Take-off planes

[Philae & Rosetta]

Hi all,

I try to fly my first plane. I tried many stuffs, read tutorials, but I still can't.

Can someone help me understand why this plane won't take off? 

Thanks

[Vanamonde]

KSP wings don't work quite like real ones. They need at least a little angle to the airflow in order to generate lift. Either angle the wings slightly upward toward the front, or adjust the landing gear so that the plane sits nose-high. Then you should start generating lift as you build up speed on the runway. 

Unrelated: the kind of fin you are using as a vertical stabilizer cannot pivot to help with steering, but your plane might be small enough that it doesn't matter. 

Good luck.

[surge]

The yellow/black ball (mass) is MUCH too far ahead of the cyan/black ball (lift). All you've made is a front-heavy dart...assuming you get it off the ground. Be sure to move the rear landing gear when you fix it (it is about right, relatively speaking for now).

https://ibb.co/LRXCwxb

Rudders rarely help with steering, since aircraft roll, then pitch, and don't just turn sideways like a car. I say leave it, you have bigger problems to fix.

 

Edited April 2, 2022 by surge

[Maria Sirona]

Slap the things more forwards so they are on top of the yellow-black gizmo

Edited April 4, 2022 by Maria Sirona

[KerikBalm]

On 4/1/2022 at 10:23 PM, Vanamonde said:

KSP wings don't work quite like real ones. They need at least a little angle to the airflow in order to generate lift.

This is actually true of real wings too.

On 4/1/2022 at 10:23 PM, Vanamonde said:

Either angle the wings slightly upward toward the front, or adjust the landing gear so that the plane sits nose-high. Then you should start generating lift as you build up speed on the runway. 

That will help, but it also must be noted that the center of lift is way too far behind the center of mass, and he effectively has no elevators.

Using those fins as the control surfaces and wings at the same time is only good for roll. Deflecting them up or down will only change the total lift, but not move the center of lift as needed for pitch control

[swjr-swis]

On 4/1/2022 at 10:23 PM, Vanamonde said:

KSP wings don't work quite like real ones. They need at least a little angle to the airflow in order to generate lift.

Adding to @Vanamonde's excellent advice: when using a fin as main wing, make sure you have pitch inactive. Otherwise SAS will basically cancel the angle you just added. Also, as main wings are usually very close to the CoM it wouldn't have much pitch effect anyway.

Your selection of parts looks very low tech, early career. I'm assuming you only unlocked TL3 with Aviation from TL4 and possibly nothing else, with part and weight restrictions for FL1 (30 and 18 t). I see a  Mk1 cockpit, a service bay, an FL-T200 tank, another service bay, and a nose cone. Two Mk0 pods with small intake and Juno on the sides. Two tail fins for wings, basic fin on top, and the fixed gear.

The tank in between the service bays (I assume containing science parts?) and the Juno pods about the same place is good thinking - you're trying to keep the plane balanced even when fuel is used. Except that only works if the CoM is also centered on that tank. The Mk1 cockpit is by far the heaviest part on that tiny plane, and pulls the CoM forward a lot. There's hardly any weight on the other side of the tank to balance things, and most of it is the fuel you'll be using, so during flight it will only get worse.

Start by emptying all the tanks: the monoprop in the cockpit, the LF and Ox in the center tank, and the LF in the Mk0 pods.

I suggest swapping the tank with the last service bay, making the order cockpit, bay, bay, tank, cone. Attach the basic fin, the Juno pods, and the rear gear to the tank. Fin and Juno pods as far back as you feel ok with. The gear towards the front edge of the tank.

Fill the oxidizer of the tank - you won't use it, but it adds ballast to that side to balance the heavy cockpit. The CoM will shift backwards a bit. Now adjust the Juno pods forward/backward to finetune the CoM position - aim to move it exactly between the two bays.

To get the fins/wings right, you will need a bit of 'trickery' to prevent them from going so far forward that it looks silly or blocks the hatch. Attach the fins to either one of the service bays, right where they touch. It doesn't really matter which one it attaches to. Select the rotate tool, make sure it's on absolute mode, then rotate the tips of the fins 30 degrees forward (with snap on, two regular 'ticks'). You will notice that the CoM shifts a bit forward too, but the CoL shifts more, bringing them closer together. Now again with the rotate tool, and holding Shift this time, rotate the front edge one fine tick upward to give the wings the angle @Vanamonde mentioned. Finally, with the offset tool, move the wings forward or backward until the CoL is inside the CoM, a little bit behind its center.

You may notice all the tankage is now behind the CoM; not ideal, I admit. But we can remedy that by attaching two extra Mk0 tanks on the nodes inside the first service bay. Rotate them vertical and offset so they fit neatly inside when closed, one a bit to the right the other to the left. If you're ok with a bit of clipping, you can even move them a bit into the back of the cockpit - as long as the center of them is still inside the service bay, they'll be drag-shielded.

Place the science instruments and other parts you want in the bays, trying to keep the weight balanced. Now adjust the wings forward so the CoL is again right behind the CoM center. Fill the tanks in the service bay (CoM will shift forward), then add fuel to the center tank until it the CoM returns where it was. All balanced.

If you followed these steps, not only should your plane be able to take off and fly... it'll actually be supersonic too and have a surprising range.

Spoiler

I know, I replaced the basic fin by two elevons to make a proper tail. Worth it.

 

Edited April 5, 2022 by swjr-swis

[magnemoe]

On 4/2/2022 at 5:44 AM, surge said:

The yellow/black ball (mass) is MUCH too far ahead of the cyan/black ball (lift). All you've made is a front-heavy dart...assuming you get it off the ground. Be sure to move the rear landing gear when you fix it (it is about right, relatively speaking for now).

https://ibb.co/LRXCwxb

Rudders rarely help with steering, since aircraft roll, then pitch, and don't just turn sideways like a car. I say leave it, you have bigger problems to fix.

 

Yes, put the rear landing legs just behind the center of mass, this is needed to lift the nose. 
On many of my planes I has 3 sets of wheels, the rear one is only used for landing and  taxing on 5 kerbal plane, for larger ones they tend to be small wheels just to protect against tail and strikes. 

[surge]

On 4/6/2022 at 7:41 AM, magnemoe said:

Yes, put the rear landing legs just behind the center of mass, this is needed to lift the nose. 
...

It's not technically for "lifting the nose", it's so that the elevators act as a fulcrum and are able to rotate the aircraft. The CoM (yellow/black ball), the rear wheels and the rear control surface should form a triangle. If you look closely at the image in my last post, the craft actually has a distinct nose-low configuration when on the ground, yet it's liftoff speed is only about 60m/s.

 

On 4/5/2022 at 5:38 PM, KerikBalm said:

This is actually true of real wings too.

,,,

There are some wings in the real world that have so much thickness and camber they can still generate lift at slightly negative AoA, like the J-3 Cub (from memory), but that's a topic for aerospace engineers, not KSP

I am quite impressed by the way KSP does it - it seems to reshape the wing (in the editor?) so that these flat planks end up being modelled as proper wings with camber, reynolds numbers and the leading edge is facing the right way.

Therefore "somehow" they end up generating at least some inherent lift. That said, angle of attack doesn't hurt... until it stalls... or you need more speed!

Edited April 21, 2022 by surge
Aerodynamics clarifications. This really bugged me.