Aerodynamics and stability

[jjackstone]

Originally Posted by Flakbadger

I mean if you look at a regular bicycle vs a velomobile, you'll find that the aerodynamic fairing on the bike makes it vulnerable to lateral winds, which is why most velomobiles are tricycles instead of bicycles. Here's a quote from the wikipedia article on them:

"The fairing on a velomobile sometimes makes it more subject to cross-winds than a similar unfaired cycle. The effect of cross-winds is complicated because the force of the wind can act as a steering force, as-if the rider had tried to steer the cycle. "Wind steer" can be a safety issue and may also hurt performance, as a serpentine path is longer and thus slower than a straight line. Thus, a design with inferior aerodynamics may be better overall. For example, it is common for time-trial bicycles to use a solid disk rear wheel for best aerodynamics, and a spoked front wheel that has worse aerodynamics than a disk, but is less likely to steer the bicycle in a cross-wind."

Something like a boattail that increases the length of the vehicle substantially could very well affect lateral stability from crosswinds, but I feel like the majority of mods centered on streamlining will not have a negative effect, since most of them focus on the front profile of the vehicle and decreasing the trailing wake.

This is not a good comparison as one vehicle has a body and the other one doesn't. A "normal" recumbent bike is about 25% more aerodynamic than most uprights yet as far as I have ever noticed the wind doesn't affect the recumbent any more than the upright.
JJ

[MeteorGray]

The Mazda3 I bought new in 2015 has the most aerodynamic rating of any car I've ever owned, at 0.26 coefficient of drag. So I was surprised to feel sidewinds more than those others that I owned through the years. I would have thought that the superior Cd would have allowed it to be more stable, and it seems to be very stable in most situations, but I sometimes do notice those sidewinds for whatever reason. I will say that most of the other cars were heavier, since the Mazda weights in at only 3000 pounds.

[wdb]

My 2008 Honda Fit got pushed around by the wakes of semis, strong sidewinds, and the like. Then I put a Beatrush undertray on it and, to my surprise, it all went away. Smoothing out the airflow under the front of the car apparently reduced lift there, and stabilized the vehicle. I give it credit for a 1 or 2 MPG improvement as well. It also hung on like a leech when I hooned the offramps!

[kach22i]

Originally Posted by sgtlethargic

I believe I've heard on this forum that aerodynamic cars have stability issues. Is that true? How so? What can be done about it?

Thanks

I looked around and could not find the old posts that would have addressed this issue.

Wish you would have started this thread with specific posts or threads in mind.

What I recall is some discussion of shifting the center of lift (often rearward), which in turn affects yawl in side winds.

That is to say a sleek design with attachment may have some lift, but a turbulent drag laden body with wild vortexes may be creating down force (and at quite a cost).

There was this one medium sized box truck example (ex. ambulance now a camper) that became the subject of discussion once. We may have mentioned it there, please look it up.

In general, a responsible design will account for shifting centers of lift and shifting center of gravity in side-winds.

To say aerodynamic design causes lift may be a misdirection, perhaps more accurate to say a box-like body takes a lot of fuel to create unnecessary down force with messy vortexes.

And yes under certain conditions this may be more stable, but not with side-winds has been my experience.

Once those vortexes start shifting their vector at askew angles, they start pulling and pushing you around which is it's own form of instability.

[aerohead]

Ford Motor Company claimed high speed stability for their 1985 PROBE-V,Cd 0.137,concept car.They incorporated a vestigial fin at the bottom of the rear slope to tune the center-of-pressure.GM/AeroVironment incorporated tail fins under the tail of the 1987 Sunraycer for the same reason.It's pretty obvious that,if you have a high speed aerodynamic stability issue,it can be resolved aerodynamically as well.Spirit of Ecomodder is rock solid under all driving conditions encountered so far.

[ChazInMT]

I know for a dead certain fact that putting the aero cap on my 2000 GMC made it hugely more stable at highway speeds. It did so by reducing lift and not allowing crosswinds to induce as great of a yaw force.

[aerohead]

Originally Posted by Vman455

Chapter 5, "Directional Stability," is 70 pages long in the 4th edition of Aerodynamics of Road Vehicles, and covers a lot of ground.

I vehemently disagree with this assessment of what Hucho* "says" (and if you think he does say that--produce it!). Quite the opposite: the chapter begins--



*Since Hucho edited Aerodynamics of Road Vehicles and most of the book, including the chapter excerpted here, are not his words, I'm using his name as I assume you are--as a stand-in for the book itself.

Unfortunate for us is a non-quantified declaration of 'high speed.' Hucho lives in Germany where there still remain portions of the Autobahn with unrestricted, unlimited speed.I drove at 124-mph,over long sections in a Mitsubishi Galant last time I was there.
Any high speed directional instability will be attributed to a center-of-pressure issue,which can be addressed by a fin if need be,as GM used on Sunraycer,Ford used on Probe-V, Fachsenfeld/Kamm used on the K-car series at FKFS,any year 550-mph streamliners at Bonneville,and faster jet/rocket cars at Black Rock Desert,Nevada,or Africa.
Hucho,as editor of the 2nd edition said instability due to lift was a non-issue.We've come a long way since the 1930s.What's your specific concern?

[aerohead]

Originally Posted by ChazInMT

I know for a dead certain fact that putting the aero cap on my 2000 GMC made it hugely more stable at highway speeds. It did so by reducing lift and not allowing crosswinds to induce as great of a yaw force.

Yes,this was a major side benefit of the GM aeroshell,which Texas Tech reported on in their SAE Paper of 1988.And the prime reason Spirit is essentially zero-lift,while relatively low drag.

[aerohead]

I went back through Hucho's 2nd Edition. It's all I have by him. Early in the book he mentions forces,moments,directional stability,lift,pitching,wheel loads,and reports that lift has only a small effect,even in cross-wind.....the wheel loading reduction small in relation to static loading....directional stability hardly affected by lift. (Spirit-I, @ Cd 0.2675,and 130-mph,generated -30-lbs downforce at the front axle, and +22-lbs lift at the rear axle,with a travel weight of 4,220-lbs,and 50-50 weight distribution).At posted speed limits and up to 108-mph Spirit is rock solid.
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Hucho mentions high speed sports cars needing spoilers.And he mentions racing cars needing spoilers and wings.
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For RV travel trailers there's mention of leading edge radii producing yawing moments.This is from early research. Emphasis is placed upon percentage of trailer weight placed on the tongue/gooseneck/fifth-wheel,and tow-bar length.Suspensions are different now. Tires are different now. We now have anti-sway technology to deal with cyclic oscillatory vibrations.
Porpoising (pitching) can be addressed with aftermarket fairings.
Some states provide active roadside electronic signage, plus radio broadcasts linked to NOAA,providing weather and wind advisories for high profile vehicles.
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Civil engineers can offer windbreaks, barriers, fencing, landscaping, etc., to mitigate side-wind gust situations.
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Later in the book,Hucho discusses tuning the vehicle nose for a forced flow separation above wind angles above 10-degrees to control yawing moments.
Kamm tail truncations are oblivious to crosswind perturbation.
Fins can be used,however better to just design more body behind the rear axle.
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Further on 'fast' coupes and sports cars and spoilers are fleshed out.Ditto for racing cars.
The greatest challenge is the driver. Reaction times before the car 'notices' feedback from the driver is 0.8-seconds,around 4-car lengths in my truck at 100-km/h.
Unless you've attended racing school, you've never had driver education.
Side gust is the most dangerous situation for Barbie and Ken.It took me one overpass on HWY-385,between Rosamond and Lancaster, California to learn that a Karmann Ghia would change lanes after bridge abutments,during crosswinds.
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The 2019 McLaren Senna has fully-computerized active aerodynamics which will take feedback from throttle,steering,and braking to control the highest downforce of any production road car in order to keep the driver alive.
Going back to the discussion of the F-1 Coanda wing,McLaren says the most dangerous thing about their Senna would be to 'slide' the car, as it would kill the downforce necessary to survive the g-loading in curves.
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G.E.Lind Walker published about cornering stability at high speed in 1950.
Chrysler Corp. began wind tunnel testing for stability in 1957.
Ford Motor opened their first wind tunnel in 1958,conducting stability tests on sedans,the Ford GT40,Indycars,and Dragsters.
In the late 1950s,General Motors basically took over Cal Tach's GALCIT wind tunnel,testing lift,yaw,drag.
In 1960,MIRA opened their wind tunnel for stability testing.
In 1968,Chrysler used the Lockheed,Georgia tunnel to prove out the Charger Daytona.
Car companies never considered lift to be an issue for passenger cars, just high speed sports cars and race cars. Walter H.Korff designed Goldenrod for 555-mph.The Summers Brothers reported that directional stability was never an issue with the car ,which they took to 409-mph,average 2-way run,and as high as 425-mph.Jet cars ended further interest in the project. Now we're over supersonic velocities.
I'm not anxious to see any member have a Darwin moment, however, if driving speeds are kept to posted limits, there's a likelihood that directional stability will be a problem lost on most drivers.

[freebeard]

Later in the book,Hucho discusses tuning the vehicle nose for a forced flow separation above wind angles above 10-degrees to control yawing moments.

What would this look like? I'm hoping Cybertruck chamfers.