Bicycle Aerodynamics for those interested.

[DavidinDurango]

 that NSU bike looks about twice the width of a Varna . . .
A lot of the bike guys ride enduros - but the bikes must have a provision for cooling that the Varna does not.  Even bikes w/ multiple riders presented more frontal area could not match that bike's speed.

Lots to learn there, I believe.\\cheers,

[Stainless1]

that NSU bike looks about twice the width of a Varna . . .
A lot of the bike guys ride enduros - but the bikes must have a provision for cooling that the Varna does not.  Even bikes w/ multiple riders presented more frontal area could not match that bike's speed.

Lots to learn there, I believe.\\cheers,

Yep, all the frontal area you don't have is 100% streamlined...    CD=0.0  CDA 0.0 

[interested bystander]

Here's a cutaway of the NSU MOTORCYCLE the Varna similarity was only comparing form, not CdA.

Another shot is a BICYCLE built for the one hour McCready prize that will have rider cooling , radio, nutrients supply. etc in a pretty compact package

[1212FBGS]

Just imagine that "bike" with a 100 cc engine in it. If it goes 80 with at best 1 1/2 hps think what it would do with 25!! That is AERO!!!

Rex

yes its aero, for 80mph..... i think it would be very lucky to go 160 with 25hp... ya see the aero is way screwed up for faster speeds... there would be mad separation on the tail do to the crazy convergence angle.... yes it was designed for 50,60 ,70 mph but over 1 buck things start getting very different
kent

[Freud]

Kent, do you have the long tail version of the Can Am bike?

FREUD

[1212FBGS]

no not yet... i started to build a replica of the complete bike last spring and got side tracked.... i will probably get back to it this year
kent

[half-fast]

I am interested in what the consensus is in regards to wheels for M/C applications. Specifically for 'naked' classes and for the front, as a solid wheel is disallowed by rule. What wheel type represents the best attainable aero? Does a spoked wheel act as a solid wheel in regards to aero at speed (from headwind not side wind), do mag type wheels act as paddles against the wind, etc?

[Rex Schimmer]

Kent,
I think that I have to disagree with you regarding your thinking that the Varna would suffer from unattached air flow above 100 mph. Looking at the pictures the angle that the body converges at is certainly less that the accepted 11-14 degrees maximum also the bike has a very small surface area which minimizes skin friction which is probably the largest component of drag for the bike. There is no doubt that all of the air flow on the VARNA is attached and probably laminar,due to the low speed, so its pressure drag is only the area of the boundary layer from both sides as they converge at the tail and being pretty short and probably laminar flow the boundary layer is probably pretty thin at the rear of the bike.

Just a note: On the JCB liner over 50% of the total aero drag was skin friction, which is why on vehicles that have limited power you want to keep them as short as possible.

Rex

[bak189]

 The Can-_Am with the long tail...it was O.K. up to 80mph after that it was not safe.
It looked great in the wind tunnel but did not work on the salt.....................................................
Also the Can-Am first ran with wheel covers on both wheels (solid wheels looked good in the tunnel) but if I recall the front wheel covers were removed for the record runs....again it was not safe to ride on the salt............................................

[interested bystander]

I'm thinking both Rex and 1212 are right- to a point -there is a relationship to finess ratio and speed -(Reynolds number?-help A2).

I was told many years ago that the Goodyear blimp is pretty close to a perfect 60 MPH shape- but looking at the WW2 drop tanks -300 mph tops -they are a bit stretched out!

Food for thought.

[Freud]

bak189, what day of the week did Can Am set the record?

I'll see what my fotos show for that day. I should also have a foto from right to left for

the return record run. Then we will know if he used the discs during the record runs.

I have fotos both ways.

The bike ran in several classes. That is shown by the number plus the letters A and C.

As 89 C it has no wheel fairing. In my opinion, the wheel discs had no bearing on class.

Dolan would know.

FREUD

[saltwheels262]

In case you haven't seen these already;

http://www.airtech-streamlining.com/landspeed/landspeed.htm

 just want to say that kent and krew did a great job on my 2 pcs.
the 2kr7501 is a real beauty along w/lsr front fender. thanks also to dutch.

franey

[1194]

In regards to Dr. Freud question on which day the 125c.c. Can-Am set the record back in the 1970"s..........I may be wrong... but wed. comes to mind.  About the "solid" front wheel..........as noted before it did not work back then.......and it did not work when I ran one on my Rotax 125c.c.
in 2001 (with special permission from Tom Evans)
But we have to remember that the 125.c.c. is a very light and short bike.....it may not be so bad on a bigger bike..........but then the rules say NO....
and I think this is a good thing...............................
Bak189

[sockjohn]

I am interested in what the consensus is in regards to wheels for M/C applications. Specifically for 'naked' classes and for the front, as a solid wheel is disallowed by rule. What wheel type represents the best attainable aero? Does a spoked wheel act as a solid wheel in regards to aero at speed (from headwind not side wind), do mag type wheels act as paddles against the wind, etc?

Everything I have seen is that there is probably not a wheel out there that is worse than spoked wheels.

Ideally you would want them to have airfoil shaped and minimal, like hayabusa factory wheels.   

[Blue]

I'm thinking both Rex and 1212 are right- to a point -there is a relationship to finess ratio and speed -(Reynolds number?-help A2).

I was told many years ago that the Goodyear blimp is pretty close to a perfect 60 MPH shape- but looking at the WW2 drop tanks -300 mph tops -they are a bit stretched out!

Food for thought.

Yes, there is a relationship between separation potential and Rn- the other way.

At low Rn, the boundary layer is very thin and has little energy in it.  This allows it to separate MORE easily than a thicker, more energetic boundary layer that we get at higher Rn.

The lowest drag fineness ratio is actually about 2.5:1.  However, this is almost impossible for any practical vehicle because the very abrupt pressure recovery is too sensitive to other effects, especially intersections with wings, tails, and wheels.  Wave drag and shock-induced-separation account for the large drag rise approaching Mach 1, but most wheel driven LSR will never get above 500+ mph where this is a factor.  We discussed this before on another thread, and based on the practicalities of axle intersections, decided that a body fineness ratio of 4 to 5 would be about optimum for a lakester.  A streamliner could be another 20% shorter.  Using this short of a design, I would add a tail aft of the main body for better stability.

Low fineness ratios allow less wetted skin area.  At subsonic speeds, this is the dominant drag besides separation.  The fact that only half of JCB's drag came from skin friction indicates a need to fix the separation that accounted for the other 50%!  Low fineness ratios also provide for more laminar flow as a percentage of wetted area;  this is the next most dominant effect after separation and total wetted area.  I worked this out from a pile of laminar flow research I have copies of and a streamliner with a fineness ratio of about 5 and the maximum cross section back about 60% of its length would have less than half of the drag of the current crop of 400 mph designs.

I hate to disappoint the streamliner crowd, but short and fat is better than long and skinny. 

Beyond Mach 1 the rules go the other way.  Wave drag is:

D/q_wave = (9pi/2)(A/L)²(EWD)

Where,
D/q_wave = flat plate equivalent wave drag
A = maximum cross section
L = total length
EWD = variation of the volume distribution vs. ideal

So beyond Mach 1, long and thin is better than short and fat.  But only beyond Mach 1.