Plane not staying in the air

[dokii7071]

I've gotten it to be in the air and then go up for a bit, but then after a bit it just starts going down. How can I stop this from happening?

[Vanamonde]

Looks like you need some horizontal control surfaces farther from the center of mass where they will have more leverage. 

Also, welcome to the forum.

[dokii7071]

48 minutes ago, Vanamonde said:

Looks like you need some horizontal control surfaces farther from the center of mass where they will have more leverage. 

Also, welcome to the forum.

That worked! But the plane now dips a ton and is really annoying to pilot, yet the center of lift is still behind the center of mass, is this normal or can I fix this?

[Vanamonde]

Got an updated pic? 

[dokii7071]

Here's the updated pic

[Vanamonde]

Hmm. Maybe larger control surfaces up front? As in, replace those flaps with a mobile fin like you're using for a tail? Oh, and on closer look, you can remove the control surface that is attached to that fin, since that kind of fin IS a control surface which pivots. 

Also, thread moved to the Gameplay sub since you're seeking help rather than offering it. 

[Stormpilot]

Possibly the centre of thrust is above the centre of mass? The two have to bee aligned for stable flight. Also the lifting surface might be just too small. You may need to add more lifting area.

Edited May 1, 2021 by Stormpilot

[Guest]

I think the reason it starts ok then goes bad is because you’re using fuel which is then shifting the CoM to a different place than in the editor.

Anyway my tips would be to add some tail fins on the sides of the tail, add more wing structures (think glider, does it look like it’ll glide with no power), tilt the wings up very slightly, the canards can work quite well on the front end on smaller planes.

I would also reconsider that tail fuselage, in fact I’m surprised it takes off at all without striking the ground.

[18Watt]

5 hours ago, FruitGoose said:

I would also reconsider that tail fuselage, in fact I’m surprised it takes off at all without striking the ground.

I thought the same thing.  However, it looks like the nose sits higher than the main gear on the ground- it sits in a nose-high attitude.  So maybe you don’t need to rotate on the runway, it could just lift off on its own reaching a certain speed.   I’ve lost track of the thousands of tails I’ve lost during takeoff, so I would also reconsider the long tail design.

[OHara]

On 4/30/2021 at 6:11 AM, dokii7071 said:

But the plane now dips a ton and is really annoying to pilot, yet the center of lift is still behind the center of mass,
is this normal or can I fix this?

Yes it is normal, and Yes you can fix it.

The standard advice for beginners is to put the CoL behind the CoM, because that is an easy way to check that the wing just a bit behind the centre of mass.  

That advice does result in a plane that is nose-heavy, as you noticed.  Gravity is pulling down at a point forward of the wing's lift.  So in flight, beginners have to command pitch-up all the time or use SAS.  

If you like, you can rotate the front elevons during assembly, so the plane is trimmed as you like.  Trimming to neutral pitch moves the CoL indicator forward to match the CoM.  If you select the root part to pick up the whole plane, you can shift-S and shift-W to pitch it slightly and see that the lift of the wing rotates the plane back to level. The aerodynamic center is still behind the center of mass so plane is still stable.

Edited May 2, 2021 by OHara

[Snark]

On 4/30/2021 at 1:37 AM, dokii7071 said:

Here's the updated pic

Looks as though you've got very little pitch authority-- i.e. pressing W and S won't make the plane pitch much, which is going to make it hard to control.

As far as I can tell, you only have two sets of pitch control surfaces, and those aren't going to do much:

• The ailerons on the wings.  But those are only barely behind the CoM and therefore have almost no lever arm to work with-- they're not going to be very effective for pitch control.
• Some tiny ailerons mounted to the fuselage, just behind the cabin door.  But those are pretty small, and also not super far forward (so not a lot of lever arm).

Suggestion:

• Take those ailerons that you've got mounted just behind the cabin door, and move them all the way forward, up so they're right close to the nose.  That will give them about double the pitch authority, just from increasing their lever arm.  (You'll probably want to disable roll authority on them-- it's not needed, you don't want them generating drag just to try to roll, and your wing-mounted ailerons should be ample for roll control).
• Add ailerons in the back.  You've got that looooong tail code sticking out the back of the craft, so use it.  Put some pitch control surfaces right back there at the very aft-most tip of the tail cone.  That's way behind your CoM and will give you a lot of control authority.
• While you're at it:  Take that vertical stabilizer and move it all the way back, too.  It'll work way better.  Also, make sure you've turned off roll authority for it; it should only have yaw activated.

 

[Spricigo]

On 4/30/2021 at 2:11 AM, dokii7071 said:

is this normal or can I fix this?

There is several things to improve in your craft:

•  The thrust axis should be aligned with Center of Mass. 
• Rearrange the fuel tanks so the Center of Mass  don't shift (too much) with fuel consumption.
• Move the control surfaces further away from the center of mass so they get enough leverage; 
• Give some incidence for our wings for better lift/drag characteristics of the craft

Designing a plane can be very tricky for a new player, it's quite easy to get overwhelmed with all the details that you need to pay attention to.  To understand those fine details is probably a good idea to look into the tutorial there is  around, there are a couple:

 

 

warning: while there is still plenty of good info in those old  tutorials, they are made for older versions of KSP and some details may just not be true anymore due to changes in the game.

[Echo__3]

I made a video detailing the basics of airplane design. 

https://www.reddit.com/r/KerbalAcademy/comments/h96cpa/lets_discuss_airplane_design/