lakester CD ?

[hotrod]

So if you make your frontal area 15% smaller can you go 15% faster with same power?

That is only true to a limit. As long as the cars form stays essentially similar scaling works over small changes but when you make large changes in size that sort of thing starts to break down.

Likewise it is possible to increase frontal area and at the same time reduce CD if you put the new volume in the right places, to improve or preserve attached flow.  What really matters is the product of frontal area x CD, as that is the major factor in total drag.

If you add a bit of frontal area and reduce the total CD by a larger fraction you could come out ahead. On the same token you can slim down the frontal area and spoil the form resulting in more total drag by increasing the CD.

Larry

[Sumner]

.........So if you make your frontal area 15% smaller can you go 15% faster with same power?....................

Using my weak math on these spreadsheets:

http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html

........and using the "Drag Force-HP-Thrust-Weight" and the "Horsepower Needed" ones my answer would be "no". 

With a 15% decrease in the frontal area if the Cd stays the same you would need 15% less HP to stay the same speed, but if you had the same HP and decreased the frontal area 15% your speed would only go up 5.55%.  This is due to the fact that it takes 8 times the HP to go twice as fast, so the relationship between frontal area and HP is not linear.

Someone else please confirm this or tell me I'm full of it  .

c ya,

Sum

[1212FBGS]

sum your full of it... ha ha i'm the first to tell him.....you asked for it..... and i think your math is right
kent

[Stainless1]

 

I can't tell you what our CD is because I don't know, but I can tell you that 167 HP will do 204mph.  The last motor we had on the dyno was the ZX-11, it made 167 and Marty went within 1/4 MPH of that number with every gear change, tuning change and weather change.  Of course it is only 1 reference point on a line, but I'm sure the aero experts on the site can deduce a CD. Now if I told ya that about 80 HP (calculated based on consumption) of N20 made the car go 228, the aero experts could easily arrive at a CD...  Unless the car still accelerating screws up the calculations, didn't have enough room to reach terminal speed. 
OK aero guys, take your hack... 

What is the frontal area overall, the body, the tires?  Was the HP at the crank?? Rear Wheels??  Countershaft Sprocket (what gear)

And here you though you were done  , but thanks for the info so far   ,

Sum

Sorry, been busy, Johnboy spilled most of the specs, although I think the top is about 11-12 inches.  That was crank HP and calculated N20.  Finished every run in 6th gear, motor self divided backing up a 230 pass, so don't really remember what sprockets or gear ratios we were running, but that should affect CD. 
The one thing I know about the 15% frontal reduction is that part of the car's CD is 0.0.  I'm not an aero whiz kid, but I know if it is not in the air it won't cause drag.  I think I read somewhere spinning wheels in open air are a greater than 1, something like 1.16 for large wheels and 1.21 for small wheels.  No, don't remember where that was, but it seemed fairly reasonable to me. 
I think the roadster guy that caused the wheel size rule change said he gained 10 MPH when he switched to the small wheels. 

[Sumner]

................................Sorry, been busy, Johnboy spilled most of the specs, although I think the top is about 11-12 inches.  That was crank HP and calculated N20.  Finished every run in 6th gear, motor self divided backing up a 230 pass, so don't really remember what sprockets or gear ratios we were running, but that should affect CD. 
The one thing I know about the 15% frontal reduction is that part of the car's CD is 0.0.  I'm not an aero whiz kid, but I know if it is not in the air it won't cause drag.  I think I read somewhere spinning wheels in open air are a greater than 1, something like 1.16 for large wheels and 1.21 for small wheels.  No, don't remember where that was, but it seemed fairly reasonable to me. 
I think the roadster guy that caused the wheel size rule change said he gained 10 MPH when he switched to the small wheels. 

In the pictures the body from the canopy down looks pretty square.  The earlier post mentioned the body in that area is 16 inches high by 24 inches wide.  Is that correct??? 

Kent posted pictures of wheels before, but I couldn't really figure them out, but then he thinks I'm full of it anyway  ,  I think I have some more info on them in one or two of my books and will try and look.  I figure there isn't much we can do about them except make them smaller as you mentioned.  Unfortunately when I bought mine at first I thought I was going to have a F or larger motored car, now the car and ground clearance are built around them. 

c ya,

Sum

[Speed Limit 1000]

In the pictures the body from the canopy down looks pretty square.  The earlier post mentioned the body in that area is 16 inches high by 24 inches wide.  Is that correct??? 

Sum
[/quote]

Yes, that is the size of it.

[Dynoroom]

Way back in the early 60s I hung with Lee and Bob Noice when they ran a Jr Fuel car. Sometime in there I went to Bonneville with them. Hooked me forever!!

When I finally lined up for my last attempt at the 2 club we were behind the Hammond roadster with Lee driving. We were walking around in our race suits and I thought "how bitchn' is this?, 40 years later and I'm with one of my heros"

We both did the record. While I was pulling off my suit the Hammond clan stopped and hugged, etc. . Needless to say a tear rolled down my cheek.

I love that place and all of you who race, want to race , or spectate.

DW

Gee, what a great snipit... speaking of heros, thanks Dan for helping keep the sport fun.

[rebelce]

"From aerodynamic point of view a lakester is a brick in the air....."

                   Then a roadster must be a tugboat out of water!

[1212FBGS]

still better looking than Bush's momma out of water.... in a bikini...yuck!
Kent

[PorkPie]

"From aerodynamic point of view a lakester is a brick in the air....."

                   Then a roadster must be a tugboat out of water!

NOT REALLY........only the air ask where to go (around)......

[tortoise]

If the drag in turbulent, unattached flow is proportionate to the size of the wake, then the narrowest possible track would seem to be the way to go, if lowest drag is the overriding concern. Wouldn't the wake of the wheels, even spaced well away from the body, still disturb the flow around the body?

[Sumner]

If the drag in turbulent, unattached flow is proportionate to the size of the wake, then the narrowest possible track would seem to be the way to go, if lowest drag is the overriding concern. Wouldn't the wake of the wheels, even spaced well away from the body, still disturb the flow around the body?

They (whoever they are) say about 1 foot away to eliminate that.  Mine will be further away than that where they are, but the insides will be 12 inches from the body if you projected a line from the inside of the front tire back to the back tire.

c ya,

Sum

[Stainless1]

I've always thought wide fronts and narrow backs were the way to go, but what the heck do I know... 
did any of the wizards figure out the CD or CDA of the Bockscar for us?    We hopefully will be back on the dyno in August. (the white one)

[maguromic]

While the front of the car is very important the rear of the car is more important. The aerodynamic performance of the racecar depends more on the design of its rear than its front end, in a proportion of 6 or 7 to 3 or 4.

As the front of the racecar pushes the air in front of it, thus creating an area of higher pressure, which tends to slow its forward movement, what is less well-known is that an area of extremely low pressure is created in the racecars wake that slows down its forward movement by applying a suction or parachute breaking effect.  This is the reason that the design of the rear of the racecar gets great attention from aerodynamic engineers.

This is also the main factor that explains the difference in Cd between versions derived from the same racecar with a cam back, square back, tapered back and others, that all have the same front end, but not the same rear, which accounts for significant differences in Cd.

The Devil Is In The Details.

[PorkPie]

This is the reason that the design of the rear of the race car gets great attention from aerodynamic engineers.

This is also the main factor that explains the difference in Cd between versions derived from the same race car with a cam back, square back, tapered back and others, that all have the same front end, but not the same rear, which accounts for significant differences in Cd.

The Devil Is In The Details.

[/quote]

This is the reason why I say - it's more important how you go out of the air than you go in....