Aerodynamics Question

"Racescort666" (Racescort666)
07/09/2015 at 10:47 • Filed to: KSP, Aerodynamics

I know there are some smart folks on here that play Kerbal Space Program so I’ve got a question that hopefully someone can answer. This pertains to the new V1.0.X versions of KSP with the new aerodynamics model and is regarding aircraft yaw stability at low speeds.

I’ve got a space plane that flies nice out of the spaceport, easily achieves SSTO, and uses Mk 2 cockpit parts. It’s got a decent sized cargo bay on it so the fuselage in front of the CG is pretty long. I understand that the new aerodynamic model accounts for body lift now (yay!) and I can definitely tell when flying different types of aircraft.

The problem I have is when I come into land, I start to get a shitload of side-slip on the nose which causes yaw that’s hard to control. (I’m rolled slightly to one side for course adjustment and I’m trying to pitch a little.) In the real world, this should be pretty easy to account for with a combination of pitch and rudder as you try to make fine adjustments in landing but since I control aircraft with a keyboard, it’s not nearly as intuitive or easy.

My solution was to add a ventral fin from the nose to the forward landing gear. It’s basically a wing strake that’s vertical and shouldn’t add any lift unless I get side slip. Was this the proper way of helping myself? Is there something else going on that I’m unaware of? I think it might be possible that I’m stalling the body and loosing lift that way but it’s more like the nose slides off it’s vector than a stall.

Begin discussion.

DISCUSSION (26)

CalzoneGolem > Racescort666
07/09/2015 at 10:56

Kinja'd!!!

These are boxer shorts.

ttyymmnn > Racescort666
07/09/2015 at 10:59

Paging f86sabre, Jayhawk Jake, and ForSweden to the front, please....

ttyymmnn > Racescort666
07/09/2015 at 11:02

Can you post a picture of your plane?

Racescort666 > ttyymmnn
07/09/2015 at 11:09

It’s very similar to Scott Manly’s plane in this video:

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I put forward canards on mine and I use the Mk2 to 2.5m adapter at the back to use one large engine instead of 2 smaller ones. He does not have the ventral fin but he has had the same yaw instability I did. He is much better at flying these things than I am though.

Racescort666 > ttyymmnn
07/09/2015 at 11:10

I usually text Rock Bottom with these questions but I figured it would be an interesting discussion for everyone.

Racescort666 > CalzoneGolem
07/09/2015 at 11:10

It works great for twin engine rocket planes.

ttyymmnn > Racescort666
07/09/2015 at 11:15

Well, sabre and Jake are both aero engineers, and Sweden is in aviation safety.

ttyymmnn > Racescort666
07/09/2015 at 11:22

The problem I have is when I come into land, I start to get a shitload of side-slip on the nose which causes yaw that’s hard to control.

Silly question perhaps, as I have no experience with KSP and don’t know how realistic it is. Are you landing in a crosswind? Can you make your rudders larger?

Racescort666 > ttyymmnn
07/09/2015 at 11:24

This is a different video but he’s getting the same yaw issue I am but he’s lined up on the runway much better than I usually do.

ttyymmnn > Racescort666
07/09/2015 at 11:29

No idea. I’m a historian, not an engineer. Looks like a neat program, though.

Racescort666 > ttyymmnn
07/09/2015 at 11:29

No crosswind. I always thought that weather would be an interesting game mechanic but it’s something that’s not in the base game. The air basically moves with the surface of the planet.

I could certainly make the rudders bigger. There are different rudders available that have more and less lift than what I’m using.

Bad Idea Hat > Racescort666
07/09/2015 at 11:36

Is this only occurring in one direction, IE slipping to the left only or right only? If so, it could be asymmetric thrust.

The problem is that, by design, that airplane is going to be a little unstable at the front due to all the weight at the back, I believe. There’s not a lot that would help, maybe with the exception of adding an SAS module to control unwanted slip.

http://wiki.kerbalspaceprogram.com/wiki/SAS

Actually, it seems like you can combine cores and make nearly an autopilot. What you really want is a control augmentation/fly-by-wire type setup to keep the plane going in the direction you want.

uofime-2 > Racescort666
07/09/2015 at 12:42

What is your angle of attack and location of the vertical tail stabalizer? there might not be enough air flowing over it if it is essentially in the wake of the body falling

Rock Bottom > Racescort666
07/09/2015 at 12:53

... and I avoid spaceplanes because I’m not that smart. Vertical launch is my jam. As a former colleague at NASA put it: “... satellites are easy, Hohmann transfer, Hohmann transfer, take picture, take picture, deorbit, fire ball, call it a day. Oh I forgot to include, (cross fingers and hope batteries last though eclipse)“. Nothing about landing without an ablator and a parachute interests me.

BorkBorkBjork > Racescort666
07/09/2015 at 12:56

Check your weight distribution. Sometimes crafts will fly fine right up until the end of the mission where all the fuel is in the very rear end of the plane and the front of the plane become very difficult to control. Do a fuel transfer to your central/forward tanks before you de-orbit.

Racescort666 > Rock Bottom
07/09/2015 at 13:35

Gee, take the easy way out. Also, anything short of an interplanetary return, you won’t really need an ablator with the stock game. The stock capsules have a pretty high heat resistance although you have to be a little careful with radial mount parachutes. I considered editing my save file to increase the entry heating from the default settings.

Racescort666 > BorkBorkBjork
07/09/2015 at 13:38

I’ll see if I can get as much fuel forward as I can next time. I have a rocket fuel tank right behind the cockpit to try to help with the balance for exactly this reason. It’s not really that the plane wants to flip out of control, it’s more like the nose just slips off to the low side.

Racescort666 > uofime-2
07/09/2015 at 13:45

AOA is pretty low, vertical stabilizer is way at the back. I guess it’s possible that could cause yaw issues. I suppose I could move the stabilizers to the wingtips. I’ve got RCS out at the wingtips so maybe it could help to turn that on for fun.

Rock Bottom > Racescort666
07/09/2015 at 13:50

Easy my ass! I have the New Horizons mod and have gotten pretty good at hitting the super far flung planets with probes without using MechJeb. It’s pretty hard to huck a probe at a planet 15 years away without wasting supreme amounts of fuel.

uofime-2 > Racescort666
07/09/2015 at 14:16

Will the game let you do CFD testing with its physics engine? that would be a nice feature.

I’m only a mechanical engineer, but I say just make the tail bigger. although using rcs could be entertaining

Racescort666 > uofime-2
07/09/2015 at 14:50

Ferram Aerospace Research mod will do a bunch of aerodynamic analysis. I’m not really keen to install it and go crazy with mods since I’m just running KSP on my laptop. If worse comes to worst, landing like a sloppy mess isn’t such a big deal, it’s just annoying and tense. The main question was whether or not the ventral fin was the right idea. I think it is but I really haven’t had a chance to try it yet.

Racescort666 > Rock Bottom
07/09/2015 at 14:52

I haven’t messed with that since I’ve been mucking around with SSTO space planes and Remote Tech.

TheOnelectronic > Racescort666
07/09/2015 at 15:07

Okay, so there are a few things that could be at work here.

If your CoM is towards the back, since your cargo bay is towards the front, your rudders are acting over a very short lever arm, while the drag from the nose of your craft has a much longer lever. When your yaw goes off-center, the drag from the nose, and especially if you put a strake on the front, will want to increase the yaw angle, not decrease it. And since your rudders are fairly close to your CoM, especially once you’ve drained your fuel, they have reduced effectiveness in both yaw authority and in simply stabilising your craft.

As I mentioned above, be mindful of how your fuel use changes your CoM over time. If you’ve still got fuel left during landing, see if you can transfer it to a forward tank to bring your CoM more forward. Remember, a craft in flight naturally wants to orient itself so the CoL is behind the CoM; if your CoM is moving close to or even behind your CoL once your fuel has drained, you’re going to have some instability problems.

As for how to improve it on your current design... maybe try adding some airbrakes to the wingtips, top and bottom, set to activate on yaw only. Or leave them inactive until you come in to land. Airbrakes are good for landing anyway, but it will also give you additional yaw authority without affecting stability like the ventral strake is.

Additionally, a single, central, large rudder tends to be more effective than two smaller rudders offset from the center. Basically, imagine how your plane looks to the airflow once you’re off-center, and imagine how it’s going to want to rotate around the CoM. Any aerodynamic surfaces in front of the CoM are going to want to increase that deviation- any behind will mitigate it.

uofime-2 > Racescort666
07/09/2015 at 15:09

I’ve never actually played the game so this might be a dumb question but, how are the control surfaces linked?

you were talking about adding a forward vertical stabilizer, is that just going to have proportional control w.r.t. the rear one?

I ask because it the rear one continues to not work correctly in certain situations that will cause some rather unpredictable behavior.

Racescort666 > uofime-2
07/09/2015 at 15:24

The front ventral fin won’t be a control surface just a way to try and keep lift up while banked.

Rock Bottom > Racescort666
07/09/2015 at 15:40

Get New Horizons. It’s awesome.