body taper (again)

[sofadriver]

This has been touched on in several threads but I still have a question.
It's been said that the correct body taper to keep airflow laminar is 7 degrees.  I'm sure that's correct - but at what speed?  I'd think that air would always rush in to fill the void behind a passing object at the same, constant speed (regardless of the object's speed).  If that's correct, then wouldn't more taper be acceptable at slower speeds where the air has more time to remain laminar?

[wobblywalrus]

Aero drag can be looked at in terms of energy.  Air has mass and it takes energy to accelerate it.  The acceleration is a change in its velocity or in its direction of movement.  The goal is to disturb the smallest mass of air possible and to minimize changing its velocity or direction of travel.

The tail works well if flow stays attached to it for its entire length.  A seven degree angle has been commonly used.  This minimizes the energy loss due to turbulence behind the bike.  None of the references I have seen have referred to velocity vs attachment.

Another purpose of the tail is to reduce the size of the low pressure area behind the bike.  It does this regardless of whether or not flow is attached near the end.  There is a lot of turbulence associated with the truncated end of the teardrop shape.  A recent photo posted on the new Charlie Toy thread shows a bike racing across the salt.  It is a crimson or red bike, as I remember.  Its tail uses a variable taper rate on both the sides and top to reduce the truncated end size.  This is the best example I have seen. 

[wobblywalrus]

The link on reply 3 on the Charlie Toy thread shows Bike #68.  A good tail on that one.  Further down in the linked attachment is an electric bike.  Another nice tail. 

[sofadriver]

 None of the references I have seen have referred to velocity vs attachment.
 

Exactly, but doesn't it make sense that velocity would affect attachment?

[maj]

i think you would need to be extremely fast to start seeing your speed effect attachment

If you google and start reading on commonly used naca shapes you can find wind tunnel data and detachment points in relation to Reynolds number

Human powered LSR bicycle guys have a lot of info on shapes around naca 0024 , and wind turbine blades are often at naca 0018 (this has a bit of info available )
on a APS bike your probably limited to a naca 0021 to a 0024 unless you dont mind a larger rear surface where you cut it off 

In my experiance it seems difficult to get the air to reattach after the leg cutouts , it can take up to3 feet to stabilise the flow , which just gives you a broad area for the turbulance to act on if you get it wrong

 

[tauruck]

I really feel for you guys with bikes.
Aero is no joke and lets be honest, there isn't much wind tunnel data floating around.

Sofadriver R&D, R&D and more.

[Rex Schimmer]

Sofadriver said: "Exactly, but doesn't it make sense that velocity would affect attachment?" My response is to get a good book on aero, read the definition of Reynolds number until you understand it and then re-think what you said.

Rex

[maj]

mixing up my viscocity with velocitys

[Nortonist 592]

mixing up my viscocity with velocitys

My velocity tends to suffer from viscosity.

[John Burk]

I'm confused on this . 66-021 profile contracts at about 16 deg per side but is one of the lowest drag profiles . How can that be ?

[sofadriver]

on a APS bike your probably limited to a naca 0021 to a 0024 unless you dont mind a larger rear surface where you cut it off 

Thanks for that tip, maj.

naca 0021 tapers at about 13% with the radius measuring 21% at 30% chord length.  That's about a perfect fit.

[sofadriver]

I'm confused on this . 66-021 profile contracts at about 16 deg per side but is one of the lowest drag profiles . How can that be ?

And that's much more than the naca 0021.

[John Burk]

Between .7 and .8 cord 66-021 tapers 19.563 deg .

[Rex Schimmer]

John,
The 66021 is one of the NACA NLF (natural laminar flow) designs. There is a great design book by a guy named Bruce Carmichael ( Personal Aircraft Drag Reduction) that discusses these kinds of things. Worth buying.

Rex

[wobblywalrus]

Don't forget about the profile of the top of the bike.