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Old Mar 02, 2010, 09:50 PM
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Wing loading -vs- plane size


I know that wing loading is an important factor in the way a model flies. Does model size enter into it? Suppose I have a 36" span plane and a 72" span plane, both with wing loadings of 16 oz/sq ft. Will they have approximately the same flying characteristics? (given a similar airfoil, power / weight ratio, etc)
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Old Mar 02, 2010, 10:02 PM
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Not really because weight is relative to cube (3 dimensions) and wing area is relative to 2 dimensions. Wing loading is ok to compare planes around the same size, but cubic loading is needed for big differences. Lots of threads here get into the debate.
Old Mar 02, 2010, 10:08 PM
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It actually does vary, due to changes in how the air flows over a big wing versus a little wing (remember, the air itself doesn't get scaled up just because the plane gets bigger).

Bigger planes can typically get away with having a higher wing loading then a smaller plane of similar design. I know from experience that this is especially true with gliders. For example, I have a 2.5m span F3F racing glider with a loading of around 18oz/sq ft. While that's not all that light, the plane will still fly in light slope lift conditions. On the other hand, if I ballasted up one of my little 50" slopers to 18oz/sq foot, it would be an absolute brick in light lift.

Obviously, this is a lot more noticeable with gliders, where we can and do vary a single airplane's wing loading...to the point where a ballasted glider can weigh twice as much then it's unballasted weight.
Old Mar 02, 2010, 10:19 PM
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Nope, not even a little bit the same.

Wing loading is only helpful if you are comparing two planes of about the same weight.

Using your example and assuming a 6" chord, the 36" plane would have 216 square inches and weigh 24oz to have that 16oz wing loading. My personal experience would be to not even consider enjoying a small plane with such high wing loading.

The bigger plane would have 864 square inches and weigh 96 oz with 16oz wing load. Now we are talking a nice flying plane.

The bigger plane would look much more authentic in the air, cruising along at a nice slow pace like the real thing. The smaller would be darting about like a toy and require you to really pay attention to not stall and crash during landing.

A much more accurate way to look at these two planes is called "cubic wing loading" which, as you might suspect, takes into account that planes are 3 dimensional. The cubic wing loading of our example large plane is 6.5. The smaller is 13.1.

To get a cubic wing loading of 6.5 on the smaller plane to match the larger, you would need to get the weight down to 12oz and have a wing load of 8oz. In theory, the two planes would look like they are flying at the same speed, and they would stall at a relatively similar airspeed. It's not a perfect system of course, but it is about 200 times more helpful than wingloading....

-Steve

PS, figuring cubic wing loading can be done on a calculator, but I like to cheat and use this great on-line calculator:

http://www.ef-uk.net/data/wcl.htm
Old Mar 03, 2010, 10:41 AM
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You can find a lot more on wing cube loading AKA cubic wing loading (CWL) at

http://homepage.mac.com/kmyersefo/sitetoc.html

Look for these articles on the site table of contents page:

Cubic Wing Loading: What it is and how to use it. Also known as - Wing Cube Loading, by Ken Myers
MODEL DESIGN & TECHNICAL STUFF: WING CUBE LOADING (WCL) by by FRANCIS REYNOLDS , Model Builder - September 1989
Aircraft Performance Parameters Revisited : WING CUBE LOADING, by Roger Jaffe, Model Builder - June 1994
3D Wing Loadings: a Better Way to Scale Models and Compare different size models easily by Larry Renger, Dec. 1997
CWL/WCL Calculator online from our friends at Electric Flight UK
CWL/WCL Calculator online at Fly RC
Old Mar 03, 2010, 11:26 AM
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Here's an example - a full scale Cessna 172 has a wing loading of 200oz/sq ft at AUW. This giant scale model could be made to fly about the same as full scale, but is 30oz/sq ft.


http://www3.towerhobbies.com/cgi-bin...&I=LXVZB0&P=ML
Old Mar 03, 2010, 12:45 PM
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Full Scale Cessna 172R, max. wt. 2450 lb., area loading (wing loading) = 225.3 oz./sq.ft., wing cube loading 17 oz./cu.ft.

Model noted above at 12.5 lb., area loading 32 oz./sq.ft., wing cube loading 12.8 oz./sq.ft.

Just for comparison.

Data for full-size Cessna 172R used as the example was found here
http://en.wikipedia.org/wiki/Cessna_172
Old Mar 04, 2010, 12:35 PM
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Hi!

Letīs look at some simple facts:

1. If you have two similar models A and B, made of the same materials (i.e. with the same density=weight/volume), and model B is L times larger than model A, then Bīs WingLoadFactor will be L times larger than A.

This is almost true with the Cessna172 in the previous posts, although the model and the real thing, are not made of the same materials.

2.And more important:

Based on the LiftFormula, (assuming that in both models the 2 constants are the same) in order to achive the same racio Weight/LiftForce, model B must fly at an airspeed that is the squareroot of L, times the airspeed of model A.
Old Mar 04, 2010, 01:35 PM
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During a storm a little boy asks his mother "what causes thunder and lightening"?
Mother answers, "ask your father he is an engineer"
Little boy says. "I didn't want to know that much about "wing loading""!!!
Old Mar 04, 2010, 02:44 PM
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I learned about it first from Ken Myers excellent articles a few years back. Made perfect sense to me, in a way I could understand on a gut level.

Chuck
Old Mar 04, 2010, 02:57 PM
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Bigger planes can get away with bigger wing loadings. That's all you need to know.
Old Mar 08, 2010, 10:23 AM
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Old Mar 08, 2010, 11:26 AM
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Quote:
Here's an example - a full scale Cessna 172 has a wing loading of 200oz/sq ft at AUW. This giant scale model could be made to fly about the same as full scale, but is 30oz/sq ft.
If the model had the same wing loading as the full size, it would have the same stall speed (probably about 60mph, but that's a guess)

In order for the model to look like it's flying at scale speed, it needs to cover it's own length in the same time as the full size plane - i.e. it needs to be proportionally lighter.
Old Mar 08, 2010, 11:51 AM
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Quote:
Originally Posted by RobinBennett View Post
If the model had the same wing loading as the full size, it would have the same stall speed (probably about 60mph, but that's a guess)

In order for the model to look like it's flying at scale speed, it needs to cover it's own length in the same time as the full size plane - i.e. it needs to be proportionally lighter.
Don't know, but that model would need to be loaded to 80 pounds weight to acheive the same wing loading. Maybe you meant cubic loading?
Old Mar 08, 2010, 12:06 PM
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Quote:
Originally Posted by victapilot View Post
Don't know, but that model would need to be loaded to 80 pounds weight to acheive the same wing loading. Maybe you meant cubic loading?
Hi victa,

He was right, the wing loading pretty much determines the stall speed. Same wing loading = same stall speed.


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