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Nov 22, 2011, 11:39 AM
I bail out, anywhere, anytime
Taurus Flyer's Avatar
Quote:
Originally Posted by kcaldwel
Actually, the pressure behind a building in a wind, or moving car or van, is not at a low pressure.

There is a high pressure in front (dynamic plus static), low pressure on the sides, and static pressure on the blunt rear side.

You can test this easily yourself, by driving a van down a dusty road with the rear door and the side windows open.

Tim, there is no doubt there is down wash behind a wing, or below a rotor. But you are only looking at a very small part of the picture. You are ignoring all the flow above and to the sides because it isn't obvious in your simple test.

Since you seem fond of misquoting NASA web pages, here is the NASA page on thrust from a propeller. Yes, the momentum change (thrust) is contained in the flow behind the propeller. Since all the thrust momentum change is contained in this flow, redirecting it 90 degrees with a flat plate will result in an equal and opposite force on the flat plate.

http://www.grc.nasa.gov/WWW/k-12/airplane/propth.html

This says nothing about how the momentum change occurred at the propeller or other thrust producing device. It also ignores all the other flows that are part of creating the thrust.

It is a very simple view that does not provide any predictive power, or aid in the design of anything.

I've also made a couple of additions and corrections to Taurus Flyer's diagram.
Kevin
kcaldwel, you write:


I've also made a couple of additions and corrections to Taurus Flyer's diagram


Not needed, I already did show more pictures in post 93 also a better one of the fortex.
See https://www.rcgroups.com/forums/show...9&postcount=93

That upwash you did draw in my photograph? Not in the Netherlands, never!


Cees
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Nov 22, 2011, 11:43 AM
I bail out, anywhere, anytime
Taurus Flyer's Avatar
Quote:
Originally Posted by Sparky Paul
.
Why make that video?
Why not?
Seeing is believing.
A long treatise on pitot tubes doesn't answer the question... What makes lift?...
Bernoulli describes the effect of a moving wing, or wind over a surface, and the pressure distributions.
Newton describes the result of the wind's effect on the wing, moving the air downwards.
It's quite interesting to stand under the approach path of a largish airplane and hear and see the effect of the downwash as it passes. That's air being pushed down.
The discussion is endless in aerodynamics, and always comes up, with mostly words getting tossed back and forth.
A "show-and-tell" adds some fact to the verbiage.
A helicopter is an ideal machine to illustrate the situation.
The blades Bernoulli away, but with the plate, Newton plants it firmly on the ground.
Sparky Paul


Read my post 121 again,

We, of instrumentation not always use pitot tubes but sometimes a PLATE by measuring the force on that plate and that is Sparky Paul showing in his video.
But measuring the "dynamic pressure of flow" with such a plate is like measuring the Voltage of your battery pack with a voltage meter with an internal resistance of 1 ohm.
Nearly nothing left of your valuable propulsion enegy.



What do you think of the dynamic pressure on your plate?

Cees
Nov 22, 2011, 11:45 AM
Cognitive dissonance
kcaldwel's Avatar
...
Last edited by kcaldwel; Jan 27, 2012 at 05:07 PM.
Nov 22, 2011, 11:52 AM
Launch the drones ...
Quote:
Originally Posted by kcaldwel
Reference?
Where I got the wall effect correction for downwash from ...

Title: Understanding Flight
Authors: David E. Anderson and Scott Eberhardt
Publisher: McGraw Hill
Chapter Referenced: Aerodynamic Testing
Page Referenced: page 211
Text Referenced:

"The wind tunnel introduces an artificial constraint, namely, walls. the wall have two effects. The first is that they interfere with the amount of air that can be pulled from above the model's wings and blocks the downwash on the bottom. This latter effect is just like ground effect and is called wall effects.

Another problem in a low-speed wing tunnel is that air will speed up in a constriction and the airplane model acts like a constriction in the venturi. In other words, the air accelerates as it moves around the model due to the blockage from the model. For obvious reasons, this is called the blockage effect. Wall and blockage effets are illustrated in (not included here).

Years of theoretical work have resulted in methods to correct for wall and blockage effects. Unfortunately, wind-tunnel corrections are not completely reliable, so wind-tunnel results must be backed up with flight test."
Nov 22, 2011, 11:54 AM
Grad student in aeronautics
Great video Kevin.

Tim, it is actually not "difficult" to study an airfoil. The flow over a 2D section of an infinite wing is equivalent to the flow over an airfoil, which is accomplished by spanning a wing between the walls of a wind tunnel. Do you agree that the effects of the walls can be accounted for sufficiently so that we can still learn a lot from the flow we observe in the wind tunnel?
Nov 22, 2011, 11:57 AM
Launch the drones ...
Quote:
Originally Posted by Taurus Flyer
kcaldwel, you write:


I've also made a couple of additions and corrections to Taurus Flyer's diagram


Not needed, I already did show more pictures in post 93 also a better one of the fortex.
See https://www.rcgroups.com/forums/show...9&postcount=93

That upwash you did draw in my photograph? Not in the Netherlands, never!


Cees
Not in my backyard either. I don't understand the upwash counters downwash argument in the least - fans blow backward, not forward.
Nov 22, 2011, 12:01 PM
Launch the drones ...
Quote:
Originally Posted by DPATE
Do you agree that the effects of the walls can be accounted for sufficiently so that we can still learn a lot from the flow we observe in the wind tunnel?
Sure. But experiments are better. And all the ones posted in this thread demostrate that lift is caused by the air being shoved down.
Nov 22, 2011, 12:17 PM
Ascended Master
Sparky Paul's Avatar
Quote:
Originally Posted by Taurus Flyer
Sparky Paul


Read my post 121 again,

We, of instrumentation not always use pitot tubes but sometimes a PLATE by measuring the force on that plate and that is Sparky Paul showing in his video.
But measuring the "dynamic pressure of flow" with such a plate is like measuring the Voltage of your battery pack with a voltage meter with an internal resistance of 1 ohm.
Nearly nothing left of your valuable propulsion enegy.



What do you think of the dynamic pressure on your plate?

Cees
.
I think nothing of it.
Not anything I'd pal around with, or take to the Mall.
The force on the plate is due to the air being deflected down by the passage of the blades through the air.
It is obviously greater in magnitude than the combined weight of the heli and the plate, so the net motion upwards is zero.
Lemme put the thing on a scale and see what happens...
Nov 22, 2011, 12:22 PM
Registered User
...
Last edited by Steve Anderson; Sep 30, 2014 at 06:14 PM.
Nov 22, 2011, 12:23 PM
Ascended Master
Sparky Paul's Avatar
Way back when, I taped some cassette tape streamers on a ZAGI. Without and with tip plates.
The bare wing tip vortex eroded the tapes immediately.
The tapes on the tip plates didn't lose any appreciable length after two flights.
Nov 22, 2011, 12:23 PM
Ascended Master
Sparky Paul's Avatar
Quote:
Originally Posted by Steve Anderson
You have to fly the helicopter at 20 Celcius for Berneulli lift. Everyone knows 68 Fahrenheit is for Newton lift.
.
And lift is measured in real units.. pounds!
Nov 22, 2011, 12:31 PM
I bail out, anywhere, anytime
Taurus Flyer's Avatar
Quote:
Originally Posted by Sparky Paul
.
I think nothing of it.
Not anything I'd pal around with, or take to the Mall.
The force on the plate is due to the air being deflected down by the passage of the blades through the air.
It is obviously greater in magnitude than the combined weight of the heli and the plate, so the net motion upwards is zero.
Lemme put the thing on a scale and see what happens...
Of course you make pictures of that while the chopper is separate placed on floor, on his own legs and the plate on the scale! So you only measure the force as result of the dynamic pressure on that plate!
Fix that chopper on the floor so it cannot fly away!!

Cees
Nov 22, 2011, 12:33 PM
Grad student in aeronautics
Quote:
Originally Posted by Tim Green
Sure. But experiments are better. And all the ones posted in this thread demostrate that lift is caused by the air being shoved down.
(Wind tunnel experiments...are....experiments).

Your propeller system is the main source or your confusion.

1. I (and others) say "there is upwash in front of a wing"
To that you say, "no, a fan doesn't blow air forward"

2. I say "wing lift is equivalent to circulation"
To that you say "show me circulation in a ducted fan"

3. You say "You can't have lift without shoving air down"
I say...."What the heck is shoving air down? Be specific!"

You can clearly see the upwash in front of a wing in a wind tunnel. Are you saying that you disagree with the results of the test? There is upwash in front of a prop blade, but that doesn't mean it is pushing the air forward! You have to examine the relative velocity between the local flow and the prop blade to discuss the downwash and upwash produced by it. (This is why studying a propeller is more difficult).

I don't think you really understand what circulation is.

I can describe to you the velocity in front of and behind a wing using circulation. Behind the wing, there is downwash....I guess we all agree on this? Is this what you call shoving air down?
Nov 22, 2011, 12:36 PM
Registered User
Quote:
Originally Posted by Tim Green
Sure. But experiments are better. And all the ones posted in this thread demostrate that lift is caused by the air being shoved down.
Nope you're still confusing cause and effect. They generally demonstrate that when there is lift happening there is also some downward air flow happening. No causality proved or even indicated.

You might just as well claim that experiments show that lift on a wing is generated by invisible wing fairies and this is clearly proved as the air being shoved down by the flapping of the fairy wings can clearly be detected .

Steve
Nov 22, 2011, 12:42 PM
I bail out, anywhere, anytime
Taurus Flyer's Avatar
Quote:
Originally Posted by kcaldwel
Actually, the pressure behind a building in a wind, or moving car or van, is not at a low pressure.

There is a high pressure in front (dynamic plus static), low pressure on the sides, and static pressure on the blunt rear side.
You can test this easily yourself, by driving a van down a dusty road with the rear door and the side windows open.
Tim, there is no doubt there is down wash behind a wing, or below a rotor. But you are only looking at a very small part of the picture. You are ignoring all the flow above and to the sides because it isn't obvious in your simple test.

Since you seem fond of misquoting NASA web pages, here is the NASA page on thrust from a propeller. Yes, the momentum change (thrust) is contained in the flow behind the propeller. Since all the thrust momentum change is contained in this flow, redirecting it 90 degrees with a flat plate will result in an equal and opposite force on the flat plate.

http://www.grc.nasa.gov/WWW/k-12/airplane/propth.html

This says nothing about how the momentum change occurred at the propeller or other thrust producing device. It also ignores all the other flows that are part of creating the thrust.

It is a very simple view that does not provide any predictive power, or aid in the design of anything.

I've also made a couple of additions and corrections to Taurus Flyer's diagram.

Kevin

kcaldwel,

I also did read your story about pressure distribution around objects.
see quote.


I show you a diagram of such an experiment, round objects.
Did you know the results of these kind of experiments? I don't think so.
Static pressure we see at 35 degrees.

Cees


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