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How to calculate wing loading.

FlyingMonkey

Stuck in Sunny FL
Staff member
Admin
#1
There's a question as to what the maximum wing loading of the Bixler is.

So that raises the question, what is wing loading, and why is it important?

The wing loading of an aircraft is the measurement of weight, carried by each given unit of area.

For our purposes, wing loading is measured as ounce (or decameters) per square foot. (oz/ft2 or dm/ft2).


So, the first step is to measure your wing area. This is done differently for the various shapes of wings. In this case we're using the Bixler so it's pretty easy. You can just treat this wing like a large rectangle. Measure the length, and the chord (front to back) and multiply the two numbers. I don't have one in front of me, but a search on the internet provides me with the information that it has a 55" wing span, and a wing area of 403 square inches. (403 divided by 55 gives me a chord of just over 7 inches.)

We still need to know the weight of the plane. The stock Bixler is just under 24 ounces, and a wing area of 403. In order to calculate the wing loading, we need the wing area to be converted to square feet (ft2).


1) Convert the area to square feet. There are 144 (12 x 12) square inches in a square foot.

403 in2 ÷ 144 = 2.79 ft2

2) We have our measurement in oz already, but if you had it in pounds, you would need to do that conversion. There are 16 ounces in a pound.

1.5 lbs x 16 = 24 oz

3) Divide the weight by the area"

24 oz ÷ 2.79 ft2 = 8.6 oz./ft2

To round this off, we'd get a wing loading of 9 oz./ft2 You can also perform the entire calculation with one formula:

(Weight x 2304) ÷ Wing Area
In this formula weight is in pounds, and wing area is in square inches.

As an example:
(1.5 x 2304) ÷ 403 = 8.57 oz./ft2

For multi wing aircraft, divide the overall weight of the aircraft by the total wing area for all wings.



 

FlyingMonkey

Stuck in Sunny FL
Staff member
Admin
#2
This still doesn't get us the answer to the question above. What is the maximum wing loading of the Bixler.

The chart below will help figure what we're looking for.

[TD="bgcolor: #FF9900, colspan: 2"]

[/TD]

Loading

Type

10 oz/sq.ft

Glider

15 oz/sq.ft

Trainers

20 oz/sq.ft

Sport Plane

25 oz/sq.ft

Fighters



So, according to this, the Bixler has the wing loading of a glider, since it is under the 10 oz/ft2 that the chart defines glider as having. The further down the chart you go, the less glide the plane will have, and the more speed and power will be needed to keep it in the air.

You could double the weight of the Bixler, and at 48 oz flying weight, you would be flying with the characteristics of something between a trainer, and a sport plane.

If you were to triple it, flying at 72 oz, you'd exceed the 25 oz/ft2 that is defined as fighter wing loading. The wings wouldn't be doing you much good, except for stabilizing, and steering at this weight. You're pretty much flying off the power of the motor alone. Your stall speeds will have increased significantly, which will make it necessary to have a larger landing area, unless you wanted to try landing it straight down. :D
 

FlyingMonkey

Stuck in Sunny FL
Staff member
Admin
#3
Since I brought up the topic of stall speed, there's a formula for calculating that too.

3.7 x the square root of wing loading.

So for the empty Bixler, that would be 3.7 x 2.9 = 10.73 mph.

Compare this to the stall speed of the 3x loaded Bixler.

3.7 x 5 = 18.5 mph.

So, you'd never be able to fly the plane slower than 18.5 mph without risk of stalling. Or another way to look at it, you'd be doing nearly 20 mph at the point right before you would expect to touch down to land. In any sort of rough terrain, this would probably do some pretty impressive damage to the plane.
 

colorex

Rotor Riot!
Mentor
#4
Now that was something new to me! I didn't know you could calculate the stall speed! I'll have to do that on my glider!

Thanks a lot, Flying Monkey! You taught me something new! :D
 

FlyingMonkey

Stuck in Sunny FL
Staff member
Admin
#5
It's mostly an estimate. It is rather dependent on the airfoil. This formula is not going to be the same for a highly efficient glider airfoil, compared to a flat sheet of foam wing.
 

ananas1301

Crazy flyer/crasher :D
#10
Awesome stuff there.

So the way I understood the formula, you just have to multiply the squre root of the wingloading with the constant 3,7.

But how did you get to that constant?
 

ljonesii

Junior Member
#11
Hey guys, great discussion. Seems almost like a duh, but the wing load calculations never mention anything about the area of the wing that is affected by the fuselage. Do you all subtract that part of the area when making your calculations. Calculators are cool, but it is not always indicated what the assumptions (if any) are that have been made in setting up the calculator. Thanks for your input!
 

earthsciteach

Moderator
Moderator
#12
Any calculator is full of assumptions. In terms of wing loading, the area within the fuselage is completely disregarded. In most cases, this is completely reasonable. If we are talking flying wings with a somewhat blended body (like the one I am building) the wing loading shouldn't discount the "fuselage" area. What do you have in mind, jonseii?
 

lobstermash

Propaganda machine
Mentor
#13
Seems almost like a duh, but the wing load calculations never mention anything about the area of the wing that is affected by the fuselage. Do you all subtract that part of the area when making your calculations.
A lot of fuselages also contribute to lift, depending on the aircraft design. Your stabilisers can also generate lift. I'd suggest that taking the fuselage and stabilisers into account is probably not going to make much difference to the wing loading calculation (in most cases).