Aerodynamicists Question
Joined: Mar 2008
Posts: 149
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From: Australia
This is awesome,
To throw yet another spanner in the works, why does the centre of pressure move aft after the critical angle is reached?
I agree with all the posts that regard stalling nothing else but exceeding the critical angle by the way. Speed is simply a tool by which we can recognise the onset of the critical angle and it will vary with varying other mentioned factors ie weight/load factor.
Bloody awesome thread mate.
To throw yet another spanner in the works, why does the centre of pressure move aft after the critical angle is reached?
I agree with all the posts that regard stalling nothing else but exceeding the critical angle by the way. Speed is simply a tool by which we can recognise the onset of the critical angle and it will vary with varying other mentioned factors ie weight/load factor.
Bloody awesome thread mate.
Joined: May 2002
Posts: 1,210
Likes: 56
From: Melbourne, Australia
This report illustrates three type of stall with chordwise pressure distributions provided.
- trailing edge stall
- leading edge stall
- thin aerofoil stall
NACA Tech Note 2502.
".. it is believed that they illustrate the stalling characteristics of most practical airfoil section."
- trailing edge stall
- leading edge stall
- thin aerofoil stall
NACA Tech Note 2502.
".. it is believed that they illustrate the stalling characteristics of most practical airfoil section."
Joined: Sep 2007
Posts: 38
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From: Southern Hemisphere
Spanner in the works anyone??
Only 'somewhat' related. But if those wing mods mentioned in the original question change the aspect ratio of the wing then the downwash will also be modified. Thus changing the geometric angle of attack for a given effective angle of attack. The Effective Angle of Attack, however, is what determines when the wing stalls (stall occurs at AoA > critical angle (effective)).
The thread is off topic already so i figured i'd help with the diversification.
And a question.....i've recently done my instructors rating and we got taught Bernoulli's Theorem etc..yet the examiner had a chat with me about coander and downwash and i've also heard of circulation...Can anyone with some experience please inform me of what the dominant theory of lift production is? If someone asked you what provided the upward force, what would you tell them?
The thread is off topic already so i figured i'd help with the diversification.
And a question.....i've recently done my instructors rating and we got taught Bernoulli's Theorem etc..yet the examiner had a chat with me about coander and downwash and i've also heard of circulation...Can anyone with some experience please inform me of what the dominant theory of lift production is? If someone asked you what provided the upward force, what would you tell them?
Fleet Manager
Joined: Apr 2001
Aviation Qualifications: ATPL
Posts: 7,445
Likes: 310
From: various places .....
If someone asked you what provided the upward force, what would you tell them?
The wing cleverly throws a lot of air down and, as a result, wants to go up .. something Newton said a long time ago, as I recall ?
The effectiveness of this process (ie lift is desirable but drag is a bore) is a measure of the cleverness of the designer and the then present literature state of the art.
When you come to describe what's going on it's no different to any typical technical thing .. you can have the simpler or the more complicated explanation according to your needs and interests.
For the typical pilot a combination of the reaction and Bernoulli seems to be reasonably adequate ? One needs to have a passing bit of an idea about pressure distributions because pressure is the way a fluid transfers a force to a solid.
The wing cleverly throws a lot of air down and, as a result, wants to go up .. something Newton said a long time ago, as I recall ?
The effectiveness of this process (ie lift is desirable but drag is a bore) is a measure of the cleverness of the designer and the then present literature state of the art.
When you come to describe what's going on it's no different to any typical technical thing .. you can have the simpler or the more complicated explanation according to your needs and interests.
For the typical pilot a combination of the reaction and Bernoulli seems to be reasonably adequate ? One needs to have a passing bit of an idea about pressure distributions because pressure is the way a fluid transfers a force to a solid.
Joined: Jan 2008
Posts: 250
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From: YLIL
The wing cleverly throws a lot of air down and, as a result, wants to go up ...
I'ts always puzzled me why we always talk about the prop producing thrust by pushing the air back (Bernoulli never gets a mention) but when we get to the wing we tie ourselves up in knots with Bernoulli, circulation, coanda and other exotica.
Are they not both aerofoils? And does not the wing, like it's smaller rotating cousin, push lots of air down?
Fleet Manager
Joined: Apr 2001
Aviation Qualifications: ATPL
Posts: 7,445
Likes: 310
From: various places .....
I'ts always puzzled me why we always talk about the prop producing thrust by pushing the air back
The considerations and explanations relate to one's vantage point.
If that be
(a) a substantial distance, then the "gross" effects are obvious - air gets thrown/pushed/shoved/whatever this way or that and there is a reaction (which is the thing we are after). This viewpoint is simple and appeals to me as a pilot as it gives me most of what I need to know to point an aeroplane in a desired direction etc ...
.. ranging through to ... being
(b) right up, close and personal, where we find ourselves looking at what is happening at/near the lifting surfaces which are actually performing the magic things. Then the mathematicians come into their own ... This viewpoint appeals to me as an engineer as I can use the mathematics to predict loads, etc., which provide useful information for doing engineering stuff.
but when we get to the wing we tie ourselves up in knots with Bernoulli, circulation, coanda and other exotica
knots - that was a pun, right ?
Bernoulli - useful for getting a feel about fluid flow and pressures - good pilot stuff
circulation - engineer stuff and not of much interest to the pilot except for handwaving whilst talking about trailing vortices at the bar on Saturday night in the presence of adoring neophytes
(Henri) Coanda useful for handwaving explanations of a number of attractive party tricks at the bar on Saturday night in the presence of adoring advanced neophytes (but not of much interest to the pilot).
The considerations and explanations relate to one's vantage point.
If that be
(a) a substantial distance, then the "gross" effects are obvious - air gets thrown/pushed/shoved/whatever this way or that and there is a reaction (which is the thing we are after). This viewpoint is simple and appeals to me as a pilot as it gives me most of what I need to know to point an aeroplane in a desired direction etc ...
.. ranging through to ... being
(b) right up, close and personal, where we find ourselves looking at what is happening at/near the lifting surfaces which are actually performing the magic things. Then the mathematicians come into their own ... This viewpoint appeals to me as an engineer as I can use the mathematics to predict loads, etc., which provide useful information for doing engineering stuff.
but when we get to the wing we tie ourselves up in knots with Bernoulli, circulation, coanda and other exotica
knots - that was a pun, right ?
Bernoulli - useful for getting a feel about fluid flow and pressures - good pilot stuff
circulation - engineer stuff and not of much interest to the pilot except for handwaving whilst talking about trailing vortices at the bar on Saturday night in the presence of adoring neophytes
(Henri) Coanda useful for handwaving explanations of a number of attractive party tricks at the bar on Saturday night in the presence of adoring advanced neophytes (but not of much interest to the pilot).
Joined: Jul 2005
Posts: 555
Likes: 2
From: South Island
I have only ever heard props described in the correct manner; as an aerofoil with lift and torque vectors , etc.
(Unless we're talking about a TIF, in which case the lift of the main aerofoil tends to get explained in rather simple terms too.)
(Unless we're talking about a TIF, in which case the lift of the main aerofoil tends to get explained in rather simple terms too.)
Joined: Jul 2008
Posts: 82
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From: Amongst the weeds and the dust
Hmm... Seems like a role-play is on order:
XXX - At what speed can I legally travel down the freeway?
Others - 110km/h
GG - No no no, that's just what the car speedo is saying. What you need to do is take into consideration the rotational velocity of the engine's crank shaft, then multiply that by the respective gear ratio (which is dependant on many things, such as which gear is selected, and if it's an auto box a factor for slippage must also be applied), then multiplied by the diff ratio. Finally you must calculate the car's tyre radius (which of course will vary depending on tyre pressure, load on the tyre, tyre temperature and tread level) and multiply that by the rotational speed of the axle, thus giving you a speed roughly equivalent to the calibrated speed shown on the speedo. The two are unrelated.
Others -
Both technically correct, only one answers the question that was asked...
XXX - At what speed can I legally travel down the freeway?
Others - 110km/h
GG - No no no, that's just what the car speedo is saying. What you need to do is take into consideration the rotational velocity of the engine's crank shaft, then multiply that by the respective gear ratio (which is dependant on many things, such as which gear is selected, and if it's an auto box a factor for slippage must also be applied), then multiplied by the diff ratio. Finally you must calculate the car's tyre radius (which of course will vary depending on tyre pressure, load on the tyre, tyre temperature and tread level) and multiply that by the rotational speed of the axle, thus giving you a speed roughly equivalent to the calibrated speed shown on the speedo. The two are unrelated.
Others -
Both technically correct, only one answers the question that was asked...
