bufordb
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Speed On The Ground

Fri Oct 26, 2007 6:51 pm

Assuming unlimited runway length, what is the estimated top speed of a Boeing 737 on the ground? Or any plane for that matter?
 
Goldenshield
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RE: Speed On The Ground

Fri Oct 26, 2007 6:54 pm

The highest a plane can go on the ground is as fast as its tires are rated for: generally around 225 MPH.
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n710ps
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RE: Speed On The Ground

Sat Oct 27, 2007 10:20 pm

In flight sim my I did 458 in a MD-11 across the desert!  duck  But serioauly, it is based on what the tires are rated for and if you trust TV commercials than Goodyear is the way to go but I would rather just listen to Maintenece Control personally. (I am in an extremely silly mood tonight)
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AAR90
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RE: Speed On The Ground

Sun Oct 28, 2007 5:03 am

Fastest I've been was 205 knots. Emergency landing after multiple bird strikes after takeoff. Landed no-flaps, heavy and with no air speed indications. Post-flight DFDR revealed a very smooth touchdown at 205 knots ground speed.
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Starlionblue
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RE: Speed On The Ground

Sun Oct 28, 2007 6:45 am

Quoting AAR90 (Reply 3):
Fastest I've been was 205 knots. Emergency landing after multiple bird strikes after takeoff. Landed no-flaps, heavy and with no air speed indications. Post-flight DFDR revealed a very smooth touchdown at 205 knots ground speed.

Did the CVR record your pounding heart?  Wink
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KELPkid
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RE: Speed On The Ground

Sun Oct 28, 2007 6:59 am

Quoting AAR90 (Reply 3):
Landed no-flaps, heavy and with no air speed indications

Does your GPS velocity update quick enough to give you an idea? I know it actually records your ground speed...  Wink

Quoting Bufordb (Thread starter):
Assuming unlimited runway length, what is the estimated top speed of a Boeing 737 on the ground? Or any plane for that matter?

The ground speed, given the wind, ambient pressure and temperature conditions, that creates the amount of lift that the flight manual calls for for a successful takeoff  Smile Even the airspeed indication itself is highly susceptible to variations in temperature and pressure...the point is here that the ultimate goal is to achieve the proper airspeed, not ground speed, for a successful takeoff. The ground speed is just a consequence of that  Wink
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AAR90
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RE: Speed On The Ground

Sun Oct 28, 2007 3:05 pm

Quoting Starlionblue (Reply 4):
Did the CVR record your pounding heart?

Cute.  Wink Like landing on a CV, I was a bit too busy to notice, but I'm sure it must have been pretty high.

Quoting KELPkid (Reply 5):
Does your GPS velocity update quick enough to give you an idea? I know it actually records your ground speed...

Never thought to look at that page in FMS. When I realized I couldn't trust the third IAS, I shifted to the AOA indicator. Very difficult as we normally don't use that so it was a hard scan to adjust to. Made more difficult when the plan shook like $%^# with the first deployment of flaps --which were quickly retracted.
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roseflyer
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RE: Speed On The Ground

Sun Oct 28, 2007 7:23 pm

Quoting Bufordb (Thread starter):
Assuming unlimited runway length, what is the estimated top speed of a Boeing 737 on the ground? Or any plane for that matter?

That's an interesting question. I think it's a case of no one knows. Eventually you are going to lose the tires. I think they would be the first to go if you had enough thrust. The loss of tires will eventually cause the landing gear to fail. Steering with the nose wheel won't work, but the rudder would have a lot of effectiveness. It would probably end messy. You'd be having to put a lot of elevator in to keep the plane on the ground.

But there would be a lot of drag and the plane theoretically probably can't go above about 300 knots. With full power and the gear down and no flaps, the 737 can just barely maintain 300 knots in level flight depending on circumstances. On the ground it would be less. There is absolutely no way that a 737 would go 458 knots. It doesn't have enough thrust.
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tdscanuck
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RE: Speed On The Ground

Sun Oct 28, 2007 8:20 pm

Quoting RoseFlyer (Reply 7):
You'd be having to put a lot of elevator in to keep the plane on the ground.

Shouldn't be...ground trim for a 737NG is slightly negative lift. As long as you didn't rotate, it would stay down.

Tom.
 
Analog
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RE: Speed On The Ground

Sun Oct 28, 2007 8:23 pm

Quoting RoseFlyer (Reply 7):

That's an interesting question. I think it's a case of no one knows. Eventually you are going to lose the tires. I think they would be the first to go if you had enough thrust. The loss of tires will eventually cause the landing gear to fail.



Quoting Tdscanuck (Reply 8):

Shouldn't be...ground trim for a 737NG is slightly negative lift. As long as you didn't rotate, it would stay down.

Could you pull up just a little to reduce the load on the tires, say to 10% of the aircraft's weight?
 
pilotboi
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RE: Speed On The Ground

Sun Oct 28, 2007 10:05 pm

Quoting Analog (Reply 9):
Could you pull up just a little to reduce the load on the tires, say to 10% of the aircraft's weight?

I see where you're going with that, but I think that would be quite difficult. Pulling back enough to make a difference would cause the a/c to start a gradual climb. You'd have to quickly apply slight forward elevator, then keep it steady there. Basically, you'd be flying the aircraft within about +/- 3 feet of a given altitude, which is hard enough for an autopilot to do, I would assume.

Also, if possible, it would reduce weight, yes. But it wouldn't change the speed at which the tires are moving across the ground. I'm not sure if it would really make a difference or not, someone else with more applied physics knowledge could answer that.
 
Analog
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RE: Speed On The Ground

Mon Oct 29, 2007 12:45 am

Quoting Pilotboi (Reply 10):
You'd have to quickly apply slight forward elevator, then keep it steady there. Basically, you'd be flying the aircraft within about +/- 3 feet of a given altitude,

It must be easier than that. I imagine that it's the equivalent of keeping between normal negative trim and level flight (well, not flight).

Couldn't you use the weight sensors on the wheels (does the 737 have these) to make sure that the weight on the wheels is more than zero?

Quoting Pilotboi (Reply 10):
But it wouldn't change the speed at which the tires are moving across the ground.

I imagine that the tires can sustain a higher rotational speed at lighter loads.
 
corey07850
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RE: Speed On The Ground

Mon Oct 29, 2007 12:56 am

With or without the plane on a giant conveyor belt?
 
tdscanuck
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RE: Speed On The Ground

Mon Oct 29, 2007 1:21 am

Quoting Analog (Reply 11):
Quoting Pilotboi (Reply 10):
You'd have to quickly apply slight forward elevator, then keep it steady there. Basically, you'd be flying the aircraft within about +/- 3 feet of a given altitude,

It must be easier than that. I imagine that it's the equivalent of keeping between normal negative trim and level flight (well, not flight).

It's not that easy. The oleo strut stroke on a 737 is something like a few feet. To keep the tires in contact with the ground you need to hold your altitude within that band. That's far tighter than any normal commercial autopilot or control system can do, since there is no need for that precise control in any normal scenario.

Quoting Analog (Reply 11):
Couldn't you use the weight sensors on the wheels (does the 737 have these) to make sure that the weight on the wheels is more than zero?

Sort of...the weight-on-wheels sensors are binary. They just say you're on the ground or not. They don't tell you how far extended the strut is. The radio altimeter is a better bet but that would require an autopilot function that, as far as I'm aware, doesn't exist.

Tom.
 
Analog
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RE: Speed On The Ground

Mon Oct 29, 2007 2:34 am

Quoting Tdscanuck (Reply 13):

It's not that easy. The oleo strut stroke on a 737 is something like a few feet. To keep the tires in contact with the ground you need to hold your altitude within that band. That's far tighter than any normal commercial autopilot or control system can do, since there is no need for that precise control in any normal scenario.

I'd imagine that keeping the aircraft in that "altitude" range is a lot easier than for any other altitude range of similar size. Keeping an airborne aircraft in a +-3ft (or whatever) altitude range requires keeping the control surfaces in an extremely narrow range and the wings cannot be allowed to effect anything but tiny variations in lift. Keeping the aircraft on the ground allows for lift variations between zero and the full weight of the aircraft. That's huge.

Quoting Tdscanuck (Reply 13):

Sort of...the weight-on-wheels sensors are binary. They just say you're on the ground or not.

Don't some aircraft have On-Board Aircraft Weighing Systems (OBAWS) that display the weight on each axle?
http://www.faa.gov/library/manuals/aircraft/media/FAA-H-8083-1A.pdf

BTW: I'm not a pilot, not an expert, etc. etc.
 
tdscanuck
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RE: Speed On The Ground

Mon Oct 29, 2007 3:08 am

Quoting Analog (Reply 14):
I'd imagine that keeping the aircraft in that "altitude" range is a lot easier than for any other altitude range of similar size. Keeping an airborne aircraft in a +-3ft (or whatever) altitude range requires keeping the control surfaces in an extremely narrow range and the wings cannot be allowed to effect anything but tiny variations in lift. Keeping the aircraft on the ground allows for lift variations between zero and the full weight of the aircraft.

Sort of. If the weight on gear goes much below the normal OEW the oleo's are going to extend to full stroke and then even a fraction of an inch of upward motion will pull the wheels off the ground. The problem is the reaction time of the controls vs. the vertical speed of the aircraft. By the time you even realize you're moving up, you'll move more than 3' before you can correct back down.

Quoting Analog (Reply 14):
Don't some aircraft have On-Board Aircraft Weighing Systems (OBAWS) that display the weight on each axle?
http://www.faa.gov/library/manuals/a...A.pdf

Yes, some do. A 737 (which is what the OP started with) normally isn't one of them, as far as I know.

Tom.
 
Analog
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RE: Speed On The Ground

Mon Oct 29, 2007 4:26 am

Quoting Tdscanuck (Reply 15):
If the weight on gear goes much below the normal OEW the oleo's are going to extend to full stroke and then even a fraction of an inch of upward motion will pull the wheels off the ground.

Do the oleos follow Hooke's law (force = -constant * displacement). If so, can't they extend half way (or anything between full and none)? That gives you several feet of upward motion to play with (still small, but better than a few inches).

If any weight is on the wheels (more specifically, the front wheels), how can the aircraft leave the ground?
 
bond007
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RE: Speed On The Ground

Mon Oct 29, 2007 7:26 pm

Quoting Bufordb (Thread starter):
Assuming unlimited runway length, what is the estimated top speed of a Boeing 737 on the ground? Or any plane for that matter?

Is the belt matching the speed of the wheels ???



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tdscanuck
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RE: Speed On The Ground

Tue Oct 30, 2007 12:43 am

Quoting Analog (Reply 16):
Do the oleos follow Hooke's law (force = -constant * displacement). If so, can't they extend half way (or anything between full and none)? That gives you several feet of upward motion to play with (still small, but better than a few inches).

The oleos don't follow Hooke's law. They're primarily variable rate dampers, not springs. The travel is on the order of ~2' on a 737, which is pretty tight.

Quoting Analog (Reply 16):

If any weight is on the wheels (more specifically, the front wheels), how can the aircraft leave the ground?

It can't. However, as the weight on wheels drops the oleos extend. If they extend to full length (which can happen when you still have weight on the wheels because the oleos push down even at full extension) then even a fraction of an inch in upward motion will pull the wheels off the ground. It's kind of like the gas shock in many office chairs...if you go below the critical weight, they will extend to full stroke without further provocation.

Tom.
 
lehpron
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RE: Speed On The Ground

Thu Nov 01, 2007 10:21 am

The problem is that once an aircaft reaches a speed where the wings can lift, a pilot would have to push the control column forward to stay on the ground.

Assuming the tires don't explode due to friction heat, then the max would be just below the dynamic pressure Vmax, I'd say around 300 ktas -- it'd be the same for most subsonic airliners.
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BAE146QT
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RE: Speed On The Ground

Thu Nov 01, 2007 11:31 am

Quote:
The problem is that once an aircaft reaches a speed where the wings can lift, a pilot would have to push the control column forward to stay on the ground.

Not if the angle of attack while on the ground is negative, which is certainly the case with a 737.



EDIT: Tdscanuck pointed this out before me in a post above...

[Edited 2007-11-01 04:32:07]
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HAWK21M
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RE: Speed On The Ground

Thu Nov 01, 2007 12:56 pm

Quoting BAe146QT (Reply 20):
Not if the angle of attack while on the ground is negative, which is certainly the case with a 737

Are you talking about angle of attack or Pitch.The B737 has a Pitch of -1 degree.
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KELPkid
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RE: Speed On The Ground

Thu Nov 01, 2007 4:39 pm

Quoting Lehpron (Reply 19):
The problem is that once an aircaft reaches a speed where the wings can lift, a pilot would have to push the control column forward to stay on the ground.

Of course, you could get around that, just remove the wings  Wink
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BAE146QT
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RE: Speed On The Ground

Thu Nov 01, 2007 6:04 pm

Quoting HAWK21M (Reply 21):
Are you talking about angle of attack or Pitch.The B737 has a Pitch of -1 degree.

Well the pitch of the fuselage is important too, but I was thinking of the angle of attack of the lifting surfaces. I believe this to be either neutral or slightly negative while sitting on the ground, and that the 73X won't attempt to climb unless you pull back on the yoke or dial in some upward trim.
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tdscanuck
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RE: Speed On The Ground

Fri Nov 02, 2007 1:19 am

Quoting HAWK21M (Reply 21):

Are you talking about angle of attack or Pitch.The B737 has a Pitch of -1 degree.



Quoting BAe146QT (Reply 23):

Well the pitch of the fuselage is important too, but I was thinking of the angle of attack of the lifting surfaces. I believe this to be either neutral or slightly negative while sitting on the ground, and that the 73X won't attempt to climb unless you pull back on the yoke or dial in some upward trim.

I worked it out a while back and the wing ends up having something like -0.5 degrees angle of attack when the 737NG is sitting on the ground (pitch is about -1.5 degrees). So, in normal ground attitude with the high-lift devices stowed (which one assumes you'd do for a high-speed ground run), the wing generates "negative lift"...i.e. it pushes down a little bit. With the slats and flaps out it probably does generate some positive lift but I doubt it exceeds the weight.

The general procedure to figure this out is to know that most aircraft are designed to have effectively zero (actually, slightly positive) pitch in cruise. Since cruise speed, weight, altitude, and wing area are all known, you can work out the Cl in cruise. At high speed, pretty much all wings have a Cl-alpha of 2-pi, so you can figure out the angle of attack the wing must have in cruise, which is basically equal to the incidence angle (since the fuselage is functionally level).

Then find the ground pitch and compare to the incidence angle to get the angle of attack on ground.

Tom.

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