
Between SnF and Oshkosh the aircraft added (what is reported to be) less
than 10-hours of flight time. Think about that. 10-hours of flight time in
4-months. If I were selling a new engine - turbine or otherwise - I would
hardly boast about 10-hours of operation in 4-months. Something was (and
is) obviously amiss.
Incredibly, I am contacted by ******* customers who have talked with ATP
... and ATP believe it or not, is giving ******* as a reference! What
balls!
After SnF 2003, ATP went "underground" at last years Oshkosh and this years
SnF. They did not have a booth, but they did promote their "fantastic,
everybody needs to have one turbine engine" at an obscure Cub display. ATP,
you will find, is not in the show program.
These people have the lowest business ethics. If you are fool enough to
give them your money, they are happy to take it.
And that's all I have to say about that.
That alone was enough to put me off buying one, but I was still curious about the technology of the engine itself, so I kept looking.
Apparently the engine is based on the Solar T-62 APU, which was designed for use in a couple of military helicopters and never offered to the general public on the commercial market.
Some information on the T-62 can be
found here. In summary, it's an APU designed to operate between sea level and 15,000', run at ~56,000 rpm and produce about 70 SHP.
How they intend to ramp that up to over 200 HP is both beyond me and something I find somewhat scary, to be honest.
Finally, there's a good write up about using APUs as primary propulsion units
here. This covers my thoughts on converting APUs pretty well, based on what I learned about them in the classes I took relating to the history, design and operation of gas turbine engines.
Important bits quoted below:
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 afte
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.
After finding out all of the above, I decided that I not only wouldn't use an Innodyn engine in any project I built, I also wouldn't set foot in any aircraft powered by one. I'm open to experimental concepts and alternate powerplants, but an old surplus military helicopter APU that is probably being run significantly out of design spec is a bit too "out there" for me.
If any of the information above is inaccurate, I welcome corrections. I had to do a bit of digging to find all of this out, and if I can save others from having to do the same or keep them from finding out "the hard way", I think that's a good thing.
Ultimately it's a matter of deciding what best suits your needs, but I'd strongly suggest staying far away from any converted APUs. They just weren't designed with that kind of operation in mind, and no amount of creative engineering is going to change the fact that the engine core was purpose built for a
very specific (and different) kind of operation in mind.