How do I make aircraft stable & smooth to control?

[Wabbit]

I've been playing around with aircraft over the past couple days, only to find that no matter how low I set the pitch authority, they tumble and crash at the slightest hold of the "S" key. Is there any way to reliably fix this? Also, I would like to know how I can tweak my aircraft to make them smooth at the controls. Thanks for any answers!

[Wraith977]

You can "smooth" controls by toggling capslock on your keyboard, the control indicators in the bottom left will turn blue from orange and they are instance dependant (if you revert your flight you'll have to toggle capslock again).

Some general pointers to make more stable aircraft, one is to set your COL just behind your COM, another is that your centre of drag should be as far back as possible - there's no way to visually show this as far as I know but a light but aerodynamically bulky rear to an aircraft generally makes it more stable. Another pointer is to make your wings larger with the same amount of control surface.

The most helpful thing to do would probably be to make a craft and share it so things you're doing wrong can easily be pointed out for you to fix. It also depends on whether you're using FAR or another aerodynamic rewrite mod that would change the way your aircraft behave.

[Val]

I wrote a post that covers similar topics. The writing is a bit disjointed, because it's been edited and restructured a few times and I'm not a native english speaker.

And there's the ever awesome 101 from @keptin.

Except for the parts regarding drag and number of intakes, everything else is still very valid. The parts regarding landing gear placement is especially relevant in 1.1.x.

Edit: After reading it again, it just seems to have a bit too many errors.

Edited September 11, 2016 by Val
Too many errors

[Plusck]

For the CoM / CoL relationship to get a stable-yet-maneuverable craft, I'd recommend doing the tweaks in two stages.

1) Before adding your pitch control surfaces, move the main wing to make sure that CoM is very very slightly in front of CoL, or exactly on the same spot (if you want more maneuverability).

2) Then add the pitch control surfaces. Unless you are using canards (not ideal for lots of reasons, IMHO), adding elevons will pull the CoL backwards since they also provide lift in their neutral position. For most small-to-normal-sized craft, you should end up with CoL sitting right at the back of the yellow CoM ball.

If you have a delta wing with a slight AoA and pitch-control elevons at the back, I also find it works well to rotate the elevons back to the pure horizontal. This reduces the amount of trim you need to apply to get the craft level in flight.

[Van Disaster]

Use Pilot Assistant - it's SAS is meant for aircraft rather than general ( read: rocket ) use.

[NotAnAimbot]

CoL (Blue ball) position relative to CoM (Yellow ball) is very important. You want to keep the CoL slightly behind the CoM, but not too much to not create torque. You can also use fine controls with the Caps key, or change control surface sensitivity and assigned movements. 

[foamyesque]

8 hours ago, Plusck said:

For the CoM / CoL relationship to get a stable-yet-maneuverable craft, I'd recommend doing the tweaks in two stages.

1) Before adding your pitch control surfaces, move the main wing to make sure that CoM is very very slightly in front of CoL, or exactly on the same spot (if you want more maneuverability).

2) Then add the pitch control surfaces. Unless you are using canards (not ideal for lots of reasons, IMHO), adding elevons will pull the CoL backwards since they also provide lift in their neutral position. For most small-to-normal-sized craft, you should end up with CoL sitting right at the back of the yellow CoM ball.

If you have a delta wing with a slight AoA and pitch-control elevons at the back, I also find it works well to rotate the elevons back to the pure horizontal. This reduces the amount of trim you need to apply to get the craft level in flight.

 

I actually like canard layouts, but I'm going to endorse this post because it's a simple, easy way to do it.

 

One point I would mention is that the more wing area you have relative to your control surfaces, the less angle of attack you can generate. You might still be able to turn very quickly, but if you don't have enough wing you won't drag your velocity vector with you, which means you force your wings into a stall and bleed energy in a hurry (and can also lose control, depending).

Also, layout matters: aircraft with a wing that runs along the length of the craft, but with a narrow wingspan (like the Mk2 fuselages or wing strakes, and to a lesser extend swept or delta wings) will take more effort to pitch over but will roll very, very easily. On the other hand, something with a wide wingspan but narrow wings (like the connector Cs) will pitch easily but need more oomph to roll.

Stunt planes tend to go for the latter layout and use powerful ailerons, because getting more roll authority is easier than getting more pitch and causes much fewer knock-on effects.

[GDJ]

Right click on the control surfaces and knock down the control authority of each item. 100% is fine for doing a pure Pilot Assisted flight but for keyboard piloting it's too much. Most of my planes I'm able to knock each of them down to 40% and still perform 7-8G maneuvers.

Also as others have pointed out, keeping the COM/COL/COT alignments proper is essential for aircraft.

[Plusck]

4 hours ago, foamyesque said:

 

I actually like canard layouts, but I'm going to endorse this post because it's a simple, easy way to do it.

 

One point I would mention is that the more wing area you have relative to your control surfaces, the less angle of attack you can generate. You might still be able to turn very quickly, but if you don't have enough wing you won't drag your velocity vector with you, which means you force your wings into a stall and bleed energy in a hurry (and can also lose control, depending).

Also, layout matters: aircraft with a wing that runs along the length of the craft, but with a narrow wingspan (like the Mk2 fuselages or wing strakes, and to a lesser extend swept or delta wings) will take more effort to pitch over but will roll very, very easily. On the other hand, something with a wide wingspan but narrow wings (like the connector Cs) will pitch easily but need more oomph to roll.

Stunt planes tend to go for the latter layout and use powerful ailerons, because getting more roll authority is easier than getting more pitch and causes much fewer knock-on effects.

Thank you.

I only very recently got into planes. I was a diehard rocketer (5k hours or so) until a month ago and then suddenly got the spaceplane bug, I can now see why some people (*cough*Val*cough*) come out with dozens of designs: it's neat when it works so well.
So thanks for the pointers - I'll take a look (and try to cure my prejudice re canards).

[KOCOUR]

Dont forget about Fule tank balancing.

What i do when i build a plane (except the tings others allready mentioned) is playing a lot with the Fuel sliders on fuel tanks in SPH and then moving the tanks and adding removing "balance mass" to the plane untill the COM doesnt move at all relatively to COL, no matter how much fuel i have with tanks.

TO achieve the easisest way to balance the fuel tanks i never put usable fuel tank in where i cant move ti easily eg. on the same axis as engines. instead i atach the fuel tanks as radial fuel tanks, or underwing droptanks, so i can slide them on the hull.

[fourfa]

For what it's worth - watching 1.2 pre-release streams, the upcoming changes to SAS are profound.  Flying planes will be much simpler, with much reduced cross-control issues.

Also stock fuel priority is in 1.2, allowing very stable CoM builds.  Similar to the fuel pump mods out there now

[Sharpy]

On 11.09.2016 at 10:41 AM, Plusck said:

For the CoM / CoL relationship to get a stable-yet-maneuverable craft, I'd recommend doing the tweaks in two stages.

1) Before adding your pitch control surfaces, move the main wing to make sure that CoM is very very slightly in front of CoL, or exactly on the same spot (if you want more maneuverability).

If his problem is the plane doing backflips at a touch of 's', I wouldn't go with 'slightly'.

I'd start with a lawn arrow that can be hardly pulled up, horrible to maneuver, with CoL way behind, but rock-stable. Then I'd go with moving it towards more maneuvrable by moving CoL towards CoM.

Also: high proportion of lifting surfaces to non-lifting surfaces. Good, strong control authority - instead of reducing the pitch control authority, make the plane aerodynamically stable, so that even if you do flip it, it will right itself. And start small. Try not to make your first airplanes longer than 3 MK1 fuselages. These tend to handle best. Once you got a hang of these, try bigger stuff.

[Corona688]

10 hours ago, fourfa said:

For what it's worth - watching 1.2 pre-release streams, the upcoming changes to SAS are profound.  Flying planes will be much simpler, with much reduced cross-control issues.

That SAS can hold a craft stable doesn't mean it's a stable craft

10 hours ago, fourfa said:

Also stock fuel priority is in 1.2, allowing very stable CoM builds.

That is something I am really looking forward to.

[foamyesque]

On 2016-09-12 at 7:15 AM, Sharpy said:

If his problem is the plane doing backflips at a touch of 's', I wouldn't go with 'slightly'.

Nah, the stabilizer will do it. If it doesn't, he can just make the stabilizer bigger; instead of turning down control authority, just add more fixed wing. Your main lift, though, should always be as closely centered on your CoM as you can manage. If I'm running something with a long delta-wing layout, I'll then use canards and have the dynamic instability to allow for manouvering; if it's something without a lot of pitch inertia, I'll go tail stabilizer.

[Corona688]

We still don't know what his craft even looks like yet...  For all we know it could be a single SRB with canards at the front.

[KingDominoIII]

1. Intakes are your friend when you use liquid fuel. They optimize use of oxidizer in SSTOs.

2. The CoM (Center of Mass) is a yellow ball on your spacecraft when you enable it in the SPH. The CoL (Center of Lift) is a blue ball. The CoT (Center of Thrust) is purple. The CoL should be in the center or just behind the CoM. The CoL should be behind the CoM on a horizontal angle, unless you're building a shuttle.

3. Control surfaces are your friend. They can be placed on wings.

4. Smaller wings on the cockpit can be good.

5. Wings on the front of the craft can cause pitching and rolling, causing you to hold W constantly. Move them back and fill them with fuel or weigh them down with control surfaces. 

 

[foamyesque]

People put waaaaaaaaay too many control surfaces on things from a lot of the screenshots I see. You can horse hundreds of tonnes, up and down both, with the standard canard pieces.

Edited September 17, 2016 by foamyesque

[Palaceviking]

A slight "inwards" tilt on your wings (5 degrees or less) will improve stability hugely and will cause your plane to slowly correct itself in flight, also if you're using large control surfaces at high speeds then the result is obvious.

 

ps: use caps lock(fine control) to reduce the amount you're actually moving the control surfaces.

[foamyesque]

8 hours ago, Palaceviking said:

A slight "inwards" tilt on your wings (5 degrees or less) will improve stability hugely and will cause your plane to slowly correct itself in flight, also if you're using large control surfaces at high speeds then the result is obvious.

Another way to manage this is to mount your wings above your CoM. They both create a dihedral effect, tending to return you to level through a roll-sideslip interaction.

 

Part of the reason I like the standard canards so much, though, is because they're an all-moving surface, which means you get the maximum possible profile changes with your commands. With a decent lever arm to work with you can be both perfectly stable in level flight and very agile in pitch and yaw controls.