Tips on building airliners?

[Actually_New_KSP_Player]

I'm not the biggest fan of planes and don't have much experience, though managed to build a nice little minmus SSTO. However whenever i want to build a heavy cargo plane that looks simillar to real life one I end up with lots of problems like wobbly wings (engines are way too heavy), huge torque, not enough lifting surfaces to push CoL closer to the CoM etc and I didn't even put cargo in it yet.

No mods installed that change the flight model, physics or the gameplay in general.

Pics.

Edited May 31, 2019 by Actually_New_KSP_Player

[bewing]

Don't attach the engines directly to the wings. Attach them to the fuselage and then use the "move" tool to make it look like they are attached to the wings. Also, use autostruts.

 

[FleshJeb]

Your CoL is WAY too far back. Tip your main wings up by 5 degrees (one tick).

Your TWR is twice as high as some real world fighter jets. Remove one pair of engines.

[Klapaucius]

13 hours ago, Actually_New_KSP_Player said:

... I end up with lots of problems like wobbly wings (engines are way too heavy), huge torque, not enough lifting surfaces to push CoL closer to the CoM etc and I didn't even put cargo in it yet.

As @bewing said, use autostruts.  Wobbly wings are almost a given without them on larger craft.  The plane is overpowered as others have said, but if your goal is aesthetic and you want it to look like a four-engine jet, that is okay. You can just back off on the thrust.  You can keep them under the wing, but again, autostrut the engines as you do the wings.

 

Your center of lift issue can be solved a couple of ways. Moving the wings and/or putting some canards on the nose of the aircraft--like on this photo of a Tu-144:

https://en.wikipedia.org/wiki/Canard_(aeronautics)#/media/File:Aeroflot_Tupolev_Tu-144_1977_Volpati-1.jpg

 

I would also reduce the sweep of those wings. That is very radical for that type of plane. Align the inside edge parallel to the fuselage, and the natural sweep built into the part will be sufficient. That will also bring your center of lift foward a bit.

 

As for "huge torque", can you elaborate?  Are you veering off the runway, starting to roll over on takeoff or just finding it very twitchy in flight?

, 

[Actually_New_KSP_Player]

8 hours ago, Klapaucius said:

As @bewing said, use autostruts.  Wobbly wings are almost a given without them on larger craft.  The plane is overpowered as others have said, but if your goal is aesthetic and you want it to look like a four-engine jet, that is okay. You can just back off on the thrust.  You can keep them under the wing, but again, autostrut the engines as you do the wings.

 

Your center of lift issue can be solved a couple of ways. Moving the wings and/or putting some canards on the nose of the aircraft--like on this photo of a Tu-144:

https://en.wikipedia.org/wiki/Canard_(aeronautics)#/media/File:Aeroflot_Tupolev_Tu-144_1977_Volpati-1.jpg

 

I would also reduce the sweep of those wings. That is very radical for that type of plane. Align the inside edge parallel to the fuselage, and the natural sweep built into the part will be sufficient. That will also bring your center of lift foward a bit.

 

As for "huge torque", can you elaborate?  Are you veering off the runway, starting to roll over on takeoff or just finding it very twitchy in flight?

, 

Torque on rockets seems simple and you should always try to minimize it or your rocket would be uncontrollable. The planes act kinda weird with it though, with 0 torque everything seem to fly normally, but when it's like 50 kN or more some crafts tend to, as you mentioned, roll over, being very unstable on takeoff and landing and some fly as usual. Same thing happened to the new version of my heavy passenger plane, despite the 1300 kNm's of torque it is perfectly stable.

[sturmhauke]

52 minutes ago, Actually_New_KSP_Player said:

Torque on rockets seems simple and you should always try to minimize it or your rocket would be uncontrollable. The planes act kinda weird with it though, with 0 torque everything seem to fly normally, but when it's like 50 kN or more some crafts tend to, as you mentioned, roll over, being very unstable on takeoff and landing and some fly as usual. Same thing happened to the new version of my heavy passenger plane, despite the 1300 kNm's of torque it is perfectly stable.

Like you said, on rockets it's easy - line everything up symmetrically on the long axis, make sure your CoM is towards the nose and your CoL is towards the engines, and you're done.

While you can get all your various force vectors lined up in this way on an aircraft, it's sometimes impractical. Instead, think of the aircraft as a flying lever, with the CoM as the fulcrum. The forces acting on it need to be balanced, which does not necessarily mean they have to be exactly opposite each other.

On jumbo jets like you have, typically the engines are slung under the wings; this produces a thrust vector underneath the CoM and tends to pitch the aircraft upwards. The CoL should generally be near the CoM, and has a more-or-less upwards vector. If the CoL is in front, the aircraft becomes unstable; this might be ok for a fighter jet that's specifically designed for it, but generally something to be avoided. Placing the CoL on or slightly behind the CoM gives the plane neutral stability and balanced handling, while placing it further back makes it more stable/sluggish. Too far back like what you have, and the plane tends to pitch down and become a lawn dart. Most of the fore/aft placement is controlled by the wing position and sweep. Tilting the leading edge up or down (angle of attack) moves the CoL fore or aft a bit, but mostly increases or decreases the total lift at a given speed, and changes the lift direction. Normally you want the AoA to be around 2-5 degrees.

Another thing to consider is the horizontal stabilizers. As a general rule, they should have a flat or slightly negative AoA. This pushes the tail down to counteract the lift pulling it up. If your plane is flying level but the elevators are deflected a lot, you should rotate the AoA to compensate.

There are more complex things to consider in some cases, but that should get you a decently performing aircraft.

[Aeroboi]

On space planes you can try to do the following to synchronize CoM and CoT.

Use Move:Tool to move the engines further up or down only if they're attached radially. This moves the dry/wet CoM up or Down so the CoM is aligned with the CoT. Use a ruler on the screen to see if it's aligned perfectly. If it isn't you can move radial attached fuel tanks up or down for the same effect, or move the contents of a cargo bay further up or down to calibrate the CoM and CoT further. You said you haven't installed cargo yet, installing it might help especially when attached to the bottom of the cargo bay to move the CoM further down towards the engines.

In your case the engines a very much down so the CoT is below the CoM quite a bit, no wonder you have control issues. Try to create a wing with the structure at the top with the engines hanging there under on pylons. Or attach the engines on top of the wing.

For flexing wings it is best to attach the end segments directly to the fuselage and then drag them to the tip. If you have multiple wing segments you get a lot of flexing and at some point even autostruts wont fix this.

On 5/31/2019 at 9:07 AM, Actually_New_KSP_Player said:

not enough lifting surfaces to push CoL closer to the CoM etc

Use more lifting surfaces... What would stop you from doing so?

Edited June 1, 2019 by Aeroboi

[Klapaucius]

I think the OP may be encountering a bigger issue here concerning his (or her?) flexing wings.  I was going to create a short video demo of some of the tweaking I do in the SPH to help out, and so I brought out this plane I built in 1.6.1.  I took it for a spin, and it flexed all over the place, even though everything is autostrutted.  Not even adding rigid attachment fixed this.

I went back to a 1.6.1 save I have and tested it. No problems at all. Check out the video comparison.

 

 

Edited June 1, 2019 by Klapaucius

[Actually_New_KSP_Player]

13 hours ago, Aeroboi said:

Use more lifting surfaces... What would stop you from doing so?

The aesthetics. I mentioned in the main post that I want a plane that looks somewhat simillar to real life one and further expanding of the wing surface or adding some control surfaces closer to the nose would ruin it.

 

13 hours ago, Aeroboi said:

For flexing wings it is best to attach the end segments directly to the fuselage and then drag them to the tip.

You can not drag things that far away from their original placement just because there is a limit.

13 hours ago, Aeroboi said:

In your case the engines a very much down so the CoT is below the CoM quite a bit, no wonder you have control issues.

Well as i mentioned in one of the posts the torque on planes acts in a weird way, sometimes it does have significant effect, sometimes it doesn't.

This is the new version of my plane. Wings now are less angled, redesigned a pair of big tail fins (they were also a reason why the CoL was that far back). Also I limited the engine thrust to 57 so it is about 1 TWR at full throttle now. The CoT and CoM are still unaligned badly, yet the only effect of this that i did noticed is that the plane tends to turn left if you leave it standing still (i'm not sure exactly why this happens though), honestly this is one of my most stable planes that i've ever built. Big problem with this craft is that the brakes are quite weak, you may consider using airbrakes, reverse thrusting and flaps if you'd like to try it yourself (KerbalX)

 

6 hours ago, Klapaucius said:

I think the OP may be encountering a bigger issue here concerning his (or her?) flexing wings.

^ Pic above, i strutted them and it's fine now. Afaik the big wings were always flexing, some even when strutted, so it's not a surprise, also the new updates very often cause old crafts to summon the kraken.

[Klapaucius]

7 minutes ago, Actually_New_KSP_Player said:

^ Pic above, i strutted them and it's fine now. Afaik the big wings were always flexing, some even when strutted, so it's not a surprise, also the new updates very often cause old crafts to summon the kraken.

Hopefully that will get sorted out. The wings on my plane were already autostrutted.  I use those wings a lot, so that may bork a lot of my aircraft.

[bewing]

9 minutes ago, Actually_New_KSP_Player said:

You can not drag things that far away from their original placement just because there is a limit.

Hold down the Shift key.

[Actually_New_KSP_Player]

2 minutes ago, bewing said:

Hold down the Shift key.

Thanks for the tip.