is there a way to figure out if my wings generate enough lift to take off?

[lammatt]

without actually going to the runway?

[khyron42]

From some testing I did long ago on a roughly 10 ton aircraft, if you get your rear landing gear just barely behind the center of gravity:

A "lift" rating of 10.6 (totalling up the lift number from all the various wings and flaps) let me tip the nose up and get airborne at 40-45 m/s. The speed at which you can tip the nose up depends on both lift, and proper placement of the landing gear. The speed at which you can raise your nose is called the "rotation speed" in the real world and is documented for most aircraft.

A lift rating of 6.8 got it airborne at 65 m/s.

A lift rating of 3.6 couldn't get airborne at max speed of 110 m/s before running out of runway, and ground, ditching into the ocean.

Based on those, if you're really leaning towards designing rather than testing, I'd aim for at least .6 lift rating per ton of aircraft mass. Beyond that I'd design for lift enough to hit whatever target rotation speed you like, keeping in mind that poorly placed landing gear could keep you from taking off no matter how much lift you have.

[HeadHunter67]

I know it's not exactly a sound aerodynamic principle, but even in real life it's true:

With sufficient thrust, lift is not a concern.

There are real aircraft that only stay aloft because of the ridiculous thrust behind them.

[BubbaWilkins]

1) Arrange landing gear so you CAN lift off easily.

2) Attempt takeoff. If you can't do so at a reasonable speed, add more wing surface.

3) If you don't want more wings, then add more engines and go faster sooner!

[Brapness]

Probably a good one for the suggestions forum. The number of times I've sat at my computer trying to encourage my planes into the air with hand gestures is beyond count. The pile of debris at the end of the runway's getting a bit depressing.

[KSK]

Thanks for this thread - I've been wondering the same thing for a while but never got round to asking. I'll see how far the 0.6 lift per ton of airplane rule of thumb gets me.

[Ferrit]

The 2:3 total lift to weight ratio is pretty reliable in stock KSP, it got me to space and back in various forms until I got addicted to FAR. Just as an aside in FAR if the wing size looks right it often is!

As BubbaWilkins alluded to above having the rear landing gear a small bit behind the centre of mass allows the aircraft to rotate to a nose up attitude earlier/at all and take off. If your aircraft only noses up when it drops off the end of the runway that's a reasonable sign that the rear landing gear needs to go further forward.

[Johnno]

Moved this to Gameplay Questions.

[tstehler1]

Someone correct me if I'm wrong about this, but I'm pretty sure the physics for spaceplanes aren't quite right in KSP, the atmosphere doesn't really give lift like it's supposed to

[khyron42]

Correct. My math was for the current game's stock physics, with all wings "leading edges" facing directly forward.

In the current physics, wings generate lift proportional to the airspeed against their expected "forward" direction, is my understanding.

I was wrong earlier - my test procedure was to pitch to 10 degrees nose up as early as possible in the takeoff roll and hold it at that angle, and record what speed was enough to take off at that pitch. My exact tests from earlier:

test name	Craft mass	 Craft lift	Rotation speed	Liftoff speed @ 10 degree nose up	Notes
lift test 1	9.79	         3.6	             10	                   N/A                        	slight downhill glide at 10 degree nose up and 100-110 m/s forward velocity.
Lift test 2	9.89	        6.8                    	10             	65 m/s	
lift test 3	10.03        	10.6          	10 to 12             	40-45 m/s                  	craft was tending to nose up and may have gotten airborne early @ 40 m/s with higher nose-up.

The test craft images: http://imgur.com/a/tsMVb

Edited September 12, 2013 by khyron42

[el_coyoto]

From some testing I did long ago on a roughly 10 ton aircraft, if you get your rear landing gear just barely behind the center of gravity:

A "lift" rating of 10.6 (totalling up the lift number from all the various wings and flaps) let me tip the nose up and get airborne at 40-45 m/s. The speed at which you can tip the nose up depends on both lift, and proper placement of the landing gear. The speed at which you can raise your nose is called the "rotation speed" in the real world and is documented for most aircraft.

A lift rating of 6.8 got it airborne at 65 m/s.

A lift rating of 3.6 couldn't get airborne at max speed of 110 m/s before running out of runway, and ground, ditching into the ocean.

Based on those, if you're really leaning towards designing rather than testing, I'd aim for at least .6 lift rating per ton of aircraft mass. Beyond that I'd design for lift enough to hit whatever target rotation speed you like, keeping in mind that poorly placed landing gear could keep you from taking off no matter how much lift you have.

Woah, I'll have to try this as soon as I get back home.

Thanks for sharing the results of your experiments, I have been looking for something like this for ages!

(but was too lazy to actually do the experiments lol)