actually the relaxation of natural 'aerodynamic' stability is in principle limited only by design-specific factorsTommy Cookers wrote:in Prandtl's time planes had poor wing efficiency in flight because the wings were too big, sized for practical runway lengthFulcrum wrote: .....wing efficiency of what is essentially an 80 year old design.
and a bigger wing is a disproportionately heavier wing
long since then this problem has been alleviated by lavish flap systems that greatly increase the lift coefficient when desired
such systems cause pitching moments that require a pitch trim authority that is impracticable without a length of fuselage ....
unless the aerodynamic stability is by design relaxed and replaced by artificial stability
the relaxation of aero stability is for public transport operations (ie airlines) severely limited by regulation
(beyond that limit a failure of the artificial stability system kills everyone)
such relaxation is bound up with potential for systems giving what we might call 'ride control' effects
eg the 777 and equivalents already have the permitted amount of these, to good effect
flap blowing (enabling smaller wings) is similarly banned for PT operations (though a key feature of the C-17 transport aircraft)
(starting with the 707 jet airliners have gyro-based 'yaw-damper', bandwidth sub 1Hz, malfunction is relatively unimportant)
aero stability eg in pitch and yaw contributes to a rough ride in 'gusts' (turbulent air)
the airliner makers have chosen with each design how far they will go to supress this roughness (still not very far)
eg the 777 has pressure transducer driven lateral gust supression systems (1.5 Hz band), and reduced natural aero stability
and an accelerometer driven system to suppress fuselage lateral bending oscillation
pitch-related gust allevation is done at various frequencies and different ways ie actively as above and/or within flight control laws
again equivalent to reduced natural stability
both Boeing and Airbus use this active damping of aerodynamic and structural forces in pitch and yaw for better ride
without reducing natural aerodynamic or structural stability to a degree that would be catastrophic with system malfunctions
typically the current large airliners have structural natural frequencies around 3 Hz
(but ever-larger conventional aircraft have lowered structural natural frequencies, then making this area critical wrt certification)
the tailless/BWB aircraft designer would want to go further, into zero or negative aero stability
eg a 20% weight saving, said Dr Howard Smith of Cranfield College of Aeronautics (they did BWB work with Boeing)
so the B-2 uses direct lift control (lift dumping etc ) driven by AoA sensors giving at best only 0.04 sec prediction
ie a high bandwidth system more relateable to ideas of (race) car 'computer-controlled' suspension
an airliner (not needing to be stealthy) could/would use beam-type sensing providing more and better prediction
about 40 years ago RR looked at direct-lift VTOL supersonic airliners needing no runway and little wing
the lift engines even then were cheap and very light and compact (40:1 thrust:wt ratio)
we might today raise useable subsonic speeds by flying at lower altitudes via a very high wing loading and ride control
lower altitude, eg in warmer climatic regions that now have a large airline presence, allows an economic 60 mph higher speed
with runwayless VTOL as above or with a conventional runway and afterburn or flap-blown takeoffs
airliners still have for takeoff and landing reasons a wing that's otherwise too big (they alleviate this by flying high)
BWB with v high wing loading as suggested above might make more sense than as a direct replacement for conventional airliners
the other big problem with BWB seem to be escape issues (the pressure hull fit to the structure being disadvantageous)
and imo high drag due to boundary layer thickening with large chord (certainly active over-body BL control is advocated)
aviation has almost always found that real boundary layer thickening is more than expected
and if engines successfully ingest the boundary layer (BLI - Boundary Layer Ingestion) there is a propulsion benefit
a significant response in BLI (at aircraft size more market-realistic than the BWB ?) is here
(note the selection of a reduced cruising speed to the low 0.7s Mach and the geared or otherwise radical turbofans)
http://web.mit.edu/drela/Public/papers/ ... 1_3970.pdf
http://aviationweek.com/awin/boundary-l ... a-d8-hopes
http://gizmodo.com/5900927/are-mits-dou ... air-travel
and for a proper and useful explanation of BLI propulsion benefits, here
http://www.lr.tudelft.nl/fileadmin/Facu ... an__LV.pdf