# No one can explain WHY planes fly...

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#### Aerowerx

##### Well-Known Member
.....
You cannot fly a heavier-than-air machine without displacing air downward.
Ever see a demo of an RC helicopter over a weight scale? The force of the air on the scale is the same as the sitting weight of the helicopter!

#### poormansairforce

##### Well-Known Member
Some cheap fuel on the fire 2:30-5:30:

#### Dan Thomas

##### Well-Known Member
Some cheap fuel on the fire 2:30-5:30:
A wind tunnel doesn't show downwash well because its floor isn't far beneath the wing, and that floor dampens the downwash in a big way, returning it to neutral. Our airplanes don't fly in wind tunnels; they fly in free air and cause downwash, which has a reactive force according to Newton's third law.

It would be interesting to see the graphic readings of sensors in the wind tunnel's floor under and aft of the wing.

As far as I can see, Bernoulli contributes to downwash. Period.

#### Doggzilla

##### Well-Known Member
HBA Supporter
The reason you don’t see it in a wind tunnel is because ground effect is only a fraction of the wingspan, and reduced with speed.

So if the tunnel is testing cruise speed the downwash and vortexes will only be like a foot from the wing.

Most of the downward force is redirected radially in the vortexes because the spanwise flow putting a spin on it.

#### dog

##### Well-Known Member
The reason you don’t see it in a wind tunnel is because ground effect is only a fraction of the wingspan, and reduced with speed.

So if the tunnel is testing cruise speed the downwash and vortexes will only be like a foot from the wing.

Most of the downward force is redirected radially in the vortexes because the spanwise flow putting a spin on it.

Most or All of the downward force is the
begining of the vortexes?

#### jedi

##### Well-Known Member
A wind tunnel doesn't show downwash well because its floor isn't far beneath the wing, and that floor dampens the downwash in a big way, returning it to neutral. Our airplanes don't fly in wind tunnels; they fly in free air and cause downwash, which has a reactive force according to Newton's third law.

It would be interesting to see the graphic readings of sensors in the wind tunnel's floor under and aft of the wing.

As far as I can see, Bernoulli contributes to downwash. Period.
Your zooming in on the contest of post #79, page 4 but out of phase and not there yet. Keep up the good work though.

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#### Norman

##### Well-Known Member
Some cheap fuel on the fire 2:30-5:30:
So both CalPoly and MIT teàch no net downwash.

#### poormansairforce

##### Well-Known Member
Maybe it all works because of gravity? Wouldn't that be ironic?

#### poormansairforce

##### Well-Known Member
So both CalPoly and MIT teàch no net downwash.
That is what Bowers is saying in effect. No net movement needed. Upflow=downflow. Simply an effect of the wing making the air do work. I'm just stupid about this stuff.

#### Norman

##### Well-Known Member
Maybe it all works because of gravity? Wouldn't that be ironic?
Actually it is about potential energy (pressure), not momentum, so in a way gravity responsible.

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#### poormansairforce

##### Well-Known Member
Actually it is about potential energy (pressure), not momentum, so in a way gravity responsible.
Thank you for getting this!

#### Doggzilla

##### Well-Known Member
HBA Supporter
Most or All of the downward force is the
begining of the vortexes?
Yes. At the tips it is inverse.

Imagine the left wing from behind. The high pressure under the wing wants to move towards the low pressure on top, so it swings clockwise around the tip. The air under the wing swings downwards, then back upwards. But by the time the flow rotates upwards it is no longer anywhere near the wing. So only half the force effects the wing.

#### Norman

##### Well-Known Member
Thank you for getting this!
Got it years ago. I used to think that it was only true in 2D flow but Prof Drela kindly explaned that I wasn't including half of the wake vortices. When you include the upwash outboard of the wing tips net downwash goes back to zero.

#### poormansairforce

##### Well-Known Member
It would be interesting to see the graphic readings of sensors in the wind tunnel's floor under and aft of the wing.
How about ahead of the wing in the floor? What would we see?

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#### poormansairforce

##### Well-Known Member
What is happening at 3:15-4:30 with the kf-1 airfoil?

#### jedi

##### Well-Known Member
Your zooming in on the contest of post #79, page 4 but out of phase and not there yet. Keep up the good work though.

This post is intended to answer the referenced quiz. If you are not interested in the quiz, and most readers were not interested as judged by follow on posts, I strongly suggest that you disregard the rest of this post. It will not explain how wings produce lift. At best it will give some background on why it is difficult to explain. The last several posts prior to this are making good progress. If you understand those you probably do not need this one.

I have an empty milk carton in my kitchen sink. I could show it to a dozen people and they would all confirm that it is “empty”. If I would strongly blow into the container and quickly plug the opening then show the same carton to the same people and demonstrate what happens (it goes “Poof”) when I remove the plug, they would say it was “full of air”.

In truth the empty container is full of air in both cases, it is just filled with less air when it is considered empty. The study of a stationary fluid in a container (air in our examples) is called statics or study of a static fluid. The “air” is a specific fluid. When I blew into the container I pressurized the container. When the pressure was released, the container was “empty”. If I use a tire gage to measure the pressure in the “empty” container it has no pressure (called gage pressure in this case). In fact we know that a barometer would measure a pressure of about 15 psi under standard sea level atmospheric conditions.

The study of static fluids in a container has presented the “Ideal Gas Law*”. Expressed as an equation it is PV=nRT. It relates Pressure, Volume and Temperature to the number of molecules (n) and a constant (R) that varies somewhat depending on the size or other characteristics of the molecules. When I blew into the container I increased the number of molecules and the pressure increased. If I were to blow into a rubber balloon the volume would increase. The pressure and volume changes are determined (or approximated) by the ideal gas law and the size of the container. In reality, there is no “ideal” gas. This is an engineering or scientific tool to estimate the real results.

When you go to the airport and preflight a Piper Cherokee 140 the pressurized air in the Oleo struts is supporting the airplane. This is a static condition. The air is in a sealed container. It can not go anywhere. The air is not accelerated or forced downward. There is no downwash and no induced drag. The same statement applies to the air in the tires.

This only applies because the container is sealed. If the tire leaks or the Oleo strut has bad seals the number of molecules changes and the aircraft is no longer supported by air. When the Piper PA-28 140 is in flight the air is free to leak from the high pressure area below the wing to the low pressure area above the wing. Understanding and explaining how that works is more complicated and more in line with the subject of this thread.

I hope to continue this conversation and talk about Bernoulli in a later post.

Any questions?

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#### henryk

##### Well-Known Member
Understanding and explaining how that works

=long distance fans-baloon inlet ?

#### BBerson

##### Well-Known Member
HBA Supporter
The air in a milk carton isn't static. The molecules are bouncing off each other and the carton at the speed of sound of those molecules.

#### poormansairforce

##### Well-Known Member
The air in a milk carton isn't static. The molecules are bouncing off each other and the carton at the speed of sound of those molecules.
Then try to create lift with it without adding any energy.

#### Norman

##### Well-Known Member
The air in a milk carton isn't static. The molecules are bouncing off each other and the carton at the speed of sound of those molecules.
The molecules outside are also zipping around at the speed of sound. What's your point?

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