Tailplanes, right way up or inverted?

[PR19_Kit]

Way back when, when I was building flying models, the tailplanes of our models were either flat or airfoil shaped and right way up, like a wing, with the curved surface on top.  In recent years I've noticed that many aircraft have their tailplanes inverted, with the curved surface on the bottom. I understand that this gives a form of 'longitudinal dihedral' which keeps the aircraft stable in pitch.

But reading up the Wiki page on tailplanes https://en.wikipedia.org/wiki/Tailplane makes me more confused. It seems that some aircraft have them right way up and some have them inverted.

I'm about to install the tailplane on my Rotocrane HC4, but the question is, which way up?

I'd be pleased to receive the committee's thoughts on the matter before I cut it to size and glue it in place.

The RW Rotodyne had its tailplanes the right way up according to the model and the few photos that show it, if that's of any help.

[rickshaw]

When was your aircraft supposedly designed, Kit?

If it was before the 1990s when inverted tailplanes became (sort of) popular, I think you have your answer... 

[PR19_Kit]

When was your aircraft supposedly designed, Kit?

If it was before the 1990s when inverted tailplanes became (sort of) popular, I think you have your answer... 

Good point Brian, yes.

The original 'dyne was designed in the late 40s and they'd have been unlikely to change anything fundamental about the design to build the Rotocrane, so I'll mount it curved side up. 

[jcf]

The Twotter has an inverted horizontal stab profile, and it aint' from the '90s, it's common
on transport aircraft, Boeing and Airbus, Boeing usage going back to the 367-80/C-135/707.

It relates to equilibrium.

9.1. INTRODUCTION TO TAILPLANE DESIGN, CONTROL SYSTEMS AND STABILIZATION
The functions allotted to the fixed and movable tail surfaces are as follows:
a. To ensure equilibrium of moments in steady flight by exercising a force at a given distance from the center of gravity.
b. To ensure that this equilibrium is stable, which implies that after a disturbance the equilibrium is restored and that
there is adequate damping for the rapid suppression of oscillations.
c. To generate forces for maneuvering the aircraft: rotation during takeoff, control of the flight path, flareout during
landing and taxying.


9.5. PRELIMINARY DESIGN OF THE HORIZONTAL TAILPLANE
9.5.1. Tailplane shape and configuration
d. Airfoil shape. The basic requirements are that the air-foil section should have a high CLα and a large range of usable
angles of attack. Frequent use is made of approximately symmetrical airfoils with a thickness ratio of 9 to 12 percent and
a large nose radius, e.g. NACA 0012. Tailplane stalling - at the lower surface - can be postponed by adopting negative camber
(e.g. NACA 23012 upside down), upward nose droop, an increase in the nose radius or by means of a fixed slot.

From Synthesis of Subsonic Airplane Design, Egbert Torenbeek, 1982 Delft University Press, Martinus Nijhoff Publishers
https://repository.tudelft.nl/islandora/object/uuid:229f2817-9be9-49b6-959a-d653b5bac054?collection=research

Inverted airfoils are also used on some helicopter stabilizer surfaces.

As to your question, with your lengthened airframe and twin boom design, a negative camber might be useful,
and, aside from RW practicalities, will look cool. 

[PR19_Kit]

Hmmmm, thanks JCF.

I've noticed that the Huey's tailplane is inverted as are quite a few other chopper's tails. Not that many pics show the tails though, so it takes a while to build up any visual evidence.

It's a pity I've glued both halves of the tailplane together already as otherwise I might have one half upright and the other half inverted! 

[jcf]

It's a pity I've glued both halves of the tailplane together already as otherwise I might have one half upright and the other half inverted! 

[Weaver]

Hmmmm, thanks JCF.

I've noticed that the Huey's tailplane is inverted as are quite a few other chopper's tails. Not that many pics show the tails though, so it takes a while to build up any visual evidence.

It's a pity I've glued both halves of the tailplane together already as otherwise I might have one half upright and the other half inverted! 

You could finish that off by gluing the ailerons at full opposite travel, then posing it with a groundcrew guy stood looking up at it and scratching his head...