Wednesday, July 7, 2010

fairing design. primers.

Recap: I want a fairing that is PRACTICAL.
That means
1) extends over at least half the length
2) is not on the sides to avoid crosswinds
3) is reasonably light
4) allow cooling air flow

D: How to design air intakes:
http://www.recumbents.com/wisil/nacaduct/naca-duct.htm

I think it's important that the vent be placed in front of the windshield as there is a high pressure zone there caused by the windshield bubble. The vent bleeds off some of this pressure. It also makes it easy to have the air blow on your face, as that's where the most blood vessels are to cool you off.

D: Doesn't this mean just put it at the nose tip.

http://www.recumbents.com/WISIL/barracuda/barracudafairingdesign.htm

Some basic "rules" are:

Avoid outside corners and areas of sharp streamwise curvature. Shaving off a few square inches of frontal area by carving a flat spot at the widest point will only increase the drag.

Avoid sudden changes in streamwise curvature. The hemispherical nose butting straight into a zero-curvature cylindrical section is a good example of a bad shape. There will usually be a large velocity spike near the curvature discontinuity, followed by a small separation zone.

Avoid aft-facing steps and air leaks at and behind the maximum-thickness point. The small recirculating zone behind a step might not be able to close in the aft pressure rise and might precipitate large-scale flow separation. Leaking "dead" air into the boundary layer over the pressure rise is very detrimental for the same reason. Minimizing the flow out of wheel cutouts is important. The best place to dump ventilation air flow is out of trailing edge.

The back edge or point does not have to be perfectly sharp. A good rule of thumb is that if the area of the flat base is less than drag_area/4, there will be no drag penalty. A good HPV fairing might have CD = 0.10 based on a frontal area of 5 ft^2, in which case the allowable base area would be 0.10 x 5 / 4 = 0.125 ft^2. This might make the fairing a bit shorter.

Surface finish is important, but there is a threshold roughness height k_min below which there is no further drag reduction. This is given by k_min ~ 4 sqrt(2/Cf) nu / V nu = kinematic air viscosity = 1.4e-5 m^2/s V = local flow velocity (m/s) Cf = local skin friction coefficient ~ 0.025 (nu/VL)^(1/7) L = distance from leading edge (m) For a typical fairing at V = 10 m/s this works out to Cf ~ 0.004, k_min ~ 0.1 mm = 0.005 in, which is not all that smooth -- like the surface of galvanized steel, say. A doped cloth surface is probably close to this threshold. For a speed HPV this roughness threshold would be quite smaller.

As far as "truck suck" or crosswind sensitivity, there is very little that can be done other than reduce the height and side area as much as possible. The "airfoil" shape of the fairing will have little effect.

D: hmm the nose-tip based air intake should try to play well with the top/bottom fairing edge.

. Recumbent Cycling Ontario claims that a recumbent's higher terminal velocity permits speeds 34 percent faster than an upright bike. Recumbent bikes climb slowly because they are heavier and the rider cannot pull up on the handlebars. All other things being equal, a road bike is faster on a hilly course than a recumbent.

Read more: Recumbent Vs. Road Bike Speed | eHow.com http://www.ehow.com/about_5457075_recumbent-vs-road-bike-speed.html#ixzz0t25sKtlh

D: but I'm using it to keep warm and/or keep the sun off/keep cool.
I am north-Euro and will MOULT in noonday sun!

D: I've always thought nosecones and tailboxes look nice and do very little.
They defy the rules of thumb for good physics.

D: right now, I"m thinking the bottom fairing and tirewell shells just stay put as basic construction.
The top part can be removed.
For slow tooling around in hilly offroad conditions, this is a good compromise.

http://www.recumbents.com/wisil/missile/missle.htm
D: - always thought wisil missile was innnovative. Apparently didn't work so well.

I'm imagining an air inlet in the nose tip.

Possibly the front half of the top drops down to just above knees, leaving head exposed. But that makes it all very complex.
Worse yet is if I want the bottom top half to optionally drop down for a 'lowracer' layout - very reclined anyway.
No real pracical reason on a tour bike.
But take off the ALICE pack and recline it on the ammo box 'n you've got yer basic layout for breaking highway speed limits...
Making me think that a crash cage might be a good idea on that layout.
Which would require a long wheelbase for stability.
The gearing is a bugger.

Suggestion:
Alfine 408% -11 gear.
18 inch granny gear.
73.44 gear inch top gear.
183.6 with 2.5x Schlumph hyperdrive.

Obviously that is overkill.
A 15" super-granny gear is likely to find much more utility.

I'm waiting on Schlumph about efficiency and the high vs hyper speed options.

5 comments:

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