Aviation Archive

Saturday, December 17, 2011

Happy Wright Brothers Day, 2011

Wright Brothers' First Flight, December 17, 1903

108 years ago today, the Wright brothers became the first humans to fulfill the dream of flight. I've written about this before, and rather than repeat myself, I'll just link to those previous entries.

So happy Wright Brothers Day. And find a little wonder in the fact that you can go out and do something that our ancestors could only dream about for thousands and thousands of years.

Monday, August 22, 2011

Fire a Little Too Close for Comfort

This was the view from my front yard yesterday afternoon*:

Helicopter Dumping Water Just Outside Tanglewood
(Click to embiggen)

There was a pretty big fire right on the edge of our development. According to police spokesman, Sgt. Joe Snyder, the fire threatened 100 houses at its worst, but thankfully in the end only caused minor damage to the siding of two houses. Around 25 acres were charred when it was all over. 20 residents were evacuated, but none were injured. Four firefighters were treated for heat exhaustion, after which they jumped right back into the thick of it fighting the fire.

We heard about the fire while we were out shopping, so we almost immediately returned back home to see what was going on. At that point, there were plumes of black smoke that we could see from a few miles away, and right near the entrance to the development, there were flames 20-30 feet high (by my estimate - my wife thinks they were higher, but it's also possible we could both be overestimating due to our excitement). The main entrance was closed, so we had to take the back way in. The picture above shows how close the fire was to our house - close enough to be a bit concerned, but not anywhere near as concerned as the people living in the houses that were evacuated. At one point, when the wind shifted, it got a little hazy around our house from the smoke, but most of the time the wind was blowing the smoke just north of us.

The helicopter showed up a little while after we got home, and by that point, the smoke wasn't nearly as bad. The helicpter stayed for around an hour or so making several dumps. Here are a couple more photos of it:

Closeup of Helicopter Dumping Water Just Outside Tanglewood
(Click to embiggen)

Closeup of Helicopter that Dumped Water Just Outside Tanglewood
(Click to embiggen)


From what I've heard, the best guess is that the fire started when an electrical transformer exploded. With as dry as it's been here, those sparks were enough to start the fire. Whether the transformer did actually start the fire or not, we were without power for several hours. With the heat wave here, it was starting to get pretty stuffy.

Thankfully, the firefighters were able to get the fire under control and prevent any major damage or injuries. And the power company was able to restore power within a couple hours of being allowed on site.

So, to the firefighters and utility workers who worked in the 106º heat to save our houses and give us our power back, I want to express my sincere and deepest gratitude.

More Info:

*Okay, technically, that was the view from my next door neighbor's yard - the tree in the bottom right of the photo blocked the view from our yard.

Monday, June 13, 2011

Directly Downwind Faster Than the Wind (DDWFTTW)

DWFTTW BlackbirdThere'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:

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

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).

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.

Free Body Diagram of 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.

Free Body Diagram of Vehicle in 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.

Free Body Diagram of Downwind Vehicle

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:


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.

Wednesday, May 25, 2011

Gamera Human Powered Helicopter

University of Maryland's Gamera Human Powered HelicopterFrom time to time, I actually read the newsletter from the engineering department from my alma mater, the University of Maryland. The latest had an interesting story. Some students built a human powered helicopter, and managed to get it airborne (around 6" off the ground for just a few seconds, but still, it took off under its own power).

Human powered flight in fixed wing airplanes, if not exactly common place, has been accomplished numerous times by now. The record holder is the MIT Daedalus, which flew 71.5 miles from the island of Crete to the island of Santorini, staying aloft for 3 hours and 54 minutes. A flight that long is more than just a hop, but human powered flight is still pretty demanding. It took an Olympic cyclist to pilot that aircraft, and the structure was so optimized (the plane itself only weighed 69 lbs), that it couldn't handle the winds in Santorini and was blown apart before the pilot landed (he did escape unharmed). Other notable human powered airplanes include Southampton University's Man Powered Aircraft (the first human powered aircraft to take off under its own power in 1961), the Gossamer Condor (which won the first Kremer prize in 1977 by flying a designated course), the Gossamer Albatross (which crossed the English Channel in 1979), and the Musculair 1 (the first human powered aircraft to carry a passenger in 1984).

But if fixed wing airplanes are a challenge, human powered helicopters are next to impossible. I've only heard of 3 that have actually managed to lift off - California Polytechnic State University's Da Vinci III, which flew for 7.1 seconds at a height of around 8" back in 1989, Nihon Aero Student Group's Yuri I, which flew for 24 seconds and reached 27.5" in 1994, and now University of Maryland's Gamera, which flew for 10.8 seconds at around 6". Although certainly impressive technically, none of those results are very awe inspiring. It just goes to show how thin air really is, and why flight is such a challenge that humans weren't able to conquer until last century.

There's an Official Gamera Website where you can see pictures of the aircraft and read more about it. There's also a Wikipedia entry. To get a better idea of the scale of the aircraft, you really should watch the video of it flying (the flight starts at around 3 minutes into the video).

From the above websites, I gathered the following data. The empty weight of the aircraft was only 101 lbs, and with the pilot included the gross weight was only 208 lbs. The helicopter had 4 rotors, each with a 42' diameter.

A metric that rotorcraft engineers like to use is disc loading. You divide the weight of the aircraft by the disc area of the rotors. This is all to do with efficiency - the lower the disc loading, the more efficient the rotor can be. When you calculate disc loading for this aircraft, you get 0.0375 lb/ft². That's incredibly low. For comparison, the Robinson R-22, a lightweight helicopter with a relatively low disc loading itself, has a gross weight of 1370 lbs and a rotor diameter of 25'-2", giving a disc loading of 2.75 lb/ft². Gamera's disc loading is more than 70x lower than the R-22's! For another comparison, the V-22 Osprey has a relatively high disc loading of 26.7 lb/ft² (part of the reason why it will never do as good as a pure helicopter in hover), 712x higher than the Gamera.

I can't help but put up a picture of da Vinci's helicopter concept here. Everyone knows that his concept wouldn't work, but I don't think most people realize just how far from a workable design it really is. You can't be too hard on da Vinci given the state of aerodynamic knowledge in his time, but compare this to the Gamera.

da Vinci Helicopter Concept

On a less technical note, Maryland's human powered helicopter was the first to be powered by a woman, 24 year old biology student, Judy Wexler. I wonder if they chose a woman just to be the first, or if it was more to do with improved power to weight ratio (she only weighs 107 lbs).

So, my hat's off to the students at the University of Maryland who built this aircraft. It was an incredible technical accomplishment. I gather that they're going to attempt more flights and try to claim the Sikorsky Prize. I wish them luck.

More Info:

Wednesday, April 27, 2011

Interesting Hovercraft Concept That Won't Work

Vortex Aerodynamic Platform AircraftAs 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.

Vortex Aerodynamic Platform Aircraft Concept

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.

My concept for an Inner Wing Aircraft from when I was a kid

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:

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?

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