Guide:Planes

Hecate's Lecture About aerodynamics and plane building in Besiege

## 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 as they can generate much more lift than the wing panels.

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 (22.875° for a small propeller), big propeller flies almost straight. 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 ID 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).

## Propulsion

Frankly, one of the most important things in the plane. In Besiege planes are usually powered by propellers, steam cannons or nives. Unlike propellers and flying blocks, water cannons are not flammable. Nives, flying blocks and steam cannons with No Bounds mod active can provide any amount of thrust.

Variable throttle can be imitated with a set of propulsion blocks on steering hinges which rotation changes the resultant thrust vector.

### Propeller Engines

Single engine airplanes (or with uneven amounts of engines) require a way to balance out the counter rotation of the main powered block. Any powered rotating block rotating in one direction will turn the rest of the airplane. To counter this effect real planes use trimming. Fortunately, in Besiege we can do this in a simpler manner. One of the ways is to place the engine on a freely rotating block (e.g. unpowered wheel or swivel joint) and attach a mass to it, ideally braces as they have a notably high annular drag, meaning braces will resist rotational movement more than compared to other blocks with the same mass.

The number of aerodynamic blocks connected to the engine will mainly affect torque, but also the overall agility of the airplane. More propeller blocks will increase torque at a slight expense of speed due to the added drag.

The angle at which the aerodynamic blocks are connected orthogonally to the direction of the flight influences the proportion of resulting speed over torque of the engine. Smaller angles (parallel to the flight direction) allow to achieve higher top speeds but have less torque. Higher angles (perpendicular to the direction of flight) reach higher torque but cause more drag and for this reason they have a lower top speed. The amount of torque will mainly affect acceleration and rate of climb of the airplane. Suggested in most cases are angled at around 30° - 45° (where 0° is parallel to the flight direction) as the resulting maximum speed far outweighs the small loss of torque.

### Thrust Vectoring

Thrust vectoring is manipulation of the thrust that allows better control over the aircraft and unlike control surfaces it works even at lower speeds. Also might be used to gain STOL (Short takeoff and landing) and VTOL (Vertical takeoff and landing) abilities. Performed by controlling the angle of the jet’s nozzles, or in Besiege, plane’s propulsion blocks with any RTC mechanism.

## COM, COL and COT

Notice that COM and COL are calculated based on all machine’s blocks. Remember to delete freely hanging out parts.

### COM

Center of mass. Indicated by the blue ball.
COL and COT were given in relation to COM.

### COL

Center of lift. Indicated by the yellow ball. COL is displayed if mass display mod is installed and on. 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 e.g. spontaneous yawning caused by intersections.
* The plane is sufficiently unstable.
* There is no “loose movement”. E.g. if the plane rolls, it does not keep rolling after the pilot let off the stick.
* Roll and Yaw are symmetrical.
* The wings do not bend too much (or brake) at high Gs.

### COT

Center of thrust, also called the thrust vector (not to be confused with the velocity vector). It is not indicated in Besiege. Balance might be tested without propellers 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).

### Propellers 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. Remember that toring angle is dependent on the wheel’s scale and its speed.

Control surfaces are becoming 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°.

In Besiege the airflow ignores most of the 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.

### Elevators

A “must have” on every plane. Control pitch. Generate more or less lift causing the plane to pitch up or down. On real planes they are usually located at the back of the plane, sometimes being a part of a horizontal stabilizer

### Ailerons

A “must have” on every plane. Control roll. Generate more lift on one side of the plane, causing it to roll. On real planes they are usually located at the sides of the wings as far from center as possible.

### Rudder

Controls yaw. On real planes it is usually located at the back of the plane, being a part of a vertical stabilizer. Some planes have two, mirrored vertical stabilizers with two rudders.

### Canards

Canards are small wings in front of the main wing. Canards as well as extended leading edge (front part of the wing) increase maneuverability, also at high AOA. In Besiege they allow to have a control surface ahead of COM to scientifically increase movability. Also might be hidden inside the fuselage (make sure that they angle in the proper direction while pitching).

### Leading Edge Slat

Does not work in Besiege as aerodynamics are not properly simulated. Might be a cosmetic feature.

### Flaps

In Besiege they are just additional Elevators. They create more lift and when the plane flies at lower speeds they allow it to maintain stable flight or slower descend, for a price of increased drag.

### Air Brakes

A surface that controls speed by significantly increasing plane’s drag. At high speeds most RTCs might not have enough power to fully extend the air brake with actual propellers or wing panels so other methods are used to imitate it, such as reversing the engine, using nives or overflow.

### Passive Air Brakes

Always active air brakes are used to increase the planes drag. They must be aligned with COM, or they will cause pitching up or down. Because the planes in Besiege can have very minimal drag in relation to their engine’s thrust, passive air brakes are added to make planes feel more natural and realistic, and allow them to bleed the energy during turns. The drag might be reduced by changing brake’s angle.

## 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.

Remember that steering hinges turn slightly upon the simulation start, and angle propellers 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.

### 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.

## Weapons

### Guns

In Besiege, cannons are usually the most important and versatile weapon on the plane. High power cannons allow to deal more damage and have higher muzzle velocity, making it easier to hit moving targets. High rate of fire (ROF) makes the recoil more consistent and easier to counter, usually done with nives.
Cannons in besiege have a random delay before shots. Because of that it is impossible to use two cannons to cancel yawing caused by recoil after shot, therefore cannons should be as close to COM as possible.

HUD or any other aiming device should be aligned with cannons. It might be tested in 0 Gs.

Crossbows might damage only wooden parts. Unless the target is wooden or the arrows hit a wooden part, crossbows are pretty much useless.

### Other Weapons

In Besiege there is a variety of easy to build A/G and A/A weapons.

Rules for building planes also apply for rockets and bombs.

Heavy payload changes COM, as well as dropping it. Heavy bombs, etc. should be attached as close to COM as possible.

Make sure that the plane’s landing gear is able to support the plane with the payload and land with it. Check if the plane is able to takeoff and land in a reasonable space.

Written with love by Hecate ❤️ Special thanks to Krautregen