UFOs, US Army and Einstein's Other Mistake

There's a story in Slashdot about this news story:
https://www.dailymail.co.uk/pages/live/articles/technology/technology.html?in_article_id=447317

Essentially, it's a craft built around the alternative explanation of lift, the Coanda Effect. Classic aerodynamics has some inconsistencies which have lead several people, Einstein included, to come to some incorrect conclusions despite remaining consistent to theory. Not his "greatest" mistake (which dark matter and energy are proving to have been correct after all), but embarrassing none the less.

Here's an explanation of the effect, written by Jef Raskin. He was the head of the original "pirate" Mac design team at Apple, now a professor at UCSD:
https://jef.raskincenter.org/published/coanda_effect.html

Nothing pleases me more in science than finding something that proves the old f@rts wrong. This explains why the boat tails I've seen tend to produce an inordinate amount of extra altitude -- they're producing lift, which here is connected more clearly to a decrease in drag. They're one in the same. The attentive rocket scientist is urged to study this carefully.

jef raskin has been dead for like 2 years


terry dean
nar 16158


ps that doens't mean what he says is wrong

Did you know that G. Harry Stine knew Coanda?? He also put Coanda Effect hovercraft in some of his fiction novels. I remember reading the first book in the series, call The Warbots.

Boat tails most certainly do not add lift - they reduce drag. This does have to do somewhat with the coanada effect however.

As for airfoils, there are many factors that contribute to the lift. Clearly it is not entirely bernoulli, though it does play a role, but it is also not all coanada. Instead, a combination of those plus newton's third law all act together to create lift.

Originally posted by cjl
Boat tails most certainly do not add lift - they reduce drag.

Click to expand...

Raskin's article is a fairly clear description of how these are the same thing. Lift becomes evident when drag is reduced asymmetrically, ie. it is not balanced against lift in the opposite direction. The concepts are reciprocal. You won't get one without the other, although you won't see lift if it's counter-balanced. It's a matter of convenience and convention to look at it one way or the other. I suspect that's why Einstein was attracted to the topic.

See also-

https://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html
https://www.grc.nasa.gov/WWW/K-12/airplane/wrong2.html
https://www.grc.nasa.gov/WWW/K-12/airplane/wrong3.html
https://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html

I read his article, and nowhere in it did it state how something of a boattail shape would add lift.

It is pretty clear how they reduce drag though. They eliminate the vortices and low pressure regions that exist behind a flat surface (such as the back end of a rocket) when it is traveling quickly through the air. There is still a low pressure region, of course, but the pressure differential is not as extreme, and the flow is far more laminar.

Originally posted by cjl
I read his article, and nowhere in it did it state how something of a boattail shape would add lift.

Click to expand...

That was my observation, not his.

It is pretty clear how they reduce drag though. They eliminate the vortices and low pressure regions that exist behind a flat surface (such as the back end of a rocket) when it is traveling quickly through the air. There is still a low pressure region, of course, but the pressure differential is not as extreme, and the flow is far more laminar.

Click to expand...

Eliminating low pressure below would have the same effect as eliminating high pressure above or creating low pressure above. It applies to any application of differential pressure, whether an airfoil or a rocket body. Applying particular terms to the effect because of geometry, number of dimensions, or axis of measurement is an arbitrary distinction. As an analogy, you can arrive at the same conclusion using calculus as you can using linear equations. Very often the former is easier to calculate or just to conceptualize, but not always. In either case, the reality being considered ("What is the minimum amount of fence the farmer would have to use..." etc.) remains the same.

I suggest that by considering it in terms of lift generated (or "reduction in negative lift") one will arrive at an estimate closer to the reality of things than if one considers it strictly as a reduction in the "vacuum" that "sucks a rocket back" as in flat base drag. I know I've consistently underestimated altitude on rockets with boat tails, and I think this helps explain why. I think it's also at the basis (from what I've read) of the difference in results achieved using Rocsim vs. more in-depth CFD software, as Rocsim uses a faster and easier estimation based on solely such base drag and drag reduction. I don't think a cD estimate is the entire answer.