Aerodynamic Drag

[Mike Doyle]

Lots of good post on this topic so far. The drag profile diagrams tell the story but it's important to remember there are a variety of ways to reduce drag some of which involve higher velocities than PWD cars travel at rendering them useless (dimples for example).

At lower speeds it's probably best to focus on clean entry and exit of the airflow. Abrupt profiles with a large frontal area are obvious to avoid, but even profiles with clean noses can fail to reach their full potential if a clean exit isn't provided for the airflow. What you end up with essentially is akin to a drag chute.

Dimples are interesting to study, but a PWD car lacks the velocity for them to generate a boundry layer and resulting laminar flow.

Last year's car featured an inverted shovel nose with a fully rounded tail and particular attention paid to the under car airflow. We used a half rounded tapering tunnel with vent holes that were sealed off between practice and the main event. It went undefeated winning the Pack and finished 5th in the Districts our first year out.

[ranman106]

Great info guys! Thanks!

[gpraceman]

Lots of good post on this topic so far. The drag profile diagrams tell the story but it's important to remember there are a variety of ways to reduce drag some of which involve higher velocities than PWD cars travel at rendering them useless (dimples for example).

Dimples are interesting to study, but a PWD car lacks the velocity for them to generate a boundry layer and resulting laminar flow.

If there is airflow across a surface, then there is a boundary layer, so I would not be so quick to assume that surface irregularities such as dimples could not reduce the overall drag at low velocities. At slower velocities the thickness of the boundary layer would be smaller but it is still there. See http://www.desktopaero.com/appliedaero/ ... ayers.html

The only way to know for sure whether surface irrgularities would help improve airflow off of a PWD car and thus reduce drag would be to do some wind tunnel testing. Of course, that is not something that most any of us would be able to do. Though, I have seen some small commercial wind tunnels but they are still pretty pricy.

At lower speeds it's probably best to focus on clean entry and exit of the airflow. Abrupt profiles with a large frontal area are obvious to avoid, but even profiles with clean noses can fail to reach their full potential if a clean exit isn't provided for the airflow. What you end up with essentially is akin to a drag chute.

I do agree that these factors should be looked at first to reduce aerodynamic drag.

[Mike Doyle]

If there is airflow across a surface, then there is a boundary layer, so I would not be so quick to assume that surface irregularities such as dimples could not reduce the overall drag at low velocities. At slower velocities the thickness of the boundary layer would be smaller but it is still there. See http://www.desktopaero.com/appliedaero/ ... ayers.html

I didn't mean to imply there wasn't a boundry layer, what I meant to point out is an increase in the boundry layer thickness or further development isn't going to result at PWD speeds.

Dimple size and design would have to be radical enough for turblence development and an increase in boundry layer thickness that it would result in a net increase in drag which we're trying to avoid.

In college we did a study on golf ball dimple patterns and how their design affected flight characteristics. What we found were very specific flow patterns at a variety of speeds depending on the dimple design. Maxfli for example used a pattern of large, small, deep, and shallow which provided stability at a variety of speeds.

I wish I had that data to support the discussion here, but suffice to say at speeds below 20 mph the dimples did basically nothing.

If you're building a golf ball car, then add dimples for realism otherwise it's all for naught.

[gpraceman]

I wish I had that data to support the discussion here, but suffice to say at speeds below 20 mph the dimples did basically nothing.

If you're building a golf ball car, then add dimples for realism otherwise it's all for naught.

I won't dispute your results. Drag reduction, if any, may have not have been even measureable. Hey, until someone has tested a concept it is worth some investigation.

Sounds like you studied aeronautical or aerospace engineering. My undergraduate degree is in Aerospace Engineering, though my career took a far different route starting with my Navy service, working in TV manufacturing and now with software development. It is nice to dust off the cob webs once in awhile.

[Mike Doyle]

Actually I was a Business major that wandered in the wrong door of the Science Hall

Today I work as an Application Engineer but my background in aerodynamics comes from a few courses and a few years working with composite turbine and ducted fan powered R/C aircraft. I sometimes wish I'd stayed on that track since the widespread use of RPV's in the Military has proven very lucrative for everyone involved.

Back on topic, I think significant gains can be had by paying more attention to the undercar airflow and how the center guide strip and wheels interact with the airstream.

Fortunately at these slower speeds "looking about right" will probably suffice since were talking about tiny gains in efficiency which added together results in a significant difference.

[Darin McGrew]

Fortunately at these slower speeds "looking about right" will probably suffice since were talking about tiny gains in efficiency which added together results in a significant difference.

There are a number of factors in derby-car speed where "looking about right" is good enough to be reasonably competitive in most derbies. The difference between no lubricant and any reasonable lubricant is huge; the differences between reasonable lubricants are tiny. The difference between a fully rigged man-o-war design and any reasonably aerodynamic design is huge; the differences between reasonably aerodynamic designs are tiny. And so on.

[Stan Pope]

It takes a real man to stand up to the derision when his Man-o-War design clocks 7.5 seconds on a 32 ft track!

[ninjarabbi1997]

Back on topic, I think significant gains can be had by paying more attention to the undercar airflow and how the center guide strip and wheels interact with the airstream.

Mike,

What do you think we can do to optimize the undercar airflow and wheel interaction with the airstream? Do you think that "fender" type designes over the wheels is best? Would the effect offset the added weight?

Thanks.

[MathGuy]

The worst source of aerodynamic drag about a typical low profile pinewood derby car is not the car shape, but rather those darn wheels. I believe I read that a rolling wheel has Cd of .58. I don't know good way of getting rid of that.

The top 5 keys to have the fast cars seem to be:

1. Have the axles perfect
2. Have the wheels perfect.
3. Have the alignment perfect.
4. Everything else.
5. Make sure 1,2,3 are done.

Under #4, low profile, smooth lines, and weight in back.

[Alex]

I concur, (a word I learned from Stan). I might ad two things though, lift a front wheel and place your weight as low as possible, if your rules allow go with NYOIL.

[vortex]

Aerodynamics is quite a bit less important than weight and preparation of wheels & axles, but it's not so trivial that it should be ignored, if you want to go as fast as possible. At our races, there is an 'Open' division, for dads and other non-scouts, with a 3.5 ounce weight limit. Aerodynamics becomes even more important in this case.

I think Stan's point was a good one; the sharp point on the wedge is not especially beneficial, and the blunt rear end is pretty bad, as far as aerodynamic drag is concerned. A 'backwards wedge', but with the new front end rounded, would be a pretty low drag shape.

There are lots of designs with fairly low drag. I would say some good general rules are:

1) the rear end should taper to a sharp edge
2) the front end should be rounded
3) the frontal area (the size of the silhouette, viewed from the front) should be as small as possible

The 'wing' and 'rail' designs seem to be pretty well optimized, if all the rear-facing edges are pretty sharp. Some fairing around the wheels, kind of like the wheel pants you see on some airplanes, could offer a pretty significant drag reduction as well, if carefully made.

vortex

[Den_Leader]

Does a rail have less aerodynamic force on the body than most other cars, since it has less total surface area?

[vortex]

Does a rail have less aerodynamic force on the body than most other cars, since it has less total surface area?

It might. If everything else is equal, reducing the total surface area will reduce aerodynamic drag. It is likely more beneficial to reduce frontal area, which is the area of the silhouette as viewed from the front. The rail design and the 'wing design' are both pretty small in frontal area, and both are probably pretty good in terms of drag, especially if all the trailing edges are tapered and all forward-facing corners are rounded. The rail design has less total surface area, so it might have lower drag, unless you cut out the middle in a way that 'messes up' the flow. I would guess that if you keep everything smooth, and taper all the trailing edges to sharp edges, the rail is pretty good in the aerodynamics department.

vortex

[Alex]

I agree Vortex. Slim as possible but stucturally stong enough. Stan is a genius and has probably forgotten more about PWD than I'll ever learn. But I have to disagree with him that a rail would create less negative airflow than a solid body. I can hear Stan thinking though, no matter how slim your car is "theres enough wood there to make 2 cars!" PM me, I've came up with a way to maximize lowering the CM, extremely rear weighted but still no rattle or loosness on the front wheel Tapered edges are important also, minute maybe but we're shooting for perfefection if your goal is speed. My 0.27" wing style cars have the same side view profile front and back. They might not be the fastest, I just can't find anyone that can beat them. I havn't ran in any official races yet this year, but have attended several, beat their winning car each time by a reasonable margin each time after the event was over. One guy accused me of buying it off ebay. I just laughed, there are no cars on ebay I can't beat on a 32 wooden track using liberal BSA rules.