Airfoil near ground "ground effect" distance (surface vehicle application)
Airfoil near ground "ground effect" distance (surface vehicle application)
(OP)
Hello all,
I believe this topic is best suited in this aerospace region of the forum although it relates closely to Automotive.
Does anyone have good details on distances from the ground in relation to drag and ground effect of an airfoil?
I wanted to build a wind tunnel in the garage this summer but found the rolling road too costly to build capable of over 120 mph. (required to remove the boundary layer)
My project looks most like an salt flats land speed vehicle. The idea was borrowed from college solar cars due to the extreme reduction in drag forces while utilizing ground effect to generate the downforce needed to keep this vehicle planted at high speeds. The basic concept is "pontoons" with an airfoil suspended between them.

I was able to find this graphic however it is not very specific on what the numbers are.

I appreciate any insights you can provide!
-Liberty
I believe this topic is best suited in this aerospace region of the forum although it relates closely to Automotive.
Does anyone have good details on distances from the ground in relation to drag and ground effect of an airfoil?
I wanted to build a wind tunnel in the garage this summer but found the rolling road too costly to build capable of over 120 mph. (required to remove the boundary layer)
My project looks most like an salt flats land speed vehicle. The idea was borrowed from college solar cars due to the extreme reduction in drag forces while utilizing ground effect to generate the downforce needed to keep this vehicle planted at high speeds. The basic concept is "pontoons" with an airfoil suspended between them.

I was able to find this graphic however it is not very specific on what the numbers are.

I appreciate any insights you can provide!
-Liberty
RE: Airfoil near ground "ground effect" distance (surface vehicle application)
RE: Airfoil near ground "ground effect" distance (surface vehicle application)
Also, check-out... https://www.sae.org/search/?qt=aerodynamics+of+rac...
Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]
RE: Airfoil near ground "ground effect" distance (surface vehicle application)
@WKTaylor thanks for the link. I believe the link you provided might get me headed in the correct direction. This one specifically has very relevant data for me:
"2008-12-02
Racecar Front Wing Aerodynamics 2008-01-2988
A numerical study of a racecar front wing is presented. The focus of the study is to investigate the aerodynamics characteristics of a wing operating in a small ground clearance. A finite-span wing with a symmetric airfoil section is used. It was found that as the wing gets closer to the ground it generates more downforce and more drag. As the wing gets very close to the ground, the downforce reaches a maximum value and after that the wing generated less downforce as it gets closer to the ground. The drag force follows a similar trend of dependency on the ground clearance. The lower surface of the wing and the ground form a convergent-divergent-nozzle shape that is responsible of all the changes in the generated forces. For very small ground clearances, the boundary layers developed on the wing lower surface and the ground get closer to each other, decrease the airflow below the wing and ultimately decrease the generated downforce significantly. The study was preformed using the CFD package STAR CCM+, developed by CD Adapco."
RE: Airfoil near ground "ground effect" distance (surface vehicle application)
on your chart "c" is the chord (width) of the airfoil.
But a huge impact for your design is going to be the very large "end-plates" at both ends of your "wing".
It seems odd to me that drag increases as the gap gets smaller (or as the wing gets wider); though I can see some optimum height.
another day in paradise, or is paradise one day closer ?
RE: Airfoil near ground "ground effect" distance (surface vehicle application)
Regarding the drag forces I believe it is in relation to the distance between the airfoil/wing and the ground causing additional drag. At some point "squishing" air into a smaller space creates some significant resistance.
RE: Airfoil near ground "ground effect" distance (surface vehicle application)
RE: Airfoil near ground "ground effect" distance (surface vehicle application)
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Airfoil near ground "ground effect" distance (surface vehicle application)
One element to consider for high speed vehicle drag is 'wetted area' and tapering or forward/aft blunt bodies.
The long-distance image of this vehicle implies a lot of wetted area and severe-bluntness/square-ness to the "wheel pontoons'.
Tapering and slenderness is 'twicky' art/science.
Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]
RE: Airfoil near ground "ground effect" distance (surface vehicle application)
This is a better angle/view to see the shape. I am open to suggestions!
I used a free online CFD analysis, but it does not have a rolling road or specs on downforce/drag so in my eyes all I got from it was some pretty graphics with blue lines ;)
One of my largest concerns is if the downforce is balanced. If I have too much in the rear and very little in the front that could be REALLY bad!
RE: Airfoil near ground "ground effect" distance (surface vehicle application)
if the flow under the body is creating drag, would it help to have vents/ducts to the outside ? possibly NACA inlets on the outside ducting air into the low pressure under the body ?
another day in paradise, or is paradise one day closer ?
RE: Airfoil near ground "ground effect" distance (surface vehicle application)
If the design creates too much drag with the suspended airfoil design then I would simply build the car like everyone else has where there is a minimal air gap under the car.
RE: Airfoil near ground "ground effect" distance (surface vehicle application)
In such a design does one need to worry about the pitch moment created by the airfoil? I could see it being a much bigger problem with such a massive single lifting surface vs multiple smaller wings attached at different points to the car like a typical open wheel car.
RE: Airfoil near ground "ground effect" distance (surface vehicle application)
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Airfoil near ground "ground effect" distance (surface vehicle application)
This was the reason I wanted to build a wind tunnel to determine the distribution of forces on front and rear axles.
Sadly setting up a function to simulate the traveling 100+mph boundary layer on the ground (moving belt or a complex air blowing system) was out of the budget.
-Liberty