Duct Duct Go

[aerohead]

Originally Posted by freebeard

aerohead -- Thanks for giving this your attention. I'm looking at a die-cast scale model and it looks like a [no more than] 4" inverted U-shape leading edge for an airfoil.

Given, say, 250cfs of flow (25 times 10x) through the duct what should it's area be? I'd address the slot opening later.

I'm comparing a minimalist vertical hoop with something more like the Bird of Prey:

https://nationalinterest.org/sites/d...s/boeing_0.jpg

I think it could have an angled intake throat if the plenum is fed from the quarter-points instead of the ends.

I'd have you look at ASHRAE and SMACNA, and consider your duct shape and surface roughness.
Friction 'pumping' losses will be a Reynolds number function, based on how smooth the duct surface walls, at a given flow velocity.
You'll see at SMACNA, that your losses vary remarkably with flow velocity.
You're at 15,000 CFM. Choose a loss factor to establish design velocity. Then dividing your volume by velocity will spit out the area.
It's very conditional.
The smoothest surface has a friction loss coefficient of 0.003 pounds/ sq-ft.
Your design velocity, in feet/minute, will give you your surface area.
I don't have the proper work books with me to go any further. I'll get that for Friday.
On a car, the surface area is approximately 10-X projected frontal area, making a frontal area-based friction drag coefficient = 10 X 0.003 = Cdf 0.03. Total drag, minus 0.03 = approx. pressure drag.
[Dynamic pressure ( 1/2 x rho x velocity-squared ) x Cdf ]= friction drag component.

[freebeard]

On a car, the surface area is approximately 10-X projected frontal area, making a frontal area-based friction drag coefficient = 10 X 0.003 = Cdf 0.03. Total drag, minus 0.03 = approx. pressure drag.

So that textured aluminum sheet I'm hoarding is right out? Friday is soon enough, I've got other targets for my attention at the moment.

[aerohead]

Originally Posted by freebeard

So that textured aluminum sheet I'm hoarding is right out? Friday is soon enough, I've got other targets for my attention at the moment.

The smoother the better, although, if it's a relatively small surface area compared to the overall area of the vehicle, we might not lose sleep over it.

[freebeard]

No further progress on this, but I did find this video:

https://www.youtube.com/watch?v=f2UCK3pCFjk

It's all speculative CGI, but it's an interesting concept. For instance it claims 15x thrust, where Jetoptera gets to that with an additional box wing. 10x from Coanda and one level of entrainment?

edit: On the subject of quadcoptera, I've been learning about the Prandtl wing and found a mention of a Prandtl propeller! It would be longer ...but quiet. All the noise comes from the tips.

[Snax]

Originally Posted by aerohead

I believe that it was the P-51 Mustang which has been reported on with respect to heated, air, coming off the oil cooler producing some thrust.
This couldn't work on a subsonic automobile, as it requires compressibility effects of transonic, or supersonic flow to be present.

Not entirely correct. I have long lost the issue, but an article in Racecar Engineering demonstrated this effect in wind tunnel testing for F1 cars. It was very small, but I suppose it could mean the difference between running out of fuel 100 feet before the finish line vs. at the line. For road cars, such gains are statistically very likely impossibly insignificant nonetheless.

[aerohead]

Originally Posted by freebeard

No further progress on this, but I did find this video:

It's all speculative CGI, but it's an interesting concept. For instance it claims 15x thrust, where Jetoptera gets to that with an additional box wing. 10x from Coanda and one level of entrainment?

edit: On the subject of quadcoptera, I've been learning about the Prandtl wing and found a mention of a Prandtl propeller! It would be longer ...but quiet. All the noise comes from the tips.

Check out the Vietnam War-era, Lockheed YO-3 'Quiet Star,' battlefield observation plane and its unique, hand-carved propeller. Large diameter, low rpm. Pima Air Museum, near Tucson, Arizona,has a restored example in their collection.
And 'Q-tip' propellers.
Virginia-Class nuclear submarine propulsor ( if you have TOP SECRET clearance )

[aerohead]

Originally Posted by Snax

Not entirely correct. I have long lost the issue, but an article in Racecar Engineering demonstrated this effect in wind tunnel testing for F1 cars. It was very small, but I suppose it could mean the difference between running out of fuel 100 feet before the finish line vs. at the line. For road cars, such gains are statistically very likely impossibly insignificant nonetheless.

That seems plausible. I'm not sure anymore what maximum velocities are achieved in F1, but Indycars, which are similar, approach transonic speed at Indianapolis Speedway, at least during qualification.
Transonic speed, and its compressibility effects would be a requirement for achieving thrust. It cannot happen in subsonic flow.