The Texas State Board of Education (SBOE) is in the news again, this time for science education. For a bit of background on the SBOE, one of my previous posts, Texas Education in the Budget Crisis, has a quick run down of the shenanigans the board has pulled in recent years, mostly on less than honest means of passing new standards for various subjects.

The latest controversy is over the adoption of new science instructional materials for our public schools. A one day hearing on the materials will be held this Thursday, July 21st, followed by a final debate and vote on Friday. Unfortunately, not all of the materials submitted for consideration are particularly good, and some are downright harmful.

Consider the material submitted by International Databases. Here are some of the passages quoted in that article.

…at the end of the instructional unit on the Origins of Life, students should go home with the understanding that a new paradigm of explaining life’s origins is emerging from the failed attempts of naturalistic scenarios. This new way of thinking is predicated upon the hypothesis that intelligent input is necessary for life’s origins.

Many scientists have adopted an unproven hypothesis upon which to build their theories regarding the origin of life and its’ diversification. This ‘foundation’ is called scientific materialism, naturalism, and so forth… There are other scientists who have adopted the unproven hypothesis that an intelligence is necessary to explain both the origin, and diversification of life on Earth. This view follows from the human experience that teaches order (complexity) results from intelligent action.

The article has more, along with screen captures to see these quotes in context.

While you might suspect that a sensible board would reject such materials that weren't based on mainstream science, not all of the board members have demonstrated themselves to be sensible. Given their past behavior and their actions leading up to this vote, there's some reason to worry.

For example, when nominating members for the review panel to evaluate these materials, several of the board members appointed creationists (including my own representative, Gail Lowe, who appointed a man who likened evolution to religious dogma).

The new head of the Board, Barbara Cargill (appointed by Perry to replace Don McLeroy, after his first choice, Gail Lowe, didn't even receive a confirmation hearing from the state Senate), has made some rather divisive comments. In a speech to the Eagle Forum, she stated:

Right now there are six true conservative Christians on the board, so we have to fight for two votes. In previous years, we had to fight for one vote to get a majority.

Not only is this counter to what should be the secular goals of our public education system (secular as in non-religious, not anti-religious), it's insulting to the other Christians on the Board who don't agree with Cargill.

So, if you live in Texas, send a letter to the Board member that represents you, demanding that they uphold sound science standards and only approve appropriate, evidence based materials (here's an example of what I sent back when it was the science standards up for debate, which obviously needs to be updated appropriately). If you don't know who your representative is, you can find out at Who Represents Me?. You can follow the links from there to get their contact information, though it appears that all board members seem to have the same e-mail address, sboesupport@tea.state.tx.us.

Hopefully, with the recent losses of the ultra-right wing bloc in the last election, there's some hope that underhanded dealings similar to previous occurrences won't take place, and that our students will end up with good educational materials.

Additional Links:

• Statesman - Education Board war escalates
• TFN Insider - TFN Insider Will Live-Blog Science Debate

Added 2011-07-27 - The debate and vote are over, and our children came out the winners. You can read more in a new entry, Texas Education - Follow Up to Science Instructional Materials Debate

Added 2011-07-21 & 22 - The debates are being live blogged by the TFN and by Steven Schafersman. So, here links to their live blogs, along with a bit more information.

• Steven Schafersman - Live Blog of the Texas State Board of Education Meeting, 2011 July 21
• Steven Schafersman - Live Blog of the Texas State Board of Education Meeting, 2011 July 22
• TFN Insider - Live-Blogging the SBOE Science Hearing I
• TFN Insider - Live-Blogging the SBOE Science Hearing II
• TFN Insider - Live-Blogging the SBOE Science Debate I
• TFN Insider - Creationists Target Publisher in Texas Adoption
• TFN Insider - Live-Blogging the Final Science Materials Vote
• TFN Insider - Science Education Triumphs in Texas
• TFN Insider - Archive of Blog Entries on This Issue

Added 2011-07-19 - I've completed my letter to Mrs. Lowe, for anyone interested in reading it.

Attn: Gail Lowe - Science Instructional Materials

Mrs. Lowe,

I am a resident of Wichita Falls, and so you are my representative on the Board of Education. I have been disappointed in some of your past actions and votes that have weakened our children’s science education, but you have a chance to vote in support of sound science this Friday when it comes time to approve the new science instructional materials. It’s no secret that some of the submitted resources are sympathetic to creationism and Intelligent Design. Such resources do not accurately reflect mainstream scientific views, and would do our children a disservice by miseducating them.

I have written you before on science education, but considering your actions since that time, I feel obliged to repeat myself on certain issues.

First of all, evolution is accepted as true by the vast majority of mainstream biologists. Consider the following statement from the summary of the National Academy of Sciences’ 2008 report, Science, Evolution and Creationism (available online at http://www.nap.edu/catalog.php?record_id=11876):

"Scientists no longer question the basic facts of evolution as a process. The concept has withstood extensive testing by tens of thousands of specialists in biology, medicine, anthropology, geology, chemistry, and other fields. Discoveries in different fields have reinforced one another, and evidence for evolution has continued to accumulate for 150 years."

Rather than go on at length listing scientific organizations and agencies that have issued similar statements endorsing evolution, I'll direct you to the following page on the website of the National Center for Science Education, which does list such statements:
http://ncseweb.org/media/voices/science

Similarly, I could go on at length describing all the evidence supporting evolution - both the fact of common descent with modification, and the various theories describing how evolution happens. Instead, I'll point out few very informative websites where you can find this information:

• http://www.pbs.org/wgbh/evolution/
• http://evolution.berkeley.edu/
• http://www.talkorigins.org/
• http://ncse.com/resources/

I hesitate to bring up the legal issues involved with the teaching of creationism, because we should simply be concerned with teaching our children the best science that we can, which evolution most certainly is, and raising other points seems a bit of a distraction. However, it cannot be ignored that when other states have provided openings to allow the teaching of creationism and intelligent design, it has resulted in costly court battles. Consider the Kitzmiller et al. v. Dover trial in Pennsylvania, the Selman et al. v. Cobb County School District et al. trial in Georgia, or the Rodney LeVake v. Independent School District 656, et al. trial in Minnesota (which is relevant to the "strengths and limitations" tactic). These types of battles are completely unnecessary, as they could be avoided entirely simply by keeping science classes limited to well founded science.

• http://ncse.com/creationism/legal/intelligent-design-trial-kitzmiller-v-dover
• http://ncse.com/creationism/legal/selman-v-cobb-county-textbook-disclaimer-case-0
• http://ncse.com/creationism/legal/levake-v-independent-school-district-656

I think it hardly needs to be said that I will not vote for a representative who puts their own biases above the recommendations of experts in the appropriate field. I urge you to vote in favor of the evidence based resources, and to reject the resources that would miseducate our children.

Sincerely,
Jeffrey R. Lewis

Posted by Jeff on Tuesday, Jul 19 | Permalink | Comments (25) | TrackBacks (0)

I received an interesting spam e-mail the other day.

Hi Jeff,

We posted an article that we thought you and your readers might be interested in having a look at, "Yay or Nay: 20 Sites Debating Evolution vs Creationism" ( [link removed - jrl] ). Just thought I'd let you know that you would be interested in featuring or mentioning it in your blog.

Thanks for your time!
Sheryl Owen

It looked superficially honest enough. Only the included url (that I deleted) gave it away. It was to a site called 'change of address dot org' (with spaces removed and the dot as a '.'). They'd thrown together a handful of links on the topic, with the pretense of it being a useful resource, but it was obviously just a hook to try to bring eyes to their site so that they could sell you something.

I'm not going to link to that site, since that would reward their practices with an improved Google ranking. However, if you want to see it for yourself, just do a Google search for 'Yay or Nay: 20 Sites Debating Evolution vs Creationism', and it will pop up along with its clones.

First of all, it's set up as if there's a legitimate debate on the issue, with 8 pro-evolutionary science sites, 8 pro-creationism sites, and 4 'Middle of the Road Options'. Now, that's fine if you're trying to understand the culture of the debate, but it's not very useful for getting at the truth. There's no real scientific question over the truth of evolution - it's entirely a cultural issue, with people refusing to accept evolution for other reasons (such as religion).

But even looking at their links to pro-evolution sites is a bit odd. I've been rather caught up in the evolution/creation debate for several years now, so I know some of the better websites that deal with the issue. Their links left off some of the best. Here are the links they chose, in the order they presented them:

• http://www.vuletic.com/hume/cefec/
• http://pandasthumb.org/
• http://www.edwardtbabinski.us/
• http://www.creationtheory.org/
• http://academicearth.org/lectures/evolution-versus-creationism
• http://www.freethoughtdebater.com/index.jsp
• http://www.scientificamerican.com/report.cfm?id=creationism-vs-evolution
• http://www.infidels.org/library/modern/science/creationism/

None of those are bad sites. In fact, if I had to make a list of the 8 best evolution sites that I knew of, I'd probably include a few they mentioned (especially The Panda's Thumb). But where is The TalkOrigins Archive? How can you possibly mention the evolution/creationism debate without including that site? In my opinion, it is the best resource for people who misunderstand evolution because of misinformation from creationist leaders. And what about Berkeley's Understanding Evolution site, PBS: Evolution, or even the National Center for Science Education?

Their pro-creationism sites were also odd. They included a link to the Institute for Creation Research (ICR) and Answers in Genesis, which I fully expected, but left off links to the most prominent Intelligent Design sites, such as Uncommon Descent and The Discovery Institute.

Oh well. I get lots of spam, and I hardly ever mention it on this blog. And this entry ended up turning into something longer than the e-mail actually deserved. It's just a bit irritating to get an e-mail that initially looks like something that may be worth posting, only to discover that it's a scam. Plus, this was a decent opportunity for me to post links to some good resources on evolution.

For a post that I didn't consider terribly interesting, this one has already gotten a troll in the comments, and an e-mail from Robert Luhn of the NCSE (that's a lot of traffic for me, so back off </Billy Madison voice>). I may respond to David Buckna in the comments, but I thought I'd include up here two more evolution resources from the NCSE e-mail.

We've excerpted a ton of evolution books on our site. Check out our archive here:
http://ncse.com/resources/free-evolution-book-downloads

Another good resource: our Antievolution Legislation Scorecard at:
http://ncse.com/evolution/antievolution-legislation-scorecard

Those are good resources worth taking a look at.

Update 2012-03-07 - If you happen to be reading this on one of the archive pages on this site, note that there was a rather long exchange between a reader and me in the comments section. You may want to click on the 'Read more' link to read the comments.

Posted by Jeff on Tuesday, Jul 5 | Permalink | Comments (39) | TrackBacks (0)

I don't generally go looking for articles at Answers in Genesis (AiG). I have written about them previously (the most explicit mentions being in I Can't Escape Fundamentalists Even When I'm Researching Pure Science, Creation Museum, Creation Museum/Creationist Rule of Thumb with the 2nd Law of Thermodynamics, AiG's Creation Museum Follow-Up, and Another Creation Museum Review), and I haven't been terribly impressed with anything coming from their organization. It would be a time sink to go looking for articles to debunk from their site, because there are so many bad articles to choose from (it would also be a bit superfluous since so many have already been covered in the Index to Creationist Claims). However, in a recent comment thread on Pharyngula, someone included a link to AiG, and I took the bait.

The article in question is Something fishy about lungs, written by Joachim Vetter. I was interested in it, because it was related to an entry I wrote previously, Book Review - At the Water's Edge.

The introduction starts off simply and accurately enough.

It is well known that creatures which live permanently in water generally breathe through gills, not lungs. The lungfishes, which are able to survive long periods when their watery habitat dries up, are regarded as a peculiar exception.

In then goes on to set up a possible scenario by which lungs could have evolved, and adds the following statement.

For a long time it was believed that this swim-bladder was a logical ‘first step’ towards the later development of lungs as vertebrates conquered the land.

I don't know the full history of scientific thought on how lungs evolved, but I'll grant them that this could be true - that at one time, people believed it probable that lungs evolved from swim-bladders. Just keep in mind that it's not the current consensus.

Next, the article points out a simple fact about the natural world, but one which I don't see why would have any negative repercussions for evolution.

The first awkward fact, usually not mentioned in high school evolutionary texts, is that there are actually many modern species of fish (not mammals, but real fish) which have lungs as well as gills.

The article then points out a fact from the archaeological record that forces one to question the 'swim-bladder to lung story', and rightly so.

An even more bitter blow for evolutionists (again seldom seen in basic texts) is that fossil evidence has come to light forcing a 180-degree reversal in the ‘swim-bladder to lung’ story. Lungs appear to be much more ‘ancient’ than swim-bladders, so by this reasoning, lungs must have evolved into swim-bladders!

In fact, as noted in my other blog entry, this is the current consensus on the relationship between lungs and swim bladders. It's not just the fossil record that demonstrates this, but cladistics as well. Only teleosts have true swim bladders. Just like this article itself noted, there are other groups of fish that have lungs. And there exist other groups with structures intermediate between lungs and swim bladders. The most parsimonious explanation is that lungs evolved first, and only developed into bladders in one lineage. And just for reference, sharks and rays, which diverged from other fish around 450 millions years ago, don't have any type of lung or swim bladder at all.

Next was another statement that seems obvious enough.

Whether endowed with gills, lungs, or a combination of both, all fish, living or extinct, appear to be (or have been) well equipped for the requirements of their way of life.

But with that in mind, read the passage that immediately followed.

The ‘obvious’ evolution of lungs from swim-bladders turns out to be a myth. In addition, a literal evolutionary- chronological reading of the fossil record shows that evolution must have had remarkable foresight. In spite of the fact that lungs are not needed for survival (fish being able to cope well with gills) they appear and are prevalent among fishes at least 100 million years (on the alleged evolutionary time-scale) before their (imaginary) migration to the land. How wonderful of evolution to develop, all by chance of course, such a ‘test pattern’, ready to be taken out of mothballs when required.

It's like the author completely forgot what he had written just one paragraph before. If he acknowledged that living fish with lungs appear to be well adapted to their environments, why would he think it must have been 'foresight' that made lungs appear in the first place. And how could he write that 'lungs are not needed for survival' of fish, when he just listed examples earlier in the article of fish that do require lungs to survive. And the author never even considered fish with swim bladders that have secondarily evolved non-lung means of breathing atmospheric oxygen (such as bettas). It seems that air breathing is a useful adaptation for aquatic life without any consideration at all for life on land (though there are probably multiple reasons for this, as noted in my previous entry, one of the most obvious is that oxygen levels aren't as consistent in water as in the atmosphere, with some bodies of water being nearly completely devoid of oxygen).

I know there's not a whole lot of new information in this entry compared to my previous entry on lungs and swim bladders, but to read an article that no only ignores evidence and sets up a straw man explanation of evolution, but which also contradicts itself so clearly from one paragraph to the next, is just really frustrating. I couldn't read something that bad without venting about it here. It's just one more example of why not to trust AiG.

Added 2011-07-06 - I should add that I only covered the most obvious errors from the AiG article. There were numerous others, such as what's actually in biology textbooks, as well as details of which animals have lungs vs. swim bladders.

Posted by Jeff on Tuesday, Jun 28 | Permalink | Comments (24) | TrackBacks (0)

There's an interesting topic that stirs quite a bit of debate in certain circles these days - whether or not a wind powered cart can travel directly downwind faster than the wind, itself. This concept is generally known by one of two acronyms, Down Wind Faster Than The Wind, DWFTTW, or the slightly longer Directly Down Wind Faster Than The Wind, DDWFTTW.

The cart in the concept has a propeller connected to wheels through a driveshaft and transmission. You might intuitively think that this is impossible. I know I did. It sounds too much like a perpetual motion machine, with the wheels powering the propeller which pushes the wheels. So, I thought I would set out to prove it impossible with a few free body diagrams, but now that I've studied the diagrams, I find myself thinking that it might actually work.

Background

First, to get a bit of background on this concept, here are some of the websites of its main proponents:

• Faster Than the Wind Team's Official Blog
• Downwind Faster Than the Wind

To get an idea of just how much passion this concept evokes, take a look at some of the discussion threads arguing over it:

• JREF Discussion Forum, Thread 1
• JREF Discussion Forum, Thread 2 (this one's still active)
• Mythbusters Fan Club Forum
• SkepticBlog
• Good Math, Bad Math

As one last link before getting into my own discussion, the Faster Than the Wind Team claims to have built and tested a car that demonstrates the concept. The North American Land Sailing Association (NALSA) witnessed and authenticated the event. While some might think that would be enough to convince doubters, many remain skeptical of the claims and suspect foul play or incompetence (such as not running directly downwind or measuring windspeed incorrectly in the more generous accusations).

• NALSA's Direct Downwind Record Attempt Page

Here's some video of the record claiming run.

Main Issues

There are two big discussions in this debate, whether a DWFTTW car is even possible, and second, whether the widely circulated 'Treadmill Experiment' is useful in demonstrating its validity. Since the first claim is more interesting, I'll address that first.

Is DWFTTW Possible?

To address this, I drew up some free body diagrams. All the diagrams are shown in an assumed steady state condition. For simplicity, the vehicles are all simplified as just a single wheel, a prop/turbine, and a transmission connecting them.

Let's start with something that we know works, an upwind vehicle.

In this case, V_wind will be greater than V_ground. The prop/turbine will be acting as a turbine, so it will be creating drag. The wheel will be driving the vehicle, so F_ground will be in the forward direction. Since the vehicle is in equilibrium:
F_ground = Drag

To calculate the power from the turbine and the power from the wheel:

P_turbine = Drag * V_wind
P_wheel = F_ground * V_ground

Since Drag = F_ground, if V_wind > V_ground, then P_turbine > P_wheel. That's what it needs to be to overcome transmission losses and the vehicle wind drag that I didn't account for. So, the diagram and analysis agree with what we expect from reality.

Next, let's move on to a case that we know doesn't work, a vehicle with no wind.

Once again, to maintain equilibrium:
F_ground = Thrust

Since I labeled the forces slightly differently, here are the power equations:

P_prop = Thrust * V_wind
P_wheel = F_ground * V_ground

Since Thrust = F_ground and V_wind = V_ground, then P_prop = P_wheel. That doesn't work, since transmission losses will sap the energy out of that system, as will the air drag. I think it should be obvious enough that if you reverse the drive direction (i.e. a turbine powering the wheels), that it still comes out to P_turbine = P_wheel, which doesn't work. So again, the diagram and analysis agree with what we expect from reality.

Now, let's move on to the DDWFTTW case.

Once again, to maintain equilibrium:
F_ground = Thrust

The power equations are the same as the previous case:

P_prop = Thrust * V_wind
P_wheel = F_ground * V_ground

Since Thrust = F_ground, if V_ground > V_wind, then P_wheel > P_prop. Like the first case, that's what it needs to be to overcome transmission losses and the vehicle wind drag.

So, it seems counter-intuitive, but unless I've made a mistake somewhere, it looks like it should work. Maybe there is something to what the propenents have been saying, that the differential velocities are the source of energy, and why this isn't a perpetual motion machine. As the diagrams show, if there's no wind at all, then the vehicle doesn't run.

The Treadmill Experiment

An early proof of concept experiment that made its rounds on the Internet was to put one of these carts on a treadmill and see what happened. The video is included in the links I gave at the start of this entry, but I'll embed it here to make it easier for you.

The treadmill experiment prompted two big questions - is it representative of a cart moving downwind over the ground, and does it demonstrate the validity of the DWFTTW concept?

The answer to the first question is a clear yes. The treadmill is an equivalent reference frame. That's how wind tunnels work – it's all about relative velocities. If a treadmill is moving at a steady 10 mph in still air, it's the same as the ground being stationary with a steady 10 mph wind.

Consider this. The Earth's surface is not stationary itself. Given a circumference of approximately 25,000 miles, and a rotation period of 24 hours, the ground is moving at just over 1000 mph at the equator (and that's ignoring the Earth's motion around the Sun, the Sun's motion around the Milky Way, and the Milky Way's motion about the local galactic cluster). In other words, the Earth could be considered a giant treadmill. But we can safely neglect that if we use a frame of reference that moves along with the earth. It's the same thing with the treadmill. As long as all the relative velocities are equivalent, then your reference frames are equivalent.

But, did the experiments in the video demonstrate the validity of DWFTTW? I think the answer is yes to that as well, but I also have an idea for another experiment.

My initial skeptical thought was that by physically holding the cart stationary on the treadmill before releasing it, they were storing energy by spinning up the propeller on the cart. When they released the cart, it would surge forward using that stored energy. It's just like a toy helicopter where you pull a string to make it take off. There's nothing surprising about that.

But, if you watch the video, once the cart is operating, they aren't holding it against the treadmill, they're holding it back against its own thrust. That, to me, is indicative that the cart wants to run faster than the treadmill. Unfortunately, given the short length of their treadmill, the videos never show the cart reaching a steady state.

My suggestion for a better experiment (aside from the full size human carrying cart) is to build a long treadmill inside a building, and put their cart on that with some guides to keep it from running off track. If it achieved a steady state forward velocity relative to a stationary observer (and was well documented by independent observers), then I'd think most reasonable skeptics would be convinced. Alternatively, the cart could be placed on the treadmill before the treadmill was started in motion, so that it wouldn't be touched by human hands at all once the experiment started. Judging by the Faster Than the Wind Team's human carrying cart, I would think they have the means to carry this out, and all remaining doubts could be put to rest.

Alternate Explanation 1

I think a better way to understand this vehicle, is rather than thinking of it as a ground vehicle powered by the wind, think of it as an aircraft powered by the ground. I have a little thought experiment that might help. Envision the vehicle suspended on some rails, with the prop aligned to propel it down the rails, and with the wheel hanging below on a caster that enables the wheel to face any direction. If you put a conveyor up to the wheel with the conveyor running sideways relative to the vehicle, it's obvious that the conveyor will turn the wheel, which will drive the propeller and push the vehicle down the tracks. Now, if you start rotating the conveyor to more closely align with the tracks, it will continue to drive the wheel. The more closely it aligns with the tracks, the higher the drag load that it will impart, but it will continue driving the wheel.

So, think of the cart as an aircraft that with no other forces acting on it would want to be 'at rest' with zero relative windspeed. But, once the ground starts moving relative to the aircraft, it provides a power source that the aircraft can tap into to propel itself.

Alternate Explanation 2

Consider a cart where one set of wheels is turning a generator used to power another set of wheels driven by motors. This obviously won't work. Since both wheels are moving over the ground at the same speed, if the force at each wheel was of equal magnitude, then the power created by the generating wheel would be equal to the power being used by the driving wheel, which doesn't work when you account for losses.

If a cart with a propeller was moving through still air, then it would be the same thing. That was what I tried to explain with the no wind case. When forces are equal and velocities are equal, then powers are equal.

What makes the downwind case work, is that the wheels and the propeller are operating in two different media at two different speeds. For a propeller, we typically look at the thrust generated for a given power, since that's the way engines operate. It's well understood that for a given power, thrust drops with airspeed. But looking at that a different way, it means that to generate a given thrust, the power requirement goes up with airspeed. So, using the no wind day as a baseline, when the thrust from the prop and the drag from the wheels are the same, the input and output powers are the same (which doesn't work because of losses). Now, if you add a little bit of tail wind, it means the propeller is not travelling through the air as fast. If you maintain it at the same thrust, it means the power requirement goes down. So, now we're getting into a regime where the power generated by the wheels is higher than that required by the prop. With enough of a tail wind, the power differential can get big enough to overcome the inefficiencies and make the system actually work.

Conclusion

So in the end, once I gave this a little thought, I surprised myself. I think my initial gut reaction to this concept was wrong, and that the DDWFTTW proponents are right. That's all part of honest skepticism - knowing when to admit you were wrong and to change your views based on new reasoning and evidence. It's certainly nice to know that the Faster Than the Wind Team is most probably honest, and that the videos probably aren't a hoax. Congratulations to them for their achievement.

Now that I've had my say, and hopefully convinced people that this is possible, here's a good article on it:

• Wired - A Long, Strange Trip Downwind Faster Than the Wind

I'll also add that the nice thing about this question is that it's testable. If I get a chance, I'll build a little cart myself. If enough people do this and test it, it should be confirmed pretty quickly. If you're one of the people that feels really strongly about this, go do a test for yourself.

Added 2011-06-13

I figured it might be fun to throw in a few real numbers to get a feel for how this would work. So, I pulled some numbers out of the air to see how the calculations come out.

I started out with a ground speed of 30 ft/s (~20.5 mph). Assuming 100 pounds of drag on the wheels, this works out to 3000 ft-lb/s of power (~5.5 HP). Assuming a 90% efficient drive train, there's 2700 ft-lb/s going into the prop. Now, the next step requires a little understanding of propellers which I've explained on my static site (Theoretical Max Propeller Efficiency). Assuming a figure of merit of 0.9*, and a propeller diameter of 15', the prop will create 170.5 lbs of static thrust. So, at the state where ground speed matches wind speed, the thrust created by the propeller will be greater than the drag on the wheels - the cart will accelerate forward. And since the cart is at zero relative airspeed at that condition, there's no aerodynamic drag to consider. Also note that there's no stored energy from a flywheel effect in this analysis, so the steady state condition will necessarily be at some speed where the cart is going faster than the wind speed.

You can play around with those numbers if you want to. For the given efficiencies and prop diameter, the break even point where thrust = drag is around 495 lbs (27 HP @ 30 ft/s). If you hold the efficiencies and drag constant, the break even point on thrust & drag occurs for a 6.7 ft diameter propeller. All of these numbers appear to be fairly reasonable, giving me yet further confidence that the Faster Than the Wind Team probably achieved what they stated.

* This figure of merit is a measure of how much of the power is going into accelerating the air. This is a more useful measure than efficiency for low speeds, since by definition, propeller efficiency is equal to zero for static thrust. A figure of merit of 1 is the theoretical limit. The propellers I've designed at work typically achieve figures of merit of 0.92 to 0.94 for static thrust.

Posted by Jeff on Monday, Jun 13 | Permalink | Comments (37) | TrackBacks (0)

As the webmaster where I work*, I field most of the unsolicited e-mail to our company. Quite a few of those e-mails are from people who would like some help developing their concepts. Unfortunately, we're too busy developing our own concept to help others, but I still get to see some interesting ideas. Sometimes, though, it's obvious that things won't work out the way the potential inventor would like, such as the several proposals for perpetual motion machines that have been sent to me.

I've just received an e-mail for a hovercraft concept that looks very intriguing, but which won't actually work. For anyone who wants to test their aeronautical knowledge, go look at the concept, and see if you can figure it out yourself before reading the rest of this entry.

Vortex Aerodynamic Platform Aircraft

The basic concept is to have a blower blow air over fixed airfoils inside a chamber, then have an auxilliary fan above the airfoils to accelerate the air further, where it gets redirected by 'annular' wings and forced downward. Below are some of the images from the above link to illustrate this. I've reduced them a bit to make them fit here, so follow the link to see them full size.

I've already responded to the person who sent me the e-mail (I hate to see someone wasting their time on something doomed to failure), so I'll adapt and expand that response here.

Sometimes, it's useful to take a step back from the details, and look at the big picture. Us engineers like to look at pressures on airfoils to calculate lift, but keep in mind Newton's 3rd Law, equal and opposite reactions, and Newton's 2nd Law, F=ma. From the 3rd Law, if you want to generate a lift force, the equal and opposite reaction is a force down on the air. From the 2nd Law, a downward force on the air must be accompanied by a downward acceleration. If the air isn't accelerated downward, there is no net force down.

So, looking at this concept, the only portion generating lift is the annular airfoils that are deflecting the air downward. The fixed interior airfoils aren't generating any lift at all. As a coworker of mine puts it, it's like trying to lift yourself by your bootstraps.

Of course, it's possible to generate lift just by ducting air downward, but it's also important to keep in mind that it's much more efficient to take a big bite of air and accelerate it just a little bit, than to take a small bite of air, and accelerate it a lot. That's why the Harrier is so inefficient in hover, and why helicopters have big rotors on top. The way to get more efficient hover is to put an even bigger rotor on top.

The reason this concept struck a chord with me is that when I was younger, back before I'd studied engineering, I had a similar concept myself. A prop would blow air over interior wings to generate the lift, and a pair of side by side nozzles on the back could aim the airflow to provide thrust and yaw control. I had dreams of revolutionizing aviation with my invention.

With the simpler layout of my concept, perhaps it's easy to look at this another way to see the flaw. Right above the wings, there will be a low pressure region, lower than the pressure below the wings, so the wings themselves will be pushed upward. However, above the fuselage, the pressure will be higher than in the duct, so the fuselage will be pushed down. When you look at all of the surfaces and the pressures on them, the forces all cancel out so that there's no lift on the vehicle. In other words, since the duct is fully enclosed by the fuselage, any change in pressure in the duct only creates forces that act internally, and won't result in any net forces on the entire aircraft.

The pressure explanation is what I thought up years ago before college which led to my abandoning the concept, but I think the Newtonian explanation is easier to follow.

This idea of an inner wing airplane makes some sense given the standard explanation for how wings produce lift (the curved top surface accelerates the air, lowering the pressure). I certainly thought it would work when I was younger, and apparently, I'm not the only one that's thought of it. It's a shame that physics has to get in the way of our imaginations.

Updated 2011-04-29 - A few slight changes in wording to improve the explanation of pressure.

*It's a small company, so we all wear a lot of hats. I mostly do engineering, not website management.

I have a couple pages on the static portion of this site that are somewhat relevant to this, though not directly related:

• Introduction to Flight
• Theoretical Max Propeller Efficiency

There's a little puzzle that should be easy to answer when you think in terms of what I've described above. If a truck driver hauling chickens pulls up onto the scales at a weigh station, and discovers that his truck is overweight, if he scares the chickens into flying around inside the truck, will it change its weight?

Posted by Jeff on Wednesday, Apr 27 | Permalink | Comments (0) | TrackBacks (0)