Odd G-III question

Ken Ibold

Ken Ibold

Final Approach
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Ken Ibold
Anybody know the elapsed time between brake release and wheels off the ground for a Gulfstream G-III at max takeoff weight?
 
At Aspen or San Jose?
Ken Ibold said:
Anybody know the elapsed time between brake release and wheels off the ground for a Gulfstream G-III at max takeoff weight?
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Ken Ibold said:
Let's go with San Jose. And not the one in Costa Rica.

Sea level.
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OK, San Jose del Cabo it is. :)

The information you want just HAS to be in the AFM, almost certainly in tabular form. That's the nominal maximum ground roll. The AFM is required to be in the aircraft, provided it was first flown in 1979 or later. 14 CFR 21.5(a) and 14 CFR 91.9(b).
 
14.3 seconds!.....................................OK that was completely made up, but it is the Internet and most facts aren't true anyway! :D:D
 
It is going to completely depend on weight and temperature with regard to how much runway it will use. The balance field length is going to be is going to be around 5,000 ft on a standard day, at max takeoff weight.
 
t = 2 * (takeoffdistance/Vr)

Vr in feet/second
 
EdFred said:
t = 2 * (takeoffdistance/Vr)

Vr in feet/second
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Beat me to it...

This does assume smooth acceleration (so spool up the engines and slip the brakes).

Just in case you want me to show my work:
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Inverted said:
It is going to completely depend on weight and temperature with regard to how much runway it will use. The balance field length is going to be is going to be around 5,000 ft on a standard day, at max takeoff weight.
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Don't care how much runway it uses. Only how many seconds it's moving on the runway surface.
 
Ken Ibold said:
Don't care how much runway it uses. Only how many seconds it's moving on the runway surface.
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How much runway is a factor in how many seconds it takes. It's how all that physics stuff works.

How exact does the number have to be?

Looks to be about 20 seconds.
 
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JeffDG said:
This does assume smooth acceleration (so spool up the engines and slip the brakes).
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It assumes CONSTANT acceleration.

It also assumes no friction, no drag, and zero moment of inertia for the wheels....in addition to a perfect thrust curve for the engines.

So, grease up that runway.

Constant acceleration is generally a poor approximation unless you're working in a vacuum. Calculate your velocity using the freshman physics above, for a fall off a 25 story building (200 feet). You'll be making sonic booms. This doesn't really happen. Models should be as simple as possible for utility, but no simpler.

What you can say is that it will take AT LEAST as long as the constant acceleration approximation, as all the corrections slow you down. But it may be a lot longer.
 
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EdFred said:
How much runway is a factor in how many seconds it takes. It's how all that physics stuff works.

How exact does the number have to be?

Looks to be about 20 seconds.
Click to expand...
I am working on an analysis that uses modeling software designed for the DoD that requires inputs in seconds, not in feet. Not knowing what the acceleration curve is for a G-III, I thought I'd try to get some real world numbers.
 
Ken Ibold said:
I am working on an analysis that uses modeling software designed for the DoD that requires inputs in seconds, not in feet. Not knowing what the acceleration curve is for a G-III, I thought I'd try to get some real world numbers.
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Trick? (Yes, that really is the name of software)
 
MAKG1 said:
It assumes CONSTANT acceleration.

It also assumes no friction, no drag, and zero moment of inertia for the wheels....in addition to a perfect thrust curve for the engines.

So, grease up that runway.

Constant acceleration is generally a poor approximation unless you're working in a vacuum. Calculate your velocity using the freshman physics above, for a fall off a 25 story building (200 feet). You'll be making sonic booms. This doesn't really happen. Models should be as simple as possible for utility, but no simpler.

What you can say is that it will take AT LEAST as long as the constant acceleration approximation, as all the corrections slow you down. But it may be a lot longer.
Click to expand...

Lets plug in our numbers into the "freshman physics" and see what we get shall we?

d = 0.5* at2
200ft = 16.05f/s t2
12.46 = t2
3.53s = t

v = at
v = 32.1 * 3.53
v = 113ft/s when you go splat.

That's about 77mph
Yea, sonic booms.

For an alleged astrophysicist you certainly are a dumb ****.

Maybe you should pull your head out of your ass before trying to pretend you know.....well....anything.
 
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Yup, you caught an arithmetical error, and it was significant.

Does it change the conclusion? Or do you still assert constant acceleration works even as the aircraft rotates and changes its angle of attack, while still on the ground?

Jets should give you nearly constant thrust at low speeds, unlike props, but this is not a frictionless problem.

Or do I have "my head up my ass" when I assert that constant acceleration is a LOWER bound? It can and will take longer.

People use simple rules of thumb all the time; a real common one is pi=3. It's a mistake to take those seriously beyond their limitations.
 
We have an acceleration meter in the Falcon 50, At the start of a takeoff roll I have seen as much as 0.31. This decreases rapidly throughout the takeoff roll.
 
MAKG1 said:
It assumes CONSTANT acceleration.

It also assumes no friction, no drag, and zero moment of inertia for the wheels....in addition to a perfect thrust curve for the engines.

So, grease up that runway.

Constant acceleration is generally a poor approximation unless you're working in a vacuum. Calculate your velocity using the freshman physics above, for a fall off a 25 story building (200 feet). You'll be making sonic booms. This doesn't really happen. Models should be as simple as possible for utility, but no simpler.

What you can say is that it will take AT LEAST as long as the constant acceleration approximation, as all the corrections slow you down. But it may be a lot longer.
Click to expand...

Actually, given constant acceleration, none of those other factors matter at all. They all impact the rate of acceleration, but if you accelerate at a constant rate, and take off in 3,000' at 120 fps, it will take you the same amount of time, regardless of wind resistance etc.
 
To find the answer all you'll need to do is find someone who still operates a G-III and who uses Ultra-Nav to calculate their takeoff performance numbers. The program gives you the number as a matter of course. But seriously, a G-III... :goofy:
 
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Nobody around at SFI that can do it. Sorry
 
Shot in the dark here, but would MS FSX be accurate enough?

I don't have FSX, so I'm not volunteering, but surely someone on here uses FSX and is willing to download a GIII add-on to test it out.
 
Dead Stick said:
To find the answer all you'll need to do is find someone who still operates a G-III and who uses Ultra-Nav to calculate their takeoff performance numbers. The program gives you the number as a matter of course. But seriously, a G-III... :goofy:
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Didn't Ken say something about government? They don't care about fuel burn.
 
MAKG1 said:
It assumes CONSTANT acceleration.

It also assumes no friction, no drag, and zero moment of inertia for the wheels....in addition to a perfect thrust curve for the engines.

So, grease up that runway.

.
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Someone please cue up the treadmill thread......;):D
 
Well, the good news is that the G-III wing is shaped somewhat like a airfoil, unlike previous versions.

Dead Stick said:
To find the answer all you'll need to do is find someone who still operates a G-III and who uses Ultra-Nav to calculate their takeoff performance numbers. The program gives you the number as a matter of course. But seriously, a G-III... :goofy:
Click to expand...
 
CJones said:
Shot in the dark here, but would MS FSX be accurate enough?

I don't have FSX, so I'm not volunteering, but surely someone on here uses FSX and is willing to download a GIII add-on to test it out.
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The simple answer to this is, and I quote: :no: :no: :lol: :lol: :goofy: :goofy: :nono: :nono: :yikes: :yikes::mad2: :mad2:
 
MAKG1 said:
OK, San Jose del Cabo it is. :)

The information you want just HAS to be in the AFM, almost certainly in tabular form. That's the nominal maximum ground roll. The AFM is required to be in the aircraft, provided it was first flown in 1979 or later. 14 CFR 21.5(a) and 14 CFR 91.9(b).
Click to expand...

I don't fly the GIII but I do fly the G450 and G550. If the AFM and performance manuals are alike (which I suspect)...you will not find any reference to time to take-off. You will find runway length required, you will find expected engine performance numbers in the form of EPR, and in the newer glass cockpits you will find expected acceleration numbers for contaminated runways but no where will you find time to take-off.

You might find this in the original certification data somewhere but it just is of no use to a pilot based on how Gulfstream presents performance data.

Kevin
 
Ken Ibold said:
I am working on an analysis that uses modeling software designed for the DoD that requires inputs in seconds, not in feet. Not knowing what the acceleration curve is for a G-III, I thought I'd try to get some real world numbers.
Click to expand...

Ken,

As others have stated, the time (like the distance) will be a factor of many variables (temp/altitude). Also, on the Gulfstream you can use two different flap settings and two types of engine thrust (max cont. and flex) that will change the data.

I don't have an answer for you, but maybe you should be contacting someone at Gulfstream. They are the experts (no offense to anyone here) and might have certification data they might be willing to share.

Kevin
 
Honestly I can't see the usefulness of timing a takeoff roll.

We start the chrono on the ND at application of takeoff thrust but that's for a different purpose.
 
Back in the day I timed the transports on hot days when departing on long trips with full cabins. 35 seconds to liftoff seemed like forever.
 
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