On todays episode of “what was Squirrel under the influence of while writing this”, we’re going to take a look at aircraft configurations where the overall appearance of the aircraft is concerned, why certain aircraft look the way they do and how configurations of an aircraft impact it’s flight characteristics.

Basically, what we did with the Engine Types series, but instead of barbequing chickens, we’re trying to help them fly. For all intents and purposes though, we’ll be looking primarily at fixed wing sky sausages.

Background

As briefly touched upon in Engine Types: Part 1 there are 4 fundamental forces involved in flight:

1. Thrust
2. Drag
3. Lift
4. Gravity

We’ve already covered the first two, which leaves lift and gravity for me to milk 5 parts on, the last of which will probably arrive in 2050.

In order to fly, we need to generate lift. Slapping wings on something is a great way to do this as they have a tendency to generate lift. How do they generate lift you ask? …or maybe you don’t because you’re a shameless nerd like me.

First of all, I should probably introduce the axis of which an aircraft can move around (you’ll see why later). These are known as Longitudinal (roll), Lateral (pitch) and the normal axis (yaw).

The axis of which an aircraft can move around

The Wings

Aircraft generate lift by abusing the laws of physics through the use of something called an aerofoil. These typically have a greater curvature on the top than they do the bottom. I’m not going to go into great detail about how it works, because it’s really quite complicated and I’m not a fluid mechanical expert, so I’d do a terrible job of explaining it.

But a long story short, because of the conservation of mass, energy and momentum the air at the forward top of the aerofoil is effectively squashed into a tighter space because of the steeper curvature, meaning it accelerates, causing an area of low pressure on top of the mid-section of the aerofoil but also due to the colander effect, the air exits at a downward angle, which generates some additional Newtonian lift (Newtonian lift is basically air being forced downward by an object).

Conversely, the air below the aerofoil doesn’t get as squashed, meaning the area of pressure is higher at the lower mid-section, but additionally, if the rear of the aerofoil is angles downward, it also generates Newtonian lift.

I’m sure I’ve described something wrong in that last bit, but my knowledge of fluid mechanics is comparable to what’s in my fridge when I’m hungry… THERE’S NOTHING THERE. But I tried, so I at least deserve a participation trophy.

So, we’ve got a means of getting off the ground and if we slap some ailerons on there, we have a means of rolling the aircraft and because wings typically take up a large area, we have some longitudinal stability as a result. But how do we stop a plane pirouetting into your minecart village or front flipping into Tully’s house?

Sure, we have flying wing aircraft like the B-2 Spirit. Presumably named after a strong distilled alcoholic beverage… I imagine a B-2 probably handles like a drunk person once you remove its flight control computers. But that’s just a theory… A PLANE THEORY.

So as alluded to in that last bit, a flying wing is generally considered very difficult to fly. There’s nothing to stabilise it along two of its axis; the normal and the lateral and thus flying wings are an inherently unstable configuration, subject to terrible stall and flat spin characteristics.

The Tail

Naturally, if a winged sausage design is unstable for use in flight, you want to introduce two aspects to it; stabilisation along the lateral axis and stabilisation along the normal axis of the aircraft. Which is why most traditional aircraft have a “tail”.

For those of you unfamiliar, the tail section of the aircraft is the aft portion which typically houses the horizontal and vertical stabilisers. Think of them like stabilisers on your bike, except instead of stopping you falling off to one side on your bike into the nearest ditch, they help stop you tumbling from 30,000 ft into the nearest hillside.

Now, in order to fly with some degree of stability the centre of lift has to be behind the centre of gravity (also known as CG). Now if we take a sausage and follow this rule when we put wings on it, the CG of the sausage will be in front of the wing.

Sausage with wings

So, if you simulate lift, by picking it up from the wings, the sausage will tip forward. That’s a problem.

It’s a bit pointless generating lift if all it does is slap your nose face first back into the ground.

So, by adding horizontal stabilisers, you’re able to use them to generate a small amount of downforce to counter the pitching down movement of the aircraft as it generates lift.

Now what happens if you blow at your assembly from the side? It’ll spin, right? Which again is a problem if your flying in areas where wind speeds are very high, because one minute you’re flying along minding your own business and then the next minute you’re god’s fidget spinner.

But, by slapping a large vertical structure at the back of the aircraft, you’ve basically created a large wind dial that always naturally wants to point into the wind instead of something that wants to pirouette everything to death.

Wind chicken

Conclusion

And that’s why the typical aircraft looks the way it does. Because these features provide a stable flying platform on which to ferry people with… Although, hopefully, they aren’t made of sausig:

Obviously, these surfaces also provide a means to mount control surfaces on. Hence you're pitch, roll and raw controls on an aircraft.

But since we’ve covered the basics, we can move into some more interesting configurations and how they compare to the “standard” layout in the next instalment.

Next Post: Aircraft Configuration: Part 2 (Wing Stability)
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