Questions tagged [stability]

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

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31 views

Reasonable values for neutral point and center of gravity

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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71 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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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
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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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10answers
12k 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
177 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
167 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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1answer
35 views

How to calculate 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) but during the modelling, I find that I need to use the ...
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3answers
122 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
97 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
64 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
191 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
84 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
635 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
118 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
151 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
123 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
170 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
234 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
210 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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447 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
195 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
189 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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176 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
662 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
290 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
458 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
135 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
384 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
784 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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216 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
41 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
159 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
639 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
336 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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74 views

Controlled stall,aileron or rudder input?

I know aileron input in a stall approach is not adviced, what if vortex generators are placed at the wingtip where the ailerons sit meaning there will still be aileron authority even at stall, at this ...
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1answer
219 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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506 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?
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1answer
203 views

Do elevators have cambered airfoils inverted or symmetrical airfoils

Do horizontal stabilizer of ga planes e.g cessna 172(or ultralights e.g lazair) have cambered airfoils inverted to provide the tail downward force or do they have symmetrical airfoils at negative aoi ...
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1answer
461 views

Where can I get a graph of lift coefficient against AOA for a symmetrical airfoil?

Where can I get get a graph lift coefficient against AOA for a symmetrical airfoil ? I am asking for a source or link to a graph or table of AOA against Cl of any symmetrical airfoil.
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1answer
368 views

What is decalage and how it affects airplane performance?

What is decalage in airplanes and how important is it in respect to airplane performance?
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277 views

Which kind of stability do aerobatic aircraft have? [closed]

I'm designing an aerobatic aircraft similar to EXTRA 330SC. I would like to know about stability: lateral and longitudinal, static and dynamic. I read that aerobatic aircraft have neutral static ...
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1answer
692 views

How does angle of incidence affect an aircraft's stability?

How does angle of incidence affect an aircraft's stability? I've tried searching the web for the simplest answer that makes sense but so far I've come up with nothing as yet. I know it sounds simple, ...
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2answers
468 views

What is purpose of engine pylon airfoils on the Hondajet HA-420?

The engine pylons on the Hondajet are aerodynamically shaped so as to provide a lift force laterally and outwardly. Engine pylon as viewed from rear Engine pylon as viewed from front Are these for ...
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1answer
662 views

Which moments are acting on a flying airplane?

I would like to ask some clarifying questions regarding the moments acting on a flying airplane. My understanding is that there are following force: Weight (pointing down and applied to the CG of ...