I would like a simple definition of aspect-ratio(if that is possible), and how it effects the aerodynamics of a flying model.
Thanks for the feedback~~~bjnorman

The ratio of the span to the chord, or vice versa, I think

http://aerodyn.org/Wings/larw.html

Since the chord may vary, you'll also see the aspect ratio
defined as the span divided by the square root of the wing area.

Martin Simons' book:

http://www.amazon.com/exec/obidos/AS...285061-3940604

... discusses the subject.

My potentially inaccurate (standard disclaimer ) recollection is:

High aspect ratio wings are more efficient than low aspect ratio. Tip vortices have a lot to do with this. As the air swirls around and near the tip of the wing it reduces the effectiveness of the the wing area near the tip. A high aspect ratio wing has less area, relatively, near the tip than a low aspect ratio, hence higher efficiency.

The cost is a more acute (steeper) lift versus angle of attack curve. This translates into greater pitch sensitivity and more severe stall characteristics.

bjnorman,

I'll just add a couple things to Jim's comments. When the chord is not constant, the AR is defined as the square of the span, divided by the area of the wing.

Higher aspect ratio wings are more efficient than lower aspect ratio wings specifically in the area of induced drag. Induced drag is the component of drag which is caused by the wing tip vortices ( a slight simplification ). All wings have some induced drag, but high aspect ratio wings have less than low aspect ratio wings. It is useful to keep in mind that induced drag is not always the most important component of drag. It is generally significant during the high-lift parts of a flight, which means flying slowly or turning sharply. Also, increasing aspect ratio may not be favorable for the other components of drag.

The lift curve slope refers to the slope of the curve that you get when you plot the coefficient of lift vs. angle of attack. As Jim points out, higher aspect wings have steeper lift curve slope, which is to say that lift increases faster as you increase the angle of attack. This can be a good thing or a bad thing, depending on the situation.

Higher aspect wings will generally have slower roll rates, which matters if you have aerobatics in mind. One other factor related to aspect ratio is chord length. If you keep the total wing area constant, then a higher aspect ratio wing will have a shorter chord, which means it will operate at a lower Reynold's number, which can be a disadvantage, especially if the plane will fly a slow speeds.

Most of the other implications of aspect ratio that pop into my head are structural in nature.

banktoturn

I thought that a lower Re equates to slower flight? So a wing operating at lower Re is adventageous for slow flyers. This can be seen in gliders which have long thin wings with high aspect ratios and that generally fly relatively slowly compared to powered planes. However, gliders have a high aspect ration primarliy due to it's higher efficiency aspects.

-Q.

a088008,

Low Reynold's number kind of implies low speed and/or small chord. Generally, model aircraft have both. What I was getting at is that models generally fly at low Reynold's number, and when they are flying at their slowest speeds, the Reynold's number can be problematically low. Gliders have high aspect ratio wings so they can have the benefit of lower induced drag, and in spite of the possibility of operating at very low Reynold's numbers. Low Reynold's number is not really advantageous, in general, it is just something you have to live with when your plane is small and/or slow. The only potential benefit I can think of for low Reynold's number is the ability to maintain laminar flow further back on the wing, to reduce skin friction drag. I doubt that model aircraft take advantage of this very often.

banktoturn

Thanks to Jim, Banktoturn,& A088008 for your replies on my question on aspect-ratios. Very interesting subject; and, I continue to learn more in my senior years.
Again, thanks, and take care..bjnorman