Semi Retired Aviator
Posts: 484
Joined: 9/20/2003
From: Melbourne Victoria, AUSTRALIA
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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.
< Message edited by JapanFlyer -- 12/29/2003 4:26:14 AM >
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RE: What stops vertical flight? - 
RE: What stops vertical flight? - 
