Will it take off?

[Ray Garrison]

couldn't resist one more attempt to make this simple.

You have a roller coaster. You place it on the downhill side of a steep slope. You've set up the rails so they can move under the car. You let go of the car. It begins to descend the slope. You start up the rails, and they start moving really damn fast in the uphill direction, opposite to the direction the coaster is traveling. I think everyone agrees that, unless the wheels get stuck, the coaster is going to continue traveling down the hill regardless of how fast the rails are moving in the other direction. Replace the coaster with a plane, and the force of gravity with the force of the engines, and you have exactly the same scenario.

[Champagne taste beer budget]

Ray, I was happy in the cave till you came along. [H]

[JetJockey]

A wing flys or supports weight because of Bernoulli's Theorem period. An aerofoil produces lift because the airflow over the top and bottom of the wing must and want to reach the trailing edge of the wing at the same time (camber). This creates a low pressure on top of the wing and high pressure on the bottom of the wing which creates an upward lift(asuming we are not inverted). Thats it plain and simple. Stick and rudder is about 50 years old and I read it when i was about 8! Great info but not legitimate theory of flight. For your Info, I am a certified Flying Instructor with 110000 hours of flying time including 5000 hours of multi engine jet time. Hope this helps.

Kind Regards, Jet Jockey.

[Ray Garrison]

I'm sorry, but that explination is incomplete. The belief that wings generate lift solely as a result of Bernoulli's Principle is a misconception that's been taught as gospel for too many years. Here's a link to a NASA educational website that does about the best job of explaining the relationship between Bernoulli's principle, angle of attack, lift, drag and stall that I've seen.

Yes, the Bernoulli Principle does contribute some lift as a result of the decreased pressure over the top of the airfoil. But the primary contribution to lift comes from the deflection of the air downward by the wing due to its angle of attack and forward motion. The primary contribution of the shape of the airfoil is to prevent the airflow from separating from the surface of the wing as the angle of attack is increased. As you know, increasing the angle of attack increases lift and increases drag. When the critical angle of attack is exceded, that aircraft stalls because the airflow around the wing goes from laminar to turbulent as the boundry layer separates from the surface of the wing. The airfoil shape allows an increased angle of attack before this critical angle is exceded.

I am *NOT* saying that Bernoulli's Principle plays no role in the lift generated, just that the forward motion of the wing through the air combined with its angle of attack contributes a larger share. If you took a flat sheet of some material that was strong enough to support itself, attached a powerful engine to it and propelled it though the air at an angle, it would fly. It wouldn't be very efficient, because it would generate a huge amount of drag, but it would fly. This is how a box kite works. The camber of the wing reduces the drag, allows an increased angle of attack, and adds some lift as a result of Bernoulli's Principle.

If Bernoulli's Principle were the *SOLE* component of lift, obviously no aircraft could fly inverted unless it could alter the shape of the wing while inverted. The fact that aircraft *CAN* fly upside down, when the contributing forces of the angle of attack and the Bernoulli's Principle force are acting in opposition to each other, indicates that the force component due to angle of attack is greater than that due to Bernoulli's Principle.

[Andy W]

I have learned two things in this thread.

1) It's no wonder engineering schools have Freshman Flunk-out Physics.

2) It's no wonder engineers can't get along with the general population -- no amount of explaining can convince those who can't grasp simple concepts.[]

Last time, I promise:

The question states that the conveyor matches the speed of the plane - NO MORE, NO LESS.

1) Imagine a 2,000 lb plane with and engine with 2,000 lb of thrust (engine type is irrelavent). On a concrete runrway the plane will take off and fly. The amount of air drag plus the friction in the wheels is not enough to keep the plane from flying. As the plane accelerates down the runway the amount of drag from the wind and the wheels increases with speed. The wind drag BY FAR is the larger force.

2) Let's say that at take-off speed, 200mph, the wind is producing 500 pound force drag and the wheels are producing 50 lb force of drag. By using Ray's pretty pictures you can see that the net force will be 1450 lb. Once the plane is airborne it can increase its speed until the drag is equal to the force of the engine, or until the drag is 2,000 lb of force.

The friction of the wheel bearings is a quantity you can measure. Stand on a treadmill wearing roller skates. Tie a rope to a fish scale. Tie the fish scale to the wall. Hold the rope. Turn on the treadmill. Write down the number on the scale. Double the speed of the treadmill. Write down the number. (Didn't change much, did it.)

Let's assume that the frictional force of the wheel bearings is linear and doubles (and that's being generous).

3) Now lets imagine the same plane with the same engine on a conveyor. Since the conveyor acts by friction through the wheels bearing on the body of the plane, increasing the speed of the conveyor adds to the frictionl force. If the frictional force is double you now have 2,000lb (engine) - 500lb (wind drag) - 100lb (wheel drag) = 1400lb.

4) So as Newton says, every action has an equal and opposite reaction. You have 1400lb of net force on the body of the plane. To oppose that force the plane must accelerate at a rate of a=F/m.

5) To keep the plane stationary (to keep it from accelerating -- and taking off) the force on the body of the plane though the wheel bearings must increase to 1,500 lb. 2000lb-500lb-1500lb = 0 and the plane will not move (edit: relative to an obsever on the ground beside the conveyor).

6) For the force of the conveyor through the wheel bearings to equal 1500 lb the speed of the conveyor needs to be ~6,000 mph.

7) The original question clearly states the the speed of the conveyor matches the speed of the plane.

Therefore:

The condidtion whereby the conveyor could prevent the plane from moving forward RELATIVE TO THE AIR is not met, and therefore the plane will take off!!!

Force of wheel bearings on the plane:

F( @ 200mph = 50lb (take-off speed)
F( @ 400mph = 100lb
F( @ 800mph = 200lb
F( @ 1600mph = 400lb
F( @ 3200mph = 800lb
F( @ 6400mph = 1600lb

[oldenough]

Sheltie Dave, okay so you did'nt like my analogy regarding the aircraft carrier, so how about these massive oil tankers where the means of getting around is a bicycle? Now i know for a fact that you can propel yourself in any direction under these circumstances. Under your theory the front end of the boat would be littered with bicycles unable to find there way back to their point of origin. Hey come to that you would only be able to walk in one direction.

Correction:I think i'm mistaken here, would'nt it be the back of the boat that is littered with bicycles? Man this is starting to confuse me!

[klipschaholik]

It looks to me as though we have thoroughly thrashed this subject and no amount of additional explanations are going to change some people's opinions. I have mine everyone else has theirs; we are all differenct and that's good. So as my wife would tell me I think it's time to - Let go, move on.

Now, if a tree in the woods falls and hits the ground and no one is around, does it make a sound?.......................................

[oldenough]

Awww, come on klipschaholik, this is probably the most interesting thread going at the moment, and not a rude word or insult has been hurled by anyone.[]

[Jay481985]

in physics world, yes

in methaphorical sense, no because sound is what we humans make into something that must be heard by another human to be actually heard.

[PhilMays]

Well Andy,

1. Booze is the #1 cause of flunking in college.

2. You guys like to talk until your blue in the face over crap that doesn't matter in the real world.[]

Engineers, don't take that wrong. My father and step fateher are both engineers and reading this thread has at times reminded me of my childhood and rather long, boring, laborious, explanations over crap that doesn't happen in the real world[].

This question is over my head but at the end of the day it doesn't matter. I'm not sure we will ever really need to know if the plane can fly.

Now, let's all go over to the Prettiest girl on the planet thread!!! Gotta know what's important boys!

[jacksonbart]

in physics world, yes

in methaphorical sense, no because sound is what we humans make into something that must be heard by another human to be actually heard.

Wait don't don't bears hear? And we all know what bears do in the woods, and its not wearing funny hats (that would be the pope), so if a tree fell in the woods you can be damn sure there is a bear near by releaving his lower intestine/colon and he would hear that tree. Might cause quite a mess actually.

[Coytee]

Champagn Taste beer budget wrote:

>Oh yeah.... I have a BIG bag of feathers waiting for someone... unless they invite me to a Jubilee audition...<

You have an invitation, just so I know I'm covered []

With that out of the way, I admit I don't know, (nor really care) what the answer is...I found this on one of my favorite sites and was amazed at how they thrashed it around... much like it's becoming thrashed around here... I thought you guys would like it! []

Here's the one concept that I can't get past my thick skull...

A plane can't fly until it transfers its weight from its wheels to its wings, right?

If said plane is sitting STILL on a moving conveyor, then said plane is going to be moving backwards, relative to its direction of thrust (presuming its facing 'forward')

So, if it THEN turns its engine(s) on, it will apply SOME forward momentum that will offset the backward movement, such that, the planes wheels are now moving forward, but the plane itself is sitting still, relative to, say a tree.

If the conveyor moves FASTER in "reverse", and the throttle doesn't move, won't the net change in the plane be, to move again, 'backwards', thereby necessating more thrust to get back to a static position? All this time, isn't the weight of the plane still on the wheels and NOT the wings?

Where in this scenario, does/can the plane build up sufficient forward motion to transfer said weight from the wheels to the wings to facilitate liftoff?

Once the weight is transfered to the wings, I can EASILY see how the conveyor is irrelevant. As long as the weight of the plane is on the wheels though, I'm confused and don't see how it could take off, as it is having its forward momentum thwarted by the conveyor, thereby eliminating the lift from the wings.

[]

Let me ask something else, presume the plane is already flying and making its approach to land. If the runway is traditional, the plane is going to need "X" feet to come to a stop.

If the runway is this mythical conveyor belt, running in reverse to match the planes forward speed, how much runway length will THEN be needed to stop? Would it be the SAME as above? Would it be zero feet?

Seems to me, if the plane could TAKE OFF from this otherwise, static position with no forward motion, then it should be able to do the opposite and land "on a dime" with no forward motion?

I had 3 room mates in college that were engineering majors... I think I annoyed them too! []

[Tom Adams]

Me be an engineer.

Me be an engineer at an aircraft company.

Me and me co-horts say plane flys (thank ged).

Now.....here's something to wrap your head around. For those who aren't familiar with the term "loads", it's pretty simple. It's the force an object exerts on something expressed in something like lbs/sq.ft. or summat.

Ok, so you have a building and the loads that the building imparts to the ground it is sitting on is what? Well....it's the weight of the building plus whatever is inside of it plus other things like wind, snow, whatever. So...all the loads of the building are taken out into the ground. With me so far??

Alright, we got this airplane with its passengers and luggage and fuel and pilots and hot flight attendants and all sorts of stuff. On the ground, all the loads are taken out to the ground. So where are all the loads taken to when in flight?

Tom

[WS65711]

Alright, we got this airplane with its passengers and luggage and fuel and pilots and hot flight attendants and all sorts of stuff. On the ground, all the loads are taken out to the ground. So where are all the loads taken to when in flight?

Ummm . . .

Other Destinations?

[IndyKlipschFan]

Alright, we got this airplane with its passengers and luggage and fuel and pilots and hot flight attendants and all sorts of stuff. On the ground, all the loads are taken out to the ground. So where are all the loads taken to when in flight?

Hopefully to an exotic place with the hot flight attendants and unable to take off again with no fuel for 72 hrs.,...

option 2?

To a terminal?

OK, I am being silly. I am sorry... One of those days here....

I still think your reading something into this word problem and missing the point here?

"A plane is standing on a movable runway." OK, stop here... It is standing... so stopped right?

The conveyor will not move at all UNLESS the plane does.. (And we know again it is stopped.) Am I missing something?

[PhilMays]

Tom,

Your argument has holes in it. I have not seen a HOT flight attendant lately.

So how can I believe anything you say now?

[sputnik]

......Now.....here's something to wrap your head around. For those who aren't familiar with the term "loads", it's pretty simple. It's the force an object exerts on something expressed in something like lbs/sq.ft. or summat..........

A "load" would be a force, as you say, but expressed as pounds or newtons (in SI units). Units of lbs/sq.ft. are used to express a "stress." I know you know that, I just didn't want to see people get more confused.

If the plane is "loaded" with both Canadians and Americans and crashes on the border between the US and Canada, where do you bury the survivors?

[Tom Adams]

Tom,

Your argument has holes in it. I have not seen a HOT flight attendant lately.

So how can I believe anything you say now?

Wait a dang minute. They might be few and far between, but there's still a few that are hotties. You should see the ones we send up on a Gulfstream with prospective customers. YOWZA.

LOL.....I had to re-read your post cause I saw flight attendant and the word holes and started to say something, but figured it wouldn't pass the minister of censorship. [:$]

Tom

[jacksonbart]

......Now.....here's something to wrap your head around. For those who aren't familiar with the term "loads", it's pretty simple. It's the force an object exerts on something expressed in something like lbs/sq.ft. or summat..........

A "load" would be a force, as you say, but expressed as pounds or newtons (in SI units). Units of lbs/sq.ft. are used to express a "stress." I know you know that, I just didn't want to see people get more confused.

If the plane is "loaded" with both Canadians and Americans and crashes on the border between the US and Canada, where do you bury the survivors?

Mexico

[sputnik]

Did you know that Mexico has laws that prohibit that?