Questions about density and being in the air

1. Jul 29, 2015

Androo

Hello all, I have been arguing about this for a few days. My friend is telling me that a helicopter flies in the air because there is a difference in density on top of the propeller and below the propeller.The air on top of the helicopter has less density and the bottom has more density. This generates a force to push the helicopter up from difference in density. He is calling this a fundamental principle of physics. I am trying to tell him that the helicopter creates a wind aka force push in the air to push it up and there is no change in density. He also has been trying to convince me for days that if i am holding a box in the air, it is a fundamental law of physics that my body density is greater than the air above it, so it stays up. I believe the box stays up because my hand pushes with a force upward greater than gravity, and thus is not pushed down, unrelated to a density difference between my body and the air. Another case he has just tried to convinced me I am certainly incorrect about is that if I blow a piece of paper into the air, he says I have manually changed the density of the air below the paper and the air above the paper is less dense and thus is moves upward. In his explanation, anytime there is a force or pressure the density has to change. I am saying absolutely not, sometimes density will cause things like a helium balloon to float, sometimes it is a force independent of density. Thank you for any explanations.

2. Jul 29, 2015

Student100

A helicopters rotors act much like an airplanes wing. The shape of the rotors causes the air underneath to travel further than the air on top. The air traveling on top of the rotor moves faster, creating an area of low pressure, pulling the aircraft towards it. This is the basic method through which lift is generated. The wind/downward thurst from the rotors does not produce the lift.

Unless I'm reading this wrong, this is blatantly incorrect. When you lift a box you aren't generating lift.

There will be some lift generated, simply due to the fact that paper is falling through the air/you blew it.

3. Jul 29, 2015

Androo

Hey thanks for the reply, I am not talking about lift so much as he is telling me all lift, pressure, or change in force is caused by a change in density which I find impossible.

4. Jul 29, 2015

ToastedMitch

I did an experiment that will actually defies the law. If you put a flat surface under the propellers like say 2 inches below the propellers the area of dense pressure pushing down (Lift) has no place to push upon because if the flat surface below it wasn't there the area of dense pressure would push against the area of regular pressure creating lift but the flat surface so when it hits the flat surface the air has no place to go except outwards instead instead of creating lift. It basically creates an fan that "fans" 360.

5. Jul 29, 2015

Student100

Huh? Lift has nothing to do with "pushing down" only the speed at which the air is flowing over the top and bottom of the wing.

6. Jul 29, 2015

ToastedMitch

One thing Helicopters have propellers and your correct but the area that the propellers are pushing down create an area of dense pressure pushing down making lift.

7. Jul 29, 2015

stedwards

You're not making sense Toasted Dude. The gate keeps may show up and ban your behind.

8. Jul 30, 2015

Staff: Mentor

In the case of the box, your explanation is correct. In the case of the other examples, the net force of the air pressure on the object is upward. Of course if the air pressure is higher, it's density is also higher, but this is a secondary byproduct, and not a cause of anything.

9. Jul 30, 2015

nasu

You can push the box up against a steel ceiling. The density of your hand is less than that of steel. What happens? Will the box fall through your hand?

10. Jul 30, 2015

Staff: Mentor

To put a finer point on it, lift is calculated in two basic ways:
1. "Pusing down the air" = momentum chage or f=ma.
2. Pressure change/difference.

The first method is totally blind to density changes.

When using pressure change/difference, density is often assumed to be constant because at low speed, the density change is too small to matter. It is totally ignored in the calculations.

11. Jul 30, 2015

Khashishi

Your friend is wrong on all of these things. Your friend probably confused the concepts of density and pressure. A helicopter hovers by exerting a downward force on the air. There's an equal opposite reaction by the air on the helicopter blades holding the helicopter up (against gravity). The propeller is basically a fan pointed downward. The blades cut through the air at an angle which forces air to move downward.

12. Jul 30, 2015

Student100

The rotor isn't a fan pointed downward. The rotor is a rotating wing. You can't put a "big fan" on helicopter and expect it to fly.

The "downward force on the air" isn't the full story. From the rotors reference the movement of the air a small distance from the rotor is inconsequential. The pressure differences explain 100% of the lift.

There is a method to calculate lift from newtownian laws. That method can get far more complicated and is of no real use to anyone outside of wing design, in my opinion.

There's really no consensus on how lift is generated or the best method to use, even between two AE's working for Boeing.

Take the newtownian method, pilots and students will neglect the flow deflection on the leeward wing surface. Which leads to the incorrect conclusion that the windward flow deflection from hitting the curved wing surface is producing the lift by simply pushing the air downward. This is false, if it were true stalls (flow separation) wouldn't exist, angle of attack wouldn't matter, and vortex shedding wouldn't even be a thing. Airplanes would fly like kites.

That's why the rotor isn't simply a big fan, if it were it couldn't generate the needed lift. I prefer the pressure differential method, and think it's the better soultion to this question. As its easier to understand conceptually and works for these speeds.

I could be wrong, if so someone correct me please so I can learn something. :)

13. Jul 30, 2015

Khashishi

A fan blade is basically the same as a wing. The blades on a propeller are thinner than a house fan because it is engineered to spin faster but it's not that different. (A thick fan blade would cause some problems with cavitation as the air could not replace itself after the path of blade in time for the next blade.)

Air is not a perfect fluid, and the concept of pressure is an idealization. But Newton's third law is not an idealization. Push air down and you go up. That's the ONLY way you can generate lift with a wing.

14. Jul 30, 2015

Student100

A fan blade has nothing in common with a wing. A fan blades job is to move large volumes of air around, not generate lift or reduce drag.

A rotor isn't an engineered fan blade that moves faster. It is there to provide high aspect ratios, produce lift, and minimize drag.

Huh on the last paragraph? That makes no sense. Newtonian flow is an idealization.

15. Jul 30, 2015

Staff: Mentor

A fan* moves air by generating lift: Khashishi is correct that a fan, rotor and wing are fundamentally the same device.

*That is, an "airfoil fan". A centrifugal fan utilizes a different operating principle...

16. Jul 30, 2015

nasu

Newton's third law is not about newtonian flow.

17. Jul 30, 2015

Student100

Newton's third law is an idealization as well.

Thanks Russ, I had just assumed they all operated like blowers.

18. Jul 30, 2015

rootone

You could make a helicopter with flat rotor blades, as long as they have some angle of attack.
It would still result in air being pushed down and therefore lift would be produced.
This would be very inefficient though, a lot of the energy delivered to the rotor would be wasted in producing unnecessary air turbulence and some heat.
The point of the aerofoil shape is to get the air to flow in a way that minimizes 'drag' - (which translates as energy wastage.)

Last edited: Jul 30, 2015
19. Jul 30, 2015

Khashishi

True, but house fan blades are also not flat.

20. Jul 30, 2015

nasu

They are, on the fan in my kitchen.
But they are not horizontal.

21. Jul 30, 2015

rootone

Sure, but why waste electricity on pushing a blade around in an inefficient way, when a simple technology can be applied with low cost to produce a more effective result it a more efficient way.

22. Jul 30, 2015

Khashishi

Oh, then it's to reduce cost

23. Jul 30, 2015

Staff: Mentor

Why did you feel it was necessary to respond to Nasu in this way when he corrected you about not recognizing the difference between Newton's third law and Newtonian fluids /Newtonian flow? He was just trying to prevent the OP from being subjected to misinformation. What is this response even supposed to mean? Your response perpetuates the idea that Newtonian flow and fluids might still be a key factor. My "misinformation warning" finger is getting itchy.

Chet

24. Jul 30, 2015

rootone

Not really that, it's to do with making best use of energy.

25. Jul 30, 2015

Student100

I was pointing out that it's an idealization as well to simply say the downward force of the air produces lift and that using Newton's 3rd law is somehow not an idealization in this case. The entire movement of the fluid is an idealization in both cases (pressure and reactionary). I thought my reply to Khashishi was clear and conveyed what I was thinking in context. When Nasu mentioned the third law, I also stated it was an idealization in the context of the conversation. To apply it to something that is inherently non-inertial is an idealization, if I'm wrong then I apologize to Nasu and I could conveyed my idea better.