Questions tagged [stability]

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

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Comparing a Flight In Turbulent Conditions At Va And Vb

First, let's cover definitions for both airspeeds: Turbulence penetration airspeed (Vb) is the design speed for maximum gust intensity (i.e. the intersection between the 66 fps gust line and the ...
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Doubt in the Coriolis component of the net force acting on the aircraft

I dig a little and got to know that the force equation in 6DOF are nothing but a mathematical depiction of the fact that for a translating & Rotating body in 3D,The net force is the Translational ...
Ravi Pratap Singh.'s user avatar
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After designing an autopilot, how does it affect aircraft modes?

Let's say an autopilot will be designed for the longitudinal axis. A pitch attitude hold will be made with a sas in the inner loop and a pid in the outer loop. How does this system change the phugoid ...
gibsonsg's user avatar
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Is the AOA on the horizontal stabilizer positive?

I am making a flight sim and would like some help. If I am correct, the angle of attack on the horizontal stabilizer should look something like this: aoa(Hstab) = aoa(Wing) - downwash + Hstab ...
SpiralingTessaract8164's user avatar
3 votes
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What are the effects of a corrugated surface on spanwise flow deviation, for a swept back flying wing?

Considering a Ju-52 wing in some hypothetical swept back flying wing configuration using the same strucural design as the Ju-52. What would be the effects of that corrugated skin, regarding spanwise ...
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Is there an aerodynamic reason for airship fins to be at the tail?

I've read this interesting Stack Exchange question about why airships have fins: Why do blimps have fins? Which has me wondering why the fins are almost always located in the tail section. After some ...
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Does pilot need to increase thrust when gust hit him at landing and where is axis of rotation?

Let's say a plane is flying above a runway in straight line, and gust of wind hits the plane from side. Do pilot need increase thrust to stay above runway and where is axis of rotation ? It often said ...
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What can cause an unstable phugoid mode?

For clarity, I am wondering about how aircraft design can lead to the phugoid poles becoming complex with a positive real part, so an oscillation with increasing amplitudes. My understanding of the ...
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Simultaneous yaw damping and sideslip control of aircraft with rudder?

Yaw damping is achieved by using the rudder to generate a yawing moment that opposes any yawing motion. Also, rudder is used to correct sideslip by applying a rudder input in the opposite direction of ...
hereyougo there's user avatar
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Confusion about center-of-lift vs center-of-gravity [duplicate]

As usually depicted, in a small aircraft (maybe larger commercial ones too), the lift due to the main wing (excluding the elevator) is depicted as on average aft of the center of gravity, with the ...
Foster Boondoggle's user avatar
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Did the dog-tooth notches in the wing of the F-8 Crusader really have anything to do with "yaw stability"?

What was the real function of the "dog-tooth" notches in the leading edge of the wing of the Vought F8U crusader? Wikipedia says:1 Vought's design team... produced... a relatively ...
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How does distance between wings of a biplane affect its horizontal stabilizer design?

When for instance pitching down, a biplane's upper wing has higher airspeed than lower wing, therefore higher drag that should dampen the pitching down rate. How significant is this, if it is, ...
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4 votes
2 answers
275 views

How does conical camber decrease roll due to sideslip?

I was reading Ray Whitford's Design for Air combat and came upon a passage that said that on the SR-71 conical camber "gives a useful reduction in the otherwise very high rolling moment due to ...
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Why do most commercial aircraft have the Center of Gravity before the Aerodynamic Center?

I've looked at the other questions concerning this topic, such as: How does an aircraft tailplane work? Does static longitudinal stability require download on the tail? But I'm still confused about ...
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Why might a canard aircraft suddenly pitch down from a level flight?

I have a canard aircraft which has pitch stability issues that I'm trying to troubleshoot. The aircraft has a characteristic tendency to depart level flight in sudden dive/ or generate a massive nose-...
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How to calculate Cm0 and Cmac,wb for an RC plane?

I am currently going through all of my stability and control calculations and am a bit confused on how to find $C_{m_{ac,wb}}$. The only things I'm finding online is that they are given through ...
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Should the planform area inside the fuselage be considered for a stability calculation?

I am using lower order tools (xflr) for calculation of neutral point and CG location.I can think of modelling the wing in three ways wing with section inside the fuse missing wing with section ...
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Without wind, would a plane go straight if the pilot let go of the controls?

If you let go of the steering wheel of a (properly maintained) car, it will typically go in a straight or nearly straight line. Would a powered airplane in no-wind conditions do the same thing?
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Estimating Moment Coefficient about Aerodynamic Center for wing-body combination

Are there any references or analytical methods to estimate the moment coefficient about the aerodynamic center, C_M,ac for a given wing-body combination? Even something rough would be useful such as &...
Seth's user avatar
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What happens when you put significant weight on the tail of a commercial airplane?

I'm curious what happens if you put some extra weight in the back of the airplane (let's say a small GE turbine or something). I'm new to Stability and Control side of things and curious how the new ...
Jones's user avatar
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Why do most canards have swept wings?

So most canards have swept back wings. Now I have been told that it is that way to ensure yaw stability (for example in a crosswind) - not sure though. But then I look at the new Lilium jet, which is ...
Vincent Cerowski's user avatar
1 vote
3 answers
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Can the flap on one wing be used to reduce a left turning tendency?

I fly a single engine Jabiru UL45 and it has a tendency to turn left during straight and level flight. Is it possible, in order to reduce a "Heavy Wing" to slightly increase the degree of ...
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How does a tailplane provide downforce if it has the same AoA as the main wing?

If an airplane is traveling through a uniform environment and its main wing is parallel to its horizontal stabilizer, how does its tailplane generate downforce? Only explanation I can think of is ...
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Is a glider on a winch directionally stable?

Let's say I were to fly a glider with the stick and rudder exactly centered laterally, while making the appropriate pitch inputs, during a winch launch. Would the glider remain wings level throughout ...
Sanchises's user avatar
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Are there any relationship between lateral and directional stability?

Directional stability is affected by the side areas such as the vertical stabilizers and lateral stability is affected by dihedrals. Is there any relationship between lateral and directional stability ...
Alexong123's user avatar
2 votes
1 answer
254 views

General Rule for Calculating Center of Gravity of any Aircraft Configuration

I'm looking for method(s) to determine the CG of any fixed wing aircraft for it to be reasonably stable. I only know for conventional aircraft, placing the CG between 1/4th and 1/3rd of MAC behind MAC ...
Kozakov's user avatar
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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 ...
Omar Khammash's user avatar
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4 answers
2k views

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 ...
Andrea Appiani's user avatar
2 votes
1 answer
336 views

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 ...
user61980's user avatar
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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 ...
Kevin Kostlan's user avatar
3 votes
1 answer
916 views

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/...
user007's user avatar
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10 votes
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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 ...
uhoh's user avatar
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1 vote
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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 ...
x3oo's user avatar
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5 votes
3 answers
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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 ...
SpeedBird789's user avatar
3 votes
1 answer
143 views

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 ...
rubemnobre's user avatar
0 votes
2 answers
555 views

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 ...
Umer Riaz's user avatar
2 votes
1 answer
181 views

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 ...
Auberron's user avatar
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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 ...
Yanick Salzmann's user avatar
7 votes
6 answers
3k views

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 ...
Lucas's user avatar
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3 votes
1 answer
213 views

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 ...
Abdullah is not an Amalekite's user avatar
1 vote
2 answers
299 views

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 ...
Helmut K.'s user avatar
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1 answer
277 views

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 ...
Patrick Tubonimi Pedro's user avatar
0 votes
2 answers
279 views

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 ...
Konrad's user avatar
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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(\...
shooshool's user avatar
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0 votes
3 answers
224 views

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 ...
Konrad's user avatar
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1 vote
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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 ...
DrZ214's user avatar
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1 vote
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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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1 vote
3 answers
544 views

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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4 votes
1 answer
975 views

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 ...
spacegirl1923's user avatar

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