Slashdot Mirror
Combined Hovercraft and Helicopter
An anonymous reader writes "Has British engineer Geoff Hatton brought us the best of two worlds with his UFO-looking machine? The US military thinks so and are investing in it. The design is sturdy (as opposed to a helicopter) and can fly high (as opposed to a hovercraft). It is based on the Coanda Effect."
Hovercraft's that use the technique you describe would be required to move a lot more air, and do not do so well at higher altitudes where the air is thinner. This is due to their lift being generated entirely by creating a low pressure zone above the craft by moving huge quantities of air from one side of the craft to the other.
This craft moves a smaller amount of air across it's surface, like the wing of an airplane, the way the air flows across it's surface creates the low pressure zone necessary to create lift.
The method you discuss, works well in situations where the rotors are very large in relation to the body of the craft, while this method works even when the rotors are much smaller in relation.
I am sure someone with a little more understanding of the physics involved could improve upon what I just said, but I'm pretty sure this is the way it works.
Sometimes the best solution is to stop wasting time looking for an easy solution.
There are two more things which you didn't mention. One is that the fan is inside a duct, which reduces tip vortices. That should make it more efficient.
The second is that it would avoid a problem which helecopters face when trying to hover out of ground effect. When more than about a rotor's diameter above the ground, the downward moving air starts to circulate down, out, up, and back into the rotor. The air moves in a circular pattern through the rotor, around, and back through the rotor again. This creates a downdraft from the perspective of the helecopter. Adding more power doesn't always help, because it just makes the air move in a circular pattern faster. The result is that the helecopter sinks when trying to hover at altitude.
If you observe helecopters hovering at altitude, you'll notice that they aren't actually hovering. They're moving forward very slowly. That's the only way to avoid that problem. You have to keep moving a little bit so you stay out of the circular rotation of air that you create behind your helecopter. If you stop completely, you're in the circular pattern and you sink unless you've got some enormous power source like a jet engine.
When you're in ground effect, the ground itself disrupts the circular movement of the air and limits how fast it can move in a circular motion. It also makes it turbulent as it deflects off the ground. The result is that you don't get a well-formed column of downward moving air that your helecopter is sitting in, thus you can efficiently hover without moving at all when you are fairly close to the ground or some other air-disrupting object like a building that you're carrying materials up to.
I would not be surprised if this device had some advantages over regular helecopters when it comes to hovering out of ground effect.
Fascism trolls keeping me up every night. When I starts a preachin', he HITS ME WITH HIS REICH!
It's interesting that you bring that first link up. The second link, Coanda Effect: Understanding Why Wings Work, is from no less than Jef Raskin, the father of the Mac. It contains a fallacious argument on why the Bernoulli effect can't explain the lift generated by a wing, which he claims he first derived as a child. It contains some child-like assumptions, the most grievous being the assumption that the ratio of the chord lengths (distance over the wing versus under the wing) is the same ratio as the speed of the air over the wing versus under. This implies that two air molecules that separate at the front of the wing, one going over and one going under, will meet at the back edge of the wing, as if joined by some invisible rubber band. In reality the ratio of the speeds is larger than the ratio of the chords, and the top molecule reaches the back long before the bottom one does. This link to a different page on the same website as the first Coanda fallacy link, shows the airflow using smoke pulses and does a great job of describing what is going on.
"I'm so moist I'm sticking to the leather." -Kermit the Frog on The Late Late Show
As for the stability of helicopters, if you look at the designs in the 50's and 60s, stability was a big goal. Look at Stanley Hiller's demonstration of hands off hovering of his helicopters. Over the years stability appears to have been less and less of a goal. Look at how the flybar on the Bell rotor systems has disappeared. I'm not sure why this has happened, but I'm guessing that agility has won out over stability, especially since stability can easily be added by electronic means.
I read the article, but I wasn't quiet sure how the Coanda effect was utilized by this design. I'm guessing that rather than tilt the rotor there are places where the downwash is attached (or not) and thus generate sideways thrust? I saw the little vanes moving, but assumed that was more for anti-torque - if you noticed, most of the vanes were fixed with a little bend in the direction of anti-torque (and, like the MDHC Notar the anti-torque force would be proportional to downwash and thus to torque). A few of them moved to give you the ability to rotate the machine and account for minor yawing forces not exactly countered by the fixed vanes.
Did anyone else notice where/how Coanda effect was used? Perhaps the moving vanes really are implementing the Coanda effect, but if so it's in a fairly different way than the MDHC Notar system. Did I miss something?
The secret of keeping helicopters in action for years is that due to very frequent checks and tests, almost every part gets regularly replaced.
I talked to a air-rescue helicopter pilot once and he told me they have helicopters in service that are 35+ years old, but the only original parts in them are their skids.