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  #1  
Old 03-12-2022, 09:45 AM
spatsch spatsch is offline
 
Join Date: Nov 2014
Location: Denison, TX
Posts: 382
Default Turbine on RV ?

Looking at the latest announcements from TurbAero I was wondering why one would ever put a turboprop on an RV (There are airframes where a turbine obviously makes sense but I am asking about RVs in particular.). Now I get the 'cool' factor. Who doesn't like the smell of burned kerosine? I also don't want to start a debate here about the merits of experimentation. If you like experminenting please do. What I am trying to understand are some of the underlying physics/costs and not being an aeronautical engineer or engine expert I am having a hard time working this out by myself.

I see 4 different areas of comparison here:

1. Takeoff roll
2. Climb to altitude
3. Cruise at altitude
4. Cost

So let's be concrete.

Let's assume we have an RV-8 with an IO-360 M1B and compare that to an RV-8 with a TurbAero Talon turbine.

Anticipated performance and cost of the Talon engine can be found here:
https://turb.aero/community/resource...sked-questions

Performance numbers for one configuration of an IO-360 M1B on an RV-8 can be found here:
https://www.vansaircraft.com/rv-8/#aircraft-details-2

Van's claims 203 mph TAS at gross with 75% power (which is typically around 9gal/hr lean of peak for me at that altitude) and 575 feet take off distance which I assume was done on a paved runway.

So coming back to my 4. areas. How can I compare:

1. Takeoff roll

Now the Talon engine sates it has 200hp at sea level which according to Van's would save me 75 feet on take off distance. That assumes that it develops power as quickly as a piston engine. Do turbo props do that?

2. Climb to altitude

I am actually not sure how to make a good comparison here. Van's claims a 15% inmprovement in climb rate at see level for another 20hp but I rarely climb at max climb rate. I typically climb at 500 feet a minute or so which already gives me a pretty decent ground speed while climbing. So how do you compare that?

3. Cruise at altitude

Van's shows his numbers at 8000 feet which is roughly the altitude I cruise at and feels quite comfortable. At 75% power that would be 135HP at 9gal/h. Now I don't fully understand how you measure power on a turbine but assume that shaft power would be the equivalent to the power spec of a piston engine. Using that assumption the Taleon engine should be able to make 180HP at 15-17gal/h at that altitude.

So that gives me 1.33 times the power. Now I was told that power and speed on airplanes is related roughly by the power of three (is that correct?). So that would provide a speed increase of approximately 10% taking the cruise TAS to 223mph. Flying higher will be somewhat better in fuel efficiency but I really don't want to have to always cruise with oxygen which limits me to 12500 feet. Also TAS is limited to 230mph per vans.

I don't know how to compare this at 12500 feet which would be the highest altitude I am willing to fly permanently and should be more favorable to the turbo prop.

4. Cost

The Taleon engine will cost 80-85k$ . Lycoming will cost 43k$ with dual EMAG.

At my typical altitude I would get 10% more speed put use more then 50% more fuel. Now I know AVGAS is more expensive then Jet A but looking around its usually cheaper by 1 dollar or so which is about 25%. Also the IO360-M1B can be operated on unleaded fuel so AVGAS is not stricly required. It's just the convenient choice in the US.

So from all I can see the turbine is strictly more expensive for a 10% speed increase. Is that the correct way to look at it?


Again I understand very little about all of this and probably made many mistakes above. The reason for posting this is to learn something.

Thx

Oliver
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  #2  
Old 03-12-2022, 10:04 AM
Carl Froehlich's Avatar
Carl Froehlich Carl Froehlich is offline
 
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Location: Dogwood Airpark (VA42)
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While I’m sure some can rationalize the cost increase, the severe degrade in range locks the plane into a limited mission profile.

While you can add more tanks, this just continues down the sub optimization curve (more weight, more fuel burn, reduced range, repeat..)

So bottom line (as are most RV build considerations) what do you want the plane to do?

Carl
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  #3  
Old 03-12-2022, 10:17 AM
spatsch spatsch is offline
 
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Location: Denison, TX
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Good point about range. As for mission profile I like flying cross country in the US VFR. I rarely fly more then 3 hours between fuel stops just because I need to use a bathroom myself …. .

It’s pretty clear to me that if you are just hovering around your home airport at a 1000 feet a turboprop makes absolutely no sense…. .

Thx

Oliver
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  #4  
Old 03-12-2022, 10:24 AM
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Ironflight Ironflight is offline
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Location: Dayton, NV
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Oliver, I think you have an excellent understanding of the puts-and-takes, and the final result which is “you do it because its cool, not because it makes sense!”

I say that as someone who owns a turbojet powered homebuilt. At least it was DESIGNED to be powered by a turbojet. But it is still based on an aerodynamic package originally designed for piston power (genetically, it is still a Sonex). It is cool, fun, unique, and great way to show off. But you can out-cruise or out climb it in a Rocket and burn considerably less fuel in the Rocket.

All aircraft design starts with a mission and an engine, and the design work grows from there. So if you decide to radically change the engine without radically changing the design, you end up with a compromised package. The RV’s were designed around the basic Lycoming engine, and using something else will compromise the performance or operational envelope. That’s just how aeronautical engineering works - no matter how much we might wish it didn’t.

Which doesn’t negate a persons dream to burn kerosene and make a splashy entrance, so long as you accept the compromises. As I say about my little turbine, on any given weekend, I can be the guy that shows up at the lunch spot in the jet……

Paul
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Last edited by Ironflight : 03-12-2022 at 02:29 PM.
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  #5  
Old 03-12-2022, 01:30 PM
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rmartingt rmartingt is online now
 
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If you don't have access to gasoline, and it meets expected cost, perhaps it might be a viable (and more supported) option compared to a diesel? <shrug>
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  #6  
Old 03-12-2022, 01:56 PM
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WildThing WildThing is offline
 
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Paul summed it up well. The only other “real” advantages of kerosene vs avgas: it’ll climb much higher (and exceed VNE), supposedly more reliable, lighter (but needs more fuel/weight to run) and no avgas shortage issues. The cool factor trumps all else though
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  #7  
Old 03-12-2022, 03:21 PM
00Dan 00Dan is offline
 
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Location: Columbus, MS
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For climb, I expect any engine that isn’t a traditional air cooled piston will outclimb most Lycoming powered RVs just by the nature of not being temperature limited. Maintaining L/Dmax AOA will be optimal compared to most who need a shallower climb profile for cooling.

In cruise, since the turbaero design will be flat rated, as I understand it, a stock RV will be Vne limited. This now comes down to flying a little different and is going to be oriented around what your mission is.

The other factor is, of course, single lever control. Being able to distill engine management down to just “move the PCL to set power” is appealing to some people compared to our traditional throttle/prop/mixture combinations.
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  #8  
Old 03-12-2022, 06:25 PM
terrye terrye is offline
 
Join Date: Feb 2010
Location: Vancouver, BC, Canada
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Default Turbine on RV ?

In general turbines (turboprop in this case) have worse fuel consumption (lb fuel/brake horsepower/hour) than reciprocating piston engines. In the case of the TurbAero, I believe they are incorporating a recuperator (takes some of the waste heat from the exhaust and pre-heats the intake air). This is not the norm for aviation turbine engines due to weight considerations. If it works as intended, it decreases fuel consumption substantially. One of the limiting factors in scaling down turbine engines is the compressor and exhaust turbine blade to casing clearance. As the turbine engine gets smaller the losses from this clearance increase out of proportion to the size scaling. So from the TurbAero website, the projected fuel consumption is 0.56 lb/bhp/hr compared to a Lycoming (I)O-360 of 0.41-0.46 lb/bhp/hr at 70% power (ie cruise at wide open throttle at 8000 ft). So the turbine will burn about 35% more fuel for the same speed and altitude. Which means range is 35% less. In the case of TurbAero I wish them good luck, but I would wait until they have a few hundred flying (tens of thousands of Lycomings flying) before I considered one.

There is at least one (maybe more) RV-10s with turbine engines. Have a look through the forums to see how the owners like them.
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  #9  
Old 03-12-2022, 07:16 PM
gfb gfb is offline
 
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Also consider the extra year in build time it will take you to redesign the engine mount, cooling, exhaust, firewall forward, cowling, etc.

Is there a prop that will work for this engine/airframe combination? What happens to W&B?

Does the company have a flying aircraft with this engine? Or even just an engine spinning on the dyno? I love experimental but I don't want to be in the group of test pilots running the first few airframes on a brand new engine. Show me a reasonably priced turbine fw-fwd package flying on RV's and at least a few hundred no-issue hours and I'm interested.

I fly my airplanes as family haulers, breakfast seekers, and trip-with-the-wife adventures. There are some of us that fly experimental as "I want to do experimental aviation and be a test pilot." Those are very very different missions with different risk profiles.
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  #10  
Old 03-12-2022, 07:30 PM
zolotiyeruki zolotiyeruki is offline
 
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Location: Oswego, IL
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Quote:
Originally Posted by gfb View Post
Does the company have a flying aircraft with this engine? Or even just an engine spinning on the dyno?
No, they're still in the design/getting to prototype stage.
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