Cooling Drag

[Snick]

Granted, I don't have an Elio hood to weigh, but I'd bet money I can make one twice as stiff, and half the weight. Same goes for the wheel fairings and back hatch. The vents (again, I have no real-world data to back this statement up), 'should' help the aerodynamics by routing the engine compartment air up and over the Elio, rather than dumping out the bottom (which in my minds eye would tend to lift the vehicle). Of course I won't know for sure until I do it (jeez this is going to be a loooong year!), and if there's no gains, or even a loss of mpg, I'll be going back the stock design...and the drawing board (CAD screen now days).

I seriously doubt that. I laugh when some young guy goes out and spends huge $$ for a carbon fiber hood for a subcompact car. Your average subcompact's hood weighs less than 8 pounds. I had to replace my hood on a Mark 4 Jetta, and it was around 7 lbs, with the liner installed. Aftermarket "carbon fiber" hoods are the cosmetically pretty variety with a lot of resin for high shine and nice appearance and are almost ALWAYS heavier than the stock unit.

Real carbon fiber is pretty ugly; if you see "CF" parts on motorcycles, cars, etcetera, it is the pretty kind and not really that light.

 

[Snick]

NSTG8R --- Vortex generators can be used anywhere laminar boundary layer airflow is separating. Separation is turbulence, turbulence is drag. VG's wind up the air, keeping it laminar longer. When used forward of control surfaces keeps controls responsive deeper into the stall. Allows an aircraft to mush along slowly fully controllable rather than stalling and snapping into a spin. Anyway on an Elio use to reduce drag.

Sorry, that's a highly disputable claim. VG's work for an entirely different reason having to do with Reynold's turbulence numbers (measure of turbulence), which in aircraft is highly dependent on the wing shape. These aerodynamics at play in an aircraft have almost zero relevance to ground-bound vehicles. VG's help create MORE lift because they increase micro-turbulence on the low-pressure side of a lifting body.

On a car, you have no designed lifting surfaces (some cars have accidental lifting surfaces, for sure, but no aerodynamicist is going to design a car intentionally with a lifting surface). Cars are designed for neutral lift or to at least minimize lift at freeway speeds because lift reduces traction and reduces stability. VG's can only create turbulence and that is the opposite of laminar flow.

And from what I've read, it is not analogous to dimples on a golf ball, that works because the Reynold's turbulence number of a spinning ball is already very high. The dimples create micro-vortices which shrinks net drag envelope (cross sectional area) in the high Reynold's flow regime. For shapes with low Reynold's turbulence numbers (like the Elio), it would hurt. The Mythbusters test of the old car with huge dimples made of clay probably saw improvements because that particular car has aerodynamics of a brick with high Reynold's turbulence to match.

 

[LonePine]

Granted, I don't have an Elio hood to weigh, but I'd bet money I can make one twice as stiff, and half the weight. Same goes for the wheel fairings and back hatch. The vents (again, I have no real-world data to back this statement up), 'should' help the aerodynamics by routing the engine compartment air up and over the Elio, rather than dumping out the bottom (which in my minds eye would tend to lift the vehicle). Of course I won't know for sure until I do it (jeez this is going to be a loooong year!), and if there's no gains, or even a loss of mpg, I'll be going back the stock design...and the drawing board (CAD screen now days).

Granted, I don't have an Elio hood to weigh, but I'd bet money I can make one twice as stiff, and half the weight. Same goes for the wheel fairings and back hatch. The vents (again, I have no real-world data to back this statement up), 'should' help the aerodynamics by routing the engine compartment air up and over the Elio, rather than dumping out the bottom (which in my minds eye would tend to lift the vehicle). Of course I won't know for sure until I do it (jeez this is going to be a loooong year!), and if there's no gains, or even a loss of mpg, I'll be going back the stock design...and the drawing board (CAD screen now days).

What you said about using body curvature enhanced with a little efficient ducting to vent used cooling air up Over the body, (hoping I read you correctly) really gave me a "Eureka" moment there. I know the reason race cars are built so close to the ground is to eliminate the turbulence from passing air tumbling around the spaces and gaps of the underside. I can just SEE an air tunnel/smoke stream test being run to perfection with the simple, (yeah, simple)re-directing of the engine cooling air. 90 mpg, here we come !

 

[outsydthebox]

......The Mythbusters test of the old car with huge dimples made of clay probably saw improvements because that particular car has aerodynamics of a brick with high Reynold's turbulence to match.

The Mythbusters episode used a late model Ford Taurus.......not exactly a "brick". Approximately 800lbs of clay was added and they saw about 12% reduction in fuel consumption. The 2" layer of clay also added (IIRC) 2" to the height and 4" to the width of the car, making the 12% reduction even more impressive.

 

[skygazer6033]

Snick --- The information you have is probably a lot more current than mine. I'm just quoting Cessna, Beech, Lear, Boeing, Mcdonald Douglas and Airbus from 20, 30, 40 years ago. Once owned a Cessna 337G Skymaster with a Robertson STOL kit. A light twin that would fly at 55 mph all day. Had vortex generators forward of ailerons.

 

[NSTG8R]

I seriously doubt that. I laugh when some young guy goes out and spends huge $$ for a carbon fiber hood for a subcompact car. Your average subcompact's hood weighs less than 8 pounds. I had to replace my hood on a Mark 4 Jetta, and it was around 7 lbs, with the liner installed. Aftermarket "carbon fiber" hoods are the cosmetically pretty variety with a lot of resin for high shine and nice appearance and are almost ALWAYS heavier than the stock unit.

Real carbon fiber is pretty ugly; if you see "CF" parts on motorcycles, cars, etcetera, it is the pretty kind and not really that light.

Oh Snick, you don't know what I do for a living (and as hobby on the side). Let me do the math for you.

The hood will be 4 plys of 7.8 oz. 4x4 twill (aerospace grade 640Ksi) cloth with a Soric core. Carbon fiber cloth is weighed by the square yard. Assuming (because I'm just going by the pictures) the hood has a surface area of 9 square feet (1 square yard), the carbon material (cloth only) would be 31.2 oz. Using a vacuum bag method (resin infusion), the resin to cloth ratio comes out around 35% (this is all by weight). So...

cloth = 31.2 oz. (4 plys of 7.8 oz 4x4 twill)
epoxy resin = 10.9 oz (35% using vacuum bag method)
for a total of 42.1oz.
42/16 (that's oz. per lb.) = 2.6lbs.
The Soric core material (3mm thickness) is basically a light weight foam with a grid pattern cut into it for flexibility. Even if the Soric weighed 1/2 lb. (highly unlikely), and the UV resistant clear gel coat (15mil), the total weight of the hood would be barely 3 lbs.

Now maybe the production model's hood will be lighter than the hand laid-up fiberglass the P4 is made out of, but the thickness required using 'anything' but carbon fiber would still have to weigh in around 10-12 lbs minimum to maintain the stiffness they'd need to keep it from oil canning at highway speeds.

Not trying to be disrespectful Snick because I agree with most things you post or respond to, but I know what I know, and I've been doing it so long I can almost retire, so I'm throwing the proverbial "BS" flag on your response to my post.

 

[NSTG8R]

Sorry, that's a highly disputable claim. VG's work for an entirely different reason having to do with Reynold's turbulence numbers (measure of turbulence), which in aircraft is highly dependent on the wing shape. These aerodynamics at play in an aircraft have almost zero relevance to ground-bound vehicles. VG's help create MORE lift because they increase micro-turbulence on the low-pressure side of a lifting body.

On a car, you have no designed lifting surfaces (some cars have accidental lifting surfaces, for sure, but no aerodynamicist is going to design a car intentionally with a lifting surface). Cars are designed for neutral lift or to at least minimize lift at freeway speeds because lift reduces traction and reduces stability. VG's can only create turbulence and that is the opposite of laminar flow.

And from what I've read, it is not analogous to dimples on a golf ball, that works because the Reynold's turbulence number of a spinning ball is already very high. The dimples create micro-vortices which shrinks net drag envelope (cross sectional area) in the high Reynold's flow regime. For shapes with low Reynold's turbulence numbers (like the Elio), it would hurt. The Mythbusters test of the old car with huge dimples made of clay probably saw improvements because that particular car has aerodynamics of a brick with high Reynold's turbulence to match.

Okay, the "vortex generators" I was referring to are the ones found on the bottom of my Porsches (914, but 911's also). They create a low pressure area just forward of the air exiting the cooling tin (aircooled engine). The low pressure area created assists the airflow across the cylinders. So, in my mind's eye, placing a conical shaped rubber boot directly in front of the rear tire, essentially creating a low pressure area in front of the tire, would be more aerodynamic than the tire itself. The vortex generators I used to install on the AV8B Harrier wing performed a completely different function, as you said, "almost zero relevance to ground-bound vehicles." Sorry for the misunderstanding.

 

[Snick]

Okay. I take it back. NSTG8R can probably build a lighter hood (but not cheaply, I wonder?)

Will your project hit the benefit/cost ratio needed?

But I still seriously doubt the production hood will weigh very much at all. It's tiny!

 

[NSTG8R]

Okay. I take it back. NSTG8R can probably build a lighter hood (but not cheaply, I wonder?)

Will your project hit the benefit/cost ratio needed?

But I still seriously doubt the production hood will weigh very much at all. It's tiny!

Well, it'll probably cost around $200 - $250 (4-5 yards) to build (I already have the mold materials, Soric, and a box full of plain weave for reinforcement at the hinge and latch points). And around 40 hrs of labor to make the first one due to creating the female mold. Once the mold is done, I could pop one out per day. Not sure what Elio would charge for a replacement hood.

As for the "benefit/cost ratio", it's impossible to tell at this point. If I got a 5 mpg advantage over stock, I'd be THRILLED! Not because of the gas savings in dollars, but because I created something that had a positive affect on my Elio...and how can you put a price on an ego boost? I drive, and will for the next 8 years or so, the exact same route to work (84 miles round trip), so it should be fairly easy to tell what affect the hood I'm picturing would have on my mpg within a week or so. If there's no affect, or I loose some mpg, I'll have to decide at that point how much a cool looking (personal opinion) hood is costing me, and whether or not to reinstall the stock one until I retire and don't have to put 400+ miles per week on my Elio. I'm just one of those people that can't leave anything the way I bought it.

I do appreciate your "Devil's Advocate" demeanor (seriously). It keeps everybody on their toes, most likely lessens the complete BS claims, and, for me at least, makes me reconsider, or at least research further, the affects of possible mods. Thanks for the input!

 

[ehwatt]

Snick --- The information you have is probably a lot more current than mine. I'm just quoting Cessna, Beech, Lear, Boeing, Mcdonald Douglas and Airbus from 20, 30, 40 years ago. Once owned a Cessna 337G Skymaster with a Robertson STOL kit. A light twin that would fly at 55 mph all day. Had vortex generators forward of ailerons.

The way I understood it was when airflow separation occurs, it becomes a gurgling brook as it were. Vortex generators have to be kind of tall to stand above the stagnant air, out into the airstream to energize the air with the generated vortex, which helps establish laminar flow.

Early Lear Jets (33s ?) had them spread out over the last 2 to 3 feet of wing to "re-laminize" the roll control devices. AH, but the life of a line-boy!