# What effect does wing loading (high and low) have on aircraft performance?

I've read that stalling speed is partly determined by wing loading. Correct me of I'm wrong - "Aircraft with high loading have more weight relative to their wing area hence the stall speed is increased (as the requirement for lift is increased)".

Wing loading is, as a North American industry standard, defined as weight/wing reference area (W/S). But then again, there is nothing wrong to define it as mass/area; so be careful of comparisons against literature.

As you have correctly cited in your OP, a large wing loading inversely affects performance, this includes takeoff performance and turning performance, all due to increased stall speed. Assuming a constant maximum lift coefficient, the larger the wing loading, the higher the stall speed:

$$V_{{s}}=\sqrt{\frac{W}{S}\frac{2}{\rho C_{L_{max}}}}$$

Large wing loading, however, is beneficial from a ride comfort perspective and offers structural benefit (i.e. weight saving) when considering gust loads. Assuming the same level of gust, the faster an airplane flies, the higher vertical acceleration (G) it experiences. A particularly salient and approximate relationship can be found in 14 CFR 23.341 (pre Admt 64), whereby the load factor from gust is given by:

$$n_z=1+\frac{K_gU_{de}Va}{498(W/S)}$$

In the above, $$K_g$$ is the gust alleviation factor which increases with W/S, $$U_{de}$$ is the gust speed in ft/s (found in 23.333 and decreases with altitude), $$V$$ is equivalent airspeed in kt, $$a$$ is the lift curve slope. As you can see, the load factor decreases with increasing wing loading and increases with increasing airspeed.

For the purposes of weight and balance and expressing wing loading values, weight and mass are the same thing and the terms are used interchangeably (you will see "Weight and Balance" and "Mass and Balance" used in documentation).

So wing loading refers to the ratio of the area of the wings relative to the total mass or weight. Wing loading is normally expressed, in North America, in lb/sqf, and in metric jurisdictions, in N or Kg/sqm.

As you go faster, the wing is able to support a given mass/weight with less and less total area, and it's more efficient overall, if your priority is to go fast, to reduce the wing area, while keeping weight/mass the same, to take advantage of that. This is because the airfoil's most efficient AOA (max L/D) is at a fairly high angle and from a speed-efficiency perspective, you want to cruise as close to that AOA as possible to minimize induced drag.

The side effect of this is still higher minimum flying speeds than otherwise, but if your priority was to go fast, you live with that. Wing flaps allow you to cheat your way, at least partly, out of this problem, by making your small wing work a lot harder at low speed than it would without flaps, then you can retract them so they are out of the picture when you want to go fast.

It's not that different really from water skiing on a small ski vs a big ski. You can go faster on the "high ski loading" small ski, but it has to go faster to get on the step.

The small ski will also ride wavelets a lot better. Same with wings; one benefit of small highly loaded wings is a better ride in bumps and you need that characteristic, since you are going faster, if you want to do it comfortably.

You see this effect clearly with light aircraft. The Thorp T-18 gets by on only 86 sqft of wing area at a gross of 1600 lbs, to cruise at 180 mph. The Pazmany PL-2 has more or less the same overall configuration and all up weight, but has 116 sqf of wing area, a significantly lower wing loading, and cruises about 25-30 mph slower on the same power and most of that is due to the larger wings and lower wing loading.

The Thorp also lands and takes off about 10 mph faster, which is bad, but it also rides bumpy air way better, which is good (I own a PL-2 and have flown in a Thorp in the past).

The aviation gods giveth, and they taketh away...

• "Weight and mass are the same thing. " No they aren't. In space, an object can be weightless, but it will always retain its mass. Weight is measured in N, mass is measured in kg. – Koyovis Dec 15 '19 at 3:46
• Well you don't have to get all science-y. For our purposes it is. Airplanes for W&B purposes aren't measured in N, only kg or lbs, and the terms weight and mass are used interchangeably. – John K Dec 15 '19 at 4:46
• Newton is weeping in his grave. – Koyovis Dec 15 '19 at 12:26
• Well you can write nasty letters to all the Weights Group managers at the various OEMs if you like. – John K Dec 15 '19 at 15:50
• Yes. "Weight" is just mass being attracted by gravity, so technically mass is the better term, but the world has been using the term "weight" forever. But like I said, in North America at least, wing loading is expressed as pounds per square foot, based on all up weight, or mass if you will. – John K Dec 16 '19 at 2:27

On stall speed, as per this answer, the wing loading is defined as: $$\frac{L}{S} = C_L \cdot ½ \rho V^2$$.

L is the lift force provided by the wing, at 1g steady flight the wing only needs to counteract gravity, so L = W. Note that it is required to use lifting force L: in manoeuvres the wing needs to provide more lifting force, and the wing loading increases.

So with a given wing profile and $$C_{Lmax}$$, at higher wing loading the stall speed is higher.

Yes indeed, at constant $$C_L$$ and wing area S, more lift can be created at higher velocity. Or less wing area is required for a given lift compensating weight.
• This is conflating wing loading $W/S$ and load factor $n$. $L/S$ = $n W/S$. Both expressions are mathematically correct but in terms of standard usage of the terms $L/S$ is not referred to as wing loading. – Chris Dec 18 '19 at 15:17