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Jerry
human power supergeek

USA
1429 Posts

Posted - 10/05/2018 :  08:09:26  Show Profile  Reply with Quote
Get yourself a low racer or build one yourself. Build a tail fairing and you can haul stuff and be fast. You can make it from coroplast, aircraft cloth, spandex, fiberglass, carbon fiber, aluminum, etc. Add a nose cone after you get acclimated very well to the low racer. make a body sock or coroplast fairing. You said before you are in high school and wanted it to look professionally made. Believe me, when you smoke everyone around, they will not care how it looks. They will be trying to make something to out run you.

This is my Baron low racer with tail box. It is plenty fast and can haul over 80 pounds of stuff in it. I could make it look better but I don't care about looks but speed!

/img]

I might build it out of fiberglass this winter to make it even faster and look better. I re-built it about 8 times until I got it right. I made it wider, longer, taller, and everything in between. The Europeans made their tail fairings very wide because they liked to ride at 35-40 degrees while my low racer is around 18-20 degrees. I found the lower the recline of the seat, the more narrow the tail fairing needs to be.

Whatever you decide, good luck.
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Matthew Martin
New Member

USA
62 Posts

Posted - 10/29/2018 :  20:13:20  Show Profile  Reply with Quote
Ok... decided to just try a model with 150mm cranks and the difference is undeniable.

In addition I talked to the guys at SimScale and got my plan upgraded, and since, have been doing some larger simulations.

I figure that if I want (and I will) to use 170mm cranks at first I can make a removable "bubble" similar to what the Milan velomobile has on the bottom to allow for the heel clearance.

Though this was not my most aerodynamic design, I think in the end the final will be very similar to this. I substituted a faster design for wider shoulder clearance, which this model has at about 47cm (18.5in).
Now for the shape,which is showing some great results:


At a wind speed of about 80 mph it only generates 6.25 Newtons of drag, which comes out to a Cd of .023. The bike has a frontal area of .34435 m^2.






I think I am slowly closing in on a shape, soon I will build a simple steel frame (square tube) and using profiles of the bike, cut in foam, I will check clearances.
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warren
human power expert

USA
6424 Posts

Posted - 10/29/2018 :  21:47:32  Show Profile  Visit warren's Homepage  Reply with Quote
That looks great!
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alevand
human power expert

USA
3432 Posts

Posted - 10/30/2018 :  08:16:49  Show Profile  Visit alevand's Homepage  Reply with Quote
Looks like the highest speed air is underneath, yet the bottom surface of the shell is blue. I think its indicating flow separation and turbulence. Maybe if the nose where lower, less air would be forced underneath or the bottom flatter to reduce the velocity. Is the lower boundary condition moving with the free stream? Can you plot vorticity, or zoom on this area with an expanded scale? I agree with Warren. Nice work,thanks for sharing, Matthew.

C:
Tony Levand

Edited by - alevand on 10/30/2018 08:33:29
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jjackstone
recumbent enthusiast

USA
266 Posts

Posted - 10/30/2018 :  09:54:26  Show Profile  Reply with Quote
"At a wind speed of about 80 mph it only generates 6.25 Newtons of drag, which comes out to a Cd of .023. The bike has a frontal area of .34435 m^2."

Are you sure of your numbers? Just asking because if these are correct, they would be in line with the some of the fastest streamliners in the world.

The following are cda's estimated by their designers.
Aerovelo Eta 0.012
U of Toronto Vortex 0.023
U of Toronto Ace 0.054
Delft Velox 0.032
Policumbent Pulsar 0.0225
Damjan Eivie 1 0.02
Damjan Eivie 2 0.0155
Damjan Eivie 3 0.0125
Damjan Eivie 4 0.0083

Your cdA comes out to .0079 from your listed values.



JJ

Edited by - jjackstone on 10/30/2018 10:19:20
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Matthew Martin
New Member

USA
62 Posts

Posted - 10/30/2018 :  17:36:33  Show Profile  Reply with Quote
JJackstone, you are right to be skeptical about the numbers, and I was too when I saw the result of the shape and tried to dismantle it and some how it made sense at the time, thanks to you I have gone back and double checked, doing some calculations and even wasting some core time just to simply go in a circle. Ironically enough I was trying to see what I could do to try to increase the result to make it more reasonable, but my simulations were set up correctly, and nothing could be changed.

I do not see any reason this bike couldn't be one of the fastest on paper, however I know it should be slower than the top BM bikes as my frontal area is larger.

So I began investigating why the computer was giving me such low numbers. Unsuccessful in trying to debunk the numbers, I tried to justify them mathematically. So I started by changing the wind speed (15m/s) which did effect the Cd values however the results were still quite low (Cda .0087). So after doing a lot of nonsense, I still could not find a reason that my number could not work. Then I began to look at the references.

I believe the Cda you have for Eta, maybe wrong. I first get this idea because one the Team Policumbent website they state "Considering a speed of 145 kph, air density of 1.18 kg/m3 and a frontal area of 0.284 m2, we get a Cd of 0.0218".

All of a sudden my numbers don't seem as unreasonable.

I also did some calulations for the Cda of based off a statistic that I found in an article which stated that Eta would only need 198 watts to go 90 kph. This would require (using a Crr of .005 and a bike weight of 6o lb and a air density of 1.22kg/m^3) a Cda value of about .007 @ 90 kph which is far lower than the cda of my design. of course this value would decrease a higher speeds.

Of course simulations do not provide a 100% accurate portrayal of the real world but can certainly help for analyzing trends from iteration to iteration.

So to sum up, I believe the Cda value of Eta is a lot lower that what you have listed. This not only would make sense based off my findings, but the results of Team Policumbents Taurus speed bike which give a Cda of .0061912, and as we know, they don't hold the world record at this time, so it may be a safe assumption to say that Eta is more aerodynamic.

I hope this provides some clarity, and thanks for challenging the results, I ended up having a nice brain workout.

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jjackstone
recumbent enthusiast

USA
266 Posts

Posted - 10/30/2018 :  18:56:36  Show Profile  Reply with Quote
Sorry, guess I should have listed my sources. Some of them may no longer be accessible.
Haven't seen the Taurus estimates. Do you have a link to that one. Actually because your shape is very similar to the top bikes I could see it being close to those numbers. Good job on the design. Hope you get the physical build done. Please bring it to Battle Mountain next year.

Aerovelo Eta 0.012 2016 Aerovelo video http://www.aerovelo.com/mission-log/2015/7/24/questions-and-long-answers
U of Toronto Vortex 0.023 2011 Vortex design report https://www.rose-hulman.edu/hpv/design-reports/2011/Vortex-Design-Report-2011-u-of-toronto.pdf
U of Toronto Ace 0.054 2010 Vortex design report https://www.rose-hulman.edu/hpv/design-reports/2011/Vortex-Design-Report-2011-u-of-toronto.pdf
Delft Velox 0.032 2011 Velox design report http://ac.els-cdn.com/S1877705812016670/1-s2.0-S1877705812016670-main.pdf?_tid=f1c0902a-1251-11e7-a25f-00000aacb35d&acdnat=1490553061_819bd7c645aed16c48543c55689cb5c4
Policumbent Pulsar 0.0225 2015 Published article by Paolo http://aec-analisiecalcolo.it/static/media/riviste/Num71_web.pdf
Damjan Eivie 1 0.02 2002 Eivie website http://www.eivie4.com/
Damjan Eivie 2 0.0155 2004 Eivie website IIRC all of Damjans cda's are based on roll down tests from Battle Mountain so may not be particularly accurate. Or maybe he did other testing also.
Damjan Eivie 3 0.0125 2010 Eivie website
Damjan Eivie 4 0.0083 2013 Eivie website


JJ
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Matthew Martin
New Member

USA
62 Posts

Posted - 10/30/2018 :  19:17:22  Show Profile  Reply with Quote
Yes here is the link for Taurus :http://www.policumbent.it/blog/category/cfd
Its towards the bottom of the page.

In the end I wont really know how fast my bike is until its done. All I can do is create the best shape relative to my designs.

Though I may not go to battle mountain next year I would like to go in 2020 to attempt to break the Junior land speed record

Edited by - Matthew Martin on 10/30/2018 19:37:52
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jjackstone
recumbent enthusiast

USA
266 Posts

Posted - 10/30/2018 :  20:16:00  Show Profile  Reply with Quote
Thanks for the link. Trying to post pix. Not working.







JJ

Edited by - jjackstone on 10/30/2018 20:19:55
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Dreamer
recumbent guru

USA
630 Posts

Posted - 10/31/2018 :  22:03:20  Show Profile  Reply with Quote
quote:
... soon I will build a simple steel frame .. check clearances.

I recommend doing this before even starting to build the frame. I always start my final build with a mock up of the internal cockpit with the actual cranks and other moving parts along with the seat temporarily installed in their design position. Using a strip construction method inside a simple frame mockup to form the inside cockpit shape takes a couple hours to build but ensures that knees, heels and toes, etc fit when cranking at full speed and also that road visibility, steering and braking ergonomics will have room to perform and/or function as intended.

Edited by - Dreamer on 10/31/2018 22:06:13
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Terry
New Member

Canada
57 Posts

Posted - 11/01/2018 :  14:41:03  Show Profile  Reply with Quote
Matthew,
That is a very fast looking shape you are developing .
Are you planning to build the shell yourself? What shell weight are you aiming for?
Would you like some help?

If you are interested, I would offer to build the shell for you. I would use wood strip Fiberglass composite construction.

Let me know your thoughts, I would love to be a part of a record breaking attempt!!
Terry
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Balor
recumbent guru

Russia
697 Posts

Posted - 11/01/2018 :  14:58:35  Show Profile  Reply with Quote
Just a thought I've had - there is a lot of talk how road vibrations kill off laminar flow, which actually makes sense.
I think bents like Eta mount their shells with vibration dampers for that reason. It might be prudent to make the bent as light as possible, yet make the shell as *heavy* as possible, massively stiff and/or introduce vibration-damping layers in layup (think automotive soundproofing).
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Matthew Martin
New Member

USA
62 Posts

Posted - 11/03/2018 :  17:11:11  Show Profile  Reply with Quote
Thank you Terry for the offer, however the main point of this whole project is to learn how to work with composites. So I am going to do all of the work myself.

Yes Dreamer, I will make a ergonomics jig first, and check clearances.

Balor, I will worry about the small sources of inefficiencies later,when I build the molds I can build different iterations, with various internal devices, however that is later.

Edited by - Matthew Martin on 11/03/2018 21:06:44
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Balor
recumbent guru

Russia
697 Posts

Posted - 11/03/2018 :  23:25:47  Show Profile  Reply with Quote
Well, it might not be a *small* source of inefficiency, in fact it might be a difference between laminar flow over 2/3 of your fairing or less than 1/3 for instance - basically, a difference between a genuine chance of setting a record and at most (not that it is a small achevement by itself) a '70 mph hat'. Notice that at high altitude you'll likely be able to generate only half the power you think you'll be able to, unless you can afford a hypobaric chamber training :) (and bent already your own personal hypobaric chamber as far as perfusion in your legs is concerned).
What's the point of carefully manipulating the shape of a fairing to achieve maximum laminar flow if you are not going to have it anyway?
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jjackstone
recumbent enthusiast

USA
266 Posts

Posted - 11/04/2018 :  00:02:47  Show Profile  Reply with Quote
Matthew, do what you want. It's great that you are learning. By the way. A 70 mph hat as a junior is a world record. The other thing is. Not one of the junior world record holders have built their own bikes. Good luck in your endeavors.

JJ
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Balor
recumbent guru

Russia
697 Posts

Posted - 11/04/2018 :  00:30:08  Show Profile  Reply with Quote
Point is, a typical unibody liner and something like ETA with entire 'laminar' portion of the shell mounted externally and suspended are very different designs - not something you can add on later.
I think this is why ETA owns speed record despite not being the slipperiest shape 'on paper'. I don't think this is much harder to do (I might be very wrong though) - just *different* and a design choice you should make from the start.

Edited by - Balor on 11/04/2018 00:30:35
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Matthew Martin
New Member

USA
62 Posts

Posted - 11/04/2018 :  07:04:33  Show Profile  Reply with Quote
Balor I completely agree, however the first fairing I will make, will probably be made with fiberglass and a thin foam core, and have a steel frame (no shell suspension)

My point is that once I have the molds I can begin to work towards eliminating sources of drag and to maintain laminar flow using internal devices. I will build another frame to accomplish this. My current thinking is, if I design a fairing that has enough clearance for the rider (what I am doing now), later on I can design a frame with various internal devices to make the bike as efficient as possible. That's what I meant by later, but you are right, I need to be thinking about these things now.
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Balor
recumbent guru

Russia
697 Posts

Posted - 11/04/2018 :  09:32:28  Show Profile  Reply with Quote
Well, theoretically, using fat(ish) low-pressure tires takes care of that as well, but practically it would likely result in too much rolling resistance/aero hit at speeds you are willing to attain. At 60+ mph speeds and given extra fairing weight rolling resistance losses are huge. Of course, fatter tires actually roll a bit BETTER everything being equal, but the best tire you can get without going custom (huge $$$) is tubeless Vittoria Corsa speed, 23mm as far as BRR site is concerned, and I tend to trust their data. Pumped up to "near" max pressure it should provide outstanding rolling resistance (nearly half that of most racing tires) and still quite adequate vibration absorption.
So far as I know, Eta used some kind of custom tires, 20mm, likely tubulars and pumped up to enormous pressures. If you play around with your tire pressure, you *might* get away without any 'shell suspension' with a relatively small rolling resistance hit.
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Balor
recumbent guru

Russia
697 Posts

Posted - 11/04/2018 :  13:48:13  Show Profile  Reply with Quote
By the way:

"I also did some calulations for the Cda of based off a statistic that I found in an article which stated that Eta would only need 198 watts to go 90 kph. This would require (using a Crr of .005 and a bike weight of 6o lb and a air density of 1.22kg/m^3) a Cda value of about .007 @ 90 kph which is far lower than the cda of my design. of course this value would decrease a higher speeds."

Given their custom tires, assuming crr of 0.002 is not unreasonable, given that you can get very close to that with Corsas (0.0023 CRR)
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Matthew Martin
New Member

USA
62 Posts

Posted - 11/06/2018 :  17:43:18  Show Profile  Reply with Quote
It all makes sense now...

I was still thinking about the cd value that I was getting from the simulation, and just knew it had to be wrong, if Eta had a Cd of .038, and my bike, which I believe should be less aerodynamic than Eta, had a lower Cd value.

So I re-started my investigation. I already knew that my simulations had no errors and the conditions were set up correctly, however I could not see what I was missing. I almost began to doubt the Open Foam solver's reliability, but was reassured that the system was capable of producing accurate results, after seeing that Mercedes F1 uses it to design there cars. So after getting frustrated I decided take a break for a few hours, when it came to me. I had forgotten to take into account viscus drag! I had previously only been taking into account pressure drag. I then solved for both viscus drag and pressure drag and came to a much more reasonable result.

The model shown had a Cda around .016, so with further refinement I managed to get this number down.
I eventually made some changes to the front wheel fairing, and tail to achieve a Cd value of .04249 and a frontal area of .3409m^2, giving a Cda of .01448.. Now I am satisfied with this result, however on the road it will more than likely be higher due to uncontrollable variables. I am still tring different devices such as different tail shapes and front wheel fairings, however I think I am set on the main body, which has nice 46cm(18.1in) at the shoulders and gives the rider at least 12.5cm(5in) width for the feet when using a q-factor of 150mm


Now I redid the calculation for Eta and,a lower rolling resistance was all it took for the numbers to make sense.

Edited by - Matthew Martin on 11/06/2018 17:46:52
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Balor
recumbent guru

Russia
697 Posts

Posted - 11/07/2018 :  01:30:35  Show Profile  Reply with Quote
That's comfy, I think even I can fit that :). Make sure that you your width does not taper too fast compared to your shoes, or you'll be scraping your toes when ankling (which is a good idea to maximise power).
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Balor
recumbent guru

Russia
697 Posts

Posted - 11/14/2018 :  04:16:54  Show Profile  Reply with Quote
By the way, an interesting tidbit from Bicycling Science:

By 'sucking in' increasingly turbulent air at and beyond transition point, you can (highly theoretically, of course, I don't think any real experiments were made) delay boundary layer detachment and 'reattach it', and extend laminar flow from 60% to 95%, which should give you much greater savings (hundred(s) of watts) in aero drag at cost of about 20 watts of 'suction power' (even given inefficiencies of generating it using some sort of human-powered fan).

Section on bicycle aerodynamics, pages 193-196
I don't recall it mentioned anywhere here, great book btw.
I wonder how this 'suction inlets' should look like... a series of Naca ducts? A simple mesh grille?

This section also stresses that minor undulations from pedalling (and balancing corections) and road vibration will wreak havoc on laminar flow. Plus, a perfectly 'laminar' airfoil will likely stall at a hint of yaw in wind direction, be it 'undulations' or side wind.

I wonder if you can have your shell not just suspended, but gyroscopically stabilized as well?

Edit:
Totally not NACA ducts, they were designed *exactly* to deflect boundary air away and dip into freestream for better airflow with minimum extra drag.

Edited by - Balor on 11/14/2018 06:20:03
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Balor
recumbent guru

Russia
697 Posts

Posted - 11/14/2018 :  06:28:36  Show Profile  Reply with Quote
By the way, you may want to take a look at this for some "expectation vs reality" pictures :)

http://sci-hub.tw/10.1016/j.proeng.2012.04.054
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Balor
recumbent guru

Russia
697 Posts

Posted - 11/14/2018 :  10:04:46  Show Profile  Reply with Quote
Interesting "to read" material concerning boundary layer suction:
http://sci-hub.tw/10.1007/s11012-015-0100-9
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alevand
human power expert

USA
3432 Posts

Posted - 11/14/2018 :  13:33:22  Show Profile  Visit alevand's Homepage  Reply with Quote
How are you going to make the fairing?

C:
Tony Levand
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