Why aren't contra-rotating turboprop engines used on aircraft more? They can produce an incredible amount of power, can increase efficiency by 6%-16%, and can go incredibly fast. Sure, they are a bit noisier, and the addition of the secondary propeller results in additional weight and complexity, but these don't necessarily outweigh the benefits. Take one of the only examples of these engines:
Kusnetsov NK-12MA Engine, used on both the Tupolev Tu-95 Bear and the Antonov An-22.
>15000 ESHP x 4 engines
>weighed 6393 lbs (2900 kg)
>sfc= 0.360 lb/shp-hr*
>turbine inlet temp: 1250K (977°C)
>single spool axial flow compressor (14 stage)
>12-chamber cannular combustor
>5-stage turbine
*Specific Fuel Consumption
For comparison sake, the specific fuel consumption for a common modern turboprop:
>Pratt & Whitney PT6T-6B
sfc= 0.591 lb/shp-hr
The Kusnetsov NK12MA is old technology, with the newer model (-M) designed in 1953. For an engine designed in the 50s, it is incredible! Why isn't the contra-rotating turboprop used more often? Surely with modern technology the engine can be modified for even greater efficiency, considering modern turbines can withstand up to 1200°C+.
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19830002859.pdf
https://en.wikipedia.org/wiki/Kuznetsov_NK-12
http://www.jet-engine.net/civtsspec.html
https://en.wikipedia.org/wiki/Pratt_%26_Whitney_Canada_PT6
Because passengers are terrified of visible propellers, and the vast majority of planes that would benefit from these are for carrying passengers who could just opt for an airline that uses non-terrifying planes.
/thread
>>890831
Ok, although I think passengers wouldn't really be 'terrified' of it, how about cargo operations?
Also, don't /thread your own post like an idiot.
since when does the airline industry give a shit about passenger comfort..
OP, I would guess that there's something you're missing in your information - service life? maintenance costs? flight characteristics? stress on the airframe?
Whatever it is, consider the aircraft used by military forces around the world: turboprops are used on medium and light transport aircraft, while jets are used on heavy transports and tanker aircraft. Clearly there's a point at which jet engines do become preferable even over advanced turboprops.
>>890860
Turbofan engines do have significant benefits over a typical turboprop engine. The power available for propeller-driven aircraft decreases as velocity increases (when the velocity of the air in front of the propeller approaches the velocity of the air behind it, the power available diminishes). This isn't as significant in the contra-rotating turboprop, as demonstrated by the Tu-95, as it cruises at Mach 0.8+. This is also a limit of turbofan engines though (albeit slightly higher speeds), because the fan is essentially a ducted propeller.
The service life has also been demonstrated by the Tu-95, as the NK-12MA engines have been used since the mid-50's, the most recent of the Tu-95's being build in the earlier 1980's.
Both the flight characteristics and stress on the airframe should improve with a contra-rotating turboprop. The use of dual propellers results in a propwash that has little to no spiraling slipstream, so there is no turning tendency caused by the engine. The opposite rotation of the props will also cancel the adverse effects of torque on the engine mounts, so there would be less stress too.
I will agree with you on the maintenance costs, however. With the addition of more propellers and the additional complexity of the engine setup, there will be a slight increase in costs. However, considering the cost of the average high-bypass turbofan engine (typically around $7-$10 million /per, not to mention routine maintenance), the cost for the CR turboprop alternative would undoubtedly be far less.
>>890860
turboprops are better than jets when the distance is under 1000miles
over that turbofan beats it
>>890942
For comparison, lets take the B-52 and the Tu-95
B-52
>service ceiling
50000 ft
>range (ferry)
8760 nm
Tu-95
>service ceiling
45000 ft
>range
8100 nm
The Tu-95 has very slightly inferior performance characteristics, but both have been incredible successes in their roles.
Now, I hate to use strategic bombers as an example, because as it stands, you are correct in that turboprops are typically more useful under ranges of 1000 nm. However, that is with the current turboprop designs (single prop, ~2x centrifugal compressors, reverse annular combustors and a few turbine sections, with outputs closer to ~1500 hp). If a contra-rotating turboprop (with axial compressors, variable guide vanes, and power outputs closer to 8000-10000 hp) were used, I think they could be a viable alternative. This design is also far superior to the "normal" turboprop design in the high-speed high-altitude flight regimes.
>>890856
Since they started dropping the saabs and the dash-8s in favor of huehuehuehue
>>890828
Howard Huges.
>>890945
I'm just going off what i remember dispatching flights
it made more sense and saved us a lot more money when using turboprops on short hops
the jets where all used for longer flights unless we need to carry a lot of people in a short distance
the closest the industry will get to using props again are UDF but I havent seen any research pop up on that in a long time
Have you ever heard a contra-rotating aircrafts engine in real life?
The tu-95 is one of the loudest aircraft ever made. It would never ever pass noise limits around airports. The bears prop tips exceed the sound barrier, they're so fucking loud. That and the fact that turbofans work better at long/medium range distances.
I fly into Barcelona regularly and the departures are specifically designed to take us away from the city because theres a fine of approximately €140,000 fine if you breach these noise regs.
Honestly a contra rotating aircraft would generate so much noise that you wouldn't be able to fly it into any airport with civilians living within a 25 mile radius.
>>891035
Yes, the KN-12 engines were loud. This was because the tips of the propellers were going Mach 1.08. Look at the shape of the propeller on the Tu-95. They are a standard, elliptical planform. Look at propellers of a modern turboprop nowadays. The propeller has a shape designed to delay the onset of a shockwave by sweeping the tips. This makes the propeller substantially quieter.
>>891044
Yeah but not quiet enough. The number of times I walk past dash 8's and the like on the ramp and they're deafening.
Honestly it's mostly about the noise. And they're needlessly complex. I'm in the airline industry and the amount of shit that goes wrong with a standard turbofan aircraft is enough to put any airline off ordering a complex contra rotating aircraft.
>>891044
It's not just the speed, the 95 is subsonic but the blades interfering with each other has an effect much like an air raid siren.
>>891054
I get that they will have constructive interference, but unless they are adequately evaluated, the only comparison we have of the noise is the Tu-95.
>>891050
A military low-bypass turbofan engine produces an incredible amount of noise in all flight regimes, especially in high power settings and slow airspeeds around an airport. I've lived for nearly my entire life in San Diego, right next to MCAS Miramar, and the noise doesn't seem to bother the vast majority of the population.
>needlessly complex
They have one additional gearbox on the front of the engine (NK-12 type engines) and an extra propeller. It's not that complex. For comparison, the Rolls-Royce Trent series engines have 3 shafts. There is no way the CR turboprops are more complex
>>890828
the problem with contra-rotating turbobrops is the gearbox weight and durability.
most of the project info I've seen like this are contra-rotating propfans. one prop is driven by one turbine shaft and the other prop is driven by the other turbine shaft, so there is no gearbox required.
>>890828
>Why aren't contra-rotating turboprop engines used on aircraft more?
Their only real benefit is on a single-engine plane. Anything large enough to have two engines can just make each engine rotate in the opposite direction to get the gyroscopic benefits without the mechanical complexity.
>>890831
Passengers don't give a shit about the plane model they are flying with.
>>893459
i do
>>890828
BSFC doesn't say jack-shit about propeller performance. It merely describes efficiency between the fuel line and the output shaft. Those numbers are completely irrelevant to your argument.
>>891071
>They have one additional gearbox on the front of the engine (NK-12 type engines) and an extra propeller. It's not that complex.
Yes, it is.
>For comparison, the Rolls-Royce Trent series engines have 3 shafts.
And not one of those shafts has any gears at all. They are completely free-floating on bearings, and the fan is direct-drive to the LP spool.
But as if the gearing alone wasn't enough (it is), add to that the fact that turboprops universally use variable-pitch props and yes, you bet your fucking ass a counterrotating turboprop is more complicated than a 3-spool turbofan.
>>893460
Bet you $1000 this was taken in Auckland, New Zealand.
Pic attached - when i stood in the exact same position as OP
>>893464
Good eye, pal. Nice pic ;~)
>>893461
It is relevant. The BSFC is a ratio of fuel consumption/output shaft hp. What I'm trying to illustrate is the fact that it, in comparison to modern conventional turboprops, it has a higher power output relative to the fuel consumed.
>>893462
Fair enough, in hindsight it would be a bit more complicated, after doing a bit more research. For the 6%-16% increased efficiency and higher SHP potential, would it not be worth the additional complication? Not to mention the initial cost of the engine being lower. Unfortunately, I cannot find any documentation on the NK-12's counterrotating gear system, so I cannot really reference it specifically, nor is there a comparable engine to reference. I am familiar with standard constant-speed prop actuation, but I'm curious to see how it is achieved with CR propellers
>Mixing unit systems
>Abusing SI prefixes and unit symbols
Probably the main reason they haven't been widely adopted.
Open props are louder than ducted props in turbofans
