Downwind turn discussion
Tolka,
True, but the weight of water that is displaced has a volume so it has to go somewhere and eckhard is correct in what he says. I believe that you need to consider the water in the aqueduct to be part of a sealed system with a constant volume of water (i.e. including the 'ocean' at either end). The weight of water displaced by the ship will have a volume that is re-distributed throughout the sealed system resulting in an overall surface level rise, including across the aqueduct. Therefore, there will be a theoretical increase in the total load on the aqueduct - but it will be veeeeery small!
So, the budgies in the truck. Once airborne, is their weight transferred back to the truck's floor by the air downflow caused by their beating wings (assuming a sealed truck)?
True, but the weight of water that is displaced has a volume so it has to go somewhere and eckhard is correct in what he says. I believe that you need to consider the water in the aqueduct to be part of a sealed system with a constant volume of water (i.e. including the 'ocean' at either end). The weight of water displaced by the ship will have a volume that is re-distributed throughout the sealed system resulting in an overall surface level rise, including across the aqueduct. Therefore, there will be a theoretical increase in the total load on the aqueduct - but it will be veeeeery small!
So, the budgies in the truck. Once airborne, is their weight transferred back to the truck's floor by the air downflow caused by their beating wings (assuming a sealed truck)?
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Crikey! Isn't Physics wonderful that its laws apply to boats and aircraft.
The boat displaces its own weight in water (the Eureka! thing). An aqueduct typically isn't an enclosed space so if you lowered the boat from a crane, water would move out at each end and the overall level of the entire canal would rise just a smidge. If the boat grounds on the aqueduct then the aqueduct is subject to an increased load that corresponds to the weight of the boat less the weight of water that it is displacing even in its sunken state.
The most marvelous incarnation of this is the Falkirk wheel which looks great from the air. (It's a good few years since I circled it, but IIRC it's under the Edinburgh CTA). It has effectively two bathtubs, one on each end of a long bar. The tubs fill with water and the bar balances. A boat sails in and displaces its weight in water. Consequently, the bar is still balanced. I don't know how much power is needed to move a boat from one level to the other but I imagine it's tiny. A true marvel of engineering.
The boat displaces its own weight in water (the Eureka! thing). An aqueduct typically isn't an enclosed space so if you lowered the boat from a crane, water would move out at each end and the overall level of the entire canal would rise just a smidge. If the boat grounds on the aqueduct then the aqueduct is subject to an increased load that corresponds to the weight of the boat less the weight of water that it is displacing even in its sunken state.
The most marvelous incarnation of this is the Falkirk wheel which looks great from the air. (It's a good few years since I circled it, but IIRC it's under the Edinburgh CTA). It has effectively two bathtubs, one on each end of a long bar. The tubs fill with water and the bar balances. A boat sails in and displaces its weight in water. Consequently, the bar is still balanced. I don't know how much power is needed to move a boat from one level to the other but I imagine it's tiny. A true marvel of engineering.
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If 2,000 dead chickens each weighing 5lbs are loaded in an aeroplane it has a payload of 10,000lbs. If the same chickens are alive and flap their wings vigorously does the payload decrease and what is the affect on aircraft performance?
decelerate. Look it up
That's what happens when you try to apply logic to English grammar
Do a search and you'll find the word used in many places, scientific papers etc.
Last edited by megan; 9th Feb 2017 at 16:14. Reason: pre not suffix
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If the same chickens are alive and flap their wings vigorously does the payload decrease and what is the affect on aircraft performance?
Are we getting more than a little abstract here? If we carry on like this someone will be asking whether a cat left in the hold is alive or dead (that one for Cabin Pressure afficionados).
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If we carry on like this someone will be asking whether a cat left in the hold is alive or dead
Now we're talking PROPER physics!
TOO
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Back to the OP's discussion...
There were letters in AOPA's magazine a while ago propounding the 'inertia' theory from someone styling themselves 'Bunbury'. I took it to be a hoax using the pseudonym employed in Oscar Wilde's 'The Importance of Being Ernest'.
Next..
'Flat Plate' theory of lift.
For those of you who aren't familiar, this is where all those fancy aerofoil sections are regarded as so much bunk and all that you need is a flat plate to generate lift.
If we keep this thread running long enough, it'll soon be 1st April, which is where all this stuff belongs.
TOO
There were letters in AOPA's magazine a while ago propounding the 'inertia' theory from someone styling themselves 'Bunbury'. I took it to be a hoax using the pseudonym employed in Oscar Wilde's 'The Importance of Being Ernest'.
Next..
'Flat Plate' theory of lift.
For those of you who aren't familiar, this is where all those fancy aerofoil sections are regarded as so much bunk and all that you need is a flat plate to generate lift.
If we keep this thread running long enough, it'll soon be 1st April, which is where all this stuff belongs.
TOO
all that you need is a flat plate to generate lift.
The fancy aerofoils result in greater aerodynamic efficiency and also help the structures and design people to have room for spars, tanks, wheels, etc.
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True! Actually I was thinking of smaller and slower things like longeze and e-Go. The tiny canard on e-Go produces a huge amount of the total lift at cruise speed.
Getting back to the original subject of this thread:
The following post which was provided by a contributor using the name “theheadmaster” in the thread concerning the Mallard crash in the Pacific Forum covers the matter very well.
For completeness the tailwind scenario is as follows:
If you are going from a 50 knot tailwind to a 50 knot headwind, the airspeed change is still 100 knots ahead to 100 knots in the opposite direction. So, 200 knots velocity change. If you want to work in groundspeed, it will be 150 knots (100 kts airspeed plus 50 kts tailwind) to 50 knot in the opposite direction (100 kts minus headwind of 50 kts). So, the velocity change is still 200 kts, the exact same value as nil wind. As mass is constant and velocity change is the same, the change in inertia is identical in all three cases.
The use of the word MOMENTUM rather than the word INERTIA in the above statements would have been more accurate, but the meaning is quite clear.
So regardless of whether we use the ground or the air mass as our reference frame, and whether we use still air, headwind or tailwind, the aircraft will experience the same acceleration.
Everything which has mass has inertia, so whenever an aircraft is manoeuvring its inertia will affect its performance. But this effect will be the same regardless of whether the air is still, a headwind or a tailwind. So it is not true to say that the inertia of an aircraft does not affect its performance in a downwind turn. It is simply that the effect is the same as that in any other manoeuvre.
The danger when turning from downwind to upwind, is that half way through the turn the aircraft will be flying across the wind. This will cause it to drift downwind. If the pilot interprets this as having insufficient bank angle, he/she will be tempted to bank further into the turn. At low speeds this risks entering a stall/spin.
Megan you have argued that the only reference frame which is relevant is the air mass, because this is the only one which affects the performance of the aircraft. It is certainly true that the interactions between the aircraft and the air mass are the only ones which influence lift and drag. But the purpose of flying is usually to get from one place to another. For this purpose the most relevant reference frame is the ground. You have also stated that you are only talking about accelerations in the fore and aft direction. The fore and aft direction is a valid coordinate if your reference frame is the aircraft, but it is not valid if you are using the air mass reference frame. Whatever reference frame we choose to use, the coordinate system must be fixed relative to that frame. The fore and aft axis of an aircraft is not fixed relative the air mass in which it is flying.
Brian you asked:
It comes from the wings. In a 60 degree banked turn the aircraft experiences a 2g acceleration. The vertical component of lift is equal to the weight of the aircraft and the horizontal component is approximately 1.7321 times the weight of the aircraft. (in the triangle of forces we have 1 squared + 1.7321 squared = 2 squared). That horizontal force of 1.7321 times the weight, is accelerating the aircraft towards the centre of the turn. One factor which is often forgotten is that the wings generate much greater forces than the engine and propeller. Any fixed wing aircraft can be supported by wing lift, but how many can hang vertically on the propeller thrust?
The following post which was provided by a contributor using the name “theheadmaster” in the thread concerning the Mallard crash in the Pacific Forum covers the matter very well.
Momentum is a vector value. The formula is p=mv, p being inertia, m being mass, v being velocity. In our example mass is constant.
Nil wind, aircraft 100 knots, turning through 180 degrees, the velocity change is 100 knots forward to 100 knots in the reverse direction (both airspeed and ground speed), so a 200 knot vector change.
If you are going from a 50 knot headwind to a 50 knot tailwind, the airspeed change is still 100 knots ahead to 100 knots in the opposite direction. So, 200 knots velocity change. If you want to work in groundspeed, it will be 50 knots (100 kts airspeed less 50 kts headwind) to 150 knot in the opposite direction (100 kts plus tailwind of 50 kts). So, the velocity change is still 200 kts, the exact same value as nil wind. As mass is constant and velocity change is the same, the change in inertia is identical in both cases.
Nil wind, aircraft 100 knots, turning through 180 degrees, the velocity change is 100 knots forward to 100 knots in the reverse direction (both airspeed and ground speed), so a 200 knot vector change.
If you are going from a 50 knot headwind to a 50 knot tailwind, the airspeed change is still 100 knots ahead to 100 knots in the opposite direction. So, 200 knots velocity change. If you want to work in groundspeed, it will be 50 knots (100 kts airspeed less 50 kts headwind) to 150 knot in the opposite direction (100 kts plus tailwind of 50 kts). So, the velocity change is still 200 kts, the exact same value as nil wind. As mass is constant and velocity change is the same, the change in inertia is identical in both cases.
If you are going from a 50 knot tailwind to a 50 knot headwind, the airspeed change is still 100 knots ahead to 100 knots in the opposite direction. So, 200 knots velocity change. If you want to work in groundspeed, it will be 150 knots (100 kts airspeed plus 50 kts tailwind) to 50 knot in the opposite direction (100 kts minus headwind of 50 kts). So, the velocity change is still 200 kts, the exact same value as nil wind. As mass is constant and velocity change is the same, the change in inertia is identical in all three cases.
The use of the word MOMENTUM rather than the word INERTIA in the above statements would have been more accurate, but the meaning is quite clear.
So regardless of whether we use the ground or the air mass as our reference frame, and whether we use still air, headwind or tailwind, the aircraft will experience the same acceleration.
Everything which has mass has inertia, so whenever an aircraft is manoeuvring its inertia will affect its performance. But this effect will be the same regardless of whether the air is still, a headwind or a tailwind. So it is not true to say that the inertia of an aircraft does not affect its performance in a downwind turn. It is simply that the effect is the same as that in any other manoeuvre.
The danger when turning from downwind to upwind, is that half way through the turn the aircraft will be flying across the wind. This will cause it to drift downwind. If the pilot interprets this as having insufficient bank angle, he/she will be tempted to bank further into the turn. At low speeds this risks entering a stall/spin.
Megan you have argued that the only reference frame which is relevant is the air mass, because this is the only one which affects the performance of the aircraft. It is certainly true that the interactions between the aircraft and the air mass are the only ones which influence lift and drag. But the purpose of flying is usually to get from one place to another. For this purpose the most relevant reference frame is the ground. You have also stated that you are only talking about accelerations in the fore and aft direction. The fore and aft direction is a valid coordinate if your reference frame is the aircraft, but it is not valid if you are using the air mass reference frame. Whatever reference frame we choose to use, the coordinate system must be fixed relative to that frame. The fore and aft axis of an aircraft is not fixed relative the air mass in which it is flying.
Brian you asked:
Force = mass X acceleration or acceleration = force/mass
From whence does the force commeth?
From whence does the force commeth?
Last edited by keith williams; 11th Feb 2017 at 14:06.
One factor which is often forgotten is that the wings generate much greater forces than the engine and propeller. Any fixed wing aircraft can be supported by wing lift, but how many can hang vertically on the propeller thrust?
I blame this on the way that the 'four forces' are displayed in most diagrams.
The four arrows are shown as roughly equal in length, whereas for a typical small propeller aircraft, the vertical forces should be about ten times bigger than the horizontal forces.
The problem is fitting the forces in to a conveniently-proportioned diagram on the text-book page and displaying them to scale.
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The danger when turning from downwind to upwind, is that half way through the turn the aircraft will be flying across the wind. This will cause it to drift downwind. If the pilot interprets this as having insufficient bank angle, he/she will be tempted to bank further into the turn. At low speeds this risks entering a stall/spin.
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If the pilot interprets this as having insufficient bank angle, he/she will be tempted to bank further into the turn. At low speeds this risks entering a stall/spin.
I was doing a standard BGA exercise in a Puchacz (which has a reputation for spinning) a couple of weeks ago. At 20, 40 and 60 degrees of bank the glider was stalled and the airspeed noted. Stall speed increased significantly at 60 degrees, but it showed no inclination to spin. A slow shallow turn with a little extra rudder and it won't hesitate.
I blame this on the way that the 'four forces' are displayed in most diagrams.
I agree. In the glider case, specifically my club's DG-1000 two-seater, the maximum AUW is 760 Kg and the best L/D is 46.5 to 1. So the total drag in 1G flight is 16Kg! I've read that opening the cockpit window and putting your hand out, can double the drag.
G'day Kieth, you're correct, but extending the argument beyond that to which I was referring. That was, changes taking place while turning from into wind to downwind while maintaining a constant ASI.
I was speaking strictly re the case of turning from upwind to downwind. Nothing else or more.
As you say usually, but not always. The time when ground reference comes into the picture is answering what's my drift, GS and ETA.
I know what you are saying, if an aircraft accelerates there will be an incremental difference seen between the body frame and air mass frame due the change in AoA. The point I obviously did not make well enough is that, there will be no acceleration sensed in the longitudinal axis, with respect to either the body or air mass plane in an aircraft making a turn from upwind to downwind while maintaining a constant ASI.
An example of taking something out of context.
Helicopters do it every day on every flight, and not only hang, but climb. 
pro·pel·ler (prə-pĕl′ər)
A device consisting of a series of twisted blades mounted around a shaft and spun to force air or water in a specific direction and thereby move an aircraft or boat.
you have argued that the only reference frame which is relevant is the air mass
the purpose of flying is usually to get from one place to another
The fore and aft direction is a valid coordinate if your reference frame is the aircraft, but it is not valid if you are using the air mass reference frame
An example of taking something out of context.
how many can hang vertically on the propeller thrust?
pro·pel·ler (prə-pĕl′ər)
A device consisting of a series of twisted blades mounted around a shaft and spun to force air or water in a specific direction and thereby move an aircraft or boat.