Questions tagged [stability]

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

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42 views

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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166 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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53 views

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
118 views

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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68 views

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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120 views

Why does spoiler extension during touchdown causes 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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260 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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2answers
114 views

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
90 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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3answers
125 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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208 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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1answer
194 views

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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82 views

What are aerodynamical reasons behind slight dihedral or no dihedral at all? [duplicate]

Regarding general aviation single engine high wing aircrafts in particular, what are the reasons for some of them to have no dihedral at all on their wing, or slight positive dihedral angle for other? ...
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103 views

What is the main source of yaw stability? [closed]

I am creating a flight simulator and I need to implement lateral and longitudinal stability. Roll stability is now working because I am computing the lift of each wing independently. But yaw stability ...
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211 views

Dynamic modes at supersonic speed

Our teacher gave us an assignment where we have to study the longitudinal stability of an X-15. I have to study it at 353.9 m/s which is Mach 1.2. Another group has to study it at 235.9 m/s so Mach 0....
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276 views

What are the disadvantages of a Tail providing lift? (tandem-wings included)

Most aircraft feature the tailplane configuration, which requires the main wing to provide higher lift than the weight of the plane itself, what seems counterproductive. Few aircraft have featured ...
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Is there an ideal value for the difference between the stick fixed and stick free static margin?

I'm calculating the static stability for a fictional aircraft and have to determine the elevator size. I have all the calculations but don't know how to choose the correct size and position. Is it ...
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1answer
94 views

How can I reduce the Short Period frequency of a Pitch Hold Auto-Pilot while maintaining stability?

I am working on a Pitch Hold Auto-pilot for F-104, which has the following damping characteristics; which clearly shows that it has degenerated and unstable phugoid mode. well, I made a SAS which ...
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162 views

How does an aircraft maintain an orientation that was actually caused by a external disturbance?

What is the difference between centre of pressure, aerodynamic centre and neutral point? At this link, there is an image used to explain the answer, but not much has been spoken about it. I ...
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238 views

First Boeing 757-200 N757A 'Catfish' was modified with a Canard foreplane. How did it behave?

The aircraft had these changes made around 2006 to act as an F-22 Avionics test bed. One would expect that the fore plane changed many performances of machine, including take off and landing attitude ...
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146 views

Would it be a good idea to use dihedral on a biplane?

My engineering class uses model plane kits to build "frankenflyers" which use parts from multiple model planes to make one plane that will carry a maximum load (pennies). I was wondering what the best ...
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0answers
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Does the 737 MAX's engine nacelle camber affect the pitching moment?

A side view of the B737 MAX's engine nacelle shows a positive camber at the top. As nose camber of any lifting shape (wing or nacelle) generally allows more lift at higher AoA, do you think this ...
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1answer
335 views

Why were trim tabs invented?

I understand the need for a trimming mechanism, but why use a secondary control surface placed on top of the first, when you could just adjust the resting position of the aileron/elevator/rudder ...
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261 views

How does the ground effect affects longitudinal stability?

I recently learned that the ground effect increases the effective AoA of the wing and thus increases lift, how does that translates in to the tail plane and it's stabilizing effects? does it reduce/...
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3answers
1k views

What is aerodynamic damping?

Could someone explain what aerodynamic damping exactly is? I'm not able to understand the whole concept. Pls try to keep the explanation simple. I tried to understand the concept from the link below. ...
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4answers
315 views

Can an automatic vane-based stabilizer really work?

Can the pitch stabilizer in the picture really work for a plane or glider? The image shows an automatic pitch stabilizer (the mechanism in the black rectangle) that is based on the vane 50 whose ...
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1answer
134 views

Is adverse yaw ever desirable?

Is there any situation during flight where adverse yaw (yaw due to roll) is desirable?
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342 views

Why is a blunt trailing edge a better stabilizer at hypersonic speeds?

Here is what I read about the X-15 spaceplane: The X-15 had a thick wedge tail to enable it to fly in a steady manner at hypersonic speeds.[14] This produced a significant amount of drag at lower ...
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1answer
307 views

Under what circumstances is the Aircraft 'Stability-Axis' system used in either design or analysis of aircraft?

I understand the definition of aircraft 'Stability-Axis', but I am unsure as to the circumstances in which forces or moments would be expressed in this axis system. Is there a convention? What I do ...
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129 views

Does the angular momentum of a jet engine affect aircraft performance?

I am interested, do jet engines have angular momentum? And how does such momentum affect airplane stability?
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75 views

what are the winglets on C17's air inlets useful for? [duplicate]

the winglets are clearly visible on this photograph, are they connected with air flow instability?
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131 views

Stability and Control of an aircraft [duplicate]

Why is the pitching moment coefficient of an airplane said to be constant about the aerodynamic centre of an aerofoil
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126 views

Did large winglets contribute to the spin behavior of the SB 13 tailless glider?

The SB-13 swept wing tailless beauty was developed in the early 1990s as a small recreational glider with relatively large upright winglets. The design proved very spin prone. It occurred to me ...
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2answers
461 views

Just how do the underbanking and overbanking tendencies work?

I understand that most GA airplanes and gliders exhibit an underbanking tendency at low bank angles and an overbanking tendency at high bank angles. According to the Airplane Flying Handbook, chapter ...
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2answers
214 views

How do high-wing and low-wing aircraft compare in terms of structural damage in a crash?

Which type of aircraft is more likely to avoid damage to the aircraft structure: high-winged aircraft or low-winged aircraft? What factors influence the type of damage usually sustained during a ...
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2answers
162 views

What are reasonable values for the position of an airplane's center of gravity compared to that of its center of lift?

I understand that if the center of gravity is in front of neutral point, it is stable. If the center of gravity is behind the neutral point, it is unstable. Based on the diagram, is it implied that h (...
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1answer
109 views

At what angle-of-attack (sideslip angle) would a symmetrical vertical fin stop “helping” a deflected rudder produce sideforce? [duplicate]

For rudder deflection angles of 5, 10, 20, 30, and 45 degrees, at what (negative) angle-of-attack does a fin-rudder combination (including dorsal fin if present) create exactly the same amount of ...
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179 views

At what angle-of-attack (sideslip angle) would a symmetric vertical fin plus a deflected rudder have a lift coefficient of exactly zero?

For rudder deflection angles of 5, 10, 20, 30, and 45 degrees, at what (negative) angle-of-attack does a fin-rudder combination (including dorsal fin if present) have a lift coefficient of exactly ...
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2answers
154 views

How do you calculate the vertical and horizontal tail volume if you have Vvbar and Vhbar?

I know the general formulas for calculating the horizontal and vertical tail volumes. Unfortunately, I only have Vvbar and Vhbar. I’ve checked through multiple textbooks and I don’t know why I can’t ...
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11answers
14k views

Why force the nose of 737 Max down in the first place?

Can someone here explain a pretty basic question that I have had ever since this controversy came to light? I don't understand why any aircraft manufacturer, engineer, software developer, would make ...
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3answers
924 views

Why are the flaperons on the 787 located so far in-board?

Why are the flaperons positioned in between the outboard and inboard flaps (see 2nd picture)? Why not have them located as far outboard as possible to achieve maximum roll authority? I would think the ...
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2answers
187 views

Can cold thrusters/roll posts be used to control/stabilize an aircraft (like a satellite)?

Aircraft use control surfaces to control their motion/direction. Would it be a good idea (i.e., more efficient, less weight, better responsiveness, less drag, etc.) to use thrusters/roll posts to ...
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2answers
138 views

How to calculate the variation of Z force with elevator deflection (Zδe) in Nelson for pitching dynamics?

I have been trying to model the pitching dynamics of a General Aircraft (Nelson, "Flight Stability and Control"). I need to use the Zδe which uses ...
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3answers
144 views

Does pumping fuel to each wingtip affect roll trim in a stabilized turn?

If an aircraft is stabilized in a constant-bank, constant rate turn with no sideslip, and then we pump 200 gallons of fuel from a fuselage tank to a tank on the tip of each wing, without changing the ...
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1answer
287 views

Helicopter tail horizontal stabilizer angle

In helicopters what factors decide the horizontal tail stabilizer angle? By angle I mean that in some helicopter they are -6 deg relative to tail boom while in some -3 deg. These angle were remain ...
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0answers
76 views

Does/Can combustion instabilities occur in SCRAM jet?

I was told that combustion occurs at Supersonic speeds in SCRAM jet. So acoustic disturbances/ pressure fluctuations can't travel into incoming jet stream. Does this avoids combustion instabilities in ...