Random Aerodynamic Oddity of the Day Thread

[freebeard]

Max. Speed: water 80 km/h

ice and packed snow 120 km/h

Always climbing out of a hole in the water.



Nice.

[kach22i]

Originally Posted by freebeard

Always climbing out of a hole in the water.

Nope, the difference in speed over differing surfaces isn't "getting over hump speed - aka the hole in the water" it's surface drag and some parasitic drag.

Ice is really flat and nice and hard, and even when the skirt accidentally touches it the coefficient of drag is fairly low.

Water can actually pull down on the skirt as if it's alive, this can cause a "plow-in" when the skirt is pulled under. Plus water unless it's a still pond is uneven and the air bearing seal (ideally a constant level) takes more energy to maintain.

The various bulbous bow skirts with encapsulated air-spring reserve and perforations to allow air to escape near the surface and help prevent parasitic drag were all developed the hard way - experience.

My hovercraft is rated as max-speed of 45 mph. In practice over water it is 30-35 mph and 50-55 mph on ice. I have no air speed indicator but rode a motorcycle for years - my estimate is that I once went 65 mph over ice on a large inland lake and strong tail wind - it was very scary as I was in fear of going head over tail should cushion pressure falter and plow-in ensue.

These things called hovercraft are pushed by a wind (thrust), and wind is quite the foe if it's going against you. Once I was trying to get on cushion in a head wind to get back home across the lake, and a storm front was moving in. I had to turn around and go on a beech, then while on land the cushion was stable and headed out over the choppy lake into the headwind with all it had and never let up. I estimate I was going less than 15 mph ground speed and barely made it home - almost ran out of gas.

Same craft that unexpectedly was doing 65 mph on ice, could barley do 15 mph, and all dependent on the wind and choppy waves.

Here is a hovercraft that has some unusual features:
https://www.neoterichovercraft.com/technology.php/

Video Link:
https://www.neoterichovercraft.com/buck-bg.webm

Neoteric’s reverse thrust system surpasses jet aircraft in efficiency: while such aircraft deliver only 18% thrust in reverse, our hovercraft deliver 60%. Neoteric hovercraft are so efficient that they can achieve a speed of greater than 25 mph (40 km/h) in reverse and are the only hovercraft in the world that can accelerate over hump in reverse.

Good descriptions on this site.

DiscoverHover CURRICULUM GUIDE #19
DRAG
http://www.discoverhover.org/infoins...de19-drag.html

A hovercraft, such as the DiscoverHover One, is able to glide or slide easily because there is so little contact friction with the surface over which it is hovering. Still, there are forms of friction which come into play, and these frictional forces are usually known as drag. Drag occurs in several forms, the most familiar being wind resistance, or form drag, which is created by the hovercraft having to push aside air as it moves forward. This effect increases more and more as the hovercraft’s speed increases. Streamlining the shape of the hovercraft decreases the wind resistance, resulting in higher top speeds. While wind resistance is always present, it becomes a much greater problem at speeds of 50 km/hr [31 mph] and above. Since DiscoverHover hovercraft usually don't exceed 50 km/hr [31 mph], wind resistance is not as noticeable as it would be on other light hovercraft.

A hovercraft operating over water is subject to three other forms of drag not experienced on solid surfaces: wave drag, skirt drag, and impact drag. Wave drag (called hump drag at low speeds) occurs when lift air under the hovercraft pushes down on the surface of the water. Some of the water is displaced from under the hovercraft, creating a depression in the water. The total weight of the water displaced is equal to the weight of the hovercraft and pilot, according to Archimedes’ Principle. As the hovercraft starts moving forward, the depression moves with it and forms a small wave in front of the bow. This causes the bow (front of hovercraft) to rise and the stern (back of hovercraft) to sink slightly. The hovercraft is, in effect, trying to fly uphill. As the hovercraft increases speed, the bow wave increases in size. Eventually, the hovercraft will reach a speed where it's moving faster than the wave and will “climb” over it. Technically known as planing speed, this is commonly referred to as “getting over the hump”. At this point the hovercraft will accelerate rapidly. The moment before planing speed is reached, wave drag is at its greatest. When traveling above planing speed, the lift air under the hovercraft doesn't have enough time to depress the surface of the water and the wave drag decreases dramatically.

[Bicycle Bob]

How do hovercraft rate on ton-mile/gallon (or other units) compared with boats, hydrofoils, cars, ground-effect fliers, aircraft, and helicopters?
Agreed that streamlining always helps, but sometimes the major losses are elsewhere.

[kach22i]

Originally Posted by Bicycle Bob

How do hovercraft rate on ton-mile/gallon (or other units) compared with boats, hydrofoils, cars, ground-effect fliers, aircraft, and helicopters?
Agreed that streamlining always helps, but sometimes the major losses are elsewhere.

If we put it in context of fair weather using Diesel power in lieu of earlier generation of gas turbines on larger commercial craft as compared to high speed catamaran ferries I'm guessing that it's similar to smaller personal craft 4-strokes as compared to mono-hull boats.

That is hovercraft use 1/4 to 1/2 of the fuel as their water displacement cousins boats/ships.

However, total operating costs must include skirt maintenance and replacement as similar to comparing sail boats to gas guzzling boats.

On the flip side, the hovercraft only need a beach or sloping concrete apron/boat launch in lieu of a port with a dock which is a considerable infrastructure cost.

As far as safety, there have been less than a dozen deaths on commercial hovercraft in the past 40 years, all because of poor weather or botched rescue efforts that made things worse.

Recreational hovercraft have suffered two fatalities, all in the past decade and none before that (that I know of). One was a freak accident on a British racing circuit when an operator was thrown from his craft and landed in between his craft and another that had broken down, a third craft hit the first craft and squished the poor guy.

In another instance a gentleman built a craft of his own design and neglected any common sense safety features such as a fan guard, it didn't end well.

One on incident on an LCAC whose large thrust propellers were also unguarded, a sailor that ventured to close to the inlet was sucked into the propeller. They now have a net draping over the ducted propeller openings.

Overall, as far as vehicles go hovercraft have the best safety record over anything else land, sea or air. And their fuel consumption is low when factoring in speed. Obviously physics rules, it's more fuel efficient to go slow typically - aerodynamics you know.

https://www.explainthatstuff.com/hovercraft.html

Where boats are slowed by hulls that drag deep in the water, hovercraft ride fully clear, which means they use less fuel and can reach blistering speeds of up to 145kph (90mph).

http://www.hovercruiser.org.uk/about...ft-v-boats.pdf

Hovercraft are much more fuel efficient than a boat - they typically consume 1/2 to 1/4 of the fuel of a comparable sized boat at the same speed.

There are exceptions to every rule, two strokes and gas turbines are rather thirsty compared to 4-strokes and Diesels.

Weight is the enemy of the hovercraft, that's why less fuel efficient power plants have been used before. How much does the extra fuel weigh, what's the price of gas today?

And then stuff happens that no one seems to be able to explain.

Giant Military Hovercraft Lands on Crowded Beach Because Russia

[kach22i]

Random TRANSPORTATION pictures - Page 2195 - Pelican Parts Forums


Aerospaceweb.org | Ask Us - Wing Vortex Devices


May 15th 2019
BAE Systems 3D printing enables first ever flight using supersonically blown air
https://3dprintingindustry.com/news/...wn-air-155452/

Drone development undertaken by BAE Systems in collaboration with The University of Manchester has reached a new landmark.

Earlier in May this year, the MAGMA unmanned aerial vehicle (UAV) successfully completed trials proving the capabilities of its “flap-free” flight technologies.

As a result, MAGMA is believed to have become the first vehicle in aviation history to be manoeuvred in flight using supersonically blown air, a technology which improves the complicated task of controlling an aircraft at low-speeds.......................

“Today BAE Systems is 3D printing our components out of titanium and we are flight testing them on the back of a jet engine in an aircraft designed and built by the project team. It doesn’t get much better than that.”...............

“We made our first fluidic thrust vectoring nozzle from glued together bits of plastic and tested it on a hair drier fan nearly 20 years ago.”

[kach22i]

I am taken back by the oddity of few if any aerodynamic improvements to achieve this record. Lots of mechanical things done, just little in the way of aerodynamics, and I find that "odd".

May 2019
1,200-HP Porsche 911 Turbo Sets Record For Fastest Car On Sand
https://www.motor1.com/news/351411/f...n-sand-record/

The car has a bespoke 4.1-liter race engine with new internals, gearbox, clutch and drive shafts, along with an upgraded E85 fuel system and sophisticated charge cooling set-up to stop engine detonation. It also has an upgraded PDK transmission and altered suspension to allow it to clear the sand...................

"The Porsche behaves very differently on sand than tarmac," he said. "The sand creates a lot of resistance and tire slip. In the end, we could only use 850 hp (1,000 hp at the engine) to avoid too much wheel spin, compared to just 533 hp (engine) from a factory car."

Eisenberg's records not only follow on from his hero Campbell, who first set the record at Pendine Sands in 1927 (174.8 mph/ 281.3 kph) in the iconic Blue Bird – a record that stood for nearly 90 years, but it also beat the one previous record set by actor Idris Elba in 2015.

NOTE: back in Feb 2019 this is the image they used (see below).

Zef Eisenberg and MADMAX Race Team to attempt 'fastest car on sand' record
https://www.racecar.com/News/90237/M...on-sand-record

It got Zef thinking, so after meticulous research into what would be needed to top 200mph on sand, Eisenberg settled on a 2015 550hp Porsche 911 Turbo S. There was just one problem: Although this car is very fast, it will ‘only do 175mph’ in a mile on Tarmac. To achieve 200mph on tarmac within a mile would, the car would need 750bhp at the wheels. Sand creates a lot of resistance and tyre slip, so we worked out that we’d need at least 1000 bhp at the rear wheels, which equates to a crazy 1200hp at the engine, compared to just 550hp from the factory.

The MADMAX race team therefore hand built a bespoke 4.2-litre Porsche motorsport engine with new stronger internals, upgraded Turbo’s, plus a new E85 fuel system, advanced cooling set-up and far more. A lot of work was done to ensure that the monstrous power would come in as smoothly as possible in order to limit wheelspin on the loose surface. To cope with such an extreme output, the gearbox and clutch needed to be extensively upgraded, and the brakes and suspension modified to allow for different wheels and tyres.

They threw a lot of money into mechanicals, and nothing into aerodynamics.

There must be a reason, some sort of rules?

EDIT:

In the other forum we decided there was a typo as far as speed in the article posted far above (motor1).

http://forums.pelicanparts.com/off-t...-mph-sand.html

All that said, I think they wanted it to look like a stock Porsche and promote the shops HP increasing talents over that of a hired aerodynamicist.

[kach22i]

How fast can one go in the draft pocket?

Sep 21st 2018
Watch a woman take bicycle speed world record behind 800 hp dragster
https://www.autoblog.com/2018/09/21/...ragster-video/

Now we have video of that incredible run, which saw Denise Mueller-Korenek pedal to an astonishing 183.93 miles per hour across the Bonneville Salt Flats in Utah during the Project Speed event on Sept. 16 behind an 800-horsepower Top Alcohol drag racer. She broke the previous world record, set in 1995 by Dutch cyclist Fred Rompelberg, of 166.944 mph, which itself is already faster than the takeoff speed of a Boeing 757.

Mueller-Korenek rode a 35-pound, 7-foot-long, carbon fiber-frame bike fitted with 17-inch motorcycle tires and a steering stabilizer and a custom-tuned suspension fork to handle the unpaved salt flat, according to Wired. The dual drivetrain is five times larger than a conventional racing bike's top gear. According to Wired, the dragster's draft pocket was only 46 inches wide, leaving little room for error. In the video, you can see her release the tether by squeezing the left hand brake at around 1:29. At this point she was reportedly already traveling at 100 mph.

"It was a crazy wild ride to 183.9 mph, but so worth the sacrifice and years of focus on becoming the fastest human on a bicycle in the world," she told BBC. "We weren't supposed to go more than 175."

https://www.youtube.com/watch?time_c...&v=wPEgG-T3zMI

[Bicycle Bob]

In motor-paced bike racing, the pedals are mainly for control. Most of the power comes from the pressure differential created by the powered device, so there is a long way to improve if people want to spend the money.

[Vman455]

Came across these images on Slowtwitch the other day. These two objects are acted on by the same drag force at a given velocity:

[freebeard]

It's a wonder those wire-rigged biplanes flew at all. Eventually they got to the Dornier Do X, a gigantic plane (with a vestigal upper wing) that could barely get out of ground effect.