Pressure thrust?

[Christ]

So, Dave - Could one vent the low pressure area along the forward mid-line surface to the aft cone area, such that would aide the vacuum effect stated as necessary to maintain proper flow and create said positive thrust, without spoiling the flow in the forward mid-line area which creates said low pressure area?

What effect would that pressure differential have on the necessity to provide additional vacuum to the rearward area of the shape in order to maintain proper flow?

[DaveBirkenstock]

Originally Posted by Christ

So, Dave - Could one vent the low pressure area along the forward mid-line surface to the aft cone area, such that would aide the vacuum effect stated as necessary to maintain proper flow and create said positive thrust, without spoiling the flow in the forward mid-line area which creates said low pressure area?

What effect would that pressure differential have on the necessity to provide additional vacuum to the rearward area of the shape in order to maintain proper flow?

It's an interesting thought... I remember reading of an airfoil that used this approach to minimize transonic shockwaves.

IIRC, the suction inlet velocity for the 'after' image was over 78 MPH while the car was at 60 MPH, so I'm not sure you could get enough suction pressure & volume.

Plus, as you wrote, changing the low pressure area might disrupt things downstream too much to get the powered pressure recovery.

[bwilson4web]

Hi Dave,

Originally Posted by DaveBirkenstock

. . .
The one thing not clearly identified on that blimp image is the suction inlet, just upstream of the concave tailcone. By adding energy in the form of suction, the mechanism can generate what Fabio Goldschmied called "fuselage self-propulsion."

Goldschmied proved a significant reduction in total power used for streamlined bodies (his baseline body was the rigid blimp, USS Akron) by exploiting pressure thrust. . . .

I remember NASA has done a lot of work on boundary layer control using perforated panels or slots and a suction pump. I see some similarity with 'pressure thrust', which appears to be pulling in a larger fraction of the air flow. Is there a rough rule of thumb about what fraction of the flow needs to be pulled in and at what velocity relative to the high-speed flow?

I just finished reading "A History of Suction-Type Laminar-Flow Control with Emphasis on Flight Research" by Albert L. Braslow. This report mentions problems with rain, ice and other surface contamination. I get the impression 'pressure thrust' pulls in more than just the boundary layer. However, I'm wondering if rain and snow might cause problems?

Bob Wilson

[DaveBirkenstock]

Originally Posted by bwilson4web

I get the impression 'pressure thrust' pulls in more than just the boundary layer. However, I'm wondering if rain and snow might cause problems?

Hi Bob, thanks for the questions.

If more air than just the boundary layer is pulled in the total efficiency begins to drop off. Boundary layer thickness will vary with speed but I'm not sure if the % of that layer will change to a great degree. The intake velocity will likely always be above free-stream velocity, thanks to the pressure drop at the suction inlet. This pressure drop may cause ice to form at the inlet even in above-freezing ambient temperatures (more on ice below).

The big difference with this vs Laminar Flow Control is that LFC is hugely dependent on absolutely precise control of the boundary layer, any contaminants like bugs, dirt, ice, gaps between panels, etc. will destroy the benefit. That is why LFC has not been put into commercial service despite multiple successful flight test programs.

Rain shouldn't be a big problem, but ice could clog the whole works. Heated parts should fix that and the heaters could be controlled so they are off when not needed to minimize their power costs.

-Dave B

[DaveBirkenstock]

Originally Posted by lunarhighway

an interesting idea... i've always wondered it it was possible to design a sort of "wing"in the broad sense of the word, that in stead of generating lift at the top would generate pressure at the end... there would naturally be a loss of energy, but if this shape could be roughly car like than you'd have a car that in stead of having drag at the end would have pressure at the end.

You are right, this does act much like a wing does*. A 'normal' airfoil uses convex curves to accelerate local flow to create lower-than-ambient pressure, which we call lift. In much the same way this setup uses concave curves to decelerate local flow to create higher-than-ambient pressure which is put to work as thrust.

With this thrust force in place, the aero design goal goes from low drag to maximum efficiency. The tests done so far point to max efficiency coming when the pressure thrust force counteracts ~90% of the aero drag & the remaining propulsion comes from the traditional drivetrain.

If only the ALMS Green Challenge races allowed the use of 'movable aerodynamic devices,' those race teams would make short work of working out the details and optimizing the different tradeoffs.

-Dave B

*I've simplified this to take it easy on my poor brain... I'm not trying to open the whole 'why-do-airfoils-make-lift' case of worms.

[jamesqf]

Originally Posted by DaveBirkenstock

Rain shouldn't be a big problem, but ice could clog the whole works. Heated parts should fix that and the heaters could be controlled so they are off when not needed to minimize their power costs.

If it's being used on a car, you could always pipe a radiator coolant loop through it, and use the "free" heat that you want to get rid of anyway. Though if it's icy out, you might not want to be driving fast enough for drag to become an issue :-)

[Christ]

I think pulling in rain and ice/snow/crapola would affect the benefit by displacing the actual volume of air that comprises the boundary layer as well.

I.E. if there is a boundary layer 1 inch thick, and you take a sample of the boundary layer that is 1x1inch, you should have 1 cubic inch of sampled air... air. Nevermind the obvious pressure gradient in measuring the exact sample. But if rain/snow/ice/other get sucked in by the vacuum which holds the boundary layer to the concave shape, you're displacing part of that sampled air, and since the boundary layer of air is creating our energy, you're displacing some of that energy.

This discussion is "water at my nostrils" in terms of what I know and can apply to it, so please, if this is wrong, correct it at will, I'd appreciate a minor education in it.

[theunchosen]

I find this extremely odd. A good rule of thumb is if the flow is inside the object read where the pressures are high and reverse everything.

So in a nozzle pressure against the walls of the nozzle is higher than elsewhere, so if the fluid is on the outside of the nozzle the pressure is lower than elsewhere(because the fluid is expanding and cooling to fill the space).

This design definitely looks like the pressure against the interior of that surface would be high and therefore the rule of thumb says the pressure on the exterior would be a low pressure area(fluid expands to cover new volume cools and pressure drops all at the same time).

I dunno the idea sounds alot like trying to lift yourself by your bootstraps and saying it makes you lighter. . .

I can almost promise you that back window is going to be a low pressure zone, which would be great for someone drafting because the air will pull on their nose to fill the vacuum(or pressure difference).

This smells like the brown's-gas alternator generators. . .

I could be well wrong, but thats my opinion.

[winkosmosis]

I don't understand the claim of most of the propulsion coming from this effect. It DOES sound like pulling yourself up by the bootstraps.

[Christ]

The idea - as I understand it - is to slow the flow down such that it creates a high-pressure area at the rear of the capsule. The high pressure area, if it had greater pressure than the high pressure area at the nose of the capsule, would essentially create movement.

Keep in mind, that this would be overunity. We know that overunity does not exist, at the current time, in technology. Rather, what's happening here, is that with the aide of suction (generated by other means), the boundary layer is slowed and kept in an inswept area (concave shape) to allow that pressure to have some effect on the capsule at large. The gross effect appears to be thrust, but the net effect (forward pressure (fP) minus rearward pressure (rP)) would obviously not be. However, the net effect, after subtraction of the opposing forces, would yield that less power be needed to overcome drag from a mechanical source. If there is high pressure somewhere, there is not low pressure in that same place, and most of drag is the wake, or low pressure area.

So, looking at the formula:

fP - rP = net effect.

If we can calculate that the pressure at fP is 200 lbs, and the pressure at rP is 100lbs, then the formula becomes:

200 - 100 = 100

The net effect is a reduction in the mechanical force necessary, to 100 lbs, to defeat fP.

Where the design still fails is:

The design still requires that a vacuum be produced to adhere the boundary layer to the concave surface of the capsule. This vacuum is being generated by mechanical means, at a cost to efficiency, also partially canceling the effective reduction in fuel used to move the vehicle.

The actual effect of the current design is to move fuel use from one job to another. However, since optimal vacuum can be generated with an engine running at peak efficiency on a constant basis, there might still be fuel savings created, if the systems are isolated from each other. (The optimal vacuum obviously depends on speed of the boundary layer, and thus would only be "optimal" at a small range of speeds... like 60 MPH, if applied to passenger cars... or 500 MPH, if applied to jets.)