How Aeroplanes Fly and Propellers Pull
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How Aeroplanes Fly and Propellers Pull
Hi All,
A point of curiosity prompted this question in the Wannabes forum.
What is the primary factor that sends us aloft: Is it Bernoulli or Newton's Third Law? I actually took it as read that it was the former. But after reading Stick and Rudder and then thinking about aerfoils such as aerobatic symetrical sections or thin delta wing designs, I'm moved to believe that Newton's Third Law is the primary and possibly the overwhelming influence with the shape and camber doing little more than tweaking the efficiency of the wing.
That's enough of my semi-blind hypothesising. Are there any educated opinions out there?
A point of curiosity prompted this question in the Wannabes forum.
What is the primary factor that sends us aloft: Is it Bernoulli or Newton's Third Law? I actually took it as read that it was the former. But after reading Stick and Rudder and then thinking about aerfoils such as aerobatic symetrical sections or thin delta wing designs, I'm moved to believe that Newton's Third Law is the primary and possibly the overwhelming influence with the shape and camber doing little more than tweaking the efficiency of the wing.
That's enough of my semi-blind hypothesising. Are there any educated opinions out there?
Originally Posted by High Wing Drifter
What is the primary factor that sends us aloft: Is it Bernoulli or Newton's Third Law?
If you find a wing that flies without causing a vortex, I'll say it's all some Bernoulli magic. There is no escaping Newton's and flow turning action-reaction.
Bernoulli's effect can be used to create the necessary pressure difference at minimum wing incidence angle. It's a matter of wing design efficiency.
Ok. How did I do?
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The two are inseparable.
Newton's Third relates [action] = [- reaction]. Bernoulli's effect changes the direction of flow around the airfoil, so that it is deflected (in the case of a wing) downward. This action is reacted by the upward force (lift). One cannot have one without the other.
For a propeller, rotate the above image 90°.
Newton's Third relates [action] = [- reaction]. Bernoulli's effect changes the direction of flow around the airfoil, so that it is deflected (in the case of a wing) downward. This action is reacted by the upward force (lift). One cannot have one without the other.
For a propeller, rotate the above image 90°.
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HWD, its funny you mention Stick and Rudder.
Having just read Kermode and a few other books on aerodynamics its refreshing to read Langewiesche version of flying. 'A plane flies by throwing the air down and the resulting force throws the plane up- forget Bernoulli'. Also referring to the elevators as flippers. Priceless.
I personally always follow the theory that money makes planes fly. Lots of money.
Having just read Kermode and a few other books on aerodynamics its refreshing to read Langewiesche version of flying. 'A plane flies by throwing the air down and the resulting force throws the plane up- forget Bernoulli'. Also referring to the elevators as flippers. Priceless.
I personally always follow the theory that money makes planes fly. Lots of money.
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Thrust is created by the engine-propeller combination.
The propeller (for arguments sake) deflects air rearwards.
If the propeller pushed the air back at say 300kt then of course the aircraft should move forwards at 300kt.
Of course the very good propeller can indeed acheive said rearward airflow but the aircraft in the air at cruise will not acheive 300kt.
Ignoring drag.
Taking proeller slip into account, exaplain lift in the same newtonian terms and apply it to the wing.
Regards,
DFC
The propeller (for arguments sake) deflects air rearwards.
If the propeller pushed the air back at say 300kt then of course the aircraft should move forwards at 300kt.
Of course the very good propeller can indeed acheive said rearward airflow but the aircraft in the air at cruise will not acheive 300kt.
Ignoring drag.
Taking proeller slip into account, exaplain lift in the same newtonian terms and apply it to the wing.
Regards,
DFC
You're all wrong.
Lift is generated by inverse proportion to the lightening of the operators wallet. The bigger and heavier the flying machine, the more money is required to make it fly.
There's also an attraction between the heavens and pretty flying machines. This is why gliders in particular need relatively little money lightening to make them fly - and conversely helicopters (which are amongst the ugliest machines invented by man) require a great deal - and tanks on the other hand won't fly however much you spend.
Weight also plays a part - since clearly the heavier an aircraft is, the more lift is needed, and thus the more money. So, microlights are cheaper to run than 2-seat light aeroplanes, and proportionally 4 or 6 seat aeroplanes are more expensive again - even if they all look as pretty.
Now my serious point. I've been working in aviation for a couple of decades, have several degrees in it and even more flying licences - and frankly I don't really know what makes an aeroplane generate lift (20 years ago, I thought I knew!). What I do know is how to predict whether it will or not, and how well. Depending upon circumstance, I pick the model that suits the job - even occasionally the rubbish I've just posted above. They are all correct - up to a point.
If you want to see the more complicated ones - such as vorticity theory, Glauert's lifting line theory, or more obscure stuff that requires serious computing power to cope with at-all, I have an office full of books on it that you're welcome to come and read. The problem is, they're all right.
So pick an explanation that does the job and makes sense (N3 or Bernoulli are both fine), and use it for as long as it continues to do so.
G
Lift is generated by inverse proportion to the lightening of the operators wallet. The bigger and heavier the flying machine, the more money is required to make it fly.
There's also an attraction between the heavens and pretty flying machines. This is why gliders in particular need relatively little money lightening to make them fly - and conversely helicopters (which are amongst the ugliest machines invented by man) require a great deal - and tanks on the other hand won't fly however much you spend.
Weight also plays a part - since clearly the heavier an aircraft is, the more lift is needed, and thus the more money. So, microlights are cheaper to run than 2-seat light aeroplanes, and proportionally 4 or 6 seat aeroplanes are more expensive again - even if they all look as pretty.
Now my serious point. I've been working in aviation for a couple of decades, have several degrees in it and even more flying licences - and frankly I don't really know what makes an aeroplane generate lift (20 years ago, I thought I knew!). What I do know is how to predict whether it will or not, and how well. Depending upon circumstance, I pick the model that suits the job - even occasionally the rubbish I've just posted above. They are all correct - up to a point.
If you want to see the more complicated ones - such as vorticity theory, Glauert's lifting line theory, or more obscure stuff that requires serious computing power to cope with at-all, I have an office full of books on it that you're welcome to come and read. The problem is, they're all right.
So pick an explanation that does the job and makes sense (N3 or Bernoulli are both fine), and use it for as long as it continues to do so.
G
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I believe (may have the wrong figures here...), but Bernoulli can account for about 80% of the lift generated by a wing of an aircraft, the other 20% is Newtonian lift generated by the downwards movement of air (3rd Law, Every action blah blah).
No idea about props! I try to avoid those at all costs!
No idea about props! I try to avoid those at all costs!
Originally Posted by PhilM
I believe (may have the wrong figures here...), but Bernoulli can account for about 80% of the lift generated by a wing of an aircraft, the other 20% is Newtonian lift generated by the downwards movement of air (3rd Law, Every action blah blah).
No idea about props! I try to avoid those at all costs!
No idea about props! I try to avoid those at all costs!
Newton's laws can account for all of it.
This applies to wings or props (although for props or fast wings you need to switch to the compressible form of the equation).
G
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Ja. In Scandinavian aircraft, Coleslaw can have a significant primary effect on the pilot, and a secondary effect on the velocity of the machine if the cockpit is open, depending on the orientation of the pilot at the relevant time - ie heading, gear up or down etc. In closed cockpits (without oxygen masks in use) the effect is on the aircrew only but can be intense depending on time since consumption and the state of the cabbage used in the dish.
So if planning a spin in the Viggen, it may be best to stick to, say, pickled herring. If considering dusting off the DH-82, Stearman etc - most foods are fine.
Let me know if you need any more help.
So if planning a spin in the Viggen, it may be best to stick to, say, pickled herring. If considering dusting off the DH-82, Stearman etc - most foods are fine.
Let me know if you need any more help.
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It's Magic.
I've been wondering about wings and lift for years and I still can't understand the magic of it. Let's say that in some fantasy land I am living on the upper surface of a monstrous wing that is generating "lift". Could I breathe normally and would gravity keep my feet anchored down. In other words what is going on in that area directly above the wing?
828a
828a
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828;
In simple terms, lift, or the net force on the wing, is the result of the DIFFERENCE in pressures between the upper and lower surfaces. Hence, the upper surface could be at one atmoshere, and still produce lift if the lower surface is at more than one atmoshphere. But in most locations, the pressure on the top surface is less than the pressure on the bottom surface. So to answer 828's question, standing on this wing could be like standing in a wind slightly greater than the airspeed of the aircraft, at an altitude greater than the aircrafts altitude. On top of Everest, for example, or at Denver, depending on the speed, aerodynamics, pressures, etc. And perhaps on a slope since the wing has camber normally. Breathing as per the wind/pressure situation, gravity has basically the same effect. Hold on.
Hawk
In simple terms, lift, or the net force on the wing, is the result of the DIFFERENCE in pressures between the upper and lower surfaces. Hence, the upper surface could be at one atmoshere, and still produce lift if the lower surface is at more than one atmoshphere. But in most locations, the pressure on the top surface is less than the pressure on the bottom surface. So to answer 828's question, standing on this wing could be like standing in a wind slightly greater than the airspeed of the aircraft, at an altitude greater than the aircrafts altitude. On top of Everest, for example, or at Denver, depending on the speed, aerodynamics, pressures, etc. And perhaps on a slope since the wing has camber normally. Breathing as per the wind/pressure situation, gravity has basically the same effect. Hold on.
Hawk
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Aircraft create lift by accelerating air downwards. The lift created is the equal and opposite reaction that is predicted by Newton's third law.
The wings acelerate the air downwards by creating pressure differences.
Bernouli's theory is one method of explaining how the wings create the pressure differences.
It is worth noting that the lift is not created by the downward velocity of the air, but by the downward acceleration given to it. This might appear to be splitting hairs, but giving the air a large downward velocity means giving it lots of kinetic energy. This loss of energy from the aircraft means drag. If however a very large mass of air is given a very small downward acceleration, it will be given less kinetic energy so there will be less drag.
The wings acelerate the air downwards by creating pressure differences.
Bernouli's theory is one method of explaining how the wings create the pressure differences.
It is worth noting that the lift is not created by the downward velocity of the air, but by the downward acceleration given to it. This might appear to be splitting hairs, but giving the air a large downward velocity means giving it lots of kinetic energy. This loss of energy from the aircraft means drag. If however a very large mass of air is given a very small downward acceleration, it will be given less kinetic energy so there will be less drag.
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I've also been pondering this for years, so I'll take a crack at it.
I think that Bernoulli and Newton's third law are both at work in creating lift. When you consider the propeller blade (which is a rotating wing with typical airfoil cross section), it does NOT create thrust by ONLY creating a low pressure area in front of the propeller. If this were so, air behind the propeller would move forward to fill the low pressure area in front of the propeller. From experience we know that propellers move air rearward when producing thrust, and more thrust means more air moving rearward at faster speeds. The fan blade of a high bypass turbofan (another wing) produces thrust with very large movements of air. This illustrates that N3 is at work.
However we also know that Bernoulli is at work in the propeller because of the propeller tip (or wing tip) vortex, which is caused by high pressure air trying to move in and fill the low pressure area on the other side of the wing. We know this can only happen at the tip because the air mass is moving over the wing, and that the wing itself is in the way, blocking the rebalance of the pressures except at the tip.
I think that at very low angles of attack, Bernoulli is mostly at work, but as AOA increases, N3 increasingly does most of the work. I think this explains the shape of the AOA drag curve for airfoils. I also think this explains the AOA stall curve of airfoils. As the stall begins, lift starts to fall off as separation and turbulence begin, but the lift falls off on a curve. I think this shows that as Bernoulli falls away, some N3 remains.
I think Bernoulli adds efficiency to a wing. Even a barn door can fly with enough power, which confirms Genghis's theory of pretty flying machines. However there won't be much Bernoulli at work here, only N3. I think that Bernoulli is very energy efficient because it is only creating pressure differences. I believe N3 takes a lot more energy because with N3 you are not just creating pressure differences, you are moving air. Again, I think this explains the drag curve.
So to summarize, my theory is that at low angles of attack, Bernoulli is mostly at work. But at higher angles of attack N3 is mostly at work, and there is a transition from more of one to less of the other as the AOA changes.
Genghis, please way in as needed. Anyone else, please way in as I would really like to nail this one down once and for all.
I think that Bernoulli and Newton's third law are both at work in creating lift. When you consider the propeller blade (which is a rotating wing with typical airfoil cross section), it does NOT create thrust by ONLY creating a low pressure area in front of the propeller. If this were so, air behind the propeller would move forward to fill the low pressure area in front of the propeller. From experience we know that propellers move air rearward when producing thrust, and more thrust means more air moving rearward at faster speeds. The fan blade of a high bypass turbofan (another wing) produces thrust with very large movements of air. This illustrates that N3 is at work.
However we also know that Bernoulli is at work in the propeller because of the propeller tip (or wing tip) vortex, which is caused by high pressure air trying to move in and fill the low pressure area on the other side of the wing. We know this can only happen at the tip because the air mass is moving over the wing, and that the wing itself is in the way, blocking the rebalance of the pressures except at the tip.
I think that at very low angles of attack, Bernoulli is mostly at work, but as AOA increases, N3 increasingly does most of the work. I think this explains the shape of the AOA drag curve for airfoils. I also think this explains the AOA stall curve of airfoils. As the stall begins, lift starts to fall off as separation and turbulence begin, but the lift falls off on a curve. I think this shows that as Bernoulli falls away, some N3 remains.
I think Bernoulli adds efficiency to a wing. Even a barn door can fly with enough power, which confirms Genghis's theory of pretty flying machines. However there won't be much Bernoulli at work here, only N3. I think that Bernoulli is very energy efficient because it is only creating pressure differences. I believe N3 takes a lot more energy because with N3 you are not just creating pressure differences, you are moving air. Again, I think this explains the drag curve.
So to summarize, my theory is that at low angles of attack, Bernoulli is mostly at work. But at higher angles of attack N3 is mostly at work, and there is a transition from more of one to less of the other as the AOA changes.
Genghis, please way in as needed. Anyone else, please way in as I would really like to nail this one down once and for all.