Bikes and air pockets (General not technical)

[briansnail]

I was just reading a members post about Neil Campbell's epic 174 mph assisted bike land speed record. Does anyone understand the origin of the force generating his high speed and where the force is applied. Its a bit like the Bernoulli principal half the people argue it keeps planes in the air, the half disagree.
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For those who need a bit more detail. A Porsche Cayenne towed (a very youthful British rider) Neil Campbell at high speed. He also helped design the bike which was built by Moss bikes in the UK. It was a fixie with small wheels.There was a expert on aerodynamics on his team. He aims in the future to get to 220mph and challenge the current bike assisted record currently held by Miss Denise Mueller Korenek. Watching the exhilarating roar of a Rolls Royce Turbo fan jet engine in full thrust I get. How exactly Neil gets his extremely high speed leaves me mystified.

[Postboxer]

If you can, maybe watch this

https://www.channel4.com/programmes/spe ... guy-martin

Episode one he cycles behind a truck if I remember correctly, I think there's a bit on aerodynamics. I would scroll through to find the relevant section but would have to watch adverts first and more if it's after any of the advert breaks. It's a good series though and a few are cycling related, ish.

[Brucey]

get the aerodynamics right and you can be pushed along at great speed whilst following another vehicle. It takes planning, effort and sheer guts to ride a bicycle that fast, and perhaps set a record, but it isn't necessarily a feat of great athleticism per se.

cheers

[gxaustin]

the Bernoulli principal: half the people argue it keeps planes in the air, the (other) half disagree.

Really? What is the counter argument?

[mattheus]

Slight clarification:
- there is no controversy that the Bernouilli effect is what it is!
- and I think everyone agrees on why riding behind a van (or a big shroud) helps you ride faster
(good HGV pic, btw Bruce)

But there has long been quite a controversy about Bernoulli and wing-lift; I think it's something along the lines of cause vs effect, but i'm not the best person to address that particular question!
(IIRC you can get lift from a wing that is symmetric in profile, which confounds the early days of lift explanation. Maybe ...)

[whoof]

the Bernoulli principal: half the people argue it keeps planes in the air, the (other) half disagree.

Really? What is the counter argument?

Air flows over the top of the wing travels faster that below the wing, the pressure difference produces lift. If the upward lift force is greater than the downward gravitational downforce the plane will fly.


But what about this?

The 'top' of the wing is now below the 'bottom' of the wing. The wing doesn't know it's upside down and neither does the air. The upward 'lift' force is now a downwards force and gravity hasn't changed. How can the plane fly upside down?

[JakobW]

(Drifting OT...)
There's a common incorrect explanation for wing lift that relies on shape of a typical aerofoil. According to this, you imagine two air molecules at the leading edge; if one goes over the top of the aerofoil and one under the bottom and they meet at the trailing edge, then the one that goes over the top has had a longer path, ergo must have travelled faster, ergo as per Bernoulli the pressure on top of the aerofoil is lower, and so you get wing lift. However, there's no reason why particles passing the leading edge at the same time must reach the trailing edge at the same time; though the ones passing over the top do indeed go faster (and therefore create a lower flow pressure), in fact they reach the trailing edge before the particles that pass below the aerofoil. Bernoulli's Theorem holds for the flow around an aerofoil, but as a sole explanation for wing lift it sets up a chicken-and-egg situation: is the pressure lower on top because the flow is faster, or is the flow faster because the wing creates a low-pressure field above it?

I once interviewed an eminent aerodynamicist who wrote the standard textbook for undergraduate aerodynamics courses and asked him what he thought the best explanation for wing lift was. He replied that he'd been thinking about this for at least fifty years, but that he still didn't have one; if he looked at the flow around a wing he could give various mathematical and flow-physical explanations of how the wing created lift, but that there wasn't an easy explanation of what set up that flow pattern. The best he could say was that if you applied the laws governing fluid flow to the flow around an aerofoil shape (or flat plate at incidence, or what have you) with the appropriate boundary conditions you got a flow field with speeds and pressures such that it would produce lift.

(Drifting even further OT, the story of how aerodynamics came to a theory that could satisfactorily predict wing lift is a fascinating one; if you're interested in the subject, I highly recommend David Bloor's _The Enigma of the Aerofoil.)

[JakobW]

the Bernoulli principal: half the people argue it keeps planes in the air, the (other) half disagree.

Really? What is the counter argument?

Air flows over the top of the wing travels faster that below the wing, the pressure difference produces lift. If the upward lift force is greater than the downward gravitational downforce the plane will fly.


But what about this?

upsidedown.jpg

The 'top' of the wing is now below the 'bottom' of the wing. The wing doesn't know it's upside down and neither does the air. The upward 'lift' force is now a downwards force and gravity hasn't changed. How can the plane fly upside down?

In that photo, the aircraft are in a dive and sinking, so the lift force is still 'down'. However, sustained inverted flight is generally feasible; even 'upside-down' aerofoils can produce lift (albeit not as efficiently as when the right way up), as long as they're at a sufficiently great angle to the incoming airflow (a google image search for 'aerofoil streamlines inverted' should bring up some flow diagrams). In sustained inverted flight the aircraft nose is usually noticeably above the horizon to provide the required angle of attack.

[Brucey]

with enough thrust behind it, at the appropriate angle of attack, a housebrick would generate lift...

cheers

[ChrisButch]

I'm more puzzled by the acceleration, and the management of the progression through the gears, between which there must be huge jumps, rather than the maximum speed. It appears that this design was a fixie, so no opportunity to freewheel?

[Tigerbiten]

Bikes that are used for high speed behind vehicles tend to be single speed.
They get towed up to a speed such that they are just in the riders power envelope.
Then slowly accelerate the rest of the way under the riders own power.

That why the HPVs have a +4 mile run up to hit +80 mph at battle mountain.
It takes that long to fully work up through the gears to hit an ultimate top speed.

YMMV .........

[peetee]

I have often wondered if the turbulant area behind a vehicle traveling at a constant speed is confirming to a regular/mathematic/random law whereby it's motion can be predicted or projected? Perhaps in the real world full of trees, buildings, other vehicles etc prevailing air conditions are too complex and influential? Having tried to 'draft' a vehicle many times over the years I remember that often it makes pedalling easier, sometimes it's harder and occasionally there is a sweet spot where you really do get pushed. Bruceys photo shows an air flow that curls around the top of the vehicle and could give that push if you rode in exactly that place. But does that place stay, erm, in the same place?

[Polisman]

I occasionally catch a waste truck (not very fast moving, about 30mph) which rolls through my winding streets of our village. I can often keep on it for about 4 miles before it speeds up and leaves me for dead. I average about 38mph behind it, which for most of the time feels pretty effortless

[Brucey]

I have often wondered if the turbulant area behind a vehicle traveling at a constant speed is confirming to a regular/mathematic/random law whereby it's motion can be predicted or projected? ………. an air flow that curls around the top of the vehicle and could give that push if you rode in exactly that place. But does that place stay, erm, in the same place?

it can be either consistent or chaotic, or contain elements of both. That is presumably why there was an aerodynamicist on the team during the recent trials.

In the example illustration I posted, I don't know just how consistent the effect would be; what I do know is that quite small differences could change things quite a lot. Aerodynamics is tricky.

In many previous attempts the limiting factors have ended up being the power output of the assisting vehicle, and the length of the track. It can take miles to get up to speed, and to create a suitably assisting wake to push a rider along takes incredible amounts of power.

As far as run-up distances are concerned, it is a bit like stopping distances, i.e. with a fixed power it goes as the square of the speed. This means that if you want to go (say) 41% faster, you need twice the distance to both accelerate and decelerate.

cheers