TO stab trim

[ahinterl 1]

To ask another of my silly questions: My understanding of TO stab trim is that less elevator is needed for rotation. But what exactly does TO stab trim, put the plane into a "neutral" state (such as no up/down forces on the elevator) at rotation time?

[scandinavian13 5,192]

It's not a silly question, as it is not completely understood across the board here, as there are two distinct views on trim.As a pilot of a small plane, trim is used to relieve pressure on the control forces. It is elevator trim (or aileron/rudder trim, as appropriate, but we're talking about pitch trim here). It works by the movement of a trim tab on the actual elevator to deflect airflow in a certain way so that the airstream holds the elevator at the appropriate angle, which is what relieves the pressure you would otherwise be holding on it through the control column. It does not increase the effective range of the elevator.In the case of larger aircraft, however, stabilizer trim serves to adjust the effective range of the elevator. Say, for example, that an elevator has a range of motion of +5 to -2 degrees. At low speeds, control surfaces are less effective than at higher speeds, so in order to effect a change in pitch, I would have to use higher deflections at lower speed than higher speed. Since, on a takeoff, you are operating in the lower effective speed range of the aircraft, you need large deflections. At some weights and CG loadings, the max deflection is not enough to rotate at the proper degrees per second (or rotate at all at Vr) - the elevator is not effective enough, even at max deflection, at that speed. In order to use higher deflections, we move the entire stabilizer to change the effective range of the elevator. If we move the stabilizer to -5 degrees (or 5 degrees aircraft nose up - ANU) itself, the elevator now has an effective range of +10 to +3, which makes it more effective at low speeds. This is in addition to the inherent benefit of the added angle of attack for the whole stabilizer itself (the stabilizer at 5 degrees is now acting somewhat like an elevator, deflecting the tail down, in addition to the elevator's deflections). After takeoff, the speed increases, so your elevator becomes more effective and trim can begin to be neutralized. You will note at cruise that the stab trim is somewhere around 0. This is because aircraft are designed for cruise flight, since that is where they spend a lot of their time. The neutral trim position is optimized for the hours of cruise, and is augmented as necessary for takeoff and landing.I hope that makes sense. If it doesn't, I can post a better explanation later, with pictures and all that.

[ahinterl 1]

Kyle, you're the man! Thank you a lot. It's that simple, just like common sense, but my brain seems to be blocked in some areas :(. Good to have experts here on board :(!I conclude from your explanation that having the proper amount of stabilizer trim set is absolutely crucial for a safe and successful takeoff. I always had the feeling that without the stab trim numbers, takeoff would be a risky venture, and in real life I woud never dare to perform any takeoff with stab trim improperly set.Though I guess there are transport planes that allow a certain "safety margin" around the right stabilizer trim setting, I have a lot of pretty good add-on planes where stab trim numbers are completely missing in the manuals, a real shame - but most simmers look like they don't care...

[scandinavian13 5,192]

You're welcome, and I'm glad that made sense. I was typing it at 0800, which can be hit or miss at that time of morning.When it comes to my add ons, if I can't fly them by the numbers, they don't get flown. I think it's called simulator, not game, for a reason.As far as safety is concerned, to a certain degree, yes, it's a safety concern. I believe if you look in PMDG's 747 manual, you can see a graphic that depics the negative effects of late or slow rotation, pertaining to climbout/obstacle clearance (it may be in one of their Type Rating courses). If trim isn't set correctly, your rotation will most likely be both late and slow. Furthermore, when it comes to a takeoff, you want to get off of the ground as soon as safely possible. First, you want to make sure you can clear the obstacles beyond the runway. If you use TOPCAT, you'll note that it'll give you the reason it is using a particular derate. It will say: Limit FIELD, or Limit CLIMB (I think those are the two terms - I'm sitting at work at the moment so I can't check). The climb part is because it has calculated that you need to leave the runway at a certain point in order to clear an obstacle, or meet some climb profile. Second, the higher and faster you get, the less fuel you burn (in general, and depending on aircraft weight and OPT alt, of course, and difference in 500' of runway isn't going to be too much of a factor for a single takeoff). Third, you must also keep in mind that there is a maximum safe tire speed for aircraft, which I believe 204 KTS in the MD-11.