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BoeingOnFinal
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The Aviation Physics Thread

Thu Aug 17, 2006 5:38 am

Aviation Physics



  • I've noticed that there is no threads here about the physics of aviation, and I though of this thread when I was studying some basic aerodynamics.

  • I hope some of you share interest in the physics of aviation, and I certainly hope some of you can contribute to the subject with more knowledge than me, cause I am very new within the field.
    Now I know there are places to read about aviation, and search for questions, but I feel that some things can be better explained if you can ask exactly what you are having trouble with understanding.

  • The way I was thinking, is that we can basically post questions and share interesting thoughts about mentioned topic.


Interesting link:

Pilot Friend Basicly everything you want to know about aviation

Now what I've been wondering about when reading the basics of how airplanes fly, is that when in level flight, with constant speed, thrust equalsrndrag, and lift equals weight.
Now I can see why lift has to equal weight, but I though thrust be greater than drag to move forward.
Now is is so that thrust equals drag because of Newtons 3rd law, that any object moving will do so until other forces play a role (I may be totally out of line here).
Anyone who can explain this for me (be as advanced as you can, the more input the better).


And please, if you have other questions and/or inputs, go ahead  Smile
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David L
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RE: The Aviation Physics Thread

Thu Aug 17, 2006 5:43 am

Quoting BoeingOnFinal (Thread starter):
I've noticed that there is no threads here about the physics of aviation

... unless you include the physics involved in bird-powered, chicken wire aircraft or impossible, magic conveyor-belt runways.  Smile
 
SlamClick
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RE: The Aviation Physics Thread

Thu Aug 17, 2006 5:45 am

Quoting BoeingOnFinal (Thread starter):
Now I can see why lift has to equal weight, but I though thrust be greater than drag to move forward.

When thrust exceeds drag the airplane will accelerate and when drag exceeds thrust it will decelarate - assuming level flight in both cases. When they are matched, no delta, no change.
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jfkaua
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RE: The Aviation Physics Thread

Thu Aug 17, 2006 5:53 am

|||| net force= mass x acceleration |||||.

If the net force is zero (drag =thrust) acceleration is therefore zero. I'm majoring in aerospace engineering so I'm kinda into this stuff...

[Edited 2006-08-16 22:54:56]
 
2H4
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RE: The Aviation Physics Thread

Thu Aug 17, 2006 7:25 am




Quoting David L (Reply 1):
... unless you include the physics involved in bird-powered, chicken wire aircraft or impossible, magic conveyor-belt runways

Ah, good. I opened the thread specifically to confirm the presence of bird-powered treadmill physics, and David L didn't let me down. Reply 1, even. He's on the ball.




2H4


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vikkyvik
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RE: The Aviation Physics Thread

Thu Aug 17, 2006 9:27 am

Quoting BoeingOnFinal (Thread starter):
that any object moving will do so until other forces play a role (I may be totally out of line here).

Yup, that basically sums it up (to be perfectly accurate, it's a combination of all of Newton's 3 Laws: F=ma; action=reaction; inertia). Sometimes I've found that people new to this sort of stuff think that to have a constant forward velocity, you must have a net forward force (aka thrust > drag). It's a very key concept that a net force in a direction will give you an acceleration in that direction, which is a change in velocity.

It's the exact same concept as lift vs. weight.....A force is a force is a force. Even if it's generated by birds inside a chickenwire fuselage. Or by engines operating in opposition to a perfectly opposing treadmill. Um, yeah.....

If you really want to provoke some interesting and possibly disputative discussion, ask questions about how lift is generated.  Smile

~Vik
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".
 
SlamClick
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RE: The Aviation Physics Thread

Thu Aug 17, 2006 9:35 am

Quoting Vikkyvik (Reply 5):
questions about how lift is generated.

Easy! The upper wing surface is so slippery that air keeps sliding off it. Because the slope is longer toward the rear the plane is propelled forward as well as being lifted.

Kind of like using the battery to run a hydraulic pump to spin the RAT propeller and fly the plane to Lajes.
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David L
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RE: The Aviation Physics Thread

Thu Aug 17, 2006 9:56 am

Quoting 2H4 (Reply 4):
Reply 1, even. He's on the ball.

It all started to go downhill from there. I had an answer all ready to go but Jfkaua beat me to it and in one sentence and I probably don't have anything left.

Quoting SlamClick (Reply 6):
Easy! The upper wing surface is so slippery that air keeps sliding off it. Because the slope is longer toward the rear the plane is propelled forward as well as being lifted.

And when it's raining, the raindrops are deflected rearwards by the slope causing the aircraft to accelerate forward even more. I have seen the light - what a forum!  biggrin 
 
Bobster2
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RE: The Aviation Physics Thread

Thu Aug 17, 2006 10:21 am

I remember reading that the energy required to pedal a bicycle is approximately proportional to the square of speed, because the wind resistance varies with the square of speed.

But airplanes are apparently a lot more complicated than bicycles, the relationship between fuel burned and speed is not simple:

"I tell you this, no eternal reward will forgive us now for wasting the dawn." Jim Morrison
 
Bobster2
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RE: The Aviation Physics Thread

Thu Aug 17, 2006 10:33 am

Another aspect of airplane physics is stopping.

It's hard for non-pilots to understand why airplanes sometimes fail to get stopped on a runway that's two miles long. We understand that airliners are heavy and it's hard to stop heavy objects that are moving fast, but we also know that two miles is a very long distance, and they should be able to stop every time.
"I tell you this, no eternal reward will forgive us now for wasting the dawn." Jim Morrison
 
kaddyuk
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RE: The Aviation Physics Thread

Thu Aug 17, 2006 11:29 am

Quoting Bobster2 (Reply 9):
and they should be able to stop every time.

But your grip on the runway is only as good as the contact between tyre and ground... If you find yourself sliding on snow, there is little you can do untill you find the grip again...
Whoever said "laughter is the best medicine" never had Gonorrhea
 
SlamClick
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RE: The Aviation Physics Thread

Thu Aug 17, 2006 11:33 am

Quoting Bobster2 (Reply 9):
fail to get stopped on a runway that's two miles long.

You would think that any pilot who had ever played air hockey would instinctively get hydroplaning.

Nil friction. No particular reason to stop, until you hit the trees.
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
Bobster2
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RE: The Aviation Physics Thread

Thu Aug 17, 2006 11:44 am

Quoting Kaddyuk (Reply 10):
contact between tyre and ground

I think the problem is that airplane wheels are so small compared to the size and weight of the plane. Consider a car, where the wheels are much larger relative to size of the car. In order to understand airplane physics you have to imagine a car with roller skates for wheels, then you can see why it's hard to stop.

They could make planes stop much faster if the wheels were proportionate to car wheels, but then they'd be too heavy to fly.  Smile
"I tell you this, no eternal reward will forgive us now for wasting the dawn." Jim Morrison
 
Bobster2
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RE: The Aviation Physics Thread

Thu Aug 17, 2006 11:50 am

Quoting SlamClick (Reply 11):
Nil friction

The braking forces may be low, but they are applied over a long distance. When you see how small the plane looks on a long runway, intuition may suggest that distance will get it stopped. Obviously, intuition is wrong, that's where physics is needed to explain it.
"I tell you this, no eternal reward will forgive us now for wasting the dawn." Jim Morrison
 
Stealthz
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RE: The Aviation Physics Thread

Thu Aug 17, 2006 2:47 pm

Quoting Bobster2 (Reply 8):
But airplanes are apparently a lot more complicated than bicycles, the relationship between fuel burned and speed is not simple:

Not really, in some ways the aeroplane is simpler than the bicycle. All the same factors come into play. The various kinds of drag, fuel flows, all have an affect.

Now designing a bike for Johnny or Johanna to ride to school may not require as much analysis of those factors as building a 787 or A380 but they are there. You can rest assured that designers of top level track racing bikes pay much attention to these things.
If your camera sends text messages, that could explain why your photos are rubbish!....well that might have changed!!!
 
BoeingOnFinal
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RE: The Aviation Physics Thread

Thu Aug 17, 2006 4:24 pm

Awsome inputs, I know see why thrust = drag in constant speed.

Quoting Vikkyvik (Reply 5):
If you really want to provoke some interesting and possibly disputative discussion, ask questions about how lift is generated.

That is interesting actually. Cause many people don't think that it is pressure difference on upper and lower surace of the wing that lifts the plane. Even I can see the logic in that. When the air travels faster over the wing because the surface is longer, the pressure in the air drops compared to the pressure uderneath, and the wings are pulled upwards.

The argument here is that military fighters uses angle of attack to get lift, even though the wings are not formed as the usual tear drop in half.

Another thing I read was that when the trailing edge of the wings point downwards and is at a lower point than the leading edge, the air that flows underneath the wings will be pushed downwards at the trailing edge, and create lift.
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kaddyuk
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RE: The Aviation Physics Thread

Thu Aug 17, 2006 4:29 pm

Quoting BoeingOnFinal (Reply 15):
Cause many people don't think that it is pressure difference on upper and lower surace of the wing that lifts the plane. Even I can see the logic in that. When the air travels faster over the wing because the surface is longer, the pressure in the air drops compared to the pressure uderneath, and the wings are pulled upwards

Another way to look at it is the wing places a force onto the air to make it bend and follow the contour of the upper surface... Because the wing applies a force to the airflow, the airflow places a force on the wing upwards, creating lift...
Whoever said "laughter is the best medicine" never had Gonorrhea
 
oly720man
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RE: The Aviation Physics Thread

Thu Aug 17, 2006 6:49 pm

Quoting BoeingOnFinal (Reply 15):
When the air travels faster over the wing because the surface is longer

The difference in length between the upper and lower surfaces is very small. It's the thickness and the camber of the wing that are the main generators of lift. A cambered wing alone will produce lift (like early biplanes/monoplanes), but stalls at a low incidence. The thickness of the wing and the leading edge radius, amongst other things, have a significant effect on the pressure variation around the leading edge of the wing and the stall behaviour of the wing.

Lift is a chicken and egg situation. It's the pressure difference around the wing that causes the flow to accelerate/decelerate that causes the pressure difference. Because of the thickness, and/or camber (curvature) of the wing the flow accelerates, reducing the local pressure. With a symmetric wing, the acceleration is the same top and bottom, so there is no net lift. If the wing is at incidence (positive), or the wing is cambered, then the airflow sees an asymmetric situation. It has to accelerate more around the upper surface, so a lower pressure is generated than on the lower surface, hence the lift.

Leading edge and trailing edge devices increase the camber of the wing and increase the lift.
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David L
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RE: The Aviation Physics Thread

Thu Aug 17, 2006 7:26 pm

Quoting Oly720man (Reply 17):
It's the pressure difference around the wing that causes the flow to accelerate/decelerate that causes the pressure difference.

It wasn't until the last time this was discussed that I was finally able to see it as the "chicken and egg" situation you decribe.

Quoting Bobster2 (Reply 9):
We understand that airliners are heavy and it's hard to stop heavy objects that are moving fast, but we also know that two miles is a very long distance, and they should be able to stop every time.

There seem to be a lot of people who don't understand how difficult it is to stop a 3/4 ton car travelling at 70 mph, hence the reason they drive 10 metres behind the car in front at those speeds.  Smile
 
2H4
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RE: The Aviation Physics Thread

Thu Aug 17, 2006 8:52 pm




Quoting Bobster2 (Reply 8):
But airplanes are apparently a lot more complicated than bicycles, the relationship between fuel burned and speed is not simple:

Ah, but the relationship between wattage produced, calories burned, and speed is a rather complex one...  Smile

Quoting StealthZ (Reply 14):
You can rest assured that designers of top level track racing bikes pay much attention to these things.

 checkmark  ....Using CFD, in fact.




2H4


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vikkyvik
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RE: The Aviation Physics Thread

Thu Aug 17, 2006 9:26 pm

Quoting David L (Reply 18):
There seem to be a lot of people who don't understand how difficult it is to stop a 3/4 ton car travelling at 70 mph, hence the reason they drive 10 metres behind the car in front at those speeds.

EXACTY! At 70 mph or so, it probably takes something like 500-600 feet at least to come to a complete stop, with medium to heavy braking. (I was actually paying attention to the 1/10 mile markers on the way to work this morning, but alas, on the 20 mile freeway drive, I didn't have to use the brakes a single time....)

As far as lift goes, there are different ways of modelling it, but ultimately, the flow wants to stick to the wing's surface. This causes it to be accelerated downwards. For this to occur, the pressure on top of the wing is necessarily lower, and the velocity higher. Call it a reaction force, call it a pressure difference; it doesn't really matter. They're all describing the same phenomenon.

~Vik
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speedracer1407
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RE: The Aviation Physics Thread

Fri Aug 18, 2006 4:06 pm

Quoting Vikkyvik (Reply 20):
Quoting David L (Reply 18):
There seem to be a lot of people who don't understand how difficult it is to stop a 3/4 ton car travelling at 70 mph, hence the reason they drive 10 metres behind the car in front at those speeds.

EXACTY! At 70 mph or so, it probably takes something like 500-600 feet at least to come to a complete stop, with medium to heavy braking. (I was actually paying attention to the 1/10 mile markers on the way to work this morning, but alas, on the 20 mile freeway drive, I didn't have to use the brakes a single time....)

Not sure which sort of "ton" you're using, but if it's the kind the spelling suggests, that should read 1.5-2 tons. Most small cars are approaching 3000lb nowadays, and big sedans frequently surpass 2 tons.

As for stopping distances, most modern cars, even SUVs, can stop from 70 in less than 200 ft. Many, with reasonably good brakes and tires, will do the job in 170 or so, and sports cars often dip into the 150s.

Contrary to the opinions quoted above, I'm always rather surprised at just how fast cars can stop if you mash the brake and let the ABS sort things out, especially when you consider that some some of the shortest-stopping "sports"cars aren't much lighter than 2 tons.
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BoeingOnFinal
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RE: The Aviation Physics Thread

Fri Aug 18, 2006 4:29 pm

Yeah, they can stop really fast, but so can the car in front of you. Sornstill, I hate people who drive right behind my back, cause even if he has a sports car and better brakes than me, it only takes about half a second if I brake HARD for him to hit me in the rear.

Probably all pilots who drive like that anyways, in their fancy sports car. Although they are insanely good at flying, they can't steer anything that is on the ground  Smile

All joking aside, what I find fascinating about lift, is how long the air is affected by it afterwards.



Wake turbulence. My instructor told me when I did a 20 or 30 degree bank, in a 360 degree turn, and hit my own wake turbulence, I was doing a good job. And I did hit my own turbulence.



But as you can se there, the "wake" falls graduately downwards, as much as 900 feet in 5 miles. So basically, for me to hit my own turbulence, I have to loose some altitude?
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oly720man
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RE: The Aviation Physics Thread

Fri Aug 18, 2006 8:47 pm

Quoting BoeingOnFinal (Reply 22):
But as you can se there, the "wake" falls graduately downwards, as much as 900 feet in 5 miles. So basically, for me to hit my own turbulence, I have to loose some altitude?

You hit the vortex wake rather than the turbulence. The vortices do sink, but how quickly depends on the speed of the aircraft generating them, i.e. the strength of the vortex. Because the vortex is rotating it generates its own flowfield which makes it move.

When I was having microlight lessons a few years ago I was told that you never do 360 deg level turns in flex wings because if you fly into your own vortex you can flip the aircraft.
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David L
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RE: The Aviation Physics Thread

Fri Aug 18, 2006 9:59 pm

Quoting Speedracer1407 (Reply 21):
Not sure which sort of "ton" you're using

It was just a number. It doesn't really matter.

Quoting Speedracer1407 (Reply 21):
As for stopping distances, most modern cars, even SUVs, can stop from 70 in less than 200 ft.

With immediate reaction, good brakes, good tyres, a good road surface and good braking technique but that's not what worries me...

Quoting BoeingOnFinal (Reply 22):
Yeah, they can stop really fast, but so can the car in front of you. Sornstill, I hate people who drive right behind my back, cause even if he has a sports car and better brakes than me, it only takes about half a second if I brake HARD for him to hit me in the rear.

Exactly. Even when you lift your foot off the accelerator the 10 metre gap starts to disappear pretty quickly.

Quoting Speedracer1407 (Reply 21):
Many, with reasonably good brakes and tires, will do the job in 170 or so, and sports cars often dip into the 150s.

A lot of things can delay the processes of realising you need to brake, moving your foot to the brake pedal and then actually starting to brake. A lot of people feel insulated in their car and aren't poised to react instantly.

Quoting Speedracer1407 (Reply 21):
if you mash the brake and let the ABS sort things out

Most cars here don't have ABS. Sorry but a 10 metre gap at 70 mph on public roads is sheer lunacy, in my opinion.
 
vikkyvik
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RE: The Aviation Physics Thread

Fri Aug 18, 2006 10:54 pm

Quoting Speedracer1407 (Reply 21):
As for stopping distances, most modern cars, even SUVs, can stop from 70 in less than 200 ft. Many, with reasonably good brakes and tires, will do the job in 170 or so, and sports cars often dip into the 150s.

Wow. That's actually is pretty damn impressive.

I did have to brake pretty hard the other day down on Cape Cod. Was going around 65 mph in the left lane, and the guy ahead of me started slowing down a tiny bit, then somewhat rapidly changed lanes into the right lane. Suddenly realize that the reason he did that is that there's a car dead stopped in the left lane, probably 300 feet from me (he was trying to make a left turn, but hadn't gotten into the left-turn lane). I Went from 65 down to around 20 with probably 60 feet to spare. I'd guess all the kinetic energy of the car converted directly into my anger  Smile

What really scares me is the winter driving. Last winter, I rear-ended someone at about 15 mph. It was snowing, and she stopped on the entrance ramp to a freeway (instead of merging). I was about 50 feet behind her, going around 20 mph. Only managed to lose about 5 mph, even with ABS. Oh well, it was my fault, you live, you learn.

Alright, enough of that crap. What really staggers me, as posted above, is how long the effects of the wings last. Anyone know how exactly the wingtip vortices keep spinning for so long? I would think they would lose their energy fairly quickly.

~Vik
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".
 
oly720man
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RE: The Aviation Physics Thread

Sat Aug 19, 2006 1:27 am

Quoting Vikkyvik (Reply 25):
Anyone know how exactly the wingtip vortices keep spinning for so long?

A vortex can only dissipate due to viscosity (or friction with the surrounding air) and for a concentrated vortex that will take a fair while.

If you watch contrails after a plane's gone by, you'll see that the twin trails start to become curved/unstable and start to break up into figure of 8 shapes as the two trailing vortices interact with each other.

If you want some light reading....

http://www.airpower.maxwell.af.mil/a.../aureview/1971/jul-aug/carten.html

Suggests that a trailing vortex can last upto 40miles behind the aircraft creating it.
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vikkyvik
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RE: The Aviation Physics Thread

Sat Aug 19, 2006 1:58 am

Quoting Oly720man (Reply 26):
A vortex can only dissipate due to viscosity (or friction with the surrounding air) and for a concentrated vortex that will take a fair while.

If you watch contrails after a plane's gone by, you'll see that the twin trails start to become curved/unstable and start to break up into figure of 8 shapes as the two trailing vortices interact with each other.

If you want some light reading....

http://www.airpower.maxwell.af.mil/a.../aureview/1971/jul-aug/carten.html

Suggests that a trailing vortex can last upto 40miles behind the aircraft creating it.

Thanks for the link - that was interesting reading.

I guess what I don't quite understand is where the centripetal force comes from that keeps the air in the vortex spinning. I understand that originally, the pressure difference caused by the wing creates these vortices. I would think the pressures would equalize fairly quickly.

Thanks.

~Vik
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".
 
SlamClick
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RE: The Aviation Physics Thread

Sat Aug 19, 2006 2:01 am

Quoting Oly720man (Reply 17):
Lift is a chicken and egg situation.

Exactly! Or perhaps eggzactly!

If a chicken lays an egg on top a wing the chances are that it will roll off the trailing edge because that slope is longer. Just like air will.
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
SlamClick
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RE: The Aviation Physics Thread

Sat Aug 19, 2006 2:06 am

What I want to know about the physics of flight is this:

Why is it that

1. The delivery of a fresh cup of hot-black-and-bitter to the pilot will cause turbulence.

2. That turbulence will cause the coffee to spill.

3. The coffee spill will find those pages in your Jepp manual which:

A. Were recently revised and therefore new.

B. You need for the next arrival.


When we get this one solved I'm going to ask why

1. Hotels put redeye crews in the noisy wing of the hotel

2. Why the hotel van driver tries to take all 14 pieces of luggage out of the hands of the arriving flight crew instead of just opening the bleedin' van door for us.
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
vikkyvik
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RE: The Aviation Physics Thread

Sat Aug 19, 2006 2:19 am

Quoting SlamClick (Reply 28):
If a chicken lays an egg on top a wing the chances are that it will roll off the trailing edge because that slope is longer. Just like air will.

So...are you saying that a wing will generate lift if it's moving through a "fluid" of eggs?

I think we need to test this in a wind tunnel. Maybe if we turn on the wing anti-ice, we'll even end up with breakfast.....
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".
 
SlamClick
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RE: The Aviation Physics Thread

Sat Aug 19, 2006 2:27 am

More of an 'egg tunnel' I'd say.

Quoting Vikkyvik (Reply 30):
Maybe if we turn on the wing anti-ice, we'll even end up with breakfast.....

Yes, and if in turbulence - scrambled.
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
David L
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RE: The Aviation Physics Thread

Sat Aug 19, 2006 2:53 am

Quoting SlamClick (Reply 28):
If a chicken lays an egg on top a wing the chances are that it will roll off the trailing edge because that slope is longer. Just like air will.



Quoting Vikkyvik (Reply 30):
if we turn on the wing anti-ice, we'll even end up with breakfast....



Quoting SlamClick (Reply 31):
Yes, and if in turbulence - scrambled.

I'm writing all this down so I don't forget this time. Any links to the maths involved?
 
SlamClick
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RE: The Aviation Physics Thread

Sat Aug 19, 2006 3:20 am

Quoting David L (Reply 32):
Any links to the maths involved?

Just remember to divide by a dozen. The rest is advanced cackle-us.
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
vikkyvik
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RE: The Aviation Physics Thread

Sat Aug 19, 2006 3:39 am

Quoting David L (Reply 32):
I'm writing all this down so I don't forget this time.

I'd be very interested to know what you managed to forget last time an egg-lift discussion came up.  Wink

Quoting David L (Reply 32):
Just remember to divide by a dozen. The rest is advanced cackle-us.

I was on the phone when I read this, and almost cracked up while talking.

~Vik
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".
 
David L
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RE: The Aviation Physics Thread

Sat Aug 19, 2006 4:20 am

Quoting Vikkyvik (Reply 34):
I'd be very interested to know what you managed to forget last time an egg-lift discussion came up.

I forgot about the "lift" part.

Quoting Vikkyvik (Reply 34):
Quoting David L (Reply 32):
Just remember to divide by a dozen. The rest is advanced cackle-us.

Credit where credit's due. I don't believe that one was mine.  Smile
 
airfoilsguy
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RE: The Aviation Physics Thread

Sat Aug 19, 2006 4:20 am

How about how long would it take an airplane to take off if it was takeing off from a conveyor running the opposite way?  stirthepot 
It's not a near miss it's a near hit!!
 
David L
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RE: The Aviation Physics Thread

Sat Aug 19, 2006 4:35 am

Quoting Airfoilsguy (Reply 36):
How about how long would it take an airplane to take off if it was takeing off from a conveyor running the opposite way?

Ooo... you're on thin ice there.  biggrin 
 
vikkyvik
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RE: The Aviation Physics Thread

Sat Aug 19, 2006 4:43 am

Quoting David L (Reply 37):
Ooo... you're on thin ice there.

...and about 35 posts late  Wink

Just busting your balls, Airfoilsdude.

Quoting David L (Reply 35):
Credit where credit's due. I don't believe that one was mine.

Strange....I never know how that manages to happen. Should have been:

Quoting SlamClick (Reply 33):
Just remember to divide by a dozen. The rest is advanced cackle-us.

I was on the phone when I read this, and almost cracked up while talking.

~Vik
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".
 
BoeingOnFinal
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RE: The Aviation Physics Thread

Sat Aug 19, 2006 8:22 am

The physics are really getting advanced here guys  Smile

And funny, no doubt about that!

And I think we covered the lift part pretty well, until we spooled off topic onto scrambled eggs  Smile
New challenge. Explain to me how you calculate how much G force you pull in a turn while maintaining altitude. And how does the G force "equipment" measure it?
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vikkyvik
Posts: 12572
Joined: Thu Jul 31, 2003 1:58 pm

RE: The Aviation Physics Thread

Sat Aug 19, 2006 8:46 am

Quoting BoeingOnFinal (Reply 39):
Explain to me how you calculate how much G force you pull in a turn while maintaining altitude. And how does the G force "equipment" measure it?

Alrighty then (boring day at work and at home). Basically, the wings produce lift straight upwards when you're in level flight. So, your lift vector is originally pointing straight upwards (we'll call it 90 degrees from horizontal). We'll call this lift L, and it exactly equals the weight of the airplane (L stays constant through the whole thing, which is why I'm not assigning it a number).

Now, you bank the airplane to a 30 degree bank. Your lift vector is now pointing 60 degrees upward from horizontal. This means that although the wings are producing the same amount of total force (L), some of the force is now horizontal and some is vertical:

horizontal force: L*cosine(60 deg.) = 0.5L
vertical force: L*sine(60 deg.) = 0.866L

Everything now revolves around the horizontal and vertical components of the force the wings are generating. Since some of the force is being directed horizontally (to make the airplane bank), you have to increase the total force so that the vertical component equals the weight (L) again. So you pull back on the yoke/stick (or the FBW computer does it for you), and the wings now generate a new force value, F, where the vertical component of F equals the airplane's weight (L).

F*sine(60 deg.) = L

So:

F = L/cosine(30 deg.) = 1.155L

Therefore, you're now pulling 1.155 G's to remain level.

At 60 degrees bank:

F = L/cosine(60 deg.) = 2L

which means at 60 degrees bank, to remain level, you must pull 2 G's.

Eh, it's a bitch to try and explain this stuff without diagrams...but I'm sure you could find some if you search a bit.

~Vik
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".
 
N600RR
Posts: 171
Joined: Tue Feb 14, 2006 1:01 pm

RE: The Aviation Physics Thread

Sat Aug 19, 2006 9:31 am

Quoting Oly720man (Reply 26):
A vortex can only dissipate due to viscosity (or friction with the surrounding air) and for a concentrated vortex that will take a fair while.

I suppose differences in air density due to altitude, humidity or temperature have an effect on dissipation times?

Quoting David L (Reply 35):
Quoting Vikkyvik (Reply 34):
I'd be very interested to know what you managed to forget last time an egg-lift discussion came up.

I forgot about the "lift" part.

Don't worry about it...most people only remember the scene where the egg gets laid anyway. biggrin 
"And the fluffy white lines that the airplane leaves behind are drifting right in front of the waning of the moon" -Cake
 
Bobster2
Posts: 1523
Joined: Mon Jan 17, 2005 3:04 am

RE: The Aviation Physics Thread

Sat Aug 19, 2006 9:36 am

Quoting BoeingOnFinal (Reply 39):
how much G force you pull in a turn

On an airliner, the passengers are not supposed to feel the turn. There should always be 1 G, pointed toward the floor of the plane. You don't want people spilling their drinks.

See "The Turn" by William Langewiesche. He explains the physics of banking and turning airplanes, and it's a fascinating story.
"I tell you this, no eternal reward will forgive us now for wasting the dawn." Jim Morrison
 
Bobster2
Posts: 1523
Joined: Mon Jan 17, 2005 3:04 am

RE: The Aviation Physics Thread

Sat Aug 19, 2006 12:04 pm

Quoting Bobster2 (Reply 42):
There should always be 1 G,

Correction. That's totally wrong. The G force varies with bank angle. Sorry.

30 degrees = 1.15 G
45 degrees = 1.40 G
"I tell you this, no eternal reward will forgive us now for wasting the dawn." Jim Morrison
 
SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

RE: The Aviation Physics Thread

Sat Aug 19, 2006 12:22 pm

Quoting BoeingOnFinal (Reply 39):
And I think we covered the lift part pretty well, until we spooled off topic onto scrambled eggs

Do you suppose that is why there were scrambled eggs on my uniform cap? They represented lift?

Actually I was just happy to get away from the "egglaying" model before I had to talk about thrust. You don't want to know!
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
2H4
Posts: 7960
Joined: Tue Oct 19, 2004 11:11 pm

RE: The Aviation Physics Thread

Sat Aug 19, 2006 1:22 pm



So can a chicken lay an egg in level, unaccelerated flight, or must it pull back on the yolk and increase the load factor to force it out?




2H4


Intentionally Left Blank
 
David L
Posts: 8551
Joined: Tue May 18, 1999 2:26 am

RE: The Aviation Physics Thread

Sat Aug 19, 2006 6:11 pm

Quoting Bobster2 (Reply 42):
the passengers are not supposed to feel the turn.



Quoting Bobster2 (Reply 43):
The G force varies with bank angle

Yes, they just shouldn't feel any side forces, i.e. their drinks shouldn't spill.

Quoting N600RR (Reply 41):
most people only remember the scene where the egg gets laid anyway

Well, I will now. Thanks a lot!

Quoting 2H4 (Reply 45):
or must it pull back on the yolk

Ah, the old "these are the yolks, folks".  Smile

Quoting SlamClick (Reply 44):
Do you suppose that is why there were scrambled eggs on my uniform cap?

Funny how it all fits together eventually.  Smile
 
BoeingOnFinal
Topic Author
Posts: 440
Joined: Fri Apr 14, 2006 6:47 am

RE: The Aviation Physics Thread

Sat Aug 19, 2006 6:45 pm

Quoting Vikkyvik (Reply 40):
We'll call this lift L, and it exactly equals the weight of the airplane (L stays constant through the whole thing, which is why I'm not assigning it a number).

How come lift is always constant?

So, when you bank the aircraft, the vertical force is reduced, and some of the lift goes into horizontal force, and that's what turns the aircraft. And so, to prevent the aircraft som loosing altitude, you have to pull the elevator up.
Then, the aircrafts tail goes down, increasing angle of attack, isn't that right?
So, then both the vertical and horizontal force increases. And in a perfect turn, the vertical force = wheight, then the altitude remain constant.

So what force is working so that the airplane now holds altitude? Cause it is not more lift that is created when pulling the yoke back.

I'm gonna have to work out that math later, havn't had math in 4 years, I'm totally blue within the field now. Off to study the multiplication table  Smile
norwegianpilot.blogspot.com
 
David L
Posts: 8551
Joined: Tue May 18, 1999 2:26 am

RE: The Aviation Physics Thread

Sat Aug 19, 2006 9:33 pm

Quoting BoeingOnFinal (Reply 47):
How come lift is always constant?

The vertical component is constant, since the altitude is constant and the weight isn't changing (even if the aircraft is full of pigeons in flight).
 
oly720man
Posts: 5813
Joined: Fri May 21, 2004 7:13 am

RE: The Aviation Physics Thread

Sat Aug 19, 2006 11:25 pm

Quoting BoeingOnFinal (Reply 47):
How come lift is always constant?

In order to maintain a horizontal turn you need to have a vertical component of lift that is constant

In reality, as you bank and turn you need to pull back on the yoke which increases the lift and also increases the drag so you need more power to maintain constant speed.

Note that in a turn you are at a higher incidence and higher power setting, but the same speed as straight and level, and closer to the stall incidence.... the cause of some accidents as you run out of either incidence or power.
wheat and dairy can screw up your brain

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