Lift drag ratio numbers seem low

[AeroGav]

In flight, you can open up a nice aero forces display by opening the ALT F12 debug menu, going to the physics tab, then the aerodynamics sub-tab,  then checking the "display aero data gui" checkbox.

I've noticed however, that the lift:drag ratio numbers seem very poor compared with real-life airplanes.  

The best I can get is 8 or 9 to one,   at 2 degrees AoA, at low speed and altitude.

At 0.82 mach and 10km,  similar to how commercial airliners fly, best seems to be 2.8 AoA and maybe 5 to one lift / drag ratio.      Real commercial airliners are pushing 20:1 at such a point,  in fact i'd bet the newest of them, the 787, is over 20 at an "economy cruise" setting.

Supersonic , things get worse.   At 1.3 Mach, I try climbing higher to use thinner air to compensate for the extra drag.   Best results seem to occur at 3.5 AoA and altitudes of 14km or more,  I might get close to 4:1.     Concorde did 7.5 to 1 at mach 2 and 60,000ft.

As we get deeper into the supersonic regime, numbers ebb steadily lower.   Above 20km  I'll start my final climb to orbit at something like Mach 4 and shutdown engines (needing only to circularise) at Mach 6.6.      During this period optimal AoA seems to shift from 4 degrees to 8 or so.    At best, I might see L/D display of 2.8, whilst pitching up to high alpha because my craft is overheating can pull it down to 1.6.

Reading a little further on wiki, it appears max lift:drag does taper off with increasing mach no matter how high the altitude and thin the air, especially for conventional supersonic/transonic swept designs.   However waveriders that rely on compression lift can do better, with designs like the Hypersoar project making 10:1 at mach 6.

I guess this is all to compensate for the overpowered nature of jet engines themselves, compared with real life.

I still find it a bit weird however, that if my L/D is so poor, why my spaceplanes are so reluctant to actually land.     I've long since given up targeting the runway and am quite happy to settle for anywhere on the KSP peninsula or in the shallow seas nearby, usually after flying back and forth across it several times.   I guess as we head below 100 m/s our L/D is getting up that of a Cessna light aircraft and we're usually coming in a tad "hot" , also the drag from landing gear, flaps, and jet intakes (with engines off!) is less than it should be .

[Red Iron Crown]

Are you using angle of incidence on your wings? I've found that drag is significantly reduced if the fuselage stack can be pointed directly into the airflow while the wings still have a positive AoA, this really helps with the L:D ratio.

[Nich]

In 1.0.5  7:1 was the best I was able to achieve with a very slow glider type craft with an AOI on the wings.  With out it I think I would be happy with 5:1.  I do think this is countered by the huge amoun t of lift created at stall limit.  Most of my space planes are able to land around 50 m/s at 25 degrees AOA without flaps.  Most commercial planes can not do this requiring 60-70  m/s at a minimum.

[bewing]

Kerbals find their wings by the side of the road. Of course they are not optimally efficient airfoils -- even if Kerbin is located in our universe (with our laws of physics), which is debatable.

[herbal space program]

4 hours ago, Red Iron Crown said:

Are you using angle of incidence on your wings? I've found that drag is significantly reduced if the fuselage stack can be pointed directly into the airflow while the wings still have a positive AoA, this really helps with the L:D ratio.

This made a huge difference for me. I also do agree with OP though that in the current air, lift seems to drop off  a fair bit more precipitously when going high and fast compared to previous versions. This is fortunately offset by less drag at lower levels, so it works to just pack on moar wings to get better performance up there.

[AeroGav]

18 hours ago, Red Iron Crown said:

Are you using angle of incidence on your wings? I've found that drag is significantly reduced if the fuselage stack can be pointed directly into the airflow while the wings still have a positive AoA, this really helps with the L:D ratio.

It adds at most 10% extra for me, at the cost of construction/handling headaches.    If you have a draggy mark 2 fuselage,  are flying fast and low, or have small wings operating at a large angle of attack, then it may help more.   I tend to fly at 3 to 7 degrees AoA , so it's less pronounced.    Also if i set 5degrees incidence on the wing  i find myself flying with a negative AoA on the fuselage, creating drag and generating downforce, during the airbreathing part of the ascent where aoa is 3 or 4 degrees for the wing.   I suppose thanks to airbreathing having much higher ISP it's worth taking a hit here if the closed cycle phase is more efficient, but it's a lot of aggro because my designs tend to have multiple wing sections and strakes and canards attached separately to the fuselage,  landing gears on wings, tailbooms on wings, dihedral etc.  

[Adelaar]

I'd be happy to get more than 2 on the L/D ratio, most of my spaceplanes do close to 1 L/D on ascent, and most of my conventional planes don't do much better. I don't have absurd designs, and work with angles of incidence on the wings to minimalize the AoA the craft has when flying. I did however notice that the way drag is calculated might not be realistic. It treats every object in the craft as solitary, even if they reside inside a fuselage. e.g., a reaction wheel (tiny) inside a fuselage (small) will generate significant drag. A better way to calculate drag is to use the surface area of the craft as a whole, not the aggregate of parts. The latter will have a significantly higher drag. Also, parts inside cargo bays STILL generate drag, which becomes most obvious when parts inside a cargo bay start overheating due to high speeds.

[Padishar]

9 minutes ago, Adelaar said:

Also, parts inside cargo bays STILL generate drag, which becomes most obvious when parts inside a cargo bay start overheating due to high speeds.

That doesn't necessarily mean they are generating drag (I'm not saying they are or aren't and won't without seeing decent evidence on a completely clean stock install).  Heat can be conducted to parts inside cargo bays but has problems radiating away and are protected from the extra atmospheric cooling by convection...

[Adelaar]

10 minutes ago, Padishar said:

That doesn't necessarily mean they are generating drag (I'm not saying they are or aren't and won't without seeing decent evidence on a completely clean stock install).  Heat can be conducted to parts inside cargo bays but has problems radiating away and are protected from the extra atmospheric cooling by convection...

You're absolutely correct. I've decided to build a test bed by placing a Rockomax brand adapter in a 0 degrees position (which should yield the highest possible drag), testing at 3000 m and 250 m/s, and then tested with 90 degrees position (which should yield the lowest possible drag), testing again at 3000 m and 250 m/s. Tests yielded the same drag (at 225.000 kN), but opening the cargo bay sharply increased drag (322.000 kN). So my previous comment regarding drag generation inside the cargo bay can be disregarded...