# Flying disk - Why not

1. Sep 14, 2014

### OutOfTheBox

I have seen the experiments done by the US after world war II on tv and understand how impossible it would have been to fly a craft like that with Levers and Knobs. However, today we use electronics, like in the stealth bomber to allow for many actions to be taken which are the result of interprating the pilots movements.

Would a saucer/disk design be more "Flyable" with todays technology? As i see the merits aerodynamiclly to having a disk shapped vehicle.

2. Sep 15, 2014

### Simon Bridge

Yes.

Usually there is something else that does the same job better.

3. Sep 15, 2014

### CWatters

In general high aspect ratio (long and thin) wings are more efficient than low aspect ratio wings. Glider and U2 wings are examples. But efficiency isn't always the priority.

4. Sep 17, 2014

Aerodynamics involves countering weight with lift by 'pushing down' on air, which pushes 'up' on the vehicle according to Newton's Third Law.
How you push down on the air makes all the difference.
The further from the leading edge of an airfoil you are, the less efficient will be the application of forces against the air. That makes narrow airfoils more efficient than wide ones.
A flying disc is a circular airfoil, so the center of the circle will provide very little lift, and the trailing half of the disc (behind its center relative to the direction of motion) will be almost useless.
Better to look for a novel method of applying force to the air - such as a Dyson fan.

5. Sep 17, 2014

### Simon Bridge

The details of how a specific airfoil produces lift is an open question ... we can also expect some contribution from Bernoullis principle.
http://hyperphysics.phy-astr.gsu.edu/hbase/fluids/airfoil.html

Wouldn't that argument also mean that delta-wings are a waste of time too?

I don't know that the dyson fan is a novel way to apply forces to air - it just uses a fan in the base. It's not an efficient way to get thrust despite appearances re the "air multiplier" effect. Remember, energy and momentum are still conserved.

The Avrocar was an alternative airflow approach, using a disk airframe, but it suffers stability issues (which could, in principle, be overcome with modern avionics). However: the expected performance is more easily achieved with a helecopter or quadcopter and there is no special advantage to having the inherent instability to the avrocar concept itself. This is pretty much the same story with each application.
Compare: https://www.kickstarter.com/projects/1524806320/hoverbike

The idea keeps coming up, i.e.
http://www.gizmag.com/go/7710/
... but it is unusual for anything to come of it.

6. Oct 7, 2014

### stinsonbr

There is no question as to how something produces lift. Force from the air hitting the bottom of the wing, and the force produced from the shape of the wing. The increased curvature on the top half of the wing forces the air to travel faster, and thus lowering its pressure. This means the air under the wing with higher pressure pushes up on the wing, trying to get to the lower pressure. This is also why planes can fly "flat", or without the nose being pointed upwards.

A disc would fly. The idea of a "lifting body" has been around for a while and is being used today. We don't really use it solely as a lifting mechanism, it's usually something we add on so the wings don't have to be as large (check out the stealth bombers).

The idea of controlling a flying disc sounds like a nightmare however.

7. Oct 20, 2014

### RandomGuy88

Its not really an open question. It is just misunderstood by most people.

It is incorrect to look at it from the point of view of Bernoulli vs Newton or even some lift from Bernoulli and some from Newton. A fluid can only generate a force on an object through pressure and friction. Friction does not have much contribution to lift so it is mostly pressure. So the pressure difference between the top and bottom of the airfoil/wing generates the lift on the airfoil/wing. In order for this pressure difference to be sustained the flow must accelerate. To sustain the pressure difference in the horizontal direction the flow velocity increases. To sustain the vertical pressure difference the flow turns downward. So you have to have a pressure difference and you have to have downward turning of the flow. They coexist.

Not entirely sure what you mean by this. By narrow airfoil do you mean a thin airfoil as opposed to a thick airfoil? Or are you referring to the length of the airfoil?

While it is true that the largest pressure difference between the upper and lower surface is near the leading edge this does not mean that the rest of the airfoil is useless. The entire airfoil determines how the air flows over it and therefore determines the pressure distribution over the entire airfoil. If you changed part of it you will change the entire pressure distribution. Increasing the chord of an airfoil (distance from leading edge to trailing edge) will increase the lift generated by the airfoil because you have more surface area and Force = Pressure *Area.

I agree with Simon that the Dyson fan is not a "novel" was to move the air.
Are you suggesting using the Dyson fan concept as a way to generate thrust or lift?

I think CWatters made a good point that the circular flying vehicle would suffer from a low aspect ratio making it aerodynamically inefficient.

8. Oct 20, 2014

### NTW

9. Oct 20, 2014

### Simon Bridge

It's a nice bit of logic, but it is incomplete as a refutation.
i.e. how does the shape of the airfoil generate the pressure difference?
If pressure differences are the whole story - how do you account for the various objections?

10. Oct 21, 2014

There's nothing "open" about the lift question. If you know the pressure distribution over the whole surface of an airfoil, you can integrate it to recover the exact lift force on the surface minus potentially some incredibly tiny contributions from viscosity.

The pressure distributaries can be calculated from Bernoulli's equation assuming that the flow is not separated and that you know the velocity field a priori. Then, as lon as you know the free stream conditions and the velocity field, you can calculate the pressure and then the lift.

If you want to know the velocity field, you need to go into any number of numerical methods ranging from inviscid approaches with corrections to mathematically impose the post important effect of viscosity (the Kutta condition), solving the Navier-Stokes equations directly, and a whole bunch of options in between with varying degrees of accuracy.

The conservation laws dictate how the air interacts with a specific airfoil shape and therefore the flow field, which dictates the pressure, which can be used to calculate lift. How this occurs has to do with the fact that the velocity around a sharp (or effectively
sharp) trailing edge cannot be infinite, and therefore the trailing edge stagnation point is enforced at the latest at the trailing edge (rather than upstream somewhere as predicted by inviscid theory). This is accomplished essentially by separating the viscous boundary layer at that point. In order for conservation laws to hold, the typical airfoil velocity profile must result. There are, of course, exceptions, as with anything, such as separated airfoils, but by and large, this is the quick version of how it happens.

Regarding the Newton argument, Newton's laws suggest that there should be an opposite reaction to accompany the upward lift force. This is where downwash comes in. The momentum change in generating downwash balances the lift force. They are two sides of the same coin. However, if you pick a random airfoil shape of interest, if you want to calculate lift, you aren't going to calculate the downwash. Thad not practically. On the other hand, you can easily calculate the velocity profile, use that to get pressure, and then use the pressure to get lift. There is no either/or.

11. Oct 21, 2014

### RandomGuy88

What do you mean "various objections".

The pressure difference is not the whole story. The pressure difference is how the force is transmitted to the object from the fluid but the pressure difference requires the change in velocity and the downward turning of the flow. All of these things coexist in a reciprocal cause and effect manner. You cannot say one necessarily caused the other.

12. Oct 21, 2014

### Staff: Mentor

I'm hoping he was referring to a long and skinny aspect ratio, not the cross section thickness.....since that would be the right/relevant one.