Why do the leading edges of aerofoils need to be rounded?

[hollus]

I borrowed this from Astracrazy, who borrowed it from Scarbs:

...you need to have depth to parts too... XX mm for example... (by looking at f1 cars).




These Kashmin threads are feeding us some unoptimized designs, which are good food for thought.


So, to the point: Aerofoils are rounded in the leading edge and sharp in the back. At the back it is important to help the two flows merge smoothly, while at the front it doesn't matter too much since the obstacle the the airflow and the resulting stagnant high pressure area will force the air around an aerofoil shape anyways, so it is better to guide it like that (or at least that is my understanding).

But: do we really need a rounded leading edge? Wouldn't it be better to have an infinitely thin wing (not just at the edges, but along the whole span), to reduce the leading edge pressure buildup? Is the roundness just the consequence of the aerofoil needing a minimum rigidity in real life to keep its shape and support the load? Would an infinitely thin foil made of flattened unobtanium (with an optimized shape) perform better than its actual chubby incarnations?

I suspect the answer is not, but why not? Is denying the air some otherwise available volume helping generate downforce (lift in planes)?

[flyboy2160]

the rounded shape of the airfoil helps keep the air attached as the airfoil angle of attack increases. and you need some depth for structure.

modern control surface aerodynamic design does not taper the trailing edge to a sharp point, but instead truncates the shape in a sharp edged flat that is a small % of the wing chord. this flat keeps the trailing edge separation at those sharp corners. older designs with rounded trailing edges or sharp points can allow the trailing edge separation point to wander around - from top to bottom even - which plays havoc with the forces on the surface.

[hollus]

the rounded shape of the airfoil helps keep the air attached as the airfoil angle of attack increases...

Do you mean that as a result of thickness the curvature of the upper surface is different than that of the lower surface? Since one flow is "pushed against" the wing and the other one is fighting "moving away" from the wing (I like to call this working in Coanda effect), it would make sense that the respective curvatures need to be different. Any guiding principles?

[shelly]

Yes curvature on upper and lower surface is different, and that gives you lift /downforce.

At the leading edge you have a stagnation point in 2d / stagnation line in 2d; from there, where the air has 0 relative velocity to the aerofoil, the flow splits and accelerateson the two sides - upper and lower. When I say accelerates, I am deliberately not considering the thin -very thin- layer closer to the aerofoil, in which velocity increases from zero (no slip condition) to the external velocity of the flow field.

If we focus on an inverted wing producing downforce, the lower split part of the flow will experience a greater acceleration, which will lower its pressure more; on the other side, the flow will accelerate too, but less. As a result pressure will be lower against the lower surface etc etc.

@f1_aero (f1 aerodynamicist posting on twitter) has recently linked this useful lesson:
http://www.cam.ac.uk/research/news/how- ... eally-work

Speaking of leading edge roudness, it is worth noting that pointy leading edges (with very small radii, almost knife edged) are needed in supersonic flight (see sr 71 chimes or f104 small wings) because at supersonic speeds air behaves in a very different way.

For a f1 car at subsonic speed you need a rounded leading edge instead - the rund shape it is needed to have an accelerated flow line following the surface.
For the air to follow a curved path you need a centripetal pressure gradient or, stated in other way, swapping cause and consequence as both things happen at the same time, if a flow pathline is curved pressure on the inside of the curve is lower than pressure on the outside

[flyboy2160]

H,

No, the curvatures don't need to be different, but they almost always are for the most lift. Aerodynamic airfoils are symmetrical top to bottom because they are supposed to 'work' either side up.

The rounding is, as you correctly figured out, a 'sticking to the surface way' of changing the direction of the flow. this curving change of direction keeps the air attached longer than would a a sharp change of direction at the edge of a flat plate.

But some planes do indeed have 'flat plate' airfoils for their empennage surfaces. The old WWI and even some modern day planes, like the Pitts and Decathlon, have, essentially, flat plate airfoils in their horizontal tails. The noses are round tubes that make up the structure. They way to keep these from stalling before the contoured wing airfoil is to make them a lower aspect ratio than the main wing and at a lower installed angle of attack.

[hollus]

...a sharp change of direction at the edge of a flat plate.

But a flat plate does not need to induce a "brusque" change of direction if it is aligned with the incoming air. Would a letter "l" rotated 90 degrees anticlockwise have any attachment issues at the leading edge? I cannot see how.

[flynfrog]

Airfoil shape is heavily dependent on speed. As you get to higher mach numbers you see a more diamond shape air foil. At slower speeds like F1 and civilian planes the round leading edge promotes attachment to the air foil. The sharp edge at the aft end insure a clean separation. you can always play around with foilsim for some basic quick analysis. http://www.grc.nasa.gov/WWW/k-12/airplane/foil3.html

[No Lotus]

A rounded edge is necessary to give a broader drag bucket. What that means is that the airfoil performs well at a broader range of angles of attack. A sharp edge is actually fine IF the airfoil is kept at its designed angle of attack.

[flyboy2160]

...a sharp change of direction at the edge of a flat plate.

But a flat plate does not need to induce a "brusque" change of direction if it is aligned with the incoming air. Would a letter "l" rotated 90 degrees anticlockwise have any attachment issues at the leading edge? I cannot see how.

H, you should study some airfoil design. A flat plate doesn't produce any lift (or drag) if it's aligned to the flow. It must be at some angle of attack to the flow to generate lift. The air rushing past the nose on the top side has to change direction immediately to follow the 'down' sloping plate. It's easier for the air to change the entire angle of the plate if it does so in a gradual manner along a curve rather than all at once.

[Lycoming]

it'll still generate drag at 0 AoA. Not very much, but not immeasurable. Perfectly flat and perfectly aligned with the oncoming flow does mean no lift though. That's true of any symmetric body. Thats also assuming you're not near any walls or the ground.

[NoDivergence]

it'll still generate drag at 0 AoA. Not very much, but not immeasurable. Perfectly flat and perfectly aligned with the oncoming flow does mean no lift though. That's true of any symmetric body. Thats also assuming you're not near any walls or the ground.

He's thinking inviscid flow. In which he'd be correct (for an infinitely thin flat plate). In reality with viscous flow, there'd be friction and growth of the boundary layer along the plate.

The reason why there are rounded leading edges is for keeping flow attached at increasing angles of attack as well as increasing the suction peak on the low pressure side of the airfoil. Of course, this should be balanced with proper pressure recovery (in the case of relatively low Reynolds numbers (not supersonic)).

[flyboy2160]

Duh, brain fart while typing. You guys are right, obviously: flat plate has no lift at O AOA, no drag in inviscid/ideal flow, drag in viscous flow.

[hollus]

I was thinking more of a section or of a cylinder's perimeter. Only the first bit would be aligned to the incoming flow, the air would then start to be deflected in a gradual manner. A 30 degree section of the letter "O" is a better analogy than a rotated "l".

Nice insight for now, thanks to all.

[Kiril Varbanov]

Yes, the explanation is simple - sharp-ish leading edge will cause airflow disruption and thus lead to drag and loss of lift (case applied to the direction of the wing).
If the trailing edge were rounded, the higher-pressure air flowing along the lower side would try to follow the rounded surface and spill upward into the lower-pressure air above the wing. A sharp trailing edge prevents this upward spill, because air cannot make a sharp turn. Instead, the air flowing off the top and bottom surfaces rejoins smoothly.

I'm recently lot into morphing wings via some de-classified NASA documents, but perhaps we need to open a new thread for that.

Edit: Further resource sharing from NASA: AirFoil sim.

[tok-tokkie]

Here is the Cambridge video:
http://www.youtube.com/watch?v=UqBmdZ-BNig

It clearly shows that the air does flow faster on the upper side.
What puzzles me is what happens afterwards?
If the leading edge of the upper & lower flows never got back into sync then there would be an accumulation of air along the upper path - because there is more air leaving the upper trailing edge each second.
Obviously that is nonsense since precisely the same quantity of air is entering each path of the system on the left so the same quantity must be leaving each path of the system on the right.
OK the video is not showing the air leaving the system on the right - that is still further to the right & it is in that unshown area that the mass balance must be restored.
What happens? Just a lot of turbulance as it sorts itself out?
But that contradicts what Kiril says in the post above this.
Going back to the statement I made that there is more air leaving the upper trailing edge each second. That does not make sense because where did the extra air come from?
The air has expanded to fill a bigger volume?
So if it has expanded then it is not incompressible flow?