How do I figure out how much lift I need?

[guitarxe]

Quite tired of back and forth trial and error any time I start designing a new spaceplane to figure out exactly how much lift (i.e, wings), I need on it to work.

Is there a way to be able to tell, with mods or otherwise, how much lift I have and how much is needed from within the SPH while I'm still building?

[Superfluous J]

Pre-1.0, each wing's lift rating was (roughly) how many tons it could lift. A good rule of thumb was for a 10-ton craft, you wanted 10 lift points.

I've only built a few planes since but it seems to hold fairly well still, and may even be too much.

Note: This is all for Kerbin of course.

[ajburges]

Pre-1.0, each wing's lift rating was (roughly) how many tons it could lift. A good rule of thumb was for a 10-ton craft, you wanted 10 lift points.

I've only built a few planes since but it seems to hold fairly well still, and may even be too much.

Note: This is all for Kerbin of course.

For what flight profile? Most of my turbojet SSTO space planes (Mk2 with two turbos and 2 delta wings) must use closer to a 1:2 (or 3) lift to ton ratio.

In thrust we trust!

There is no other way to evaluate lift other than trial and error. Too little, and you need too much of an AoA for level flight and induced drag robs your accent efficiency. Additionally your glide slope approximates a falling brick. Too much, and you have too much wing drag in atmosphere. Additionally, the extra mass eats your dV.

Edited June 6, 2015 by ajburges

[guitarxe]

Mine is more like 1:4, no wonder I'm having trouble keeping up my pitch in the upper atmosphere... how am I even flying...

[cybersol]

Wings create lift but also a lot of drag in 1.0.2. I find 1:4 up to even 1:6 to be good for SSTOs that need to lift a lot and still get through the transonic drag spike. In my opinion, if you have enough to take off before the end of the runway and land, then you are good to go.

[n0xiety]

I use very little wings compared to the craft. As long as your craft is able to reach 1500m/s+ in atmosphere up to 30k the faster you go more lift you will get. My planes are hard to controll while going slow because of this and hard to land but they gain speed alot faster and able to keep that speed because of less drag.

[Warzouz]

Too much wings : harder to go to orbit

Too little wings : harder to return from orbit

I did a lot of tweaking before finding a "pleasant" compromise.

[guitarxe]

Hm, maybe wings are not my issue then. I'm building MK3-sized spaceplane, with an orange tank as payload. I can lift it off the ground, but by around 10-12km I lose pitch authority to the point that at max pitch up, the plane will still nose down and dive.

I've already tried adding a lot more control surfaces, and far away from the COM/COL, but at around the same altitude I still experience the same issue.

Then I just slapped some huge wings for the heck of it just to see if maybe it was lift, and hey, it worked wonders! ... up to a point. Do I need more lift? Or more speed? I try to keep my speed below 200m/s while under 10km.

[cybersol]

Post a pic for the best help, but meanwhile for me it been about more pitch control when I've had similar problems. First of all make sure tail rudders are only yaw controls and your other controls do not yaw. Its much less critical, but if its a big plane that needs lots of control, I also end up disabling roll on about half the other control surfaces. Everything horizontal has pitch enabled, because it always seems to be the limiting factor. Other than that its about more controls, with more control lift, further out and forward or backwards of the CoM (not CoL). Do you have canards or elevators at the very front, as those really seem to help? The BigS elevators are also really great for being high lift controls.

As for speed, the drag spike really only starts around 290-300, so for a lifter I keep around 270 until I'm ready to level off an go transonic at 12-14 km.

[Mic_n]

"enough lift"

ie: "it depends". Speed & altitude are the biggies. The higher the speed and lower the altitude, the more lift your wings will generate. With enough thrust and angle of attack, ta-daaa it's a rocket and who cares about wings?

NB KSP (stock, at any rate) wings aren't airfoils and don't generate "lift" as you might think of it. If they're pointing prograde, they're not pulling you up. They'll slow your 'slip' through the air column though and will help you move your heading vector, but unless you mount them 'pointing up a bit', they're not actually going to make your aircraft rise.

A craft with wings bolted straight ahead and sitting level on the runway will not lift off of its own accord, no matter how fast you go or how many wings it has.

In other words, there's no real answer. How much lift you need for what? just try it and see what feels right for your purposes.. it's massively unscientific but I honestly can't think of a complete answer to satisfy all the options.

Edited June 6, 2015 by Mic_n

[Michaelo90]

A craft with wings bolted straight ahead and sitting level on the runway will not lift off of its own accord, no matter how fast you go or how many wings it has.

Actually, it will, if you can get to orbital speed without burning up. But realistically, no

[Mic_n]

smart a.s.s.

Absolutely true though... you you can get up to ~2600m/s, you'll soon find yourself going so damn fast that gravity can't keep up with the curvature of the ground you're moving across, and you get to be flying HitchHikers Guide To the Galaxy style... falling but missing the ground. Which is also known as "in orbit".

[Windspren]

There's a button in the SPH next to symmetry mode called center of lift. If you have enough lift the blue arrow will point upwards.