Open Rotor
 

Engines, or anyone else who may know - am trying to get my head around Open Rotors - as per Safran's design.
By eliminating the cowl, you expand the volume of air that can bypass the hot core?
I assume that as the aircraft accelerates to flying speed, and then cruising speed the slipstream around the engine`shapes' the flow of bypass air (much as the cowl would) ensuring a large mass of cool air is dumped into the hot exhaust from the hot air - thereby increasing it's density and thrust?
Or have I got it completely wrong... :confused:

As the old saying goes, there's nothing new under the sun.

Whatever happened to propfans?

Yes, yes I know the history- noise etc. It’s the theory I’m trying to understand. Anyone?

While the concept clearly works, it does assume that current cowl technology only provides extra weight and drag, which is clearly not the case. Cowls also provide a degree of protection against ice shedding and blade separation too.

The theory of operation is similar to venturi systems where a high pressure fast moving central stream causes a lower pressure boundary at its edge which sucks in surrounding air which is then accelerated. This serves to create a duct that confines the central jet and improves efficiency while mitigating noise travelling sideways out of the stream.

on the theory: http://turbomachinery.asmedigitalcol...icleid=1672746

Check the Russian research, they have been keeping this going.

http://cdn-www.airliners.net/aviatio.../5/1270539.jpg
https://upload.wikimedia.org/wikiped...fan_Engine.jpg

Had them on the Electra. Used to reckon those steel blades were UDF.

Big props are still being developed, but I think the Mach limits are the biggest problem and also noise.

A400M for example- Mach .72

Lot of extra weight for all those blades

Ahah - venturi effect - I see - thank you G0ULI.

Tartare, if you look at propulsion theory, you get maximum efficiency by accelerating an infinite amount of air an infinitesimally small amount. Pure jets accelerated a small amount of air a huge amount - turbofans accelerate a much larger amount of air a smaller amount and hence are way more efficient. That's the reason for the ever increasing bypass ratios of the turbofan engines. Problem is as the fan gets bigger, the weight of the inlet, fan duct, containment ring, drag of the cowling, etc. tend to cancel out the improved propulsion efficiency.
A turboprop or unducted fan gets rid of all that weight and drag, but creates other problems. Of course there is the noise (especially with counter-rotating props which is pretty much a worst case for noise) and blade out. But efficiency wise, having an inlet provides a huge advantage for high-speed flight. With an inlet, the speed of the air at the fan face is pretty much independent of the flight speed - the engine fan doesn't see much difference between takeoff and Mach 0.8 cruise. Hence you can keep the fan speeds sub-sonic most of the time even at high speed cruise (good for efficiency) where as unducted props go supersonic at high speed cruise (very bad for efficiency). The newer prop designs have moved that cruise speed at which they go supersonic quite a bit higher, but still well short of ducted fans.

tdracer,

I hope this doesn't count as thread hijack, but your explanation of the effect of the inlet brought home a point that was eluding me on the AF66 thread. If a fan separated intact from its shaft in cruise, it would at the instant of separation be spinning fast enough to produce significant thrust. Unrestrained, it would tend to pull itself forward through the inlet.

If it have this right, though, once it cleared the inlet (and even if its rotation had not decreased substantially and it had remained undamaged) it would become in effect a UDF turning too slowly to make thrust at cruise speed. So it seems like whatever forward velocity it had relative to the aircraft when it cleared the inlet would be all there was.

It's important also to consider incident angle on the blades. As soon as it changes (air speed and airfoil twist) the blade stops pumping (as an airfoil) and is just a free body against windage... It can be modeled to predict free body trajectory

Where the fan goes after something like a fan shaft separation is pretty much indeterminate. Initially the thrust vector will pull it forward, but then it will immediately start chewing into the inlet and surrounding structure - and that will start to re-direct the fan depending on the nature and density of whatever structure it's impacting.
As I noted earlier, the only comparable event I can think of was an RB211 on an L1011 center engine back around 1980. IIRC there was some sort of bearing problem that caused the bearing to overheat to the point the lube oil caught fire and cut the shaft - the fan went forward and down and tried to cut the aircraft in half :eek:. Although they managed to land safely, it was an obvious safety concern and Rolls came out with a 'fan catcher' (sort of a disc brake like device) that would retain the fan in the event of a future shaft failure. A while later there was a similar RB211 fan shaft failure but on a 747 - the fan catcher worked, but the LP turbine quickly over sped and burst, cutting the back of the engine off and peppering the wing and fuselage with shrapnel...:sad:

The forward screw forward is a natural result of the blade spinning in a plane against the friction of a soft outercase. The fan catcher concept simply puts a hard 360 degree ring ahead of the fan (streamlined) so that the blade screwing action is stopped.

The early RB211 fanshaft separations were attributed to dirty Titanium and both P&W and RR traded processes to eliminate it.

The RB211 fanshaft failures were due to LP location bearing (intershaft) ball bearing wear/break-up that allowed excessive axial/radial movement that resulted in heavy rubbing of the air-oil seals. This resulted in oil leakage outside the bearing chamber that ignited and overheated the shaft which then failed.


The LP turbine overspeed was addressed by a fuel cut off system on later marks.

There were two episodes for fan failure on the RB211-22B. The early episode was failure of the titanium fan disk. I met the first Eastern Air Lines aircraft that returned after losing the Nbr 3 LP fan. Fortunately the major pieces went away from the fuselage but one fan blade had penetrated the leading edge slats and was lodged in a passenger window. It penetrated the outer pane and was just touching the inner pane. They set a very low cycle life limit on the fan disk there being different lives for disks made from the upper or lower halves of the billet. TWA also experienced one of these failures.
The later failure were as described above involving the LP Location bearing. That one was an EAL Nbr 2 engine . I viewed that aircraft at JFK the next day and it was only fate that allowed the aircraft survive for three hydraulic systems were severed and the fourth had the line damaged but not punctured. With the flying stabilizer the A/C would be uncontrollable if all four systems had failed. Fortunately all later versions of the RB-211 had improved the LP bearing.