Expert: Paul Soderman - 4/8/2010

Question
QUESTION: Hi Paul.

I know that there are two types of ground effect.
1st one is air cushion effect (ekranoplans)
2nd one is venturi channel effect (race cars)

Can you explain the difference between the conditions of two types?
I mean what conditions must happen to say that "yes, it's an air cushion effect !" and "yes, it's a venturi effect!"

thanks,
Burak

ANSWER: Burak
A ground effect aircraft or ekranoplan flies close enough to the ground that the ground effect increases lift and reduces lift-induced drag.  An aircraft in normal flight creates a pressure field all around the aircraft.  In fact there are flow upwash and downwash regions around the wing.  When the aircraft flies close to the ground (or water surface) the ground changes the pressure field in such a way to add lift to the wing.  You can think of it as a pressure reflection, though we always modeled the effect by pretending that there was a mirror image of the aircraft flying upside down below the ground boundary at the same distance below the boundary as the real aircraft was above the boundary.  The pressures from that mirror image change the flow field of the actual aircraft to give it more lift than it would have in free air.

The venturi channel effect on race cars is completely different in that pressures are reduced below the race car causing a download on the vehicle.  First, you need to read about venturis and how they work.  As a flow channel area in a venturi is reduced, conservation of mass requires that the airspeed increases and the pressure decreases due to the Bernouilli effect.  Race car designers have shaped the underside of race cars and sometimes added side plates such that a crude venturi is created under the car.  As the airspeed under the car is increased the pressure is decreased relative to the pressures on the upper sides of the vehicle.  Hence, a download is created to help keep the car on the road during high speed racing.

Just to confuse things, we could say that a poorly shaped race car that developed lift could generate ground effect like an ekranoplan at the same time it generates a venturi effect under the vehicle.  The stronger of the two effects would determine whether the car stayed on the ground or not.

I hope this is all clear.  If not, contact me again.
Paul

---------- FOLLOW-UP ----------

QUESTION: Thanks for the reply Paul.

But the thing I wonder is a little different.
Forget about the race car thing.

Lets assume a solar car body (see picture).
Airfoil=NACA 0010
Length=5m
Width=1.8m (picture is 0.9m, with symmetry plane(yellow))
Ground height of chord line=0.7m
RE~=8m

According to a simple analysis, at 25m/s velocity, it creates a ~45N downforce. Here we see the venturi effect.
When I use a NACA 1412 airfoil(Cl~=0.12), downforce/lift is almost zero.

Now, looking at ekranoplans, they fly 1-5m above the ground. Their wing area is much larger. Like a bigger scale of a solar car body.
But the concept "looks like" same as above.

What I wonder is, what is the difference of these concepts?

*Both are airfoils with low aspect ratios
*Both are close to the ground
One has increased lift, one has decreased lift !

Can you tell me what is different between these two?

I hope it is not a tough question.
Thanks in advance,

Cheers,
Burak

Answer
Burak
The NACA 0010 is a symmetrical airfoil that will have zero lift in free air at zero angle of attack.  So I would expect it to have little air cushion lift in ground proximity if it has little or no angle of attack.  The venturi effect could be substantial in ground proximity, so the down force could be substantial.

The NACA 1412 is a cambered airfoil that does have significant lift at zero angle of attack.  So I would expect it to have significant air cushion lift in ground proximity that might cancel much of the venturi effect.  The resultant force could be nearly zero; i.e., lift and down force cancel more or less depending on the geometry of your simulated installation.
Paul