Over the last couple of decades L/D ratio has increased significantly. Is it possible to determine an "upper limit" on how much more potential there is for common tube-wing aircraft to improve L/D. I am looking here at commercial Aircraft. I am aware that their is already a post regarding gliders.
For a jetliner flying without doing any acrobatic manoeuvres, $L$ equals weight for most (if not all) of the flight. That means that the only ways to improve $L/D$ is by reducing $D$.
At subsonic speed, drag is basically given by the sum of two terms:
- induced drag, mainly drag due to lift;
- and parasite drag, mainly due to skin-friction.
Induced drag can be reduced increasing wing span while skin-friction can be reduced reducing... well the skin i.e. the external surface of the aircraft and improving its smoothness.
These tricks have obviously limitations:
- bigger wing span means more structure and weight and is anyway limited by space availability in airports;
- smaller external surface means less space for the payload;
- and however the surface smoothness is obtained, after some takeoffs and landings it is anyway gone due to dirt.
G load tolerance will be strongly influential in determining a L/D maximum for tube wing commercial aircraft.
Expected cruising speed will be another.
With improvements in strength and flexibility (without fatigue) advances have been made in L/D ratios, as they go hand in hand with improvements in fuel economy. Modern airliners are in fact excellent gliders, with L/D ratios more then twice that of a Cessna 172.
This glider sports a lift to drag ratio of 70.
Higher lift to drag ratios can be had at speeds where Mach effect is not significant. Generally, wings that can bend air without forming a transonic shock wave are most efficient. This is usually at Mach 0.5 or less.
In raw physics terms there’s no theoretical upper limit. In reality there are mechanical limits such as the strength, stiffness and durability of materials. Also highly relevant to your question is what is considered to be a ‘common tube-wing aircraft’, which would presumably rule out high-performance designs. So the question really comes down to how far you’re willing to stretch the definition of the term.