Aerodynamics of a bug

[Humble]

Jason, if you order one, double it for sure and I'll grab it off you. Thanks!

[beetle1303]

Love to see this topic raised from the dead

I would like to correct/ increase definition on some things said before.
*First take a look at the link Rich gave us which is very helpful.
*Consider looking at the right side of a car (ie right is front, left is back) and for the ease of explanation consider the car as 2d (the overall profile on the centreline)
*Stagnation point is a theoretical point ahead of smthng that moves through
and the air starts to split at this point in order for the part to move through the air. WE, OUR CARS move THROUGH the air, not the other way...

1. As we move toward left the car starts to show its profile. our bumper being the foremost point, it has a stagnation point just ahead of it. Since the bumper isn't solid as it moves through air, the air suddenly gets sucked by the rear of the bumper, creating a vacuum effect and spiraling vertices.

2. next is the handle on the front hood. lets say it just splits the air left to right, so it is negligible at this moment(we could use it if talking 3d)

3. next is the profile of the front hood. specifically on the 1303 and less on the older bugs it has a hump facing forward. this has another stagnation point that the spirals from the bumper interfere with.

THERE is the first actual problem area due to the LOW PRESSURE POINT BEHIND the bumper and the HIGH PRESSURE POINT IN FRONT of the hood.

IN GENERAL every clean profile that the air gets to "see" first is a HIGH PRESSURE AREA, analyzed in HIGH PRESSURE POINTS and the opposite is for the LOW PRESSURE/VACUUM AREA/POINTS

Evil, the reason the auto makers put the air intake between the windshield and the hood is that the create a strong vacuum zone by raising the rear lip of the hood (to further promote the effect) so air can be SUCKED in.
Also the fan at the back actually works, because it lays in a low pressure area (engine compartment) that is connected to the "laminar" air flow at the rear with the grills below the rear window.

Don't confuse ram air that can work with carbs, efi with the actual fan
that is if we could put a big hose supplying air directly into the fan the fan would spin, but there would be no air flow.

same applies to turboprop engines in aircraft where u have a turbine spinning and a gearbox actually a huge reduction box that the prop is connected on to. if the ratio of turbine/prop was 1:1 the prop would go supersonic and the effect will be the same as spinning the same prop in space at zero pressure
this a real example from the cessna super cargomaster caravan aircraft where the turbine runs at 33000rpm @ 75% thrust and the prop at a mere 1750rpm.this is why u cannot go supersonic with a prop and only with a jet

sorry for the non germanlook example, it was the most recent i had from a personal experience


Chris

[Wally]

So, from all this knowledge/experience above, what would you suggest to be the first/best action to do on our little bugs Chris?

[beetle1303]

I will reply as soon as i get home.iphone isnt very easy on writting emails.what im going to do is get a rake on my 1303, get some kind of spoiler @ front to reduce the air gioing under and try to evac as much air as possible from wings.
Chris

[beetle1303]

back home...
well, the best for me, even for a daily is to get a flat bottom for the pan. this would extend forward under the head (1303 talking) and expand toward the sides of the car, leaving exposed the wheel wells and the control arm mounts on the head. at the back it should stop at the torsion bar housing, leaving the rear exposed.

the even more extreme solution would be to conseal the tranny as well (bottom) and sides, trying to form two wells for the rear wheels. In order to cool the tranny, i would use naca ducts starting at the torsion housing and blowing through two eclipses at the sides of the tranny box towards the axle mounts.although there will be a need for a means of cooling the bottom and getting access to the drain plug and fill plug of the tranny.

That's where differencies start, depending on rear tire size, offset and engine/exhaust configuration, the size of the rear wheel wells will have to vary for the rest of items to fit.

best would be to fabricate a diffuser at the back, keeping the tire and part of the suspension to the outside of each side of the car, tranny in the middle, but narrowing both diffuser paths (extending away from the centreline) for the engine/exhaust combo to fit.

Depending on the angle of the diffuser the fender/rear apron/fender combo will have to be chopped in order to for a bit of a proper profile for the 2 diffuser paths.

This only some idea, based on physics, NOT calculations. it also provides a means of "hiding" all enhancements from others, and not increasing drag the biggest aero prob of the bug imo is the rear, where we got an engine aft of a huge well ( where the tranny sits) and two wings that perform in a "parachute way" for the car

In the end it comes to the owner to decide which way and how far to go...

Chris

[volkdent]

Just wanted to add this link for searches:

Bug Aerodynamics

Jason

[Gerrelt]

Thanx Jason for posting the link here!

I made my own Roof Spoiler that's attached to the rear window rubber:






See this article on how I made it: Creating a Roof Spoiler.

[Steve C]

Originally Posted by beetle1303

prob of the bug imo is the rear, where we got an engine aft of a huge well ( where the tranny sits) and two wings that perform in a "parachute way" for the carChris

Hi

I was driving my sons 1303 on a freeway today and I was thinking of this subject, is that weird?

Anyway I wondered what the affect would be on the "Parachute" area when you run a non air cooled motor, Subaru etc which has no sealing around the engine to body area, the otherwise trapped air would be able to escape through vents in the engine lid or the under rear window vents.

Steve

[Humble]

I think everyone's seen my aero by now, hard to miss...



Large vents in the front fenders seem to help a bit but I won't know for sure until I take it to a race track. The rear wing is almost necessary above 120, and can be felt as low as 50-60mph. I've also trimmed up the rear fenders to help them air out a bit as well. Currently I'm working on front aero which is not present and the bug understeers as a result. With the airdam and splitter in place the balance should be a lot better and I'll be able to dial in the rear wing properly.

For street use something above the rear window is best. A small strip like carbon joe did but a little bit taller, 2.5"-3" from drip rail to drip rail, would be ideal. Vents in the top of the front fenders might work for the street but I don't think they will be as effective, however they won't ruin the paint on your rear fenders.

[Jadewombat]

Originally Posted by Humble

I think everyone's seen my aero by now, hard to miss...



Large vents in the front fenders seem to help a bit but I won't know for sure until I take it to a race track. The rear wing is almost necessary above 120, and can be felt as low as 50-60mph. I've also trimmed up the rear fenders to help them air out a bit as well.

How did you decide to put the vents there, vs. like along the rear near the bottom of the fender, or the middle or ? Just wondering if you did some pressure readings in the fender beforehand or if this is a known trapped area of air along the inside near where the fender bolts to the body.

I ordered a punch and flare tool to do some holes on the fenders on my bug (autocross car).