Suction Cups: How?

[Jim Harbison]

Most of the tops of the containers i saw at FIRST were cracked and some of ours are as well..........so dont count on sucking on the top of the container .......it really sucks.
Jim

[GregT]

Originally posted by Matt Leese
What you're talking about is the Bernoulli Effect. Basically, it means that as the speed of air increases, the density of the air decreases. In the case of an airplane, the curve of upper section of the wing causes the air to travel more quickly over the top than the bottom. This creates a lower air pressure on the top so the airplane lifts up.

Now, many modern planes use an "angle of attack." What this means is, that as you fly, depending on how much lift you want, you change the angle of the wing, which causes the curve "seen" by the air to change. This gives a change in the amount of lift.

Matt

Very little of any planes lift actually comes from the Bernoulli Effect, this is just a widespread mis-conception. Yes, it does contribute, but it is not the largest lift component.

All planes change their AOA (angle of attack) not just modern planes. To climb a plane lowers it's tail and points up, increasing the angle of attack and gaining more lift.

Greg

[ChrisH]

Originally posted by frumious
I'm fairly sure using the bernoulli principle would fall under the 'custom pneumatic components' section, anyway. I think the only way to make a vacuum legally involves actuating one pneumatic cylinder with another, creating a vacuum air connection of the cylinder being moved. If you attach this to a decent suction cup, you should be able to form a vacuum between that and any surface.

Tyson

There are at least two other legal ways of creating a vacuum, though one only applies to suction cups. The first is simply hooking tubing up to the compressor inlet. Of course this makes it difficult to pressurize our pneumatic system at the same time, but it works and is legal. The pneumatics guy at the LA Kickoff said you should get about 24 inches of mercury vacuum from the compressor.

The other way is to push down on a suction cup until it's flat. Push down on the cup with a ring and then pull up on the center. How hard a vacuum you get depends on how hard you pull. But there isn't much volume so a slight leak could be trouble.

[ChrisH]

Originally posted by GregT
Very little of any planes lift actually comes from the Bernoulli Effect, this is just a widespread mis-conception. Yes, it does contribute, but it is not the largest lift component.

If you want to start an argument, go to one of the aero newsgroups and ask "what creates lift?". Even the aero guys can't agree on an explanation that other people can understand, even other engineers. But they sure can calculate it.

[Jim]

While we are all aware of the *top secret* "Magic Smoke" factories that produce that crucial component installed in every electrical and electronic device; few people are aware of the small "Homesick Angel" installed in every type of aircraft.

Unfortunately (for aircraft designers) they have no say in which angel they get and some are more "homesick" than others. Early attempts a flight were often thwarted by the fact that most of the angels found here on earth were in no hurry to get back home. However, as time has passed and more and more aircraft take to the skies, more angels have been lured to earth.

By the way - I'd consider this thread officially *Hi-Jacked* <G>

Why can't you hook your suction cups up to the intake of the compressor. It is on the left side when looking at the motor end. It has the white felt filter. Does anyone see how this would violate the rules????

[nkrumm]

The first is simply hooking tubing up to the compressor inlet. Of course this makes it difficult to pressurize our pneumatic system at the same time, but it works and is legal. The pneumatics guy at the LA Kickoff said you should get about 24 inches of mercury vacuum from the compressor.

Are you sure? when tried this, we found that both the connectors on teh compressor provide air going OUT, and that the intake was simply the air vents around the compressor.

Seems like the only way to create a vacuum is to have a motor (or another cylinder) pull a cylinder.

You can't use the venturi effect to create a vaccum. What we might do is have to opposing pistons, one using the compressed air to have the other piston create a vaccum.

As for the airplane debate, I think it's the bernoulli effect. Or at least that's the most important component. The angle of attack does make a difference though but it's important with the wing shape, that's what the Wright brothers figured out. The curvature of the surface of the wing has low pressure, the bottom end has high pressure. Since you've got a plane with wings that have a large surface area, you've got a lot of lift force overall. The important part to lift is airspeed. If planes relied entirely on angle of attack, then in theory, your flaps that angle down would make the plane lift up on landing. (not too sure about that)

Wll when I took the compressor apart it looks like the only place air comes in is from the white felt area. Port A is discharge and B is the relief. Air should not normally come out of B. Some how you would have to hook an air sitting up to the white felt area but then this might be constued as altering the compressor.


So what we are going to do is use the fisher price motor to pull a cylinder to create vacumn. If you look at the threaded rod that came with the kit it appears as if it and the black plastic nut were designed to fit with the fisher price motor gear box.


It sucks that you just can't use an eductor.

[Chris Hibner]

I had to jump in on this one - sorry.

The equation for the lift generated by an airfoil is:

Lift = 0.5 * CL * (AirDensity) * (Wing Area) * V^2

Where CL is the coefficient of lift and V is the airspeed over the airfoil.

From the above formula it is obvious that as airspeed increases lift also increases.

What is not obvious is how CL is determined. CL is a function of the wing shape AND the angle of attack. For actual airfoils, CL is determined experimentally in a wind tunnel for all angles of attack (i.e. they make a CL vs. angle of attack graph). For thin airfoils and small angles of attack, you can approximate CL with the following formula:

CL = 2*pi*(Angle of Attack) [angle of attack must be in radians]

How exactly is lift created? That is a debate that will go on forever. It is partially Bernoulli effect and partially Newton's 2nd Law - how much of each is subject to debate. Actually, some aerodynamicists have suggested that in the case of an airfoil, one principal can be used to explain the other. In all honesty, it's not really all that important as long as lift can be calculated accurately.

Someone made a statement about flaps. Flaps are used to increase the angle of attack of the airfoil and change its camber (shape). This is done so that the airplane can get larger amounts of lift at slower airspeeds and lower deck angles (like what is required during landing). By changing the wing camber, flaps also increase the stall speed of the airplane (which is also good for landings since it allows the plane to land at a slower speed). Flaps also add a lot of drag which allows the plane to make a steeper approach without having to do a forward slip (which are fun for the pilot but they tend to freak out the passengers).

-Chris

[sleepyprogs]

We created a setup of two cylinders attached together by their threads and bolted it to a piece of lexan. the only problem is when you create a vacuum the lexan sheet will bend. we'll probably use aluminum instead. the suction cups we have work great on the sides of the boxes, but they are horrible on the top of the box, mostly because the tops crack too easily and the texture is diiferent on the top than on the bottom.