What are the props actually doing
- Thread starter Bathy
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ianwood
Taco Wrangler
ProfessorStein said:
Not yet. But check out quantum compass. It's a theory that you can use sub-atomic fluctuations to measure position changes. It would be accurate to the millimeter if not less and would make GPS obsolete.
Also note that GPS satellites move through time more slowly than people on the ground do (Einstein's special relatively, I think it is) and their clocks have to be adjusted for it.
If the downward air from the props had nothing to push against, there can be no lift. The shape of the props make them efficient but the props could be flat and still provide lift or thrust. It's the angle of attack to relative air. An airplane could fly with flat wings with no airfoil shape . The angle of attack provides the lift. Wouldn't fly very well though. I do have a bit of a clue since I have messed with aircraft all my life, got my pilots license in 1985. Been building and flying rc aircraft for the better part of 30 years.
Depends on what you call "flying", Monte. Just moving through the air off the ground for any length of time? Then sure. But if you're talking about sustained flight, powered or unpowered, you need an airfoil. The angle of attack establishes how much slower the air above the wing moves than the air below, and thus the amount of the pressure differencial between the two... and, thus, the amount of lift. A flat wing will provide very minimal lift... and if a flat wing aircraft is flying, it's more the innertia created by the engine powering the craft than any sort of lift created by the wing.
Monte55 said:
im not arguing they could be flat with no airfoil design and still provide lift (very poor and inefficient but it would still be there). But if your theory that its the air pushing against something would be true then there would be more lift at ground level and none once in flight. And this is the point at which you make the argument that the air from the props is pushing on the air beneath the props and holding its self up that way. But air moves like a fluid and once you start moving it it continues to move. So in your theory the air below the quad would be continually pushed down and because it is being pushed down the new air being pushed down from the props now has no resistance to push against thus causing a vortex that would drop the quad out of the sky like a rock.
But in reality it is the lack of air above the props or vacuum that sucks or lifts the quad up. Hence the term "lift". The term "thrust" is actually better suited to jet engines where you are forcing air through and because those jet engines are attached to a wing that causes lift the jet airplane can fly. This is the reason there are jet drag cars with TONS of thrust but no lift and they stay on the ground
Monte55 said:
Actually....
lol they're absolutely right. When the propellors push air from above them to below them, that creates low pressure above them...not a complete vacuum, but the pressure difference is the magic.
poor Bathy....your brain never stood a chance LOL. Sorry for the super in-depth conversation. But essentially the props work because the difference in air pressure above vs. below and as air is moved downward there is a low air pressure region formed just above the prop. This causes the prop to be pulled up into that region more providing lift and picking the quad up into the air. when the blades spin faster a greater pressure difference is caused making the quad go up faster. When they spin slower there is less pressure difference and not enough lift for the weight of the quad to overcome gravity thus causing it to descend. its more about what happens on top of the prop than what happens under it. to more left the left motors decrease in speed and that side of the quad dips slightly because that side of the quad can no longer overcome gravity and it causes it go left. It gets more complicated from there on out...but i hope this helps you understand it???Bathy said:
chuddly said:
Eh. There is lift on those rocket cars, simply from the body. Most will have an inverted airfoil to attempt to push them downward into the ground. I used to think that's why they called them "drag cars"... but I dont think that's true. But it IS why even those crazy petrol drag racers are so high in the back (that, and to use larger wheels to increase torque to the track). The trick is to have just the right amount of downward thrust so it doesn't add TOO MUCH friction and drag to the wheels, unduly slowing you forward progress.
But some of those rocket cars... if they hit so much as a stone, they pop off the ground and go flying. Again... most of that is the innertia created by the engine, though.
ProfessorStein said:
I get what your saying and yes they do use downward force via spoilers/wings/airfoil designs to create downward force. but that is because they can not set the body directly on the ground and the compressed air under the car would (and does on occasion) lift the car up. And when it does lift that car up is when you see the car become the new airfoil and the pressure under the car (now facing forward) become greater than the pressure on top of the car (now facing backward) and it turns the car over backwards. To add to that the natural rotation of the tires is helping to lift the front of the car thus the need for wheelie bars. but this is all off topic LOL
actually i have...against a 4 wheeler....a dog ...and my kids LOL I won all the races. The Phantom is undefeated
I think what people don't understand is the term SUCTION. It's just a word we use. It doesn't really exist, like a vacuum. What lifts the quad is the higher air pressure below the prop as opposed to the lower pressure on top. When this pressure is great enough, we get thrust or lift. In order to get higher pressure below the prop, the air has to be compressed somewhat. The air on top has pressure also but not as great as the bottom. The higher pressure on bottom of prop pushes up as in the wing of a plane. If we are close to the ground and in ground effect it is easier to compress the air below so it takes less power to achieve the same lift and a given rotor speed. Quads and helis are pushed up ...not sucked up. A flat blade at 0 angle of attack generates no lift but a blade with an airfoil shape will at 0 . If you increase the angle you'll get more lift until you reach the max angle for the design and it begins to stall or the drag is greater than the lift.
Monte55 said:
I agree that it's the terminology that causes a lot of the confusion. It really doesn't matter which side of the pressure difference we use to describe it (pull/push), it's still the same net effect and for the same reason.
Well, it does help to understand. Let's take a vacuum cleaner. It doesn't suck dirt in, the outside of hose pressure pushes it in. The vacuum motor only reduces pressure inside the vac and the much greater outside pressure takes over. It's like using a vacuum pump on a closed vessel. You don't suck anything out. If you lower the pressure outside the internal will push itself out. No wonder we can't achieve perfect vacuums. There is no pressure to push the last molecule of air out. Imagine a room with 50 guys in it. This is the vessel. Now start the vacuum pump...each man pushing on one another as they leave the vessel. Who will push the last guy out?
ianwood
Taco Wrangler
According to NASA, lower pressure on the upper surface is not the primary source of lift and AoA and deflection of air downward is the primary source of lift.
http://www.grc.nasa.gov/WWW/k-12/airplane/wrong1.html
http://www.grc.nasa.gov/WWW/k-12/airplane/wrong1.html
ianwood said:
Ummmm... while that page is very interesting, and, indeed, somewhat disproves the Bernoulli's effect, altering my understanding of lift (which is pretty cool), NASA also somewhat disproves the idea of deflection as the primary source of lift as well.
http://www.grc.nasa.gov/WWW/k-12/airplane/wrong2.html
So, in actuality, both sides are wrong. And it's actually a combination of these two theories, to some extent, coupled mainly with the behavior of viscous gases and Euler's equations (which I must admit, I was never able to fully get through without tearing my hair out) as the primary source of lift.
But... that's really neither here nor there. Whatever your understanding of lift, the individual motors of the Phantom speed up or slow down to create more or less of it, controlling it's altitude, pitch, yaw, and roll... which was the OP's original question ;-)
ProfessorStein said:
It was my understanding that yaw is induced as a result of the net torque imbalance on the airframe caused by varying motor speed...?
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