The Ever Again? 3

[dik]

Just in... sounds familiar... A question for Alistair, "I learned from the Ever Given" event there are Bernoulli forces on ships cruising near shorelines... I didn't know that earler. Are there similar effects when aircraft fly past an object? Is this a concern?

https://www.dailymail.co.uk/news/ar...locking-traffic-18-months-Green-stranded.html

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[TugboatEng]

Ships run of ground all the time. Where Ever Given was unique in how fast it was going when it did it. The solution to this problem is more tugboats and lower speeds. However, this would likely reduce the capacity of the canal more than a few ships grounding here and there.

 

[Alistair_Heaton]

Yes there is with aircraft but it usually comes into play in two situations.

1 flying high over mountains and safe terrain separation needs in creased when the wind is high. In fact we can't fly over the Alps when the wind is more than 40 knots I think it is. Our single engine drift down alt is lower than the adjusted minimum safe altitude.

2. Last 30ft off the ground landing we get into what's called ground effect. Angle of attack of the wings changes and drag is reduced. Its a lengthy subject which you are best to google. But if you get a firm landing in still air conditions its more than likely because the runway is hot creating lift and ground effect is in full force and the pilot is struggling to get her down as everything is in balance and they have levelled off. Those conditions you need to drive her down and not stop descending.

So yes it is a concern and its factored into aircraft performance when required.

 

[TugboatEng]

The thing about ships is that there is minimal redundancy. Once the engine is lost the rudder has minimal command initially and has even less command as the ship slows.

 

[1503-44]

I think ground cushion force is a bit different. Its in the opposite direction to that of ships against a canal wall, or two ships on a parallel course, where the water moving relatively faster between those two surfaces create a pressure lower than that of the opposite side of the hull, moving the two surfaces together. Ground cushion occurs on the bottom of the wing where pressure is higher than that on the wing's upper surface, the normal lift condition. Descending lower towards the runway, that higher pressure region's boundary contacts the runway surface and interrupts the usual dissipation flow of that high pressure air below the wing. The high pressure air volume trapped between wing and ground is reduced as the aircraft descends even further, compressing that air and increasing its pressure even more, causing the ground cushion float effect. As such that does not appear to have much to do with Bernoulli, but rather more the result of Boyle's law of pressure and volume.

We had a seaplane base at the field where I learned to fly at Hooks Field north of Houston. Lake Corp started their operations there.

http://www.lakeamphib.com/lakeamphib.htm

Anyway I took a couple of lessons, which were my intro into a true Bernouli effect on aircraft. The seaplane base was a dredged canal "strip"; narrow and long, consequently very still with no waves. The lack of waves was a problem. The typical shape of seaplane floats are curved on the bottom, in the case of Lake seaplanes, that was the entire hull. As you takeoff, the relative velocity of Hull to water causes Bernoulli effects that tend to sink the aircraft, especially if not disrupted by waves creating air gaps. Bernoulli forces persist, become greater, act in the downward direction and can Swamp the aircraft. Next time you see a seaplane take off on still water, watch the pilot trying to break the suction by dolphining the aircraft with rapid up/down action of the elevators, making his own waves while attempting to get some airflow under the floats or hull to break that suction.

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

Is this the same incident? Very interesting animation in this article of the oil tanker going aground.

https://www.theguardian.com/world/2022/sep/01/suez-canal-ship-stuck-blocked-again-affinity-v

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Alistair_Heaton]

It is different I agree.

They still haven't decided why planes fly.

Personally I think it's Newton gives the majority of the lift and Bernoulli the control.

The main pain in the arse is the reduction in drag with ground effect.

 

[1503-44]

I could give equal credit to both, depending on airspeed and attitude at the moment.

Wing tip vortices might contribute ground cushion too?

Is the drag reduction caused by the slower airspeed, or is it a force multiplier from the higher pressure escaping from the trailing edge? Perhaps both.

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Alistair_Heaton]

From memory it reduces wing tip vortex's by blocking the rotation. Which leads to reduced drag. As well as the reduced Cl and Cd due to the effective reduction in angle of attack.

Vortex's are the lateral movement of flow due to the swept wing.

 

[1503-44]

Flying the smaller types i was taught to assume a high nose attitude on landing to near stall (keeping in mind wind speed). Slow speed reduced the drag, but hi angle increased it. I think the hi angle drag increase was more than the drag loss from reduced velocity, at least in the lower speed range. Drag being a function of v^2, was more significant in the higher speed range, cruise to approach, rather than between approach and touchdown, where v^2 was proportionally much smaller.

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Alistair_Heaton]

Your right.

But things change a bit when you get within half the wingspan of the ground which for a Cessna high wing makes it pretty hard to feel.

Swept low wing with 32 meters of wing flaring at 30 ft and the wings 12 ft off the ground it's much more of an effect. Especially when there is a microclimate circulation going between a hot black runway and cooler grass next to it. Or even better a big lump of cold water just in front of the threshold which is pretty much every Mediterranean island airport.

 

[1503-44]

OK. Yes, I didn't start really feeling it until I got to the Cherokees. In Houston summertime there was pretty strong balloning over the road landing south, plus with GC, I think I actually had to get below stall speed to finally mush down. I recall lowering the nose landing on hot black tarmac and feeling like it was going to glide forever unless I just pushed it down onto the ground...then bouncing back up into it again.
I used one field in the rice paddies where you'd get a strong sink rate over the rice growing between a hot road and the landing strip. That was often kind of exciting. I thought I'd hit the numbers, but then wound up floating to mid field.

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Alistair_Heaton]

Yep that's the effect.

You can cheat and drop a wing and put some rudder in and use it as an airbrake but you won't find that in any manual and there will be a fair few who won't understand what your doing and get upset if they see it. Those that are fellow bodger's will just laugh and call you cheating illegitimate person.

They don't like it if you go much lower that Vref -5 below that if you have an engine failure off rejected landing you can be below Vmca and with the machine giving you full power it can get outside the average pilots ability to recover.

 

[1503-44]

By that time the chief was getting too hot and opening the door. That worked. :-(

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Alistair_Heaton]

Old instructors trick I used to do the same on the pa38. Just push the door a bit with your knee.

 

[dik]

Thanks, Alistair, and 1503...

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik