Questions tagged [stability]

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

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1answer
47 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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0answers
82 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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72 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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2answers
125 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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2answers
95 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
137 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
172 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
125 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
75 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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2answers
93 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
59 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
13k 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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2answers
276 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
171 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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70 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
125 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
142 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
65 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 ...
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5answers
228 views

Why does instability in pitch lower trim drag for an aircraft with a tail?

I have a hard time figuring out why a longitudinally unstable aircraft has a lower trim drag. This seems to be a thing especially in fighter aircraft, so does it have to do anything with supersonic ...
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1answer
89 views

What causes 'wing rock' and how does it work?

Currently studying for a course on fighter aircraft. I came across the phenomenon of 'wing rock', which occurs at very high angles of attack, but I couldn't find any good sources that explain how this ...
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3answers
834 views

How does this RC helicopter keep itself upright?

I have this (cheap, beginner-level) RC helicopter: It has 3 controls: one for climbing and descending (collective throttle), one for yawing (differential throttle), and one for pitching (the tail ...
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3answers
120 views

Sensitivity of aircraft stability with respect to the CG location

I am working on the design of an aircraft (2 m long, wingspan = 3 m). The platform is longitudinally stable but it is very sensitive to the variation in CG location. For example, 1) CG_location = 100 ...
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1answer
159 views

Why was the Akron’s spy basket installation so remarkably unstable?

A spy basket (also known as a cloud car) is a type of airship appurtenance consisting of a streamlined one-(usually)-person gondola attached to its host airship by a long cable; to use it, the host ...
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2answers
130 views

Contribution of wing vertical location to roll stability

Recently, I was reading about the factors that contribute to roll stability of an aircraft and came across a point stating that the wing's vertical location (Distance of wing from CG) contributes to ...
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9answers
2k views

Is a vertical stabiliser needed for straight line flight in a glider?

My team is designing a glider and I am unsure about whether or not to include a vertical stabiliser. The task the glider must complete is to achieve the largest possible displacement in a straight ...
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1answer
154 views

Would knowing the exact weight that has been loaded on a commercial plane and it's distribution be useful? [duplicate]

There's been a bit in the media about weighing passengers in order to get more accurate take off weights for commercial planes. I'm guessing the delta between a heavy set of passengers and a light ...
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1answer
184 views

How does engine geometry affect flight stability?

News items around the recent aviation accidents involving Boing 737 MAX 8 aircraft suggest, that bigger engine diameter demanded for a different (longer) nacelle strut for proper ground clearance ...
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3answers
1k views

How does having a bigger engine placed slightly more forward and higher impact the plane in pitch?

I'm asking about general aerodynamics for under-wing twin-jets – I'm not after information/speculation on the Max 8 crashes/systems, but I mention the crash below because of what I see as an unclear (...
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1answer
240 views

What is the stability of a 777-like plane if the engines flameout and the autopilot disconnects and the pilots don't make any inputs?

According to Wikipedia: If no control inputs were made following flameout and the disengagement of autopilot, the aircraft would likely have entered a spiral dive and entered the ocean within 20 ...
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2answers
231 views

Are there any airplanes with the Centre of Gravity behind the Centre of Pressure? [duplicate]

We are used to seeing a forward CG in all airplanes, from a small Cessna to a large Boeing. I wonder whether any airplane with CG behind wing CP exists in production? Is it even possible? If so, what ...
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2answers
457 views

What prevents BFS from becoming unstable when pitching from horizontal to vertical?

In this video from (SpaceX's BFS landing) there is a point where the ship tilts from horizontal « belly first » terminal velocity freefall, to vertical « engine first » landing position. The graph ...
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32 views

How would a sudden change in weight affect a plane? [duplicate]

If I have a plane trimmed for perfect straight and level flight and then suddenly I removed some mass from the airplane (let’s ignore any momentums introduced by mass release) – how would the aircraft ...
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2answers
199 views

What would happen if the elevators were set to opposite angles?

For example left elevator angle = +30 deg, right elevator angle = -30 deg. Would these settings be able to induce a rolling moment on the aircraft? What would happen with the pitch?
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1answer
190 views

What is the source for the “rule” forbidding configuration changes below 200ft AGL?

I keep hearing from countless instructors, and even some DPE's, that in single engine aircraft, no configuration changes are allowed below 200AGL (i.e. flaps). Some have also stated that this is ...
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0answers
200 views

What are stick position stability and stick force stability?

What are stick position stability and stick force stability? What are the difference between them? How will changing trim of an aircraft affect them?
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5answers
796 views

Is the aircraft neutral point a function of the tail incidence angle?

While using XFLR v5 for a basic analysis I am facing this issue. For a fixed configuration I varied the tail incidence angle and XFLR is displaying a change in the Neutral point location. But as per ...
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1answer
318 views

Why is the delta wing naturally stable in pitch?

Any thoughts on why the delta wings are naturally more stable in pitch, so they don't require any tail surface? What's the physics behind this?
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2answers
511 views

What happens aerodynamically when we trim an aircraft?

I'm trying to understand stability & control. Please correct me if I'm mistaken. An aircraft will have longitudinal stability if the aerodynamic center (AC) is behind the center of gravity (CG). ...
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1answer
142 views

What is the location of AC for typical modern airplanes?

Some sources state that AC must be located behind CG, so that AC is stable. There is a source, which throughout the study of stability and control consider AC located ahead of CG and positive cambered ...
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1answer
429 views

Why do some Mach trimmers move the elevator?

A pop-up rod on a the FO's control column on a DC-9 that shows the Mach trim position. (YouTube) Regarding the title, I'm not sure if it's just some or all jetliners (I tried to research it). On the ...
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4answers
909 views

Do high-wingers have dihedral?

I know that high wingers have an inherent dihedral effect. Are there any examples of high wing aircrafts with a dihedral? If so, what is the reason behind it?
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1answer
246 views

Is aerodynamic flutter speed dependent?

Is it possible for control surface flutter to occur at low Reynolds and low speed? If yes,why don't Ultralights have counter weights or tail sting mass to immune the control surfaces against flutter ...
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0answers
42 views

What happens to the neutral point of a fighter jet during supersonic flight? [duplicate]

I understand that for a fighter aircraft, it would have to be inherently unstable. To do this I believe that the neutral point needs to be aft of the cg. I was told that during supersonic flight this ...
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1answer
175 views

Is there a difference in stability between a tail downforce (cg ahead of ac) and a tail upforce (cg behind)?

So from what I understand, having a - 1.cg ahead of the neutral point(ac) with a tail downforce and 2.cg behind ac with a tail upforce are both stable configurations.(Please correct me if I'm ...
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3answers
651 views

Is this paragraph about the dihedral effect in the FAA's Glider Flying Handbook correct?

I've been reading the FAA's Glider Flying Handbook, 2013 (FAA-H-8083-13A). In chapter 3 ("Aerodynamics of Flight"), the book is discussing stability. On page 3-12, it states: Dihedral is the upward ...
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5answers
7k views

Why do most biplanes have their top wing slightly forward of the lower wing?

In most of the biplanes, the top wing is located a little forward of the lower wing. What is the use of this? Also, how does this affect stability of the aircraft.
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2answers
364 views

What is the airplane with the best flying characteristics? [closed]

I'm reading Stick and Rudder by Wolfgang Langewiesche, and in Chapter 7 titled, "What the Airplane Wants to Do" he discusses stability. In discussing it, he says that no plane yet built is completely "...
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2answers
158 views

Could vortex generators be used to preserve aileron authority during a stall?

I know that using aileron input in the approach to a stall is generally inadvisable; what if vortex generators are placed at the wingtip (where the ailerons sit), meaning that the ailerons will still ...
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1answer
245 views

At what angle of deflections do rudders become inefficient?

This article (picture) says rudders for ships can deflect 35° max,so at what aoa does an airplane rudder lose its efficiency,at what " angle of deflection " does it start to stall ?
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1answer
521 views

Can an aileron stall in-flight?

Horizontal stabilizers stall, so I was wondering, is it possible for an aileron to stall? If yes, at what angle of attack?