Aviation Archive

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?

Friday, December 17, 2010

Happy Wright Brother's Day, 2010

Wright Brothers' First Flight, December 17, 1903

107 years ago today, the Wright brothers became the first humans to truly fulfill the dream of flight. You can read what I wrote about the significance of this from my Wright Brother's Day, 2007 entry. On a related note, you could read my entry, Flying, from last year, where I marvel at just how cool it really is to be able to fly.

Wednesday, October 13, 2010

XKCD Takes on the Wing Myth

XKCD finally did a comic relevant to my degree (it's usually computers or physics):


For a good explanation, check out my page, Introduction to Flight. If you're really interested in aviation, I have a whole section on Aviation Theory.

Tuesday, November 17, 2009

Woo Hoo!

Woo Hoo!Carter Signs UAS License Agreement with AAI

Carter Aviation Technologies LLC (Carter) of Wichita Falls, TX is announcing that they have completed negotiations with AAI Corporation, an operating unit of Textron Systems, a Textron Inc. company, of Hunt Valley, MD on an exclusive licensing agreement for Unmanned Aircraft Systems (UAS) using Carter's revolutionary Slowed Rotor/Compound (SR/C) Aircraft Technology - a combination of rotorcraft and fixed-wing aerodynamics. The 40-year exclusive agreement covers all UAS programs worldwide.

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Selling Out