APS body taper?

[debgeo]

Google Brett and look at images for several pictures of his bike.

[sofadriver]

The nose was made as blunt and rounded as practical so the wide part was as far forward as I could get it.  This allowed for a longer tail taper. The sides gradually transition back at an increasing taper rate with the greatest taper, 10%, being right near the tail end.  I figure, what the heck, it is too much taper but it is doubtful the air would be attached at the tail that far back, anyhow.  Along the sides and forward part of the tail the taper rate is less than 7 percent.

Exactly.
I guess I'm not the only one to have this little dilemma.
I still wonder, though. Is 10% too much at, say, 200mph but acceptable at 130mph?

[sofadriver]

Google Brett and look at images for several pictures of his bike.

I just did and the shape of that tail is fantastic! My problem is that my feet are very close to the rear of the rear tire leaving a pretty short distance to the end of the body. Even with a 10 degree taper, that's going to leave a pretty large Kamm tail. It's not too late to extend the wheelbase at the rear. That would lengthen the distance from my feet to the end of the tail. I'll check it out tomorrow

[bak189]

Tunnel testing that Can-Am did on their 125c.c. effort in the early 1970's.....showed 7 degrees on the back end worked best......( 136mph...125cc.....22hp)

[John Burk]

NACA 66-021 wing section tapers over 16* per side and is one of the lowest drag drag shapes .

[Sumner]

NACA 66-021 wing section tapers over 16* per side and is one of the lowest drag drag shapes .

Interesting.  I wonder though if you can't make the whole car that shape as viewed from above would the 16* work as well as say 7*?

In most of our cases trying to make the top view of the car a true NACA profile just won't work considering all of the stuff we have to package in the car.  There are lots of compromises that have to be made.  My car has gotten quite long trying to stay at the 7* though so now has more wetted surface  .  Still it looks like I'll end up filling the whole works and need all of that length....compromises compromises    .....,

Sumner

[bbarn]

You can fit all the internals in a NACA-66 from plan view. I have seen it somewhere.....

[Sumner]

You can fit all the internals in a NACA-66 from plan view. I have seen it somewhere.....

I'm not saying impossible but still going to depend on what you have and what you will except for a total width.  If you increase the width (frontal area) to get what you want in the car (wheels/tires if a streamliner) then the overall length might be shorter but with more frontal area.  Still if the Cd is better it might make up for it.  Lots of options one just needs to consider as many as possible.

In the smaller engine classes, say bike motors, this is easier.  If you are in the larger engine sizes and running blown gas and have turbo/turbos, intercoolers, intercooler ice-water tanks, plumbing, main water tank, maybe a rad-in-a-box and so forth then I think you are ahead with a longer narrower car with the 7* taper at the back and a teardrop shape at the front vs. a wider car just to maintain the NACA profile, but that is just my opinion  .  One thing I love about the lakester/streamliner (and some degree comp coupe) classes is we have a lot of leeway to mess around and at worst go home with egg on our face .

I had the lakester set up with a teardrop shaped nose and an expanding width to just behind the driver and then it went to a 7* taper from there back.  Now that I'm switching to a car engine I have to add 3 feet and will have a 3 foot length now just behind the driver (firewall) that will be a constant width/height.  Not what I originally wanted but will now have  ,

Sumner

[bbarn]

The packaging of the #496 Carbiliner is the example I was referring to. That is a twin turbo, 540 cid, ice water intercooled full size car. It is a series of NACA-66 foils where the maximum width of the fuselage is roughly 30 inches while the length is 28 feet.

Testing shows that the airflow is highly laminar through the first 33% of the chord and very still through the last 66%. Oddly enough, there is very little penalty shown if the fuselage was shorter and wider.

I know that goes against what most people say about aero, but the numbers are there. The "fineness ratio" we have could be tweaked to shorten the length and widen the fuselage and there would be negligible change in the disruption of the air beyond the 30% chord mark. Meaning no drag penalty for that adjustment.



 

[John Burk]

From the things I've see at 300 mph skin friction is 70% of total aero drag . That says within reason adding width to reduce surface area pays off .
The 16* thing may be true for the motorcycle body we were talking about but not a roundish 4 wheeler body . Somebody said to equate 2 dimensional (wing) information to 3 dimensional body shape add 50% to the length . That would change a 66-021 to about 11* per side .

[Stan Back]

Just want to see if the degree sign shows up here -- Option Zero on my keyboard -- ººººº.

[saltwheels262]

°°°

[Sumner]

The packaging of the #496 Carbiliner is the example I was referring to...

I figured it was , but you don't have the back wheels packaged in that shape and if someone wants to do that it is going to be hard.  You guys might be on the right track  only time will tell,

Sumner

[bbarn]

It would be difficult to pull them in unless you adjusted the fineness ratio. If you went shorter and wider you would have a chance. I think the issue is the rears would need moved farther forward which would wreak havoc on cg.

[Sumner]

The best chance of doing this (NACA profile) with a larger car is John's front wheel drive where the back whee/tires could really be sucked in.  John do I remember seeing a rendering of you car before?

Also the front drive Salttarus lakester approach is headed in the right direction so I think it could be done but unfortunately us lakester guys still have the wheels/tires hanging out there in the wind.  But hard with a rear-wheel drive car or 4 wheel drive car.

If I was starting from scratch and at my age won't be, I'd go frontwheel drive even though there are some things that have be be carefully considered doing that and it is going to be more expensive,

Sumner