Geared Turbofan & Contra Rotating Turbofan

[Obi Offiah]

Does anyone know if the ratio between the fan and LPT will remain fixed? I ask because if not there may need to be another indication on the engine format e.g Fan, N1, N2

Also with SNECMA's Contra Rotating Turbofan (CRTF) they mentioned reduced engine speed and hence less noise for equal power. Is there also a design concept limitation which prevents running the engine at typical speeds and thus producing substancially more power?

Obi

[barit1]

A geared turbofan has (doh!) a fixed ratio between LPT and fan rpm, and is thus no different in concept from a free-turbine turboprop. One tach should suffice.

A contra rotating turbofan, assuming the two fan rotors are not mechanically linked, would require two tachs under FAR rules (one tach required for each rotor).

And a fan designed for lower rpm ops will have a relative high blade pitch (aka a low "stagger angle"), and thus it would require a excessive driving torque to spin it up to "normal" rpm. The core could probably not deliver that much torque, and if it did, it might twist the shaft in two. :eek:

[singpilot]

I was 'involved' with the UDF (UnDucted Fan) demonstrator in an MD81. Several fan configurations were tried, all without success.

Twin rows of aft, conta-rotating external fans, different pitches and lengths, leading and trailing fans were tried. (Vibrations amomg several magnitudes and frequencies).

A scimitar single row fan with different pitches and blade lengths. Similar results.

Fuel flows were unpredictable. Several medium and high altitude 'unstarts' were recorded, but no definative cause was ever singled out. An 'airstart' was never a given.

When the tests were complete, the airframe (never intended for commercial service) was scrapped because so many rivets were on their own flight plan.

There was more single engine MD81 time than not, probably. Thank Gawd for that other JT8D-217.

[keesje]

New R&D programs on Counter Rotating Open Rotors (CROR) are in full swing again. Computional Fluid Dynamics (CFD) play a major role. Progress is made on reducing noise and limiting vibrations. The big OEMS are in doubt to CROR or not to CROR. It promises an additional fuel saving over advanced conventional engine configurations.

Even GE, while developping the more conventional LeapX is still looking at CROR. Photo I took at Paris Airshow:

http://i191.photobucket.com/albums/z160/keesje_pics/OPENROTOR-26PCT.jpg


During recent years aircraft configurations with the engines on top of the aft fuselage were considered to shield off noise. Engine containment issues / regulations (1 fan blade taking out the other engine) let to configurations with engines in more convention wing and T tail configurations, like singpilot's MD80 20 yrs ago..

http://www.nlr.nl/eCache/DEF/12/623.bD0.jpg

[singpilot]

Amazing. I sure hope lessons have not been forgotten.

[barit1]

singpilot - There was another UDF testbed - a 727-100 operated by GE. Don't know what became of it, though I suspect it too was scrapped.

[Obi Offiah]

Thanks :ok: