can you explain wing loading?

[jetman12]

Hi all, I was trying to get some help if someone could explain wing loading and the effects it has on performance? Thanks

[wangus]

If I was asked this in an interview tomorrow......!

Wing Loading = Aircraft mass divided by wing area. The more loading of the wing, the more lift the wing must produce. To produce more lift, you must either increase your angle of attack, thus increasing induced drag, or increase your velocity (as per the Lift formula), increasing your parasite drag (unless you are on the left side of the drag curve and about to die.) The stall speed in all configurations will be increased, as will VMU, VR, V2, Appraoch Speed, VRef, etc. While performing steep turns, the stall speed can be much higher than anticipated. More thrust will be required to maintan a straight and level speed, such as Mach 0.79. An empty 737 flying next to a 737 carrying it's maximum payload at the same speed will be at a lower thrust setting. To fly faster requires more thrust, which means more fuel, which means more cost to the operator.

(Am I talking absolute nonsense????? That's how I would explain it to the HR people at interview. Please be gentle in your criticism!)

[Rj111]

Wing Loading = Aircraft mass divided by wing area.

I think you should have stopped there.

[fly_antonov]

Wing loading - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Wing_loading)

Higher wing loading drastically reduces take-off and landing performance.
That' s why you use Fowler-type flaps during these phases on airliners with high wing loadings.

The fuel economy is not necessarily worse for aircraft with a higher wing loading.
Otherwise we would see A320' s with A380 wings.

The reasons for this are:
In the formula of drag, increased S can significantly increase the induced drag.
Or another problem is that an increased S can increase mass.
Increased S can also decrease CL, depending on how the additional surface is won.

It all comes down to the right wing loading for the right speed.

[Rj111]

In the formula of drag, increased S can significantly increase the induced drag.
Or another problem is that an increased S can increase mass.
Increased S can also decrease CL, depending on how the additional surface is won.

The only reason is increased weight, presuming both wings are of similar design/technology. An A320 with A380 wings which somehow weighed the same as a regular A320 would be far more efficient.

[fly_antonov]

I' m not so sure about that :ok:

What happens to parasite drag and induced drag when the surface increases?
The bigger the wing, the bigger the vortices and the bigger the parasite drag.
How much bigger will also depend on the overall mass, there I agree, but it will be bigger anyway.

How does the A319' s efficiency (fuel burn per seat) compare to the A320 with the same wing? How does it compare to the A321? There may be a slight advantage but it will be dodged under the advantage of decreased parasite drag per seat of the longer airframes.

Extended Fowler flaps do not increase mass and yet they increase induced drag as well as parasite drag (through added camber and surface resisting to dynamic pressure).

[Rj111]

Induced drag would be higher for the same IAS but you could fly at a lower IAS at a ludicrous height.

Parasitic drag is negligible compared to form/induced.

[fly_antonov]

Form drag is part of parasite drag.
Parasite is dominant over induced above the minimum drag speed.

You are correct when saying that increased area (S) will decrease induced drag as long as the aspect ratio stays the same or increases (going towards the "infinite wing"). But I am correct in saying that the parasite drag is the dominant factor over the induced drag above the minimum drag speed.

A bigger wing area will add friction and form, hence increase parasite drag.

Graphically speaking, by adding surface area to reduce wing loading, your parasite drag curve will go higher on the Y axis.
Your induced drag curve will go lower on the Y (drag) axis but nowhere as much as the parasite drag curve goes up. This will shift your minimum drag speed to the left on the X (airspeed) axis.
That will bring your total drag curve higher on the Y axis and shift it to the left.

Plot it on a graph and you will see what results.

Thrust = total drag, so in the end you will cruise significantly slower at the same thrust setting, doing less distance per hour at the same hourly fuel burn. When flying above the minimum drag speed of the wing with the smaller surface and higher wing loading, your aircraft will be less efficient.

[Rj111]

It's simple...

You wing's L/D ratio won't change just because it's bigger - yes there will be more drag but there will be more lift too.

You'll be able to fly much higher reducing form drag on the fuselage.

Your IAS will be slower but your TAS will be the same.




Disclaimer: This may or may not be completely wrong.

[fly_antonov]

Ok, let' s take an example.
Imagine a Cessna 172 with a wing the size of an A380 wing, but the same mass and design as a 172 wing. This results in a very low wing loading.

Your stall speed is going to go down to 5 kts, which opens up your flight envelope, or so you think.
The problem is that with the same engine/power, the 172 will reach a top speed of 15kts because the dozens of times bigger wing is going to create more parasite drag. The increased drag will be such that the small engine will barely be able to keep up with it.

If you maintain the same design and the same lift to drag ratio as you suggest, the glide ratio stays the same, only the gliding speed will be different.

Your coffin corner or buffet boundaries won' t get any bigger because for the same wing design but X times bigger, your Mcrit will be lower.

When manufacturers design an aircraft, they first determine a small range of cruise speeds they want to work in, after conducting studies. Once they determine this range, they adapt the wing loading to meet the requirements of this range. Hence, the right wing loading for the right cruising speed and altitude.

There is a multitude of factors including engine performance that goes into determining the right cruise speed and altitude. Turbofan engines are most efficient at high subsonic speeds.

I do agree though that there is a benefit to low wing loading and that is the faster climb to cruising altitude.