This page covers Aerodynamics and basic principles thereof.
In Besiege aerodynamics concern all fast moving machines that have propellers (or other blocks affected by drag). Its principles apply in cars and planes engenering.
Angle of Attack (AoA)[]
Angle of attack (also called alpha) is an angle between the velocity vector and an imaginary line that goes through the middle of the wing (or whole plane to simplify). The higher the AOA the higher the lift generated, until stall. Low α might mean the plane flights straight at high speed while high α might mean the plane is performing a narrow turn or is flying at low speed and had to increase its lift.
Propellers and Flatangles[]
From a plane building perspective, it is better to use Propellers rather than Wing Panels, Wings, or Build Surfaces as they can generate much more lift.
Changing the angle of the propeller changes it’s drag and lift. At the default 0° angle the propeller drops down slowly. Angled up flies backwards, down, forward. Notice that mirrored propeller and one rotated by 45° behave completely different despite looking the same.
At 22.845° from the default angle, big propeller’s drag and lift vectors cancel out allowing it to fly straight. It also keeps accelerating during free fall. Used in wings to generate lift.
At 23.06876° from the default angle (22.845° for a small propeller), big propeller flies almost straight, this angle is called flat angle. It also keeps accelerating during free fall, but even faster (due to zero drag). Used as stabilizers. Flat propellers used in mirrored sets provide lift in planes and allow fast and stable flight.
One of the hidden blocks is Block 52. It is a small propeller that acts like a big propeller with a connection point at the end (used in helicopters rather than planes).
Geometric Centers (CoM, CoL, CoT)[]
Notice that CoM and CoL are calculated based on all machine’s blocks. Remember to delete unconnected/unused blocks and parts.
Center of Mass (CoM)[]
Center of mass. Indicated by the blue ball, and visible in vanilla (though not accurate; the Instrumentality mod improves the accuracy). CoL and CoT are tuned relative to the CoM.
Center of Lift (CoL)[]
Center of lift. Indicated by the yellow ball. CoL is displayed by the Instrumentality mod. It affects the stability of the aircraft. If it is too much to the left or right from the CoM, it will cause rolling. It must be aligned with CoM. CoL ahead of CoM causes more instability. CoL behind CoM causes more stability. The further they are the stronger the effect. In besiege CoL should be close to CoM. It doesn’t really matter if the CoL is too high or low, though the high wing placement is technically more stable laterally compared to the low wing placement.
Building with CoL[]
When adding the first propellers to the plane and its control surfaces, make sure the CoL is around CoM and keep adding them until enough lift is generated. Keep checking that:
- The plane is properly trimmed and there is no spontaneous yawning caused by intersections or roll caused by the engines.
- The plane is sufficiently unstable.
- There is no “loose movement”; i.e. if the plane rolls, it does not keep rolling after the pilot let off the stick.
- Pitch, Roll and Yaw are equally strong.
- The wings do not bend too much (or break) at high Gs.
Center of Thrust (CoT)[]
Center of thrust, also called the thrust vector (not to be confused with the velocity vector). Indicated by the red ball. CoT is displayed by the Instrumentality mod (though it doesn't account for propeller engines).
It affects the stability of the aircraft. Balance might be tested without propellers (except those that are parts of the engine), in 0 Gs.
- If it is too much to the left or right side, it will cause yawing.
- If it is too high or low, it will cause pitching.
- CoT at the back (like in jets) makes the plane slightly less stable . Remember, it is possible to hide nives anywhere in the fuselage.
- CoT in front (like in propeller planes) makes the plane slightly more controllable.
There is not that much of a difference if CoT is far back or in front and it can be easily countered with propellers.
Trim[]
In Besiege trimming a plane is a very time consuming process. On a real plane, it can be adjusted to e.g. to counter one side of the plane being heavier, or to counter the propeller’s roll. In besiege only pitch should be adjusted. If your plane rolls or yaws (and there are no expected CoM deviations such as dropped payload from one wing) this means there is something wrong with your plane. It might be a problem with connection triggers, some propellers intersect with something, some blocks not being mirrored correctly or other random thing.
Because of the way triggers work in Besiege, rotating your machine by 90° might help control what connects to what. Deleting every block symmetrically and undoing it after is another possible solution, plus it may reveal blocks that have not been mirrored properly or mirrored at all.
Pitch trim might be performed by rotating some propellers responsible for pitch on both left and right side of the plane to increase or decrease their lift. Notice that both propeller angle and distance from CoM matters for the trim’s effectiveness. Variable throttle planes might not be able to be trimmed perfectly at all speeds and it might cause more issues. It is best to avoid trimming with propellers. Making the plane lighter or heavier, or moving CoL also trims pitch and makes it independent of plane’s speed.
- CoL forwards and decreased mass cause more pitch.
- CoL backwards and increased mass cause less pitch.
Stability[]
Though one could guess that the plane should be “stable”, we must first learn that e.g. pitch stable plane will try to maintain its pitch angle all the time, and so it will be hard to maneuver. If the plane is too unstable, it will overreact to the pilot’s actions and also will be hard to fly. A high level of maneuverability requires a low level of stability.
What we want to achieve is such instability that e.g. when the plane pitches, it does not increase nor decrease the pitch angle after the pilot lets off the stick. A plane should be sufficiently unstable in all the axis (in Besiege).
Propeller Placement[]
Propellers located behind CoM increase plane’s stability while propellers ahead increase instability. The further they are from CoM the stronger their influence. E.g. the horizontal stabilizer in real planes is located at the back, making the plane very stable in yaw. In order to avoid and keep the original look, it is possible to hide the propellers in front of the plane in a fuselage to make it less stable in yaw.
Control Surfaces[]
A plane can rotate along three axes: pitch, roll and yaw, thanks to the three different types of control surfaces. In Besiege they are usually an RTC mechanism with flatangled propellers.
Control surfaces are more effective:
- The further they are from CoM.
- If there are more propellers on the control surface.
- If they angle more (bigger angle causes also bigger drag). Usually, the values are around 15° - 45°.
- Remember that toring angle is dependent on the wheel’s scale and its speed.
In Besiege the airflow ignores most blocks, therefore propellers (and wing blocks) may and often are hidden inside other blocks or braces. They might be inside the plane’s fuselage or wing. In fact, even a cube with propellers will fly. Drag in besiege is not that realistic, and so, the shape, size and placement of the wings does not matter (as long as it is not made of propellers or other flying blocks). Any wing configuration might be used.
- To make a plane more resistant to move in the chosen axis, place propellers further from CoM and or place more of them.
- Propellers at the back and front control pitch resistance.
- Propellers placed in wings control roll resistance.
- Vertical propellers control yaw resistance.
- More propellers generate more lift and drag at high AOA. Planes with less propellers might feel like with higher momentum, with slower velocity vector changes. * More propellers will cause faster velocity vector changes, but may cause stalling if the direction change is too fast. It will also cause huge stress on propellers and my break them off.
Troubleshooting[]
General Rules[]
If not trimmed correctly in pitch, change the lift of some propellers (remember which one for later changes). Both angle and distance from CoM matters. May not work with variable throttle planes and at different speeds. More consistent effects are achieved with weight trimming and moving or adding more propellers. Check if everything is placed symmetrically. Wrongly mirrored propellers might cause huge issues. Some inconsistencies in symmetry may be fixed by rotating the machine by 90° and or symmetrical deletion of every block and undoing it after.
To make a plane harder to turn and less likely to e.g. keep rolling after pilot rolled the plane in the chosen axis (but will also require more force applied from control surfaces), place the propellers further from CoM or add more of them.
Control surface’s effectiveness can be increased by placing them further from CoM, changing their max angle or adding propellers to them. Make sure your RTC mechanism has enough power to rotate the propellers. Avoid placing them far from the axis of the rotation.
Steering hinges turn slightly upon simulation start; you may need to tune the angle of propellers or engines attached to them.
Always check CoL and CoT and if they are properly aligned.
Look for intersections, CoM offsets caused by braces with length, asymmetry caused by block placement order and or glitches. Intersections might be checked at 0% timescale if some block move after starting the simulation.
Yaw[]
A plane should not roll during and after using yaw. If it does roll to the opposite side to the yaw it means that vertical propellers are too low. If it does roll to the same side as yaw it means the vertical propellers are too high. Move propellers accordingly to counter that or have them all aligned with CoM.
Roll[]
Pitch on roll might be caused by pitch trimming done with angled propellers. Slight roll might be caused by steering hinges that move slightly at the start of the simulation. Check control surfaces if its symmetrical or trim it.
Pitch Energy Gain[]
Light planes might experience energy gain while pitching. It is due to unfortunate alignment of the flatangled propellers in the elevators that produce enough thrust to overcome the energy bleed. It can be easily fixed by duplicating propellers in the elevators and then mirroring the duplicated propellers upside down to negate the effect. Mind the changing COL.