Expert: Paul Soderman - 10/28/2008

Question
QUESTION: First, thanks for doing this!
I'm writing a book about great inventors and their failed inventions called Thomas Edison's Concrete Piano. For the chapter on Buckminster Fuller's Futuristic People Carrier, I have several questions. Fuller's original vision for the vehicle was a personal car/plane that used jet propulsion principles akin to a duck's wing.  In his words the car is“the land-taxiing phase of a wingless, twin orientable jet stilts flying device.” Technically speaking, do you think it is possible today to design an operable land vehicle with retractible wings and this concept of "duck wing engineering" that could drive and fly? Why or why not?
Secondly, the Dymaxion car Fuller ended up with had significant aeronautical features - its streamlined shape and rear steering (as he points out boats and planes steer from the rear, front steering is a holdover from horse carriage days).  I know of at least one modern car that uses these feature - the Aptera (http://www.nytimes.com/2008/06/15/automobiles/collectibles/15BUCKY.html) but I'm wondering if you can think of any technical reasons why these design features would not work for modern cars?
Thanks in advance, Judy


ANSWER: Judy
Yes, you could design a car that could fly.  But why would you - it would be a lousy car and a lousy airplane.  Consider the wing - many home-built aircraft have removable wings for easy transportation.  But mounting the wings is a big deal because of the high stress at the wing root. The attachment is heavy duty. It would be difficult to design a safe, retractable wing that somehow folds back and stows.  That would be possible but impractical.  And driving around with heavy, stowed wings would be a nuisance.  Imagine if you got a ding in your wing parked at the grocery store but didn't discover it until you were in the air. Airplanes require a much higher level of care than cars do for obvious reasons. As for flying, the engine would have to drive a propeller, or you would carry a jet engine around.  More weight. And you would have to deploy a tail for pitch control.  It sounds like a big headache to me.  It is much easier to build a car-boat than a car-plane.

A rear steering car is odd and rare because to make a turn you steer the rear wheels in the opposite direction and rotate the rear end until the car is aimed in the right direction.  Watch a fork lift in action and you will see the rotation.  In tight traffic, that rear end rotation could be a disaster for most full-sized cars.  The Aptera gets away with it because the tricycle body is so tapered that the rear wheel probably stays within the tracks of the front wheels.  

Streamline shapes is a big plus for cars as long as it is done right and the car does not get lift at high speed.
Paul

---------- FOLLOW-UP ----------

QUESTION: Great answer. Can you elaborate on this, "In tight traffic, that rear end rotation could be a disaster for most full-sized cars."  From everything I've read the Dymaxion car worked remarkably well. Fuller did change the steering for his later model - so the driver sat at the front - but still controlled the back wheel, the "rudder". Would this solve the difficulty you envision?  DO you know of any other streamline cars by any chance ? (I know your expertise is aeronautics so don't expect so).
May I quote you in the book? I could send you final text for your approval via a private question.
Thanks again,
Judy

ANSWER: Hi Judy
When I said full-sized car I should have said standard rectangular car or truck.  The problem with rear steering on a rectangular car is simply a problem in geometry. Consider driving straight down the road - the rear wheels are following in the tracks of the front wheels.  Now turn to the right, which requires turning the rear wheels to the left.  As the rear wheels track left, the car starts to turn right because the front wheels are being forced in that direction.  In the process, the rear wheels swing to the left until the driver corrects the steering, which causes the tail of the car to swing left for a short time.  If you were close to another car that made the same maneuver in the opposite direction, the two tail ends might collide.  This cannot happen in a front-steering car because the rear wheels  try to follow the front wheels and track a little to the inside of the turn.  If you can find someone driving a fork lift you will see the rear end rotation easily.   Or simply have someone drive your car straight backward and then turn the wheel.  You will see the front bumper swing away from the turn momentarily.  This is why people driving backward often bang the front of their car on an obstacle to the side.

The Dymaxion, like the Aptera, had a tricycle wheel base with tapered aft end.  Rear-end rotation occurs here also, but the tapered geometry allows room for the rear-end rotation without fear of collision.  But you have to give up the two-person back seat. I guess it would be perfect place to put mother-in-law.  Incidentally, I understand the driver is in front.  Driver location is not relevant to rear-end rotation.

Almost all cars today are streamlined to minimize drag and gas consumption and many have been tested in wind tunnels.  They have low profile, carefully rounded edges, recessed door handles, hidden rain gutters, shielded mirrors, tapered trunk area, streamlined head lights and tail lights, etc.  No sharp corners or bad recesses.  I see one boxy car on the market that has sharp corners.  The drag must be lousy, but the design must appeal to someone.

Incidentally, the Dymaxion is streamlined, but probably never tested in a wind tunnel. I see some obvious room for improvement.

Please get back to me if you have more questions.  This is a very interesting subject.  You may quote me in the book.
Paul


---------- FOLLOW-UP ----------

QUESTION: Paul, thanks for the quoting privilege. I do have more questions! I understand now - after reading this, and watching youtube videos of forklifts. As you suggest in your reply, it wasn't an issue for the Dymaxion. Neither was there an issue of no room in the backseat; the car seated 10. So I guess I need to reask the question in light of this fact, for the car was not so much a car, but a small bus.  It was nearly 20 feet long.  SO - can you see any technical reasons why this large vehicle would not be practical today? Incidentally, the third and final model had steering in all three wheels and could actually move sideways, like a crab. It also had gas mileage of 40-50 miles per gallon and could go 120 miles an hour.

In terms of aerodynamics, I'm wondering how a "fish" shaped Dymaxion car might compare with a rounded but rectangular car. The diagrams of air flow related to the dymaxion literature show no turbulence at the back end of the dymaxion, but do show turbulence at the back end of a rectangular car. Does this "compute"?

Also, the front end shape of the dymaxion car is reminiscent of fast train. While you point out nicely the plentiful aerodynamic features on modern rectangular cars, can you make any comparisons between the shape of the fast train front end versus the fast rectangular car front end? Or is this splitting hairs and wind tunnel data is required?
When we're done with this, I have another question about Robert Hooke's flying machine :>)  

Answer
Hi Judy
You have a lot questions piled up.  Let me start with the first one - yes, I can think of reasons why the Dymaxion would be impractical today.  I want to be quick to say I admire Fuller's innovative ideas.  I suppose he was at the cutting edge of technology in the 1930's.  But with 75 more years experience in research and technology, we (the engineering community) could find a number of ways he went awry.  

1.  It is a little unclear from the one photograph you cited, but it seems he has a more or less round nose on the car.  An elliptical nose would be much better aerodynamically.  Imagine a quaterback throwing a football and a soccer ball.  I bet the first one goes much farther.  The drag on a body with round nose is higher than an elliptical nose because of flow separation aft of the centerline.

2.  Modern cars are designed to minimize the flow under the body for two reasons - a) the underbody is cluttered with axles, mufflers, hoses, etc. that are high drag items, and b) lift of the body is undesirable because it reduces the tire/road contact force.  Modern cars are low to the ground, shaped appropriately, and have air dams in some cases.  Race cars have inverted airfoils to keep strong wheel/ground force. The Dimaxion has a nicely rounded underside of the nose, and more importantly is at angle of attack relative to the stern.  At high speed the wind would tend to lift the tail off the road with disastrous consequences.

3.  In cross winds due to natural wind or a passing truck, the side pressure forces would be better resisted  by the front wheels than the single aft steering wheel.  This would tend to turn the car into the wind with bad consequences.  The actual dynamics would be best evaluated in a wind tunnel.

4.  The turbulence at the aft end of the Dymaxion might be better than that of a rectangular car, but only wind tunnel tests could verify that. If the flow separated early because of bad nose shape, the wake turbulence could be strong.

I qualify all this by saying that a tricycle car such as the Aptera could be designed to have good performance.  I just don't think the Dymaxion was an example of good design so much as it was an example of innovative design that needed much more work.

Incidentally, three wheel independent steering is cute but way too complicated for the task.
Paul