What is creating the downwash on a wing?! 1

[jerkosovo]

If the coanda effect is causing the induction of air above the wing to be pushed downwards, then what is actually doing this work? Is it just the boundry layer that induces the rest of the air movement above the wing? Thanks!

-Mark

 

[mohr]

Hi Jerkosovo:

The danwash represents the "wing induced angle of attack"
why ? because the real wing has a finite span.

If you have a wing with infinite span, your downwash angle
will be near zero ( the wing induced angle of attack = 0)

The downwash can be determined by integrating the contribution of the elemental trailing vortices (vortex theory) over the wingspan.

Regards
Mohr

 

[GeraldV]

Down wash is created by a spanwise movement of air over the wing, due to pressure difference between the top and the bottom wing. For equilibrium at the tip, vorteces are created, the familiar contra rotating vortex you see on formula one car. For infinitely long wings - as said above, the effect is not prominant.

 

[JeanAlesi]

Just to add to this, the primary purpose of winglets is reducing the 3D effects of a finite wings (downwsh).

 

[vortexman]

jerkosovo,

I think there might be some confusion about your question. In aerodynamics, the term downwash is used quite specifically. I think you are getting answers to a different question than you mean to be asking.

The way I read your question, you want to know what is the phenomenon that causes the air above the wing to be forced down. There are several ways to answer this. In a way, you've already answered it: the Coanda effect is simply the tendency of air ( and other fluids ) to follow a curved boundary, such as the upper surface of the wing. Like you, I find it useful to have a more intuitive understanding than that. Kind of a bottom line view is that the flow of the air is 'driven' by pressure differences and momentum. The motion of the air around the wing simply creates pressure differences and relatively high momentum regions that cause it to want to follow a curved boundary, under the correct conditions. If the angle of attack exceeds a certain value, of course, the conditions are not correct, and stall ( separation ) occurs, reducing or eliminating lift. In a way, just accepting that air is going to follow that curve, and consequently impart downward momentum in the air, is the most useful view.

Downwash is, speaking somewhat imprecisely, the relatively small downward component of the velocity of the oncoming flow which is due to the wing generating lift. This results in a small decrease in the actual angle of attack, which varies along the span of the wing. It is not caused by air spilling from the high pressure bottom of the wing to the low pressure top, it is caused by the very act of generating lift. This is very different from the downward velocity of the air above the wing as it approaches the trailing edge, which must exist if the flow remains attached.

vortexman

 

[jerkosovo]

Thanks for you help, everyone.

I guess I really didn't ask the right question, but vortexman understood what I was trying to say. Thanks!

I was trying to understand this better because I was trying to figure out if there was an alternative method to make a wing generate lift, such as small spanwise nozzle which would force compressed air above the wing. Similar to how a round, or curtain transvector amplifies the amount of air in/around it by forcing a relativly lower amount of compressed air through it. You can see some examples of these "airflow amplifiers" by doing a google search on 'air transvector'. I haven't seen this method being used to generate any kind of lift on an aircraft, so I was just wondering if it was even a viable option.

-Mark S

 

[vortexman]

jerkosovo,

You can definitely increase lift by blowing air rearward from a nozzle on the upper surface of the wing. There are several different flavors of this. If you want to do some searching on the web, try the following terms: "upper surface blowing", "boundary layer control", "blown flaps", "jet flaps". Generally, I would not characterize any of these methods as alternative ways to generate lift, but as ways to generate additional lift.

Good luck,

vortexman

 

[cawiddison]

Mark
The simplest approach (in my opinion) is that downwash behind a wing comes in two flavors :
(1) vortices that trail from (near) the wingtips cause downwash inboard of the tips and upwash outboard.
(2) Newtons's law that for every action there is an equal and opposite reaction - the lift on the wing is due air properties (speed, density and viscosity) and the wing section geometry. The lift is effected by pressure exerted on the wing surface by the air surrounding it - relatively low pressure on the upper surface and higher pressure on the lower surface. Newton's law mandates that the wing exerts an equivalent downward force on the air. Since there is nothing to hold the air stationary against this reaction it gains some downward momentum - downwash.

 

[Miper]

It is interesting to see the various physical mechanisms that can be used to explain different aspects of wing lift and the flow around an airfoil.

Yes, for a wing lifting up, the integrated pressure over the upper surface is lower than that on the lower surface.

Yes, there is a vortex flow pattern surrounding the wing. Lanchester first described it over a century ago. This vortex flow is superimposed on the wing velocity and results in an overall net downward flow aft of the wing.

Yes, the lifting wing and the surrounding air experience a momentum transfer, which keeps the airplane up and forces the air down. This downward moving air is the same air described above in the vortex paragraph.

 

[davidjh]

jerkosovo;

As Vortexman said your scheme could increase lift. However there are many pitfalls. I was involved in a Flt Test program some 35 years ago where jet engine bleed air was exhausted from a slot along the trailing edge of the wing's inboard leading edge high lift device. Orientation, smoothness of surface, and precision of the slot dimension were critical, not to mention symetry of left wing versus right wing. The lowering of stall speed was evident, however the stall characteristics became erratic and sometimes unsafe.

 

[Sparweb]

My favorite web reference on this topic is:

http://www.av8n.com/how/htm/airfoils.html

Once you can picture the circulation, both the downwash and the tip vortex become more obvious.


STF

 

[PuterUzr]

In my reading I belive I came across a wing design that used a horzontal fan in the upper surface of the wing that would exactly describe and define what is being discussed. The last I knew the inventors were building a scale model for flight testing.

PuterUzr

 

[edmacs]

It's interesting to cite that the additional lift generated using Coanda effect has been used in ekranoplan where small engines on top of canard wings help during take-off.

These small engines work only during take-off because the main engines are rather weak: optimized for the "plane" large L/D due to ground effects.

Eng. Edmar Silva