Adjustable tailplane

[Faileas]

Many aircrafts are fitted with variable incidence horizontal tailplane. Considering a mechanical controlled airplane, I can't understand if, to trim the aircraft at a certain speed, only the taiplane is rotated, with the elevator that is perfectly aligned with it (but in this case the hinge moment on the elevator is not zero) or if also the elevator is rotated, with respect to the tailplane, so to cancel the hinge moment and relieve the force on the yoke.

[Mad (Flt) Scientist]

Yet another "it depends" answer I'm afraid.

For aircraft with powered controls (and I suspect that is the majority of the cases with trimming tailplanes, though I'm happy to learn otherwise) either the elevator is indeed kept in a fixed position as the tailplane rotates - which might not be the "faired" position, depending on the column input - or alternatively the elevator is "geared" to the tailplane in order to give a bit more trim authority than the tailplane alone can generate. I'm not aware of any powered control aircraft where the elevator is used as one might a tab, to offload tailplane hinge moments.

In the case where the elevator is unpowered, there would indeed be a change to the elevator hinge moments as the tailplane moved, and with constant force on the column the elevator would indeed move to the new balance. If this was not a desired behaviour then either some for of compensating tab mechanism might be used, or the elevator/column gearing could perhaps be a function of the tailplane position (its actually quite difficult not to have it vary a little, due to the way the various geomtries interact).

In terms of the actual practise of using the trimmable tailplane, for the first case what happens is the pilot stabilizes the aircraft using the column/elevator, holding some force. He then uses the trim switch to move the stabilizer, and as it takes up the control requirement the column force is reduced until eventually an acceptable hands-off trim is achieved. So you'd start off with a significant elevator deflection, and end up at either zero or the geared value, depending on the design, with the column centred in both cases.

[Machinbird]

Faileas,
On an unboosted, non-irreversable elevator control aircraft, the elevator will desire to fair with the local airflow.

If the aircraft is out of trim in pitch (for speed change or cg change reasons), we will have to hold some elevator displacement to compensate.

If you rotate the big surface that the elevator is attached to (the horizontal stabilizer) we change the net downforce generated by the HS. If we rotate the HS (trim) in the proper direction, we can take out the need for elevator displacement.

By rotating the HS, we maintain the net downforce supplied by the HS & elevator combination (and trim out any corrective elevator deflection we may have had to apply before trimming.) The angle between the HS and elevator change, and the yoke or stick will move slightly in the process.
The HS is now carrying all the downforce and the elevator is neutral (faired with the local airflow).

[FE Hoppy]

sub-sonic doesn't really need a flying tail to retain control authority but once you get into trans-sonic/ supersonic regime then the ability to trim mach on the stab leaving full authority on the elevator becomes useful.

[Machinbird]

sub-sonic doesn't really need a flying tail to retain control authority but once you get into trans-sonic/ supersonic regime then the ability to trim mach on the stab leaving full authority on the elevator becomes useful.

Hoppy, a THS doesn't really give you access to supersonic.
Once you are supersonic, the significant aft shift in CP (Center of Pressure) requires significantly more nose up right now than a slow moving trimable stabilizer can promptly deliver. For that reason, full flying slab tails are the preferred means of control once supersonic. That reason and flutter are the primary speed limiting factors on our modern THS equipped airline flying machines.

Even the Korean war vintage F-9 Cougar switched pitch control method once gear up to a full flying tail with the elevator locked in line with the HS, and it could only reach supersonic in a vertical dive.:)

[Faileas]

Thank you for the answers.
So generally, for an unpowered control system, in trim condition the elevator floats to a balance position and it is faired not with the rotated tailplane but with the local airflow, so that the force on the control column is relieved.
Is the misalignment relative to the adjustable horizontal stabilizer small?
I suppose that for maneuvering the stabilizer actuator is too slow and the elevator is used for this purpose, right?

[Mad (Flt) Scientist]

Thank you for the answers.
So generally, for an unpowered control system, in trim condition the elevator floats to a balance position and it is faired not with the rotated tailplane but with the local airflow, so that the force on the control column is relieved.
Is the misalignment relative to the adjustable horizontal stabilizer small?

Not always - depends on the position of balance/trim tabs, which are used to bias the elevator neutral position, in order to give the appropriate aircraft-level pitch balance.

I suppose that for maneuvering the stabilizer actuator is too slow and the elevator is used for this purpose, right?
Yes, Typical numbers are stabilizer rates of up to 0.5 deg/sec, which for 10-15 degrees stab range means 20-30 seconds to move the stab full range. Compared to typical elevator control speeds, which are of the order of full travel in 1 second, as a rule of thumb.

Obviously you'd never do either full travel change in flight, but it illustrates that the elevator is about 10x faster than the stab, so even though it's a less powerful control per degree of movement, you get better control from the elevator.

Of course, these numbers are for a THS+elevator aircraft. The stab rate for the all-flying-tail case is of course somewhat more sprightly.

[Faileas]

But if the aircraft has a trimmable stabilizer are the trim tabs usually not present?

[Mad (Flt) Scientist]

As a general rule, you only have trim tabs on an aircraft with an unpowered elevator. (because if the elevator is powered you don't need a device to adjust the hinge moment characteristics, you just power through it)

As another general rule, you usually don't have a trimmable stabilizer and a manual elevator system. (Because trimmable stabs become most useful, as mentioned somewhere up above, for the transonic/high subsonic case, and those aircraft typically have powered flight controls)

So in general you won't find tabs on the elevator if the stab is trimmable.

[Machinbird]

As another general rule, you usually don't have a trimmable stabilizer and a manual elevator system. (Because trimmable stabs become most useful, as mentioned somewhere up above, for the transonic/high subsonic case, and those aircraft typically have powered flight controls)


Except on aircraft like the J-3 Piper Cub and its relatives. :}

[westhawk]

As well as IAI products like 1124 Westwinds and 1125 Astras (now the Gulfstream 100) and the entire Learjet series. All have unpowered elevators with electric motor/jackscrew driven movable horizontal stabilizers.

Edited to add that these airplanes have movable stabilizers rather than elevator trim tabs.

[FE Hoppy]

Quote:
sub-sonic doesn't really need a flying tail to retain control authority but once you get into trans-sonic/ supersonic regime then the ability to trim mach on the stab leaving full authority on the elevator becomes useful.
Hoppy, a THS doesn't really give you access to supersonic.
Once you are supersonic, the significant aft shift in CP (Center of Pressure) requires significantly more nose up right now than a slow moving trimable stabilizer can promptly deliver. For that reason, full flying slab tails are the preferred means of control once supersonic. That reason and flutter are the primary speed limiting factors on our modern THS equipped airline flying machines.

Even the Korean war vintage F-9 Cougar switched pitch control method once gear up to a full flying tail with the elevator locked in line with the HS, and it could only reach supersonic in a vertical dive.

THS are used on all modern BOEING/AIRBUS/EMBRAER airliners for exactly the reason stated. I'm not familiar with the F-9.

[gas path]

Good example of trimmable STAB and manual elevators is/was the Boeing 707.
Internally aerodynamically balanced and assisted by tabs.:8