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        Discussion Cord vs Span

#1 Roger Rocket Jan 28, 2013 11:52 PM

Cord vs Span
 
Always trying to learn something, and I guess this is the place. So, here's my question : In models as well as Full Scale I kinda thought high Aspect Ratio was what designers seem to prefer. Also, as in Cessna and Piper and some others, Tapered T.E. BUT, after seeing a Van's homebuilt that uses short span and wide cord, and they seem to have much better performance than most other light aircraft. Van's are very fast, but have almost STOL qualities, and what I hear or read, the best control Harmony than any other light aircraft. Plus, there made to be aerobatic. Now in models, most of what I see, in Sport, or contest type aircraft is the Higher Aspect wings. So, the question, is short span, but wide cord something that will be ' re- Discovered ' in the future ?? I know this might be a simplistic question to some, but I with friends are trying to figure out why Van's wing design works so darn well, but goes against current design trends. Is Long Span, Short Chord always going to be the best way to go. ?? ( except for Van's aircraft )

#2 BMatthews Jan 29, 2013 12:39 AM

You don't look at many of the fun fly 3D aerobatic models at all do you? They all have low aspect ratio wings. And most sport models are using what I would consider as low to medium aspect ratio wings.

So first off let's look at what is low, medium and high aspect ratios. To me low is anything up to 5:1. Medium starts at 6:1 and runs up to 8 or 9:1. High is 9 or 10:1 and over.

Models intended to have sporty fast roll rates will always tend to have medium or lower aspect ratios. You simply won't find any aerobatic designs that are over about 8:1. And most will be lower than that. Now gliders such as hotliners and slope soarers are certainly aerobatic and often have higher than 8:1 aspect ratios. But they simply do not have the roll rates of the lower aspect ratio "proper" aerobatic designs. That's because the longer wings tend to damp the roll rate both aerodynamically and due to long mass arm moments.

On the other hand aircraft intended to fly far on low amounts of fuel or that are designed to fly well on low power and do so efficiently will have higher aspect ratios. That's why sailplanes all have long skinny wings. It's so they use their "fuel", which is their altitude, in the most frugal manner possible. Similarly you see long skinny wings on such specialty aircraft as Rutan's round the world flyer.

So all in all I would suggest that there's simply nothing hidden or awaiting re-discovery. It's simply that the aircraft were designed to work the best at what their intended missions might require.

#3 packardpursuit Jan 29, 2013 11:18 AM

In addtion to what Mr. mathews said, ALL aircraft design are compromises. Some design feature are opimised at the expense of others. While designer try to opitmise most of their aircraft features it is quite evident that an aircraft cannot optimise them all. Van's "Hershey bar wings" are optimised toward ease of fabrication, with the single rib constant section, matched skin drilling etc. Their performance is great for what they do. But STOL qualities? Really?? A SuperCub has better stol quallites but cannot match th RV's for performance top speed etc. For travel to Alaska, you may want the RV-8, but once you get there you'll want the SuperCub to get into the back country!

#4 Curare Jan 29, 2013 10:11 PM

Back in the 50's Steve Whittman figured out that a low aspect ratio, wide chord wing could be made to fly pretty damn fast, which became the basis for the bonzo, dfr and tailwind racers and a bunch of other 50's and 60's racing design. They're not only quick, but easy to build, (constant chord) and can be built in a household workshop. Important stuff!

Fast forward 50 years and with mode detailed CFD programs, specialist airfoils and planforms have increased on that efficiency, but at the cost of making them much harder to build, and correspondingly more expensive.


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