Contra Props

Well i too wasn’t sure how this would work, but as a mere enthusiast of the type who am i to question someone else?

When he ends a convoluted article with “…I hope I am at least 50% right. 8:-) D” and he is challenged on much of the simpler stuff he has writtn about, then I think you can safely question it!
It’s not a piece that you can rely on for sound information, this is one problem with t’internet!

Like you though I hope someone here knows alot more about the Tu-95.

Well i too wasn’t sure how this would work, but as a mere enthusiast of the type who am i to question someone else?

Does anyone on here have a better understanding of the Kuznetsov NK-12turboprop and this propeller system.?

I’m not sure I can trust a source with this to say, and I quote,

“One interesting feature of these coaxial props is that they are mechanically
independent. One can rotate them together in any direction or counter-rotate
them in any direction too. All of this with the small fingers. Each propeller
is driven by its own free-turbine (four stages each, if my memory does not
betray me.) The counter-rotation is achieved gasodynamicaly by the flux
turning the rotors in opposite directions…

…I hope I am at least 50% right. 8:-) D”

I’m not the best person to describe this and I’m away from my sources.
A free turbine is not an extra power source, or extra shaft etc, it is merely that the main turbine assembly and shaft (which drives the gearbox) is not physically connected to the compressor stages of the engine. This is the most common layout for all turbo-prop engines, and one feature of the system is not having the external torque forces from the propellors causing the compressor to stall.
This was a major problem for the Gannet and Wyvern early on, the Mamba was redesigned to become a free turbine, unsure about the Python.

Thus all free turbine engines you can freely spin the propellor from outside with your hand with minimum force, unless it has been braked or physical stop locks are in place.
Notice on a lot of turbo-prop aircraft there is a ground ‘sock’ which goes over one or more of the blades and tethers it to a part of the airframe somewhere, as it is undesirable to have the props turning over in the breeze all the time.

The number of stages is the number of discs with the blades attatched, ie a four stage compressor constitutes four discs of blades.

I don’t know enough about the Tu-95 or its engine to say the above passage is bunkum, but I certainly have never heard of such a set up, or seen it alluded to elsewhere. I just cant see how (or even why) the two propellors of a single driven contra-prop assembly can act completely independantly of each other.

The author is suggesting that each prop has its own free-turbine, so each engine has two free turbines, I would like to see proof of that set up.
Also noticed the author has been challenged on other issues that he has written, the classic ‘supersonic’ prop tip speed turkey.

Think about it, if the engine stops how can one propellor keep running?
Also check out Bear footage with the engines being started, watched some on video today and those props were definately connected to each other.

As Wiki would say, ‘citation needed’!

Each set of props is driven by its own free turbine. Information was gleaned from the following..

http://www.aviation.ru/Tu/95/Tu-95.html

James, the prop sizes quoted seem to vary between 16′ – 18′.. depending on where you look, but as i have never measured one i cannot say for sure.

P-01, I think we`ll have to wait for Wyvernfan to explain whether the aircraft was either an in-service one ,or an old static model. Anyone know if `Flypast`/Aeroplane/Flight ever did an article on the Beast,or seen any engine `cutaway ` diagrams. I`ve also seen the bit about the props being up to 18 ` diameter,but that may be engine model/ aircraft specific.

This 575mph giant has propellers measuring a massive 16′ 4″, and when on the ground each one can be rotated in either direction.. they are mechanically independent of each other.

Actually thinking on, that independant bit has got to be wrong, look at any picture of a Bear and the props are always set in a certain relation to each other, either when they are parked together neatly at ‘+’, when they are spread to all eight blades visible, and anywhere in between.
My PC won’t allow me to watch vids at the mo, but if you do a youtube search there is loads of TU-95 footage there and I seem to remember it clearly showing both props powering up and turning absolutely in unison with each other.
I just can’t see why, or how the props would be independant when driven from a single engine, but willing to be corrected if the info is out there.

Also I’m seeing references to the props being just over 18 foot in diameter (but you know my history on prop sizes Rob:D)

If you `google` Kuznetsov NK-12`,you can see a photo of the Bear`s engine in all it`s g(l)ory detail.Further trawling gives the info that it`s a 14 stage axial flow compressor,driven by a single 5-stage turbine, with standard inlet guide vane control and compressor `blow-off` valves. It would be interesting to know/see a `cutaway` diagram of the gearbox,to see how that works,as the combination is `a fixed-shaft turboprop`,but according to Wyvernfan`,each prop can be turned independently of the other…unless of course the aircraft had all the geabox bits removed ? Anyone know any more?

Got me confused aswel, there are plenty of free turbine turbo-props about (in fact preferred layout to prevent torque stalling) so apart from the size the engine is conventional, but to have the props being able to swing about independantly from each other seems odd. When it’s all running you would want the props to be changing pitch together for sure, and if one engine shuts down you will still lose the pair of props anyway, even if they are independant. All I can think of is some sort of clutch system which allows this, but why?

Going back to basics, you have the componant (?) that physically takes the energy from one prop and drives the second prop in the opposite direction, and the Translation Unit that makes sure that the two sets of props achieve the same adjustmanets in pitch, or are these combined?

If you `google` Kuznetsov NK-12`,you can see a photo of the Bear`s engine in all it`s g(l)ory detail.Further trawling gives the info that it`s a 14 stage axial flow compressor,driven by a single 5-stage turbine, with standard inlet guide vane control and compressor `blow-off` valves. It would be interesting to know/see a `cutaway` diagram of the gearbox,to see how that works,as the combination is `a fixed-shaft turboprop`,but according to Wyvernfan`,each prop can be turned independently of the other…unless of course the aircraft had all the geabox bits removed ? Anyone know any more?

Wyvernfan will know more, but I think that the Eagle engined Wyvern crash that killed Sqn Ldr P Garner in 1947 was caused by a translation unit failure. The props appeared to have done what Camlobe describes above, ie the rear set went to fine pitch.

Yes.. although differering accounts put both props at fine pitch, hence an almost vertical descent to maintain airspeed.

With regard to the DeHavilland contra-rotating props on the Shacklebomber, please allow me to inform, hopefully without sounding pedantically picky…The front protrusions engaged into the base of the cylinder and were retained by nuts, duly splitpinned. The rear protrusion of the rack bolts engaged through the front of the translation unit and were retained by nuts duly splitpinned. This front part of the translation unit was connected to the inner race of the ball bearing. …Therefore, the rear prop was six inches less in diameter, but to compensate for the reduced blade area, the rear blades were made a little fatter.
camlobe

“Why a four year-old child could understand it.
Run out and get a me a four year-old, I can’t make heads or tails out of it.”

–Groucho Marx, Duck Soup, 1933

On the Tu-95, it appears to have had each engine’s props independently driven, probably by its own free-turbine and gearbox with just a dual CSU to keep the two in unison. With 15.000 hp to split two ways and the potential for big trouble should they lose an engine or props on a long range cruise you can see they made the right choice.

I don’t quite understand that bit, surely with the Bear each engine drives one set of contra-rotating propellors, so that if one engine had to be shut down then you would lose that set (ie both props) of propellors?

The Americans produced several beefy fighter/attack single-engine, contra-rotating prototypes around 1943-45:-
Curtiss XP-60C, XP-62 & XF14C-2
Fisher P-75A
Northrop XP-56
Republic XP-72
Boeing XF8B-1
Rather like the look of the Boeing, a sort of single-engined B-29 crossed with a Skyraider.
Then there’s the twin engined, contra-prop Hughes XF-11 & we know what happened to that.
Bill

Wyvernfan will know more, but I think that the Eagle engined Wyvern crash that killed Sqn Ldr P Garner in 1947 was caused by a translation unit failure. The props appeared to have done what Camlobe describes above, ie the rear set went to fine pitch.

Shack Incident

I hope the FE got the recognition for his skill that he deserved – not least amongs the crew of the aircraft in the bar that evening.

This helps to illustrate why twin-engines on separate co-axial props are a better bet than lots of noisy gears and (gory) complex translation units and attendant linkages on one.

Thanks again Camlobe, good story. I note that the TU oil was separate from the engine system. Did the CSU and pitch-change piston use engine oil or did this have its own (clean) supply too?

On the Tu-95, it appears to have had each engine’s props independently driven, probably by its own free-turbine and gearbox with just a dual CSU to keep the two in unison. With 15.000 hp to split two ways and the potential for big trouble should they lose an engine or props on a long range cruise you can see they made the right choice.

Anon.

Fortunately, I was forced to leave the office last night (‘tea is on the table’ – and I live 45 minutes away from my business), otherwise, my diatribe would have never ended.

Gory bits?

OK. Try this.

Previous experience had demonstrated that should a Translation Unit fail, the rear prop immediately went to fine pitch while the front prop almost feathered in an attempt to control rpm. Depending on preselected power settings, the engine would rapidly overspeed leading to catastrophic failure of one (or more) conrods through the side of the crankcase. Hot oil straight onto an even hotter exhaust. Fire. Two minutes later the rubber engine mounts have burnt away and aerodynamic forces then take the power plant (engine assembly complete with propeller assembly) over the top of the wing and away. If it is an inboard engine, it takes the tail off on that side.

Not a nice thought.

Needless to say, we were very concerned about the slightest sign of clean oil on the rear spinners.

One nice day in the late ’80’s, the unthinkable happened.

‘Mayday, Mayday, Mayday. Shackleton aircraft, 9 POB with TU failure’.

Those of us on the ground were stunned and impotent. The aircraft was over the North Sea about 40 minutes away.

It made it back.

The Flight Engineer had noticed that the airspeed had lost about 15 knots. He had a scan of his panel, then the pilots panels. Then he went forward to look back at the aircraft from the nose. Nothing of note. Then he went to the rear of the aircraft to look forward.

Due to the need to reduce weight at the rear of the AEW Shacks, all soundproofing behind the galley had been removed during the AEW mod.

Whilst the FE was in the rear of the aircraft, he noted that things didn’t sound right. He went forward, sat on the back of his chair (so he could see out of the side windows), and looking starboard, saw nothing untoward. He looked to port and the blood drained from his head as the realisation sank in. # 2 engine rear prop was in fine pitch while the front prop was almost feathered.

The crew carried out all the correct drills which included shutting down and feathering the affected engine. Unfortunately, due to the TU failure, only the front propeller would feather. The rear prop remained in fine pitch due to ATM and generated enormous drag while windmilling.

The engine didn’t overspeed, and the reduction gearbox didn’t fail.

Rolls-Royce’s greatest and longest serving front line piston engine, the mighty Griffon, had brought the nine souls back safely.

As a postscript, I had the team drop the oil filters, visually examine the engine, and give everything a minute and detailed going over. We couldn’t fault the engine. We changed the broken TU for a serviceable unit, and ground run the engine for over two hours. All indications and responses were normal. Still nothing from the filters.

In the end, I couldn’t snag the engine as no limits had been exceeded and no damage or fault could be found. More ground running followed by local flights. More oil filter drops. This engine was not ready to give up yet, so I signed it off for continuation of service.

It made it to TBO.

Well done and thank you Rolls-Royce.

camlobe

TU-95 Bear.

This surely has to be the ultimate contra-prop equipped aircraft ever. This 575mph giant has propellers measuring a massive 16′ 4″, and when on the ground each one can be rotated in either direction.. they are mechanically independent of each other.

This aircraft has always fascinated me. First flown in 1955 and the holder of several records, one of which was said to be the loudest aircraft at one point… and it can also out accelerate an F.3 Tornado at altitude.

Don’t forget Wyvern 1 x RR Clyde Jan ’49. 😉

Didn’t forget it, it just seemed to be a complete disaster! However it was the first designed use of a turbine powered contra-prop so worth a mention and I will add it.

Also I haven’t mentioned the various test-beds which were earlier with first flight dates, Lancaster TW911 first flying with two Pythons in January 1949.

We could do with some clarification on Spitfires and Seafires with this set up.
All I’m aware of is Seafire MK.46s and 47s with, and very vague references to some Spitfires having it.

Brabazon and Princess installations.

Almost certainly each prop could have been featherable independently of the other sharing the same axis.

In aircraft of this size and in the cause of safety you would not want to shut down any more engines than were necessary were there a problem with any one power unit or propeller.

To lose two off one side (even though you may have ten engines total) could make performance very marginal should a problem occur soon after take-off.

Anon.

Don’t forget Wyvern 1 x RR Clyde Jan ’49. 😉

Yes that is very interesting info Camlobe, thanks for posting.
I didn’t realise that Martin Baker made the translation units, I wonder if they really did pioneer this set up with the MB.5. It is the first British application of this particular system that I can find.
It’s funny but I never considered the Shack to be that important in the world of contra-props, but it is certainly being revealed here to be so.
the Shackleton does seem to be the the most produced and successful designs to feature this prop layout.

As far as I can see the Short Sturgeon was the first British aircraft to be designed and produced (all be it in limited numbers) from the outset to feature contra-props, and seems to be the only Merlin powered application.

This thread is also surprising me in its revelations of how few pure, ie single engine powering a gearbox driving two propellors, contra-prop designs there actually were.

MB.5 1 x Griffon May ’44
Fury 1 x Griffon Nov ’44
Seafire 1 x Griffon c.’45?
Sturgeon 2 x Merlin June ’46
Wyvern 1 1 x Eagle Dec’46
Seagull ASR 1 x Griffon July ’48
Wyvern 2 1 x Clyde Jan ’49
Shackleton 4 x Griffon Mar ’49
Wyvern 4 1 x Python Mar ’49

Similar but not quite the same are the following designs, they featured pairs of engines driving contra-rotating propellors. What I can’t fathom out on these, and the centre of wether they are true contra-prop or not, is if one half of the powerplant, and associated propellor could be independantly controlled, shut down, and feathered. If so they are not a true contra-prop design.

Bristol Brabazon 8 x Centaurus 4 x gearboxes
Saro Princess 10 x Proteus 4 x co-axial and 2 x single gearboxes