Why can planes travel faster than trains (or automobiles)?
At a quick glance, airplanes expend a lot of energy just staying aloft. You would think trains or some other kind of wheeled vehicle could easily beat them, but this is not true.
My guess would be air density disparities. Drag being the square of the velocity.
Maybe rolling friction? But I would assume steel wheels on a steel track do not present much rolling friction.
I thought this would be an interesting discussion.
Pretty hard to keep a train on a curved track when you’ve got a jet engine strapped to the back. Theoretically, there’s no reason you can’t build tracks and trains sturdy enough to deal with those kinds of forces, but dealing with 500mph vehicles at ground level can be a hassle. You’re also going to spend a tremendous amount of energy accelerating a vehicle which weighs hundreds to thousands of times as much as the biggest airplane to that kind of speed. Planes are good at delivering a little stuff very fast. Trains specialize in delivering a whole lot of stuff slowly but efficiently.
All other things being equal (which they’re not of course), air is denser on the ground vs. 35,000 feet up, which means more drag and thus lower speed. On the other hand, build a vacuum tunnel for your train, and that problem magically goes away, and it could end up going Mach 5+.
At some point you’ll end up with structural problems related to wheels spinning at very high RPM’s, and addional problems related to the smoothness of the track; small bumps create BIG impact forces when encountered at high speed.
High speed at low altitude, as you note, requires lots of power, even with a very aerodynamic shape. At some point there’s not enough traction force available at the wheel-ground interface and you have to resort to other methods of driving the vehicle forward. Famous examples include the rocket-powered Blue Flame, and later the jet-powered ThrustSSC. Note the size of the engines in relation to the vehicle.
Modern commercial airliners cruise at 550-600 MPH at altitudes of 35,000-40,000 feet.
The Concorde was able to cruise at 1300+ MPH, but at altitudes of around 60,000 feet.
The SR-71 was able to cruise at 2000+ MPH, but at altitudes around 80,000 feet.
One other issue is noise. You don’t hear airplanes cruising at 40,000 feet. a high-speed train, OTOH, makes quite a racket a ground level; if you were able to drive one at, say, 600 MPH, it would be really noisy, causing major problems for anyone living anywhere near the track.
Trains operating in vacuum tubes have been proposed, but the expense makes them seem unlikely.
Is it really true that airplanes exert energy to stay aloft?
Obviously, they need a certain airspeed to make the wings generate sufficient lift, but it’s my understanding that this lift is “free” when you’re already exerting the energy necessary to go at very high speeds. If my understanding is correct, a plane at level flight is not exerting any energy to maintain its altitude, merely to maintain its speed.
Actually, not so much. Induced drag (the drag associated with producing lift) is proportional to the inverse square of the airspeed - double your speed and that part of the total drag goes down by 75%.
It takes power to stay aloft. It is called induced drag. Arguments will likely ensue, but you essentially generate lift by using the wings to keep flinging air downward, and it takes power to do that. You can fling a lot of air mass downward slowly with a long wingspan, or a lesser amount down quickly with a short span or biplane, which is less efficient. The faster the airplane travels the more air it interacts with, so the wing becomes more efficient, as the greater air mass can be flung downward more slowly and still generate the needed lift.
It is true that most aircraft at cruise suffer MORE drag from sources other than lift-related induced drag, but induced drag never drops to zero. As a rule of thumb, an aircraft’s Vmax L/D (best glide speed) is when induced drag is equal to that from other sources, and cruise speed is normally quite a bit higher than Vmax L/D.
A fully loaded 747 weighs about 500 tons. The weight of a train is going to be a lot more variable but 5000 tons would be a reasonable average and 15000 tons would not be that rare.
Trains don’t have to be heavy, and I don’t think anyone suggests that if we cut a train down by a factor of 10 that it can go 747 speeds.
I guess another variation of my question is “Why did we have to revert to flying just go to fast?” Everyone knows that going fast and flying in the air are both neat, but the connection between them is not blatently obvious.
From the answers so far, it appears that the lesser air drag at higher altitudes allows faster speeds. If you want to go fast without having to be high in the air (with all of its associated difficulties) then you will have to make a tunnel with thin air in it.
To keep the train from overturning, the curves need to be banked, yet they must not be banked so steeply that the train can’t stop on the curve. Regardless of bank, the train pushes the track toward the outside of the curve, and this is related to the square of the speed. The track must be anchored solidly enough to resist this.
This creates a pretty severe radius/speed limit for trains.
Airplanes can bank to whatever degree is needed, (within structural G-limits) and there are few limits to how large a radius they can use when turning.
Beyond this, even if the track were a straight line, in order to stay centered on the track, the train’s wheels must not slip on the track, so the track must be very smooth (under load) and the suspension very good so that the wheels never “skip” over what bumps remain. Suspension that does this tends to create dynamic stability issues, and dealing with THAT is the biggest trick to making high speed trains work.
To tie a couple of loose-ish ends together …
At airliner cruise altitudes, the air is a bunch less dense. While going ~500 mph the “airspeed” is only about 300 mph. That means the measured impact pressure is the same as 300 mph at ground level. In other words, while going 500 mph in cruise we’re experiencing the same drag force as a 300 mph train would at ground level. Given that drag goes up at the square of the air speed, that 40% discount on effective speed is a big help.
You’re not going to get trains much faster than we have now until they run in a reduced pressure tunnel. Which isn’t going to happen for a host of reasons, mostly economic.
Is there also additional aerodynamic drag from the interaction with the ground? (“Ground effect” is probably not the right term, but something akin to that?)
For comparison, modern “bullet trains” can travel at 150-200 MPH, and maglev trains have reached speeds of over 350 MPH.
Pretty damn fast considering all the abovementioned obstacles to high-speed ground transport.
It’s not just the difference in air density, since you see the same thing in animals, even a few feet above the ground. The slowest birds are about as fast as the fastest land animals.
That seems… wrong. Every kid knows that cheetahs can hit 70 mph. According to this, that puts it in the top echelons of birds in level flight.
I guess if you use sustained speed, though (say, time taken to travel 1000 miles) birds are going to be substantially faster.
I wonder how much damage a supersonic train would do to any towns it passed through.
If it was running through the forementioned vacuum tubes, none, as there would be no air to transmit the sound/shock waves of its passing.
Actually the French TGV, which is a conventional railed train, has done 357 mph!
I wonder what it could do at a theoretical 30,000ft altitude?
Since we’re comparing trains to planes, here, it seems unreasonable to insist that the train be safer than the planes. If a plane loses power, it (probably) crashes. You could have faster trains/tighter turns if you didn’t require that a train that lost power didn’t also crash.
Obviously, there are tradeoffs to be made between safety and efficiency, but it only seems fair to choose them as close to the same when comparing the two technologies.
Ground effect is the correct term for what happens to a plane when it’s within about a wingspan of the ground (or ocean). But it results in a reduction in (induced) drag, not an increase.
This would not apply to a train - only to a vehicle with wings generating lift. It is of course inportant to an Ekranoplan.