How do stock Aerodynamics work?

[Coam]

I'm not particularly knowledgeable about aerodynamics, but stock aircraft handling has always felt off to me - aircraft handle very differently to my expectations from other simulators. In a rough description, I would say they handle almost like rockets - during a turn, they seem to maintain speed in the direction they were previously heading in, instead of maintaining forward speed but relocating it to the new direction of flight, among other oddities. I've seen a few videos of people flying with modified aerodynamics like FAR and those aircraft appear to operate roughly as I would expect.

I was testing a new plane earlier today and flew it up a few thousand metres high, then glided it back to KSC. Even during steep dives the plane lost speed without thrust, and sometimes appeared to gain speed when pitching up. My expectation was that one could trade altitude for speed when gliding, but this did not seem to occur, with speed loss remaining constant. I tried another test prior to making this post, this time using a different design, and placing it into a vertical dive with engines powered down. Again, it didn't gain speed and proved unable to pull up before crashing despite remaining fast enough to fly. (Contrary to expectations.)

That specific example is possibly an oddity of the speed indicator rather than aerodynamics, but it lead me to wonder. How do stock aerodynamics actually work? What are the general rules? What traditional ways of flying (such as trading altitude for speed) work differently in stock aerodynamics?

I understand this is a broad question (or series of questions), but hopefully it can yield some explanations that are useful to keep in mind for future plane designs and future flights when using stock aerodynamics.

[Alshain]

It is off, you really can't even call it "aerodynamics". Basically each part has a drag value assigned to it. Those values are added to all the other parts on the craft (regardless of where or what they are behind) and that sum is used in a formula to calculate a drag factor. In simple terms: More parts = more drag, so your craft with aerodynamic nose cones, fairings, and cargo bays are worse than an otherwise identical craft without them.

This will change in 1.0. For now there are mods you can use such as Ferram Aerospace Research.

[Motokid600]

How do the stock aerodynamics NOT work... Is the question, lol. I'm not very knowledgeable on the subject, but what I will say is something you'll hear over and over.

Welcome to the souposphere where planes fly like there under water. Because on Kerbin.. there's water at the bottom of the ocean. Carrying the water. Moving the water.

Edited February 10, 2015 by Motokid600

[RobotsAndSpaceships]

*not work.

[Pecan]

Kerbal SPACE program has just enough of an aerodynamics model that you can wave bye-bye to it as you pass through on your way to or from SPACE.

Kerbal SPACE program was never intended to be an atmospheric flight-simulator and I think Squad were surprised at how many people cared even about realistic rocket-handling in atmosphere.

Kerbal SPACE program never even had plane parts until comparatively late in development because it was intended to be a SPACE simulator.

There probably wouldn't have been such pressure for an accurate aerodynamics model if it weren't for all the people who spend so much time below SPACE.

The next version of KSP will have a new model, so don't worry too much about the current one. Why not go to SPACE instead?

[capi3101]

The major difference between stock aerodynamics and real aerodynamics (to my understanding anyways) is the way that the game's current drag model is set up. In reality, the formula for calculating the amount of drag force an object experiences is dependent upon the cross-sectional area of that object. More cross-section means more drag acts on the object. In KSP, this cross-sectional area relationship is replaced with the object's mass - i.e. more mass means more drag on the object. This simple change has some pretty substantial effects - for example, with stock aero, an plane that has a sleek nose will generate more drag than one with a blunt nose, because the nosecone is a separate part and adds mass to the craft. This is, of course, completely un-intuitive and is one of the major complaints that veterans of flight simulators (to say nothing of folks out there with real world flight hours) have with the game in general.

I'd have to go back and check my notes to see how much the atmospheric pressure of Kerbin's atmosphere changes with height and how it compares to Earth's atmosphere in that regard. I do know that on average the atmospheric pressure on Earth at 10 kilometers up is roughly 100 hPa (roughly 0.1 atm). There may be a difference there that also contributes to differences in how the atmosphere behaves.

The developers have said that they intend to update the stock aerodynamic model prior to the release of KSP version 1.0. What I got from that announcement was that the new system wouldn't be entirely like any of the existing aerodynamic models available (stock, NEAR or FAR); I of course have no details on how it would operate.

So yes, if you are dissatisfied with stock aero, NEAR or FAR are options that will make planes behave more realistically and you're more than welcome to check them out. Me, I fly stock, and I consider someone's preference of aerodynamic model to be akin to the choice of whether or not to use Mechjeb - totally up to the individual, no right or wrong choice there.

[Coam]

In simple terms: More parts = more drag, so your craft with aerodynamic nose cones, fairings, and cargo bays are worse than an otherwise identical craft without them.

This will change in 1.0. For now there are mods you can use such as Ferram Aerospace Research.

I assume the drag rule includes additional wings. Is there any rule of thumb for when "More wings" ceases to be useful for lowering takeoff speed?

How do the stock aerodynamics NOT work... Is the question, lol.

*not work.

To be honest that is also my question - basically, what are the rules of the odd aerodynamics we find ourselves flying in using stock, so that I can adapt my flying habits to this new way of doing things? At the moment I basically apply my understanding of normal flight and apply corrective measures when that doesn't work... which often doesn't work.

The next version of KSP will have a new model, so don't worry too much about the current one. Why not go to SPACE instead?

Other users seem to manage flight (indeed, even SSTO spaceplanes) with stock aerodynamics. My aim isn't to complain about the current model (which I could just mod out), it's just to ask what rules I should use for operating below space. In the end, this information might be useful for stock SSTO designs.

As for the specifics of why I'm not going into space... it involves a problem with the Unit-E life support on a Minmus rocket.

[LethalDose]

Like the posters above have stated, KSP's aero model doesn't really work. It's a big simplification that calculates drag per part based on the parts mass and a part-specific multiplier.

The good news is that they're improving it for release.

Kerbal SPACE program has just enough of an aerodynamics model that you can wave bye-bye to it as you pass through on your way to or from SPACE.

Kerbal SPACE program was never intended to be an atmospheric flight-simulator and I think Squad were surprised at how many people cared even about realistic rocket-handling in atmosphere.

Kerbal SPACE program never even had plane parts until comparatively late in development because it was intended to be a SPACE simulator.

There probably wouldn't have been such pressure for an accurate aerodynamics model if it weren't for all the people who spend so much time below SPACE.

The next version of KSP will have a new model, so don't worry too much about the current one. Why not go to SPACE instead?

You mad, bro?

You gotta go through the atmosphere to get to space, and then you gotta come back through to get home, not to mention the surface of several planets and a moon. NASA's done tons of atmospheric work with aircraft to improve their abilities to explore space. Or, SPACE, if you have to shout it every time.

Aerodynamics are also important for building SSTOs, that go to space.

Atmospherics and aerodynamics are an important part of spaceflight.

Squad decided to include it, I'm glad they did.

[cantab]

Remember that the current stock aerodynamics is going away in the next release of KSP. With that out of the way:

The drag on a part that isn't a wing, air intake, or control surface is dead simple. It is directly proportional to the mass of the part, including any fuel or other resources in it, and the atmospheric density, and proportional to the square of speed. Orientation makes no difference, whether it's in a cargo bay makes no difference, a nosecone on a booster doesn't reduce the drag of the booster and does add the drag of the nosecone. There is a drag coefficient that's multiplied in but almost all parts have that as the same value, except for deployed parachutes that have a drag coefficient thousands of times higher to work.

This is behind most of the unrealism in the aerodynamics model. For example almost all craft have the same rather low terminal velocity, the fuselage on a plane or body of a rocket will contribute nothing to stability, and the centre of mass and centre of drag will almost always coincide. It explains why picking up speed in a dive doesn't really work - the plane's "streamlined" fuselage offers no advantage compared to a block of fuel tanks, and the overall drag level is so high it robs the speed.

Lift is better. Wings behave largely as you'd expect, giving maximum lift at a certain angle of attack, though they don't really stall beyond that. Nearby wings don't interact at all, so don't worry about that. If you want more lift from the same wing span just slap two copies of the wing right on top of each other! Control surfaces, on the other hand, gain maximum lift at 90 degree angle of attack and can create thrust when deflected - the so called "infiniglide" bug. This especially affects lightweight and/or low-powered craft, but it might account for your unexpected speedups.

Despite all the issues people do fly planes in stock, but if you want a more realistic experience get FAR. Besides a vastly better aerodynamic model (albeit with its own quirks), it includes analysis tools to inform you about your plane's stability before it even leaves the hangar, and the ability to set control surfaces as flaps or spoilers. If you get annoyed by your planes breaking apart in FAR just turn aerodynamic failures off.

PS: One more thing. SAS tries to hold your heading in space. That can be helpful for level flight but is counterproductive when turning an aeroplane. If you hold down Alt (or RShift on Linux) and press WASDQE, you can set trim on the controls, which is my preferred way to fly. Neutralise trim with Alt+X.

Edited February 10, 2015 by cantab

[Postron]

I'd have to go back and check my notes to see how much the atmospheric pressure of Kerbin's atmosphere changes with height and how it compares to Earth's atmosphere in that regard. I do know that on average the atmospheric pressure on Earth at 10 kilometers up is roughly 100 hPa (roughly 0.1 atm). There may be a difference there that also contributes to differences in how the atmosphere behaves.

Actually atmospheric pressure at 10 km is roughly 26 kPa. As a rule of the thumb you can say atmospheric pressure halves roughly every 5500m.

[Alshain]

I assume the drag rule includes additional wings. Is there any rule of thumb for when "More wings" ceases to be useful for lowering takeoff speed?

I don't have one for you. I'm afraid the rule of thumb that I play by is: "Don't play stock aerodynamics."

[KerikBalm]

Ok, so here were go:

As many have said, one part will not shield another part from drag. Two fuel tanks side by side have the same drag as two that are inline!

Adding a nose cone just adds drag for the nosecone part without reducing the drag of what is beneath it!

They use mass as a stand in for cross sectional area: they include fuel mass.

So an empty fuel tank has the same terminal velocity as a full fuel tank!

I'm not exactly sure how they behave... but they are all screwed up... infinigliding, floating...

Then there are the control surfaces... they never stall, and lift peaks at a 90 degree AoA? ... some people have abused this to create "blimps"

Watch this, it explains quite well... especially 2:20 and beyond:

Next is the lift formula.... and the drag formula...

Lift is proportional to velocity... while drag is proportional to the square of velocity....

that means except at very low flying speeds, your craft will generate much less lift than it should, and this is why it wont turn very well and will tend to keep going in the direction ti was going.

Lastly... there are the reaction wheels, which torque your craft around ... which is a behavior that is not common for most aircraft...

Earth's Scale height= ~ 6 km

Kerbin's= ~ 5km

So.. 1/10th scale radius, 5/6ths scale atmosphere

[Coam]

Firstly, Thank you to everyone else who replied as well. The additional information is helpful, I just can't think of relevant ways to reply to large amounts of it.

This is behind most of the unrealism in the aerodynamics model. For example almost all craft have the same rather low terminal velocity...It explains why picking up speed in a dive doesn't really work - the plane's "streamlined" fuselage offers no advantage compared to a block of fuel tanks, and the overall drag level is so high it robs the speed.

I hadn't considered that the simplification of drag into mass would change this aspect of gaining speed by diving.

If that method of generating speed when engine power is gone (whether the engine fell off, or ran out of fuel) doesn't work, is there an alternative way of gaining speed for a glide (beyond perhaps wiggling the control surfaces.), or is the best thing to do simply to dive low to the ground and try to manage a controlled, slow crash rather than try to move as far forward as possible?

As a general question:

Would it help to simply consider airplanes a special class of rocket, as opposed to actual planes, with regard to piloting? This came to mind when I remembered another oddity, that planes can often be pointed straight up and climb better than they do when angled.

[RobotsAndSpaceships]

Kerbal SPACE program has just enough of an aerodynamics model that you can wave bye-bye to it as you pass through on your way to or from SPACE.

Kerbal SPACE program was never intended to be an atmospheric flight-simulator and I think Squad were surprised at how many people cared even about realistic rocket-handling in atmosphere.

Kerbal SPACE program never even had plane parts until comparatively late in development because it was intended to be a SPACE simulator.

There probably wouldn't have been such pressure for an accurate aerodynamics model if it weren't for all the people who spend so much time below SPACE.

The next version of KSP will have a new model, so don't worry too much about the current one. Why not go to SPACE instead?

Are you being serious or not?

[tadswana]

I think the aerodynamics are handled quite well for a beta stage, and looking forward to the 1.0 release.

Sure it's a rough approximation assuming a constant Cd for all parts, however it gets you there. In reality, drag is a lot more complicated, comprising of form drag (the shape), parasitic drag (friction with the skin), and induced drag (tip vortices created from lift). Adding to this stall , and we start to get non-linear behavior.

It gets to a point where the aerodynamics become very computationally intensive, and it becomes a real flight simulator (like Xplane) - there needs to be a balance between the accessibility and playability of KSP (which is it's great strength imho) and the fidelity of the simulation. I think 1.0 will have the aerodynamics 80% right, so let's see!

[Nathair]

It gets to a point where the aerodynamics become very computationally intensive, and it becomes a real flight simulator (like Xplane) - there needs to be a balance between the accessibility and playability of KSP (which is it's great strength imho) and the fidelity of the simulation.

Like... FAR?

[Vanamonde]

Answer the question or skip to the next thread, please. Let's not make this yet another another dump-on-aero thread.

[cantab]

I assume the drag rule includes additional wings. Is there any rule of thumb for when "More wings" ceases to be useful for lowering takeoff speed?

I overlooked this before, but actually they don't. I'm not sure how drag on wings works, but they often have much less drag than normal parts. This thing that's almost all winglets hit the ground in freefall at over 1500 m/s: https://flic.kr/p/oP8yBt

If that method of generating speed when engine power is gone (whether the engine fell off, or ran out of fuel) doesn't work, is there an alternative way of gaining speed for a glide (beyond perhaps wiggling the control surfaces.), or is the best thing to do simply to dive low to the ground and try to manage a controlled, slow crash rather than try to move as far forward as possible?

I'd say enter a steady glide at an angle you're happy with. Which is pretty much what you'd do in real life - until you have identified your landing site and are sure you're in range of it you won't descend faster than you need to.

[numerobis]

If you have fixed thrust, there is a tradeoff: wings have a minimum speed below which their mass exceeds their lift. At that speed they exert a certain amount of drag, so eventually you wouldn't have enough thrust to overcome that drag.

On the surface, we're talking about a few mN of drag per wing versus over 100 kN of jet engine thrust, so it's not really relevant. But at altitude, for wings to get enough lift, they have to go faster, and drag increases at a higher rate, so the tradeoff is much more relevant.

I should maybe calculate some related numbers for each wing.

[MarvinKitFox]

STOCK AERODYNAMICS:

The Void is filled with Ether. And the Ether shall pervade everywhere, and shall not be moved by mere matter.

Kerbal attracts Ether, thus it gathers near the surface.

Passing Mass through Ether causeth drag upon the Mass. yeah, to the second power of ye speed shall the Drag be.

And if the Mass hath the attribute "lifting surface", then shall the Passage of Mass through the Ether create a Force, and the Force shall be the multiple of the "lifting surface" and the Speed of passage, and directed to right angles to the Surface.

That's about as accurately as one can describe KSP aero.

In simple, less-religious terms:

1) Air causes drag on mass. NOT on surfaces, NOT on cross-section, but purely on Mass. Thus a filled fueltank has much more drag than an empty one.

2) Some things drag more than others. Plane cockpits are very non-draggy, air intakes are very draggy.

3) Drag is proportional to speed^2

4) Lift is proportional to speed, and angle. BUT ONLY for objects with "lifting surface" in the description.

5) Air gets thinner with increasing altitude.

As a result of these wildly wierd rules, you get monstrosities such as this:

A "zeppelin" build from fully filled orange tank, "flying" around at less than 10 miles per hour.

As a result of this complete abortion of the laws of physics (hereafter called the Lore Of Fizzics)

one should not apply any real-world experience to flying in KSP.

or

Install FAR, and enjoy a believable facsimile of actual aerodynamics.

[Pecan]

Are you being serious or not?

(And @LD) No, I'm not being completely serious with all that SPACE stuff, just making the point (on the first and second lines) that atmospheric handling is a minor part of things. Given that, and the planned replacement in the next release, there isn't a lot of point in effort spent learning the stock aerodynamics.

@OP: "Other users seem to manage flight (indeed, even SSTO spaceplanes) with stock aerodynamics. My aim isn't to complain about the current model (which I could just mod out), it's just to ask what rules I should use for operating below space. In the end, this information might be useful for stock SSTO designs."

Whatever you want to do is fine, there's no wrong way to have fun. The summary of stock aerodynamics are - the atmosphere is really thick below around 20km so drag will make you lose speed, even in a dive quite a lot of the time. If you want to make SSTOs the first rocket in my tutorial is a Single Stage To Orbit and for more useful things, a Skipper can SSTO 5t, a Mainsail 10t and KR-2L 24t. Hopefully jets will be nerfed to something realistic, since realism's what air-lovers seem to want, but as they are you need a different launch approach to getting them near orbit before switching to rockets. Again, see the tutorial, it contains designs for four planes, three of which are for *hehe* SPACE!

[pincushionman]

Everybody's hung up on the drag model, but lift is off-kilter too, what with the lack of body lift and the linear-V garbage. Both play havoc with your abulity to balance and the needed angle of attack.

[numerobis]

I find the stock model is really, really easy to balance, because every part can be considered in complete isolation of the rest of the spacecraft. Is the CoL too far forward? Just move a random wing further back, doesn't matter where!

One thing that's critical, and which wasn't obvious to me at the start: rotate the root node. That will tell you how your aircraft changes as the angle of attack changes. If by pitching up you end up with an angle of attack moving forward, your aircraft is going to be unstable. Also useful for seeing how lift changes with roll and yaw.

[juanml82]

Kerbal SPACE program has just enough of an aerodynamics model that you can wave bye-bye to it as you pass through on your way to or from SPACE.

Kerbal SPACE program was never intended to be an atmospheric flight-simulator and I think Squad were surprised at how many people cared even about realistic rocket-handling in atmosphere.

Kerbal SPACE program never even had plane parts until comparatively late in development because it was intended to be a SPACE simulator.

There probably wouldn't have been such pressure for an accurate aerodynamics model if it weren't for all the people who spend so much time below SPACE.

The next version of KSP will have a new model, so don't worry too much about the current one. Why not go to SPACE instead?

So when do they add the submarine parts?