View Full Version : Trim in Widebody Airplanes

20th Mar 2002, 02:11
Ok- I have never flown anything larger than a Cessna 172, but am wondering how the trim works say in a Boeing 747? In the cockpit I have seen the trim tab, but now kind of trim wheel like in a 747. So is the trim automatic or how do you work it?. . . . <small>[ 20 March 2002, 15:58: Message edited by: shon7 ]</small>

20th Mar 2002, 16:20
There's a pair of levers on the left hand side of the centre console, and they work like the trim wheel that you're used to, except that you push them forwards to trim nose-down and vice-versa.. .There's also an electric trim switch on the control yoke, and you move the switches both down (they're again paired) for nose-up and again vice-versa.. .When the autopilot is engaged, it works the trim all by itself, from elevator position information. (It tried to keep the elevator in the neutral position, and does this by moving the entire horizontal stabiliser)

20th Mar 2002, 19:21
As another PPLer, why are there two levers? Can the tailplane be trimmed seperately left and right? I always thought that the tailplane was effectively one piece, trimmed by one jack.

20th Mar 2002, 20:40
One is to arm the system the other to operate it, this is to prevent inadvertant operation. No it is a one piece Horizontal Stabiliser i.e. one solid unit and is trimmed with one ballscrew actuator which is normally operated by 2 hydromechanical motors with 2 brakes, it can still operate with one motor only but at half speed. <img border="0" title="" alt="[Wink]" src="wink.gif" />

23rd Mar 2002, 11:28
shon,. .. .for big jets, the trims trim the horizontal stabiliser instead of a trim tab as in a c172. but basically, it has the same function.. .. .SR

23rd Mar 2002, 18:59
SuperRanger,. .. .I had noticed that, from looking at some of the big jets. Interestingly, this is the same method that the Piper Super Cub uses (and, I think, all Cubs - but I've never taken a close look at any other than the Super Cub). I thought this was a rather clumsy way of doing it when I first started flying the Super Cub, but apparently not if that's what the big boys use too!. .. .FFF. .------------

23rd Mar 2002, 20:32
Not clumsy at all. Increased efficiency compared to a tab alone ie less drag for the desired effect.. .. .Mooney use a novel variant of it by pivoting the whole empennage - tailplane, fin & tailcone.. .. .The aft fuselage fits like a tight glove over the empennage, providing an almost gapless seal and reducing drag.. .. .About the only gap I can remember is a very small one between the dorsal fin extension on the aft fuselage & the fin itself.. .. .Just a bit of germaine trivia... <img border="0" title="" alt="[Razz]" src="tongue.gif" />