Farsight
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Silly question

Fri Jun 28, 2019 4:21 am

Here goes:

Forgetting tire limits and runway length, what do we reckon would be the maximum speed of a commercial turbofan powered plane on the runway if it was to never liftoff? I randomly wandered this the other day, did a google search but to no avail.

There must be a maximum speed at which engine thrust can’t overpower drag. Any estimates?
 
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Starlionblue
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Re: Silly question

Fri Jun 28, 2019 7:23 am

For this exercise, it might be useful to think of the aircraft simply flying very low, e.g. 100 feet AGL.

High subsonic would be my guess. That's where drag starts increasing more rapidly.
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RetiredWeasel
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Re: Silly question

Fri Jun 28, 2019 2:33 pm

As Starlionblue stated, your fantasy would have to assume the landing gear was raised, or it would never attain anything close to supersonic..or the gear/doors would rip off.

Certainly not a commercial plane, but a turbo powered vehicle: ThrustSSC supersonic car which set the record in 1997. ("1,228 km/h (763 mph) and became the first land vehicle to officially break the sound barrier") from Wiki: https://en.wikipedia.org/wiki/ThrustSSC.
 
mxaxai
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Re: Silly question

Fri Jun 28, 2019 4:54 pm

RetiredWeasel wrote:
As Starlionblue stated, your fantasy would have to assume the landing gear was raised, or it would never attain anything close to supersonic..or the gear/doors would rip off.

Certainly not a commercial plane, but a turbo powered vehicle: ThrustSSC supersonic car which set the record in 1997. ("1,228 km/h (763 mph) and became the first land vehicle to officially break the sound barrier") from Wiki: https://en.wikipedia.org/wiki/ThrustSSC.

Just for clarification: No current commercial aircraft can go supersonic in level flight. Commercial aircraft engines all feature convergent nozzles, which have a maximum exhaust velocity of Mach 1 (or less). As jet engines stop producing net thrust once the aircraft speed matches the exhaust speed, this puts a definitive limit on your top speed.

Also, airliners typically have a Vmo of ~ 340 - 370 knots, which is well below Mach 1. Vmo is set by structural limits, though, so the aircraft could probably go faster when limited by thrust & drag alone.
 
spacecadet
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Re: Silly question

Sat Jun 29, 2019 2:39 am

I'm not sure that continued acceleration beyond a certain speed without liftoff would actually be possible. I'd have to believe that the actual maximum speed of this exercise would be limited by things other than thrust and drag. A plane's wings generate lift proportional to the relative wind. To a certain point, you can keep a plane on the ground by pushing the nose forward, but in smaller planes, at least, this pretty quickly becomes unsafe and/or impossible due to ground effect. The basic aerodynamics of larger planes is the same so I'd assume it'd be the same in an airliner.

If what you really want to know is the max speed of an airliner at sea level, I think you'd have to actually take off and get out of ground effect to find that out. If it's really about max speed on the ground, I think the limit would be determined by either the speed the plane lifts off on its own, or the pilot risks an accident by forcing it on the ground.
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fr8mech
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Re: Silly question

Sat Jun 29, 2019 3:09 am

I think the OP was also looking to disregard lift in his scenario.

Basically, how fast would an aircraft travel down the "road" given a) tires/under-carriage don't fail, b) there is infinite runway and c) there is no lift produced.

Is there a treadmill involved?
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mxaxai
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Re: Silly question

Sat Jun 29, 2019 9:11 am

Well, lift depends on the angle of attack, which depends on the angle of incidence as long as all wheels touch the ground. If an aircraft's fuselage slopes downward on the ground (e. g. the A330), the angle of attack could possibly be just right to generate zero lift while on the ground.

Alternatively, you could deploy spoilers do destroy most of your lift but the added drag probably means that you can't go very fast.
 
Farsight
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Re: Silly question

Sun Jun 30, 2019 12:22 am

Thankyou for the responses, I feel slightly more satisfied now...

So perhaps 450ish MPH?

If indeed we did add some downward elevator deflection, is it possible the tail would rise up off the ground at high enough speed? Pushing the nosewheel into the ground?
 
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Starlionblue
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Re: Silly question

Sun Jun 30, 2019 12:33 am

Farsight wrote:
Thankyou for the responses, I feel slightly more satisfied now...

So perhaps 450ish MPH?

If indeed we did add some downward elevator deflection, is it possible the tail would rise up off the ground at high enough speed? Pushing the nosewheel into the ground?


It is possible the tail would rise, but unlikely. The CG is rather far back from the nose gear. Also, I'd use the stabiliser instead of the elevators. More power and less drag.

I think you might get up to 500 knots. But that's purely a WAG.

Also... Statute miles? ;)
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Farsight
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Re: Silly question

Sun Jun 30, 2019 1:01 am

Yes, very statutish. Shame we will never get to test this out. I don’t think mythbusters have the budget.

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