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Got Jet Fuel? Turbine Helicopters > Turbine's

SemiArticulate
Veteran
Location: On Location

"The most efficient way to extract energy from a gas turbine is to use the "gas turbine" as a gas generator and use the exhaust to drive a "free turbine" this is how all modern gas turbine engines operate. "

That is false. A full scale heli is driven from the turboshaft. Not a jet blowing exaust on another external turbine. Why would you do that? The gas is already blowing on a turbine that is inside the jet engine. Using that turbine to blow on yet another external fan is a giant waste of energy. The only reason RCers did it was because no one was making Turboshaft gas-turbine engines when the ball started rolling.

02-24-2002 Over year old.

Doug
Elite Veteran
Location: Naples Florida....

The reason why is to achieve the best "impedance" match and to extract the maximum energy from the exhaust. Very few (like no) modern gas turbine engine directly drive the rotor, the most common the Bell Jet Ranger uses the Allison (now RR) 250-C20 (my mistake) This engine has a three stage Axial Compressor and a single stage Radial compressor being driven by a single stage axial turbine the free turbine is also single stage. The maximum efficiency in a turbine is achieved by expanding the exhaust gas to the greatest extent possible, Since a pressure ration of about 1.9/1 achieves supersonic flow (which must be avoided) engines with pressure ratios of over 40 to one (common now days to improve overall system efficiency) require many stages to fully expand the exhaust efficiently, it can't go from 590 PSIA to 14.7 PSIA in one stage. I think that other considerations such as being able to idle with the rotor system stopped or at very low speed is also an advantage.

02-25-2002 Over year old.

SemiArticulate
Veteran
Location: On Location

Ok we are talking about two different things. I thought you were defending the use of a Turbine to blow a fan, like the old style heli turbines. What you are talking about IS the Turboshaft Engine. It is one unit. Let me also say that I do not know the specific inner workings of the JetCat Turboshaft engine. I've never had my hands on one. There maybe something about it that keeps it from being a true "Turboshaft" but from what I understand it works like the full scale and the efficiency is much better than prior designs.

02-25-2002 Over year old.

ed vega
Key Veteran
Location: nyc, queens

I'm looking to see whats going on and who is developing the kits .. I have no issue other than prices , as it's very new aspect to the hobby.

everyones got a different configuration in mind .. so far I'm seeing a few different ways to turn the rotors ..

over time we will see the different variations .. which one works best has yet to be determined .. with all the different jet manufacturers hard at work improving on the ecu automated systems , we'll see what happens with heli's . i'm not partial to anyone type of heli or jet .. even if it's the first of it's kind(one off) doesn't make me partial to it .. as I find more pics , i'll put them up for discussion .. I don't think if you've seen one you've see them all yet .. hopefully theres more to come ..

regarding serious jet scale, also does anyone in the top gun masters actually have a drogue chutes(2) installed or actually use it on landing? or fully functioning ejection seats .. or fully operational radar dome?, or a simulated carrier landing with using arresting hook ?, I've seen alot of other functioning items but i'm curious about wether these things have been seen at the masters .. serious scale ...

rgarding the elite , I don't agree with the elite owners concept - well if thats true then there probibly not going to be much parts inventory to be made and would not consider a heli that will eventually not be supported due to parts availibility for the elite only ,one offs etc.. , manufacturer specific one of a kind part .. it needs to be be universal and availiable in the open market to survive ..

also the pic I put up is from simjet and it's on their website as a complete kit and have no idea if it will be deployed .. the more observations the better .

http://www.simjet.com/heli.htm

02-25-2002 Over year old.

Doug
Elite Veteran
Location: Naples Florida....

The "Jet Cat" system uses a multiple stage toothed belt reduction then conventional gears to achieve reduction since the power required from the turbine is "small" compared to what is used just to run the engine this works out good.I would post a picture of the system but I see that it's now "sat only".

02-25-2002 Over year old.

ed vega
Key Veteran
Location: nyc, queens

my question is where should the engine torque load get applied before it reaches the rotor shaft main drive unit .. in front of the jet engine shaft or back of it ..

having seen both , how does the load change between front or back of the rotor engine shaft as it delivers the output torque to the main rotor shaft ? .. is the front bearing working harder than the rear? or are bearing loads different fore/aft?, bearing sizes would help too, the section of the shaft during or after combustion creating torque , will it will get higher torque along the part of the shaft that combustion takes place? ..

if it has to travel from back to the front impeller shaft as drive torque will the shaft lose something along the travel and continue to lag alittle as the front bearings and shaft will have to absorb it , then depreciate while delivering the torque values to the front of the shaft , including the starter resistance .

with front mounted electric starters , does front starters have any parasitics to the main engine shaft rpm ? , if only two bearings are used what are the bearing sizes between front and back?

02-25-2002 Over year old.

Doug
Elite Veteran
Location: Naples Florida....

Where its cool

02-25-2002 Over year old.

SemiArticulate
Veteran
Location: On Location

"The most efficent method to get power from a turbo shaft engine is to extract the most heat and velocity from the gasses before exhausting."

"Before exhausting", yes I agree with that 100% and that is how full scale turboshaft jet engine works.

"They are and will remain a novelity reserved strictly for people with lots of money, tinker time and an strong urge to have something different... abiet being somewhat impractical."

I don't fully agree with that, but I bet thats what was said when the first full scale turbine was put to use. Heck, thats what they say about our hobby in general.

I don't know what was meant by "elite" because I didn't write it. But "elite" does not need to mean rich or skilled. I read that and thought it meant for those that wanted the very top end of scale looks and operation. It probably isn't a heli that you would "3D" on a regular basis nor is it for someone that just wants to slap on a Funkey fuse and go.

But the cost is not a problem because you are not forced to pay it. But the best variation will be the one that comes closest to a real Turboshaft Engine.

02-25-2002 Over year old.

Doug
Elite Veteran
Location: Naples Florida....

Here's a good picture of the Jet Cat

02-26-2002 Over year old.

ed vega
Key Veteran
Location: nyc, queens

I'll just have to get one .. thanks guys for helping me understand the jet helis alittle better ..

02-26-2002 Over year old.

SemiArticulate
Veteran
Location: On Location

When you get a turbine heli, you'll let us fly it won't you?

02-27-2002 Over year old.

ed vega
Key Veteran
Location: nyc, queens

will do .. it'll be as much fun watching it fly as it will be flying the jet .. cause I'll want a video too .

02-27-2002 Over year old.

mr_matt
Heliman
Location: Oak Park, CA

JetCAT turbine mechanics

Hi guys,

My name is Matt Carroll, I have been flying planks with JetCAT turbines for the last few years. I would like to get into helis myself one day, I have a raptor 30 but I have not quite finished it yet, .

Anyway the question has been asked about the relative merits of the JetCAT direct drive system vs other older free turbine designs. Now I cannot speak at all about full size, I have learned from propulsive turbines that many things that relate to fullsize do not apply in model sizes and vice versa!

Anyway for those not familiar with the JetCAT PHT3 system I will describe some of it (my apologies if I do not use all of the correct heli terminology!)

As I said the JetCAT PHT3 is direct drive. After the turbine core starts the user flips the aux turbine control swith from "start" to "run" and then the Electronic Control Unit (ECU) brings the headspeed up to whatever RPM has been preset by the user (with a small device included int he package, the Ground Support Unit - GSU).

At this point a series of control algorithms (well engineered algorithms, proven for years in the planes) regulate the power to the turbine and maintian the headspeed closely. I have noticed that changes in the collective setting do not cause the big drop in motor RPM I have heard in some of the helicopters I have watched fly.

Herein lies the main advantage to the direct drive. The time constant of the turbine is much less than a free turbine, therefore all of the regulation algorithms perform better. You get a much more repsonsive control of the power to the rotor. I think they are rated at like 5 or 6 horsepower, that is very conservative and allows the ECU to really maintain tight control of the turbine without over temping the core, very important!

BTW, note that there are only 2 authorized service and support facilities for these units in the world...one at JetCAT in Germany, the other at JetCAT USA in California. These are complex devices, and they will need service. If you buy from JetCAT USA you ship it to them and they fix it and send it back. If you buy it from anyone else it will have to go to them and then to Germany to fix and back! Could take awhile.

Also, JetCAT USA offers a 1 year warranty, not sure if any other distributor is, you should check

Sorry if this came out like a commercial, hope I did not break any rules, sorry in advance if I did.

Matt

03-01-2002 Over year old.

SemiArticulate
Veteran
Location: On Location

Thats sounds pretty cool but you will need to send me one for free so I can check it out for myself. I'll report back the results.

Well, I had to try.

I read the the Jetcat system was derated from 20 hp to 6 and had a 64 to 1 gear ratio. MHW had a good article on it but I can't seem to find it.

How do you think the Jetcat systems work compared to AMT as far as plank jets? AMT seems world class but Jetcat is new to me. I don't fly any turbines yet but I like to keep my eye on them.

03-01-2002 Over year old.

JCadwell
Key Veteran
Location: Richland WA/ Morro Bay, CA

How are the turbines decoupled from the main blades? Do they spin a conventional clutch somewhere in the gear reduction system? How does the starting speed of the turbine affect that operation.

Thanks, John Cadwell

03-01-2002 Over year old.

mr_matt
Heliman
Location: Oak Park, CA

Yes

there is a clutch on the back of one of the main belt drive cog wheels, I know they spent a lot of development time on that clutch. I am pretty sure there is another clutch in the system, I will take a look tonight or tomorrow

I have only seen the turbines remote start, I am not sure if you can start it and hold the blades still. The core must spin to about 55K rpm to stabilize then back to idle with I think is about 33K.

I will check with the factory and post for sure!

Oh I guess I did not mention the JetCAT PHT3 uses their patented autostart system. The turbine is controlled with a single channel and a 3 position switch. The positions are for:

Start
Run
Shutdown

During startup, the ECU lights the glow plug, engages an onboard starter motor, and injects a propane/butane mixture, Upon ignition of the propane/butane the motor rpm starts to build and if all is correct in the sequence the kerosene pump starts and ramps up, and some point the starter is disengaged, the propane feed is cut, the motor stabilizes and then waits for you to set the switch to "run". THere is an LED array on board that gives visual indication of the modes during startup, but it is all outomatic

During this time the blades do slowly turn, kind of like full size starting

03-01-2002 Over year old.

mr_matt
Heliman
Location: Oak Park, CA

JetCAT vs AMT

I better indicate that I am sort of a rep for JetCAT so I am biased. I have always maintained however if there were no JetCAT I would go with AMT. They are very reliable turbines, a bit heavy but very well built. You have to start them on compressed air from a scuba tank. Not sure if they plan a heli or not

03-01-2002 Over year old.

SemiArticulate
Veteran
Location: On Location

What is the Waiver status on these? I don't need to fly a pattern plane over 130 mph 50 times to fly a turbine heli do I?

03-01-2002 Over year old.

mr_matt
Heliman
Location: Oak Park, CA

Waiver

THey are working on that, I think they have talked to Joe Amato over at the AMA.

I will confirm and post, but the intent is to not have any plank flying requirements

03-01-2002 Over year old.

mr_matt
Heliman
Location: Oak Park, CA

OK I got clarification

1.) The unit has a single clutch under one of the cog wheels as I said. In addition there is a one way bearing on the turbine output shaft

2.) You can start it and hold the blades. The stabilizing RPM is lower than what we run on the plank turbines and peaks just at the rpm that the clutch starts to engage.

3.) The Turbine heli proposal to the AMA was to Carl Mulroney, and suggested that a pilot had to be able to take off, hover, go into folward flight return to hover and land. There may be another requirement for nose in hover. None of these proposal has been acted on and will not be unitl after Top Gun in April. Initial indications are favorable however.

Regards,

03-01-2002 Over year old.

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