The wikipedia articles on the enginesfor the Tu-95 doesn’t explain how this is done. Are there 2 separate turbines in those engines, each one rotating the other direction, or does it drive a gigantic transmission to get the opposite rotation?
If you follow the ‘contra rotating propellers’ link in the article, it goes to another page that discusses the mechanism (with further links) - to planetary gear or spur gear transmissions.
I don’t know how the Tu-95 does it, but contra-rotating propellers are usually driven from a single engine with a gear arrangement on the front of it so that the single input shaft spins an inner shaft in one direction and an outer (hollow) shaft on the same axis, in the other direction.
This image shows the basic concept.
http://cdn.instructables.com/F3T/7OUM/HINOB4Q3/F3T7OUMHINOB4Q3.LARGE.jpg
In the center you have what is called the sun gear. As that rotates, it spins the planetary gears, which in turn spin the ring gear. The ring gear ends up spinning in the opposite direction from the sun gear.
Are you sure the Tu-95 does it this way? 6,000 horsepower is a lot of energy per second to run through a mechanical gearing system that can wear and heat up.
The actual photographs of the engine show me 2 exhausts, which made me wonder if it’s actually 2 separate driveshafts and 2 separate power-take off turbines (maybe they share a central main turbine) rotating in opposite directions.
I don’t think so. the split exhaust duct is probably like that just to get around the pylon.
I’m hesitant to place too much authority on a strict parsing of wiki text, but a wiki paragraph (near the bottom) reads
says pretty clearly that it’s a single spool design with the power takeoff coming from the front of the single shaft. That would require a transmission to convert the single input shaft into two concentric counter-rotating shafts and to reduce the shaft RPM. Single spool engines are famous for turning at insane RPMS, 12K-20K being common, whereas prop RPM on something like that would be probably between 1K & 2K RPM. Also bear in mind this thing was designed and perfected in the early 1950s. Current turboprop technology or common design practice isn’t relevant.
A careful reading of wiki’s source 2 at Creation of the TV-2 (NK-12) Turboprop Engine makes it clear the transmission is indeed planetary.
BUT … that article describes the immediate predecessor engines (2TV-2) as consisting of two turboshaft core engines side by side driving two input shafts into a single combining planetary transmission with concentric outputs.
They then say that didn’t generate enough power or reliability so they uprated the turboshaft core/cores to create the TV-12. They do not make it explicit whether the TV-12 has one or two cores. But looking at the rest of the specs in that article my cautious conclusion is the TV-12 is a single large core, not a dual core.
Which would support the contention that the transmission is a one shaft in, one shaft out coaxial planetary system.
Here are some cutaways of the NK-12:
As well as a closer view of the contra-rotating hub:
With or without contra-rotating propellers all turbopropaircraft use gear reduction of some kind. A turbine engine has to spin much faster than the propellers can (or need to).
The first pic is almost certainly a different engine. The other two look like the right thing, and support my suppositions from the wiki sources.
Superb find there Dorjän. Thank you.