Why does a stabilizer move downward more than it does upward

[InFlames]

Hey there. I am an AME student and I was just wondering if you could help me with my homework.

Why does an adjustable horizontal stabilizer travel further in a downward direction compared to upward?

I just don't know.

Thanks

[crazy_aviator]

The stabilizer is there to provide downforce lift, therefore , for nose down commands, LESS down lift is required, for nose up commands more lift is required

[InFlames]

Hey there. Thanks for the reply. At first I was confused because I was thinking that as the stabilizer moved down, so would the nose, but now I realize that was backwards, because it is more like a trim tab.

If I understand you correctly, I think you are saying that in a neutral position it will already provide downward force on the nose, therefore, for nose up commands it will need to travel further (down) to compensate for that.

Thanks.

[RenegadeAV8R]

Hey there. Thanks for the reply. At first I was confused because I was thinking that as the stabilizer moved down, so would the nose, but now I realize that was backwards, because it is more like a trim tab.

If I understand you correctly, I think you are saying that in a neutral position it will already provide downward force on the nose, therefore, for nose up commands it will need to travel further (down) to compensate for that.

Thanks.

The Horizontal Stabilizer is always producing a downward lift - the Horizontal Stabilizer provide a downward force on the tail of the aircraft. The Leading Edge of the Horizontal Stabilizer is lowered to trim the aircraft in a Nose Up attitude (see Figure 5-23)

From a pilot perspective, The Horizontal Stabilizer Trim effectively perform the same function as a trim tab.

Trimmed Aircraft.jpg (114.17 KiB) Viewed 2239 times

Adjustable Stabilizer.jpg (67.88 KiB) Viewed 2239 times

[photofly]

The Horizontal Stabilizer is always producing a downward lift

Actually that's not true. For C-of-G near the rear limit, in many aircraft the tailplane provides lift (upwards) just like the wing.

[RenegadeAV8R]

The Horizontal Stabilizer is always producing a downward lift

Actually that's not true. For C-of-G near the rear limit, in many aircraft the tailplane provides lift (upwards) just like the wing.

This is interesting. Is it happening in Cruise? With which aircraft?

[photofly]

Certainly with a C182. It's not hard to see that the rear cg limit goes back significantly past the centre of lift (very close to the quarter chord). That means there's a nose-up moment that needs to be resisted by the tailplane pushing up.

What's important for stability is that the angle of attack(*) of the main wing is higher than the angle of attack of the tailplane. If the main wing flies at +2 degrees, then the tailplane can fly at +1, or +1.5, or anything up to +2 degrees also. John Denker explains it very well, references on request.

It could be that jet transports never fly with the cg that far back, but it's not a stability issue with the tail.

(*I'm using the convention of 0 AoA = zero lift. It doesn't matter, aerodynamically, where you measure AoA from, but the stability condition is easier to state this way.)

[RenegadeAV8R]

Certainly with a C182. It's not hard to see that the rear cg limit goes back significantly past the centre of lift (very close to the quarter chord). ...

Since the original question was about movable Horizontal Stabilizer, I was thinking about large jet transport aircraft. Where, when the speed reachs the neighborhood of 0.7 Mach, the Horizontal Stabilizer has to be trimmed aircraft Nose Up (which is Horizontal Stabilizer Leading Edge down) to prevent the Mach Tuck effect.

On a high speed jet aircraft, the center of lift moves aft as the speed is approaching the speed of sound.

I was involved with the design of the Pitch Trim system of only one jet aircraft, and if I remember correctly, the Mach Trim function was automatically activated when the aircraft was reaching something like 0.68 Mach; the Mach Trim was trimming the aircraft Nose Up, always, no matter what was the weight or the CG location.

[photofly]

Since the original question was about movable Horizontal Stabilizer, I was thinking about large jet transport aircraft.

The Cessna Cardinal C177 and the Cherokee PA-28- series have all-moving tailplanes too. I believe the same applies to them.

I accept entirely your comments about mach tuck, etc; I just wanted to point out that it's widely taught, and wrong, that the tailplane of an aircraft always pushes the nose up.

[iflyforpie]

Yes, there are several circumstances where the horizontal stabilizer pushes up, not only because of C of G, but also because of lift coupling and thrust-drag coupling as well. Canard aircraft are perfect examples of aircraft that require the horizontal stabilizer to push up, because the horizontal stabilizer is also the wing!

WRT to Mach Trim and Mach Tuck, this can be reduced somewhat by fuel tanks in the horizontal stab. This is similar to how the Concorde (another aircraft that requires all positive lift) trimmed for Mach Tuck. Mach Trim can decrease efficiency quite a bit, especially on aircraft like the 737 where only the elevators move for Mach Trim rather than the stabilizer.

[ottopilot]

Falcon Jet has a pretty good explanation of the mach trim system in their manuals or type course books. Anyone have something they can post. Interesting system.

[InFlames]

Hey there, good discussion here. My original question was in relation to a Cessna 180, which I should have specified.

Thanks for putting up the figures. I think I at least know enough now, to at least same something that sounds reasonable.

[photofly]

But a Cessna 180 doesn't have an adjustable horizontal stabilizer, it has a horizontal stabilizer that's fixed, and elevators (with a trim tab)....

[iflyforpie]

The 180 does have a trimmable horizontal stabilizer (ask me how I know, those dual trim jacks are a pain in the arse to re and re) as does the 185 and the early straight-tail 182s (the 180-2s).

[photofly]

The 180 has an all-moving tailplane, like the 177? Who knew!?

[iflyforpie]

The 180 has an all-moving tailplane, like the 177? Who knew!?

Not quite. It isn't a stabilator like a 177, Cherokee, or Musketeer. It is just an adjustable stabilizer. Many (all?) of the Piper Cubs have them as well.

[photofly]

Instead of a trim tab?

[TeePeeCreeper]

Instead of a trim tab?

Yes, the whole assembly moves with a jack screw... Some call it a "flyable tail".... The trim wheel moves the whole elevator... No tabs...

As Pie already said they are a bitch to maintain but I've never had any issues with mine... So far that is! You might have read or seen (Mayday) about Aloha flt (?) an MD-11 (was it?) That lost control because of a jackscrew failure... Same kind of design although in small GA airlplanes such as the J-3/C180...