Help! First try with a very big spaceplane, but I can't get it to pitch up

[-Orion-]

I've been stuck on this for hours. 

My idea was to create the biggest spaceplane I could come up with just for fun. Problem is it just won't lift, and I don't mean off the ground, I mean at all. 

I hacked the gravity to give it some speed and height, but once I turned it back on it started to nosedive uncontrollably. I changed the design of the wings a few times but the problem remains, it just doesn't want to pitch up. I can't really figure out what's wrong. Is it the control surfaces? The wings? 

Edited April 20, 2020 by -Orion-

[steuben]

A quick guess might be your centre of mass relative to centre of lift. Post a pic in the hanger with those two showing.

[-Orion-]

Here it is. I made sure it was behind the center of mass

[UncleManuel]

Definitely a combination of wrong CoL and insufficient control surfaces. Activate the 3 balls during construction. These show CoG, CoL and CoT. The first two should be close together to prevent the plane to pitch down or up. 

Another factor: weight and thrust. The upper layers of the atmosphere are getting thinner and thinner, so you need speed to stay afloat. If your plane is too heavy it will fly like a brick. A good way is to use the Whiplash engines with supersonic intakes - the faster you go the more air they cram into the engine the more thrust you'll have. Supersonic jet engines are good for up to 20000 meters and 900 m/s. Above that you'll need rockets or the switchable Rapier. 

 

Edited April 20, 2020 by UncleManuel

[AHHans]

3 hours ago, -Orion- said:

I made sure it was behind the center of mass

Yes, that is important to make sure that the plane is stable in flight, i.e. that it doesn't want to flip around and fly backwards. But this stability also means that it wants to pitch down, and you'll need aerodynamic control to counteract this tendency to pitch down. Your plane is "too stable": it wants to pitch down more than you can counteract with the control surfaces that you have available. You need move the CoM and the CoL quite a bit closer together for it to fly well.

But to make things even more complicated: check how your CoM shifts when you burn fuel. If that causes the CoM to move behind the CoL then you'll get an unstable plane. Another issue that doesn't affect airplane much but is an issue for spaceplanes is that when you fly high and fast the center of the aerodynamic forces is less dominated by lift and more by drag, so if you have lots of lift at the back of the plane and comparatively more drag at the front, then it may happen that the plane becomes unstable when you get above  - say - 25 km and mach 3. And finally: the FAT-455 wings are not very heat-resistant, so there is a good chance that they overheat and explode either already during ascent to orbit or during reentry.

[-Orion-]

12 hours ago, AHHans said:

Yes, that is important to make sure that the plane is stable in flight, i.e. that it doesn't want to flip around and fly backwards. But this stability also means that it wants to pitch down, and you'll need aerodynamic control to counteract this tendency to pitch down. Your plane is "too stable": it wants to pitch down more than you can counteract with the control surfaces that you have available. You need move the CoM and the CoL quite a bit closer together for it to fly well.

But to make things even more complicated: check how your CoM shifts when you burn fuel. If that causes the CoM to move behind the CoL then you'll get an unstable plane. Another issue that doesn't affect airplane much but is an issue for spaceplanes is that when you fly high and fast the center of the aerodynamic forces is less dominated by lift and more by drag, so if you have lots of lift at the back of the plane and comparatively more drag at the front, then it may happen that the plane becomes unstable when you get above  - say - 25 km and mach 3. And finally: the FAT-455 wings are not very heat-resistant, so there is a good chance that they overheat and explode either already during ascent to orbit or during reentry.

Hm, I moved the center of mass and center of lift at pretty much the same place, and moved the engines further back so they could provide more torque with their gimbal but the problem remains pretty much the same. I could add more control surfaces but a well designed plane is supposed to fly straight without any input right?

[steuben]

17 minutes ago, -Orion- said:

...a well designed plane is supposed to fly straight without any input right?

Yes and no... without a maybe, and depending on what you mean by input, and what you are designing for.

An easy to fly plane will hold a near arbitrary orientation and heading with minimal input. For that you want center of mass ahead but not too far ahead of the center of lift. 

You want a plane that can dance through a corn field without taking an ear off, well for that the CoM and CoL relationship is going to be a bit... off. You're either going to have to keep both hands on the stick or have some nice computer aided flight control. This has been used in RL aircraft, the one that comes to mind is the Avro Arrow. A canuk plane that would have shown those yanks and ruskkies a good what for, if it had made production.

You may have to take an iterative approach to the design.  Strap on control surfaces, hide some reaction wheels, etc until it does what you want. Then start trimming things away until it fails.

 

[AHHans]

3 hours ago, -Orion- said:

a well designed plane is supposed to fly straight without any input right?

No. All aircraft - real world or KSP - need to be trimmed to fly straight and level without any additional control input. As I wrote: an aerodynamically stable airplane has a pitch-down torque from the placement of the CoM and CoL which needs to be counteracted by torque from the aerodynamic forces. With a given mass distribution  (position of CoM) a certain trim setting will keep a plane flying level at a certain speed. If it flies slower then the torque from the aerodynamic forces is weaker and the plane pitches down, thus gaining speed which increases the "aerodynamic torque" pitching it up again. And vice versa. (And, yes, oscillating around that is a common issue.)

You can apply this trim by designing it into the craft (e.g. by pitching the elevators in the SPH), by setting manual trim during flight (with <Alt>-W/<Alt>-S on windows), by using SAS, or by manually "pulling up the stick" (well, that wouldn't really be "trim", but it would work).

 

[Fraktal]

On 4/21/2020 at 3:19 PM, AHHans said:

You can apply this trim by designing it into the craft (e.g. by pitching the elevators in the SPH)

I personally pitch the main wing about 1-2°, then pitch the smaller tail/canards in the opposite direction to fine-tune until the CoL and CoM almost overlap and the plane wants to very slightly pitch up while flying at normal cruising speed without SAS. I don't know how much extra drag this causes compared to trimming the elevators, but I don't think it's that much since the elevators are almost completely flat against the wing 99% of the time due to the plane naturally wanting to stay in the air without having to be forced.

Edited April 23, 2020 by Fraktal