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Loose Change - Part IV

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Peace, you guys. The fully-fueled aircraft will have a higher terminal velocity, as being denser increases its effective ballistic coefficient.

But it is also correct to say that the lighter aircraft will accelerate quicker in response to thrust of engines being spooled up.

It's hard to guess, off the top of one's head, which will hit faster. It will depend on airspeed and whether inertia or drag is dominant. At high speed, drag is dominant. At steady-state (diving), the fully-loaded plane will have a higher top speed, but it might take longer to get there. Flat and level, top speeds will be the same.*

I wouldn't expect the difference to be much, in any event.

*: More or less. L / D = T / W and all that. Wing efficiency becomes a factor...
 
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R.Mackey said:
Peace, you guys. The fully-fueled aircraft will have a higher terminal velocity, as being denser increases its effective ballistic coefficient.

But it is also correct to say that the lighter aircraft will accelerate quicker in response to thrust of engines being spooled up.

It's hard to guess, off the top of one's head, which will hit faster. It will depend on airspeed and whether inertia or drag is dominant. At high speed, drag is dominant. At steady-state (diving), the fully-loaded plane will have a higher top speed, but it might take longer to get there. Flat and level, top speeds will be the same.*

I wouldn't expect the difference to be much, in any event.

*: More or less. L / D = T / W and all that. Wing efficiency becomes a factor...
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I totally agree with that. I disagree with Cuddles statements, which are mathematically and physically false.
 
rwguinn said:
I bow to your superioror knowledge.
I surrender my Engineering degree, 40 years of physics study, Professional license, and 350 years of physical science accumilation since Gallileo to your hands.
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The descent of the aircraft is not determined by gravity because an aircraft is not in free-fall. You must consider aerodynamics.

To maintain altitude, an aircraft must generate enough lift to maintain the aircraft's weight in the air. The lighter the aircraft, the less lift is required - hence why a 747 needs a long runway and a high rotate speed, whereas a Cessna 172, for example, requires a much shorter runway and lower rotate speed.

A fully laden aircraft requires more lift than one with little fuel on board. As such, given a full and an empty aircraft, both in the same attitude at the same thrust level, the heavier aircraft will descend faster - because the shortfall in required lift will be greater.

-Andrew
 
I'm sorta starting to be able to laugh at the LC people. Mostly though, they still only make me angry.

Luckily, though, my church has some resources I've found theraputic, one of which is this sound clip.
 
I also wouldn't expect much difference due to the fuel load. The heavier weight would help get over some of the air drag, but it would also be harder for the engines to accelerate.

A data point: at full throttle, a fully-loaded airliner accelerates from zero to takeoff speed (what, 150 kts?) in about 40 seconds. This is going on level ground, without the benefit of a descent, and there is some increased resistance due to rolling tires, and the wind drag of the gear and flaps. In a clean configuration, in a descent, at full throttle, I would expect it to accelerate at a similar rate.
 
R.Mackey said:
Peace, you guys. The fully-fueled aircraft will have a higher terminal velocity, as being denser increases its effective ballistic coefficient.

But it is also correct to say that the lighter aircraft will accelerate quicker in response to thrust of engines being spooled up.

It's hard to guess, off the top of one's head, which will hit faster. It will depend on airspeed and whether inertia or drag is dominant. At high speed, drag is dominant. At steady-state (diving), the fully-loaded plane will have a higher top speed, but it might take longer to get there. Flat and level, top speeds will be the same.*

I wouldn't expect the difference to be much, in any event.

*: More or less. L / D = T / W and all that. Wing efficiency becomes a factor...
Click to expand...

Yeah, when I brought this up, I was strictly speaking from my experience talking to 767/757 pilots about VNAV autopilot overspeed disconnects, which typically happen when a fully loaded aircraft is descending and simply cannot slow down or even maintain speed.
With a level attitude, I'd imagine the fully loaded airplane will need more thrust to maintain speed and this certainly is this case when climbing. A fully loaded plane airplane can't climb or accel nearly as fast as as empty one.
 
gumboot said:
The descent of the aircraft is not determined by gravity because an aircraft is not in free-fall. You must consider aerodynamics.

To maintain altitude, an aircraft must generate enough lift to maintain the aircraft's weight in the air. The lighter the aircraft, the less lift is required - hence why a 747 needs a long runway and a high rotate speed, whereas a Cessna 172, for example, requires a much shorter runway and lower rotate speed.

A fully laden aircraft requires more lift than one with little fuel on board. As such, given a full and an empty aircraft, both in the same attitude at the same thrust level, the heavier aircraft will descend faster - because the shortfall in required lift will be greater.

-Andrew
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Again, no problem.
That was NOT what was originally said.
Originally Posted by apathoid
Good points DarkRotor. Dont forget that the airplane was full of fuel, meaning it would accel faster in a descent.
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and from cuddles
A 10000lb aircraft will have the same air resistance as a 5000lb aircraft but a larger force from gravity, therefore it will accelerate faster.
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CurtC said:
I also wouldn't expect much difference due to the fuel load. The heavier weight would help get over some of the air drag, but it would also be harder for the engines to accelerate.

A data point: at full throttle, a fully-loaded airliner accelerates from zero to takeoff speed (what, 150 kts?) in about 40 seconds. This is going on level ground, without the benefit of a descent, and there is some increased resistance due to rolling tires, and the wind drag of the gear and flaps. In a clean configuration, in a descent, at full throttle, I would expect it to accelerate at a similar rate.
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All very true. But desending, the fully loaded airplane's extra weight(not drag) speeds it up at any given descent rate. The opposite is true for a climb..
 
Look! Its our hero Do Over Dylan the cartoon character, with his buddies, and the Clenched Fist Salute!

(Uh, does it matter which fist?)

Those signs in the background are just ripe for monkeying with - maybe there will be alternate ones at GZ:

Ask stupid questions of unqualified people. Obfuscate for Peace. No Justice! No Pizza!

http://loosechange911.blogspot.com/

Oh, and by the way, big sale on T-Shirts - marked down 2 Bucks!

Honor the Fallen!
 
Clenched Fist Salute!
Click to expand...

One tin soldier rides aloooooone.

Good lord, they imply that Firefighters are complicit in the cover up of the attacks, especially those that knew that WTC7 was going to fall, and they don't expect hubbub? Do they really see themselves as some sort of Malcom X?

Do these guys even have a permit to march?
 
pgwenthold said:
Are you suggesting that things that are heavier fall faster?
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IN air, and given that all other factors are equal, I'll say it (for the case of intact airplanes under powered flight).

Because it's true. The higher density means that a greater force from gravity acts on it, but this is cancelled by the greater weight (so acceleration is identical). However, the force applied from drag (air resistence) as well as the amount of lift (from the wings) are identical...while the weight (more accurately, mass) are higher. Therefore, the wings provide less upward acceleration, and the drag produces less deceleration.

So the plane "falls" (more accurately, descends) faster.
 
pgwenthold said:
Are you suggesting that things that are heavier fall faster?
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This has nothing to do with freefall.

I guess I need to clarify. We have:

Airplane 1: A 757, at 80 tons, descending at idle at 2000 fpm starting at 400 mph
Airplane 2: A 757, at 125 tons, descending at idle at 2000 fpm starting at 400 mph.

The extra weight will will cause airplane 2 to speed up because more gravity is pulling it down, the rate of descent is the same in both cases. Again, this has nothing to do with freefall velocity.
 
apathoid said:
This has nothing to do with freefall.

I guess I need to clarify. We have:

Airplane 1: A 757, at 80 tons, descending at idle at 2000 fpm starting at 400 mph
Airplane 2: A 757, at 125 tons, descending at idle at 2000 fpm starting at 400 mph.

The extra weight will will cause airplane 2 to speed up because more gravity is pulling it down, the rate of descent is the same in both cases. Again, this has nothing to do with freefall velocity.
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I'd argue that it's not so much gravity, but that the opposing forces that are based on the shape/structure (rather than mass), such as drag and lift, produce identical forces that act on a larger mass.

Gravitic force increases, yes, but acts a higher mass, and the effects cancel out.
 
gumboot said:
The descent of the aircraft is not determined by gravity because an aircraft is not in free-fall. You must consider aerodynamics.

To maintain altitude, an aircraft must generate enough lift to maintain the aircraft's weight in the air. The lighter the aircraft, the less lift is required - hence why a 747 needs a long runway and a high rotate speed, whereas a Cessna 172, for example, requires a much shorter runway and lower rotate speed.

A fully laden aircraft requires more lift than one with little fuel on board. As such, given a full and an empty aircraft, both in the same attitude at the same thrust level, the heavier aircraft will descend faster - because the shortfall in required lift will be greater.

-Andrew
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rwguinn is in the right.

Vertical speed and velocity vector are not the same for an aircraft that still has lift. The aircraft was not falling vertically, it was flying a roughly 5 degree glide slope.

Given the overall weight of the aircraft, the marginal difference in airspeed vector due to fuel would account for a knot or three (five?) of difference in airspeed in a shallow dive like that. The nose/pitch attitude, and angle of attack on the arifoils (below the horizontal) would have more to do with that, as you'd get additive thrust from gravity (a small resultant vector) to add to the thrust from the engines. Less than a percent, if impact was at 400+ knots.

The Kinetic Energy at impact would be calculated at the point of impact to include the vector sum of the forces along the line of travel: gravity's contribution to this is minimal, since the plane had lift, and plenty of it, all the way to impact. Lift is a force that counteracts gravity, or generally works in opposition to gravity.

DR
 
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Huntsman said:
I'd argue that it's not so much gravity, but that the opposing forces that are based on the shape/structure (rather than mass), such as drag and lift, produce identical forces that act on a larger mass.

Gravitic force increases, yes, but acts a higher mass, and the effects cancel out.
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Yeah, I dont doubt you're right. But I always thought drag opposed thrust and lift opposed weight. Meaning more weight wouldnt necesarily lead to more drag, thus not slowing the plane down.
 
apathoid said:
Yeah, I dont doubt you're right. But I always thought drag opposed thrust and lift opposed weight. Meaning more weight wouldnt necesarily lead to more drag, thus not slowing the plane down.
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Well, drag is based on area and shape, not weight, so weight shouldn't increase drag. Because the plane is more massive, the drag (which exerts the same amount of force as on a similar plane with a lighter load) has more mass to decelerate, so slows it at a lower rate. Basically, all this boils down to is the heavier plane has more inertia.

But drag opposes thrust is correct, and gravity opposes lift is correct, but only in perfectly horizontal flight. As you raise the nose you have thrust and lift opposing drag and gravity. As you nose down you have thrust and gravity opposing drag and lift.
 
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