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I know the Russians seems to be planning some sort of catapult then ski-jump system, but I am not talking about that.

In the history of all aircraft catapult, they all seems to be launching the aircraft leveled instead of pitching upward. From old seaplane launcher to modern supercarriers. Is there a reason why no one is doing it? Does it make it cope harder in rough seas? Does it make it more complex, procedure and mechanism wise? All I can find is someone on Quora saying that they work better leveled without any explanation or citation.

R3G3N3R4T0R
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    What benefits do you think there might be from an angled deck? – BowlOfRed Jan 20 '23 at 09:01
  • I think it can give the plane a bit more angle of attack. From what I have seen there seems to be some kind of procedure that requires you to quickly rotate the aircraft at the end of the launch. I also think it may allow launching heavier aircrafts as the upward momentum should give a little leeway before reaching take off speed. – R3G3N3R4T0R Jan 20 '23 at 09:11
  • You get greater angle of attack, but at the expense of speed and mass, which means it is worse for heavier aircraft. – Rory Alsop Jan 20 '23 at 11:55
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    are you asking about the entire flight deck or just the catapult area? – selectstriker2 Jan 20 '23 at 15:15
  • I can only imagine the engineering complexity to have the catapult track bend to go up the ski jump. Additionally, you'd have the nose gear of the aircraft compressing as it hits the bottom of the ramp instead of staying fully extended as it's pulled. Also, since all carrier aircraft are already designed for flat-deck operations (except Harriers), every carrier aircraft would have to be redesigned to work with a catapult/ramp launch system. The inertia says stay with a flat deck. – FreeMan Jan 20 '23 at 17:22
  • @selectstriker2 Just the catapult area. Slanting the entire deck might just bring trouble to parked planes and landing. – R3G3N3R4T0R Jan 20 '23 at 19:26
  • @FreeMan I do think a curved "ski-jump" deck wouldn't be practical for engineering purpose, but perhaps a flat but non-level pitched up catapult wouldn't? If it is designed to be workable in rough sea, then a mere 5° pitch wouldn't hurt right? – R3G3N3R4T0R Jan 20 '23 at 19:29
  • So now, instead of the catapult shuttle having to negotiate a gentle slope, you're expecting it to make a sharp bend while under the massive load necessary to launch an aircraft? That's an even greater engineering challenge. – FreeMan Jan 23 '23 at 13:46
  • @FreeMan Why does it have to bend? Is it that hard to tilt the entire catapult? Is it because it might damage the keel? Maybe the support will reduce hanger space? Problem getting the aircraft in position? – R3G3N3R4T0R Jan 24 '23 at 19:28
  • Ah, so now the entire runway with the catapult is angled up? I suppose that eliminates the challenges I thought of, but the answers still stand - it's not necessary. – FreeMan Jan 24 '23 at 19:35

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There's no benefit to a ski-jump catapult, and plenty of drawbacks. The purpose of a catapult is to rapidly accelerate an airplane up to flying speed in a short distance. Having an upward sloped angle makes it harder to get an airplane up to speed.

Angle of attack is going to vary aircraft to aircraft and be adjusted for performance factors such as weight, so any catapult ski-jump would have to be rapidly adjustable to allow for every launch, which would be complex.

There's just no point: once the airplane is up to flying speed its control surfaces will be used to adjust angle of attack, and if it isn't up to speed the crew will be reaching for ejection handles anyway, having a bit of a slope up at the end isn't going to help.

GdD
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To answer your headline question:

A level deck allows for faster take-off with a greater payload than a ski-jump.

The ski-jump system has been used for decades, with the Royal Navy developing them in the 1970s. The Harrier "jump-jets" used in the Falklands in 1982 perfected the ski-jump launch - see this pic from Wikipedia's ski jump launch page for example: Harrier launching from an aircraft carrier with ski-jump

The ski-jump is useful for reducing the take-off speed required to launch, but it has some risks for aircraft that cannot thrust vector as airspeed reduces after launch...

And that leads to other downsides, including reduced payload:

However, ski-jump launches cannot match the payloads made possible by high-speed catapult launches. While aircraft such as the F/A 18 that are normally catapult-launched can make use of a ski-ramp, this typically comes at the cost of a reduced capacity for either fuel or munitions, and thus negatively impacting mission scope significantly.

(also from that Wikipedia link)

So...why not use a catapult and a ski jump? Well, the catapult would have to stop before the ski jump, as the mechanical stresses involved in trying to have a straight then curved catapult make it impossible to maintain, so the highest speed would be at the base of the ramp. Which means the ramp then has to change the vector into an angled up course - that's a lot of strain on the undercarriage. So much so that even without a catapult, it broke various aircraft undercarriage when higher speeds were attempted.

Additionally, you don't want to expend energy to then have the speed reduce during the ramp phase - you get better bang for buck accelerating the length of the flat runway.

Rory Alsop
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    you're answering catapults vs ski-jump. I read the Op question as being, "why aren't catapults used together with ski-jumps?" – Federico Jan 20 '23 at 12:53
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    In re-reading the question I see what you mean. I don't think you could easily do it. You'd have to halt the catapult before the start of the ski jump, otherwise the engineering becomes horrible - will try and work out a useful edit to include that – Rory Alsop Jan 20 '23 at 12:56
  • How about non-curved "ski-jump"? I.e. flat runway angled pointing up? This option is not discussed in the answer. – Tero Lahtinen Mar 02 '23 at 13:25