Beginners Guide to Aerodynamics
I am currently having diagrams and graphs drawn up for inclusion on this page to help ease the learning curve and facilitate understanding of these sometimes highly complex issues!!
There are 4 forces which act on an aeroplane: LIFT, DRAG, THRUST and WEIGHT.
If THRUST is equal to DRAG the plane will maintain a constant speed, if the wings generate enough LIFT to equal the WEIGHT the plane will maintain a constant altitude
However, increase THRUST with the above model in mind and the following will happen:
At this point in time another 2 effects will come into operation:
Drag, the two main types...
There are 2 main types of Drag worth knowing about, these are:
Prasitic Drag is a force most of us have experienced when we've stuck our hand out of
the window of a moving car! Asides from the fact that it's a dangerous thing to do, it is
a good way to examine the drag imposed on a 25 ton fighter. What is obvious is that as
speed increases so does Parasitic Drag, BUT it does not do so proportionally -
double the speed and you will effectively QUADRUPLE Parasitic Drag! Aeroplanes
with swept wings experience a reduction in Parasitic Drag.
Induced Drag is somewhat different from Parasitic Drag, and is associated with Lift. Induced Drag is a result of Air Molecules 'sensing' that they are about to flow over a Wing. As air flows towards an wing it is described as the Remote Free Airstream - the undisturbed air ahead of the wing. This airstream does not cause Induced Drag. The airstream which causes Induced Drag is the Local Airstream 'sensing' its arrival over the Wing.
As the Remote Free Airstream becomes the Local Airstream it 'senses' its arrival over the Wing, it therefore comes in from a higher angle than when it had come in from the Remote Free Airstream. The Wing must therefore increase its AOA to get the air coming across its surface at a perpendicular angle i.e. straight across it. This air is called the Relative Airstream (that coming straight across / perpendicular the wing). This increased angle moves the Lifting Force back a little way reducing the Effective Lifting Force. The difference between the Total lift on the Effective Lift Force is Induced Drag.
Vorticies also aggravate Induced Drag - As the positive pressure on the underside of the wing is GREATER than the negative pressure on the upper surface of the wing it tries to move into the area of less pressure on top. It does so by trying to slide over the tip of the wing and hence creates Vorticies. This Vortex mixes with the Relative Airstream and aggravates Induced Drag.
The formula for lift and drag show how these factors come into effect:
L = Cl ½p V2* S
D = Cd ½p V2* S
These are some of the simpler equations available to to allow Lift and Drag relationships to be explored. There are however 2 other factors which effect Lift and Drag:
Alpha is defined as the angle at which the Wing cuts local wind coming across it
With this all in mind it can therefore be seen that Cl and Cd alone can be used to compare different wings for the simple reason that all other factors cancel each other out. (see above equations)
Having said that it should be remembered that Cl and Cd are both functions of Alpha, and as such, different wings have different characteristics at differing degrees of Alpha:
Yeah, Yeah...but what does all this mean to me: a humble PC Pilot...?!!
L/Dmax has three important implications on your plane and the way you fly it:
Bear these three factors in mind when evaluating the 'Realism' of a flight sim and whilst evaluating your aircrafts performance - But to gain a deeper understanding of how your flight sim should handle read on...
Right, back to technicalities...
Maximum 'G' is the number of times the pull of Gravity a wing can take before it either:
Often referred to as Instantaneous G it is often achieveable only momentarally,
as airspeed and thus lift will decay rapidly during high G manouvers (Some aircraft are
exempt from this rule, for example the latest block F-16, which can maintain a force of 9G
and acclerate whilst doing so)
G creates more drag and it should be remebered that:
Ok, so lets try and simplify things - if there isn't enough thrust to overcome drag at Maximum G then one of 2 things will happen:
If the latter occurs, we call this point Sustained G. Whilst Instantaneous G is usefull in a situation where you need to pull the nose of the plane up for a quick snap shot, or to break hard in order to shake a bandit off your tail, Sustained G is more Advantageous Tactically: the ability to sustain a given G is a measure by which one fighter will be able to continuously outurn another fighter
Right, lets start looking at the real application of all this...!
W/S or Wing Loading (W = weight, S = Area of Wing in square feet) ties in with what i've just mentioned above: Sustained G is also a function of Cl divided by W/S
Therefore: If you have two fighters both weighing the same generating the same thrust, with the same aerofoil - the fighter with the larger wing area will be able to sustain higher G's, i.e. will have a higher Sustained G ability and will therefore be able to outmanouver the other fighter.
Again, comparing 2 aircraft in terms of their Sustained G performance at any given altitude and Mach number is really a kind of shorthand for Cl, T/W Ratio ( Thrust to Weight Ratio) and various other parameters.
Specific Excess Power or Ps is a measure of the difference between an aeroplanes Drag and its Thrust
Wings generate Lift, but they also create Drag as we already know:
A small wing does the following:
A large wing does the following:
Regardless of wing size, when Thrust exceeds the amount of Drag
generated, the aeroplane can climb or increase speed or do both and it is this excess wich
is labelled Ps.
(Ps is measured in Feet per Second)
Ps Formula =
Ps = Thrust minus Drag x Velocity, divided by Weight
Tactically speaking, an aeroplane with a high Ps should be able to out accelerate, out climb and outurn an opponent with a lower Ps