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Trim - what is it, and what does it do in a WWII airplane?
No, it's not the little fringe things hanging from the top of the
windshield in my Jug. ;) I won’t get into the fancy
aerodynamic explanations (they are really long and complex) why trim works the way it does, but instead concentrate on the resulting effects and how to deal with them in Warbirds.
As your plane flies, at a particular altitude, weight, and speed, your plane will fly straight and level with the stick perfectly centered. As you go faster, the nose tends to rise, the plane tends to yaw to the right (in the single engined planes), and the machine will tend to roll to the right as well. As you go slower, the nose tends to go down, the plane tends to yaw to the left, and roll slightly to the left. Now since you need to be able to control a plane at a wide range of speeds, those thoughtful engineers included little deals called trim tabs that would let you adjust the plane to fly straight and level for many different sets of conditions. When we refer to a plane being in trim, we mean it is trimmed for straight and level flight with the joystick centered at that airspeed and condition. When a plane is out of trim, it’s joystick center is far from the middle of it’s travel, meaning you must use input to make the plane fly straight and level. What would happen in a real WWII plane, is that you would feel the center of force of your joystick changing as you went faster or slower. Eventually you would have to trim the plane to get your center of force back close to the center or you would have your stick mashed against one side of it’s travel and be unable to change direction any further in that direction. At the very least, it would limit your stick travel and hence the amount of control you could exert.
The trick is that we don’t have real joysticks connected to flight surfaces, we have simulated joysticks that use springs to pretend there is a center of force and give you some resistance. When the plane starts to go out of trim in warbirds, the logical center of force moves but our simulated sticks stay right where they are! In WarBirds if you go faster your stick’s logical center of force goes forward but your stick stays still so you start to climb. Generally, the WarBirds pilot will simply push the stick forward to keep the plane flying straight and level. The movement of the logical center of the joystick creates two problems for WarBirds pilots. First, since the logical center is not the same as the physical force center it makes flying straight and level tricky, and it makes small gentle adjustments all but impossible. Your movements will tend to be jerky and really inaccurate. This presents a problem in WarBirds simply because those movements are exactly what we need to do when lining up for a guns shot on an enemy plane. If your logical center of force matches your physical one on your stick, you are in trim (trimmed properly for that speed) and you will find gunnery much easier and smoother since you can "feel" that your physical center of force matches your logical one. In this case, if you let go of the stick your plane will fly straight and level. The second problem comes into play when your logical center of force gets too far away from your joystick’s physical center of force. For example, you have your plane trimmed out at 500 IAS and you get into a slow turning fight in your P51 Mustang. As you slow, your logical center of force moves backward, if we slow to 200 IAS the logical center of force will move quite far back, possible ¾ of your joystick’s travel. At this point, we need to have the joystick ¾ back just to maintain level flight, meaning we only have ¼ of our travel left when we need to turn hard! In this case, you will simply run out of physical stick travel before you have full elevator authority. In fact, you would barely be able to turn at all. The solution to these problems is to trim your plane for a speed close to that at which you will be fighting. You can use manual trim keys to move your logical center of force back closer to the physical middle in the heat of battle so you can get full control of your plane. In WWII airplanes, you would have to wind trim wheels or move levers to trim your plane. Since iMOL did such a great job modeling WWII aviation, in WarBirds you can manually adjust these tabs with the following keys:
I - nose down elevator trim
Fortunately, iMOL gave us auto-trim to take the tedium out of trimming a plane for a given speed and condition. The X key is Automatically Trim for Level Flight. What this does is trim the plane to fly straight and level for the speed you are travelling. You can do this adjustment manually if you like with the keys mentioned above, but it is much quicker to let the computer do it. Let the plane settle out (it takes a few moments for the computer to finish trimming) and release the auto-trim by hitting X again or moving your joystick. You will notice that the plane flies straight and level, but will start to drift after a bit. This happens as the plane speeds up or slows down, it will start to nose up or down accordingly as I mentioned. What you accomplish with this is to trim the plane for level flight with the stick centered for the speed at which you released the auto-trim. (The computer keeps adjusting as speed changes as long as the auto-trim is engaged.) What you want to do is to auto-trim the plane for the speed you will be fighting at. It doesn't matter what your cruise speed is, you need to trim for fighting speed so your stick is centered right when you really need it to be. If you are going to be turnfighting slow in a Zeke, trim it to 170 or a low speed like that, if you're doing the Boom N Zoom in a P-51, you may want to trim for a much higher speed. Auto-Trim for Speed
Now lets discuss the other two types of auto-trim.
First Auto-trim for speed. This is a very useful
tool, with this function you can have the computer keep trimming
the plane until the desired airspeed is reached. The most common
use for this is an auto-pilot climb out. Set your
speed (.speed xxx where xxx is your speed in IAS) and then engage
auto-trim for speed by hitting shift + X. The
auto-trim will trim the plane constantly to maintain that
airspeed, the result being that if you use a low speed and enough
power the auto-trim will have to keep the plane climbing to
maintain that speed. Hence, to start most people select a speed
between 150 and 175 and use auto-trim to climb out until they
reach the desired altitude. In the Warbirds 2.01version you may have to use a higher speed for your climb out. Currently, roll trim doesn't seem to be able to counteract the torque of many planes at low speeds, so your climb out speed may have to stay above 200 IAS to keep the bird's wings level.
You can use speed trim for another important thing in WarBirds, you
can use it to dive and escape! Because the speed trim keeps
trimming the plane to get to the desired speed, it will keep the
plane trimmed for acceleration, usually nose down if you pick a
high speed. Example: Before a fight in my P-47, I set
.speed 450. Now assuming I have enough altitude, when
I get in trouble I hit WEP, point it for home or friendlies and
engage the speed trim. <G> The trim will keep adjusting the
trim to attain and keep a speed of 450 IAS, but be careful, it
will merrily fly you into the ground attempting to do this!
You will however find that the effect is that the speed trim is a
great way to dive the plane, although you can accomplish the same
thing diving and adjusting the trim manually. The key is that if
you know your opponent is flying a bird with an effective top
speed of X, you can set speed trim to X+10 and relatively sure he
won’t gain on you. In the P-47, I’ll set speed trim for
500 IAS if I have enough altitude and wave bye bye to most planes
in the game. A handy trick!
Auto-Trim for Angle
The third type of auto-trim is auto-trim for
angle. You engage this auto-trim by hitting
ALT + X, it will trim the plane as necessary to
maintain the pitch angle you are at when you engage the
auto-trim. As you fly, the auto-trim will keep trimming the plane
to fly at exactly that angle as long as it can. Generally, if you
auto-trim for angle in a steep climb, the auto-trim will
disengage when your airspeed goes below 150 IAS. If you dive too
fast with auto-trim for angle, your plane may begin to pull
out as it loses control due to compression effects, but
more likely you’ll just go barreling into the ground. Once
again, this is a great way to dive your plane since the auto-trim
can generally let you execute a nice smooth dive without the
extra control inputs a human pilot would use. Control inputs do
create a bit of drag in WarBirds, so in this sense the auto-pilot
can more easily maintain an efficient dive. If you are carful with your stick inputs, likely the best dive will be maintained by diving smoothly and keeping your G meter at 0 for as long as possible. This should result in your best accelleration to your desired speed while still letting you get the separation you need for escape maneuvers.
There is one more use for auto-trim that relates
to aileron or roll trim. When your speed changes dramatically,
your plane will roll (usually only in single engined planes a
bit one way or the other. We have manual trim keys (M and ,) for roll
in WarBirds, so you can either fix up your aelieron roll manually or use
auto-trim. When you use any of
the auto-trims, they also adjust your roll or aelieron trim. Fortunately, most planes are not very sensitive to
roll trim, so you are usually far out of trim in
other areas before roll becomes a problem. Planes with more power and torque such as the F4U or Spit XIV, will experience more changes in roll trim than others. Remember that the auto-trim modes don't hold roll trim very well below about 200 IAS, so use the manual trim keys to level it out if need be or trim at a higher speed.
Now lets apply this "trim" thing in the game. Ok, let’s start with a BnZ sortie... especially since I mainly fly the P-47 now. And yes, I fly it as an Air-to-Air fighter platform. What it lacks in maneuverability, it makes up for in speed and firepower. <G> Take off... do .speed 165 to set speed trim to 165 IAS. Engage auto-trim to speed and let the beast claw it's way to 7 - 10 k otw to target area. Once I reach my desired cruising alt., I'll engage auto-trim to level flight, and let the engine haul the beast up to 300 mph or so. Ok.. now to engage a target. Assuming BnZ, I'll be diving onto my target. Hopefully target is 2k or so below. I'll dive into the target, using my I key to keep the plane trimmed fairly neutral as I gain speed in the dive. I'll use both I and K to trim as needed for a nice smooth pass on the bogey, make any hard breaks as necessary to get a shot. Assuming I miss, I'll pull up into very high yo-yo's and look for a snap shot. I'll use the K key to trim the nose up more as I start to lose speed. As the fight gets slower, I keep adding a bit of K (nose-up) trim to keep it roughly trimmed and add flaps if needed. Now lets say I kill the target, but a co-alt spit shows up... time to run like hell! I'll throw the Jug into a steep dive with WEP and engage auto-trim for angle (ALT + X). This will keep me diving fast and keep trimming the plane as I pick up speed to maintain my steep dive angle. I'll dis-engage the auto-trim as I pass 500 IAS or get too low.. whichever comes first, then level out at 100 feet and auto-trim level for the speed run along the deck. (OK, sometimes I’ll push it to 600 just to see if I can suck the other silly bugger into auguring. <G> NOTE: Don’t try this with other planes, the Jug is one of the few that can dive this fast and not lawndart.) Now lets look at a TnB sortie where things are a bit different. Lets take a 1v1 dueling situation for example. I'll climb out using speed trim just like before and level at cruising alt. Next, I'll climb sharply to very low speed and then hit auto-trim for level flight. I'll then dis-engage the auto-trim at the speed I plan to be fighting at. For example I'll dis-engage the trim at 225 for a P38, as it turns amazingly well at this speed. Note that the condition at which you disengage the auto-trim is what counts. Airspeed, flaps, and even altitude all affect the planes trim setting at this point, so have those things roughly set where you will want them in the middle of a fight. Don't worry about ALT too much, speed and flap position are the critical ones when dealing with trim. Now the rest depends on how I get into the fight from here. I'm trimmed for the proper speed, but I'll possibly go much faster diving onto a target. I'll count the I strokes I use to stay trimmed in a dive onto a target, and then dial the same number of K key strokes in as I slow down into a turnfight. The end result is that I know my plane is trimmed for a certain speed and flap position, and can return it to that position easily. Note that for slow stall-fighters, your trimmed speed may be much lower than your cruising speed. Also, flaps can greatly affect your trim position, so be sure to take that into account. Many top pilots have their sticks programmed to add a certain number of K key strokes whenever they lower flaps a notch to compensate. The rest of us just add a few more K strokes until you can ride the stall horn and still have enough stick travel left to stall it out. One other trick to try is when you initially trim the plane for your ideal fighting speed, do it with flaps popped if you plan to fight with them deployed. This way you can count I and K trim strokes and still return the plane to a known position with the flaps popped. I actually do this in a duel situation, but in the arena tend to just go by "feel" as to how many I and K strokes I need. As long as you can get full elevator authority to get maximum turn, you should be fine in a turn fight, so don't worry about being perfect. Close is good enough. The trick to this is to have the stick close to being centered so you have full elevator authority, and so that when you are pulling for the shot, it's easier to be smooth. If you are way out of trim, making small adjustments for a shot can be very difficult since the stick will be so far off center. It’s always best to be as close to in trim as is possible given the situation. In a TnB fight, you simply need to have enough elevator authority to get maximum turn rate. Generally, this means you should have enough stick travel to spin the plane out or at least go deep into the stall horn. In a BnZ type situation, you need to be very smooth when lining up for the shot, so being as close to in trim as possible will make the line-up easier and smoother. You can use the auto-trim angle and speed functions to make nice smooth efficient diving escapes if you do get in trouble. This explanation of trim and how to use it will give you the basic knowledge you need to implement it in WarBirds. It may seem to be a pain at first, but a with a bit of practice it becomes second nature. Many WarBirds pilots program an extra 4 way hat to handle trim functions for quick access, so try your own combinations until you get a configuration you are comfortable with. Try re-mapping the trim keys to the arrow keys since the number pad is used for views now, I find that very easy to use. Keep practicing and remember that just as in real WWII air combat, having your plane in trim can make the difference between life and death.
Physics of Flight Trimming your Aircraft in WarBirds
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