Aero - Boundary Layer??

[Modo]

How thick is the boundary layer on waxed or unwaxed fiberglass after crawling across 5 feet of glass at 100 mph?? That is to say, how far out from the skin, is say, 90 to 100% of the 100 mph airflow or clean air?? ....Mike O

[provamo]

it depends

[Tifosi]

I'm pretty sure theres an answer to this question.

[Rick Iverson]

On helicopter main rotor blades, it's about the thickness of a dime.

[gmmls]

Not sure if this is the answer you are looking for, but look at the distance a pitot tube stands off a small aircraft, that should give a general idea right?
Mike

[DMac]

Make inch marks on your hand and stick it out while you are driving.

Not sure if this is the answer you are looking for, but look at the distance a pitot tube stands off a small aircraft, that should give a general idea right?
Mike

Yes, it should. My best estimate for boundary layer thickness is three inches or less. Without out running a model on ANSYS, I can't say for sure.

[Charles Warner]

http://www.desktopaero.com/appliedae...s/blayers.html

[Modo]

for Mr. Piper and Mr. Cessna?? (applied)....Don't have my spruce goose catalog handy and they probably don't have installation instructions for the airspeed pitot....Tnx Charles for the link to the equations....suppose I can google the pitot install

[George Main]

How thick is the boundary layer on waxed or unwaxed fiberglass after crawling across 5 feet of glass at 100 mph?? That is to say, how far out from the skin, is say, 90 to 100% of the 100 mph airflow or clean air?? ....Mike O

Unlike an airplane, where the pitot tubes tend to be before the main chassis structure to accurately measure airspeed, placement of the pitot is determined by it's use.
On an airplane, the pitot's reside on or near the proximity of the nose before the area where the striaght portion of the fuselage begins.

In answer to your question, placement of a pitot has everything to do with what's in front of it. Wings, open wheels, body openings all effect the boundary across the whole car. At a 100mph it may even make it to the rear wings from the front tires.
On a F2000 car with a pitot on the front chassis section (in the front rocker area) you may have to be as high as 6" to 8" to reach clean air. Side pods tend to be directly behind some of the dirtiest air on the car, coming from the front wheels. An uncovered wheel will pump air in both directions, which will turn and form an envelope of dirty air across the side pods. This can cause a less defining point of where the boundary layer actually begins/ends and it's width.

On an S2000 or a CSR car, measurements are calmer, though vents over the front wheels and front wings can still throw a wake across the following body. Sharp edges to the body (less than 2" radius) also create turbulance and seperation.
Pitot height can be less to find clean air, depending on what's in front of it. S2's would be lower with the smoother body shape. A CSR with wings, you will need longer pitot's.

To measure accurately the boundary layer, you will need not only an adjustable pitot (in height and in unison with one in clean air) but also a couple of "flush" aero pressure port taps mounted on the body work to determine seperation points. (Unused rivets heads work well for this) and make for a movalbe tap (by moving the hoses to a different taps each time, that way you don't need to buy 20 aero sensors. Takes more time, but achieves the same results. The pitot will tell you about head/tail winds, provided it's one in clean air.

BTW, when mounting pressure sensors, insulate them from the chassis vibration and mount on shock mount of some sort lest you recieve bogus readings from chassis vibrations and shock intervention.
IMHO

[Mike Allison]

For not too much money, you can get the answer to your question here. www.a2wt.com

Gary Eaker is the owner, superb aero guy. Dave Salizar can get you started.

Mike

[jabro1000]

Guys...although I'm not positive, I'm pretty sure there's at least a convention, if not a rule, that threads on this forum are supposed to be in English, TIA

[preform resources]

I'm not an aerodynamicist nor could I do the calculations, but my take on boundary layer is that for short objects like autos, that a large form drag would not occur such as in aircraft(long fuselages,large wings),having said that I would qualify that premise involving race cars that the car were reasonably smooth and clean,not like the F-1 cars
that don't weigh a lot and have a lot of horsepower to overcome excessive drag in order to promote downforce.
Dave Craddock