Will it take off?

[TraceZ]

Oh boy. It's not the rotating mass of the tires...it's not the friction produced by the bearings. Again, your completely missing the amount of drag between the tire and the runway surface. Distribute the weight of the plane respectively amongst the tires and you have an enormous amount of force being pushed down upon each tire. Enormous. I suspect that the bearings and tires could likely handle the doubled rotating velocity, but the friction would be the decisively limiting factor.

Ok, that is another source of drag. The casings of the tires flex at the contact patch as the tire rotates. That flexing produces some heat and drag under even normal circumstances.

Is it a significant amount at normal takeoff wheelspeed (X)?
At 2(X) is it a significant amount?

I seriously doubt it. Again, I dont think the pilots would even notice it.

[Kevin - Blown 95 TA]

I've seen a Harrier go straight up in a vertical takeoff with no runway at all.

[fastTa]

How about this. A harrier weights about 12,500 pounds and puts out 23,800 pounds of thrust. They can take off without a runway. With a thrust to weight ratio of 2:1 they have no problem picking their own weight up off the deck. Marine carriers don't have a catipult.

What's your point? When the Harrier lifts itself off the ground it eliminates all previously discussed and relative friction induced by the relationship between the aircraft tires and runway.

Is there a thrust to horsepower conversion?

I asked my dad. He said this is a commonly debated issue at work and not an easy mathematical task given the appreciable circumstances and physics. But, a general rule of thumb is 1 HP per 1 pound of static thrust at 30,000 feet. That is generally the altitude that turbofans are tuned for for this type of aircraft.

But you have to understand some fundamental physics and Newtons laws of reciprocal actions and dynamics associated with jet propulsion. Attempting to perceive jet propulsion as a measure of horsepower is very deceiving. Molecules such as air molecules react very differently than lets say a molecule of a sold substance such as concrete. Given this, the law of reciprocal actions dictates that the air obviously responds very differently to an exerted force than say the runway surface would..a solid substance. The net affect of forward motion is substatially less when comparing jet propulsion to that of a car's propulsion.

Most passenger jets have a pretty low specific thrust. This is done to make them quiet and easy on fuel. This has a consequence of making what they call the bypass ration exceptionally high. This is when the bypass flow exceeds the core flow.

The following is obtained from a website dedicated to aeronautical physics:

"At low flight speeds the nozzle is unchoked (less than a Mach number of unity), so the exhaust gas speeds up as it approaches the throat and then slows down slightly as it reaches the divergent section. Consequently, the nozzle exit area controls the fan match and, being larger than the throat, pulls the fan working line slightly away from surge. At higher flight speeds, the ram rise in the intake increases nozzle pressure ratio to the point where the throat becomes choked (M=1.0). Under these circumstances, the throat area dictates the fan match and being smaller than the exit pushes the fan working line slightly towards surge. This is not a problem, since fan surge margin is much better at high flight speeds."

The above is commonly referred to as "Ram Rise" and is a major contributor to a jet engine's dynamic power output. Basically this means that a jet engine has substantially less capacity to produce usuable and effective thrust at slower speeds such as that of runway and taxiing speeds.

Even the mathematical formula used to obtain a value for thrust is dependant upon a value of aircraft velocity and intake mass air flow.

Fnet = m(vjfe - va )

m = intake mass flow
vjfe = fully expanded jet velocity (in the exhaust plume)
va = aircraft flight velocity

[Kevin - Blown 95 TA]

If all those variables and constants aren't given in the original question, then don't we need to assume, for the sake of argument, that they are not intended to be part of the answer, and that the answer needs to be based on general theory? Otherwise, it can never be answered because you don't have any specifications. The question is general and needs a general answer, not one jansky equals a flux of 10-26 watts per square meter of receiving area per hertz of frequency band (W/m2Hz).

[fastTa]

If all those variables and constants aren't given in the original question, then don't we need to assume, for the sake of argument, that they are not intended to be part of the answer, and that the answer needs to be based on general theory? Otherwise, it can never be answered because you don't have any specifications. The question is general and needs a general answer, not one jansky equals a flux of 10-26 watts per square meter of receiving area per hertz of frequency band (W/m2Hz).

Just answering Jeff's question as vaguely as I knew how. Maybe the flux capacitor from the DeLorean in Back To The Future can get this plane off the ground.

[thomas]

"Ahh, which way did he go george?"






This conversation went passed my capability's after the first original question...

[TraceZ]

Here is another interesting tidbit:

To taxi a 747, the throttle levers are moved to 1/3 throttle until desired taxi speed is met (around 30 mph). They are then backed off to almost idle to continue.

so, lets say the resistace in the tirecasing and the rotational inertia of the wheels were to double....

Would it require noticably more than 1/3 throttle to accelerate to taxi speed?
Again, I doubt the pilots would even notice it. Compared to the force required to move the mass of the airplane, the added frictions would not be noticable.

It's not like the brakes are being applied, the wheels still freewheel.

[fastTa]

Here is another interesting tidbit:

To taxi a 747, the throttle levers are moved to 1/3 throttle until desired taxi speed is met (around 30 mph). They are then backed off to almost idle to continue.

so, lets say the resistace in the tirecasing and the rotational inertia of the wheels were to double....

Would it require noticably more than 1/3 throttle to accelerate to taxi speed?
Again, I doubt the pilots would even notice it. Compared to the force required to move the mass of the airplane, the added frictions would not be noticable.

It's not like the brakes are being applied, the wheels still freewheel.

I can see this hapening with a relatively small aircraft, just not anything larger.

What was the question again?

[Jeff_in_Atl]

What's your point? When the Harrier lifts itself off the ground it eliminates all previously discussed and relative friction induced by the relationship between the aircraft tires and runway.

My point is it does it with ease and I'm 100% positive it is many times harder to lift the plane than to roll the plane.

I asked my dad. He said this is a commonly debated issue at work and not an easy mathematical task given the appreciable circumstances and physics. But, a general rule of thumb is 1 HP per 1 pound of static thrust at 30,000 feet. That is generally the altitude that turbofans are tuned for for this type of aircraft.

So lets say the F14 were not in afterburner and each engine produces about 15,000 pounds at full throttle times 2 because it has 2 engines and 1 pound of thrust approx equals 1 horsepower battling the arresting wire with a 1.75 hp motor on it and it can't move it. 1.75 hp against 30,000 hp. Something doesn't add up there. There's more going on there.

[Jeff_in_Atl]

If all those variables and constants aren't given in the original question, then don't we need to assume, for the sake of argument, that they are not intended to be part of the answer, and that the answer needs to be based on general theory? Otherwise, it can never be answered because you don't have any specifications. The question is general and needs a general answer, not one jansky equals a flux of 10-26 watts per square meter of receiving area per hertz of frequency band (W/m2Hz).

Amen f brother.

[TraceZ]

What was the question again?

OOHHHH you funny man!!

I guess we are going to have to agree to disagree. No big deal, it's not like any of us can actually prove our positions.

Here is a question... How much would it cost to actually build a runway long enough for a 747 that could move like a conveyor belt and was still as durable as concrete.

I can't afford it.

[Jeff_in_Atl]

Most passenger jets have a pretty low specific thrust. This is done to make them quiet and easy on fuel. This has a consequence of making what they call the bypass ration exceptionally high. This is when the bypass flow exceeds the core flow.

Boeing 777
P&W 98,000 lb
RR 90,000 lb
GE 93,700 lb
http://www.boeing.com/commercial/777...technical.html
So that's 180,000 lb min
"low thrust" WOW

[thomas]

Has anyone seen Nepolean Dynomite?


G o d........

[fastTa]

Here is a question... How much would it cost to actually build a runway long enough for a 747 that could move like a conveyor belt and was still as durable as concrete.

I can't afford it.

You and me both.

[fastTa]

Boeing 777
P&W 98,000 lb
RR 90,000 lb
GE 93,700 lb
http://www.boeing.com/commercial/777...technical.html
So that's 180,000 lb min
"low thrust" WOW

Low specific thrust, yes. That's what they call it. It's just a good measure of efficiency and velocity differences.

You guys win, the plane flys!