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Old 07-14-2010, 01:58 PM   #1 (permalink)
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Aerodynamics questions related to recumbant HPVs

I was wondering, what would be the most aerodynamic shape for a recumbent bike shell would it be better to make a larger shape with a smooth surface, or a smaller shape with an protruding "pod" for your head to fit into?
I'm wanting to design a shell, and was wondering which of these would be the best place to start.
This
adventuresofgreg.com/HPVlog/photos/05-19-06/CPfloating.jpg
adventuresofgreg.com/images/CPstrip.jpg
This
instructables.com/files/deriv/F4U/AT7F/FTDBN5DD/F4UAT7FFTDBN5DD.MEDIUM.jpg
or this
instructables.com/image/F87FLXYF0H63OGW/Video-Part-Making.jpg
I can't post images yet

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Old 07-14-2010, 02:59 PM   #2 (permalink)
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Quote:
Originally Posted by snowluck2345 View Post


This

or this

I can't post images yet
I can post images.

My hunch is a small "head pod" is ever so slightly higher drag than if the head pod were extended all the way to the front of the vehicle. But I'd go with a small head pod, for the sake of simplicity and light weight.

It depends on your budget and goals, I suppose. Why not start with an open top like so:

Last edited by RobertSmalls; 07-14-2010 at 03:04 PM..
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Old 07-14-2010, 03:02 PM   #3 (permalink)
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The shapes above may be the best there now is, but not the best there could be.

For that, reading Bruce Carmichael may be necessary.
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Old 07-14-2010, 03:09 PM   #4 (permalink)
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There's a ton of info here: The Recumbent Bicycle and Human Powered Vehicle Information Center

You might want to take a look at this list and pattern your fairing after one of the winners: IHPVA - International Human Powered Vehicle Association

This is the winner of the .Decimach prize: http://www.ihpva.org/land.htm

Last edited by Patrick; 07-14-2010 at 03:32 PM..
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Old 07-14-2010, 05:07 PM   #5 (permalink)
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There are a lot of trade-offs involved. A two-lump shape has more drag than a continuous curve, all else being equal, but by so little that the skin drag difference may matter more if you don't need all the volume. As for HPVs, You are dealing with problems such as solar gain, demisting access, vision through imperfect plastic, which is terrible at low angles with bad lighting and/or rain and so on, as well as doing trade-offs between frontal area, rider position, wheel placement and size, vision, cooling, surface area and so on.
A pure laminar shape tempted the HPV pioneers, but surface flaws and vibration generally defeated them, while adding considerably to frontal area in some examples. Lately, gains have been found by using small cranks to reduce frontal area, raising RPM 30% or so, until a 1 or 2% power loss wipes out further gains- until someone who has learned to spin a Hot Wheel at 300 RPM shows up.

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Old 07-14-2010, 05:16 PM   #6 (permalink)
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Plus, in the real world even bikes are subjected to x-winds. The small pod could very well have the drag advantage there, and almost certainly "weathervane" less.
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Old 07-14-2010, 07:58 PM   #7 (permalink)
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So, what would be the most aerodynamic shape possible for one of these? A reverse teardrop with a reverse teardrop head pod?
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Old 07-14-2010, 09:02 PM   #8 (permalink)
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Well the Varna Diablo has had the absolute HPV speed record for a while.
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Old 07-14-2010, 09:49 PM   #9 (permalink)
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Quote:
Originally Posted by snowluck2345 View Post
So, what would be the most aerodynamic shape possible for one of these?
A single teardrop is a hair better, but you're not going to achieve it. Better question: what's the right shape for a homebuilt HPV fairing used for (commuting, competition, setting a specific record)?
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Old 07-15-2010, 01:33 AM   #10 (permalink)
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If you want the ultimate, skip the head bubble and use remote viewing. Oval pedal paths are nice, too. It is not too unusual for a team to show up at an IHPVA race with a brand-new streamliner, designed on the very best theory. They come in trying not to sneer at the greasy gypsies who have been hacking away for years, knowing they will blow them all away at the awards banquet. Then they place in the lower half of the field. There is so much you need to know, I'll quit trying to make a dent with custom essays.

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