Spaceplane handling

[Raideur Ng]

I've got a standard heavy lifting SSTO that does it's job pretty darn well. The only issue is that it has some very wonky and downright difficult flight characteristics during descent and landing that make it almost a pain to get on the ground. I've done everything I know to try and get to fly better, but Im stumped. CG is perfectly balanced for all cargo/fuel states. Wings have proper AoA and CoL is behind and slightly above CG for stability. The CG is even balanced so there is no rotational torque.

The entire craft is optimized for low drag ascent performance. Not sure if that would create descent instability, but I figured I'd throw the question out and see what smarter people than me think.

Craft file:

https://nofile.io/f/ZJPtdzjjgum/Heavy+Lifter.craft

Any ideas?

[Vanamonde]

Could you be more specific about the kind of problem you're having? 

[Raideur Ng]

Poor roll control, nose droop when maneuvering, and general instability that requires you to fight it to achieve desired attitude.

[Scarecrow]

Well I downloaded your craft and had a play, and managed to get it to a 75k orbit and back again, without too many untoward flight characteristics.  I did make a few changes before being able to do that though.

Changes I made include resetting the angle of incidence on the wings back to neutral.  This means taking off is a little more difficult, and to help that I moved the front wheel down a little to lift the nose a touch, and raised the rear wheels to lower the rear.  I also moved the rear wheels forward slightly.  Ideally they should be just behind the CoG to aid rotation, but you would be in serious risk of a tail strike if you moved them much further forward from where they are.

For the control surfaces I changed the front ones so that movement is up to 150, and changed the response to pitch and roll only.  I also changed the rear controls to pitch and roll only.  The tail fin was changed so that it had yaw authority only.

The craft you supplied for download doesn't have any oxidizer on board, so I had to fill some of the tanks to enable the engines to work in recycle mode to get it to orbit.  However the fuel does seem a bit marginal, so I am not sure what weight you would be able to put in the cargo hold and still manage to make orbit.

If you would like to have a version of the craft with my changes, I can upload it to dropbox.

[bewing]

In general, I think the main issue is going to be that your nose has somewhat higher drag than the back end (because the back end is occluded to some degree). So I'd be trying to take some wing off the front, and moving it to the back, I think. This would change the CoL and require some redesign -- but I think it would change the drag characteristics enough to make a difference.

 

[Xd the great]

I think it is due to the consumption of fuel and making your planes CoM change.

[AeroGav]

On 7/3/2018 at 10:25 PM, Raideur Ng said:

Poor roll control, nose droop when maneuvering, and general instability that requires you to fight it to achieve desired attitude.

Stability wise, you are doing most things right.   However,  the mod "CorrectCoL" shows that your airplane is trimmed to fly about 5 degrees nose down when "hands off" ie with no inputs from you or SAS.

I presume you are trying to correct this by using SAS in its default "stability assist" mode.    The problem is SAS is a terrible fly by wire system.    It just tries to hold the nose at the same angle (relative to the sun)  that is was at last time you touched the controls.     So,  over time it "learns" how much nose up input to apply to fight the plane's aerodynamic tendencies,  but every time you touch the controls (such as to roll the wings) it resets so the nose falls quite some way.

Fix the excessive nose down tendency and stop using Stability assist mode as a crutch,  everything else falls into place.

This is my "minimally changed" version - https://www.dropbox.com/s/r0nm6cashlw6v30/Heavy Lifter MINMOD.craft?dl=0

What I did was use the fine rotation tool to reduce the incidence of the rear wing.   It makes a bit less lift,  but the front wing is at a bigger angle of attack, and starts to stall first when the airplane is at a high AoA.    As a result the plane flies 2 or 3 degrees nose up with SAS off and no-one at the controls,  but if you yank the stick back hard you can't get it to pitch more than about 10 degrees.  It's a space plane, not a fighter,  no need to pitch that much even on landing.       Most of the time you'll probably use Prograde Hold   SAS mode in flight, which cuts the AoA down to about 1 degree nose up.

I made a more heavily modded version too. 

https://www.dropbox.com/s/pt2emjl7fdmxs05/Heavy Lifter2.craft?dl=0

The original has a lot of empty tanks that are just making drag.    Even the above version is crazy overpowered.  Full nose up until it's climbing 40-45 degrees on takeoff to stop speed getting stupid down low,    then go prograde, it levels off about 18k by itself,  gets over 1500 m/s air breathing.      In rocket mode, TWR is over 1.15 

[Lego_Prodigy]

On 7/3/2018 at 4:25 PM, Raideur Ng said:

Poor roll control, nose droop when maneuvering, and general instability that requires you to fight it to achieve desired attitude.

Poor roll can be caused probably by the tiny wingtip surfaces you have, which are only most effective when they are at the very tips, and the ones you have are a little on the long side. Nose droop could be caused by the fact that the CoL is a little too far behind the CoM, always have it as close as you can get it to improve overall handling. And instability could maybe be solved by smaller vertical wingtip surfaces.

[Boris-Barboris]

15 minutes ago, AeroGav said:

However,  the mod "CorrectCoL" shows that your airplane is trimmed to fly about 5 degrees nose down when "hands off" ie with no inputs from you or SAS. 

Fyi non-retracted gears affect the graph.