airfoil vs thin wing

[justin]

does airfoil wings really make a big difference in speed? or thin wings would be better? i know that airfoil wings will have more lift. however, there would be more drag becuase the width is quite big.

so should i get airfoil wings or thin wings?
do not bother about their strength . pls advise. thx

[oceanus]

You don't determine speed by the wing's thinkness alone. What you want to achieve is an airfoil that can produce the lift required and at the same time, with minimum drag. A thin airfoil has less drag than a thicker wing, but it produce very little lift, and so thw airframe requires comparitively more wing area than one with thicker wing, thus more overall drag.

Although a thicker wing is supposed to create more drag, but it produce a lot more lift than thin airfoil. Thus, the airfarme only requires a small wing to achieve the lift, and so less overall drag.

Engineers usually choose an airfoil based on lift/drag coefficient rather than the wing thickness.

BTW, a flat plate wing, although has very thin cross-section, produce very little lift and plenty of surface drag. The boundary air usually formsturbulent airflow and creates a lot of skin drag. You may not feel it when you fly full power all the way, but the moment you retard the throttle, you'll find your plane drop like a brick.

[justin]

eh.. the wing area is fixed . u cannot change wing area.
what i meant was cross section of 0.4 cm(flat) vs 0.7cm (airfoil)

the extra 0.3cm of airfoil can create a faster speed? does it affect the speed greatly? or issit just a small factor.

[oceanus]

Justin, you do not compare airfoils this way.

If you are asking me whether a gust of wind blowing a across a 1 meter high wall and a 2 meter high wall, I'll tell you that the 2 meter high wall will definitely produce more drag. But to compare airfoil, you must understand how the air molecules travel along the wing surfaces.

In fact, the thicker wing with an airfoil will be less draggy than your thinner flat plate wing, as the air can run smoother across the surfaces. The flat plate, although being thinner, will create many vortices at the boundary layer of the airflow, which creates more drag.

Okay, assuming the wing are is fixed, since the flat plate wing produces less lift, you'll also need to fly at higher angle of attack , for the wing to produce the same lift compared to the airfoil wing. So do you think that the drag will also increase?


If you have time, why not build both and see for yourself?

[xhaohaox]

A thin airfoil has less drag than a thicker wing, but it produce very little lift, and so thw airframe requires comparitively more wing area than one with thicker wing, thus more overall drag.

so in a way, yes, thinner airfoils will yield faster speeds as the plane need to fly faster to produce the lift needed to lift the weight of the plane.

[justin]

okay.. there are 3 options. airfoil wif cross section 0.4 cm, 0. 7 cm or flat wing cross section 0.2cm.
which one is the fastest?

[xhaohaox]

my advice, try all 3 and tell us the results! out of gut feeling i think the airfoil with 0.4cm cross section will be the fastest. Gut feeling only, dun blame me if its not.

[justin]

dun wanna waste time dats y i'm askin for advise
theres actually a formula to calculate lift budden i lazy to work out cuz its super complex..
btw heres a simple way to calculate
bud i dunno how to use it

http://www.lerc.nasa.gov/WWW/K-12/FoilSim/index.html

[thong]

Airfoil Simulation

Found interesting web page regarding Airflow...http://www.lerc.nasa.gov/WWW/K-12/freesoftware_page.htm
Hopes it helps.THANKS

[oceanus]

It's not as simple to compare this way. Yes, in upfront it looks like the 0.4cm may be the fastest. But you do not know the all up weight of the airframe yet.

The equation is as such :


Lift = CL x r x (v²/2) x A

where : r = air density
v = velocity
A = wing area
CL = Coefficient of Lift

CL = depends on airfoil design and angle of attack which is usually plotted as a graph by experimenting in wind tunnels.

For level flight, lift must be equal to the weight of airframe. Assuming the air density , velocity and wing area are constant, the only factor left to compare is the Coefficient of lift. So now, you arrange the equation and find out what is the Coefficient of lift needed to achieve level flight.

CL = weight(lift)/ [A x r x (v²/2)]


Once you found this figure, you have to draw a line across the graph plotted by using wind tunnel testing which coincide with the CL you worked out. From the curve, you can find out the angle of attack to achieve. Apart from CL, the CD ( Coefficient of drag ) of the same airfoil is also plotted by using the wind tunnel. With the angle of attack you found out form the previous CL graph, draw a line across the CD which coincide with the angle of attack found earlier.

Now you know the Coefficient of the drag it creates to achieve level flight of the given velocity, wing area, air density,weight.

Next do the same for the other airfoil and compare the Coefficients of drag for both airfoil. the one with higher CD means more drag created for the same speed, which also mean more power is needed to overcome it. So for the same power rating, you can say that the one with higher CD will be slower, right? If you are interested, you can even work out the CD equation to find out how much drag force is being yielded in figures.

So, am I confusing all of you? All I want to say is that it is not a simple comparison like what you think. You must see all the factors that acts on the entire airframe before you can come to a conclusion. If you wings are not meant to produce lift and just to be dropped vertically downwards, then I can tell you that the thinner wing will travel faster.

[xhaohaox]

wah...very confusing. much simpler to build all 3 and test.

[oceanus]

Yah, when I said to build and try, that's what I mean. You want to compare on paper, I give you formulae. Now you are confused with them.

[oceanus]

If you want to save youself some wind tunnel testing trouble, use proven airfoils and compare the graphs. But of course, workmanship does causes some deviation. If you just want some proven airfoil, I reccomend MH30, and MH43, proven pylon racer airfoils.

[justin]

wads mh 30 and mh43?
got any example?

[justin]

issit also true dat a larger wing area would be better in gliding after the engine has cut