Building with FAR 101

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Building with FAR 101 – Barebones Basics

First off, I''d like to thank tetryds, both for advice and corrections he provided in the course of writing this tutorial, as well as the awesome BAD-T tournament he is running that led me to write this tutorial in the first place, that it might be of some use to those who want to enter but might not be familiar with FAR . With that out of the way, lets begin.

Ferram Aerospace research is a mod that aims to bring real-world aerodynamics and aerodynamic principles and mechanics to KSP. As a result, building an aircraft is now a bit more complex than in the rather more forgiving stock aero model, as planes must now be built to take into account various real-world aerodynamic design considerations. This will not be a comprehensive overview, it won't tell every trick in the book on how to make a competitive performance dogfighter, but it should adequately cover the basics sufficiently to build a successful subsonic aircraft that avoids plowing into the ground seconds after launch.

Lets start by building a basic airframe:


This consists of a KAX D-25 radial engine, a structural fuselage, an inline cockpit, a liquid fuel tank, and a tail adapter.

Now, before we add some wings, we should first know where the Center of Mass (CoM) is. This is important, as it will greatly affect wing placement and aircraft balance later on. On the sample craft, the CoM is here:


Also activated are the Center of Thrust and Center of Lift indicators. Now, the CoL indicator here looks different from stock; there's no arrow. In FAR, the CoL functions similarly, but not identically, to Stock. In stock, it indicated the net direction of lift provided by lift forces from the wings and other lift surfaces, in FAR, its simply the Aerodynamic Center. It can still be used to get a rough approximation of craft balance, but for learning precisely how a craft is balanced requires the FAR editor readouts, covered later.

So lets add some wings:


With wings on, it's looking more like a plane, but there's still some work to do. Control surfaces need to be added, and the plane balanced – as seen the CoL is slightly ahead of the CoM; this plane will want to pitch up, potentially leading to a stall and possibly crash. For a successful, easy to fly craft, the CoL should be behind the CoM, the aircraft should want to naturally pitch slightly downwards while in flight without control surfaces.

To correct the imbalance, there are a few available courses of action: The mass of the craft could be shifted forward, the wings moved farther back, or the empennage lengthened to shift the tailplanes back and offer better leverage on the craft once control surfaces are added.

A quick craft re-arrangement later, and :


A bit unorthodox, perhaps, but serviceable. The wings were moved back, and the structural fuselage behind the engine was moved to behind the fuel tank, which puts all the heavy things at the front of the craft. As a bonus, the main wings are now on the CoM, which will help with performance, as the main wing is the point the craft will want to pitch up or down; the closer the CoM is to this axis, the less leverage needed to affect pitch. Additionally, the main wing is also located on the fuel tank, which means as the craft files and the tank is drained of fuel, the overall CoM of the plane will more-or-less remain where it is. Knowing where the Dry and Wet CoM on the craft is good to know. If the CoM shifts too much as fuel is used up, it may result in the plane becoming unstable.

To control the aircraft, control surfaces are needed. Lets add some:


The craft now has a pair of ailerons, a pair of elevators, and a rudder. The ailerons are located at the main wing tips, and provide Roll authority, the elevators are on the horizontal tailplane and will control Pitch, and the rudder on the vertical one will control Yaw. keep in mind the principle of leverage; ailerons/elevons (control surfaces that control both Roll and Pitch) that are located near the fuselage will be less effective at rolling the craft then if they were placed further down the wing, likewise, the further from the CoM the elevators are the more efficient they are at pitching the craft. Elevators placed inline with the CoM won't have any effect at all (i.e. if the ailerons in the picture were set to control pitch, they would have negligible effect due to how close to the CoM they are).. By default, a newly placed control surface will be set to respond to Pitch, Roll, and Yaw commands; while leaving control surface inputs default generally speaking won't keep you from flying, it may cause the craft to do weird things, since the rudder will be trying to pitch or roll the plane, the ailerons will be trying to adjust the craft's Yaw, and so forth. To adjust the control surface's inputs, right click on it to get a context menu:


Std. Ctrl is the control settings needed here. Flp/splr controls flap and spoiler settings, covered later, and curWingMass is how much the wing weighs, with Mass-Strengt... 1 is the wing/control surface structural strength, also covered later.

Clicking on the Std. Ctrl button opens a new menu:


A few more options than in stock, no? Pitch %, Yaw %, and Roll % are pretty explanatory, these control that respective input. The 100 means that at present this surface will fully deploy in response to an input. This number can be changed from -100 to 100. having a Pitch at less than 100 means the surface will only deploy partially in response to an input, and a negative setting will have the surface deploy upside down, useful for things like front canards and so forth.

AoA% is the Angle of Attack percent. Instead of reading a P/Y/R input, it reads the crafts current AoA while in flight, and dynamically deflects the surface in response to it. This can be set from 200 to -200; useful for things like wing slats or having a plane that could automatically pull out of a dive, that sort of thing.

Brake Rudder sets if the control surface can be used for affecting Yaw like an A.I.R.B.R.A.K.E.S. part – perfect if you want to make a Ho-229 /B-2 type flying wing, this lets you maneuver without vertical rudders.

Ctrl Dflect is important. This determines, in degrees, how far a control surface deflects. The lower the number, the less deflection, and the slower the pitch/roll/yaw effect propagates in flight. Because FAR models aerodynamic stress failures, or in other words, too aggressive a maneuver and the wings rip off, being able to adjust the control surfaces so they don't result in a 15 G turn and Rapid Unplanned Disassembly of the craft is vital. The other, less immediately fatal thing adjusting Control Deflection will do is help prevent stalls from overaggressive maneuvering. Too much control authority for your elevators and you risk pitching the craft's AoA too far from prograde, resulting in at best higher drag, or worse, throwing the aircraft into a stall. A lifting surface stalls when its cL falls too low; in other words, when a lifting surface stalls, it drastically reduces the amount of lift it generates; too little lift, and the craft falls out of the sky.

Now, what about the Flp/Splr setting? Lets take a look:


Clicking the Flap setting will designate the control surface as a Flap; flaps are useful for take off and landing, when extended, flaps provide greater lift at lower speed; they also cause more drag. Flaps shouldn't be extended much or at all in normal flight. When a control surface is a flap, extending/retracting can either be done via right click menu while in flight, or via action groups.

Clicking the spoiler button actives that surface as a spoiler. In KSP, spoilers are basically airbrakes; they are automatically added to the Brake action group, and when the brakes are activated, the spoiler will deploy. Be careful when placing spoilers; place a spoiler in the wrong place or upside down, and you may discover that rather than a brake it is instead acting like a flap or an elevon and affecting the craft's flight differently than intended.

Now, lets talk about Mass-Strength. Mass strength adjusts both the mass and the strength of a wing. The stronger the wing, the more aerostress it can take before catastrophically failing. The slider goes from 0.05, for a wing that is basically made out of paper mache and prayers to 4.0 for a wing made out of adamantium. For a a spaceplane, a wing setting of 0.4 or so should generally be sufficient, for a subsonic stunt/sport craft doing acrobatics at low altitude, a higher wing strength is recommended, something like .6 or so for a light weight craft. Keep in mind, that a wing strength that works for turns at 100m/s might not be sufficient for puling out of a dive at 240 m/s. While there is nothing wrong with a higher wing strength, keep in mind that stronger wings are heavier; the standard Wing Connector A, with a strength of 0.05, weighs 15 kg, the same wing at 4.0 strength weighs 1237 kg!

With a basic plane built (yes, it still needs things like landing gear, but those are unneeded for now), its time to see how well it will fly (theoretically). To do this, open up the FAR editor window in the SPH/VAB by clicking on the FAR icon in the toolbar. When this is done, it brings up this:


This is the Flight analysis graph, which will display some information on the flight characteristics of the craft in various flight regimes. Currently here it is set to AoA analysis, which will show how the craft will handle at various angles of attack. It can also be set to show how the craft will perform at various mach numbers by clicking on the Switch to Mach Sweep button. The numbers at the bottom adjust the parameters to be tested. By default it is set to examine the craft from 0 to 25 degrees AoA, at an airspeed of mach 0.2

The side bar buttons are craft and environment settings. Want to know how the craft will fly on Laythe? Select it from the celestial bodies menu. You can also see how the craft will fly when flaps and/or spoilers on the craft are deployed. Lets see how the example craft will fare:


The mach value has been adjusted to 0.35, (around 120m/s), and the upper AoA limit has been increased to 50 degrees. The graph displays a number of lines:

The green line what the craft's lift/drag ratio is as the craft sweeps from 0-50degrees AoA. Looks like the craft will get the best lift for the lowest drag at around 5 degrees AoA.

The Blue line is coefficient of Lift – how much raw lift does the craft generate. The blue line goes steadily up until about 35 degrees AoA, and then starts falling. As the line goes up, the more lift the craft is generating. The tip of the cL curve at 35 degrees is important, this is the point the craft will begin to stall.

The red line is drag. As AoA increases, more wing surface area is exposed to oncoming air, generating more drag. Here, the line increases until about 40 deg. Of note is the area around the intersection of the red and blue lines. At 35 degrees, L/D increases slope slightly, and at 40 degrees it begins to plateau, combine that with the the correlating decrease in cL and it shows the aircraft stalling at 35 degrees and coming to a full stall at 40, pitching the aircraft that far up should be avoided. With stalls, main wings are the easiest to stall out, other lifting surfaces like tailplanes are a bit harder.

The final line, the yellow line, is a measure of craft stability. The lower the line, the more the craft will want to return to a neutral state. Here it steadily goes down, looks like the craft will automatically recover from an AoA induced stall of the wings and bring the nose back down to a prograde vector. The value of the line factors into this; the steeper the slope of the line, the more aggressive the return to a neutral state. the more aggressive this movement, the higher the G force the airframe is subjected to, the more G's, the stronger the wings will need to be.

The second page of the FAR editor window that is important is the Data/Stability Derivatives. This page can be access from the drop down menu in the upper left currently reading Static Analysis going to it, and:


A bit scary and technical, isn't it? Lets walk through it. At the top there is a planet selector (again, if you want to know how the plane will do on Laythe/Duna/Eve, or for the suicidally insane, Jool). The next is altitude, in kilometers, from sea level. As atmospheric density and temperature change, so to does aero performance. The last is speed, in mach, for the same reason; higher mach results in different aerodynamic considerations and fun engineering challenges like dealing with the trans-sonic region supersonic drag, which won't be covered here. Lastly, flap/spoiler settings can be set.

On to the numbers. The Calculate Stability Derivitives button has already been clicked, so the stats have data to them. The first set of numbers at the top are technical data about the craft. Of note is the wing area, which can be used to determine the craft's wing loading. The higher the wing loading, the more mass per square meter the wing is supporting, and the stronger the wings have to be, but higher wing loading also means less drag. Lower wing loading means the craft has more drag, but is also potentially more maneuverable, dependent on control surface settings. Also of note is the level flight numbers,which tell the the chosen mach speed in m/s, AoA, and the coefficient of lift for level flight at the selected planet, altitude, and speed – in this case, the craft needs to have an AoA of ~2.4 degrees when flying at ~119m/s (the input mach 0.35) for level flight at sea level. The coefficient of Lift can also be used in conjunction with the static analysis graph, go back and look, and the cL of .157 shown here will correlate with the blue line on the graph.

Next up are the Longitudinal and Lateral Derivatives, each of these reference a particular type of movement about an axis. These should be green, any numbers in red indicate the craft will exhibit an unwanted behavior, and the higher the green number, the less that particular kind of negative craft behavior is a concern. In particular, lets look at Mw. This number tells how much a craft will pitch up in neutral flight. The number is -.114, which means the craft will very slowly pitch down in normal fight, which is to be expected with the CoM head of the CoL. But what if we moved it?


Lets see what happened:


Mw is now red, with a value of 0.08. the craft will now want to pitch up in normal flight. It wont pitch up very fast, but it is still a motion that will have to be actively corrected through trimming the elevators or applying active inputs to them every so often.

As with higher numbers being good when green, higher numbers when red is bad, the higher the number, the worse the detrimental effect. When a number is red and very low, it isn't the end of the world, and can easily be corrected by the control authority the plane has. For numbers bigger than around 0.5 - 1.0, a redesign of the craft is suggested to reduce the number, or even better, make the value in question green. In many cases this will be minor tweaks to wing and control surface placement. In the case of large numbers (3.0 - 4.0 and above) major structural redesign will be necessary.


Edited by SuicidalInsanity

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On 2/25/2016 at 2:21 AM, SuicidalInsanity said:

Building with FAR 101 – Barebones Basics

 ... In particular, lets look at Mw. This number tells how much a craft will pitch up in neutral flight. The number is -.114, which means the craft will very slowly pitch down in normal fight, which is to be expected with the CoM head of the CoL. ...


First, thank you for the tutorial. I hope you decide to write further installments. I could for one use a good AoA% example.

Secondly, you current coverage is very appropriate for an "101", so I do not want to try and 'correct' your information, I just want to add to it.

Lastly, while the above statement about slowly pitching down in 'normal flight' is correct and appropriate in the given example, there is a potential gotcha. The pitfall has two important facets to it.

  • Most people can agree what the yellow (CoM) ball means. If you read some of the FAR threads around you might notice that people have a lot more trouble defining the meaning of the blue (CoL) ball in FAR. I do not know what it means either, but I do know that not knowing sometimes causes me a few surprises.
  • The Mw number is found on the derivatives tab because it is a derivative. That means it is a number describing the extent of an effect caused whenever the flight conditions change by a tiny bit. Ok, what I am trying to say is that while a negative Mw is good, fine, stable and all it may not necessarily imply that your plane exhibits a 'controls at neutral' pitch down force in regular cruise flight.

To illustrate the second bullet I created a (slightly contrived) counter example:


This plane the has the blue (CoL) ball behind the yellow (CoM) ball and a negative Mw. But because I tilted the front wings up by 5 degrees it will pitch up all by itself at regular speed (even more so at high speed and not so much at low speed).


Notice how in regular cruise (in this case at 85 m/s at sea level) the control surfaces must be trimmed to keep the nose down (and by kept down I mean kept level).

Edited by Rodhern
minor edit

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