• Ratty's Ramblings - Trim and Trimming

    Ratty's Ramblings

    Ratty's Ramblings - Trim and Trimming

    By Ian Radcliffe

    There are many aspects of flight that are not necessarily obvious, and one of these is the role of trim. Not everyone understands trim, and a lot of people who think they do don't. It doesn't help that in our simulations it's often poorly implemented. I've also found that amongst sim pilots there are two common misconceptions that I should disabuse you of right now. The first: trim is not a flight control. The second: it's not an "elevator booster". Let's take a look at the purpose of trim, and then at how to use it.

    Most modern aircraft are dynamically stable; that is, once established in a steady flight state they will tend to stay in that state, or return there if displaced. In cruise flight, the pendulum stability of high-wing aircraft and the dihedral of (most) low-wing aircraft help maintain wings-level flight, and help restore it if the aircraft banks. Apply rudder briefly and the nose will swing from side to side until the vertical stabilizer brings the airplane back to straight-ahead flight. Raise the nose a little and release the controls and the nose will drop and rise a few times before settling back into level flight.

    Early aeroplanes with no throttle had one cruise speed; the power options were only All or Nothing. If, at cruise speed, your plane had a tendency to roll or climb or turn it could be rerigged on the ground to fly straight so you didn't have to hold control forces all the time. But add a passenger behind you and suddenly your plane is tail heavy, and heavier, and your elevator forces have changed.

    The biggest variations in control forces take place in pitch. Changes in speed, weight, power, and airfoil configuration all contribute to how hard you have to pull or push. Increase power in level flight and the plane will start to climb; if you want to stay level and go faster you'll have to hold forward pressure on the stick. Decrease power, and to maintain level flight you'll have to hold constant back pressure. And that's what trim is for: to stop your arm getting tired.

    The first trim device apparently was developed in 1910; all I've been able to find out is that it was "ground adjustable", but I'd bet it was created to accommodate changes in pitch. There are many ways that trim is implemented, most being aerodynamic and involving adjustment to the elevator and/or the entire horizontal stabilizer. The simplest system is a bungee or spring device connected to the stick.

    Ratty's Rambles

    Aileron and rudder trim appear mostly on larger aircraft, because planes are most stable in roll and yaw. About the only way to make one wing heavier than the other is to mismanage fuel in the wing tanks. Propeller-driven airplanes do have yawing tendencies that vary with different power settings and speeds, but they're usually rigged for cruise, so a P-51 Mustang REALLY wants to swing left on takeoff, but bores straight ahead in level flight.

    In practice, I find I can fly for quite a long time holding control pressure, like holding a fixed accelerator postion when driving, so my use of trim may be different than others', and I encourage you to look around and see what others have to say. For what it's worth, here's how I use trim:

    Before flight I set the elevator trim to the takeoff position. After takeoff, I establish a climb at the desired airspeed; to maintain that speed I need to maintain a particular stick pressure, but if I do it too long my arm will get tired. This is where I adjust the trim until I don't need to push or pull any more. In a real airplane you can feel the pressure going out of the controls. I have nose down/nose up buttons on my stick, and as I run the trim I can almost feel the pressure going out of the controls.

    When I get to my cruise altitude and level off, I lower the nose first, then adjust the throttle as necessary, then wait (because it can take a while to settle on a speed) and then retrim.

    After that, as I burn fuel my airplane will get lighter and want to climb. If I drop ordnance or fuel tanks my airplane will want to climb. If I increase power my airplane will want to climb. If I reduce power, my airplane will want to descend. If I add weight (as in air-to-air refuelling, for example, frankly the only example I can come up with) my airplane will want to descend. To maintain level flight, or establish a particular climb or descent rate, I will now have to apply force to the stick again. I wait for the speed to settle and then retrim.

    In the landing phase there can be numerous changes in the stick pressure you use. Slowing, power adjustment, lowering gear and flaps, all change the trim of the plane. I generally trim when established on downwind at a steady speed with the gear extended. Once cleared to land, reducing power puts me into a descent without the need to touch anything else. Speed adjustments will require pressure on the stick, but since I still have the flaps to deploy I don't touch the trim again until I'm established on a steady final approach with everything hanging and no major power changes in the offing.

    And then I won't touch the trim again; unless I have to go around, in which case I will apply power, clean up the aircraft, establish a climb...and trim.


    13 Comments
    1. mekim's Avatar
      mekim -
      Thanks for sharing your experience. I'm having trouble with trimming for cruise and extreme changes in trim for landing with some aircraft. There seems to be a lag in trim perhaps due to CPU overload, but I also have a problem with runaway trim. Also, thanks for noting the scenery addon you use; I often see pictures and video and wonder what software they're using.
    1. sfgarland's Avatar
      sfgarland -
      As your previous writings, interesting, informative and most enjoyable
    1. lnuss's Avatar
      lnuss -
      A nice article, Ian. You chose another area of sim flying that so many misunderstand and misuse, and you've done a great job explaining the need for and the proper use of trim, and your way works fine for you and for many others. But as you noted "my use of trim may be different than others'," so here's my take on it.

      One thing I've noticed over the years is that holding stick pressure is easy for a while, but holding it accurately is tougher and tougher after the first few moments, to the point where it soon takes too much concentration, and shortly after that loses accuracy, the length of time being aircraft dependent, primarily dependent on how much pressure is needed at any given time*. So I'll duplicate your comments on takeoff, but modified to show my way. The portions in red are mine.

      "When I get to my cruise altitude and level off, I lower the nose first, adding nose down trim as needed to minimize the pressure needed,, then repeat as necessary until the aircraft is stable in level flight, but wanting to accelerate too much." Finally I adjust the throttle as necessary, then maybe one more touch of trim or fine tuning of the power.

      For the rest of the comments, though my use is similar in many ways, I use trim much more frequently (always to take the pressure off of the controls, of course) but in smaller amounts, so as I slow entering the pattern, I do a lot of trimming on faster aircraft, less on a Cub or such.

      ===================================================================

      * A somewhat extreme example is a Cessna 206 with only fuel and front seat occupants, where with flaps in the flare it is almost impossible to keep the nose up without a substantial amount of trim applied, and that amount is a lot more than is needed coming down final at normal approach speed, so a LOT of adjustment is needed for that flare. There are, of course other areas of flight where it's still prudent to use trim very frequently even though the pressures aren't quite as strong as during the flare on landing.

      On the other hand, in a Cessna 150 or 152 I often can do fine with less frequent trimming, though the pressures still get somewhat heavy with full flaps until they are trimmed off.

      Of course in the sim the pressures aren't all that strong, but the stick pressures are more difficult to gauge accurately, so there, too, I tend to use the trim much more frequently, partly to improve the accuracy of my trim settings but also partly because the sim doesn't have the fine, almost infinite analog adjustments of a real aircraft, so I often have to trim until I'm as close as I can get, then make very tiny adjustments in power to allow the aircraft to actually be properly trimmed.
    1. N069NT's Avatar
      N069NT -
      "Not everyone understands trim, and a lot of people who think they do don't."

      I learned about trim long before learning how to fly from growing up in a boating family. I had my own boat with a trimmable outboard engine at age 15. Basically if the motor was trimmed own as inward towards the transom causing the bow to dig in more it would create torque steer to the left similar to P-factor. Trimming it to neutral to get less of the bow in the water or even positive to minimize hull contact would eliminate torque steer.

      Taking that experience made it real easy to transition to flight and dealing with trim. For non-boater and non-pilots and trying to explain aircraft trim I like to use a classic automobile problem with steering and having an alignment out of whack causing a pull to the left or the right you have to constantly hold the steering wheel against. I tell them just imagine if you had an option to reset that alignment problem in the steering wheel and alleviating that pull. They get a much better understanding and appreciation for what it is and how to use in a flight simulator. You don't fly with the trim: you fly with the yoke or trim then increase or decrease the trim to neutralize the input.
    1. nortopj's Avatar
      nortopj -
      Nice article on Trimming, where you have summarised the purpose of the Trim Tab, on old style basic systems or servo neutral offset on modern hydraulic systems, perfectly. Without using the trim switch or button a pilot merely resists a heavier load and doesn't select a new neutral. Nor, in fact, does he/her gain any change in Range-of-Movement (RoM), despite what happens in FS9, FSX or MS2020.

      Why then does Flight Sim., even up to MS2020, have the trimmer as a sort of boosted surface with more RoM effect when it is used? On another point: Why do the engines always start in the wrong order, left to right. Real aircraft start engines right to left, i.e. 4, 3, 2, 1. There IS a Good Reason for this but I won't bore you further...
    1. toftedal's Avatar
      toftedal -
      Another great article ... 'Stick & Rudder' for the flight sim community ... thanks Ian!
    1. N069NT's Avatar
      N069NT -
      Quote Originally Posted by nortopj View Post
      On another point: Why do the engines always start in the wrong order, left to right. Real aircraft start engines right to left, i.e. 4, 3, 2, 1. There IS a Good Reason for this but I won't bore you further...
      Not true at all. It is up to the airline flight ops manual. There is no Boeing mandated start sequence for the 747. If you've been hitting the auto start in MSFS (or any previous FS version for that matter) instead of manual start then you are letting the computer program to start your engines and not real world ops. For the record I don't have MSFS yet and don't know if it has a manual start option. I'm just responding to the real world comment and referencing a 747.

      I do know however that many pilots of 747s since the 200 series like choosing 1 and 4 first for start because the outboard engines control the hydraulic pumps that power nose wheel steering and brakes. I have no idea what mandates a 4-engine A380 or A340 start sequence having never flown on one or known a pilot of one (my neighbor is a retired Delta 744 captain).

      This of course all revolves around what ground crew operations are doing. Bottom line: in the real world, any engine sequence can be chosen to start in any aircraft. Nothing is mandatory that I know of in modern airliners, corporate, or private when it comes to start sequence in multi-engine aircraft. Now if you are talking about classic prop airliners like a DC-4 or Constellation or Electra in days when passengers boarded from the ground and there were no APUs, then yeah there's a good reason the starboard engines were started first to power up the aircraft!
    1. quint60's Avatar
      quint60 -
      Trim is important to understand, and while you do a good job of explaining your technique, I’ll be very clear about the root cause of why it works. You trim for airspeed. Period. Try this, climb to any altitude, and establish and maintain a constant altitude, cruise power setting and airspeed. Then trim off the stick pressure. Once stabilized, reduce the power to idle. What does the plane do? It begins a descent. It will oscillate a bit as it noses down to start, but after a few moments, it will re-find and maintain that same airspeed you had trimmed in cruise flight. All by itself. Now add full power. What does the plane do? It starts a climb and seeks out and eventually maintains that same airspeed. Why? Airspeed directly affects the amount of lift being produced by the wing. So does weight and flap configuration, but for the purposes of this explanation, we’ll say those are kept constant. The horizontal stabilizer balances the upward lift of the wing by applying a DOWNWARD aerodynamic force (downward lift). Like the lift the wing produces, the tail’s negative lift is controlled directly by airspeed. The elevator changes the amount of downward lift that the horizontal stabilizer produces in order to control the aircraft around the lateral axis (that stems from the CG and extends through the wingtips). To maintain level flight, you must maintain the overall amount of lift being produced- positive by the wing and negative by the tail. The faster you fly, the more lift the wing produces, etc... so, you would trim the elevator to provide more nose down lift on the hotizontal stabilizer as you accelerate in level flight. It works the opposite way as you slow down in level flight.
      In a large turbine aircrafft, the elevators are often hydraulicay controlled, so there isn’t really a natural back or forward pressure that you feel. So, if trimming was about pressure alone, how would they do it? Well, on takeoff, for instance, we set a trim setting before takeoff that will facilitate a climb at V2 (or V2 + ?) in the event of an engine failure during takeoff. V2 is a single engine takeoff safety AIRSPEED. So, in heavy aircraft, we set our pitch for a specific airspeed, because that’s what trim does.. Try it in the C172- in balanced and trimmed cruise flight, set your power to idle and trim for the takeoff trim setting. What airspeed does that give you? Probably about the same as the rotation speed - say 55-65 kts, somewhere in there?
      So, in summary, just trim for airspeed. Yes, that will manifest itself as a release of pressure, but it is really all about the airspeed. That is all.
    1. quint60's Avatar
      quint60 -
      Correction- sorry, I mispoke but ran out of time to make an edit. The faster you go, more nose down force you require (lift of the wing is typically agead of the CG, which is why the nose pitches up when you add power). In some cases, you actually add upwards lift to the horizontal stab (which is a nose down moment) to counteract that force.
    1. ianhr's Avatar
      ianhr -
      Quote Originally Posted by quint60 View Post
      Correction- sorry, I mispoke but ran out of time to make an edit. The faster you go, more nose down force you require (lift of the wing is typically agead of the CG, which is why the nose pitches up when you add power). In some cases, you actually add upwards lift to the horizontal stab (which is a nose down moment) to counteract that force.
      I think you'll find that lift is generally AFT of the CG. In some GA planes, for example, adding passengers in the back can actually make the plane go faster because reducing the downforce required from the horizontal tail to hold the nose up means less drag.

      Also, pitch up with power application is partly a result of forward movement of the Center of Pressure (reducing the nose-down moment and increasing the tail-down moment), partly due to the increased airflow over the horizontal tail, and partly a function of the location of the thrust line.
    1. quint60's Avatar
      quint60 -
      you are absolutely right. I stand corrected. Thanks.
    1. ianhr's Avatar
      ianhr -
      Quote Originally Posted by quint60 View Post
      you are absolutely right. I stand corrected. Thanks.
      Thank you! I appreciate all the input. Since I wrote this piece I've been reexamining how I use trim and wondering if I need to write some sort of addendum in my next offering, because I've noticed that I adjust it far more often than I thought.
    1. uokbob's Avatar
      uokbob -
      This just shows the need for a trim wheel. Buttons on the sim are hard to adjust. I wish someone would make one at a reasonable price.
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