This a build guide for working single-rotor helicopters. I've spent quite a few months on and off, figuring out exactly what does and doesn't work for helicopters within Kerbal Space Program. This is the culmination of that effort. Please feel free to leave comments and feedback. Also, I'll post a link to the helicopter seen in this guide for anyone to reverse engineer.
A conventional helicopter generates lift with a single main horizontal rotor, which produces a torque known as reaction torque. The vertical tail rotor produces thrust opposing this torque. This prevents the body of the aircraft from spinning, also known as the stator.
The three sizes of helicopter blades differ greatly in performance and flight dynamics.
Large blades seem to have disproportionately more drag and although providing more lift, higher airspeeds are usually unreachable due to said drag and instability. Furthermore, greater blade mass puts additional strain on the rotor system. Stretching robotic joints and attachment nodes, which exacerbates lift and yaw instability.
Small blades are often too close to the centre of mass in the horizontal axis, which contributes to cyclic/input instability; due to a bug/game limitation. I recommend building a medium sized helicopter at first, to avoid the inherent problems that come with the small or large blades.
Collective pitching of the main rotor blades produce lift and control altitude. When configured correctly, the collective pitch should be bound to the main throttle group with the use of a KAL-1000. Helicopters don’t generally incorporate negative collective pitch but in Kerbal Space Program it’s helpful when descending and also to successfully autorotate.
Set blades to deploy and set the blade deploy angle to 0 in the right click menu. This ensures the rotor does not produce lift before fully spooled.
Reduce the play-speed of the KAL-1000 to make collective throttle proportionally slower, as rapid changes in collective may have undesirable effects on pitch stability.
When setting the KAL-1000 deploy angle limits, stay within the stall limits of the rotor blades. I recommend between -3, +12 (for medium blades)
Cyclic is the differential pitching of the rotor blades throughout the rotational cycle (advancing/retreating). This reduces and increases lift on opposing sides of the rotor disk. This provides control in the pitch and roll axes.
On the main rotor blades, enable pitch and roll and disable yaw.
High blade authority angles can reduce overall lift during intense manoeuvres. This causes blades to exceed their stall angle, resulting in a rapid reduction in lift/altitude. I recommend between 1 and 3.
Thrust from the tail rotor prevents the helicopter spinning, known as counter-torque. The tail rotor, also provide yaw authority or control in the yaw axis. Manual control of counter-torque is quite common and often trim is used to find the neutral point. Although it is possible in Kerbal Space Program, manual control of the counter torque is very difficult without a flight stick or H.O.T.A.S.
Helicopters’ mechanically couple the main rotor torque to the counter-torque rotor, also called mixing.
To couple collective pitch and the counter-torque rotor in Kerbal Space Program, add blade deploy angles for both the main and tail rotors to a KAL-1000. It may also be possible to simulate torque-coupling or mixing by adding tail rotor torque in place of the blade deploy angle to a KAL-1000.
In the propeller/tail-rotor graph, evenly distribute points along the timeline.
Build a rig using launch clamps and free spinning rotors; in order to roughly tune the counter-torque on the ground. Then, in-flight precisely tune the counter-torque. Later copy these settings to be saved in the SPH (editor) Again, set blades to deploy and this time, set the tail rotor blades deploy angle to the neutral angle found during tuning. [craft v1.4.1 update: Retuned counter-torque in-line with adjustments to the tail height.]
Finally, disable pitch and roll, and enable yaw.
As mentioned earlier, Kerbal Space Program has a critical limitation in relation to control surfaces (rotor blades) orientation and distance from the centre of mass. To avoid complications with cyclic inversion/mixing, use a medium or large rotor hub to properly space the blades.
To create a reliable start-up sequence. Add rpm and torque limits for the main/tail rotors to a KAL-1000. Set the rpm/torque values as required, adjust the graph curve and reduce play-speed for a gradual spool up sequence.
Although, directly adjusting rpm in-flight is not common, managing torque to regulate rpm is. To simulate this functionality, add the main rotor torque limit to a translation action group for in-flight adjustment.
I recommend binding play-sequence/set-forward-direction to the stage action group for engine start-up and play-sequence/set-reverse-direction to the abort action group to shutdown.
Single rotor helicopters are inherently asymmetric and come in two configurations; clockwise or anti-clockwise spin. The highlighted colours on the top surface and silver leading-edges seen on rotor blades, indicate the direction of spin.
The effect of asymmetry on lift is known as “dissymmetry of lift” or the difference in lift between advancing and retreating blades in relation to the direction of travel.
Dissymmetry of lift is responsible for the continuous roll instability experienced by all single rotor craft. This imbalance, even when properly compensated for, will at the absolute limits of a helicopter's high-speed flight, result in a fatal uncontrolled roll.This phenomenon, a roll towards the retreating blade side is known as a “retreating blade stall” and a stall on the advancing blade side, is known as an “transonic stall”
The effects of gyroscopic precession, will cause any force acting on the rotor disk to be offset by 90°. With the exception of blade deploy angles which are calibrated relative to the centre of mass within Kerbal Space Program.
Dissymmetry of lift however, is subject to the gyroscopic effect. The imbalance of the advancing/retreating blades imparts a force on the retreating blade side, which offset by 90°causes a pitching moment about the centre of mass. This effectively raises the nose of the aircraft, returning it to level flight.
To achieve static stability, a helicopter needs to compensate for the effects of dissymmetry of lift and gyroscopic precession. This is done through a combination of flapping and feathering hinges built into the rotor assembly.
When pitch/roll are carefully balanced, the helicopter will be stable across a range of dynamic flight regimes, and retain control authority in extreme bank angles and at high speed.
As with fixed-wing aircraft, helicopters longitudinal stability is affected by the relative position of centre of mass/lift forward and aft.
Helicopters are exceptionally sensitive to changes in mass, but when balanced will fly straight and level for some distance. For a basic design, having the main rotor centred directly above centre of mass is preferred but more advanced helicopters may position the main rotor slightly forward or aft.
The addition of a vertical tail rotor inhibits the usefulness of the centre of lift overlay in the SPH (editor). Instead, reference the main rotors' centre of spin as the true centre of lift.
Cyclic Authority is the ability to control pitch and roll. The height of the main rotor relative to the centre of mass determines the degree of control; although not exclusively. (higher, more authority and lower, less authority)
The tail rotor (in addition to the main rotor) has a massive influence over roll stability. Its height relative to the centre of mass determines the roll bias of the helicopter; either neutrally stable or with a left/right bias. The influence the tail rotor has on roll changes across flight regimes and at differing airspeeds. The addition of feathering, flapping and hunting hinges/rotors help to equalise any bias or changes that might occur at differing phases of flight/airspeeds.
Any changes to either the tail rotor or main rotor will drastically affect cyclic authority and overall static stability, so take care when adjusting. [craft v1.4.1 update: I raised the height of the tail significantly. This required a slight retune of the counter-torque and the left-roll bias had to be accounted for, with changes to the roll-angle of the main rotor and feathering/flapping hinge settings.]
Fully Articulated, Semi-Articulated and Rigid
A fully articulated rotor system has three axes of blade articulation: feathering, flapping and hunting. Each blade can move independently in each axis.
A semi-articulated system usually has two axes of articulation: feathering and flapping. Often blades will only be independently articulated in a single axis (feathering), and mast-articulation will make up the other axis (flapping). Less commonly, a semi-articulated system will have three axes of articulation, for which mast-articulation would make up at-least one.
A rigid system has zero axes of articulation. Instead rigid rotors often use flexible blades in place of hinges. Rigid systems will also sometimes include a stabilising fly-bar.
Rotor blades in Kerbal Space Program are not flexible, so a statically stable rigid system is not possible. This leaves two options: a fully and a semi-articulated system. Building a system strong enough to withstand rotational forces is essential. Unfortunately however, due to the strength limitations of the robotic parts, assembly options are even more limited and a fully-articulated system is unlikely.
Therefore, I recommend a two hinge semi-articulated system, with mast-hunting. More than two hinges can result in a significant amount of robotic drift, making their use impractical.
Most articulated helicopters will also have an articulated tail rotor, with independently articulated blades to allow feathering and flapping. This ability compensates for dissymmetry of lift across the tail rotor and equalises out the counter-torque thrust.
Feathering is the blade twisting about the chord length, changing the pitch angle. The advancing and retreating blades are free to move in the pitch axis, which actively compensates for dissymmetry of lift. Working in conjunction with flapping and hunting, feathering absorbs torque generated by the collective and cyclic. During an autorotation, a slight negative pitch helps to sustain rotor rpm, storing energy for the descent.
Flapping is the up and down motion of the rotor blades. This movement allows blades on the advancing side to flap up, reducing lift and down on the retreating side to increasing lift. This contributes to a reduction in dissymmetry of lift. Cyclic authority is improved by the addition of flapping, as the rotor disk is able to tilt in the direction of travel. This dampens gusts that would otherwise disrupt the static stability of the helicopter (KerbalWeatherProject) and or rapid changes in direction; which would increase drag and reaction-torque.
Hunting is the leading or lagging of the rotor blades or rotor system, adapting to changes in torque. Additional drag on the rotor system generates more torque, which is then dampened and prevented from transmission to the stator. This works dynamically at different airspeeds as drag is increases. Usefully excess counter-torque forces can also be dampen; allowing the tail rotor to operate at a higher thrust.
Hunting can also become saturated which causes a phenomenon known as the speed wobble. This occurs shortly before rapid aerodynamic instability and is a helpful warning to reduce speed.
How to build a semi-articulated three axis system!
Once the basic layout of the helicopter is worked out. Add three robotic components: two small G00 hinges and a single F12 rotational servo.
[update: Found a simple alternative to rotor hinges. By using a single grip pad in place of the G00 hinges. One grip pad can provide enough movement, to effectively do the job of both the feathering and flapping hinges. I recommended using rigid attach for the blades, and grip pads. Also, strut between the rotor hub and the rotor-blades as before; to provide strength. The rotor-blades are more prone to spin separation/drift but offsetting parts can help with this issue. The counter-torque will also require a retune to account for altered main-rotor loading. However, overall, this alternative is about 80-90 as effective, as separate hinges. Top speed is reduced and fly characteristics are significantly different.]
Attach the F12 rotational servo under the main rotor to provide mast-hunting, allowing the entire rotor system to lead/lag. Set the F12 to its maximum power output and a traverse rate of at-least 1.0. This will allow torque-transmission to the stator.
I recommend reinforcing the connection between the F12 servo and the stator (fuselage) with struts. Robotic drift under load will cause the entire rotor system to stretch and compress; however, strutting both sides will prevent rotation, nullifying its usefulness.
Flapping and Feathering
Next, attach both sets of G00 hinges between the rotor hub and blades. One set will provide flapping and the other feathering.
Flapping hinges are fairly simple. They allow the blades to move up/down. Set power output to maximum, traverse rate to maximum, target angle to 0.0 and the angle limits to -1.0,3.0, biasing the angle limit in the upward direction. [craft v1.4.1 update: settings changes... angle limit -2.0,2.0, target angle 0.0, traverse rate 2.0, motor size & output 0.00. This accounted for a heavier left-roll bias and effectively increased level flight top speed by 10mp/s (to 60mp/s) and high-speed stability.]
Feathering hinges are a bit more complex. Use the rotation and translation tools to orient/centre the hinges, such that they can twist about chord-length, in the blades’ pitch axis. Set the power output to 10%, traverse rate to maximum, target angle to 0.0 and the angle limits to 3.0,3.0. [craft v1.4.1 update: Settings changes... angle limit -2.0,2.0, target angle 0.0, traverse rate 1.0, motor size & output 0.00. This accounted for a heavier left-roll bias and effectively increased level flight top speed by 10mp/s (to 60mp/s) and high-speed stability.]
I do not recommend the use of rotor-servos for high drag applications, as they seemingly experience more robotic drift than hinges. Although they do work for low drag applications, as on Duna.
Finally, add struts between the rotor hub and blades. The minimum angle limits of hinges are actually more coarse than the minuscule angles required.
Using struts successfully reduces their range of motion and prevents too much flapping, which would destabilise the helicopter. This extra stiffness also increases overall cyclic authority.
Static wing surfaces will have minimal impact on stability during low speed flight but throughout differing flight regimes, they can be used to make the helicopter dynamically stable; even partially compensating for dissymmetry of lift.
An asymmetric tail plane angled up on the retreating blades side, will lift the tail and lower the nose. Resisting the helicopter's tendency to level out at high speed and roll in a retreating blade stall.
Rotor Bias and Autorotation
It is possible to bias the main/tail rotors’ for high speed flight, to assist auto-rotation or for dynamic stability in conjunction with the static airframe. In both cases changing the main/tail rotors' leverage over the centre of mass or rotating them about the pitch/roll axes.
Translating and rotating the rotor forward and aft will impact the longitudinal stability or pitch stability.
Translating and rotating to the left or right will impact lateral stability or roll stability. Bias in the longitudinal axis can help keep the nose down. Bias in the lateral axis will compensate for a retreating blade stall, lateral drift caused by thrust from the tail rotor and mitigate a death-roll during auto-rotation. [craft v1.4.1 update: Tilted the main rotor roll-right toward the retreating blade side, in order to eliminate side slip caused by the tail rotor thrust. This is only possible due to the angle and height of the tail-rotor; which is heavily biased left-roll towards the advancing blades side. Also, the feathering and flapping hinges had a settings change to allow more freedom of movement in the rotor system to account for this bias.]
Translating and rotating the rotor in the vertical axis will impact lateral stability or roll stability. Translating and rotating through the horizontal axis will impact yaw authority. Adjusting the height of the tail rotor will bias roll stability at low-speed (unstable) and compensate for a retreating blade stall at high-speed (stable). [craft v1.4.1 update: The vertical tail and rotor were tilt as seen in the imagine bellow (formerly straight vertical) this was to account for the main-rotor adjustments.]
Rotating the tail rotor like the above imagine, reduces the bank angle needed, to prevent side slip caused by the lateral thrust of the tail rotor.
These changes are usually made at the expense of low-speed stability. However, the use of three axis articulation increases the envelope where the helicopter remains statically stable, working in reverse to compensate for bias and enable a dynamically stable design.
This capacity for damping forces, can allow for an increase in counter-torque thrust at the high end of the collective throttle. The addition of mast-hunting dampens the effectiveness of the counter-torque at low speed, when drag on the main rotor is low and at higher speeds when drag is high, releasing its full potential. This only works because the hunting servo can become saturated.
Some bugs to be aware of….
Cyclic mixing/inversion, when rotor blades are too close to the centre of mass/centre of spin.
KAL-1000’s can bug out, if attempting to bind the authority limiter and deploy angle.
Excessive uncontrollable roll. Occurs across multiple craft saves. Is unaffected by reverts and most unfortunately looks exactly like the effects of dissymmetry of lift on helicopters. I believe it’s related to the memory leak Kerbal has and the only known fix is a complete game restart.
Node strength and robotic drift really limit helicopters. (I really wish Squad had implemented a set of helicopter specific hinges)
It is my opinion that the drag configuration of all rotor blades is excessive and is the root cause for a lot of helicopter related bugginess.