Questions tagged [stability]

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

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Control surface deflection during takeoff

What is a reasonable (or typical) control surface deflections required for takeoff? I would like to know two ball park values if possible elevator deflection for conventional transport aircraft ...
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Why does static directional stability decrease with altitude?

At constant CAS, high altitude means lower air density and therefore higher TAS: this decreases the aerodynamic damping during a rotation. For this reason, I cannot understand why directional ...
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The Effect of Pitching Moment on Aircraft Stability

I'm designing a small RC plane and I'm currently at the first iteration of airfoil selection. I have converged to Clark-Y, NACA-2412, NACA-2415, and S-8036. In comparing the four, I noticed that all ...
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What is the location of the CG on the T-38?

Is the Northrop T-38 Talon's CG forward or aft of the center of lift (like the F-16)?
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Are there any aircraft where wing, horizontal empennage and optionally canards produce positive lift?

Planes are more stable if the center of mass is ahead of the center of lift. But this means that the tail must provide downforce to keep the nose up, which is inefficient. Are there any designs where ...
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Why is the CG assumed to be between the CP and the tail?

While discussing stability (Longitudinal, Lateral, Directional), invariably, all texts assume CG to be between CP and tail. Ex: Look at the specific image in wikipedia https://en.wikipedia.org/wiki/...
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What does it mean exactly when an aircraft is "35% unstable"? (aircraft stability factor)

CNN's Grumman X-29: The impossible fighter jet with inverted wings contains several interesting photos, and says: It was unflyable -- literally -- without a digital flight computer on board, which ...
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How do boxwing and tandem airplanes balance moment

I want to build a hydrofoil like this picture: I discovered that you can pump it with a leg motion and moving the center of mass. Like this: I discovered xflr5 and ...
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How to correct the design of an aircraft that is not speed-stable during approach?

I was just wondering if an aircraft is found to NOT be speed stable during the approach flight phase, what might a designer be able to do to stabilize the aircraft without adversely impacting its ...
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Angle of Attack influence on Adverse Yaw

we are analyzing an aircraft's lateral-directional control characteristics with self-made code. We have observed that the aileron's influence on the body axis yaw moment is quite sensitive to the ...
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How can I calculate the tail area required to achieve longitudinal static stability?

I've got a longitudinal stability question. I've not attempted this question yet as I didn't know where to start from, also my lecturer hasn't explained static stability to us clearly and I needed a ...
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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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