I don’t have much technical understanding of jet engines so don’t take it that way but I always thought that a jet engine propelled its self forward by the expansion of gas caused when fuel is ignited similarly to how a rocket propels a space ship forward. However the difference being that a rocket fuel contains its own oxidizer much like a smokeless gun powder. On the other hand the fuel of a jet engine requires oxygen from its surrounding known and that’s why they are referred to as “air breathers.” So if the fuel could be ignited without an external source of oxygen then a jet engine would still provide thrust in a vaccum. The wings provide lift being shaped to cause a higher pressure below as air is moved over them.
So if that’s true and I’m not sure it is, then thrust isn’t really determined by air pressure or density for a jet engine neccesarly right? It’s more that the jet engine must only be able to extract enough oxygen from the atmosphere to the ignite the fuel. So if that’s correct wouldn’t a jet engine produce the same amount of thrust all the way up to the minimum oxygen density ceiling and then just stop working? If the drag is reduced with altitude then this would explain why a Jet is more efficient the higher you go.
Unlike a jet engine that uses the expansion of fuel to create the gases that provide thrust, a prop driven motor like on the Mavic produces lift by creating a higher pressure environment below it and a lower pressure environment above it thereby “floating” in its actual environment. Much like a ballon that is released underwater It’s not so much that it is expelling gas to provide thrust but rather the DIFFERENCE in density it creates. The thrust and the lift vectors are the same and indistinguishable.
So at lower air pressure wouldn’t it require the same amount of work to create this difference in pressure (lift/thrust) up to a certain point where there is no longer enough air for the difference in air pressure to be enough to float in anymore? Like how the balloon reaches the surface of the water and stops?
Again this may all be totally wrong but wanted to run it by you guys and figure out where I’ve gone wrong.
Thanks guys!
The drone does not actually create a higher and lower air pressure environment, like you may have in your minds eye, of how it is flying. It is the props that do the work. The prop blade is shaped in a similar fashion to a wing. Without going into too much detail, lets take too wing shapes, that is cross sections of them, if you were to cut a piece off and look at its thickness.
So, we shall look at a fat airfoil shape and a thin shape. The thin shape is a jet airliner, for argument sake and the fat one would be, say, an old Piper Cherokee 140. The fatter shaped wing cross section has more length of metal on the top surface, that the bottom surface. The jetliner does too but at a much lower ratio, so just a little bit more on top, compared the bottom surface. Because the top surface is curved and the lower surface is straight (keeping this shape thing simple, for explanation purposes) when air hits the leading edge and is forced to split itself up, the air going over the top has a greater distance to travel, than the air flowing underneath.
Air does not split itself up and stay divided or there would be a vacuum created at the trailing edge. Therefore the air travelling over the top surface of each wing must speed up in order to catch up to the air flowing under the wing sections. The faster the air travels on top, due to a thicker, or fatter shaped wing, the lower the air pressure created. The lower the pressure on top, the greater the lift will be on the wing. Now a fat shaped wing will be much more draggy too, thereby being less aerodynamic, so not as fast through the air. With that said, it does generate greater lift, so can get off the ground more quickly, but is less efficient, the faster it travels.
A Jetliner has thin wings, so are designed for speed but do not have as much lift generating design to them at low speeds, and will not get off the ground as quickly, meaning it would need an awful lot of runway to lift off, if left like that. To help with this, the jet wing will have slats and flaps built into them, which extend out the front and the rear of the wing on take off and landing. What these two pieces of metal do, is fool the air into thinking this is actually a fatter wing shape, so it creates greater lift, when configured this way at lower airspeeds. That way it gets off the ground sooner and can slow down more for landing.
Once the jet lifts off and continues to increase its airspeed, those slats and flaps are brought back into the wing, thereby changing the shape back into the fast and streamlined shape it was designed to be, becoming more efficient but requiring greater speed to work best. This being ideal for highspeed flight. A prop blade is the same thing as a wing, just tiny little thin wings. So when they spin, they also create the high and low pressure regions of air, right above and below each blade. As they spool up, the increased speed creates the increased lift, so it climbs or goes forward faster and faster.
If you want to see how high and low pressure works right in front of your eyes, just cut a strip of paper from a standard 8.5x11 inch sheet. Cut it so that it is about an inch or two wide and about 8 inches in length. Hold the thin end with two hands an place it just under your bottom lip, letting the rest of the length just droop down over your chin. Now take a deep breath and start to gently blow over the top of this piece of paper, increasing as you go. As you blow harder you will see the paper begin to rise in front of you, because your blowing of the air on top is creating the lower air pressure, compared to the air below that strip of paper. Therefor that whole piece of paper will act like a wing or a prop blade and rise up to a level position as you blow harder.
I hope that is all understandable for those who were not aware of these basic principles.