Hi Mikey,

Just happened to be browsing after a hiatus of a couple months from this site. Regardless, it amazes me the things some guys think and truly believe as fact.

I have discussed the main reasons before a bunch times but not everybody reads it and comprehends it I suppose. One more time seems in order.

When it comes to snaps and spins, it totally depends on what kind of model you want to fly. Do you want a spirited, spritely, agile aerobat or do you want a slow to respond, slow to exit, clumsy model? Do you want the model to initiate and STOP exactly where you want it and when, or do you like a horsed entry and over-rotated exit? In car terms, a Ferrarri of a Truck??

Stabs ....well...stabilize. Stabs are either horizontal of vertical, it matters not at all...same rules apply. Very large stabs stabilize exceedingly well and smaller stabs stabilize to a lower degree. Large stabs work great on trainers.

Lets talk horizontal stabs for a moment. Large stabs, those that are greater than about 26-27% of the wing (rule of thumb), are really larger than they should be for a really agile, effortless, efficient aerobatic design. When a model uses larger stab area than this rule of thumb, it will require unecessarily large amounts of elevator command (and elevator area as percent of the stab area) to overcome it's designed in stability. A stronger, quicker servo will help you, but my point is, why work you equipment that hard? Your exits will not be as predictable and precise and you will likely wind up chasing the exit. My personal preference is stab area at around 20-22% of the wing. Typical elevator area is approx 40-45% of the stab area so a 200 sq in stab has about 80-90 squares of elevator.

Wing leading edge radius plays an important role. The outboard 12-15" of the panel (about 1/3 span) should have a fairly sharp LE...I prefer no greater than 1/8"-3/16" radius. The inboard LE is less critical to spirited performance.

Rudder and fin typically stabilize to a greater degree in yaw than the horizontal stab and elevator do in pitch....there is much less vertical fuse area to stabilize (compared to wing vs stab area), so the amount of vertical area in the tail is almost always large. HOWEVER, this is necessary to counter the spiralling prop blast and it's effects. Have you noticed some of the guys using some vertical area at mid-ships? My good friend Nat Penton pioneered this idea some 25 years ago. Well, that essentially destabilizes the fuselage in yaw a bit, again making for spirited performance from the model. It makes rudder input more effective. Think of it as having the identical purpose as dorsal and ventral fins have on a shark at mid-body. These surfaces make the fish extremely agile and without them, it would not be capable of turning on its prey as quickly as it can. Nature invented the concept eons ago.

This is getting as long winded as some of Troy's stuff so I'll cut it short. Just busting you Troy, so don't be offended...please!!

I'll leave this topic with a key parameter to chew on regarding pitch stability for aerobats. Tail Volume Coefficient, TVC, is a dimensionless number that brings wing, stab, and tail moment together in a neat package that is easy to calculate. It is essentially the ratio of the stab to wing areas times the ratio of tail moment to wing MAC. The tail moment should be determined aerodynamic center (AC) of wing to aerodynamic center (AC) of stab. It's generally accepted as being located at 25% MAC for either surface. MAC for either stab or wing can be determined graphically....or look it up on the internet. I really like the way my designs fly at TVC of around 0.65 and all who have flown my stuff can attest to their effortless flying ability. Many present day designs have TVC greater than 0.8 which would not be my first choice. From about 0.6 to 0.75 is a good all around range to shoot for.

There are some other factors that trim this main factor but once you have a handle on this major factor you will begin to understand why you model behaves one way or another. For example, I have found that high taper ratios on wings, as high as 3:1, tend to help the model's agility. Some fellows have opted to reduce the model's span instead. I prefer to keep the span as long as possible but use a high taper ratio. I get larger wing area and greater agility at the same time, since the tip chord is much shorter than usual which means I can drive to a smaller LE radius.

Hope this helps guys and doesn't confuse

MattK