This may have been said further up. I didn't read all posts.

If the aircraft will go vertical, the power/thrust:weight ratio is 1:1 or greater. Many aircraft will go vertical but if it is to be sustained vertical the power:weight ratio must be greater than 1:1. I've seen model aircraft held vertical at very low levels with no forward speed, and that is a thrust:weight ration of 1:1 being demonstrated very dramatically.

The reason an aircraft with a power/thrust:weight ratio of 1:1 or greater can't keep going vertical is because the engine loses efficiency the higher it gets.

The principle remains the same for an aircraft climbing vertical after gaining speed in a dive though. Sooner or later, gravity will take over, and the climb can't be sustained

This is really evident with a fighter aircraft with a thrust:weight of much greater than 1:1. It can go vertical till the engine runs out of air, but it continues vertical because of the intertia it has, and then the controls start to lose effectiveness as well and it tumbles out of control, but still going up, until gravity takes over, i.e., inertia runs out, and it falls back toward earth under the pull of gravity. On the way down, first the controls begin to function as the air becomes thicker and the engine will restart some time later. The altitude at which the engine restarts will depend on engine design.

This is a simplistic explanation. When the aircraft is going vertically, the drags (skin, aerodynamic, induced, etc.) are acting in the same direction as weight, directly down and add to the demand on the engine, so to remain vertical, in reality the aircraft must have a power/thrust:weight ratio of >>1:1.

When the model aircraft leaves the vertical and no forward speed position and accelerates upwards, a greater level of lift is generated which is at right angles to the wing surface, and parallel to the ground. An elevator input is required to counter this and that creates drag across the airframe. If a down elevator input is not made, the aircraft will tend to 'climb horizontally', i.e., it will move across the ground whilst travelling vertically. The faster it accelerates, the greater input is required and the greater the drag generated. This contributes to the weight which is pulling the aircraft vertically downward.

Only slightly related to this topic is that a jet transport aircraft, with the engines operating at as high an efficiency as can achieved at sea level (probably not much over 80%), will not go much faster than about 350 knots in level flight. Once again this is simplistic because there are other things like wing design that come into the equation. However, at 40,000', with the same engines operating at efficiencies as low as 25%, it will reach almost 500 knots in level flight, but at fuel flows about a quarter of those required to fly at 350 knots at sea level. This is an indication of the density of the air at that altitude and the lack of resistance it offers to the progress of the airframe.

If the jet engine had not been invented by Frank Whittle, aviation would probably never have progressed beyond the big radial piston engined aircraft like Constellations.

The only other information I can offer is that the carburettors on model engines aren't very sophisticated and can't cope with huge changes in altitude and the engines run out of grunt sooner than we might expect. I'm only guessing on this though, and may be wide of the mark.

So ultimately, the answer to the question is gravity, but other things contribute.

I think it's important to note that the power:weight ratio does not tell you the thrust:weight ratio. In order to sustain vertical flight you need: (Thrust-Drag)>Weight. It is possible for an aircraft with a power:weight of less than one to be able sustain vertical flight (and for one with a power:weight of more than one to be unable).

It's also worth noting that because power and weight do not share the same units, you can only make a meaningful comparison with an implied reference velocity. I don't think it means much to say that an aircraft has power:weight>1. It certainly doesn't tell you if it can sustain vertical flight.

You're quite right Shoe, and that's why I used power/thrust. Not everybody is familiar with the what some might call the subtle difference, although it isn't all that subtle. I intentionally didn't intend going too deep for a couple of resons; First is that it's very complex and takes a lot of space and time, and second is that I don't know it all and may well knot myself up. I probably did know it many years ago, but it's not terrribly important in day to day flying of airline category aircraft. Not too much vertical flight is done in 737s/747s etc. I would have used a slightly different equation, and that is thrust > weight + drag, because the drag and weight vector resultant points vertically downwards, but it means the same thing.

gravity has nothing to do with it mother earth just sucks harder

There`s a little more to it that we hadn`t considered, and that`s propeller efficiency. The pitch of a fixed pitch prop is set for optimum efficiency at a predetermined speed, and guessing, that would be about 30-40 mph I`d say for a not too high performance model.

After the model starts to climb, more elevator down input is required to counter the lift which is horizontal and parallel to the ground. That creates more drag as I mentioned above. The propeller also creates its own turbulence and therefore drag and starts to lose efficiency, spinning in turbulent air. See the discussions elsewhere re three and four bladed props versus two blades.

As the aircraft slows, less lift is generated, so less elevator input is required, but by tha time, the aircraft is on the back of the lift/drag curve, and is unable to accelerate out of it.

Once again, this is probably a simplistic explanation, but is essentially what happens.

the simple answer is the sports hall roof...

Another thought might be the throttle curve of the engine/servo/transmitter. If your plane is hovering at 1/3rd stick, that could possibly be around 50~70% of your power from your engine. Your plane could be heavier or your engine is not putting out the power you expect.

Just a thought.

It has been my experience that vertical flight, when going down, is always stopped by the ground.

NASA has recently discovered the "EMO quark", which is a particle type that actually conveys an "emotion" of sorts by increasing it's own mass and thus exerting more gravitational pull, on those objects it "likes". What is happening is that the ugly-but-overpowered planes do not seem to be affected by the EMO quark, but the more expensive and beautiful composite, scale, jet, etc., planes have a stronger EMO effect. That is, the EMO quark "misses" them as they climb higher, and increases it's own mass, to pull the planes back down. Emperical testing proves that ugly planes last longer than beauties, and now we know the reason.

By applying the following formula, you can predict some of the outcomes:

B = "Beauty" (Interchangeable with C (cost)) - 1 is low, 10 is high
E = EMO quark effect, in standard EMO units, equating to a "G multiplier"
Thrust, lift, drag, etc., are no factor, since the EMO quark overrides them.

So, the formula, which gives the results in terms of G units that the aircraft feels, is:


B(or C)*E=G (Gs perceived by the aircraft)

As of May, 2003 (in the 8/03 Model Aviation mag I happened to pick up to look at) the F3A (Aeroplane, piston motor) NE143 Height Record, held by Maynard Hill, was 8205 meters, set 09/06/70. I calculate 26,919 feet, 5.091 miles. As I recall, the Navy loaned a rangefinder to track the flight.

I suppose that is where vertical flight stops.

Jim

As of 2004, vertical flight in the US stops when the plane is not visible to unaided vision...

Not sure if this has been said, but the reason planes and spaces shuttles move forward is because of the matter the push in the opposite direction. As the air gets thinner at higher altitudes prop planes and jets are no longer pushing as much matter backwards with the prop or fan spinning at the same speeds. When in space, space shuttles have boosters that affect the pitch, yaw, and speed of the aircraft. The control surfaces no longer have any effect on the aircraft when there is no more matter

4.1 Definitions
The main purpose of this chapter is to clarify the concepts of lift, drag, thrust, and weight. Pilot books call these the four forces.

It is not necessary for pilots to have a super-precise understanding of the four forces. The concept of energy (discussed in chapter 1) is considerably more important. In the cockpit (especially in critical situations like final approach) I think about the energy budget a lot, and think about forces hardly at all. Still, there are a few situations that can be usefully discussed in terms of forces, so we might as well learn the terminology.

The relative wind acting on the airplane produces a certain amount of force which is called (unsurprisingly) the total aerodynamic force. This force can be resolved into components, called lift and drag.


Lift is the component of aerodynamic force perpendicular to the relative wind.
Drag is the component of aerodynamic force parallel to the relative wind.
Weight is the force directed downward from the center of mass of the airplane towards the center of the earth. It is proportional to the mass of the airplane times the strength of the gravitational field.
Thrust is the force produced by the engine. It is directed forward along the axis of the engine (which is usually more or less parallel to the long axis of the airplane).
These are the official definitions.

Figure 4.1 shows the orientation of the four forces when the airplane in ``slow flight'' — descending with a nose-high attitude, with the engine producing some power. Similarly, figure 4.2 shows the four forces when airplane in a high-speed descent. The angle of attack is much lower, which is consistent with the higher airspeed. Finally, figure 4.3 shows the four forces when the airplane is in a climb. The angle of attack, the lift, and the drag have the same magnitude as in figure 4.2.



Figure 4.1: The Four Forces — Low Speed Descent



Figure 4.2: The Four Forces — High Speed Dive



Figure 4.3: The Four Forces — Climb

Note that the four forces are defined with respect to three different coordinate systems: lift and drag are defined relative to the wind, gravity is defined relative to the earth, and thrust is defined relative to the orientation of the airplane. In level flight these coordinate systems usually don't differ too much, but in figure 4.1 you can see the difference.

This situation seems a little complicated, and it is: for instance, thrust, lift and drag all have vertical components that combine to oppose the weight; similarly the thrust and lift both have forward horizontal components.

In ordinary cruising flight, the situation is simpler. When all three coordinate systems coincide, lift must balance weight and thrust must balance drag.

Interesting site http://www.lerc.nasa.gov/WWW/K-12/airplane/short.html .

Nope!
If you're standing on a ladder 100 miles tall, at the top, your scale will still show *** pounds.
You have to -moving- at a speed which has you going around the earth, to get weightless.
It's the centripetal force that nullifies the gravitational force, not the altitude it self.

If you're standing on a ladder 100 miles tall you better be able to hold your breath a REAAAAAAAL long time, otherwise you are dead.