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On a 737, trim adjusts the pitch of the entire horizontal stabilizer. As such, in the extreme, it actually has more control over pitch than the elevator (which
by mhandley 8y ago
On a 737, trim adjusts the pitch of the entire horizontal stabilizer. As such, in the extreme, it actually has more control over pitch than the elevator (which is controlled by the pilot's yoke). If the trim runs out to the limit, and the pilot doesn't correct it, the elevator does not have enough authority to override the trim.
- dreamcompiler 8y agoWow. Didn't know this. I can understand why they do it this way--it's probably the lowest drag solution to the problem. But it sure swaps my connotation of what "trim" and "elevator" mean.
- AWildC182 8y agoThere's a common GA plane (M20) that moves the entire tail for trim, both the horizontal and vertical stabilizer as one piece. Basically conventional (not a canard) stable (not an F-16) aircraft have the CG in front of the center of lift of the main wing so it wants to nose down. The tail then acts as a counter balance, pushing down to keep the nose up. Because airspeed over the smaller surface effects its downforce and the center of thrust is not always in line with the center of drag, they add trim to prevent the pilot from having to maintain constant control pressure. Some do it by biasing the control surface itself, either in the control mechanism or with a tab that aerodynamically moves the surface, or by changing the angle of attack of the horizontal stabilizer.
- village-idiot 8y agoI’m no pilot, but one of the first things you learn in Kerbal Space Program is that CoG behind CoL is a completely unflyable configuration.
- AWildC182 8y agoHa, yea. I was deliberately using that terminology hoping people might be able to draw some parallels with their KSP experience.
- zymhan 8y agoDo you have a source for this claim? Two different comments in this thread are stating opposite claims https://news.ycombinator.com/item?id=18409562 https://news.ycombinator.com/item?id=18409562
- mhandley 8y agoThis explains a bit about how the THS works: https://www.skybrary.aero/index.php/Trimmable_Horizontal_Stabiliser https://www.skybrary.aero/index.php/Trimmable_Horizontal_Sta... The Rostov-on-Don crash in 2016 appears to have been caused by downwards trimming for 12 seconds. This caused a -1g dive from which the pilots were unable to recover. It's inconceivable they wouldn't have been attempting to pull up with the elevator. You can't not notice -1g. https://en.wikipedia.org/wiki/Flydubai_Flight_981 https://en.wikipedia.org/wiki/Flydubai_Flight_981 Now the final report is not out yet, so it's not clear if the downwards trim was somehow manual (which doesn't really make any sense), or a similar problem to the Lion Air crash. Either way, it's worried Boeing and the FAA enough to issue this AD.
- ryanmarsh 8y agoIs the trim setting (pardon my lack of knowledge) easily visible to the pilot?
- jzwinck 8y agoIn a Boeing, absolutely. The designers know it is important, and they devote a slice of cockpit space to a comically large "trim wheel." It has a white stripe to highlight automatic movements, and a large handle to allow manual control. See: https://www.quora.com/What-are-the-spinning-wheels-in-an-airliners-cockpit https://www.quora.com/What-are-the-spinning-wheels-in-an-air...