I have a few questions regarding the sweep of the LE on stabilizers and the size of control surfaces. On some models I've seen the stab have a constant chord i.e. no sweep to the LE or TE, but on most designs the LE is sweept. What factors determine how much sweep to use?

I'm mostly interested in aerobatic models. I have been looking at very general design materials (from the web) regarding proportions to control surfaces. Most references size the elevator area, for example, as somewhere between 20-30% of the total stab. area. If this works on a lot of designs, why do some aerobatic models have huge control surfaces? I've seen a lot of profile 3D designs like this. What makes these huge surfaces work? Do they require less deflection to work properly?

I'm just learning model aerodynamics, and the answers to these questions either aren't in the book I'm reading or are not yet clear to me just yet. So any help on these questions would be great.

Thank you,

Dan

I'll take a stab at this (no pun intended).
The sweep in the leading edge of the stab as far as I know is mostly a function of asthetics. Wing sweep in full scale aircraft is generally employed to delay the onset of critical mach number (this is when airflow goes supersonic over the top of an airfoil and sets up a shock wave). It works because the speed of the approaching air becomes a trigonometric function of the sweep angle. Obviously, in models this isn't a factor especially on the stabilizer.
The size of a control surface is really determined by the speed the model flies at and how much force you want the surface to exhert. First, let's start with a small surface that is very wide. Let's say your flying slow but you want to do some abrupt manuever. The force the control surface produces is a function of the surface's area and angle of deflection. If you don't have much area then you'll need a lot of deflection. The problem is, if you deflect the surface too much then the flow over it will seperate. The solution is to give the surface more area. Consider the profile planes you mentioned. These aircraft are designed to be highly aerobatic at slow speeds, hence the large control surfaces.

There is a slight structural advantage to a tapered stabilizer, It moves the center of pressure closer to the fuselage reducing the bending moment at the root, and since the thickness is usually tapered as well as the chord (at least on full scale aircraft) the structure is more efficient. Aerodynamically it makes little difference if the trailing edge is swept forward like the Mooney or the leading edge is swept aft like on most or even if both leading and trailing edges are swept and the spar is straight. Like much in airplane design it is a compromise between the stubby tapered planform which is structurally more efficient and a wide span narrow chord planform that is aerodynamically more efficient.

For most .40 size and smaller planes, the stab shape is really for appearance only.

Larger planes, especially Pattern competition models and others expecting high loads on large stabilizer surfaces do have the strucural/weight advantages of the sweep/taper as a major consideration.

At the relatively low maximum speed of models, we practically never need to consider Mach effects. The wings and tailplanes just aren't goig to start creating a Mach buffeting effect at the models size and airspeed. ( a major reason for sweeping wings/tailplanes of 400+ mph full scale aircraft) A model aircraft's prop though... can have the tips exceed .6 Mach, and that can create a LOT of noise. (that noise is wasted power...)

One reason for sweep on tailplane leading edges, (and fins) on aerobatic aircraft is that the surface keeps working at higher angles of attack due to "vortex lift". Frinstance look at the attitudes of Deltas landing and taking off.