A quiz for people

[freebeard]

The airflow is not attached, and the lift is Bernoulli caused. Others will find nits and pick them, but this is simple and succinct

Picking nits is what we do here. My best shot:

What causes wing lift in free flight is controversial. This may or may not apply to half-bodies that leak air through the connection to the ground plane.

Search term : duckduckgo.com/?q=wing+lift+not+explained

What Creates Lift - How Do Wings Work? - Aerospace ...
https://aerospaceengineeringblog.com/how-do-wings-work/
However, this does not explain how asymmetric airfoils with pronounced positive camber at zero angle of attack, as shown in the figure below, create lift. In fact, such profiles were successfully used on early aircraft due to their resemblance to bird wings.

Incorrect Lift Theory
https://www.grc.nasa.gov/WWW/k-12/airplane/wrong1.html
The lift predicted by the "Equal Transit" theory is much less than the observed lift, because the velocity is too low. The actual velocity over the top of an airfoil is much faster than that predicted by the "Longer Path" theory and particles moving over the top arrive at the trailing edge before particles moving under the airfoil.

JulianEdgar — When do we get the result of the not-poll? It's been ten days.

[redpoint5]

My question is, does shape matter in supersonic flight with regards to drag, or is it all about frontal area?

[skyking]

Shape does matter. Look at the significant narrowing between station F and H on this drawing of the T-38/F-5


https://aerofred.com/details.php?ima...7bdu5srop13943

That was done due to the complex interaction of the wing to body airflow at supersonic speeds.

[freebeard]

Sonic Wind a rocket powered land speed record attempt on ice

This site features rocket powered land speed record vehicles and rocket cars which use bi propellant rocket engines burning rocket fuel and liquid Oxygen. Our goal is to exceed the speed record set by Richard Noble, Andy Green and and their British team and to eventually become the fastest car on Earth.

"Wheels? We don't need wheels where we're going!"

[redpoint5]

Not related at all, but still that rocket car made me think of it... I just learned that the 37 million combined horsepower of the space shuttle engines, gimbal. Can't imaging thrust vectoring that amount of power.

https://www.youtube.com/watch?v=CdHcj_kp1UQ

[freebeard]

My thought process went:

Gimbaled thrust > Bucky 'Trim Tab' Fuller > duckduckgo.com/?q=orcas+attack+sailboats

[JulianEdgar]

Originally Posted by hat_man

I'm willing to learn something here but I'm going to need some guidance.

Is the lift of the Dream created by the attached airflow over the top of the profile or because the design of the underside creates more pressure on the underside than is created on the top (underside pressure >topside pressure. Lift created by a specific design.) or us the lift created by the basic shape resembling an airfoil and adds as such (Bernoulii ?) because it is positioned high enough off the road that it acts as a "wing" (not a specific underside design to create lift, but placement of a basic design in relation to the road to create lift)

I thought unattached airflow (flow separation?) was more turbulent and caused more drag. If the Dream is low drag then wouldn't it have more attached air and less turbulent air moving around the body?

1. I don't think it helps much to try to work out the mechanism by which wing lift occurs - ie at a purely theoretical level. As Freebeard correctly later posts, theories of lift are contentious and so for amateurs like us, just cause confusion.

2. To have lift, a vehicle must have a lower average pressure on the upper surfaces than the underside surfaces. That's all.

3. Pressures causes by attached airflow can be greater than caused by separated airflow. Low pressures occur where airflow speeds are highest. Where the airflow follows a surface that, for want of a better way of putting it, causes the airflow to wrap around a curve, low pressures are generated. On normal cars (eg a fastback) these low pressures occur, therefore, at the curved leading edge of the hood, at the windscreen header, across the curved roof and down the first (most curved) part of the hatch.

4. Low pressures can also be generated under the car, where the air flow travels quickly between the smooth underside of the car and the road. However, without mirroring the curves of the upper body, these pressures will generally not be as low as on the upper surfaces.

5. The Honda Dream would have attached flow everywhere. It would have, I would guess, lowest pressures on the upper surfaces where the curves are greatest - ie where the front axle line is. Conversely, except when there is yaw airflow, the relatively flat area at the back of the car would produce relatively little lift. The underfloor area would also be developing a low pressure. In the case of the front, top surface lift is much greater than underside downforce (CLf is positive), while at the back, the rear underfloor downforce is greater than the top surface lift (CLr negative), except in yaw conditions.

6. Where the flow separates, a low pressure is also developed. However, except for occasional small separation bubbles, the top surfaces of all modern cars have attached flow. Under the car, separation is much more common, and relatively few cars have attached flow here. But we can talk about what happens under the car another time!

[JulianEdgar]

Measuring the aero pressures for yourself is always far better than reading theory. It opens your eyes in a way that nothing else can.

So, the measured panel pressures on my Honda Insight, equipped with effective undertrays to develop low pressure under the car:



Then, looking only at lift/downforce, the vectors (arrows that show magnitude and direction of resulting forces):



Then a diagram that shows (by squiggly areas) the rough integration of force x area. (To get better accuracy you would take more measurements, and also split the forces into their X and Y axes, but this gives you the general idea):



If you look at the squiggly lines, you can see a greater area of squiggles under the car than above ie this car heavily modified car develops downforce (don't forget to include the downwards pressure in front of the windscreen).

These measurements can be done for any car, on the road, easily and pretty cheaply (under US$100 for the tool kit that you can use forever).

[JulianEdgar]

The "low pressure when attached airflow wraps around curves" can be seen on many vector* diagrams of pressure distributions on cars.

(*force strength and direction)

Here's a classic one for the Mazda RX7 (but note it doesn't show undercar pressures - back then, no one much cared about that):



When examining the vectors, we can use the 'triangle of forces' to work out what is the horizontal (thrust/drag) component, and what is the vertical (lift/downforce) component.

From my book:

Mercedes in wind tunnel...



Developed vector force:



Yellow arrow shows thrust vector, and green arrow shows lift vector.



So, in pretty well all cars, the airflow at the top of the windscreen helps propel the car forward, and lift it off the ground. But, because the area where these pressures are applying is very small, both forces are also relatively small.

Now mentally apply the same triangle of forces to the vectors on the roof and hatch of the Honda:



You can see that the horizontal components are small, and that thrust and drag (including all the hatch) probably cancel each other out - that is, the roof and hatch are not causing any pressure drag.

Resolving the horizontal and vertical forces allows you to see how lift/downforce, and thrust/drag, are being created by the pressures acting on the body.

And a reminder: you can easily measure all these pressures for yourself by doing some simple on-road testing.

[hat_man]

In my simple mind list can only happen if the pressure underneath is greater than the pressure over the top. (Under > over). Maybe it's simplistic but I'm seeing a wing "at rest" as under = over. Is my thinking so far correct?

Assuming a wing moving through the air at X mph, is the air moving under the wing equal to x and the air moving over the wing > x ? Or is it the other way around with the air over the wing equal to x and the air under the wing <x ? I'm assuming the former only because the air under would be relatively flat and the air over being the variable because of the "detour" it has to make over the top to reach the same point on the trailing edge?

Or is that the crux of the nit being picked?