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klagldsf's (beginner's) guide to airplanes and aerodynamics
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Author:  klagldsf [ July 27th, 2012, 3:57 am ]
Post subject:  klagldsf's (beginner's) guide to airplanes and aerodynamics
This is a quick and dirty guide to how to draw aircraft - not how to design aircraft. I'm not looking to make aerospace engineers out of all of you (save the ones who already are), just in what an actual properly flying aircraft looks like.

I'll start with the quick and dirty basics and go up to more advanced concepts in progression. But first let's start with the most basic principle of all:

[ img ]

You may recall that a ship has certain forces that come into play. If you push down on the bow of a ship, the stern will raise up. Well an airplane isn't much different: if you push down on an airplane, it will go down. We generally refer to this force as "weight," along with other forces known as "lift," "thrust" and "drag":

[ img ]

These are the four basic forces that act on an airplane. When all four are equal (in equilibrium), the airplane is in straight flight (or sitting on the ground). Notice that I didn't say straight and level flight - all four forces are still in equilibrium if the airplane is in a steady climb or dive of constant speed.

Thrust is supplied by the engine and weight is supplied by gravity. As drawers and amateur aerodynamics, we are primarily interested in the other two - lift and drag (namely, how to minimize the latter and maximize the former - a bit hard to do since drag is a byproduct of lift, but we'll get to that some other day). Right now we'll concern ourselves with the very basics of drag.

The most obvious observation of drag is that the more surface area that is perpendicular to airflow, the more drag that is created:

[ img ]

This BTW is called "profile drag" and it's the one form of drag we will be concerned here with on Shipbucket. Anyway, the less surface area an object presents (or the more "sleek" it is, the more "aerodynamic" it is, whatever language you are most familiar with) the less drag will be produced. Most of you already know this.

However, there are other considerations for a low-drag aircraft. The smooth flow of air needs to be maintained in order to prevent unnecessary drag production. Smooth airflow is closely related to the concept of the "boundary layer":

[ img ]

The boundary layer is the layer of air in closest contact with the surface of the object. If the boundary layer is kept intact, the airflow will not be interrupted and minimal drag will be produced. However, if the boundary layer is interrupted (through unnecessary breaks, for example), the boundary layer will be lost and the airflow will become rough and produce vortexes, which for the most part will create drag (there are numerous and extremely important exceptions, but we don't need to concern ourselves with this just yet).

Anyway, that's it for today, I'll have to see how well people take this info to progress further.
Author:  indiajuliet [ July 29th, 2012, 1:26 am ]
Post subject:  Re: klagldsf's (beginner's) guide to airplanes and aerodynam
Great idea, klagldsf. Do you have any background in aeronautics, or aerodynamics specifically, or are you self-taught?

I'll be going into the third year of my Aerospace Engineering degree in September, so this is a little basic for me having spent quite a long time studying the principles of flight and aerodynamics, and in my opinion, in places you have simplified a bit too far, and some definitions differ from what I know and how I would try to explain the concepts, but a great start to help other members try to understand the theory behind aircraft design. I would be very happy to put additional explanations here or diagrams that I have or could make, just let me know. There are some great books on Amazon too, I've got a few on aircraft aerodynamics, motorsport aerodynamics, flight mechanics and fluid mechanics if anyone wants recommendations.

There are just a few little things I'd like to pick up on if that's okay, just for clarity.

I'll use the following notation: L=lift, D=drag, T=thrust and W=weight.

In straight and level unaccelerating flight, ie. in equilibrium, this does not necessarily occur when all 4 forces are equal. It is infact when L=W and D=T as when you sum forces with respect to a particular axis, such as horizontally, L and W would not come into it as they are perpendicular forces (in this case, vertical). If you think about the difference in magnitude between the thrust an aircraft produces and its weight, then these are often quite different values.

With regards to drag, only lift-induced drag is a by-product of lift, as the parasitic drag depends on other factors, consisting of the separate profile, skin friction and interference drags.

I think your 'smooth airflow' explanation could do with a bit about the difference between laminar, transition and turbulent flows, which links in with the boundary layer theory and it transitions from laminar to turbulent and the devices used to control/delay transition. The boundary layer is actually the only air in contact with the surface, defined as the height at which the local velocity is 99% of the free stream velocity, with the no slip condition stating that the velocity at the surface is zero, and therefore, a velocity profile of the boundary layer goes from zero at the surface to the height at which 99% free stream velocity is found, which then becomes the constant free stream velocity. Technically, the boundary layer is not lost, but separates. Although a very thin region, it is very very important.

Once you get past the definitions and concepts, then it's basically all about using equations to calculate and quantify design parameters, whilst trying to optimise the design for efficiency (civil) and/or performance (military combat), depending on the main role it is being made for.

What do you plan to write about next?
Author:  klagldsf [ July 29th, 2012, 11:17 pm ]
Post subject:  Re: klagldsf's (beginner's) guide to airplanes and aerodynam
I suppose it's useful but like I said, I'm not trying to give an aerodynamics course, but to coach through how to draw more realistic aircraft. This is why it concentrates on shapes so much, and why it's greatly watered down.

And as for my personal background, I'm basically an aerospace engineering dropout. I suppose that shows.
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