Wing Loading: What is it really?
 

Hello Everyone,

I have a few general questions about wing loading. I have a general idea of what it is and what it does...but not really. :eek:

So...wing loading is the loading on the wings (OBVIOUSLY). It has units of oz/in^2 (which may vary). It explains how much weight the wings must generate lift for relate to the size of the wing. In general, the greater the wing loading the worse the flying characteristics. BUT what is wing loading really???

How does greater wing loading really affect an airplane's flying characteristics? And how much can the wing load increase before negatively impacting the flying characteristics?

I ask these questions because I am trying to chose an engine for a plane and am worried one may be too heavy, resulting in too much wing loading and poor fly characteristics.

Any help would be appreciated.

-Matt

RE: Wing Loading: What is it really?
 

I find it helps to think of wing loading relating to how fast the plane must fly to stay in the air, rather than flying poorly.
Different aircraft behave better with very light wing loading such as 3D, aerobatic, etc.
But what I have found is that some scale I have built behave better with higher wing loading, ie: they fly more scale like.
You do have to be careful with flying speed, with a high wing loading you will stall out much sooner.

So I guess what I am saying is that light wing loading is not the be all and end all of aircraft design.

RE: Wing Loading: What is it really?
 

Sooooo many variables here and so little info to go on in your particular case.

Except, perhaps for slope-soaring gliders (?) extra weight is ALMOST ALWAYS, as a stand-alone variable, BAD. It increases stall (hence t/o and ldg) speed. It lessens climb rates. Increases descent rates. On and on, but you already have the gist of it.

If a smaller engine will provide the pwr to do what you want (whatever that may be, hugely dependent on the type of flying U do etc...) then use it. Extra weight just for the sake of "wasted" power does you no good, and some of the extra pwr will be "absorbed" just to carry around the extra weight anyway. If you're not sure how much pwr you need for whatever U want to do w/ a particular model just ask around these forums and execute searches to see what others have done and what the results were.

If you NEED extra pwr, well, extra weight is often (but not always) just the trade-off you have to make.

Thinking in terms of percentages may help. If you can add 30% more pwr for a 5% weight penalty (don't forget the extra fuel you'll need for a given length of flight) then that's a pretty good trade in a lot of cases. But if the model could already climb straight up after a 20 ft roll w/ a smaller, cheaper, more economical motor, then what's the point?

On the other end of the spectrum, I've totalled more than one model because it had only "enough" power, without the reserves to pull me out of trouble when I did something dumb.

I hope I provided more answers than questions for you. I'd be happy to expand on or clarify my opinion if you like?

RE: Wing Loading: What is it really?
 

Wing loading when stated as weight pre unit area does not adequately describe the flight characteristics. For instance, a 1/4 scal with a wing loading of 26 ounces/sq.ft. would most likely be a floater while a 1/2A model with that wing loading would probably be a lead sled and nearly unflyable. A better description is "wing volume loading" which is defined as (weight of the model in ounces) devided by (wing area in square feet raised to the 1.5power). Anything under 15 will fly quite well, if you are down near 7 it will be a real floater.

RE: Wing Loading: What is it really?
 

It generally is only useful to compare wing loadings to models of the same size due to the strong scaling effect. That is because air molecules do not change size as we make our models larger or smaller.

RE: Wing Loading: What is it really?
 

Thank you all for your responses.

mboland:

Everything you said about viewing wing loading as flying speed necessary to maintain flight makes sense. So I'm assuming an aircraft's speed must affect its lift. It must. Right?

So if your airplane has greater wing loading than it will need more power for slower flight than a plane with less wing loading? Is that what you're saying?

And also, with these fully symmetrical wings that are commonly used on 3D/Aerobatic models, the wings don't produce lift in level flight. Bernoulli's principle describes how a faster moving fluid over the wing relative to under the wing will cause a change in lift, BUT there is no change in speed of the fluids at level flight as the distance from the leading edge to trailing edge is the same. So, in order to produce lift with a fully symmetrical wing at level, slow flight is to orient the plane upwards which in turn would change the velocities of the moving fluids and then a change in lift. (I think everything I have said is at least semi-correct :)) So my secondary question is: since we must adjust the wing attitude in slow flight to generate lift and prevent a stall, how come we don't have to do something similar in fast, level flight? I have noticed when I "punch" the throttle, the plane will maintain altitude. Why wouldn't the plane be falling as it's not producing any lift to counter its weight?

Wow, there was a lot there. I got a little sidetracked and went on a tangent. I suppose one question led to another. :D

highhorse:

Everything you said about "absorbed" power and buying only what's necessary makes total sense. However, could you go in more detail about the trade-off you were speaking of?

When you talk about the trade-off of power vs. weight (i.e. 30% more power for 5% more weight), would you calculate that with difference in horsepower (a percentage, not actual horses) vs. difference weight (again a percentage), and then try compare them?

Rodney:

I'm not quite sure what you mean with wing loading affecting airplanes differently from plane to plane, like a 1/4 IMAC plane to 1/2A. I thought wing loading was supposed to be a generalized statement about the loading on the wing. So, wouldn't a wing loading of a 15 oz/ft^2, for instance, affect the 1/4 IMAC plane the same as the 1/2A? If you could develop on this that would helpful.


Thank you all for your help,
-Matt

RE: Wing Loading: What is it really?
 

Dave, thank you for your response.

Is there anyway you could clarify what you mean with "strong scaling affect"? I realized that the air molecules don't change size, but how exactly does this affect the wing loading and more importantly the plane's flying characteristics.

Thanks,
-Matt

RE: Wing Loading: What is it really?
 

You can always increase a planes wing loading for BETTER flying.

You can never decrease a finished models wing loading very much.


Build light, add engine size and more battery.

RE: Wing Loading: What is it really?
 

cyclops2,

When would higher wing loading result in better flying characteristics??? I have never heard of this.

Thanks,
-Matt

RE: Wing Loading: What is it really?
 

A 5 lb airplane must have 5 lbs of lift to maintain altitude. The heavier the airplane is the higher the AOA needs to be to generate the needed lift to maintain altitude. As speed increases the AOA needed to generate the needed lift decreases. Lift = drag and drag require power.

A high lift wing could reach a speed where a negative AOA was required to maintain altitude and that would waste power. That is the beauty of the symetrical wing for fast airplanes because it never reaches that point.

RE: Wing Loading: What is it really?
 

This ought to drive the scholars nuts:
lift is generated anytime the pressure is different from bottom to top of wing

no theories needed here .
more difference - more lift
size of lifting thing makes a big difference
why
air is lazy - it takes path of least resistance
just like students --------
if the air can go around the lifting thing -instead of over/under it (remember- easiest path) lift is reduced or is nonexistant. the smaller the lift thing - the easier it is for the air to simply move around -away (compress a little if that is easier ).
so - a great big piece of cardboard 10ft x10ft moved slowly- will move a LOT of air -a big pressure difference as the air can't get around it - easily stand a sheet on edge - let it drop - you will see.
But take a little piece-it drops fast - the air can movearound it much more easily
don't get caught up in airfoil theories - all of th millions of foils are simply shapes wich are more efficient under set conditions
my answer won't win you a score on a test but if you actually do some flying models -in widely varying sizes -it will allquickly become apparant to you.
in a nutshell as the lifting thing gets smaller and less capable of directing flow-- it will lift/carry/ support
LESS weight disporportionately-- the curve steepens downward
given two wings both 3-1 aspect ratio
the one at 400 squares and 6 ounces to the foot loading will seem to fly much the same as one 2000 sq inches at 30 ounces to the ft loading
In this example - 1/5 the area has about 1/5 the wing loading

RE: Wing Loading: What is it really?
 

yes, absolutely.

No. Mixing apples and oranges here. More loading means more speed required for the wing to lift the load, period. You can't lower a wings stall speed just by putting more power on it. The min flying speed for a given weight is what it is. (OK you guys, don't argue that technicality w/ me here and confuse the poor guy even more, it won't be constructive for this aero 101 lesson) More power will get you faster flight, but not slower.

We know what you mean, but to make things more clear for this and future discussions, let me help you say it better. Do not confuse LEVEL FLIGHT w/ ZERO ANGLE OF ATTACK. Any wing, even a symmetrical wing, IS making lift in level flight or it wouldn't be flying, right? Right. It's the angle of attack that you meant to refer to. But you were on the right track.

Hmmmmm. Well we DO need to do that ! Think of it this way: A symmetrical wing has 2 variables that determine how much lift it makes, AOA (angle of attack) and speed. They are in a CONSTANT trade off during flight. You wanna fly slow without descending? Gotta increase the AOA. Ya wanna fly fast without climbing? Gotta decrease the AOA. It's impossible, tho to see a change in AOA from, say, 3deg to 1deg as you speed up while maintaining level flight. It's there, just not noticable as your model rockets by. And believe me, there IS at least SOME small positive AOA or you're right, the model would definitely be falling. And re-think your understanding of "stall". Increasing the AOA is the ONLY repeat ONLY (seriously, ONLY !!!) thing that can create a stall. It aint (directly) a loss of speed. Research the term "critical AOA" and it's relationship to lift. I'd run it all down for you, but frankly, I'm tired.[&:] It's not a difficult concept, but it's a critical one.

Sure. If I have a model that's not climbing well but is lightly loaded and glides just fine then I know I have some room to bargain. I won't mind adding some more weight in that case because the stall speed wont increase by much and the climb might easily double (climb is a function of "leftover" pwr above "minimum" reqd for a given speed. If a 1 hp model is using up .9 hp just to stay level it only has .1 hp leftover to climb with. Bumping up to a 1.2 hp engine, igoring for simplicity the weight penalty, means that the model now has (1.2 - .9) .3 hp to climb with, a 300% improvement in climb from a 20% improvement in pwr. An extreme but valid example.)

Not to speak for Rodney, but I think I'll speak for Rodney!:D (Correct me if I'm wrong, Bro)
Think in terms of wing VOLUME, not just area. If a wing is doubled in area as the THICKNESS remains the same then it's apples to apples. Your manipulating twice as many air molecules. But the thickness of the wing usually increases along with span and chord, right? Sooooo, It's more of a cubed inches thing that a squared inches thing. Example: my Ultra Stick and my Giant Ugly Stick have roughly the same wing loading in terms of oz/sq in (giant slightly higher loading), but the Giant seems to have a slightly lower stall speed! For 90% of what we're doing tho, comparing sqares to squares works just fine.

Wow, long post. I need a nap! Hope this helped, Don.

RE: Wing Loading: What is it really?
 

Matt,

Is there anyway you could clarify what you mean with "strong scaling affect"?

The smaller wings in small model planes do not produce the same lift in scale as full sized aircraft. They produce less lift because as you said, air molecules do not change size with the scale of the model. They fly slower, (in most cases), and do not generate the pressure differential between the bottom and top of the wing that faster larger planes generate.
The smaller wings also have a greater percentage of lift-loss at the wingtips. The higher air pressure under the wing escapes to the lower air pressure above the wing at the wingtips creating vortices. This happens on all wings, but is a larger "percentage" loss in smaller models. Lift also increases exponentially as speed increases. Double the speed, and approximately tripple the lift, (this is a general statement and there are a lot of other factors - This number is not a fixed number. I just used it as an example)

I'm not quite sure what you mean with wing loading affecting airplanes differently from plane to plane, like a 1/4 IMAC plane to 1/2A. I thought wing loading was supposed to be a generalized statement about the loading on the wing. So, wouldn't a wing loading of a 15 oz/ft^2, for instance, affect the 1/4 IMAC plane the same as the 1/2A? If you could develop on this that would helpful.


A full-scale Piper Cub has a wing loading of about 99oz/ft^2 and stalls at about 27 mph. The Great Planes .40 size plane has a wingloading of about 18oz/ft^2 and stalls at about 16 mph.

The wingloading of the model is 1/5th the full scale, but the stall speed is 2/3rd the full scale plane.
If aerodynamics were linear with scale, the stall speed of the model should be just over 5 mph.

Here is a link to a site that explains it very well:
Look at the last section explaining airspeed vs. lift near the bottom of the page.
http://www.djaerotech.com/dj_askjd/d...wingcalcs.html

With that said, wingloading becomes more critical with smaller slower flying models, and airfoil selection is less critical as you get smaller.
A small change in wingloading can make a big difference in the handling of a smaller plane, affecting the stall speed and even higher speed extreme maneuvers.

I hope that this help, instead of raising more questions...:D

Bob

RE: Wing Loading: What is it really?
 

well done, Bob

RE: Wing Loading: What is it really?
 

Thank you, Highhorse.
I started posting, I guess while you were posting your explanation...
Nice post.
I think you covered everything just fine, and would have reserved my post if I had seen yours first.

Bob

RE: Wing Loading: What is it really?
 

Between the two of you I think you nailed all the "101" concepts here.

UMD, you're obviously trying to come to terms with all this and I gather it's all turning your mind to pablum.... :D A slick little tool that may help you putit all together is Foilsim. Google for it and then go to the link and spend some time playing with it. Set the parameters for a model sized wing and then play with the thickness and camber and flying speed and angle of attack. To see how the angle of attack changes for the shift from landing to hot passes with given airfoils alter the speed and then adjust the angle until the value for lift is what you expect your ficticious model to weigh. Note that this is ONLY for level flight. Try to think about all the things that are happening to your model. Come back and ask more questions.


And just to add some real numbers to the whole wing loading scale issue .... Examples of my own models over the years show that a 36 inch span 1/2A power sporty but not full on 3D models work best at an 8 to 10 oz/ft^2 loading. A .25 powered model with the same sort of apparent performace will have a loading of around 12 to 15 oz/ft^2. A little 20 inch span .010 or electric sporty model to fly in the same apparent way would need to be down around 5 to 6 ox/ft^2. Meanwhile the Boeing 747 lifting off in the distance is flying with a loading of well over 200 POUNDS/ft^2. That's your whole scaling issue at work. Granted part of this is because the preffered flying speed gets a lot lower as the size changes but most of the issue is about how the size of the wing interacts with the air.

RE: Wing Loading: What is it really?
 

As I noted -- my model which is 1/5 the area-- needed about 1/5 the wingloading to flyand react in much the same way.
unless you actually fly large and small -in aerobatic models-you will not see how well this works

RE: Wing Loading: What is it really?
 

My statement of increasing the wing loading to fly better is based on the fact, that a Cement brick lands in a straighter line on the runway, than a foam brick.

Flying and landing in a gust is easier with the Cement brick.
Calm day, fly any wing loading. Not sure you should fly day. Take off in the brick.


A 747 is a overpowered brick.

RE: Wing Loading: What is it really?
 

Cyclops2,

While that statement is true, a brick is much harder for a novice to control than a 747. :D

In gusty conditions, yes, a higher wingloading handles better than a lighter wingloading, and I am using general terms as to try to keep it simple, mainly because of the inertia to surface area aspect.
If you throw a 2 pound rock, it is less likely to be deviated by gusty winds than 2 pounds of loose leaf notebook paper.
This is because the rock has a much lower "footprint" in the wind than paper.

A 747 can land in 50 mph wind, mainly because of its weight and speed required to stay aloft.
The stall speed of a 747 empty is only 86 knots, or 98 mph, but fully loaded it is up around 150 knots, or 175 mph.
This is a considerably faster stallspeed than any model plane that of which I am currently aware.

I have a model with a wing configuration, much like UMD Pilot, Matt.
It is a scratch-built model that has a span of 48 inches, and a chord of 13 inches. The wingloading is about 14oz/sq ft.
The stall speed is about 15 mph, so if the wind is 8 gusting to 10, I could very easilty lose the plane on landing.
On the other hand, I have a Katana S 72" that weighs 15 pounds. The plane flies like a dream, wind or no wind, 10 - 20 mph, (it is also overpowered with a G38 in the nose).

Since the wingloading of my scratch-built plane is so low, and I have a OS .40LA in the nose, I took up a digital camera with an additional servo to take aerial photos. This was not the main purpose of this aricraft, but a bonus.
http://www.dutchessrc.com/pictures/sky1/index.html
This increased the wingloading, and made the plane handle very differently.
Landings were a little hotter, but where I noticed it most is in accelerated maneuvers.
High speed turns were noticeably wider, and when I stalled the plane, (intentionally), recovery took longer.
I also entered a few accelerated maneuver stalls, where you are at cruise speed and pull back hard, then the plane falls off to one side, (tip stalls, or snaps). This never happened without the camera, and is partly due to the added weight, or higher wingloading, but the plane could handle higher winds/gusts.

In normal flight, and high wind conditions, yes, a higher wing loading flies more predictably than a lower wingloading, but is not as much fun, in my opinion.


Bob

RE: Wing Loading: What is it really?
 

Even in gusty conditions I prefer a moderately lighter model. A heavier model may well appear to be flying smoother but when it does get into trouble it's harder to get it out of trouble and uses more room and needs more control input. So while I do not like flying featherlight models in gusty conditions I definetly don't recomend an overweight model. Like so many things in life there's the usual happy middle ground.

RE: Wing Loading: What is it really?
 

simply put wing area/wt in^2/oz

RE: Wing Loading: What is it really?
 

Bruce,

Very nicely put...
I have a tendancy to get too technical sometimes. :D
I actually like to fly lighter myself. The planes seem to be more agile, and I like to be on the edge when it is windy, but that's just me...

Bob

RE: Wing Loading: What is it really?
 

UMD.
Are you still with us ?

On the subject of a heavy plane. If the plane is tail heavy with a recommended engine in place. By all means put in a engine size that will need no ballest or butchering of anything. You can get the engines weights on a place like Tower Hobbies. Build a plane Put a weight in the engine area that is the recommended engines. Tailheavy ? Add weight till it is balanced. Buy a engine that weighs that much. Works everytime.

I found that all the wood ARF's took engines 2 X as large as recommended to balance.
Now we have planes that are scale, correct COG and fly vertically out of your hand. I like planes like that.

RE: Wing Loading: What is it really?
 

All,

Thank you all very much for your responses. It all makes sense. I will research some of these topics in a little more depth and when I have more questions I will return and ask.

cyclops2,
The tip about the weight and the engine is a great one. I will have to try it.

Thanks,
-Matt

RE: Wing Loading: What is it really?
 

In the early days of RC sailplanes, light wing loading was the goal (such as 4 oz/sq ft) and we were even advised to fly with the trims, so as not to increase drag by control surface deflections. These were "floaters," which might stay up a relatively long time but wouldn't go anyplace in a 5 mph breeze. It was less of a go-find-lift to a sit-there-and-wait-for-a-thermal. That was dying out by 1980, and we began to realize that the same models, ballasted to 7-8 oz./sq. ft., would stay up almost as long in dead air but would actually move around the field in search mode. We started to build them stronger and heavier, able to tolerate zoom launches without folding, getting extra altitude which translated to extra air time. It takes about 1 oz/sq. ft. of wing loading to make a perceptible difference. With an adequate and accessible ballast compartment, it was easy to experiment--find the best compromise and best performance. I'm talking about gliders in the 100-120 inch size range, in particular.
John Agnew