Lift cakculation

[Astraph]

Hello fellow Kerbonauts,

There is one thing that keeps bugging me, but I'm unable to find a satisfying answer. How to calculate (or even just estimate) the amount of lift generated by wings? I know each aerodynamics part has a fixed lift rating, but how does this relate to lift force? It can't be newtons or even kN (since, well, 0.9 canards would be useless then), but I have no idea what equation should I use to estimate how much wing area would my plane need...

[Claw]

The lift force generated is dependent upon a lot of things, including airspeed, air density, and Angle of Attack. So, that being said, this thread discusses how to calculate the lift. It might be more complicated than what you're looking for.

If you want a rule of thumb as to how much wing to use for an airplane, then you can aim for somewhere around 0.5 to 1.0 lift rating per ton of aircraft. Go on the 0.5 side for smaller craft or higher thrust to weight ratios, or on the 1.0 side for lower thrust to weight ratios. There isn't a "required" number, so play around a bit and see what kind of setups fit your personal design style.

Good luck

[Geschosskopf]

I don't do any calculations. The way the game works right now, the only real effect various amounts of lift have for a plane is determining how slow it can fly. The slower it can fly, the less runway it needs both taking off and landing. Thus, the amount of lift required is really a function of the plane's intended use. If you're making a spaceplane that will always takeoff and land at KSC's runway, as long as you can get it off the ground before you run out of runway, you're fine.

But if you're wanting to land out in the countryside on Kerbin or other planets, being able to take off and land on short fields might be essential to survival. In such cases, keep adding more wing parts until you get the short-field performance you want. And note that it's perfectly acceptable to make hypersonic biplanes to keep the wingspan short. Pretty much everybody does.

My own self, I usually strive to have enough wing that I can land at 30m/s and in fact have to force the plane to quit flying. This allows landings in ridiculous places like near the peak of the small, spike-like island near Airbase Island. Then combine this with a TWR > 1 and you can take off from the bottoms of deep, steep valleys.

[Jouni]

Lift also affects ascent and re-entry. If your spaceplane doesn't have enough lift, it has to use thrust to fight gravity, making it slower to reach orbit. During re-entry, planes with less lift are more forgiving, as the control surfaces are more likely to be able to counter the spin caused by poor balance or ridiculous angles of attack.

[numerobis]

My KSP-scripts include a module for calculating lift with excruciating precision.

https://github.com/numerobis/KSP-scripts/blob/master/lift.py

Example: to figure out the lift from a small control surface when you're going down the runway at 50 m/s and you pulled up to a pitch of 10 degrees, you type:

import lift
lift.smallControlSurface(10,50, altitude=70)

And you get the answer: that surface provides 4.28 kN and change in lift.

One unit of lift from control surfaces gives you about 8.5 kN of lift when you're on the runway going 50 m/s at 10 degree angle of attack. The equation is sin(AoA)*pressure*speed*lift.

One unit of lift from a wing gives you about 7 kN in the same circumstances. The equation is sin(AoA)*(1-|sin(AoA)|)*cos(AoA)*pressure*speed*lift.

That all assumes you are flying level, no slip. The direction of lift is perpendicular to prograde; if you're climbing, some of the lift will be slowing you down. I haven't fully worked out what slip does in KSP, but I think it just reduces your lift.

Edited February 7, 2014 by numerobis

[Geschosskopf]

Lift also affects ascent and re-entry. If your spaceplane doesn't have enough lift, it has to use thrust to fight gravity, making it slower to reach orbit. During re-entry, planes with less lift are more forgiving, as the control surfaces are more likely to be able to counter the spin caused by poor balance or ridiculous angles of attack.

I dunno. The time required to get a spaceplane to orbit is more a function of TWR than lift. All spaceplanes go through 3 stages of ascent: initial climb to ~20km, hold at ~20km to build up speed, then final rocket burn to get out of the atmosphere. The better your TWR, the less time required for each stage. In the initial climb, thrust will get you up to 20km faster than wings will. Then the time required at 20km is a function of acceleration, which is also a function of TWR. The more lift you have at this point, the harder it is to keep from accidentally reaching flameout height before you're ready. Rockets get to orbit in a couple of minutes compared to 10-20 or more for spaceplanes. Thus, the more your spaceplane flies like a rocket, the faster you get to space. And the faster you get to space, the less time you have to run your engines, so the less fuel you need to get there.

As for entering the atmosphere, I've never had a problem with this even for planes with the insane amount of lift necessary to fly on Duna. Just come in with the nose right on or slightly below your prograde vector.

[Jouni]

I dunno. The time required to get a spaceplane to orbit is more a function of TWR than lift.

And the lack of lift directly translates into smaller TWR. I'm not talking about the difference between planes that have a lot of lift and planes that have even more lift, but the difference between planes that can barely take off at KSC and the kind of planes that most people prefer to build. The amount of fuel a spaceplane uses depends almost exclusively on the speed it can achieve with air-breathing engines. When you are trying to reach orbit efficiently with a plane that requires a 20 to 30-degree angle of attack for level flight above 20 km, reaching the top air-breathing speed takes a lot of time.

As for entering the atmosphere, I've never had a problem with this even for planes with the insane amount of lift necessary to fly on Duna. Just come in with the nose right on or slightly below your prograde vector.

This is less of an issue with passenger planes, because you can easily balance them for re-entry during the design process. (And with 0.23 and tweakables, the balancing no longer takes many hours and many failed re-entry attemps.) With cargo planes, you don't know in advance where the center of mass in going to be, so you either have to use ballast tanks (leading to a heavier and more complex design), or you can use less wings, which leads to a more forgiving design.

[numerobis]

At 30 degrees angle of attack you're only losing 14% of your horizontal thrust.

Lift falls faster with air pressure than you can compensate in speed. By 20km, you typically have about a quarter of your takeoff lift. Luckily, you also have nearly twice the thrust.

[Jouni]

The actual loss of horizontal thrust is more than 14%, as intakes also become less efficient. This in turn forces you to accelerate at lower altitudes, increasing drag.