General spaceplane questions and ranting.

[CorruptDB]

I am attempting to build a reusable spaceplane to shuttle Kethane from the surface to orbit and I have been running into a number of difficulties. My latest incarnation is a canard biplane. My current problem with this plane is on the climb out when I get it to 6000-8000m it suddenly noses up and goes into an unrecoverable spin. I have plenty of speed and have the nose at about 25 degrees when this happens. I designed the plane to be stable according to aerodynamic principles (I design, build and fly a lot of R/C airplanes so I understand the basic concepts) but the planes in KSP don't follow aerodynamic principles.

What is required for a stable plane in KSP? My first attempt was scapped when I was unable to reenter from orbit without an uncontrolled spin. I thought this was because I was more top heavy on re-entry (fuel tanks were slung low and had nearly emptied) so I designed this plane to be balanced in the pitch axis as well as the roll axis but it seems to be even less stable than my first one. For both planes, I always have to maintain at least a 15 degree pitch or I lose altitude, is that normal? Are their any parts with airfoils or does everything depend on the angle of attack? Do people regularly angle the wings up on the fuselage or does everyone just fly with their ass dragging through the air? In what units is the "lift generated" parameter on wing parts? How do you calculate the amount of "kerbal lift" to lift a plane of a certain weight? Does the KSP aero model even have a concept of a stall speed? It feels like the planes fly exactly like the rockets, the only thing that seems to matter is the direction of thrust, the wings dont seem to interact with the atmosphere at all. With this model, what makes a plane stable or unstable?

[CorruptDB]

One more question: The wiki says that the intakes produce less drag when closed, is there any other documentation on that? I don't seem to see any difference no matter how many intakes are open or closed and KER reports the same amount of atmospheric drag. Is there any official documentation from Squad? Where do the users that edit the wiki get their information? Is it just from experimentation or is it stuff gleaned from the devs on the forums?

[allmhuran]

Point 1: In the VAB make sure your center of lift is just behind your center of mass. Putting the centre of lift even more than a few "CoM ball widths" behind the CoM is OK too *if* you have enough control surfaces to compensate. Never put the center of lift in front, this is a common cause of uncontrollable pitch-up.

Point 2: Be aware that as fuel is drained from the craft the center of mass is going to shift. Usually people put a long fuselage of fuel tanks with the engine or engines at the back. In this configuration, the centre of mass is going to shift backwards as fuel drains, potentailly moving far enough to put it behind the centre of lift. Then see point 1.

Point 3: As your velocity increases drag becomes a significant factor in the behaviour of the vessel. An aircraft that uses the scoopy, surface attached radial air intakes on, say, the underside of the fuselage might seem fine at low speed, but when you start hitting 1000 m/s or more you will discover that the extra drag under the plane makes it difficult to get the nose up. On the other hand, if the scoops are on the top it might be difficult to keep the nose down. Note, also, that the drag forces are tricky... intakes way at the back will tend to make the plane want to point its nose along the velocity vector, since any deviation from this will create a torque as drag is produced off the centerline.

Note that this also means that if you have a lot of air intakes near the front of your craft and you are pitched up, the drag from the air intakes will want to pitch the craft up even more (unless you have zero AoA)

To answer your second paragrph: There is no stall speed in KSP, but wings do generate more lift (up to a point... I think someone figured out the exact value, maybe about 40 degrees?) and more drag as the angle of attack increases. If you have too high of an angle of attack the drag will be quite high, but KSP includes mass in its drag equation, and the mass of wings is very low, so this is usually not an issue. Long story short, it *is* beneficial to attach your wings to the fuselage with 5 or 10 degrees of angle of attack (hold down shift to rotate parts in 5 degree increments instead of 90 degree increments).

Intakes definitely generate much more drag when open than closed. The most efficient intakes (in terms of intake-air-to-drag-ratio) are, by far, the blue-textured high performance intakes. You can test this yourself... build a plane with heaps and heaps of air intakes (like, 20), and put 18 in an action group to toggle them open and closed. Take off and start climbing. Keep an eye on your speed and how fast you are accelerating. Now toggle the action group to close 18 of the intakes... your acceleration will be MUCH higher.

As far as knowing whether you have enough lift: I don't bother. Build what looks cool to you, and if can't get off the runway, add more wings

Edited September 18, 2013 by allmhuran

[Sirine]

General question with general answer:-

1) The sudden spin mostly cause by your engines out of air. Check your air-intake.

2) Maintain 15 degree pitch to maintain altitude is normal...to over come this, just add more wings (lift value).

3) KSP aero model currently are underdevelopment. Don't expect too much from it. Use mod if you want realistic.

4) Intakes currently have little to no effect on atmospheric drag...reason same as above.

[Tiron]

Some things to keep in mind:

KSP's drag and lift models are not aerodynamically accurate. The drag model in particular uses the weight of the part multiplied by the drag coefficient (which is what the 'drag' stat is). One side effect of this is that all parts produce drag regardless of position, based mostly on their weight. So things that are normally used to reduce drag at the expense of weight, like nose cones, actually INCREASE drag.

The lift model is similarly less than spectacular, and does NOT include stalling in any form.

The most likely causes of uncontrollable Pitch-Up are either the Center of Mass being right on/ahead of the Center of Lift, or Center of Drag being above the Center of Mass. Note that because of the way the game calculates drag that empty fuel tanks have less drag than full ones, due to their change in weight.

Center of Drag being ahead of the Center of Mass can also result in pitch-up in some situations, particularly while aerobraking/re-entering. This is usually caused by forward-mounted air intakes, exacerbated by having empty fuel tanks. What happens is, any deviation from straight forward is accelerated, as the too-far-forward center of drag suddenly has enough deviation from the CoM to start pushing the nose of the craft around. In such a case the craft will only pitch up if you're in a nose-up orientation to start with: If you're nose down it'll go down, if you yaw it'll turn into the yaw. In any of these cases without intervention it'll come to rest flying backwards.

There's an addon called Ferram Aerospace Research that exists specifically to update the drag and lift models to be fairly realistic, but it requires much more attention to the aerodynamic qualities of your design, and most designs that work fine in stock will not work or will work poorly with FAR. It also makes nosecones a requirement for efficiency, and requires changes in rocket flight path to deal with the changed aerodynamics. A counter to all this is that it actually REDUCES overall drag, and in particular removes the link to weight. It also makes some changes to jet engines that limit your top speed to about mach 5 (1600 m/s, roughly), and reduce their overall power.

Edited September 18, 2013 by Tiron

[CorruptDB]

> 1) The sudden spin mostly cause by your engines out of air. Check your air-intake.

I don't think it is an engine-out condition, none of the engines appeared to go out ( I have seen those engines flame out, it does some particle effects and the engine goes dark)and resources reported plenty of IntakeAir. Speaking of that, how much IntakeAir is enough? The TurboJet engine in the SPH says it requires 15 units of IntakeAir but the Ram Air Intake only provides .2 units so do I really need 75 intakes to feed each TurboJet engine? That seems a little unreasonable, I must be missing something here. The plane in question has 6 engines, with 12 Ram Air intakes and 34 radial intakes. My previous plane didn't have as many intakes and it was able to reach around 25k before flaming out so I don't think it is a lack of air.

> Point 1: In the VAB make sure your center of lift is just behind your center of mass. Putting the centre of lift even more than a few "CoM ball widths" behind the CoM is OK too *if* you have enough control surfaces to compensate. Never put the center of lift in front, this is a common cause of uncontrollable pitch-up.

>Point 2: Be aware that as fuel is drained from the craft the center of mass is going to shift. Usually people put a long fuselage of fuel tanks with the engine or engines at the back. In this configuration, the centre of mass is going to shift backwards as fuel drains, potentailly moving far enough to put it behind the centre of lift. Then see point 1.

The plane goes up a little nose heavy and should stay that way as 4 radial tanks in the back will empty before any fuel is used in front of the CoM.

> Point 3: As your velocity increases drag becomes a significant factor in the behaviour of the vessel. An aircraft that uses the scoopy, surface attached radial air intakes on, say, the underside of the fuselage might seem fine at low speed, but when you start hitting 1000 m/s or more you will discover that the extra drag under the plane makes it difficult to get the nose up. On the other hand, if the scoops are on the top it might be difficult to keep the nose down. Note, also, that the drag forces are tricky... intakes way at the back will tend to make the plane want to point its nose along the velocity vector, since any deviation from this will create a torque as drag is produced off the centerline.

Is the aero model in KSP different depending on if you take off from the runway vs. the launchpad? My understanding of the model is that each part had a value for drag (0.2 for the scoop intake) and that value combined with its mass determined the drag of the part regardless of orientation. The drag of all parts is summed to determine the drag of the vessel. I'm almost positive that is the way it works for rockets, does it not work the same for planes? At any rate, the intakes in question have radial symmetry and should balance out fairly well.

> The most efficient intakes (in terms of intake-air-to-drag-ratio) are, by far, the blue-textured high performance intakes.

Is this just accepted lore or has it been tested? The SPH reports more drag for the ram air intake (0.2 vs 0.3) and it masses only 0.0015 tons less. The scoop says it supplies 0.5/1 units of intake air but the ram air only supplies 0.2. Does it really perform differently at altitude or is the difference cosmetic? The part.cfg file says:

dragModelType = default

maximum_drag = 0.3

minimum_drag = 0.3

it doesn't seem to me like it should act any different than any other part.

I guess I will just keep testing different wing configurations on this plane. Does the orientation of the wings even matter or does each wing part jusr provide "lift" no matter how it is placed?

[CorruptDB]

OK, I did a little test and confirmed that wings don't need access to airflow for lift. It's useless to make conventional planes, why make a plane 100 meters wide and prone to touching a wingtip when you can just have this:

It's a dream to fly. With a few more wing panels I think it will turn into an airship, it almost floated off the runway. Definately a plane Jeb would love to fly I think I just solved all my wing problems.

[WafflesToo]

Off-center drag seems to make a big difference. A lot of my spaceplanes perform fine flying by themselves but when I have a load strapped to their back they want to flip end-over-end exactly like you described. I think its because your flight surfaces rapidly lose effectiveness as you approach and pass 10km and the drag from the probe is off-centered from the thrust-line.

Here, watch the pitch-control indicator in the bottom-left. The plane flies nose-heavy at take-off, but as I pass the 10k mark it's almost all the way pitched-down trying to keep the plane from flipping nose-over-tail to the point I have to turn on the RCS system to keep the plane controllable. The probe only masses about 600 kg, but the landing gear alone mass 1.5 tons so it can't be the off-centered mass causing the trouble, the only thing I could ever figure out is it must be the off-centered drag from the probe.

I don't know if it's much assistance, but I hope it at least helps you understand the problem you're facing.

[Sirine]

@CorruptDB yes..MORE wings did make your plane float. Is a bug that the Dev need to solve.

[WafflesToo]

*snip*

It's a dream to fly. With a few more wing panels I think it will turn into an airship, it almost floated off the runway. Definately a plane Jeb would love to fly I think I just solved all my wing problems.

/facepalm

I really can't decide if that is the single most awesome thing I've ever seen or the most pathetic

Squad? Game's broken... you gonna fix that?

[allmhuran]

Is the aero model in KSP different depending on if you take off from the runway vs. the launchpad?

Something is definitely handled differently, but AFAIK nothing related to what we're discussing here. (things like what the game considers "up" and "forward" change, see thread linked shortly for more info)

at value combined with its mass determined the drag of the part regardless of orientation

Negative. Wings produce different amounts of lift and drag depending on orientation. There was a recent thread on this, see from this post forward: http://forum.kerbalspaceprogram.com/showthread.php/29788-How-to-calculate-lift?p=593280&viewfull=1#post593280

The drag of all parts is summed to determine the drag of the vessel

Similarly negative. Try adding a bunch of intake scoops on a structural girder that sticks out way above the vessel... your plane wll do loop-de-loops because of the torque generated by the scoop drag.

Is this just accepted lore or has it been tested?

This has been tested. For air intakes the drag equation is more than just what you see in the part description. There was an excellent thread on this (similar to the one above to do with AoA) where someone dug into the game to figure it out, but I think it was eaten by the forum crash. In any case, the only air intake worth using if you are shooting for optimum performance is the ram air intake. Of course sometimes the nacelles, scoops, etc, are fun to use for aesthetic reasons. But if you're "min-maxing" your plane... stack ram air intakes. Scoops are the worst. What it basically comes down to is the ratio of mass to intake area. Ram air wins.

Edited September 18, 2013 by allmhuran

[Hodo]

CorruptDB,

If you want realistic atmospheric dynamics in KSP, I suggest downloading the FAR plugin. It adds actual drag and flight physics to the game and makes it actually quite a bit more fun for aircraft design. You actually need to have a craft that would work, not these winged blimps.