Contra-rotating props and Bernoulli etc

[freewheeler]

I'm interested in how CR props work! On a mechanical level I can visualise some sort of planetary gear lash-up to effect reverse rotation off the crankshaft; but I am stumped by the thought of the 2nd prop (ie the one nearest the engine) taking bites out of an airflow disturbed by the 1st prop which bites into "clean" air! Doesn't this result in a real mess of an airflow? perhaps this aerodynamic conflict explains the racket which the "Bear" created? Perhaps not. Also, I find Bernoulli counter-intuitive, in that I think of lift being created when a control surface projects into the airflow and the pressure of that airflow pushes against it, forcing the wing up (eg in the case of an aileron). I also think of a propellor in the old sense, ie an "air-screw", biting its way through the air like a boat's screw through water. On reflection, when I see a boat's prop spinning, there appears to be an area of air behind it, so is this the vaunted area of lower pressure which is the source of propulsion? can hydrodynamics be akin to aerodynamics? are air and water different as a matter of density only or by a lot more? Why do we have NOTAR and not CR rotors a la Kamov? why does the BAE 146 have 4 engines and the 777 get away with 2? Finally, when the Pakistan earthquake hit and they were crying out for helos, where were the Mi-8's etc from India, Russia and China? Am I being naive (yes) but wouldn't a multi-national helo task force for disasters be great? What could be better, challenging flying but not in combat, but to save lives!! Ah well, dream on. :ugh: :confused:

[skiingman]

I'm interested in how CR props work! On a mechanical level I can visualise some sort of planetary gear lash-up to effect reverse rotation off the crankshaft; but I am stumped by the thought of the 2nd prop (ie the one nearest the engine) taking bites out of an airflow disturbed by the 1st prop which bites into "clean" air!

I'm interested by it as well. It would appear empirically to be a bad choice since its so uncommon (both in propellers and helo rotors) but I'd be interested in knowing what advantages it does present and how bad/interesting the aerodynamics are. I wonder if a modern propeller could replace the CR deals on the Bear and perform similarly, and if not, why not.

Re: the noise of the Bear, I've never heard one but I've often read of that phenom. I've wondered if it has much to do with the contra-rotating design or just really high tip speeds.

why does the BAE 146 have 4 engines and the 777 get away with 2?

I can't answer any of your questions, but the answer to this one is rehashed weekly in this forum and a simple search out to turn up all sorts of (valid) reasons for the trend toward fewer engines when feasible.

[Doodlebug]

Mmm,interesting. Wouldn't CR-props get rid of the critical engine on a twin, for one thing? (downgoing blade produces more thrust than upgoing, hence on a twin with two 'regular' props one engines' thrustline is offset closer to the centre than the other, etc, etc) Also, surely the airflow behind a prop, whilst definately tremendously accelerated relative to the still air in the surroundings and also possessing a vortex-like swirl, is still simply a mass of air meeting the prop at a certain velocity, and not a sea of crazy turbulence? Therefore, all that is necessary is to have the blades at the appropriate pitch angle and you're ok?

Must be a complete and utter bitch to build and maintain, though, for just a little bit more power.

[W.R.A.I.T.H]

With the contraprops and messed up airflow, I'd say it's the other way round. Back in my uni years we had a very old lecturer - aerodynamicist, who used to come into close touch with russian technology in his younger years. He had a hell of a lot to say, so in really simple terms - the overall propeller efficiency is a product of its mechanical and aerodynamic efficiency. The aerodynamic efficiency is the amount of forward thrust generated from a unit of power. Conventional propellers actually lose quite a lot of this power by turning it into the swirl energy, which rotates behind the propeller with no apparent use, I believe even the most modern ones like the curved ones on the C130J do lose about 30% on the swirl. The second propeller in the contraprop utilizes mainly the swirl energy, turning it back into the direct stream - theoretically. Quoting a book on Tupolev, the aerodynamic efficiency of the contraprops mounted on the NK-12 (Tu-95, An-22) is 96% - compares to 60-80% of a modern conventional prop. the biggest problems of this kind of a design are but significant added weight, complexity and maintenance requirements, since it is all a heavy piece of machinery. Also as you guys mention, the added noise is tremendous - i don't know where this one is generated, but having heard the whine of the mighty Bear myself on an airshow, it really cuts into the bone. There are stories of submarine crews who could hear the whine of a Bear cruising 10 km above.

BAe-146: this was discussed somewhere around here not long ago. Allegedly the initial design from 80s counted on operations to remote airstrips, where an engine out on a twin would ground the craft and have it wait for a replacement unit. On a quad, that would constitute a deferred defect under the MEL and warrant at least a technical non rev flight back to the base.

B777: that's more of an a.net stuff here :ok:

[ionagh]

As far as the noise is concerned, the blade tips were normally in the transonic region when operating at max cruise speed on the Bear.
The shock waves generated would result in a level of noise that was often heard by the pilots of jets sent up to intercept them :ooh:

[john_tullamarine]

Re the 146, I recall reading an RAeS paper some years ago which looked at the history of the Type. As best I can recall, the original design was for a twin but then shelved when the engine failed to eventuate. It was dusted off some years later when the little vacuum cleaner engine became available and transformed to a quad ..

[411A]

Propellors.
Generally speaking, they are quite efficient at what they do, transform mechanical energy to useful thrust, to propel the aeroplane.

I can't speak about newer designs, but I do have numbers for one older type, the DC-6B.
Propeller efficiency...86%, give or take.
Surprised?
I'm not.
Hamilton Standard put a lot of hours/thought into propellor design, and the 45E60 blade is very efficient.

[chornedsnorkack]

Wouldn't CR-props get rid of the critical engine on a twin, for one thing? (downgoing blade produces more thrust than upgoing, hence on a twin with two 'regular' props one engines' thrustline is offset closer to the centre than the other, etc, etc)

Yes, they would. But twin does not need critical engine anyway - you can have two conventional contrarotating props, and no critical engine.

But if you have two contrarotating props which are not coaxial, then you are still shedding propwash and wasting energy on swirl. The point of coaxial contrarotating props is that they are eliminating the swirl.

[Dick Whittingham]

If contra-props are so efficient then why didn't everyone use them? I would need some convincing. IMHO they are chosen because for a given diameter they can absorb more engine horsepower (EHP). Prop diameter is usually limited by either tip speed or ground clearance, though bending stress in long blades also has to be taken into account.

The Bear is the only long range aircraft I know of that uses contra-props, and they have hugely powerful engines. Anyone know the EHP? 7000? More? That is a lot of power for a single disc.

Dick W

[xsbank]

Effeciency vs. weight/complexity? I would think CR props only become economical when you are talking huge engines like the bear. How big/how many blades of a conventional prop would you need on a Bear with engines that big?

Ailerons 'deflecting' the slip-stream? I was taught that the downward aileron increased the angle of attack of the wing and increased the lift of that wing while the 'upward' aileron deflected the slipstream.

[John Farley]

freewheeler
To save you pressing the search button
The 146 was originally conceived by deHavilland long before BAe. Back in 1966 I was told that if the Harrier did not get ordered then I could expect to go to Hatfield and deal with it. It was designed as a hub and spoke aeroplane to open up Africa where there were no facilities at the end of the spoke. Literally none.
Because they did not want it stuck at the end of a spoke with an engine problem they decided it had to have four engines and be certificated to RTB without pax on three.
Since some strips were expected to be pretty rough the high wing was chosen to keep the intakes as far from the surface as possible.
Eventually BAe happened and decided to launch the design on the back of super quiet for parts of the US and the rest as they say is history.

[smith]

The Bear is the only long range aircraft I know of that uses contra-props,

Now obsolete but the RAF Shackletons had CRP's.

[Whitehatter]

http://www.pprune.org/forums/images/editor/separator.gifYou can also see contraprops on aircraft like the Illyushin 18 if you go to the dusty corners of places like Sharjah. The big Antonovs also used them.

There's also this more recent take on the contraprop done by Antonov
http://upload.wikimedia.org/wikipedia/commons/6/61/Antonow_An_70_prop_detail_rvb_jno_MACS_2001_0037.03.jpg

[Brian Abraham]

Anyone know the EHP? 7000? More?

Dick, 14,795 shp
The Tu-114 Rossiya airliner was of the same general layout as well. With the noise wonder what the ride was like. from Wiki - The Tu-114 had a fairly short commercial service life, being operated from 1962 to 1976. While in service the plane was known for its reliability, speed and fuel economy (it used less fuel than the Il-62 that replaced it). In regular service with Aeroflot, the plane was first used for flights to international destinations like Copenhagen, Havana, Montréal, New Delhi, Paris, Belgrade and Tokyo (in co-operation with JAL). When it was replaced by the Ilyushin Il-62 on these routes, it was frequently used on long range domestic flights. After the end of commercial service, it was used by the army and airforce until the early 1980s.

[chornedsnorkack]

Prop diameter is usually limited by either tip speed or ground clearance

Ground clearance is obvious. But what does tip speed have to do with limiting propeller diameter? Obviously, a propeller of whatever diameter suits ground clearance should be rotating at whatever rpm gives the optimal tip speed!

[ICT_SLB]

When I worked on the BAC 1-11 and the 146 was just being mooted, we were told that it started life as the HS681 with twin Pegasus. This was intended as a V/STOL supporter to the Harrier II (P1154) force. The Peggy gives you two effective jets per engine (see the QSTOL Buffalo) so they needed to be replaced by the 4 Lycomings.

Some of the last Western contra-prop research was done by Boeing using a Fairey Gannet in support of the UDF concept - it was curtailed when they ran out of tires! - or so I was told by an aerodynamicist involved. I beleive they found about 10% increase in efficiency mainly from removing the swirl in the propwash (as above).

[Dick Whittingham]

Tip speed is limiting because you cannot slow down the prop below a reasonable RPM. Or you could, but it is called feathering.

Dick W

[Evening Star]

Cannot comment on the technology, but with regard to the noise once met a TU-95 flight engineer and he assured me that deafness is quite common amongst former TU-95 crews. Suppose 'hearsay' is perhaps the wrong way of describing this piece of information.:hmm:

[WHBM]

Apparently the one remaining Antonov 22 visited the UK last week, routing to and from Brize via Detling and Woodley. I didn't know it was here until afterwards but can tell you I heard something "strange" from my bedroom in Central London in the night although it was presumably well to the south. That resonance carries for miles.

The Monino museum near Moscow has examples of all three of the big Soviet long-range contra-rotators, the An-22, Tu-95 and Il-114. Interesting to look at them close up, as well as things like the Mil-12 helicopter, which looks like they took the drawings for the Chinook but accidentally mounted the two main rotors perpendicular instead of in-line, then had to dream up some support structure to hold them in place ! The Soviets must have had very different research.

Whitehatter, the Il-18 is not a contra-prop. Straightforward 4-blader.

Evening Star, Defaness in later life was a feature among ex-BOAC Argonaut crews as well apparently.

[The late XV105]

The Wikipedia take on the subject: http://tinyurl.com/2f9uo8

As a young lad of six or so, to this day I recall the distinctive sound of Fairey Gannets ( http://tinyurl.com/yq6fog ) clawing their way in to the night sky at Changi, Singapore. I also remember asking Dad why such a small aeroplane could be so loud, and his reply about contra-rotating propellors. In true young lad fashion of course that would have lead to "why?" but I don't recall the rest of the conversation!

[Avi8tor]

The way i understood it, the 4 engine layout has 2 advantages.

1 - could spread the weight of the engines over the a larger section of the wing, hence making the wing lighter to construct and

2- in the advent of engine failure at V1 you only lost 25% of the thrust not 50%. I know the climb gradient required is larger, but the net result was you put smaller engines on.

A small ps on the CR story, i like the theory of energy 'recovery' as u smooth out the helical flow, kinda like stators behind the N1 fan.

[chornedsnorkack]

A small ps on the CR story, i like the theory of energy 'recovery' as u smooth out the helical flow, kinda like stators behind the N1 fan.

Indeed.

Is there any reason a propeller aircraft cannot use stators?

[GOLF_BRAVO_ZULU]

Unshrouded stators? I think it's called drag.

[chornedsnorkack]

Unshrouded stators? I think it's called drag.

And why don´t shrouded stators cause drag?

[barit1]

The "shrouded stator" bit works best when it's a convergent duct, yielding a higher exit velocity for greater cruise efficiency. This can be done with a smaller rotor diameter, bringing the drag down.

And by the time you do all this, you might as well drive it with a gas turbine and call it a turbofan.

But if you want really efficient lower-Mach cruise, the counter-rotating fan (e.g. GE36) (http://en.wikipedia.org/wiki/General_Electric_GE-36) is the record-setter.

[chornedsnorkack]

The "shrouded stator" bit works best when it's a convergent duct, yielding a higher exit velocity for greater cruise efficiency. This can be done with a smaller rotor diameter, bringing the drag down.

And by the time you do all this, you might as well drive it with a gas turbine and call it a turbofan.

Turboprops are also driven with gas turbines.

Why do turbofans find the swirl-compensating stators to be efficient, rather than a source of extra drag?

[barit1]

Why do turbofans find the swirl-compensating stators to be efficient, rather than a source of extra drag?

Like everthing else, there's a bit of tradeoff (airfoils have lift AND drag, don't you know).

But to merely call the stators "swirl-compensating" misses the point. They convert the swirl or vortex energy (which would otherwise be wasted in the slipstream) into increased static pressure, which results in a higher nozzle pressure and velocity, which can in turn be optimized for best cruise thrust & SFC.

A propeller designer doesn't have so many "knobs to twist" in optimizing his design, so he is stuck in a lower-speed cruise regime.

[Avi8tor]

I think I made a faux pas, should have used the word 'guide vains' not stators. Stators in the engine have a totally different function.

All engines have guide vains behind the N1 fan

[barit1]

All engines have guide vains behind the N1 fan

It's a matter of semantics. Different donkmakers have different terminology; Pratt's tower shaft is GE's radial drive shaft.

But vains, veins, and vanes all have distinct and separate meanings. :}

[chornedsnorkack]

Like everthing else, there's a bit of tradeoff (airfoils have lift AND drag, don't you know).

But to merely call the stators "swirl-compensating" misses the point. They convert the swirl or vortex energy (which would otherwise be wasted in the slipstream) into increased static pressure

Precisely.

They take the swirl downstream of a (shrouded) fan, and convert it into (nonswirling, more direct) jet blast. They spend some of the energy for drag, but they still gain more energy by converting swirly slipstream to direct than they lose to drag.

Why cannot open propellers also install open stators behind the propeller? Converting swirl to more direct slipstream?

[barit1]

http://www.airwar.ru/image/i/attack/fantr-i.jpg
Theoretically possible, and occasionally implemented.

[Avi8tor]

Well spotted barit1, spell check can't fix that.:} I take the red face.

Man, but the world hates a smartar:mad:

:)

[ICT_SLB]

Avi8tor,
At least you only did it once. We once let our (ex-) secretary loose on a Stall Protection System flight test report - Angle of Attack veins?

[freewheeler]

Fascinating stuff....I followed the wiki links to the Bear and was amazed!! and so much more food for thought as well; thanks!!:ok:

[Dick Whittingham]

As a seeker after knowledge, let me put this to you:

Considering the airflow through the prop as in a tube, the prop generates thrust by accelerating the air in the direction of flight. The only way for the stators to generate extra thrust is to further accelerate the air. How do they do this?

Dick W

[Anotherflapoperator]

Crackin' thread this one. Thanks for a great read so far.

As to the 4 Vs 2 bit on the 146, all I know is that having 4 hair dryers means an engine failure won't cause one of our old skippers a heart failure! Just a minor inconvenience.....I look forward to more of this,

Thanks again!

[barit1]

Considering the airflow through the prop as in a tube, the prop generates thrust by accelerating the air in the direction of flight. The only way for the stators to generate extra thrust is to further accelerate the air. How do they do this?

Excellently put question, Dick. The extra thrust comes from a two-step process:

1) Capture the vortex energy by turning the air back parallel to the direction of flight. In the process of doing this, the stator vane array has a larger exit area than its entrance area, so the air is slowed. The energy capture is reflected in the static pressure rise.

2) Re-accelerate the air through an annular nozzle, maintaining the axial flow direction.

Both steps are necessary!