SkyGazer
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Horizontal Stabiliser Question

Wed Jun 20, 2007 11:03 pm

Excuse me if this sounds a bit elementary, but I've been wondering about this for a while.

As you can see in this photo below:


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Photo © Jean-Pierre Bonin



The horizontal stab clearly is mounted on a mechanism which allows the whole unit to rotate up and down.

Now I know that it is usually elevator/tailplane that pushes the plane's nose up and down, but does the movement of the whole stab ever play any role in this too?

Cheers.
Types flown: B738, B772ER, B773, B77W, B744, A310, A320, A321, A332, A333, A343, A388
 
Tristarsteve
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RE: Horizontal Stabiliser Question

Wed Jun 20, 2007 11:23 pm

Yes, on some aircraft notably the Tristar the whole stab moves to control pitch. The elevators on the Tristar are really geared tabs. The stab is moved by four huge hydraulic jacks.

On the B737 shown, pitch movement is by the elevator and pitch trim by the stab. In steady flight the stab will always be trimmed to leave no elevator deflection. This system is much more common on airliners.
 
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Starlionblue
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RE: Horizontal Stabiliser Question

Thu Jun 21, 2007 12:20 am

Skygazer, I get the impression you don't know the difference between trim and "direction changing". If you do, ignore the following.

In a typical commercial airliner, the elevators (moving surfaces at the rear of the horizontal stabilizer) change the pitch of the aircraft. However, weight distribution (drinks cart and what not) and pitch demands change constantly. It is inefficient to use the elevators to counter such changes in "trim" since you get more drag if the elevators are not in line with the rest of the stabilizer. So jackscrews move the entire horizontal stabilizer.

Airliners have automatic trimming, so there is no need to continually adjust trim. In light aircraft, trimming is done manually. You would use the yoke/stick to get to desired pitch (moving the elevators), then trim the aircraft so it remains in that state.

Quoting Tristarsteve (Reply 1):
Yes, on some aircraft notably the Tristar the whole stab moves to control pitch.

Sometimes known as an "all flying" stabilizer, this arrangement is frequently found on fighters. Furthermore, fighter elevators are often elevons, acting as elevators and ailerons as well as providing pitch trim control.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
411A
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RE: Horizontal Stabiliser Question

Thu Jun 21, 2007 12:41 am

Going just a bit further with the discussion, and looking at the Lockheed TriStar in particular, some might ask just why did Lockheed design the horizontal stab the way they did?

My understanding is that the particular design was selected so that in conjunction with the other quite unique TriStar design feature, direct lift control (DLC), is that this particular feature allowed for very stable approach paths to be flown, for automatic approach/land (autoland) operations.

The TriStar was the FIRST widebody civil jet transport to achieve CATIIIB certification, and it did so directly from the factory.

A superb design, quite unique in many respects....and liked a LOT by the folks who still fly them, myself included.
It is quite apparent that a huge amount of design effort was put into the flying qualities of the TriStar, and it paid off very well.

An absolute delight to fly.

Ahhhh, Lockheed!
 
SkyGazer
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RE: Horizontal Stabiliser Question

Thu Jun 21, 2007 12:45 am

Quoting Tristarsteve (Reply 1):
In steady flight the stab will always be trimmed to leave no elevator deflection.



Quoting Starlionblue (Reply 2):
It is inefficient to use the elevators to counter such changes in "trim" since you get more drag if the elevators are not in line with the rest of the stabilizer.

Ahh yes, that explains it. Thanks guys.  Smile That makes sense, I can imagine what a pain it would be to constantly adjust trim on a plane that big for such durations.
Types flown: B738, B772ER, B773, B77W, B744, A310, A320, A321, A332, A333, A343, A388
 
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Starlionblue
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RE: Horizontal Stabiliser Question

Thu Jun 21, 2007 12:59 am

Quoting SkyGazer (Reply 4):

Ahh yes, that explains it. Thanks guys. Smile That makes sense, I can imagine what a pain it would be to constantly adjust trim on a plane that big for such durations.

Indeed. In Jurassic and Classic 737s, the trim wheels are "exposed" and you see them spin back and forth in the cockpit.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
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SEPilot
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RE: Horizontal Stabiliser Question

Thu Jun 21, 2007 3:04 am

Just to expand on what has been said on previous posts, light aircraft (low end ones, anyway) usually rely on a trim tab, which uses aerodynamic force to bias the elevator one way or the other. This causes slightly more drag than moving the whole elevator, but is much simpler. There is more weight involved in a moving horizontal stabilizer as well. However, when you are traveling at jet speeds the drag issue dominates and so all jets use moving stabilizers, as do many higher end light planes.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
N231YE
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RE: Horizontal Stabiliser Question

Thu Jun 21, 2007 5:53 am

Quoting Starlionblue (Reply 2):
In light aircraft, trimming is done manually. You would use the yoke/stick to get to desired pitch (moving the elevators), then trim the aircraft so it remains in that state.

Or, if you feel like cheating (like me  innocent  ), you use the trim to control pitch, not the elevator! (but that only during cruise, not during takeoff or landing, or any maneuvers)
 
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Starlionblue
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RE: Horizontal Stabiliser Question

Thu Jun 21, 2007 9:53 am

Quoting N231YE (Reply 7):
Or, if you feel like cheating (like me innocent ), you use the trim to control pitch, not the elevator! (but that only during cruise, not during takeoff or landing, or any maneuvers)

Is that really kosher? Didn't some 727 pilot almost buy the farm like that?  Wink
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
rwessel
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RE: Horizontal Stabiliser Question

Fri Jun 22, 2007 5:58 am

Quoting Starlionblue (Reply 8):
Is that really kosher? Didn't some 727 pilot almost buy the farm like that?

Autopilots on many small aircraft do just that... Whether or not this is a good idea on an airliner, I have no idea.
 
pilotpip
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RE: Horizontal Stabiliser Question

Fri Jun 22, 2007 1:23 pm

N231YE,

If you were my student, we'd have some issues. I would NEVER use trim to control an airplane, including the jet I'm about to get in. Always use the controls to set the desired pitch, then trim to relieve backpressure. Why? Using the trim makes for a lot of overcontrolling in many aircraft on the good end. On the bad end, having excessive nose-up trim on approach could easily lead to a very nasty pitch up moment and stall when you don't want it on a go around.

Yeah, autopilots use trim to change altitude and whatnot but remember, the pilot still has a number of ways to disconnect it. You tell the box to do something, it's going to try and do it until it can't, then it will try some more.
DMI
 
Tristarsteve
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RE: Horizontal Stabiliser Question

Fri Jun 22, 2007 6:07 pm

Quoting Pilotpip (Reply 10):
Yeah, autopilots use trim to change altitude

The standard airliner autopilot controls pitch with the elevator. It has a feedback from the elevator position, and if the elevator is displaced x ins for y amount of time, it then trims the stabilisor to fair the elevator.

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