Last night, I was watching a video about the Tu 114 airliner. When running, the two props on each engine ran opposite ways; counter-rotating. 15,000 shaft horsepower needed two airscrews to get all that power turned into work, and so I understand the reasoning for two props. The counter-rotation also canceled out a lot of torque issues.
My question is this: in at least two clips, a Tu 114 was shown taxiing in to the gate. The two inner engines remained running; the two outer ones were just about completely wound down. What I noticed was: both props on the outer engines were turning the SAME direction rather than in counter-rotation. This was not an artifact of film frame rates; I looked carefully.
What is/was going on?
Thank you.
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- BowlingShoeDC9
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Re: Tu-114 prop question
Aliasing is the word you’re looking for, not artifact. Even if you do look carefully, the frame rate could still reverse the direction of rotation. It would still appear that way even if you looked at it frame by frame.
I know nothing about the TU-114 (other than the basics), but I don’t think you can rule out aliasing since it’s a phenomenon that affects the recorded data, not just your perception of it.
I know nothing about the TU-114 (other than the basics), but I don’t think you can rule out aliasing since it’s a phenomenon that affects the recorded data, not just your perception of it.
Re: Tu-114 prop question
The OP might be referring to this video:
https://www.youtube.com/watch?v=FE8k12c8pnc
For a few seconds starting at about 7;20, there's a shot of a Tu114 taxiing with it's no.1 engine nearest the camera, it's 2 props nearly stationary but both clearly turning slowly the same way.
Strange - I'd have thought that the gearbox arrangement for contra-rotating props would preclude them both turning in the same direction.... perhaps there's some sort of 'differential' unit like on a car axle that allows this?
https://www.youtube.com/watch?v=FE8k12c8pnc
For a few seconds starting at about 7;20, there's a shot of a Tu114 taxiing with it's no.1 engine nearest the camera, it's 2 props nearly stationary but both clearly turning slowly the same way.
Strange - I'd have thought that the gearbox arrangement for contra-rotating props would preclude them both turning in the same direction.... perhaps there's some sort of 'differential' unit like on a car axle that allows this?
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- eeightning
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Re: Tu-114 prop question
Without doing the research to accurately answer your question with references, I'll say that co-axial props can be either counter-rotating, or contra-rotating. I can never remember which is which, but in one case the props are connected to the same gearbox so that if one moves, the other has to move in the opposite direction. In the other case the props can move independently of each other. This could be done with two engines driving through a common axis, or one engine with independent shafts/power sections.
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- WesternDC6B
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Re: Tu-114 prop question
DH106 wrote:
That’s the video!
Re: Tu-114 prop question
Looking at that video, you’ll see a shot where on the ground with all shut down, someone on the ground spun one prop, but not the other. That gives the impression they are not connected, nor geared to the engine, just free powered by a turbine stage via a connecting rod. Each prop with its own turbine.
This is just a guess though, as it is hard to find specifics about the NK-12s.
This is just a guess though, as it is hard to find specifics about the NK-12s.
Re: Tu-114 prop question
CrewBunk wrote:That gives the impression they are not connected, nor geared to the engine, just free powered by a turbine stage via a connecting rod. Each prop with its own turbine.
This is just a guess though, as it is hard to find specifics about the NK-12s.
Wikipedia - although not the most reliable source - says, on the NK-12: "The contra-rotating propellers and compressor are driven by the five-stage axial turbine."
Not conclusive, but implies that there's only one turbine set driving the compressor and propeller gearbox. Any more would require a complex set of concentric shafts, which although modern turbines (many double spooled + RB-211 tripple spooled etc) use, would probably be impractical when the NK-12 was developed (late 40's - early 50's).
As I mentioned above, perhaps the propeller gearbox has some sort of 'differential' arrangement allowing each prop to have freedom of movement.
Re: Tu-114 prop question
The NK-12 engine has a planetary type reduction gear to achieve different rotational direction of its two props.
Here are some schemes and pictures of it from a russian source:
https://reaa.ru/threads/soosnyj-reduktor.19525/page-9
The principle of possible rotational directions for both propellers is described here:
ru.wikipedia.org/wiki/Планетарный_редуктор
Here are some schemes and pictures of it from a russian source:
https://reaa.ru/threads/soosnyj-reduktor.19525/page-9
The principle of possible rotational directions for both propellers is described here:
ru.wikipedia.org/wiki/Планетарный_редуктор
Re: Tu-114 prop question
My guess is, that the props could be disconnected from the gearbox. Same as an ATR-42/72, which has no APU, can disconnect the prop from the engine no.2, so engine no.2 can run on the ground and be used as an APU for air condition and electrical generators, while the prop is not turning.
So if I am right with my guess here, the the props were disconnected from the gearbox while taxiing into the gate, then it is likely the both props (forward and rear prop on the outboard engines) despite usually turning into different directions, would turn into the same direction because oft he wind forces propelling them in the same direction. (Because when disconnected from the gearbox, the props can turn freely on their axle in both directions as long as there is no prop bake installed/activated or the prop is secured by a rope/line.)
So if I am right with my guess here, the the props were disconnected from the gearbox while taxiing into the gate, then it is likely the both props (forward and rear prop on the outboard engines) despite usually turning into different directions, would turn into the same direction because oft he wind forces propelling them in the same direction. (Because when disconnected from the gearbox, the props can turn freely on their axle in both directions as long as there is no prop bake installed/activated or the prop is secured by a rope/line.)
Re: Tu-114 prop question
CARST wrote:My guess is, that the props could be disconnected from the gearbox. Same as an ATR-42/72, which has no APU, can disconnect the prop from the engine no.2, so engine no.2 can run on the ground and be used as an APU for air condition and electrical generators, while the prop is not turning.
So if I am right with my guess here, the the props were disconnected from the gearbox while taxiing into the gate, then it is likely the both props (forward and rear prop on the outboard engines) despite usually turning into different directions, would turn into the same direction because oft he wind forces propelling them in the same direction. (Because when disconnected from the gearbox, the props can turn freely on their axle in both directions as long as there is no prop bake installed/activated or the prop is secured by a rope/line.)
That is not at all how the ATR Hotel mode works. The prop is not disconnected from the gearbox. The gearbox simply has a brake installed that brakes the whole prop/gearbox assembly. Because it is a free-turbine engine, the power turbine can simply stop while the hot air washes around it. The moment the brake slips or is released, the prop starts turning at a heck of a rate.
Any free turbine (PT6-type engine) will exhibit the same behaviour when you feather the prop in the air and the free air stream keeps the prop still or at least very slowly turning. Or by holding the prop when starting the engine, the engine would start up fine, until the residual torque around the free turbine is sufficient to break your hold on it. Exactly the same is seen with a helicopter: the rotor blades are still, until there is enough power from the engine to start turning them.
I would rather guess the NK12 is built similarly to the T56 of the C130: constant speed fixed shaft with a massive reduction gearbox, but all physically connected. The gearbox then makes provision for the contra-rotating props, but if you turn any one of them, the rest of the assembly would turn with. Even the Shackleton had a similar set-up.
A clutch system is just something that is likely to fail, and with the amounts of torque/power in a system like that, a lot of unneeded weight.
The T56 has got a disconnect system, but it is designed to decouple the prop/gearbox assembly from the engine driveshaft in the event of engine seizure, not 'operational' use.
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