Solved: Aerodynamic issues

[Thomot512]

Today I wanted to try aerodynamics. I ran into a lots of issue. The first one is that the shape of the airplane does not seem to influence the ability of the airplane to break the sound barrier.

As can be seen from the image below the airplane fly in low atmosphere at Mach 2.6, without any shape optimization. Either the aerodynamics does not simulate the shock wave drag and its relation with area rules or the thrust of this motor is way too high!

Second issue I had is about trimming. As can be observed in the screenshot below, the airplane is gliding at around 250 m/s, with all control surface at 30° deflection to increase lift and pitching moment. It is generating around 170kN of lift force and weight about 40kN. Even with a lift that is enough to fly a 4G maneuver, the airplane can almost not pitch up.

When the engine is running it is perfectly trimmed at those speed but without engine it is  flying like a stone.

 

All of the above is extremely counter intuitive and seem very unrealistic. I suppose some more work is urgently needed in the aerodynamic simulation section.

Edited March 26, 2023 by Thomot512
Solved

[Greyflex]

Fully agree, control surfaces seem to be almost useless right now.

I tried about 5 or 6 airplane concepts before getting one that could reliably turn with no engine running.

Also i couldn't tell if that was a bug or just mistakes on my part so although I love that aerodynamics window, i'd love to have the visual arrows showing lift and drag per part like in ksp1 (the default keybind was f12 or f11 if i recall)

[Thomot512]

I ran a few other test. I wanted to trim an airplane into level flight and look at the its weight and lift.

Here is the airplane used:

It's total mass is 12720 [kg]. It's weight is therefore 9.80665 [m/s^2] * 12720 [kg] = 124746 [N].

If we assume that the angle of attack needed for a level flight in trim condition will be small, and therefore the component of the thrust adding to the lift is negligible, we can assume that the weight must be compensated by a lift force of equal intensity.

Therefore the expected lift force in level flight must be 124746 [N].

During flight, the airplane was trimmed at three different altitude and speed.

The three condition represent three different flight condition at different Mach number and air density:

Air density [kg/m^3]	Mach number []	Lift [N]
1.131	0.502	274582
0.615	2.852	113105
1.172	0.269	275441

What can be observed is that below the speed of sound the airplane needs roughly 2.2 times the expected lift in order to achieve level flight. Yet at supersonic speed this requirement drop below 0.9 times the expected lift.

This behavior cannot be considered otherwise as a bug.

If I had to guess I would say that the aerodynamic forces displayed in the AeroGUI represent roughly what should be expected for this airplane in those condition, including the huge increase of drag at supersonic speed. But those forces are not well linked with the mechanics of the airplane. It behave in a really weird fashion.

[Thomot512]

In order to investigate further the aerodynamic model of the game I designed a simple plane with detachable rectangular wings in order to investigate the effect of aspect ratio on the aerodynamic drag.

Spoiler

It does not seem to be simulated.

Here's the design:

I created the first design with a relatively high aspect ratio expecting a lower drag from this design.

In trimmed level flight at around 130 m/s 550 m above sea level, I got the following results:

Right after separation I got those results:

Then I changed the aspect ratio for something ridiculously small expecting a steep increase in drag.

For very similar speed and altitude I got:

And after separation:

 

As can be seen once the drag of the body is removed from the total drag, the drag of the wing can be read, and in both cases it is ~2.1 [kN], This proves that the aspect ratio does not have any effect on drag, or on L/D ratio. Which is quite sad in a game such as this one.

It is such a fundamental of aerodynamic that I would classify that as a bug.

[FlazeTheDragon]

I dont think the game indicates wing drag, it basically adds it up to the ''lift'' part. But i can assure you that bigger wings will indeed increase your drag overall based on all the ssto's ive built

[dukeb]

I made this account to report that wings seem to be working completely inverted and control surfaces and stabilizers seem to do almost nothing but instead ill just add on here because I would assume you're pretty much having the same problem as me.

Wings: steering appears to be partially inverted for pitch but correct for roll/yaw. this makes it incredibly difficult to even test out the other worse problem with wings which is that they seem to produce lift but in the downwards direction. im not savvy enough of a builder to figure out how to flip the wings over to test this but that is what it looks like to me.

Stabilizers: seem to just be incredibly weak but for pitch only? After giving up on wings and using large stabilizers as wings I can roll the plane over with a hair trigger but slam the pitch up button and its years before anything happens. changing the control angle does nothing visually and im unsure if it does anything as far as aerodynamics.  

Man this is really disappointing considering the new wing/stabilizer/control surface system seems to be the only thing added to the game over the last couple years compared to ksp1... 

[FlazeTheDragon]

Thats normal, if you look at any real plane, youll see that the elevator is going to be substancially larger then either roll or yaw controls.

Out of these 3 axis, pitch takes by far the most energy to affect, which means that you need a much bigger control surface to affect it.

Also, your center of mass/lift might be off, if you want really snappy pitch control, youll want your center of lift juuuuust behind the center of mass, the further it is, the harder it will be for your aircraft to pitch up or down. Aka, it wont be able to pull as much aoa

Edited March 24, 2023 by FlazeTheDragon

[Thomot512]

@Buzz313th

 

Disregard that post. I forgot that my thrust was vector was below the CoG and therefore producing a pitch up moment which changed my trim setting and therefor my angle of attack. This lead to the reduction in Lift.

I'm therefore unable to reproduce the bug observed in the first version of the Early access.

Following your answer on an other post, I decided to do an other test.
I was not able to reproduce the bug where the lift needed for level flight was completely off, as observed in my post of February 28th. It might have been a bug of the aero GUI that is not present in the new version of KSP2. Now the announced lift correspond to the actual flight condition.
But I still have the bug where cutting the engine change dramatically the lift produce without the airspeed changing.

The aircraft design is the following:

• Almost neutrally stable;
• No thrust vectoring;
• mass: 16.42T, Fg = 161.02kN

 

The issue that can be observed in the pictures above is the following:

• The airplane is trimmed for level flight and produce the expected ~160kN of Lift at an airspeed of 258 m/s when the engines are running;
• The airplane still with the same trim setting does only produce ~66kN of Lift at an airspeed of 255 m/s when the engines are cut off.

Expected behavior: Lift is linked to the airspeed, mach number, reynolds number, density, geometry, and angle of attack, but not to thrust.

Observed behavior: Lift is linked to thrust!

Edited March 26, 2023 by Thomot512
Made a mistake

[Thomot512]

I flew the design used in my post of the February 28th and re-flew it. Same issue as before.

Then I remade the same (as much as possible) design, and there problem disappeared.

No idea what is going on. But the problem seems to be solved for new design.

Good enough for me will mark the issue as solved.