Realism of flight

Model Airplane News, Oct 2000 by Leu, George

I receive a lot of mail from readers, and the topic that comes up more than any other concerns the scale realism of flight; in particular, one reader and some friends fly .60size aircraft, and they feel that they do not receive fair flight scores at competitions compared with those who fly larger aircraft. This results from a lack of understanding by both modelers and judges of what "scale flight" means. Noted scale modelers Bob Frey and Kent Walters compete with what are considered "small" aircraft by today's standards, but they still do extremely well in competition. I hope contestants and judges alike can use this column to gain a better understanding of scale flight characteristics and how scale models should be assessed.

To make the following points more clear, let's review basic aerodynamics. For better flight realism (and higher flight scores), most scale competitors prefer 1/4 scale for WW I and Golden Age aircraft, 1/5 scale for WW II fighter aircraft and 1/6 to 1/8 scale for ducted-fan jet aircraft; turbine-powered jets are now even bigger.

No matter how large or small an aircraft is, its ability to fly is governed by the laws of physics. Thrust, drag, lift and gravity are the forces that make flight possible. In straight and level flight, gravity is a constant. Drag is related to aircraft volume (displacement of air and airflow) and is usually related to the aircraft's surface area, some of which provides lift. A larger plane (one with more surface area and volume) creates more drag than a smaller aircraft. Drag is countered by thrust (forward air speed) to keep a plane airborne.

Lift counteracts gravity but can be varied by altering the wing's angle of attack and the control surfaces, but only at the cost of increasing drag. Prop pitch and engine rpm affect thrust; when drag is changed, thrust must be adjusted accordingly.

As model airplanes become larger, their ability to fly more efficiently is improved. A model's overall weight does not automatically double as you double the size of the airframe; i.e., a 72-inchspan model doesn't weigh twice as much as a model with a 36-inch span; this, in turn, reduces the relative wing loading of the larger model. Also, without getting too technical, larger models tolerate higher wing loadings better. All this and more contribute to the relevance of "Bigger is better."

With larger airframes, drag also plays a lesser role. If we compare a 1/6-scale model with a 1/5-scale one, the larger model's wing area may be 25 percent larger than the smaller one's, but its drag will not be 25 percent greater. Bigger means more efficient.

So when we talk of scale flying for realism, it is important to remember that smaller planes need to fly faster than larger ones just to overcome drag. The small plane has less lifting area, and this, too, means that it has to fly relatively faster to stay airborne.

DEPTH PERCEPTION

Another consideration when flying large and small models is how our brains perceive distance. The closer things are to us, the bigger they look. As they travel away from us, models appear to get smaller. We see a small, 50-inch DR-107 One Design as farther away from us than a 108-inchspan, Y4-scale Piper Cub, even though the two are the same distance from us. This perceived distance change affects our perception of speed. Since the DR-107 looks farther away, we think it must be flying faster to cover the same horizontal distance as the closer Piper Cub has traveled. (See Figure 1.) False depth perception affects how we perceive our models.

To sum up, scale fliers and judges-should be aware of a model's size prior to a scale flight round. Small planes need to fly faster than scale speeds to do scale maneuvers. Larger models appear to fly more realistically. Everything being equal, flight judges should understand that planes of both sizes are being flown correctly, and both pilots should be assessed accordingly. Factors such as flight attitude, loop size, angles of descent/ascent, banking angle and turning radius are more important for realism of flight than perceived flight speeds.

As a scale contestant, before you fly, take the time to explain to the flight judges how your model will look to them. Make this explanation part of your preflight explanation of maneuvers. While I love large aircraft, there is no reason they should automatically be declared better modeling subjects for scale competition.

BUILDING JIGS

What is a jig? It's a device that holds something in place while you work on it. I use a jig to hold my fuselage crutch as I add formers and stringers. Jigs all but guarantee perfect alignment and offer easy access to the fuselage members during construction. Jigs are ideal for working on round fuselages such as the Ercoupe shown in the photo. I built mine from /4inch lumber-store plywood and used glue and screws to hold everything together.

I first attached B (the backboard) to the rear of A (the base) to hold the vertical C parts in place and prevent them from moving around. I next added the three parts C to the base and backboard. Use a level and a 90-degree square to keep everything square during construction. Check to make sure the jig is sturdy, then add a few coats of shellac to make it look nice. I have used my jig for quite a few years, and it has served me well. Make one for your next project, and see what a difference it will make.