Do headwinds affect your speed more than tailwinds?

[Balor]

Laugh all you want, but one of my (admittedly, remote) goals is to build a 'venturi tube' bike fairing, or 'ramjet' as I call it.

In *theory* it should allow for a fairing that is much smaller laterally (chopped off nose and back) and still retain much benefits of a full fairing, and if you add a relatively low-powered EDF 'jet engine' (as in - fan):

https://www.turbines-rc.com/en/ejets/105-ducted-fan-edf-jetfan-80-ejets-80mm-carbon.html

In *theory*, it should provide much more benefit than either alone, kinda like boundary layer suction (hey, Beavis, he said "Suction"!) - where a relatively small power used for sucking away turbulent portion of boundary layer can allow for massive fuel savings. Unfortunately, latter is really hard to get right, this should be much simpler.

 

[tiltmaniac]

Laugh all you want, but one of my (admittedly, remote) goals is to build a 'venturi tube' bike fairing, or 'ramjet' as I call it.

In *theory* it should allow for a fairing that is much smaller laterally (chopped off nose and back) and still retain much benefits of a full fairing, and if you add a relatively low-powered EDF 'jet engine' (as in - fan):

https://www.turbines-rc.com/en/ejets/105-ducted-fan-edf-jetfan-80-ejets-80mm-carbon.html

In *theory*, it should provide much more benefit than either alone, kinda like boundary layer suction (hey, Beavis, he said "Suction"!) - where a relatively small power used for sucking away turbulent portion of boundary layer can allow for massive fuel savings. Unfortunately, latter is really hard to get right, this should be much simpler.

Seems like there would be more skin drag, as you've added significant surface area (especially if you avoid the human inside)?

 

[Balor]

Remember that skin drag is a small fraction of a pressure drag unless we are talking about perfectly streamlined bodies. CFD show that without a fan to help the flow along it indeed makes more drag...
Unfortunately, a 'perfectly streamlined' form and practicality rarely go hand in hand. I like this idea might eventually 3D print a scale model and test it by chung method, because CFD have serious limitations.

 

[Osiris]

I rode from the Northern end to the Southern end and then onto the Southern Lakes (?) trail. Around 50 miles. I agree, the section from Winter Park past the golf course is the best with few other riders and nice trees. The left turn after Winter Park going north by the wooden bridge has some incredible old oaks that must be 400 years old. Only downside were lots tricycles riding side by side (several groups were 3 riders spread across the trail) and of course, the stop signs. Great pavement.

The group of trikes you encountered was likely the "CRABS", as they call themselves. That roaming gang of octogenarians is infamous on Florida trails. Many of them now have fast electric powered trikes, so it's not as easy to pass them as it once was.

Yesterday I spent much of the afternoon riding just that 3 mile portion of trail shown in my video. I was testing out numerous modifications I'd made to my M1 Lowracer, and by the time I was finished I had ridden a total of 32 miles, just going back and forth. I got lots of "What the hell is that thing?" comments from other trail users, some of whom insisted on having their picture taken with it.

 

[bladderhead]

If you ride a bent in the Sunday ride they complain about drafting. Imagine how they would react to a propeller.

 

[Balor]

If you ride a bent in the Sunday ride they complain about drafting. Imagine how they would react to a propeller.

That's not for racing, or even brevets. That's just for fun .
But yea, drafting someone with an EDF should NOT be fun, ehehehe.

 

[bladderhead]

Everyone in Florida needs to know the Crabs are on e. There is someone with knackered lungs who keeps saying to himself "At least I can still overtake the Crabs".

That guy is just going to sit down and give up.

 

[Balor]

Well, you can also embrace the 'e'. Though it is unlikely that your total ride enjoyment will be considerably more in the long run (unlike resulting expenses and weight). Or maybe it would. I guess it is not about flat speed, but being able to climb stuff without keeling over (quite literally, ehehehe).

 

[bladderhead]

There is a road that goes downhill where I can do 40mph, but then I have to get up the other side. The really annoying thing is the tight bend at the lowest point. I still do not trust myself to get round without drifting into the oncoming traffic so I have to brake and lose a load of momentum before I even start climbing. From top gear to bottom, and keeling over starts to seem like a distinct possibility.

But no e. No propeller. I am not joining the ranks of the Essex Crabs. Not yet.

 

[Osiris]

Remember that skin drag is a small fraction of a pressure drag unless we are talking about perfectly streamlined bodies. CFD show that without a fan to help the flow along it indeed makes more drag...
Unfortunately, a 'perfectly streamlined' form and practicality rarely go hand in hand. I like this idea might eventually 3D print a scale model and test it by chung method, because CFD have serious limitations.

How do you plan to route the air passage around the rider? Could you use a splitter like on the Mig 15?

 

[ed72]

I had a lot of time to contemplate wind on Sunday's 300K in Florida that went East to the Atlantic Ocean with a 15-20 mph tailwind that became a 30-35 mph headwind going back West to the start. An aspect to consider is how and when to spend one's limited energy in Joules or Calories or mental reserves, however one wants to characterize. My choice was to ride fairly moderately downwind and then balls to the walls coming home. During winter rides, I pick the route based on wind. I start into the wind and come back downwind (hypothermia insurance) because my body at least starts to put out less and less power as it gets cold (under 94-95F). E would have been nice for the 90 miles into the wind the other day.

 

[Balor]

How do you plan to route the air passage around the rider? Could you use a splitter like on the Mig 15?

Inside uninterrupted frame tube. Using a splitter would likely make thing much better (stiff, less resistance), but I don't have a similar budget, unfortunately .

 

[Osiris]

Inside uninterrupted frame tube. Using a splitter would likely make thing much better (stiff, less resistance), but I don't have a similar budget, unfortunately .

I once read about someone modifying a race car this way, by essentially running an air shaft all the way from front to back. Apparently it won so many races that officials decided to ban the design from competition.

 

[ed72]

One of the Velomobile racers uses a powered fan to suck air thru the body of the velo but probably just for cooling although I did wonder about aero effects.

 

[Balor]

One of the Velomobile racers uses a powered fan to suck air thru the body of the velo but probably just for cooling although I did wonder about aero effects.

Once you get into 'advanced' aerodynamics things get very interesting (and very, VERY complex).
Anything that helps with reducing frontal pressure 'buildup' and increasing pressure recovery will, obviously, help with air resistance. At extreme end of the spectrum you get a jet .
The trick is to do it without adding on too much turbulence (that dissipates energy without contribution to pressure recovery) and skin friction. Airfoil shapes have optimal width to length ratio for very same reason - what looks more streamlined does not necessary have less resistance because of skin friction, just like tiltmaniac noted.

Playing with CFD and visualising results taught me a LOT about aerodynamics, thought I still don't know a single formula... but derivation of aerodynamics from 'ground up' (as in - Kolmogorov microscales) is a hopeless takes even for today's supercomputers.

I once read about someone modifying a race car this way, by essentially running an air shaft all the way from front to back. Apparently it won so many races that officials decided to ban the design from competition.

Really? Where?
I've heard of two banned aerodynamics improvements, both related to generating downforce - 'venture tube' floor shape and 'vaccuming' the air from underneath the car and ejecting it in the back (ostensibly for engine cooling ).
Downforce in car racing is MUCH more important than top speeds, because keeping your speed in a curve without flying of the track is much more important than top speed... it is not that F1 cars have aerodynamics of bricks, but actually pretty close . They do generate so much downforce that can be safely driven on the ceiling once up to speed. Their tires have simply atrocious rolling resistance as well.

 

[Osiris]

Really? Where?

It was done decades ago, back in the 1970's or even 60's from what I recall. It was a "stock car" that had been stripped bare of all the internals except for a racing seat for the driver. With all that empty space available, the builders decided to run a big air shaft through it to see if this would make the car faster by reducing frontal air pressure. Apparently it worked so well that future designs of that sort were outlawed in this racing class.

 

[Balor]

It was done decades ago, back in the 1970's or even 60's from what I recall. It was a "stock car" that had been stripped bare of all the internals except for a racing seat for the driver. With all that empty space available, the builders decided to run a big air shaft through it to see if this would make the car faster by reducing frontal air pressure. Apparently it worked so well that future designs of that sort were outlawed in this racing class.

Makes sense I guess - 'stock class' in 1960th cars are basically blunt bodies. Adding a huge hole in the center would significantly decrease frontal area with greatest Cd (I dareday about "1").

 

[McWheels]

Cars have generally worried about being grip-limited. The F1 Fan-car was all about generating downforce.

I might have to dig back into my aerodynamics lecture notes, but reducing drag uniquely is rarely the aim in the things I've studied. Lift and stability are also large parts of what you trade for. Bikes should not generate lift; in fact a little downforce, while inefficient for ultimate pace might just keep you out of the green stuff.

I'd also counsel that tripping the airflow into turbulence early isn't always a bad thing. It becomes a lot more predictable if you need a known amount of lift/drag etc.

Powered fans had to be banned, or limited in power maybe, for the Pedal Prix cars in Australia (thanks LBBR!). They were originally to keep the windows un-fogged, but the bigger they got, the greater the advantage.

 

[Balor]

Bikes should not generate lift; in fact a little downforce, while inefficient for ultimate pace might just keep you out of the green stuff.

*Arguably*, no. For a few reasons:

I really don't think you can generate either much either way (we just don't output that much power), and either lift or downforce indeed comes with inevitable drag penalty.
Unlike cars we don't rely purely on slip angles in cornering, we lean into turns. We also have to lean into the wind, etc. That changes your geometry, sometimes unpredictably.

Even in MotoGP, 'winglets' for downforce is considered a very mixed blessing and is mostly about preventing wheelies under heavy acceleration, not for cornering last I heard. I don't think MOST bents have problems with wheelies... ok, maybe FWD ones... but at climbing speeds it will not work.

Hence, it makes sense to simply disregard either lift or downforce and concentrate on minimizing drag. In my case, some (lots) of lift would be quite beneficial, my front wheel is overloaded enough as it is . Yet again, there is no free lunch and there is no free lift (or downforce).

It is MUCH of a problem with streamliners that, indeed, have HUGE lateral area, and once leaned into the wind generate enough lift that some lighter HPVs (like Tetz foamshell or Easy Racer streamliner) can simply be picked up and flung off the road given stronger wind, and narrow, 'fish-like' streamliners should have lateral Cd close to that of a flat plate.

Only example of a very succefful and much used 'streetliner' - Rotator Coyote - had 'interrupted' fairing with open side 'windows', that were closed in only for track racing.

All in all, you should have CG as HIGH as possible and CP as LOW as possible (and located after your CG laterally).

 

[ed72]

But.....tripping airflow cab be a good thing.....some special TT clothing (socks and skinsuits for instance) have surfaces to improve flow after the body part. I *think* Zipp rim dimples might serve this purpose as well although it could be mostly to make it more predictable in the wind with a higher stall angle. I have the fancy socks but not the skinsuit.