How am i supposed to determine the speed of air?

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Discussion Overview

The discussion revolves around determining the speed of air for a glider project, specifically in relation to Bernoulli's Equation. Participants explore how to calculate the velocities of air above and below the wing of the glider, as well as the implications of Bernoulli's principle on lift.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation
  • Homework-related

Main Points Raised

  • One participant suggests that the velocity of air over the wing could be the same as the glider's velocity, while the velocity underneath could be obtained from weather data.
  • Another participant asserts that Bernoulli's principle does not explain lift, prompting questions about alternative explanations for lift.
  • Some participants argue that while Bernoulli's principle can describe lift, it cannot be used to calculate it accurately due to the complexities introduced by viscosity.
  • A participant mentions that vortices form at the rear edge of the wing, affecting airspeed differences above and below the wing, and references historical theories on the topic.
  • One suggestion is to use Bernoulli's equation to calculate the freestream velocity of the air by measuring pressure with a pitot tube.
  • A later reply indicates that the original poster feels less concerned about needing help, as they are nearing completion of their project.
  • There are light-hearted exchanges about personal anecdotes and historical references, but they do not contribute to the technical discussion.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of Bernoulli's principle to explain lift, with some asserting it does not explain lift while others argue it can describe lift under certain conditions. The discussion remains unresolved regarding the best approach to calculate airspeed and the role of Bernoulli's principle in lift generation.

Contextual Notes

There are unresolved assumptions regarding the conditions under which Bernoulli's principle applies, particularly concerning viscosity and flow complexity. The discussion also reflects varying levels of understanding about fluid mechanics and its implications for lift.

MolotovMonarch
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Please Help!

How am i supposed to determine the speed of air??
I need it for Bernoulli's Equation, see i built a glider and i have to present on Wednesday, and i want to show that i did at least one calculation.
As if things weren't bad enough already i have to figure out two velocities, one of the speed underneath the wing and the other of the velocity on top of the wing.
how do i do it?? I was thinking that maybe the velocity of the air over the wing is the whole glider's velocity which would be really easy to calculate. and perhaps the velocity of the air underneath the wing would be the air's velocity at that time. (i can probably get it online at a weather website)
Anyone have any suggestions?? R.S.V.P.!
This is driving me nuts!
 
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MolotovMonarch said:
How am i supposed to determine the speed of air??
I need it for Bernoulli's Equation, see i built a glider and i have to present on Wednesday, and i want to show that i did at least one calculation.
As if things weren't bad enough already i have to figure out two velocities, one of the speed underneath the wing and the other of the velocity on top of the wing.
how do i do it?? I was thinking that maybe the velocity of the air over the wing is the whole glider's velocity which would be really easy to calculate. and perhaps the velocity of the air underneath the wing would be the air's velocity at that time. (i can probably get it online at a weather website)
Anyone have any suggestions?? R.S.V.P.!
This is driving me nuts!
Tell your teacher that Bernoulli's principle does not explain lift. Have a look at:
http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html

You might try calculating the lift that your glider creates. Have a look at the NASA site on glider flight:

http://www.grc.nasa.gov/WWW/K-12/airplane/glider.html

AM
 
hmm.. the bernoulli principle doesn't explain lift... so what does?
 
newbie101 said:
hmm.. the bernoulli principle doesn't explain lift... so what does?

What does what?

What does explain lift?----> Fluid Mechanics Theory.

What does Bernoulli Equation say?----> Several things, but not everything.
 
Bernoulli principle does explain the lift, the problem is that you cannot use Bernoulli equation to calculate the lift.
Bernoulli equation deals with ideal nonviscous liquid. For that liquid no lift can be generated, neither air can provide a retarding force. However, if the viscousity is taken into account, the flow became complex.
The rough explanation is that at relativelly low speeds vortecies are forming on the rear edge of a wing. Then, because the momentum should be conserved, an opposite vortex is formed along the wing. That attached vortex provide the necessary difference of the air speeds above and below the wing. After that we can use the Bernoulli principle to calculate the lift.
That theory was developed by Zhukovsky and Chaplygin in Russia before WWI.
 
shyboy said:
Bernoulli principle does explain the lift, the problem is that you cannot use Bernoulli equation to calculate the lift.
Bernoulli equation deals with ideal nonviscous liquid. For that liquid no lift can be generated, neither air can provide a retarding force. However, if the viscousity is taken into account, the flow became complex.
The rough explanation is that at relativelly low speeds vortecies are forming on the rear edge of a wing. Then, because the momentum should be conserved, an opposite vortex is formed along the wing. That attached vortex provide the necessary difference of the air speeds above and below the wing. After that we can use the Bernoulli principle to calculate the lift.
That theory was developed by Zhukovsky and Chaplygin in Russia before WWI.

You're exposing yourself to replies of Arildno & Co. who has been discussing this topic very long long before. I am not going to reply to you in order to don't start it again.
 
Clausius2 said:
You're exposing yourself to replies of Arildno & Co. who has been discussing this topic very long long before. I am not going to reply to you in order to don't start it again.
Am I a dangerous man? :rolleyes:
Can't say I disagree terribly much with shyboy, since he notes the crucial importance of viscosity.
 
Last edited:
I believe it is better to be exposed to critics than to be safe like this:
What does explain lift?----> Fluid Mechanics Theory.

why not Newton's laws ;)
 
Easy guys - this looks like a question that needs a simple answer. MM - how about just using Bernoulli's equation to calculate the freestream velocity of the air(essentially, the airspeed of the glider) at the front of the glider by measuring the pressure with a pitot tube: http://www.du.edu/~jcalvert/tech/fluids/bernoul.htm
 
  • #10
well thanks guy for trying to help, but i don't really need it anymore. I have to present on Wednesday and I'm not really worried about it cus I'm pretty much done...i just got to do the PowerPoint presentation.

and by the way, you can find the NetForce using Bernoulli's Equation.
First you take the change in pressure using Bernoulli's equation.
Then, since Pressure is equal to Force over Area, you solve for force and get
F=(pressure)(A) the area used is the wing's total area. you calculate Fnet and Voila, you're done!
we did an example problem like this, and if the teacher was wrong you better not ride in another Boeing cus that's where he worked. hahaha
 
  • #11
arildno said:
Am I a dangerous man?

You know, Arildno, you have a Toledan Knife just below your PC table. I saw it when you invited me to take dinner that day... :biggrin:
 
  • #12
Toledan knife?
Are you sure you don't mean the saracen sword my Viking ancestor got from Al-Mansur for helping to subjugate soft Spaniards of his time? :wink:
 
  • #13
arildno said:
Toledan knife?
Are you sure you don't mean the saracen sword my Viking ancestor got from Al-Mansur for helping to subjugate soft Spaniards of his time? :wink:

:smile: :smile: Not really.

Toledo is a medieval city of Spain. It is famous for its steel and swords:

"All European armies knew the superior quality of Toledo's steel and many great warriors relied only on sabers of Toledan provenance."
go here:
http://www.tf.uni-kiel.de/matwis/amat/def_en/articles/steel_collector/swords_from_toledo.html

The best steel? In Toledo. The best swords? In Toledo. :wink:

Some time you come here I will show Toledo to you, if you want. :rolleyes:
 

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