About a year ago, I wrote about a project at the University of Maryland, Gamera Human Powered Helicopter. They built a helicopter that was powered entirely by its pilot, Judy Wexler. She managed to keep in the air for 10.8 seconds. That may not seem like very long, but it was only the third human powered helicopter in history to even make it off the ground (I discussed the challenges of human powered flight in that previous entry, so there's no need to go over it again here). Judy was also the first female to power such a vehicle.
Gamera II
Now, the team from UMD is back with an improved aircraft, Gamera II. At only 71 lbs empty, it's 35 lbs lighter than the first Gamera. The informational handout from the official website also claims that the aircraft only requires 0.62 HP to hover, a significant improvement over the 1.03 HP for the previous machine (both calculations with 135 lb pilot).
So what have these improvements allowed? A flight time of 50 seconds with Kyle Gluesenkamp at the cranks. That's not quite the full minute required for the Sikorsky prize, but it's a big improvement over the previous record of 24 seconds set by the Nihon Aero Student Group's Yuri I. And the website says that they plan more flights in August, so they might yet hit the minute mark.
Here's a video of the record setting flight:
I found a certain chart from their informational handout to be very interesting. Here's the chart.
Of course, the top is interesting to see how much they've improved with this new design. But look at the bottom part, where it compares Gamera II to some other aircraft. It has more disk area than a CH-53E, which has a max takeoff weight of 73,500 lb (per Wikipedia). It's comparable in dimensions to a Boeing 737, which has a max takeoff weight of between 111,000 lbs and 187,700 lbs, depending on the model (again, per Wikipedia). It really goes to show just how hard it is to fly, and just how much power we can get out of the small powerplants we install on aircraft.
So once again, congratulations to the Maryland team, and best of luck in the coming months.
For Christmas, here's a poem my great uncle wrote and sent to a few of us last year. In case you're wondering on the choice of aircraft in the poem, he was writing it for all the guys based out of the same airfield as he is. If you want the full effect, you can read a scan of the original. (BTW, if you like the picture for this entry, click on it and support the AOPA's Air Safety Foundation by buying a Christmas card with that as the front, or pick one of their many other aviation themed cards.)
A Plane Christmas Greeting by Bud Eichel
T'was the night before Xmas,
At Finleyville "Airdrome".
Not a creature was stirring,
Human, elf, or gnome.
All Aircraft secured,
In their Hangar "stalls".
The Xmas shoppers,
Home from the Malls.
From atop the Hangar,
The wind-sock hung low.
And bathed in moon-light,
The runway was aglow.
The rest of the field,
Was snowy and white.
This flyer's home-base,
Was a beautiful sight.
Then quick as a wink,
Dark shadows appeared.
Following moon-beams,
As they all neared.
Big ones and small,
These shadows all grew.
Twisting and turning
As by me they flew.
They made a "formation",
The shape of a "V".
Now as they pass,
They are plain to see.
Stearmans and Wacos,
A Stinson went by.
T-Crafts and Luscombes,
All on the fly!
Home-builts, a Mooney,
A new Carter-copter.
A Cessna amphibian,
An L-2 Grasshopper.
PT's and BT's,
From World War Two.
And old-style craft,
Like the Wright Bros. flew.
A "Cub" and a Grumman,
A sleek Monocoupe.
Can you believe this?
A pretty, '47 Ercoupe!
Aeroncas and Cessnas,
A Beech Musketeer.
Of all these Planes,
Not one, could I hear!
Are they "ghosts" of the past?
Am I tired and weary?
Wait, just a minute,
I have a theory!
That Angels exist,
I have no doubt.
And on Christmas Eve,
I'm sure they're about.
Did they take the form,
Of things that I love?
Is this my "gift",
From Heaven above?
If this was a gift,
I'd sure like to share.
Merry Christmas, to All,
I wish you were there!
Happy Holiday's, and Happy Landing's, to all my Pilot friends, & families.
No matter how you celebrate this time of year (or even if you don't celebrate at all), I still hope you have a good time.
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.
This was the view from my front yard yesterday afternoon*:
(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:
(Click to embiggen)
(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.
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:
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).
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:
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:
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:
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.
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.
