Questions tagged [stability]

The physics of maintaining a specific state in the dynamic system of aircraft flight.

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What are the stability criteria of helicopters?

In airplanes, longitudinal stability is maintained if centre of lift lies aft of center of gravity. Like in the airplane shown in the figure below, do helicopters also have stability criteria? They ...
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How to calculate Longitudinal dynamic stability derivative for a wing? Mq and Mα

How to calculate Moment due to rate of change in pitch(Mq) and Moment due to rate of change in AOA(Mα) manually and How to extract from CFD software? Consider a fighter aircraft is in Case 1- Cruise ...
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Center of Gravity and Center of Lift stability

I have recently been reading about the center of gravity and center of lift. On the German Wikipedia there are many different graphics about the relation between the center of gravity and the center ...
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Is a trimmed flight not always an unstable system?

As soon as I am flying in trimmed flight condition, my pitching moment around the center of gravity becomes zero. So my center of pressure has to be located in the center of gravity, so that there is ...
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1answer
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How is it an advantage for the L1011 to not right itself?

A friend who flew the L1011s loved them because of their stability. His joke was that you could put the plane into a banked turn , go to the bathroom, come back, and you would still be in the same ...
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What happens if an aircraft is only stable at higher angles of attack?

Does anybody know what will happen to an aircraft, if it is stable (in longitude) for let's say: -8° > angle of attack > 8° ? So for angle of attack 4° the aircraft is not stable (centre of ...
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1answer
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How do I calculate the centre of gravity on the lateral (transverse) y-axis?

I have a real issue with determining the centre of gravity for the y axis of an aircraft, especially with selecting the reference/datum point and how to measure the displacement. Please I would really ...
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2answers
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What is the purpose of the fin sweep on small piston airplanes? [duplicate]

Why some small piston airplanes, like Cessna 152/172, Tecnam P2008 and many more, have the vertical stabilizer with a sweepback? I don't think that those airplanes may encounter compressibility ...
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1answer
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What does gravity compensation do in flight control?

In flight control designs for lateral directional movement, there is a block named "gravity compensation". This block changes yaw rate measurement from "$r$" to "$r-g/Vcos(\...
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3answers
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Wing moment - providing stability or not [closed]

It is said that wing alone is unstable, because it provides a nose up moment, so tailplane is needed to damp it. But normally wing provides a pitch down moment (as far as I know, apart from very high ...
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How does the F-16 achieve negative stability? [duplicate]

F-16 has negative stability at subsonic conditions. How is this achieved? To me it looks like a very streamligned, very normally shaped aircraft. Is the Center of Mass actually behind the Center of ...
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1answer
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Why hang glider dont over pitch when inrease AoA?

Glider change AoA with changing c.g. To do that,you must push control bar forward,your c.g. move back,so we have torque in clockwise direction which wants rotate glider.. Situation can be even worse,...
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Is C.G. of wing itself in line with hang point (hang glider)?

Where is c.g. of hang glider wing,at hang point ,in front or behind hang point? Is control bar "neutral" when glider is at rest and there is no wind or flip backward or forward? At 0:45 ...
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What happens when the eigenvalues of the longitudinal A matrix in dynamic stability analysis, are NOT two sets of complex conjugate pairs?

The complex conjugate pairs of eigenvalues of the longitudinal A matrix characterize phugoid and short period dynamic responses, this information is well established. My question is - what happens ...
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Is this glider stable?

In level flight all forces must be in balance,that mean center of overall pressure(lift) is in line with C.G. Disturbance comes in such way that shift C.P. back. Is this glider stable? If I use ...
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Why it doesn't matter if tail produce lift or downforce for neutral point position?

I am really strugle what neutral point represent and what it depends on.. In this link we can see how neutral point is calculated.Thay use wing and tail areas and distance between aerodynamic center (...
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What is the manoeuvre point?

a centre of gravity position at which stick force per g becomes zero What does this mean? If the centre of gravity happened to be at the maneouvre point, would there be stick be very light in the ...
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2answers
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Calculate pitch balance of RC glider?

Today, I played with styrofoam gliders with my kids; I made an asymmetric wing, asymmetric horizontal tail, symmetric vertical tail, and attached these to one stick for the fuselage. I've had big ...
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2answers
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How does an aircraft designer increase dynamic stability?

This question explains that an aircraft can be statically stable (it will seek to return to equilibrium) but dynamically unstable (the amplitude of the oscillations increase) if there isn't enough ...
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What does it mean when an aircraft is statically stable but dynamically unstable?

I am reading this book: "Flight Stability and Automatic Control", second edition, Dr. Robert C. Nelson. In the chapter 2, page 42, the author wrote: Note that the vehicle can be statically ...
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Is the longitudinal stability achieved the same for all aircrafts or does it differ? [duplicate]

If there is a difference in longitudinal stability how are they achieved?
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How does a transonic aircraft gains longitudinal stability [duplicate]

How is the transonic aircraft lift augmentation help in achieving stability
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1answer
634 views

How is an aircraft designed to be longitudinally stable?

How do aircraft designers ensure that a specific aircraft is longitudinally stable? Are there any different considerations for subsonic and transonic aircraft?
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Can a helicopter be stable without moving the control inputs?

I have only flown helicopters in a flight simulator (X-Plane) and I have found it exceedingly hard to fly in certain regimes (especially hovering and close to hovering) because it seems like there is ...
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How do fighter jets stop pitching motion after a elevator deflection?

I'm working on a flight simulator , In which the jet has only ailerons , rudder , and two elvators at the back . When I nose up my aircraft via elevator deflection , it just keeps pitching up ( even ...
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What happens without trimming a statically stable aircraft?

if I have a statically stable aircraft, it will always return to its neutral position. What happens now if I do not trim my aircraft? Will it move/osicallate around its neutral position and be ...
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What is the purpose of downthrust?

Many light general aviation aircraft-- and perhaps many other aircraft as well-- are designed with some of amount of downthrust in the engine mounting, relative to the centerline of the fuselage. ...
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How would a fighter be changed from having static stability to having relaxed stability?

A good portion of aircraft, especially those designed before the 1970's, possessed static stability. However, modern fighters (and some airliners) have been build with relaxed static stability. This ...
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302 views

What is rotor damping and how does it affect helicopter stability?

What do we mean by rotor damping (in terms of helicopters)? How is it affected by air density? How does helicopter stability depend on rotor damping?
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Why is the aerodynamic center for most airfoils located at the quarter chord location?

I've been reading several sources on the location of the aerodynamic center https://web.mit.edu/16.unified/www/FALL/fluids/Lectures/f03.pdf https://en.wikipedia.org/wiki/Aerodynamic_center John ...
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Does dihedral improve roll stability in gusty conditions?

Against roll instability, should I add more dihedral/a high wing? I'm trying to build an RC plane that can handle windy conditions. For that I want extraordinary aerodynamic stability. For ...
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2answers
103 views

Why do semitailless swept-winged aircraft, but not delta-winged aircraft, suffer from (primarily) pitch oscillations at high mach?

Semitailless1 swept-wing aircraft without active stability enhancement2 generally3 have extremely poor handling characteristics, most notably a tendency towards violent pitch oscillations at high ...
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1answer
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Explaining development of sideslip due to bank [duplicate]

In lateral static stability: "Once an airplane banked due to a disturbance, the airplane develops a sideslip in the direction of the bank because of a spanwise component of the weight w*sin($\phi$...
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Why didn’t the Me 163 experience the serious handling problems suffered by other similarly-designed semitailless aircraft?

Although the Messerschmitt Me 163 rocket-powered interceptor had a number of frequently-lethal features (its extremely-toxic, violently-reactive, enthusiastically-explosive hypergolic propellants; its ...
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Can an airfoil have multiple aerodynamic centers?

Aerodynamic center is a point where the moment is independent of angle of attack. Can we have 2 or more such points? Does it necessarily lie inside the airfoil?
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1answer
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Does the stability of a mass change if the center of gravity is above or below the point of thrust? [duplicate]

I was wondering if it makes any difference if the mass is balanced above or below the point of thrust. See image below for clarification: In the image you can see two configurations. The red arrow / ...
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2answers
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How far away should the vertical stabilizer be from the leading edge of the main wings?

I am wondering what the advantages and disadvantages are of placing the vertical tail nearer to, or farther from, the main wing. For instance, when the vertical tail is just behind the wing, do the ...
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236 views

How attitude, speed, and altitude vary when increasing pitch?

I have been struggling with the following question for a while, would greatly appreciate some help and explanation why. Scenario: I'm cruising in my Cessna- cruise attitude 2300 RPM trimmed for 95. ...
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Stability Analysis of Aircraft

How can we do the complete lateral, longitudinal, and directional stability analysis of an aircraft by drawing appropriate geometry? I tried DATCOM+ but it has certain limitations. Is there any other ...
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3answers
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Why does a banked aircraft create sideslip?

Below is an excerpt from Raymer (Aircraft Design) where he explains the dihedral effect. He states that a banked aircraft (due to say a roll disturbance) will have a sideslip which leads to a ...
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Will a quad fan arrangement on an aircraft help with pitching moments during VTOL flight transition?

The phenomena of ducted fan assemblies inducing pitching moments whilst in transition from vertical to horizontal flight paths is a problem I have seen mentioned by many designers and engineers of ...
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Why does directional stability decrease at supersonic speeds?

As an aircraft accelerates through the transonic region from full subsonic to to full supersonic flow its directional or yaw stability decreases. What causes this reduction?
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Why does spoiler extension during touchdown cause a pitch up moment?

There are reports that incorrect flight technique associated with spoilers extension during touchdown were contributing factors to pitch increase after landing, causing tail strikes in jet transport ...
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4answers
365 views

How can an up-lifting tail have normal force direction and gradient when generating UP force?

This article includes this statement about tail forces to validate the concept that the horizontal tail in conventionally tailed aircraft can be generating force DOWN or UP at different times and ...
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Do horizontal stabiliser force direction change with CG variation for a given aircraft? [duplicate]

I am wondering about the direction in which horizontal stabilizer provide force. I know that it can provide either upforce or downforce. But I am wondering whether is it determined for a particular ...
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1answer
97 views

How does the RQ4-Global Hawk improve stability? [closed]

I have seen several videos recorded from an UAV in which the stabilization of the camera is almost perfect. Cameras are installed in a gimbal, of course, but do new drones introduce some new ...
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242 views

How does vertical stabilisation work?

I know that similar questions are available all over the internet, including this website. However, none of the answers make full sense to me. I'm looking for simple answer (explain-like-I-am-5 type ...
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222 views

Would this hovercraft be stable? [closed]

Would this two-rotor hovercraft design work? The bold black line is weighted metal on the lower to stable it. If it helps, this is a human sized hovercraft. Would it work if: prop1 lift > prop2 lift ...
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Why did the Wright brothers take steps to increase the amount of vertical surface area forward of the CG on some of their aircraft?

Why did the Wright brothers take steps to increase the amount of vertical surface area in front of the CG on some of their aircraft? Could this have actually improved the flight characteristics in ...
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When a paraglider pilot applies brake on one side and maintains the brake pressure, why does the roll rate eventually come to zero?

I recently started to study paragliding and I am confused about the rolling movement created when a brake is maintained. I was reading this answer How do paraglider controls work? and I am not clear ...

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