(tail)Fairings again, with !!science!!.

[BalorNG]

Both human power and typical electric motors (or I should say - batteries less than that of Tesla) benefit a lot from aerodynamic efficiency. A road bike-like 'tucked in position' is not terribly effective and uncomfortable. A fully recumbent position is effective and comfortable, but compromises control and visibility/vision.
So, an upright position + fairings seems like an interesting idea.

A full fairing is great, but VERY tricky to pull off and has ingress-egress issues and rider cooling issues in case you want to pedal.
Front fairings leads to 'wind steer' and are not terribly effective because they do not do anything about 'pressure recovery' - eliminating 'suction' behind the trailing edge.

Tailboxes should work - but anecdotally few find them worth it, due to giving only 5, at best extra 10% cruising speed at most.

Problem with typical tailboxes is that human body is a terrible leading edge, and without a proper leading edge there is almost no chance of pressure recovery - air is 'splashed to the sides' instead, generating wide wake.

So, here is my take on the problem... It all started with this picture from NASA:

And running CFD simulation of a similar 'geometric primitive' that confirmed that, here is how it looks like:

Speed field:

HUGE separation going on. No chance of pressure recovery.

Turbulence:

Basically, we have huge vorticity created by separation bubble.

Resulting Cd 0.87 Less than 1.14 in Nasa example - that's because of the bevel. Without it, things are basically the same.



Talking 'airfoil shapes'...

Naca 0025, full:

The wake is tiny, and CD 0.05


Now, let's try lopping the tip off a bit...

Well, I'll be damned. Drag is higher, but only 0.055 now! This can work!

But resulting airfoil is damn huge, if made to the size of human body (and the flat section represents a human, so it must be 40-45cm wide at the 'base' and ~60cm at the widest point! Even wider in my case, unfortunately... )

Let's try and Kamm it to get it to 140cm:

Ok, a bit too much, drag coefficient shoots up to 0.1 - nearly double.

ell, I can use a different naca - 0030, it's shorter for it's width, and kamm a bit less, leaving total length at 150cm - which is something I can live with provided that tailbox does not stick out past the rear wheel too much.

Well, hot damn x2!
Calculated drag coefficent is actually LESS than 'whole' NACA 0025 - 0.045!

This actually might be a modelling artefact, but how it comes to this conclusion kind of makes sense - there is more pressure drag, but much less viscous (surface friction) drag as well, and I've read that Naca 0030 (which is about 3 times as long as it is tall) is kind of optimal when it comes to classic airfoil shapes so far as pressure/skin friction is concerned, so yea, that might be true.

Well, I don't have much resources left on my free CFD account, let us try to use this shape to create an actual 'Mother of all tailboxes'!

I have created a monster! Yes, this is a coaxial MBB arrangement that would work just great for this purpose, resulting in even weight distribution, and low BB that I think is very important because flailing legs in your crossection might work well on unfaired bent, but would likely actually disrupt air flow and prevent tailbox from doing it's just with utmost efficiency.

I've run this model in CFD and got... slightly disappointing results - the tailbox does seem to work, but instead of 0.045 I now have 0.4 Cd (an order of magnitude more) and resulting CdA is 0.2 (total frontal area is almost exactly 0.5 m^2) - which is actually pretty cool, like a well-optimised highracer with a medium-sized rider in a highly reclined position or a TT bike (also pretty well optimised), but not exactly ground-breaking. Not that I expected Cd like that of a BM speedbike, but still...

But what's this?

Round tubes of forks and bars create THIS much drag (and turbulence)?!
It looks like it generates like half the drag of entire bike!

https://commons.wikimedia.org/wiki/File:Hoerner_fluid_dynamic_drag_coefficients.svg

Well, apparently a long round cyliner does create a ton of drag!

Also, underbody flow with flat bottom seems not perfect either. Let's try a version two...

So, I've basically changed the forks and bars to NACA 0030 airfoils and reshaped the tailbox a bit.

Speed:

Pressure:

Turbulence:

Ok, there is clearly some interference drag going on (talking about tendency to make aero fork 'legs' much wider for marginal aero gains), but at least not a whole lot of turbulence.

But main point is, Cd dropped to 0.25 and my CdA is now 0.125 - which almost exactly that of a NoCom lowracer and close to older generation 'fast' velomobiles!

Of course, this a simulation, and I don't have effect of pedalling, aero drag of of transmission bits, spinning wheels, even feet (pedal fairings are a must, I think)... but this must be close to truth and pretty damn cool for a bent with completely upright position (body english!) and wide-ish bars, zero ingress-egress and rider cooling issues. Also, since tailbox is wider than the body, if made sturdy enough - it will still offer protection from road rash and will be VERY visible on road.

So, preliminatry finding is, if you want your tailbox to be REALLY effective, it must be wider, and considerably, than your body, very long and shaped as a proper airfoil, and be somewhat '3d', so there would be no turbulence at the top and bottom, also it should include head fairing because head is pretty un-aero as well.

For someone who want 'legal-ish' and effecitive ebike, simply using overruning pedal hub would likely work great - you have option to help with pedal on very steep hills and during starting (very low single gear), and once up to speed the hub freewheels. No chain at all.

I'll be trying to implement this construction I think - this should be easier than a full fairing by far.

 

[red]

BalorNG,

Thank you! I hate aerodynamic drag. I do have a question, though. I know that drag operates at the square of velocity. At low speeds, I would expect almost no benefit, and maybe even a small loss due to the extra weight. In the real world, at what speed does the tail-box streamlining become noticeably effective, for your designs? Do you have any suggestions on materials and construction?

In the early days of hang gliding, all of our single-surface gliders had large round crossbars out in the breeze, keeping the wings spread. Modern hang gliders usually have the crossbars inside the airfoil now. Anyway, I got very good results with 4:1 (thickness ratio) streamlining on the crossbars, at a fixed Angle of Attack, from about 20 mph and up. So, I'm a believer. 8)
These crossbar fairings are nine inches in chord.

 

[BalorNG]

BalorNG,

Thank you! I hate aerodynamic drag. I do have a question, though. I know that drag operates at the square of velocity. At low speeds, I would expect almost no benefit, and maybe even a small loss due to the extra weight. In the real world, at what speed does the tail-box streamlining become noticeably effective, for your designs? Do you have any suggestions on materials and construction?

In the early days of hang gliding, all of our single-surface gliders had large round crossbars out in the breeze, keeping the wings spread. Modern hang gliders usually have the crossbars inside the airfoil now. Anyway, I got very good results with 4:1 (thickness ratio) streamlining on the crossbars, at a fixed Angle of Attack, from about 20 mph and up. So, I'm a believer. 8)
These crossbar fairings are nine inches in chord.

Well...
https://www.gribble.org/cycling/power_v_speed.html

Let's say we have 80 kg rider on 15 kg bike like pictured (BB much higher than upright bike, pretty low seating but upright position), that translated to about 0.42 CdA and 0.8 Cd, CRR of 0.005 is quite decent, but not all-out racing tires.

So, it takes just 17 kmh for air resistance to overcome rolling resistance, and you'll move 32.5 kmh while pedalling 200watts (serious, but sustainable effort for a fit recreational rider).

You will also climb 3% hill (pretty common) at that wattage at speed 18.5 kmh.

https://www.gribble.org/cycling/power_v_speed.html?units=metric&rp_wr=80&rp_wb=15&rp_a=0.42&rp_cd=0.8&rp_dtl=2&ep_crr=0.005&ep_rho=1.22601&ep_g=3&ep_headwind=0&p2v=200&v2p=35.41

Now, add, say, 10 kg (which is actully pretty damn huge, but assume you've didn't use carbon fiber and other high tech materials, but sheet plastic, PVC tubes, aluminium wire, etc) of fairing.
Also it will inflate your area to 0.5 but reduce Cd to 0.25 (assuming that this estimation is correct).

Let's plug in the numbers:
At 200 watts, you'll ride at 43.3 kmh (almost 27 mph, which is about equal to maximum legal ebike speed for cat 3 ebike) and climb 3% hill... at 18.5 kmh!
https://www.gribble.org/cycling/power_v_speed.html?units=metric&rp_wr=80&rp_wb=25&rp_a=0.5&rp_cd=0.25&rp_dtl=2&ep_crr=0.005&ep_rho=1.22601&ep_g=3&ep_headwind=0&p2v=200&v2p=35.41

So, it will not slow you down on hills except those very steep, but will *greatly* increase your speed or, assuming you have a cat 3 ebike and travel at maximum speed will increase your mileage:

With fairing: 218w at 28 mph
Without fairing: 476w at 28 mph

More than two times, more than doubling your range, so you can, say, get away with much smaller battery. Incidentally, it will greatly increase your capacity for harvesting energy from regen braking, because otherwise you will 'coast forever'.

Yea, that's a bit optimistic, but than there are precedents of extremely fast tailfaired semi-lowracers with position that is less reclined than usual, but use similar tailbox principle:

https://speedbikes.ch/birk-piloten

The main point here is 'integrate' the rider into a fairing, so he becomes part of the leading edge.

 

[nicobie]

That's very interesting, especially the Birk RT Comet fairing.

 

[BalorNG]

That's very interesting, especially the Birk RT Comet fairing.

Based on my CFD experiments, it does not seem *quite* wide enough either
But than, I'm no stranger to overly optimistic CFD predictions (I've had one basic one give me very low drag for a nose cone fairing that turned out to... basically do nothing at all)...

 

[PaPaSteve]

I know that drag operates at the square of velocity. At low speeds, I would expect almost no benefit, and maybe even a small loss due to the extra weight. In the real world, at what speed does the tail-box streamlining become noticeably effective, for your designs? Do you have any suggestions on materials and construction?

Being a "practical" engineer I find coast-down testing to be rather revealing.
Literally have done hundreds of coast-downs with dozens of different bikes and aero combinations.
Starting the test from a dead stop at a marked spot on a hill.
The method uses bicycle speedometers with wired sensors that have the ability to download data to a computer.
A model that has a file type of .csv is most useful as it goes straight into a spreadsheet.
The coast-down data is placed side by side in a spreadsheet to compare distance verse time.
Each wheel revolution is counted so a precise calibration for the wheel roll out is necessary for accurate results.
Anyway ...
What I found is : seemly minor aerodynamic improvements can be seen within the first few revolutions of the wheel.
Meaning, to me at least, that aero improvements start at almost zero wind speed.
As you note, if there is a significant weight increase from the aero device then over the long haul it can be a hindrance more than an asset.
For a human powered vehicle (powered by me) the cut off point is around 45 pounds.
Meaning a streamlined vehicle over 45 pounds is ... overall, in real life roadworthy conditions, slower.
A 30 pound aero improved streamlined vehicle can yield "run away from the pack" speeds.

 

[BalorNG]

I know that drag operates at the square of velocity. At low speeds, I would expect almost no benefit, and maybe even a small loss due to the extra weight. In the real world, at what speed does the tail-box streamlining become noticeably effective, for your designs? Do you have any suggestions on materials and construction?

Being a "practical" engineer I find coast-down testing to be rather revealing.
Literally have done hundreds of coast-downs with dozens of different bikes and aero combinations.
Starting the test from a dead stop at a marked spot on a hill.
The method uses bicycle speedometers with wired sensors that have the ability to download data to a computer.
A model that has a file type of .csv is most useful as it goes straight into a spreadsheet.
The coast-down data is placed side by side in a spreadsheet to compare distance verse time.
Each wheel revolution is counted so a precise calibration for the wheel roll out is necessary for accurate results.
Anyway ...
What I found is : seemly minor aerodynamic improvements can be seen within the first few revolutions of the wheel.
Meaning, to me at least, that aero improvements start at almost zero wind speed.
As you note, if there is a significant weight increase from the aero device then over the long haul it can be a hindrance more than an asset.
For a human powered vehicle (powered by me) the cut off point is around 45 pounds.
Meaning a streamlined vehicle over 45 pounds is ... overall, in real life roadworthy conditions, slower.
A 30 pound aero improved streamlined vehicle can yield run away from the pack speeds.
Cherry-Pie-Race.JPG

Have you ever tried a similiar concept of huge tailbox that is wider than the rider?
This is not a new concept, when it comes to aero wheels, 105% rule applies:

https://silca.cc/blogs/silca/part-5-tire-pressure-and-aerodynamics

But than, a human body is much worse leading edge than even tire is!

I think in case of human body, rule 120% should apply, as my tentantive CFD expriments show...

 

[Warren]

There is a ton of stuff out there. There was/is? a European racing class for recumbents that only allows tail fairings. I recall a lowracer with an enormous, very aero tailbox. The rider sat into the tailbox...sort of a centaur/mermaid. Can't find an image now....found it!

http://recumbents.com/wrra/records.asp?MoreInfo=Yes&ID=73

https://scontent-iad3-1.xx.fbcdn.net/v/t45.5328-4/15541832_1068070093302027_7965906406878478336_n.jpg?_nc_cat=106&ccb=1-7&_nc_sid=c48759&_nc_ohc=kKFC6dj7OMwAX9JZZHv&_nc_ht=scontent-iad3-1.xx&oh=00_AT_ZSLCnvDtM64wR_MWHjyCUxSSIi7ZUYgsKCznMGv1Tyw&oe=6353301B

 

[BalorNG]

There is a ton of stuff out there. There was/is? a European racing class for recumbents that only allows tail fairings. I recall a lowracer with an enormous, very aero tailbox. The rider sat into the tailbox...sort of a centaur/mermaid. Can't find an image now....found it!

http://recumbents.com/wrra/records.asp?MoreInfo=Yes&ID=73

https://scontent-iad3-1.xx.fbcdn.net/v/t45.5328-4/15541832_1068070093302027_7965906406878478336_n.jpg?_nc_cat=106&ccb=1-7&_nc_sid=c48759&_nc_ohc=kKFC6dj7OMwAX9JZZHv&_nc_ht=scontent-iad3-1.xx&oh=00_AT_ZSLCnvDtM64wR_MWHjyCUxSSIi7ZUYgsKCznMGv1Tyw&oe=6353301B

Yea, Birks were an inspiration, but airfoil shapes are 'stock' from nasa.
Point is, I don't like their seat angle and BB height. In my experience, it leads to worse recumbutt and tailbone soreness than completely upright position! The body creates a 'kink' called 'sacral position'.

Basically, completley upright or just a bit of recline with low BB is fine. Very reclined with high BB is also fine FOR MOST, not me though... between those positions there is a weird no-mans land where you end up with recumbutt and sliding on the seat a lot, which sucks even with very wide and custom my butt-shaped seat - something I notice even in office seats.

I think using 'office chair ergonomics' should work for long similar multi-hour rides provided inherent 'unaero' of this position is compensated for by a proper tailbox.

https://www.work-fit.com/blog/how-to-sit-properly-at-your-desk

 

[Warren]

Check out Ron Thompson's efficiency study results!

View attachment Speed Comparison 1 Nov 22.pdf


https://youtu.be/EnxLkYtfpjg

 

[BalorNG]

Check out Ron Thompson's efficiency study results!

Speed Comparison 1 Nov 22.pdf


https://youtu.be/EnxLkYtfpjg

Yea, I've discussed it with him. His tail box does not seem to be *particularly* effective to be frank, given that I've got similar results on my MBB with similar position, also full disk wheels, considerably higher weight, similar fitness (well, two years ago...) but actually full suspension, much less aero frame, higher CRR and much lower BB... it might actually ruin his aero, in fact
Does not seem anything particularly wrong with his setup otherwise, should be much faster I think...

Btw, recent hour record on upright TT bike is almost 57 km per hour and CdA after all the refinements was ~0.15 m^2 I think.

That's considerably better than all bents but best of partially faired, extreme lowracers and fully faired, of course.

 

[JeffH]

One of the biggest (latest) advances in paragliding is an air inflated, lightweight fabric fairing for the harness.

May be worth a look for bike applications

https://flyozone.com/paragliders/products/harnesses/submarine/

 

[BalorNG]

Yea, a buddy of mine was talking about this idea for 'bike applications' a long time ago.
But idea is one thing, and actual implementation is an other...

 

[red]

b548dc30decfac965ba233300b64fe23ade8a0b2-1941x1300[1].jpgOne of the biggest (latest) advances in paragliding is an air inflated, lightweight fabric fairing for the harness. May be worth a look for bike applications

Streamlined paraglider harnesses are not that new, but they have been made from semi-rigid foam sheet, covered by ballistic cloth.

The inflatable pilot "submarine" covering is much lighter, but paragliders are always going forward at about 20 mph (30 kph), so ram-air inflation is no problem. I have to wonder how you put road wheels on the ground with an inflatable skin, and what does it look like when you go slow or stop for traffic control, and start off again.

You could maintain inflation with small fans. The inflatable airfoil on the Whoopi-Fly (no kidding!) hang glider has such fans, in the ram-air scoops that inflate the airfoil in flight. The Whoopi-Fly is a low-performing hang glider that fits into a backpack. Still, as vehicle streamlining, any inflatable bike skin would be complex to make, with poor personal ventilation. It would not be one-size-fits-all, either, but custom-made to fit every different bike. The simpler paraglider "submarine" pictured in the post above now costs more than US$ 1700.

 

[Warren]

His tail box does not seem to be *particularly* effective to be frank

Btw, recent hour record on upright TT bike is almost 57 km per hour and CdA after all the refinements was ~0.15 m^2 I think.

That's considerably better than all bents but best of partially faired, extreme lowracers and fully faired, of course.

Yes. His tailbox is way too small to be of much use...buried in his wake.

I was interested to see he verified research done years ago, about high bottom bracket having better aero. You want your cranks, feet, and legs, in front of your torso.

Got a link for the TT bike? The problem with aero uprights is they are horribly uncomfortable, and you are looking out of the top of your skull. Life is too short to put up with that. I recall somebody marketing a pair of riding glasses with prismatic lenses in them, so you could leave your neck straight on aero bars, and still see where you were going.

 

[Warren]

The Aero Edge body sock, and nose bubble was about as good as you were ever going to streamline a human on an upright, and still be semi-comfortable, and in control. I saw Mike Mowett racing his at WISIL events years ago. He may still be at it.

http://4.bp.blogspot.com/_LWGcbKlwPjU/TC9EoHK-YsI/AAAAAAAAATY/m19YKJC-tAA/s1600/R1-+7A.jpg

 

[BalorNG]

His tail box does not seem to be *particularly* effective to be frank

Btw, recent hour record on upright TT bike is almost 57 km per hour and CdA after all the refinements was ~0.15 m^2 I think.

That's considerably better than all bents but best of partially faired, extreme lowracers and fully faired, of course.

Yes. His tailbox is way too small to be of much use...buried in his wake.

I was interested to see he verified research done years ago, about high bottom bracket having better aero. You want your cranks, feet, and legs, in front of your torso.

Got a link for the TT bike? The problem with aero uprights is they are horribly uncomfortable, and you are looking out of the top of your skull. Life is too short to put up with that. I recall somebody marketing a pair of riding glasses with prismatic lenses in them, so you could leave your neck straight on aero bars, and still see where you were going.

https://www.cyclingnews.com/news/euro75k-per-hour-filippo-gannas-full-gear-and-kit-list-for-his-hour-record-attempt/

And I've actually tried a pair of lens like this, but promptly got nauseus

 

[Warren]

https://www.cyclingnews.com/news/euro75k-per-hour-filippo-gannas-full-gear-and-kit-list-for-his-hour-record-attempt/

As I thought. I don't see anything there that is any different than what was being done decades ago. It is useless as a real bike, for normal people. Twenty five years ago, I did dozens of long ride events on a 1995 RANS V-Rex, a not very aero recumbent converted to dual 559 x 25, and coasted away from guys on triathlon bikes with aerobars. ^They couldn't maintain the needed position for mile after mile. Unless you are a tiny person, in a skin suit, who can endure real pain, such bikes are a joke.

 

[BalorNG]

https://www.cyclingnews.com/news/euro75k-per-hour-filippo-gannas-full-gear-and-kit-list-for-his-hour-record-attempt/

As I thought. I don't see anything there that is any different than what was being done decades ago. It is useless as a real bike, for normal people. Twenty five years ago, I did dozens of long ride events on a 1995 RANS V-Rex, a not very aero recumbent converted to dual 559 x 25, and coasted away from guys on triathlon bikes with aerobars. ^They couldn't maintain the needed position for mile after mile. Unless you are a tiny person, in a skin suit, who can endure real pain, such bikes are a joke.

Well... I've personally been doing 600 km brevet with a guy on a road bike with aero bars... as in - I've *started* with him, he promptly shot away.

He finished the brevet in a bit less than 24 hours... AND got a few hours of sleep. His average speed was close to 20 mph.
Do not underestimate 'upright' form-factor... and limits of human endurance, heh.

Personally, I find recumbutt on 'not fully reclined' recumbents not that much less annoying than seat pain on uprights, and when it comes to 'fully reclined - like I already posted somewhere, my 400 km hilly brevet I did one year on my hybrid MTB bike with fat tires were completed in 18 hours or so, while trying it next year on a recumbent I've bought (and actually spent a very considerable time training on before the event) was 27, barely managing to fit into time bracket. I've had to walk a few hills that were a breeze on upright.

But yea, in theory I can actually exceed this CdA with this fairing and retain position that should be comfortable in the long run, and offer perfect vision, visibility and no power penalty.

 

[Warren]

Well... I've personally been doing 600 km brevet with a guy on a road bike with aero bars... as in - I've *started* with him, he promptly shot away.

Ah! Randonneuring is not for normal humans. Possibly 1% of people can do what you are doing. Even in my twenties I was totally incapable of such a thing. Count yourself as extraordinarily fortunate

Our son suffered a debilitating life-long nerve condition from commuting on an upright roadster in Japan in his twenties.

 

[Chalo]

Our son suffered a debilitating life-long nerve condition from commuting on an upright roadster in Japan in his twenties.

What did he do wrong? Considering the billions of people who've gotten on just fine with normal bikes, there must have been something.

 

[BalorNG]

Our son suffered a debilitating life-long nerve condition from commuting on an upright roadster in Japan in his twenties.

What did he do wrong? Considering the billions of people who've gotten on just fine with normal bikes, there must have been something.

Technically, neuropathy from pinched nerves due to lots of weight on your *hands* is pretty common in ultra-distance randos, some have it for weeks after 1000+ km brevets, I can see how one can have permanent damage this way.
Funny thing is, aero bars that are not very extreme (low) are a remedy for this.

 

[Warren]

What did he do wrong? Considering the billions of people who've gotten on just fine with normal bikes, there must have been something.

You are a disgusting human being.

 

[Warren]

Technically, neuropathy from pinched nerves due to lots of weight on your *hands* is pretty common

His neuropathy developed in his pelvic region. It is rare in men, but it does happen. He had to leave his job, and his girlfriend in Japan. He spent several years seeing specialists, and undergoing multiple procedures and tests. The pain was so severe, he was in a very dark place. Ultimately he ended up on a drug, not an opioid, they were of no help, to relieve most of the pain. That, moderate exercise, and meditation has allowed him to go back to teaching, and build a new life, with a loving wife in the US.

 

[Chalo]

What did he do wrong? Considering the billions of people who've gotten on just fine with normal bikes, there must have been something.

You are a disgusting human being.

Why? Bikes are proven tech, optimized for generations. If merely riding one were capable of inflicting that kind of damage, we'd know it, and what we consider normal bikes wouldn't be normal. So there's something else to it.