True up to a point. But here’s a couple of radically different situations to consider:
A steel ball is flying through space. You switch on an electromagnet, brake the ball to a stop, and send it back in the opposite direction. You had to spend energy to do this, yes?
A steel ball is flying through space. It bounces off an enormous steel wall (which it moves by an insignificant amount) and rebounds back the way it came and at nearly the same speed in the opposite direction. You had to spend no energy to do this. Instead the energy of the ball itself was used to provide the motive power.
The rocket is more like the first case, the plane is more like the second.
The problem is that in any real world process (and even in theoretical thermodynamic cycles) there are always energy loses that do not contribute to the primary operation. In rockets, for instance, there is the post-nozzle thermal expansion and lateral components of motion that of propellant that don’t contribute to vehicle momentum change. The higher v[sub]e[/sub] is, the higher the vehicle specific impulse is, but usually at a tradeoff to power efficiency, so either you carry a lot of propellant (low I[sub]sp[/sub]) or a carry a very powerful energy source (high I[sub]sp*) like a nuclear fission reactor.
Of course, if you add up the energy in the entire system (including propellant kinetic energy, thermal heat energy, et cetera) along with the change in V of the rocket or airplane, the total energy remains the same. But the efficiencies can be wildly different.
Wouldn’t it be more efficient to a) execute a roll until the plane is upside down, b) pull back on the stick and loop downward until you’re at a lower altitude and rightside up again, but now going the opposite direction from how you started? Isn’t that more efficient than fighting gravity by looping upwards? Granted, you’d still need to ascend back to your original altitude.
That “granted” at the end is the killer, so far as any hopes of efficiency gains are concerned. Otherwise, both are valid manoeuvres but if you start off going down, you’re beginning your downward step at high speed, so you need more room to execute - and you’re losing a lot of altitude, and kissing energy goodbye all the time you’re doing it. (Doing anything with the nose except keeping it pointed at the horizon costs energy, and the faster you’re going the more energy you’re wasting.)
Plus the g-forces thing, which guarantees that you can’t make your half-loop all that tight ('cos the faster you’re going, the more gee you have to pull to tighten up the loop).
I guess I meant ‘efficiency’ at turning 180 degrees, not total energy. Sorry for being unclear.
As far as speed, throttle all the way back, open the flaps, and shed airspeed; put down the landing gear if you must. You’re diving at the ground, speed won’t be a problem. My flight sim lets me do this, and it works like a charm, so I’m sure this can be done in your average aircraft, right?
Yes, if you just want to turn around in a hurry, that’s fine. It’s just that if you were hoping not to waste energy, bumping up your drag coefficient like that wasn’t the way to go about it.
G-forces actually work for you. The more g you pull, the more drag the aircraft has and the less you’ll speed up. So rather than deploying flap and gear and closing the throttle, you can just pull the nose up a bit to wash off some speed then roll inverted and pull through at about 4 gs. The speed will be maintained within limits and the half loop will be reasonably tight. It is safer to the other way though, loop up then roll off the top.
Dead right. Again, it’s a means of turning speed into height, which you can then turn back into speed, rather than just throwing speed away. And yet another means to the same end is the stall turn - pull up into a vertical climb, and as your speed drops close to the stall, kick the tail around with the rudder until you are facing straight down, drop back down and pull up to the horizontal. Another way of reversing course that depends on lift and gravity, and which isn’t available to the astronaut in his rocket.
The major difference between turning a space vehicle and an aircraft is that you have to bring the space vehicle to a complete stop first, then accelerate back to the original speed in the opposite direction. An aircraft simply changes the direction it is pointed, which takes a lot less energy.
hdc_bst’s analysis is correct, but assumes that the aircraft stops and restarts again. Malacandra is closer to the mark, but even in the bounce case, there is a significant transfer of energy to and from the surface of the bounce.
The aircraft has to continue to move “forward” to maintain enough speed for lift, so the turning process is done in such a way that the actual momentum doesn’t change much, except for direction. In a space vehicle, you could gradually turn the vehicle so that it’s pointing in the opposite direction of its movement, but that doesn’t alter the movement itself, only the orientation of the vehicle. Changing the orientation of the aircraft also changes the direction of travel.
In the vertical part of this maneuver, the aircraft is ballistic, the wings are completely unloaded, and therefore stall speed is zero. When the maneuver is done with the turn at zero speed, it is known as a hammer-head turn. Prop wash over the tail allows some rudder authority even though the airspeed is zero. There is usually some trouble keeping the airplane from rolling due to engine torque, as the ailerons become ineffective. Thus it usual to cheat, and kick the rudder a little early.
I think you are all making this more complex than it needs to be.
A rocket coasting through space can turn or reverse direction by expending its own rocket energy or by using the force of a massive body like is often done. A rocket can be made to swing past a planet and change or even reverse direction of travel. The earth is constantly changing its direction of travel and spends no energy in the process. In only six months we’ll be going in the opposite direction as we are traveling now. But if a body has no force from outside, gravitational, electromagnetic, mechanical or otherwise then it must rely on ejecting some of its own mass.
A train on tracks will only stop because of friction but if there were no friction it would continue to move and it could turn only because the rails push it to one or the other side. That force is compensated by the mass of the earth. If the train was coasting at 80 MPH going east and the tracks have turned it so that it is now going west then the earth has also experienced a miniscule change in its rotational speed.
The airplane is just like a train on tracks. It does have something external with mass to hang on to: air
Turning an airplane definitely consumes energy. Ask a fighter pilot. Energy management in a dogfight is a big deal - the more you turn, the more energy you lose, which can give your opponent an advantage.
A sailplane pilot experiences this more intimately as well. without an engine to add energy back, sailplane pilots learn that to turn means to lose energy, and therefore to lose altitude. They have to turn a lot to stay in thermals, but they learn to do it smoothly, at the right speed for minimum energy loss, and to stay coordinated in the turn for maximum efficiency.
Remember, lift causes drag. When you move the lift vector out of the vertical to affect a turn, you are increasing drag (or losing altitude if you don’t increase lift to keep the airplane level, thus giving up potential energy). If you increase the load factor, the airplane will begin to slow down, costing kinetic energy. Either way, you’re losing energy.
To summarify my post even further: the [only] way for a body in motion to change direction is to create a force which pushes (or pulls) against something. If you have a large mass you can use (the Earth, a planet, etc) then you just push or pull (using gravity, of mechanical link if possible). But if you are in the deep vacuum of space, happily (or unhappily) coasting along and have no way to exert force to any large mass outside then the only way to change direction is to exert force against some of your own expendable mass. The basics are the same in both cases. Moment is conserved etc.
All true, and so is what Sam Stone said. But the bottom line is, an aeroplane can reverse without any additional energy input, though it will lose some part of its energy in the process owing to inefficiencies. If this were not so then a glider could not reverse direction at all. A spacecraft without a nearby mass to curve space for it cannot reverse direction without spending at least as much energy as it took to get up to its present speed, plus an additional amount to head off in the new direction.
That’s a strange way to look at it. An airplane can turn without using its engine because it has stored up kinetic and potential energy it can use. A rocket has stored up chemical energy you can use.
Either way, if you want to turn, you have to use energy. If you maintain a constant load factor in an airplane and commence a turn, you’ll be shocked at how much altitude you lose. If you fly an airplane, try it sometime. Start your turn wthout pulling back on the yoke, and using just enough rudder to keep the turn coordinated. You’ll be in a spiral dive before you know it. If you load up the airplane to maintain altitude, your airspeed will drop rapidly. To prevent that, you burn more fuel in your engine.
I can’t see why it’s a strange way to look at it. The whole question and difference is that both an airplane and an interplanetary rocket have kinetic and potential energy stored but the rocket cannot use it in the same way an airplane can and the difference is that the airplane has something to hold on to while the rocket does not.
Put a person on a sliding mat and propel him across an ice rink and they can not stop or change direction.
Now place a pole in the middle of the ice rink and the next person can turn direction by grabbing the pole.
