Written by Rob Caso
Scale Park Pilot
As seen in the Spring 2013 issue of Park Pilot
>> Scale models are nearly always heavier per square foot of wing area than the average sport model, and are therefore flying closer to the edge of the flight envelope. What this means is that the heavier model will have a higher stall speed, and be more sensitive to how it is handled in the air by its pilots. Compounding this is the scale builder’s need to use scale or close-to-scale airfoils so that the model correctly resembles the prototype.
It is appropriate to pay attention to some of the more critical factors that make our scale models harder — or easier — to fly, and you don’t have to be an aerodynamicist to understand them.
I suggest reading Wolfgang Langewiesche’s Stick and Rudder. This book is a primer on full-scale flying, but many of its aerodynamic rules and examples apply directly to our models. How many of us were taught that a wing lifts from its upper surface? Well, it’s not true. All wings fly at a slight upward angle to the wind that is created by the aircraft’s movement. This up angle, or angle of attack (AOA), is provided by either a noticeable angle of the wing relative to the breeze, by the shape of its airfoil or by a combination of the two. Try to pick your model up by using only the upper surface of the wing. Can’t do it, right? This positive AOA produces force on the underside of the wing when it’s in motion. If this force exceeds the weight of the aircraft, the airplane will fly. If it does not, the airplane will descend, or stall (Figure 1). 
Skipping a rock across a pond demonstrates this rule perfectly. As long as the force of the water (provided by the rock’s forward motion) exceeds the weight of the rock, it will skip. Once the rock slows down, it sinks, or in essence, stalls. It is the bottom of a surface that provides lift, not the top. When an aircraft is slowed, its wing will pitch up. The model will still be flying because the wind’s force against the higher angle of the wing will still be in excess of the model’s weight. The wing is now simply deflecting more air to generate the same force as it was when in normal flight. The wing will continue to pitch up until the force of the airflow diminishes to a point where it cannot sustain the weight of the model, and a stall occurs. This is why the elevator simply controls the pitch of an aircraft and the engine controls altitude. Don’t believe me? Cut the engine and try a sustained climb using only elevator. A high-speed stall acts in the same manner, except in this case, some force has magnified the weight of the model. The wing was perfectly level and the model was zooming along fine, but it stalled — why? Centrifugal force is the answer. It can happen at the bottom of a loop or in highly banked (high-G), tight turns. In these cases, the model incurred G-forces that made it temporarily heavier. These forces exceeded the lift being provided by the wing, causing a stall.
Here is how washout is being built into the wing for Rob’s 51-inch Bulldog. Note that the tip is shimmed up, while the rest of the wing is pinned to the board.
Get them slow enough and all wings can be made to stall. Still, some heavily loaded models have gentler stall characteristics than similar models with a lighter loading. Often, a model will stall all at once, usually dropping a wingtip in the process, whereas others will stall more gradually and do so straight ahead. Why is this? A significant contributing factor is washout, which is a deliberate warp or twist in the wing so that the wing root’s AOA is always higher than that of the tip’s (Figure 2). The wing roots will stall first, but the tips will still be flying and the wing will stall gradually, from root to tip. Since the tips are where the ailerons are, some level of lateral control can be maintained into the stall.
I have a number of World War II warbird ARFs, none of which has washout, and although they are lighter than some of my wooden models, when they stall, look out. They do so all at once and drop a tip. All of my wooden models have some measure of washout. Heavy models are usually equipped with flaps, and these act in much the same way as washout (Figure 3). Because the flaps are inboard, lowering them increases the effective angle of the wing at the root, thus creating washout because the tips don’t move. So, a wing can have built-in washout, washout using a variable airfoil (flaps) or a combination of both.
I build washout into all of my wooden wings, and the procedure is fairly easy. If the wing is a D-box structure, sheeted top and bottom from the main spar forward, a twist may be induced by shimming up the rear of the wingtip before applying the top sheeting. This locks in the twist because the upper sheeting follows the curvature of the ribs (Figure 4.)
I am only scratching the surface here, but these are the basics. Happy flying.
Written by Rob Caso
