Streamlining. More important, the front or the back?

[sendler]

Originally Posted by ChazInMT

Finding someone online willing to let you play with their 4,096 CPU's

FoilSim works fine but you can't change the shapes.
.
FoilSim III Student Version 1.4d beta
.
Someone here was previously modeling a semi with solid works/ flow but hasn't chimed in yet. This sim I am asking for would be a piece of cake in the same vein. Not a big deal.

[sendler]

Originally Posted by ChazInMT

Yes, someone designed a full fairing on the front of a motorcycle that had the shape returning in before it ended

Vetter is way beyond this for four years now. But no one has ever had a chance to get in a wind tunnel yet to test whether it is really necessary to use the full tail or if a Kamm will get most of the benefit.
.
.

.

.
.

[freebeard]

****** WARNING ******** WARNING ********* WARNING ********** WARNING ******* WARNING *******

FoilSim is a simulation. It is not reality.
FoilSim is an educational computer program. It is not a design program.

Danger, Will Robinson! That's a 2D sim of free air movement, not 3D in turbulent ground conditions.

CFD and wind tunnels are both resource intensive.

The vetter example is good. The next step is to have the front slide forward for ingress/egress/putting your foot down. The example used to be on the Ecomodder home page.

But no one has ever had a chance to get in a wind tunnel yet to test whether it is really necessary to use the full tail or if a Kamm will get most of the benefit.

Start here.




Or, the one you posted in #9.

[sendler]

Originally Posted by freebeard

Danger, Will Robinson! That's a 2D sim of free air movement, not 3D in turbulent ground conditions.

Something like foil Sim would be more than good enough to give us an idea if we could manipulate the shapes to suit our questions in general.

Originally Posted by freebeard

Start here.

I already commented on the fuselage chart. It is way too long in form factor. By the time you take away 40% of the length, the sides are already straight. It is no longer a Kamm. It is a round nosed bullet.

Originally Posted by freebeard

Or, the one you posted in #9.

That is the correct shape but doesn't make sense. Where are they measuring "l"?

[Grant-53]

Looks like we have some choices. First is to try a search of free CFD software online. Second is to contact an area college that has a Mechanical or Chemical Engineering department that would have a student run some simulations on a mainframe. Third is to make a quarter scale model and put in front of a large window fan with thread for tuft testing.

[aerohead]

Originally Posted by sendler

Something like foil Sim would be more than good enough to give us an idea if we could manipulate the shapes to suit our questions in general.



I already commented on the fuselage chart. It is way too long in form factor. By the time you take away 40% of the length, the sides are already straight. It is no longer a Kamm. It is a round nosed bullet.





That is the correct shape but doesn't make sense. Where are they measuring "l"?

The chart and its values are only for a fuselage of the fineness ratio shown.I selected it because I didn't do the research.It was tested at Germany's NASA,the DVL,where Kamm spent some of his time.
The 2.5:1 fineness ratio fuselage has the lowest drag.Cd 0.04
In ground effect its drag rises to Cd 0.08
When 'gross' wheels are added,it goes to Cd 0.12.
If you truncate it at 60% you should see the same 28% drag increase,or Cd 0.15.(drag of the body is figured separately with drag of wheels held constant).

[aerohead]

Originally Posted by sendler

Thanks for the replies. My question stems from discussions of whether streamlining the front or the back of a motorcycle will make the most improvement if you were only to do one or the other. And, is it worth the trouble to run the tail at full length or will a Kamm truncation to 60% of the ideal length be only a marginal loss. The hemisphere in front of a cone was just my way of over-simplifying the shape so it would be still useful for analysis but also very easy to draw.
.
The fuselage example above is much too long to for our discussion of Kamm truncation since most of the length along the sides is nearly straight. By the time you get to 60% of the length, you have made a long round nosed bullet with almost no angle remaining and haven't satisfied the other rule of thumb for Kamm which is to end with a width that is no more than half of the max width of the airfoil. The angle of my tail I have started to build is about 10.5* per side, 18 inches at the front, truncated to 8 inches at the back by 28 inches long.
.
That it only takes 4% of the width for the radius of a round over on the front of my tail to harbor attachment is good news as that is only .75 inches which should be very easy to exceed with a pool noodle cut in half.

Kamm's 50% frontal area wake body truncation was a concession to 'practicality' and had nothing to do with ideal streamlining.He knew they were still leaving 50 % or more of the drag on all the K-cars.

[aerohead]

Originally Posted by sendler

Vetter is way beyond this for four years now. But no one has ever had a chance to get in a wind tunnel yet to test whether it is really necessary to use the full tail or if a Kamm will get most of the benefit.
.
.

.

.
.

*The full tail IS a Kamm tail.It just happens to be the whole thing.
*The fuselage chart is from the wind tunnel and shows exactly what happens when you chop portions of the tail away.
*Arado Flugswerke has published results of truncated fuselage wind tunnel studies.
*Most known aeronautical companies studied fuselage and wing truncation though the 1920s up and through WW-II.
*Lay did this wind tunnel research with truncated car models in 1933.
*Breer actually did the very first 'Kamm' tail in 1934 on a DeSoto Airflow test mule.
*K-Fachsenfeld wind tunnel tested the K-tail in 1935 and received a patent for it.
*K-Fachsenfeld was brought into FKFS to do research under Kamm.They repeated Lay's truncation research and refined the K-tail
--------------------------------------------------------------------------
From the results of what's already been published we may be able to make fairly high confidence performance predictions without additional studies.

[aerohead]

Originally Posted by Grant-53

Looks like we have some choices. First is to try a search of free CFD software online. Second is to contact an area college that has a Mechanical or Chemical Engineering department that would have a student run some simulations on a mainframe. Third is to make a quarter scale model and put in front of a large window fan with thread for tuft testing.

You'll need 80-mph out of that fan to get the tufts right since they're dependent upon a turbulent boundary layer.

[aerohead]

Originally Posted by sendler

This drawing would be a good example of the tail I am building and what effects a truncation will have Though I am having trouble correlating the ratio that is displayed into our concept.
.
.
Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - aerohead's Album: Book illustrations - Picture
.
.

This is W.A.Mair's boat tail.
CAUTION!!!!!!!!!!!!!!!!
The values he presents here are with skin friction removed.If the friction drag is re-introduced the curve takes on a very different appearance.
I've posted an image of Mair's entire wind tunnel model elsewhere.