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  #31  
Old 01-09-2012, 09:15 AM
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johngoodman johngoodman is offline
 
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I'll throw a little fuel on this fire (g)...

I would think a very small turbine driving a generator for an electric motor would make more sense for an RV-10. Fire away....

John
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  #32  
Old 01-09-2012, 09:38 AM
ksauce ksauce is offline
 
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Quote:
Originally Posted by johngoodman View Post
I'll throw a little fuel on this fire (g)...

I would think a very small turbine driving a generator for an electric motor would make more sense for an RV-10. Fire away....

John
You've still got to generate the power (HP or kW). You would need batteries to provide the high power needed for takeoff and climb. This would mean you would rely on the turbine to recharge the batteries while you're running at reduced power (in cruise, or if you had a fancy/complex enough system, you could regenerate power in descent). Let's assume you want the ability to cruise at 75%, that means you're going to need a turbine that can provide a bit more than that amount of power to provide power for level flight AND recharge the battery simultaneously. So now you're carrying a battery array and fuel for the turbine and increasing the complexity by an appreciable amount for what is likely a negligibly smaller turbine/fuel burn.

A few months before the Chevy Volt came out, people thought that it was essentially going to use a similar system of a gas engine powering a generator that drives electric motors. It was to be the first hybrid where the gas engine was not directly coupled to the drive wheels. This sounded great, but it amounted to another great internet rumor. I'm sure the Chevrolet (and Toyota, and Ford, and ...) engineers are very smart and had a look at this type of system and found the design compromises weren't particularly favorable.

There are a few instances of systems like this. Off hand, the two I can think of are diesel electric locomotives and the new Navy destroyers/frigates that use turbines. That shows that there's efficiency in such a system, but in both applications, I think gross weight is less of a concern than in our RVs.
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  #33  
Old 01-09-2012, 09:57 AM
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RVbySDI RVbySDI is offline
 
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Quote:
Originally Posted by ksauce View Post
You've still got to generate the power (HP or kW). You would need batteries to provide the high power needed for takeoff and climb. This would mean you would rely on the turbine to recharge the batteries while you're running at reduced power (in cruise, or if you had a fancy/complex enough system, you could regenerate power in descent). Let's assume you want the ability to cruise at 75%, that means you're going to need a turbine that can provide a bit more than that amount of power to provide power for level flight AND recharge the battery simultaneously. So now you're carrying a battery array and fuel for the turbine and increasing the complexity by an appreciable amount for what is likely a negligibly smaller turbine/fuel burn.

A few months before the Chevy Volt came out, people thought that it was essentially going to use a similar system of a gas engine powering a generator that drives electric motors. It was to be the first hybrid where the gas engine was not directly coupled to the drive wheels. This sounded great, but it amounted to another great internet rumor. I'm sure the Chevrolet (and Toyota, and Ford, and ...) engineers are very smart and had a look at this type of system and found the design compromises weren't particularly favorable.

There are a few instances of systems like this. Off hand, the two I can think of are diesel electric locomotives and the new Navy destroyers/frigates that use turbines. That shows that there's efficiency in such a system, but in both applications, I think gross weight is less of a concern than in our RVs.
Ok, I do not desire to sound argumentative so I apologize before hand if my post sounds as such. Really, I just wish to question something from this post.

Not being an engineer I am just thinking about this logically. For any engineer out there with knowledge and/or experience feel free to set me straight. So, I fail to see where your logic of needing additional batteries is necessary. The query about a turbine generating electricity to me would imply the turbine would have enough HP to generate all the electricity needed for the electric motor to power flight. If the turbine is a high HP one such as 500-600-700+ in output and the airframe can successfully climb and cruise at 200+ why would there need to be storage batteries? Given a powerful enough spec-ed turbine/electric motor combination why would you also need to carry around storage batteries?

I know diesel/electric locomotives have been successfully generating massive amounts of electricity to pull tons and tons of weight around for a century or so. I know those diesel/electric locomotives weigh tons too but I think the weight is an issue of the job the engines must perform. It is not inherent to the fact that the engine design requires it be heavy.

The truth is I cannot see any RV being a good platform for turbines but John's query about turbines driving electric motors is a legitimate idea that may have some merit.

My .02.
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  #34  
Old 01-09-2012, 11:07 AM
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Default Where'd I park the Rolls...

What ever happened to this one?

http://www.youtube.com/watch?v=j2nD7Nqh7B4

Smokey
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  #35  
Old 01-09-2012, 01:02 PM
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John Clark John Clark is offline
 
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Default IO540 vs RR300

While working on an unrelated project for a client, I came across some interesting numbers for the turbine discussion. Robinson produces two very similar helicopters, the R44 and R66, the difference being mainly the powerplant. An IO540 Lycoming in the R44 and a Roll Royce RR300 turboshaft in the R66. The performance numbers are interesting. The gross weights are within 200 lbs, the R66 is 2700 and the R44 is 2500. The cruise speeds and range are similar with the turbine holding 74 gallons of fuel and the piston 49 gallons. Now for the costs, the estimated hourly cost, including overhaul allowance, is $300 for the turbine R66 and $183 for the piston R44. The purchase prices are $805,000 for the R66 and $348,000 for the R44.

I fully appreciate the reliability issue especially for a "working" aircraft but have a problem with a $457,000 difference in price for personal, noncommercial one.

John Clark ATP, CFI
FAAST Team Representative
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  #36  
Old 01-09-2012, 01:06 PM
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Flyfalcons Flyfalcons is offline
 
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Quote:
Originally Posted by John Clark View Post
While working on an unrelated project for a client, I came across some interesting numbers for the turbine discussion. Robinson produces two very similar helicopters, the R44 and R66, the difference being mainly the powerplant. An IO540 Lycoming in the R44 and a Roll Royce RR300 turboshaft in the R66. The performance numbers are interesting. The gross weights are within 200 lbs, the R66 is 2700 and the R44 is 2500. The cruise speeds and range are similar with the turbine holding 74 gallons of fuel and the piston 49 gallons. Now for the costs, the estimated hourly cost, including overhaul allowance, is $300 for the turbine R66 and $183 for the piston R44. The purchase prices are $805,000 for the R66 and $348,000 for the R44.

I fully appreciate the reliability issue especially for a "working" aircraft but have a problem with a $457,000 difference in price for personal, noncommercial one.

John Clark ATP, CFI
FAAST Team Representative
EAA Flight Advisor
RV8 N18U "Sunshine"
KSBA
Yep. I recommend those unfamiliar with feeding and care of turbine engines to have a seat before checking out the pricing on them (and parts, labor, etc). It's a whole different world and far removed from the mission of most homebuilders out there.
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  #37  
Old 01-09-2012, 06:10 PM
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Long live the Lycosaurous
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  #38  
Old 01-09-2012, 06:54 PM
ksauce ksauce is offline
 
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No problem. I assumed "driving a very small turbine" from the post to indicate something quite a bit smaller than the comparable lycoming (260hp). Basically, I was saying that if you have a turbine that can generate 100hp and have an electric motor that can produce 260hp, you would still need to come up with the additional 160hp. Batteries are the most readily available form (super capacitors are also available, but they're used for short durations) of electrical energy storage.

If you use a turbine of comparable power to the Lyc, you are correct in that no batteries are necessary. In that case, the benefits really come down to how efficient it is to convert the turbine mechanical energy into electrical energy versus simply mechanically coupling the turbine (free air or direct) to the propeller. I have no data in this area, so I can't really offer an informed opinion, but I imagine that modern turboprops can be very efficient.

If we assume relatively comparable levels of conversion efficiency and a comparable power level, you still have the additional complexity of the conversion power stack, controller, and the electric motor itself. This is on top of the complexity of the turbine with its own (fadec?) controls.

Additionally, you have to provide equipment to account for regeneration of the motor. If a motor is turned by an outside force (by a windmilling prop for instance) it becomes a generator. This can generate a non-trivial amount of power that has to be dissipated in some way. Hybrid cars actually send this electrical energy back into the batteries. If you don't have batteries, you can use braking resistors that essentially act like big heaters and convert the electricity into heat.

I'm not saying it's impossible, I'm just saying that I don't think with current technology it isn't practical. Perhaps in the future, technology will allow power densities and efficiency to go up while weight goes down. It may be that we're all flying behind fuel cells and filling our tanks with hydrogen pellets.

Quote:
Originally Posted by RVbySDI View Post
Ok, I do not desire to sound argumentative so I apologize before hand if my post sounds as such. Really, I just wish to question something from this post...
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  #39  
Old 01-11-2012, 01:17 PM
rv7charlie rv7charlie is offline
 
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The efficiency question (and what happened to the product) is basically answered in the link posted by SmokyRay. The Rolls rep said in the video that the burn was "mid 6's". That means ~0.65 lbs of fuel per hp per hour. Most a/c engines will run (real world operation)
around 0.45-0.46 lbs/hp/hr, a slight bit better with fuel injection & high advance rate electronic ignition. Since that Rolls engine was probably about as good as it gets for a small turbine, that's about a 50% penalty in fuel burn to run a small turbine.

I'm no engineer, but I've been told (by engineers) that the problem with small turbines is that it's very difficult to get pressures up enough to make the engine efficient (basically like compression ratio in piston engines). Supposedly, the ratio of leakage area at the perimeter to the overall area of the small wheel is poor, and multiple compression stages are difficult from both weight & cost standpoints in small turbines.

Charlie
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  #40  
Old 01-11-2012, 04:14 PM
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Yep...........SIZE MATTERS
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