Naked Science Forum
On the Lighter Side => Science Experiments => Topic started by: simplex on 29/02/2020 21:45:11
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The Wright brothers’ patent: "Plane stabilized in roll by a pendulum" (US Patent no. 1,075,533). It seems impossible.
"The pendulum rocket fallacy is a common fundamental misunderstanding of the mechanics of rocket flight and how rockets remain on a stable trajectory." Robert Goddard fell into this trap as late as March 16, 1926 when he tested a liquid fuel rocket. The Wright brothers filed their US Patent no. 1,075,533 in February 1908, tested the stabilizer in December 1913 and it worked flawlessly. Orville Wright even got a prize for his achievement (see the book below).
How is it possible? A pendulum with its pivot in an accelerated movement (like inside an airplane) does not tend to align to the vertical and can not be used as a reference like a gyroscope.
Is there another case when somebody succeed in stabilizing a plane in roll using a pendulum?
The 5 Wright brothers' patents:
- The US patent no. 821,393, granted on May 22, 1906, and its foreign versions, claim: (1) the method of wing warping, in particular, and the ailerons (already invented in 1868 by M. P. W. Boulton), in general, for stabilizing an aeroplane type machine in roll, (2) a movable vertical tail aimed at counteracting the adverse yaw generated by twisting the main wings, (3) a flexible front elevator for maintaining the pitch stability of the same machine, (4) various constructive details.
- The French patent no. 384.124, published on March 30, 1908, and its foreign versions, claim two more vertical rudders, placed in front of the main wings, one fixed and the other mobile. They were aimed at better counteracting the adverse yaw.
- The French patent no. 384.125, published on March 30, 1908, and its foreign versions, claim two additional vertical rudders, placed close to the tips of the main wings. Their purpose was also for eliminating the adverse yaw.
-> The US Patent no. 1,075,533, granted on October 14, 1913, and its foreign versions, claim automatic stabilization mechanisms: in roll, driven by a pendulum, and in pitch, governed by wind vanes (two models are proposed).
- The US patent no. 908,929 - “Mechanism for Flexing the Rudder of a Flying Machine or the Like”, granted on January 5, 1909, and its foreign versions, claim systems aimed at flexing the rudders of an aeroplane type machine for the purpose of modifying their lift.
Download link for "The Wright brothers’ patents and their low importance for aviation" (http://wright1903dec17.altervista.org/Wright-patents.htm)
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwright-brothers.wikidot.com%2Flocal--files%2Fstart%2FThe-Wright-brothers-patents-and-their-low-importance-for-aviation-Book-by-Bogdan-Lazar.jpg&hash=ac54120ab524a6fcf00477f7916f9d67) (http://wright1903dec17.altervista.org/The-Wright-brothers-patents-and-their-low-importance-for-aviation-Book-by-Bogdan-Lazar.pdf)
The book contains the patents of the Wright brothers in full.
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Pendulum stabilisation is indeed used in some aircraft instruments and simple autopilots. It works because we are talking about real engineering, not theoretical physics!
Roll stability (the "wing leveller") is the simplest autopilot for light aircraft: once you have established a stable course (by steering) level and speed (by pitch trim and engine speed), there is no acceleration for several minutes or even hours, so a pendulum reference allows you to let go of the control column whilst you fiddle with your phone, chat to the cabin crew or put out the fire. It can also provide adequate roll stability in a cruise climb or descent, which takes some of the stress out of instrument approaches and departures.
It's also used to erect the artificial horizon - worth studying the technical manuals because the system is dead clever but too complicated to describe here!
Given the low speed, complicated engine management and general instability of early airplanes, a pendulum wing leveller could usefully reduce the number of arms and brains required to fly them.
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The pendulum of the Wright brothers is not about the acceleration the plane has along its longitudinal axis but about the angular acceleration when the aircraft randomly rolls about its longitudinal axis. The Wright planes were unstable in roll and the two inventors were looking for means to stabilize their flying machines. The US Patent no. 1,075,533 is about automatic stabilization governed by a pendulum whose pivot continuously goes to the left or to the right in a plane perpendicular to the longitudinal axis of the airplane.
Anyway, any example of a working device with a pendulum that is able to correct the bank angle of a plane, is welcomed.
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Look for a picture of "turn and slip indicator".
The turn indicator part (usually at the top) is a tethered gyro, but the slip gauge looks like an inverted spirit level. If the lateral axis of the airplane is not horizontal, the black ball of the SG slides along the tube. In an intentional coordinated turn you apply bank, then use the rudder to "kick the ball into the middle" so that the effective g force remains aligned with the lift vector. However in straight and level flight, with the rudder trim correctly set, a gust under one wing will also displace the ball so you can use that to indicate roll (and thus correct it manually) if all the gyros have failed in zero visibility.
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... any example of a working device with a pendulum that is able to correct the bank angle of a plane, is welcomed.
I can supply one to the contrary ...
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Look for a picture of "turn and slip indicator".
This a very good remark.
"The turn and slip indicator can be referred to as the turn and bank indicator, although the instrument does not respond directly to bank angle." (https://en.wikipedia.org/wiki/Turn_and_slip_indicator)
If the plane takes a turn in a coordinated way the bubble of air will rest in the middle of the tube regardless the bank angle. A pendulum hanging from the ceiling of a plane will do the same thing. It will point toward the floor of the airplane (the rod stays perpendicular to the longitudinal and transverse axes of the airplane) no matter of the roll angle, assuming the centripetal acceleration during the turn remains constant. However, neither the bubble nor the pendulum can say anything about the bank angle and in consequence they can not be used as governors which correct the unwanted roll.
In the case of the Wright brothers, they correct the roll with a pendulum!
The turn and slip indicator is one more piece of experimental evidence that a pendulum or a bubble can not be used for measuring the bank angle of an airplane.
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I think the bubble slip gauge was abandoned around 1920. Most aircraft use a heavy ball in a viscous liquid.
To lose height quickly without losing speed, e.g. approaching in turbulence, short runway, tailwind, failed flap motor, 747 up your backside, or bored controller saying "expedite or go around", you can use a forward slip. Roll left (to maintain best view of the ground) and prevent the turn by feeding in right rudder. The slip ball and attitude indicator will indicate bank but the turn indicator should stay central. It's difficult to get right but can be a lifesaver. Similarly you can cope with a lot of crosswind by sideslip: start the turn into wind, hold the bank angle and apply opposite yaw with the rudder to get the ball indicating bank but the aircraft flying sideways in a straight line. Scares the bejasus out of the passengers but will get the slipperiest of jets and gliders into a remarkably small field.
Note the difference between a turn indicator and a turn coordinator with an angled gyro - the latter responds to roll rate.
Early aircraft really didn't make significantly banked turns. If you're hugging the ground with marginal power, all sorts of things can go wrong in a banked turn, so even up to 1914 elementary flying manuals recommended flat skidding turns and the idea of a pendulum stabiliser was to correct for asymmetric gusts in straight flight.
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In a straight flight the pendulum does not work even if the speed of the plane along its longitudinal axis is constant and the plane flies in calm air parallel to the ground. As soon as the airplane starts to roll, even at constant angular speed, the pendulum gets a chaotic movement and has no tendency to align to the vertical or oscillate about the vertical of the place.
(https://www.maplesoft.com/view.aspx?SI=4888/PendMovingPivot_127.gif)
This is a simulation of a pendulum with a pivot rotating at constant speed along the circle of center (0,0) and radius=1 (as if it is inside a planes that turns continuously about its longitudinal axis while flying at constant speed). There is no tendency of alignment to the vertical. The bob has a chaotic trajectory from the moment it starts to move.
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Try fixing the pivot at the center of rotation of the aircraft, give the pendulum some inertia, and add some damping. And don't attempt a complete barrel roll, never mind continuous rotation. Then use the pendulum deflection to correct the roll. Remember the object is to sense the initial roll and correct it. Also think about roll rates: with full aileron deflection, very few aircraft can exceed 100 deg/s. For wing levelling in cruise we are looking to correct about 1 - 2 degrees of wander.
Or try flying a plane. It's a lot more satisfying than computer simulations, even of flying. We disconnect the roll damper for aerobatics or combat, in which situations everything that isn't actually bolted to the floor comes up and hits you in the face, chaotically.
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No matter how you run the simulation, the trajectory of the pendulum is always chaotic regardless the damping ratio.
The pivot can not be placed in (0,0) because the center of rotation of an airplane is unknown. It depends on the number of people inside, their weight and position, the quantity of fuel, etc. If, I put the pendulum, in the simulation, only 1-2 mm away from (0,0) the bob has the same chaotic movement as in the animation above.
Even if I bank the plane only 5 degrees to one side, as the trajectory of the bob is dependent on the angular speed at which the plane rolls and its lateral acceleration, I get the same chaotic movement.
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If the plane rolls left, which way does the pendulum initially swing? Why?
Worth remembering that whilst airplanes can rotate fairly quickly about all three axes, linear acceleration is very slow.
Anyway, you have comprehensively proved to your own satisfaction that the instrument everyone has used for the last 100 years, and the simple wing leveller I've been using for the last 50 years, doesn't work, which is why planes fall out of the sky all the time. Thank you for enlightening thousands of pilots and engineers who don't know any better!
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"Anyway, you have comprehensively proved to your own satisfaction that the instrument everyone has used for the last 100 years, and the simple wing leveller I've been using for the last 50 years, doesn't work"
If by wing leveler you mean the glass tube and the air bubble inside this device:
(https://upload.wikimedia.org/wikipedia/commons/thumb/f/fa/Turn_coordinator_-_coordinated.svg/440px-Turn_coordinator_-_coordinated.svg.png)
which is basically a pendulum, it is well known that it is unable to indicate the roll angle. In the picture, the plane is banked to the right and the air-bubble (the bob of the pendulum) rests in the middle of the tube! The fact that you see the plane banked to the right has nothing to do with the air bubble (the pendulum) but with the gyroscopes inside the device. They detect the roll angle not the bubble.
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There is no bubble in your picture. It is a standard rate turn coordinator, not a wing leveller.
The "tilted airplane" indicates that the aircraft is turning right at 3 degrees/second. The central ball (the pendulum) shows that the turn is correctly balanced with no slip or skid. It's all in the elementary flying training manuals, or you could attend my groundschool lectures where we dismantle the instrument and discuss how it works.
As I pointed out earlier, if you apply bank and hold off into a slip with opposite rudder, the ball will indicate slip (offcenter) and the gyro of a turn indicator will show no turn ("level"). The turn coordinator you have shown will initially indicate roll as you apply bank, but will gradually centralise if the airplane isn't actually turning.
Failure to understand this instrument can seriously shorten your life expectancy.
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1) Then post a link or a picture of that "simple wing leveller you have been using for the last 50 years" that works using the principle of the pendulum. I simply do not find such a basic device.
2) In a coordinated turn the rod of the pendulum will always have the direction of the total lift (or the resultant of the weight and the so called centrifugal force), see the picture below. In the case of the ball, this will always stay in the middle of the tube.
(https://i.stack.imgur.com/Z3EVn.png)
As long as the turn is coordinated, no matter of the value of the bank angle, the pilot will always see the pendulum or the ball in the tube in the same position as if the plane flies non-accelerated in a straight line. How can such a device be used for measuring the roll angle?!
3) A simple example with a train and a pendulum inside it:
This is a rail-car in an uncoordinated turn (the resultant of the centrifugal force and the weight does not point perpendicular to the plane of the rails)
(https://qph.fs.quoracdn.net/main-qimg-673bd770868c4c4ddbd967516390e043)
A pendulum in that rail-car will have the direction of the above mentioned resultant but its angle with the blue line will have nothing to do with the angle between the railway and the horizontal. The pendulum will just show to somebody in the car that the train is moving too fast, above the optimum speed of that turn.
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The whole point of a wing leveller is to prevent roll, not to measure it. You have to switch it off for an intentional turn, because it will fight against your roll input. Most modern units actually run from the turn coordinator, an angled gyro that responds to rate of roll, and the directional gyro, so you can use its proportional response to alter course, but you may find some precursors of the "Navomatic" that used a simple pendulum.
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but you may find some precursors of the "Navomatic" that used a simple pendulum.
When you find one by chance please post a link to it here.