RE: Can It Take Off??  
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RE: Can It Take Off?? - 8/7/2006 1:41:14 PM   
mesae



Posts: 568
Joined: 12/16/2002
From: Edmond, OK, USA
Status: offline 		Here we go again. The joke is on anyone who thinks the plane will be prevented from taking off.

How about this: An airplane with wheels, not skis, is on a smooth, hard-frozen lake of sufficient size. The pilot locks the brakes and takes off. Get the irony and the joke?

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RE: Can It Take Off?? - 8/7/2006 5:02:21 PM   
air mail rcu


 

Posts: 522
Joined: 8/3/2003
From: Pittsburgh, PA, USA
Status: offline 		Gee I missed the irony...... and the joke.

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RE: Can It Take Off?? - 8/7/2006 9:39:18 PM   
da Rock



Posts: 6376
Joined: 10/11/2005
From: western, NC, USA
Status: offline 		chuckle.....

How about this......
Airplane on floats is on a raging river tied up to a speed boat running at the same speed as the river.
Airplane starts it's engine and drops the towline.
How long before there is a horrible boat/airplane collision?

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RE: Can It Take Off?? - 8/7/2006 10:23:54 PM   
air mail rcu


 

Posts: 522
Joined: 8/3/2003
From: Pittsburgh, PA, USA
Status: offline 		Raging.....I have that on my speedometer.

If the boat and the plane only have enough power to go the speed of raging then the boat and plane would never run into each other. And yes the plane would not take off.

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RE: Can It Take Off?? - 8/8/2006 12:12:15 AM   
kbear



Posts: 295
Joined: 9/11/2004
From: Denton, TX, USA
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quote:

ORIGINAL: darock

Can you catch a greased pig?

Not with the "interface" present. The interface is the grease.

The wheel bearings on the airplane is the grease. The wheel/axle structure exists so that an airplane cannot be stopped from moving on the ground by the ground. As long as there are wheels on the axles, the ground or a treadmill ain't gonna stop the airplane from movign.

This treadmill question is definitely a joke question.


Are you saying that if a plane didn't have breaks, it would never stop. If the runway were long enough any plane would eventualy roll to a stop.

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RE: Can It Take Off?? - 8/8/2006 12:16:46 AM   
kbear



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quote:

ORIGINAL: mesae

Here we go again. The joke is on anyone who thinks the plane will be prevented from taking off.

How about this: An airplane with wheels, not skis, is on a smooth, hard-frozen lake of sufficient size. The pilot locks the brakes and takes off. Get the irony and the joke?


I have already said that if you skid the tires you can take off. An iced over lake is not moving in the opposite direction of the plane. Yuou only have to have enough thrust to ovecom the frictional force on the wheels. The conveyor increases its speed and force as the wheels turn. Add thrust and the wheels turn and the belt reacts. Skid and the belt doesn't react.

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RE: Can It Take Off?? - 8/8/2006 12:21:04 AM   
kbear



Posts: 295
Joined: 9/11/2004
From: Denton, TX, USA
Status: offline 		This has been very fun. I will say this and let it go. I think that you cannot just toss out parts of the equation like the force that the belt puts on the plane. We will just have to agree to disagree. Thanks for the debate guys. Now I've got a plane to build. Blue skies everyone.

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RE: Can It Take Off?? - 8/8/2006 3:47:51 AM   
da Rock



Posts: 6376
Joined: 10/11/2005
From: western, NC, USA
Status: offline 		Fun? not actually

For the belt to keep the airplane from moving it must create a force. For that force to work on the airplane, it must be transmitted to the airplane. The belt can only transmit a force into the tire/wheel. The force must then be transmitted through the wheel bearings to have an effect on the airplane.

The belt can only do one of three things to the wheels. Rotate them "with" the airplane, rotate them "against" the airplane, or not rotate them.

If the belt does not rotate the wheels, there is no force against the tires. No force against the tires? No force to transmit through the bearings. No force? No retardation. (except for some of us)

If the belt moves the wheels "with" the airplane, any force that could possibly be transmitted through the bearings would obviously help the airplane take off.

If the belt moves the wheels "against" the airplane, it will be doing just what the runway of every airport in the world does to the wheels, spins them.

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RE: Can It Take Off?? - 8/8/2006 5:28:20 PM   
ptulmer



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Joined: 4/28/2003
From: Brunswick, GA, USA
Status: offline 		I've got to admit that I only skimmed this thread. It's amazing to see how many people don't understand that the propeller will determine airspeed, not ground speed. These same people probably think the downwash from the wing propels the airplane into the sky like rocket thrust...

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RE: Can It Take Off?? - 8/8/2006 9:17:12 PM   
mesae



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From: Edmond, OK, USA
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quote:

ORIGINAL: ptulmer
..These same people probably think the downwash from the wing propels the airplane into the sky like rocket thrust...


First, the belt will have very little influence on the plane (none, if bearing friction and tire momentum are ignored), in case there is any doubt about my position on the matter.

Now, your apparently facetious statement above isn't entirely untrue:

Rocket thrust is a force in reaction to the acceleration of gases.
Propeller thrust is a force in reaction to the acceleration of gases.
Wing lift is a force in reaction to the acceleration of gases.

All three statements are true.

Also, for kbear who wrote that an iced lake isn't moving: If the airplane is facing west, at rest with respect to the lake, then as it begins its takeoff roll, it will accelerate with respect to the air over the lake, but it is decelerating with respect to a non-rotating reference frame at the center of the earth. So while it is true that the lake isn't moving in response to the airplane, it can be considered to be moving under the airplane, in the direction opposite the takeoff run, just as truly as it can be said that the airplane is moving with reference to the lake and/or the air over the lake.

Put another way, an airplane flying 1000 ground mph westbound over the equator, is staying approximately stationary with respect to a non-rotating reference frame at the center of the earth, while the surface of the earth rotates under it.

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RE: Can It Take Off?? - 8/8/2006 9:29:41 PM   
ptulmer



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quote:

ORIGINAL: mesae

Now, your apparently facetious statement above isn't entirely untrue:

Rocket thrust is a force in reaction to the acceleration of gases.
Propeller thrust is a force in reaction to the acceleration of gases.
Wing lift is a force in reaction to the acceleration of gases.

All three statements are true.


Hmmm... bold statements. You'll have to back the last two up, though. That's not my understanding. The wind behind the propeller isn't doing anything 'cept ruffling your hair. The work is done at the prop itself. I'd say lift happens by all those air molecules hitting the bottom of the wing. Whether that wing is a propeller or fixed wing makes no difference, the "wind" created is purely an effect. Proof of work having been done, then on to cleaner air.

At least we can agree on the question at hand! That airplane wouldn't have even been slowed down.

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RE: Can It Take Off?? - 8/8/2006 10:43:13 PM   
dick Hanson



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Status: offline 		oh oh

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RE: Can It Take Off?? - 8/9/2006 3:38:03 AM   
mesae



Posts: 568
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From: Edmond, OK, USA
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quote:

ORIGINAL: ptulmer

quote:

ORIGINAL: mesae

Now, your apparently facetious statement above isn't entirely untrue:

Rocket thrust is a force in reaction to the acceleration of gases.
Propeller thrust is a force in reaction to the acceleration of gases.
Wing lift is a force in reaction to the acceleration of gases.

All three statements are true.


Hmmm... bold statements. You'll have to back the last two up, though. That's not my understanding. The wind behind the propeller isn't doing anything 'cept ruffling your hair. The work is done at the prop itself. I'd say lift happens by all those air molecules hitting the bottom of the wing. Whether that wing is a propeller or fixed wing makes no difference, the "wind" created is purely an effect. Proof of work having been done, then on to cleaner air.

At least we can agree on the question at hand! That airplane wouldn't have even been slowed down.



Well, let's ignore vertical component of thrust for now, and also tail down-force, just for simplicity, and let's assume that all the lift necessary to support the weight of the airplane is produced by the wing. At higher speeds, this usually isn't very far from the truth.

The propeller/jet engine produces thrust that just matches drag. The airplane is flying at a constant altitude and airspeed and not turning; it weighs 1000 lbs. What this means is that the wing exerts a net force of 1000 lbs-force on the atmosphere. This also means that the atmosphere is exerting 1000 lbs-force of lift on the wing, otherwise the airplane would not be maintaining a constant altitude. The only way this can happen is if the wing imparts a change of momentum to the fluid equivalent to net 1000 lbs-force. It does this by constantly and steadily accelerating air downward. It also accelerates air a little bit upward in front of the wing but much more goes down so the net reaction force just balances the weight of the airplane. You see: action and reaction. Some of the change in fluid momentum is the result of simple deflection against the bottom of the wing. Some is caused by angular acceleration over the top of the wing. Again, the net result is the equivalent of 1000 lbs-force of air being constantly accelerated downward, the reaction being that the airplane stays up.

The air behind the propeller moving aft and the air below and behind the airplane moving downward is doing so because of the force applied to it by the propeller and the wing.

Cheers!

p.s. Funny, Dick!

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RE: Can It Take Off?? - 8/9/2006 4:21:16 AM   
aflipz


 

Posts: 932
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From: Montreal, QC, CANADA
Status: offline 		ROFLOL this is still a debate!!!

Gee i feel a little sick, i was giving the world so much more credit than it diserves


Seriously i think the question is deliberately vague and open to interpretation/misunderstanding of the conditions to cause this very debate. Otherwise their wouldn't be one.

And i'm about as far as you can be from being a rocket scientist too. If I we were given serious figures such as thrust, weight, airfoil size etc.. I wouldn't be in any position to answer "it will fly" as easily as I can now.

that's my opinion, and everybody is entitled to my opinion now aren't they?

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RE: Can It Take Off?? - 8/10/2006 8:40:22 PM   
Tim Green


 

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quote:

ORIGINAL: ptulmer

The wind behind the propeller isn't doing anything 'cept ruffling your hair.


You apparently have never stood behind a full size airplane's propeller. Any way you slice it, that moving air has a lot of force to it. And if you do chance to stand behind a full size spinning prop, you can ponder this as you pick your butt off the ground.


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