Motorcycle aerodynamics, a slippery and windy, slope?

[sendler]

Originally Posted by Cd

I skimmed over the many pages here, and just wanted to ask if using window screen on the sides of a fairing / tail would allow sidewinds to have less effect on stability while still providing a surface for air to slip over from straight onward.

Possibly with 1/4 inch square wire. Something a little bigger than normal window screen? Even still, the bubble of displaced air around a bike seems to become somewhat solid.

[Ken Fry]

Originally Posted by Cd

I skimmed over the many pages here, and just wanted to ask if using window screen on the sides of a fairing / tail would allow sidewinds to have less effect on stability while still providing a surface for air to slip over from straight onward.

It's worth mentioning that driving north at 60 mph with a 15 mph crosswind from the east, the apparent wind angle is 14 degrees off the nose at 62 mph. If the shape of the fairing is like that of an airfoil (when viewed from above) the fairing could be generating significant lift at this angle, and the shape would be close to the stall angle of attack. The lift direction would be perpendicular to the apparent wind direction, so 14 degrees forward of abeam (an angle that is pretty effective in pushing the bike around).

On the leeward side of the bike, much of the flow on the back half of the bike will be detached, and the center of lift will be at a point about 25% back from the nose. On the windward side (the east side) the flow will be attached all the way back to the tail.

The above applies to a lesser or greater extent, depending on how much the fairing is like a symmetrical airfoil section standing on end, and the degree to which flow from the high pressure side can travel over to the low pressure side, over the top of the bike or under it.

So... to reduce side force, the screen would need to be fairly far forward on the sides. Most laminar flow from that point rearward would probably be disrupted. Transition grit is used in wind tunnel studies as a boundary layer trip. (You can read bout it here.)
Window screen is probably of a roughness that would work as a trip.

You can search for info an "porous wing surfaces" -- there is quite a bit available, with most of it having to do with reducing drag or increasing lift on airfoil sections.

I think it would be an interesting area to pursue, although a difficult one. To get meaningful results, you'd need a wind tunnel -- although you could probably learn a lot from scale models in the small tunnels available at a lot of universities -- which can often be accessed for free if you can make a student project out of it.

My gut feel is that window screen is about the right size, although perhaps something thinner would be better.

Of course, you'd want to first build a streamliner and get a feel for sidewind sensitivity. With the right distribution of lift (which in this case is acting horizontally, rather than vertically as in an airplane wing) you may find that the bike is self correcting, and that there is no need for porosity. There are many bikes like Sendler's, in which it feels like the wheels are getting pushed out from under the bike, causing the bike to bank into the wind.

If you found the need, then a pressure distribution map across the area of the fairing could be produced (best in a wind tunnel, because you would want to look at constant angles of attack of 10 - 15 degrees) and the venting areas would be best positioned at the highest pressure areas, and communicated to the lowest pressure areas on the opposite side. (The reverse of this is done in some porous wings studies.)

[COcyclist]

Originally Posted by jkv357

In Land Speed Racing, rudders work because they add straight-line stability, and the amounts of side loads are insignificant compared to the forward velocity.

...and they shut down the course when it's windy so they don't deal with cross-winds.

[sendler]

It was very windy yesterday. A perfect day to experiment and experience the profound ability of the CBR250R to self correct it's line in gusty side winds. I had been challenged to prove that I am not subconsciously steering the bike as it magically leans itself into the wind by riding no handed. It still works just the same. I still don't have a camera but will post some video proof eventually when I get one. When gusts of side wind hit the bike, the nose stays right on line while the wheels only, seem to ride out from under you with the wind to lean the top of the bike into the wind by the just right amount. I have been asking all of the riders I hang out with and am surprised that very few have ever taken notice of this. Maybe because it works much better if you lower and anchor yourself by riding on the tank. Some people vehemently argue the opposite saying that you must always counter steer the bike to actively fight the wind at all times. Not so for me. It is actually fun to go out on windy days. Riding a really aerodynamic motorcycle at speed is not unlike flying in a light aircraft. To be connected to the wind and a part of it all and let the bike work it's magic.

[FXSTi]

Sendler,

Not that it is in any way streamlined, but I have experienced this on my Harley. I just assumed that I subconsciously did the right thing, but from what you are saying the bike does it itself. It's an odd feeling when a gust hits and suddenly you are leaning 20 degrees but still going straight.

[sendler]

I've never had one but I would have expected that cruisers would also self correct better than high riding adventure bikes. Cruiser riding position has the rider much lower so your torso doesn't get blown over and the high grips have none of the body weight to support. The chopped out front ends probably have more trail also.

[Grant-53]

Welcome to the club!

[sendler]

Originally Posted by Ken Fry

It's worth mentioning that driving north at 60 mph with a 15 mph crosswind from the east, the apparent wind angle is 14 degrees off the nose at 62 mph. If the shape of the fairing is like that of an airfoil (when viewed from above)

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Incorrect Lift Theory
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Again Ken Fry comes through with an important concept we have been missing. I have been imagining the streamliner going 60 mph down the road as a steady state with a side wind blowing on it being a new and completely separate force pushing straight on the side area of the bike. As he said, this is not the case when the bike is moving forward fast. The side wind will seem to combine with the 60 mph head wind to make a new wind that is at an angle just off the straight ahead direction. Giving the symmetrical airfoil of your streamliner an angle of attack.
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I found a computer simulator at Nasa.gov that lets you play with an airfoil's angle of attack and probe the pressure and velocity at any point on the screen. And observe the movement of all of the air particles. There is also a guided experiment to show how the "equal transit" theory of air flow over a wing that we have all been brought up with has been proven wrong. The air particles that are next to each other before splitting up to go over and under the wing do not speed up and slow down to meet at the back. And Pluto is not a planet. But anyway. The speed IS different at the top and bottom but the pairs don't have to meet. Interesting but not what I was getting at.
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A symmetrical airfoil with an angle of attack creates lift. Or in our case, side. You can see from the simulator that there is a pressure difference between each side of the airfoil which is greatest in the area of the fat part of the wing. This is why Craig Vetter discovered from actual road tests that his streamliner was much better in sidewinds without the "door" covering the entire right side even though the left side was open. By leaving the entire streamline open in front of his legs and chest he has completely vented the majority of the pressure differential right at the fat part of the wing.
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This also explains why, contrary to my intuition, he has found from road tests that the full, cd improving tail is no problem in sidewinds. It is only really a side wind on the tail when the bike is parked. Great work Craig and Ken.

[PeterS]

Thanks Sendler.... that's a well worded explanation .One that I reached myself a while ago !

The wind off the nose is called "apparent wind" IIRC ,as sailors and sail designers refer to it . It seems that the opening behind the screen is effective in either spoiling the lift by essentially having a big gap in the foil or allowing pressure to equalise between the two sides ....and spoiling the lift!

There doesn't seem to be too many options to this though as a good equally balanced aero shape will be very unstable at highway speeds. One minute we will have an apparent wind off our nose and we will be compensating for it with lean, then a truck will block it and our compensation will be to much (?). It seems likely that a shell that generates lift both sides has the potential to be very unstable.

Here's one I designed earlier It's too suit a Royal Enfield fitted with a diesel engine that I could run on biodiesel .The engine would only produce 9hp so a decent drag coefficient is essential. I'd like to achieve .5 but I'm not sure it's possible on a road going bike.



and Craig's for comparison.



BTW, I wouldn't bother with Craig's extended tail.The Kamm tail is quite effective .

[sendler]

Speaking of a Kammed off tail ( for garage storage space issues if nothing else) does it help to have a pronounced, concave shape inside of the termination like an overhang of a couple inches with a fillet? Or does just sawing the airfoil off flat at the back work the same.