PURPOSE:
(If you don’t want all the theory and practice flying, just skip to the summary for basic rules of thumb.)
This tutorial will cover different types of flight controls and how you can adjust your flight controls so that your airplane / space plane flies how you want. We will also see the effects of inertia on an aircraft’s ability to roll, pitch, and yaw and how you can adjust your flight controls to compensate.
As with my other tutorial(s), I will try to demonstrate the basics of design as opposed to telling you how to make a specific aircraft. This tutorial does use an aircraft to demonstrate the principles in flight.
This is a bit of a long article, but some of the airborne steps sort of repeat themselves (you’ll see what I mean). So after you do the steps a couple times, you should get the hang of modifying the test aircraft to see the different effects.
Flown in Stock KSP v0.23
TOPICS:
For easy reference, here’s what you’ll find below:
BACKGROUND
Section 1: Pitch Control (Elevon, Stabilator, Canards, Torque, Thrust Vectoring)
Section 2: Roll Control (Aileron, Differential Stabilator, Torque)
Section 3: Yaw Control (Rudder, Torque, Thrust Vectoring)
Section 4: Inertia (Pitch, Roll, Yaw)
SUMMARY – (Includes basic rules of thumb if you get tired of reading the rest)
This information might be basic or slightly advanced based on your level of knowledge. You should be able to jump into any section you wish.
BACKGROUND:
We will cover two areas in this tutorial: Flight Controls and Rotational Moments of Inertia. I’ll cover some of the background theory before we dive into flying.
Also, while there are some Kerbalisms that you can use to optimize your design, I will try and stick to basics so that you can get the idea, then you can tweak your own aircraft to do what you want.
For this tutorial, we will use the aircraft pictured below. This airplane is stable enough and has enough gas that you can (hopefully) fly through the whole tutorial.
Feel free to try and copy the design, or just steal the .craft from here. It is a mid-mounted, delta wing design with no dihedral to keep aero effects from confusing our experimentation. Of note, only the cylindrical monopropellant tanks at the root of the wings (one of the left side and one of the right side of the fuselage) are full. The wing tip cylindrical tanks and all spherical tanks are empty. Also, all flight control surfaces start out disabled except for the large ailerons.
So, what the heck is all that stuff?...
Control Surfaces:
If you are unfamiliar with the types of control surfaces, check out this article by Keptin. It has a lot of other useful information in it as well, but you can just read up on the control surface section for this tutorial. Also, I tried to give some short definitions below as a refresher for our basic test airplane.
One general thing to note before we dive into controls: The further your control surface is from the Center of Mass, the more effective it will be.
- Aileron – Controls roll. Typically placed on the wings.
- Elevator – Controls pitch. Typically placed on the trailing end of the fuselage (behind the wing), but can also be found in “T-Tail†and other configurations. In the case of the test craft, the elevator is actually called a vertical stabilator because the whole thing moves.
- Canard – Controls pitch. Typically placed on the forward part of the fuselage (in front of a wing).
- Rudder – Controls yaw. Typically placed on vertical tails.
- Elevon – A combined Elevator & Aileron which controls roll and pitch. Typically placed on the trailing end of a delta wing.
- Trim – Removes forces from the flight controls during flight. If you fly with SAS on, this is pretty much taken care of for you. It is not something you exactly design into your Kerbal airplane / space plane, but certainly affects the flight characteristics. I mention it here because we will use the Yaw/Pitch/Roll (YPR) indicators in flight to show us what kind of “trim†the SAS is providing, thus telling you if you need to adjust your design. If you want to fly without SAS, then you’ll need to use [Alt + WASD] to adjust your trim. [Alt + X] resets the trim to zero for all three axes.
To demonstrate all of these, we will turn on/off the control surfaces in flight. To turn on/off the control surface, right click and disable each axis (yaw/pitch/roll). In flight, unlike the SPH, you have turn on/off the flight control on both sides. It does not automatically do symmetry (KSP v0.23).
Inertia:
Inertia is the resistance an object has to being moved. Translational Inertia is usually pretty obvious to most people in how hard it is to push something around. For example: it is harder to push a large rocket with an LV-45 than to push a small rocket. Once that large rocket is moving, it’s harder to slow down.
Airplanes/spacecraft not only has translational inertia, but also rotational inertia. If you have already flown large rockets in Kerbal, you probably already know what I’m talking about. If you haven’t, we will do a little inflight demonstration for how this can affect your airplane / space plane design.
Another note: an airplane has three rotational moments of inertia. For simplicity, we will call them “Rolling Inertia,†“Pitch Inertia,†and “Yaw Inertia.†I will spare you the math and details. Just know that roll inertia is how resistant an aircraft is to roll, pitch inertia’s resistance to pitch, and yaw inertia’s resistance to yaw. Pitch and Roll Inertia are usually much more obvious in aircraft / space plane design because you don’t usually fly in a lot of yaw.
Simply put, the more mass an aircraft has out on the wings, the more resistant it is to roll (more roll inertia). The more mass an aircraft has on the nose and tail, the more resistant it is to pitch (more pitch inertia). More mass on the wings and/or nose/tail tends to make the aircraft more resistant to yaw (more yaw inertia).
To demonstrate all this, we will use the monopropellant tanks to transfer mass around, much like an ice skater moving his/her hands in and out while spinning.
LET’S GET STARTED:
Section 1: Pitch Control (Elevon, Stabilator, Canards, Torque, Thrust Vectoring)
Sooo, pitch control… When I’m designing Kerbal planes I find that I spend a lot more of my flight control tweaks on pitch than anything else. I think that’s because when flying a space plane you do a lot of pitch work so changes are more noticeable. Plus, it takes a lot of tweaking to get the feel you want when the CoM is moving around, lift is changing, and you get a sudden kick of thrust when trying to leave the atmosphere.
Fortunately there are a lot of options available to help with pitch control. Unfortunately most work on the same principle so you don’t necessarily get magical perfection using a canard vs. elevon vs. stabilator. On the plus side, you can use any of these in your design if you know how and where to use them which makes it possible to create effective designs.
1.1 Elevon
Elevons are ailerons that can also act as elevators (brilliant!). So they are attached to the trailing edge of a wing, and that wing is traditionally a delta wing (although it doesn’t have to be and in our case, isn’t). Because wings are generally not as far back on the airplane as say, a tail, the pitch control surface is not quite as far away from the CoM as what we get with a tail. This means it is (generally) not quite as effective as a stabilator or canard. (Lift rating also factors in.)
Also, in reality an elevon often becomes an important lift contributor since it is an extension of the wing. This means that when you use the elevon to pitch up, you are also giving up some of your wing’s lift. Depending on your Kerbal design, this can cause the aircraft to want to descend or slow down while you’re trying to pitch up (because you are losing lift and creating drag).
So, let’s actually go fly!
Flight:
1) Fly level at about 2000m, full throttle.
2) Disable all flight controls in yaw/pitch/roll (leave torque and thrust vector on). To disable a flight control, right click on it and deactivate the yaw/pitch/roll. Remember to do the other side. All flight controls (except the large elevon) are disabled by default for the supplied .craft.
3) Activate the yaw/pitch/roll authority for large control surface on the wings (remember to do both sides). This control is active by default for the supplied .craft.
Starting around 2000m, fly the airplane through a loop (just hold back until the plane flies through straight up, straight down, then ends up back facing where you started). When you fly the loop, notice how fast and far your nose pitches up, then sort of springs back. Also pay attention to the maximum and minimum altitudes. You should reach a max altitude about 3500 m and end the loop at about 900 m. After that, notice how it isn’t very “springy†and bouncy it is as you fly the loop. It’s slow, but nice and smooth.
You can play around with this for a while to get the feel of it. When you’re done, fly on to the next part.
Modifications:
1) Fly level at about 2000m, full throttle.
2) Disable the large elevon’s yaw/pitch/roll control.
3) Activate the yaw/pitch/roll for the small control surface on the wings (remember both sides).
Starting around 2000m, fly this airplane through a loop just like before. Notice how springy this one is and how slow it pitches. You should reach a max altitude about 4000 m and end the loop at about 600 m. It still flies nice and smooth.
The small control surface is about as far behind the CoM as the large control surface, but has a slightly lower lift rating. The end result is that it takes longer to pitch through a loop than with the large surface.
1.2 Horizontal Stabilator
This is the classic pitch control surface for high performance fighter type airplanes. In the example airplane, this is further away from the CoM than the elevons but has slightly less lift rating (KSP v0.23). Stabilator effect on the wing’s lift is not the same as for elevons, but can still be a bit of a factor if your plane is maxed out on weight. You might end up in a spot where you don’t have enough pitch authority, but it won’t steal lift from your wings since it pitches by pushing down on the tail. So you can generally keep the nose up until you simply run out of lift or your stabilator can’t turn any further.
Modifications:
1) Fly level at about 2000m, full throttle.
2) Disable all flight controls in yaw/pitch/roll (leave torque and thrust vector on).
3) Activate the yaw/pitch/roll for the horizontal stabilator (remember both sides).
Starting around 2000m, fly this airplane through a loop just like before. Again, compare the feel in springiness and how fast it turns with the elevon. You should reach a max altitude about 3400 m and end the loop at about 1200 m. Even though it has a slightly lower lift rating than the large elevon, the stabilator is further behind the CoM than the elevons and pitches a little faster.
Again, feel free to fly around a bit with this and go on to the canards when you’re ready.
1.3 Canards
This is the forward set of winglets. It has been used as a pitch control surface for a few high performance fighter type airplanes and many private airplanes. In the example airplane, the canard is further away from the CoM (forward) than the stabilator and has the same lift rating. Canard effect on the airplanes lift is different than for a stabilator or elevons. Because the canard is in front of the CoM and the CoL is behind the CoM, canards actually provide lift upward instead of downward (as with a stabilator).
So this means if your airplane or space plane is at maximum lift angle of attack (around 25 degrees in KSP v0.23) and you try to pitch up, your canard may actually lose lift and the nose will drop. This is because the canard’s AoA goes above 25 degrees causing the canard’s lift to decrease. This is slightly different than how it works for an elevon, but the effect look similar.
Modifications:
1) Fly level at about 2000m, full throttle.
2) Disable all flight controls in yaw/pitch/roll (leave torque and thrust vector on).
3) Activate the yaw/pitch/roll for the canards (remember both sides).
Starting around 2000m, fly a loop just like before. Again, compare the feel in springiness and how fast it turns with the stabilator. You should reach a max altitude about 3200 m and end the loop at about 1500 m. Because the canard is further away from the CoM than the elevons and stabilator, it pitches faster. Also notice how far you can get the nose to move initially, then it sort of bounces around.
KSP tends to like canards because of the above factors. In reality, the equipment needed to put canards on the front end of an airplane is complicated because that pesky co*ckpit is in the way. KSP doesn’t suffer from the same limitations so you can put canards wherever you want. However, that doesn’t guarantee that your plane will fly like you want. Plus, once you get into space, canards turn into more inertia that you have to rotate around with RCS. (See Section 4 for inertia.)
1.4 Torque
When I mention torque here, I’m referring to the gyros available in your command module. In the case of this test airplane, it’s the venerable Mk-1 co*ckpit. Up till now we have left it on all the time. We’re still going to leave it on here, but fly with everything else turned off.
Modifications:
1) Fly level at about 2000m, full throttle.
2) Disable all flight controls in yaw/pitch/roll (leave torque and thrust vector on).
That’s it. Fly around and see how it does. Do a loop if you want, but it probably won’t go very well.
This airplane is stable enough that torque alone is enough for the SAS to maintain stability. In fact, we have been relying on the torque when switching around the flight controls. You probably wouldn’t want to fly an airplane or space plane this way, but there’s no reason you can’t! If you disable torque to control your space plane in the atmosphere, remember to turn it on (or your RCS) at high altitude because you will need it when on the fringes of space.
1.5 Thrust Vectoring
The TurboJet engine we are using has up to 1 degree of thrust vectoring. It isn’t much, but like torque it can help out in controlling your plane.
Modifications:
1) Fly level at about 2000m, full throttle.
2) Disable all flight controls in yaw/pitch/roll (leave thrust vector on).
3) Disable torque control for the Mk-1 co*ckpit (right click on the co*ckpit and select “toggle torqueâ€Â). Make sure you’re nearly level.
The plane is stable enough to maintain level flight, but there isn’t much thrust vector authority to pitch up since the engine is about maxed out with trim to maintain level flight. Realize the nozzle is pointing up to provide a down force (like a stabilator). This is because the Center of Lift is behind the CoM. If your craft is neutrally stable, thrust vectoring will point up less.
When you’re done, make sure you turn the torque back on. Also, feel free to turn on several control surfaces and see how it flies.
Section 2: Roll Control (Aileron, Differential Stabilator, Differential Canards, Torque)
I think roll control is a little more straight forward, but some people might not realize that you can control roll with more than just ailerons. There aren’t quite as many options as with pitch, but you can still design different effects to suit the style you want.
2.1 Ailerons
On this plane we have big inboard ailerons, and small outboard ailerons. The lift rating doesn’t really scale well with the physical size (in KSP v0.23 anyway), but we’ll still get the idea from this demo. Generally speaking, ailerons out near the wing tip will be better at rolling the aircraft than ailerons near the fuselage. However, as with canards, ailerons further out on the wings creates more inertia that uses more RCS to rotate in space. (See Section 4 for Inertia.)
Modifications:
1) Fly level at about 2000m, full throttle.
2) Disable all flight controls in yaw/pitch/roll (leave torque and thrust vector on).
3) Activate the yaw/pitch/roll for the large ailerons (remember both sides).
4) Make sure the Mk-1 torque is back on if you did Section 2.
Fly it around, note how fast the airplane rolls. Do 3 or 4 full rolls left or right and see how it does.
Modifications:
5) Disable the yaw/pitch/roll for the large ailerons (remember both sides).
6) Turn on the yaw/pitch/roll for the small ailerons (remember both sides).
Fly it around, note how fast the airplane rolls. Do 3 or 4 full rolls left or right and see how it does. Even though the control surface is smaller, it is much more effective out on the wingtip.
In addition to the inertia I mentioned above, if you get to aggressive with aileron placement or your lift is maxed out, using ailerons can cause your wings to twist asymmetrically. This will give you all kinds of yaw/roll/pitch problems that might not make sense. Strutting the wings can help here, especially if you have large or funny angles on your wings.
2.2 Differential Stabilator / Canards
The concept of differential stabilator is the same as canards, so for the sake of shortness (which this article already isn’t) we’ll discuss both here. Since the stabilator and canards are closer to the fuselage, you can (hopefully) imagine that they are less effective than the ailerons as demonstrated with the inboard/outboard ailerons above. So let’s try it out.
Modifications (hopefully you’re getting the idea):
1) Fly level at about 2000m, full throttle.
2) Disable all flight controls in yaw/pitch/roll (leave torque and thrust vector on).
3) Activate the yaw/pitch/roll for the stabilator (remember both sides).
Fly it around, note how fast the airplane rolls. Do 3 or 4 full rolls left or right and see how it does. Compare to the ailerons.
Modifications:
4) Disable the yaw/pitch/roll for the stabilator (remember both sides).
5) Turn on the yaw/pitch/roll for the canards (remember both sides).
Fly it around, and compare to the stabilator. Is there much difference?
Realize that at higher Angles of Attack, your craft may respond differently to rolling with canards versus stabilator, especially at high altitude. So it’s good to check that out too if you plan on using them to help your craft roll. KSP (v0.23) allows you to selectively cut out controls to tweak it the way you want.
2.3 Torque
As with pitch, you can roll your aircraft with torque only. Torque tends to be more effective in roll than pitch because of inertia (see Section 4 for inertia), but this can vary dramatically between designs.
Modifications (bet you can guess):
1) Fly level at about 2000m, full throttle.
2) Disable all flight controls in yaw/pitch/roll (leave torque and thrust vector on).
How well does the roll compare now?
Section 3: Yaw Control (Rudder, Torque, Thrust Vectoring)
We finally made it to yaw. There isn’t a whole lot to mention here because for the most part in KSP (v0.23) yaw doesn’t play a huge factor in most (smaller and flat) symmetric designs. There are certainly designs you can make where it’s a bigger factor, but the limited options make it more straight forward. As your craft grows in overall size or is really long/wide, control placement becomes more important.
3.1 Rudder
Ahh, the mighty rudder. It can mess up your aircraft in a big way if you get too crazy with it. But it can also help a lot to recover from that bothersome asymmetric thrust.
By the way, putting a rudder really high up on your airplane can cause it to act like an aileron. Imagine sticking another wing straight out the top and plunking an aileron on it. So rudders tend to be close to the body and near the tail, so that it acts in an intuitive way. It certainly doesn’t have to be there, and you can use stabilator/canard type designs for a tail. It all depends on what you need and want. If you have a really wide aircraft, you might need rudder further away from the body to make it effective.
Modifications (hmm, is this new?):
1) Fly level at about 2000m, full throttle.
2) Disable all flight controls in yaw/pitch/roll (leave torque and thrust vector on).
3) Activate the yaw/pitch/roll for the rudder (remember both sides…oh wait, there’s only one).
Yaw that thing around. Loads of fun and useful for steering a little if you want to stay flat. High AoA rudder control isn’t very realistic yet (KSP v0.23), but you can still use it.
Concern about yaw is hard to demonstrate with this airplane because it has a pretty basic shape. If your airplane is really wide, you may need to move rudders out away from the body (and away from the CoM) to make them more effective.
3.2 Torque
And just like pitch/roll, you can yaw with torque only. Give it a try! (I’ll let you figure out the steps…)
For small airplanes and space planes that aren’t too unusual, the torque provided by the command module is usually plenty of authority to deal with yaw, unless you end up with asymmetric thrust. If you make a flat, flying wing design with no vertical tail, you can use torque to change your heading slightly with yaw so you don’t have to roll the airplane.
Section 4: Inertia
Okay, so we talked in the background section about what inertia IS, but why should you care about pitch/roll/yaw inertia?
When designing your aircraft flight controls, overcoming (and stopping) the inertia is a big factor. If an airplane or space plane has most of its mass (or most of its parts) concentrated near the Center of Mass (a lower roll/pitch inertia), then you won’t need as much flight control area to get the control authority you want. In fact, if you over do it with the amount of control, you risk putting your aircraft / space craft out of control.
In the case of an aircraft where the concentration of mass is more spread out in the wings or along the fuselage (a higher roll/pitch inertia), you will need more effective flight controls to get the authority you want. Also, this type of design will be slower to START turning and slower to STOP turning.
4.1 Pitch Inertia
If you recall, pitch inertia is how resistant the airplane or space plane is resistant to changes in pitch. So what parts are we going to use for the pitch inertia? Since we have already explored pitch flight control systems, we will start off with the test airplane in a controllable state and move monopropellant from the middle of the plane to the nose/tail to increase the pitch inertia.
Modifications:
1) Fly level at about 2000m.
2) Disable all flight controls in yaw/pitch/roll (leave torque and thrust vector on).
3) Activate the yaw/pitch/roll authority for the canards and the horizontal stabilator (remember to do both sides).
Before we do any more modifications, let’s fly this around again to see how it works out. Start out at 2000 m and do a loop (just hold ). You should reach a max altitude about 3000 m and end the loop at about 1800 m. Note how “springy†the nose is right after you start pulling. It initially pitches up to 60 degrees nose high, comes back down to 45, then back up to 90 degrees.
No we’ll move some weight around. So we can see what is happening to the SAS pitch control, we’re going to first disable the canard/stabilator pitch controls and watch the Y/P/R trim indicators for changes in pitch. Disabling the flight controls makes the trim indicators more sensitive because only thrust and torque are giving input.
4) Disable the yaw/pitch/roll authority for the canards/stabilator (both sides).
5) Take a look at the pitch trim. It should be about 4 notches up, depending on your fuel.
6) Transfer monopropellant from the LEFT fuselage monopropellant tank to the monopropellant tank on the NOSE. Make sure you fill the forward tank completely. (Right click on the left fuselage cylindrical monopro tank, then hold [Alt] and right click on the nose sphere tank. Transfer fuel in.)
7) Take a look at the pitch trim, it should now be about 7 notches up. You might also notice the plane is flying with about 3 notches of yaw. If you’re not sure why, it’s because we have shifted the Center of Mass slightly right. The Center of Thrust no longer lines up with the Center of Mass which results in flying a little sideways.
8) Transfer monopro from the LEFT fuselage tank to the tank on the TOP of the aircraft. (We’re putting monopro up here to make sure the fuselage tanks end up balanced left and right.)
9) The pitch trim should hardly change, but the airplane will be flying with a little more yaw.
10) Transfer monopropellant from the RIGHT fuselage tank to the REAR tanks on both sides. (Using the right wing tank will keep the plane balanced left/right.)
11) Take a look at the pitch trim, it should now be back to about 1 notch up, better than before we started. Also, there should be no yaw.
12) Activate the yaw/pitch/roll authority for the canards/stabilator (both sides).
Start out at 2000 m and do another loop. You should reach a max altitude about 3000 m and end the loop at about 1800 m. So it basically pitches at the same rate. However, note how the “springiness†of the nose is different. This time it jumps to 70 degrees nose high, back down to 55, then up past 90. This is because the airplane has enough control authority to get the plane pitching, then the higher inertia makes it want to keep pitching. This can make it easier to lose control, and harder to get it back if things start moving fast.
Note that there are two spherical monopro tanks on the tail and only one on the front. If you recall from the SPH, the tail monopro tanks are about half as far from the CoM as the nose tank is. Filling one in front and two in back helps leave the CoM unchanged between the lower and higher moment of inertia configurations. Basically this means when designing a plane, the CoM and Inertia are affected more by weight placed further from the CoM.
More Modifications (if you want):
1) Leave the canards/stabilator active, turn on the large ailerons (both sides) and fly around.
2) Activate all the surfaces (small and large elevons, stabilator, and canards – both sides) and fly around. In this configuration, when the aircraft pitches up hard, it gets close to going out of control. If you pitch down hard, it goes out of control. Why? Because gravity helps in the pull down and it’s just enough to go past the point of stability. This is harder to do when the pitch inertia is lower (monopro in the cylinder tanks).
When you are done flying around, make sure you transfer the monopropellant back into the cylindrical tanks on the fuselage body.
4.2 Roll Inertia
So roll inertia is how resistant the airplane or space plane is to changes in…(wait for it) roll! Since we just did this in pitch, hopefully you can guess what happens with roll. For this exercise, we will use the inboard (large) ailerons plus the monopropellant tanks on the tips of the wings. Similar to the pitch inertia experiment, we will move some of the monopropellant from the middle of the plane to the wingtips to change the roll inertia.
Modifications:
1) Fly level at about 2000m.
2) If you haven’t already, make sure all the monopropellant is transferred back into the cylindrical tanks on the fuselage body.
3) Disable all flight controls in yaw/pitch/roll (leave torque and thrust vector on).
4) Activate the yaw/pitch/roll authority for the large aileron (remember to do both sides).
Before we do any more modifications, let’s fly this around again to check out how it rolls. Start out at 2000 m and do a few aileron rolls. Note how quick it is to start rolling, and how long it takes to stop rolling.
No we’ll move some weight around. While we transfer monopro this time, we will watch the Y/P/R trim indicators for Yaw changes. There is no need to disable the flight controls (like we did in pitch), since the ailerons are not providing yaw assistance.
5) Transfer monopro from the LEFT fuselage tank to the LEFT wingtip tank. Yes, transfer all of the monopro. The aircraft will yaw a lot. The aircraft will fly stable if you started out level and don’t mess with the pitch too much. Without rudder, the SAS can’t keep up with that much mismatch between the Center of Mass and the Center of Thrust so the airplane skids around in a turn. (If you are having problems with the aircraft going out of control, transfer half of the monopro for the left side, then all the monopro on the right, then return and finish the left side.)
6) Transfer monopro from the RIGHT fuselage tank to the RIGHT wingtip tank. Make sure you get it all. The aircraft should no longer be yawing.
Start out at 2000 m and do more aileron rolls. The airplane rolls a bit slower, but not dramatically. But if you start and top several rolls, you will hopefully see a bigger difference in how long it takes to start and stop rolling.
You can mess with the aileron (small/large) configuration and see how things change. When you are done flying around, make sure you transfer the monopropellant back into the cylindrical tanks on the fuselage body.
4.3 Yaw Inertia
I bet you can guess what yaw inertia is by now (resistance to yaw maybe…). Since we just did pitch and roll, you are hopefully getting the idea.
Yaw inertia is usually not a big impact to smaller airplanes or space planes (flying in atmosphere) because they are symmetric left/right. Still, if you want to see how it can affect your craft (especially when encountering things like asymmetric thrust), then you can mess with the yaw inertia.
Realize that yaw inertia for an airplane or space plane is basically affected by the weights of pitch and roll that we just talked about, but at the same time. Why? The reality is that pitch and yaw are relatively unaffected by vertical weight in the airplane because smaller airplanes tend to be flat. So in our discussions of pitch/roll inertia, we basically ignored the up/down weight distribution. In yaw, you can’t simply ignore the left/right or nose/tail weights unless your craft is very short or very long. And in these cases, where you put your yaw control can be very important.
Modifications:
Hopefully you’ve gotten the idea from the Pitch and Roll Inertia sections on how to work through this. I’m going to leave it as an exercise for you to try out. Activate the rudder/canards/stabilator in various orders and move monopro out to the wings (or nose/tail) and see how it goes!
Good luck!
SUMMARY:
This is such a long article, that I tried to pull together some short reminder points here.
-- Flight Controls in General --
- Flight controls placed further from the Center of Mass (CoM) are more effective.
- Flight controls with a higher lift rating, at the same distance from the CoM, are more effective.
- Putting on heavier control surfaces or placing them further from the CoM adds to rotational inertia, which costs more fuel to rotate in space.
- More flight control authority is not automatically better. It can cause over-controlling in the atmosphere and may lead to an out of control aircraft.
- You can use flight controls as an added lifting surface instead of adding another wing, which can keep your overall design size smaller/lighter. As of KSP v0.23, just right click and deactivate the surface. It will not move and becomes part of your wing/tail/etc.
- Flight control chatter (SAS): Also, I didn’t dive into it because this aircraft doesn’t chatter. But if you have an airplane or space plane that chatters around (where the flight controls go crazy because of the SAS) you can selectively cut out some of the control surface functions to try and make it stop. This tends to happen to me when I have too many flight control surfaces that end up fighting with each other to stabilize the craft.
-- Pitch Control --
- Elevon – Can give plenty of pitch ability for craft that don’t need to be highly maneuverable. If your airplane or space plane is heavy with little wing area, using elevons can cause lift problems.
- Horizontal Stabilator – Allows you to pull the nose up by pushing down on the tail. Generally you can keep the nose up until the wing runs out of lift or the stabilator can’t turn any further. Typically less effective than canards due to being closer to the CoM.
- Canards – Are generally more effective than an elevon or horizontal stabilator because it is further from the CoM. You can still run into pitch control issues if your craft relies on the canards for lift, or if it flies at high angles of attack.
- Torque & Thrust Vectoring – Generally not strong enough to give you all your pitch control needs from just the command pod. If you stack up a few control wheels they can have a big effect on small craft. The benefit here is that all the torque is available for use in space but doesn’t add to drag due to lift. Also, they could add less rotational inertia than flight control surfaces, depending on where you place them. The downside is they have a lot higher mass than most control surfaces (KSP v0.23).
-- Roll Control --
- Ailerons – The primary roll control source. If you have a strangely shaped wing or it carries a lot of weight, ailerons may cause the wing to warp causing yaw/roll/pitch problems. Strutting or moving flight controls can help this problem.
- Differential Stabilator / Canards – Work as well as ailerons, but add less inertia since they are typically attached to the fuselage and are closer to the CoM (although they don’t have to be).
- Torque – Might be enough to control your airplane or space plane, depending on size and how maneuverable you want it to be. You generally need a canard/stabilator/elevon for pitch control too, so why not just go ahead and use those for roll control also? Well, if torque gives you enough control, not using the canard/stabilator/elevon for roll can minimize asymmetric lift issues (making yaw and pitch easier to control).
-- Yaw Control --
- Rudders – Not horribly important on smaller aircraft, but placement becomes more significant for larger or oddly shaped airplane and space planes. If your aircraft is really wide, you might need to move the rudders out to the wings and away from the body (away from the CoM) to make them more effective.
- Torque – For smaller airplanes and space planes, torque from the command module can be enough to provide yaw control unless you have asymmetric thrust problems. You may still need a vertical tail to provide stability, but you don’t necessarily have to attach a rudder to it.
-- Inertia --
- Rotational inertia is a measure of the airplanes resistance to rotation (yaw, pitch, or roll).
- More mass located near the CoM gives a lower rotational inertia.
- Moving/adding mass out on the wings increases roll and yaw inertia.
- Moving/adding mass to the nose or tail increases pitch and yaw inertia.
- Moving mass out to the wings/nose/tail increases RCS fuel consumption for rotation, but doesn’t change fuel consumption for translation.
One example of how to combine all this: If you have a space plane doing a lot of translation but very little rotation, lower mass control surfaces that are placed in more effective places (i.e. small ailerons out on the wingtips) are better.
- Lower mass parts (generally) have lower drag factors, helpful in atmospheric flight.
- Lower mass parts consume less Delta-V.
- Placing them out further increases pitch/roll/yaw inertia. This needs more RCS fuel for rotation, but we aren’t planning on doing a lot of rotation so the drawbacks are lower.
Good luck and happy Designing!
Edited by Claw
