64
The main thing to avoid in aeroplane stability & control, is an aerodynamic nose up moment that is not commanded by the pilot. The uncommanded nose-up moment would not auto-stabilise, but rapidly get progressively larger with increasing angle of attack, and run away to a stalled aeroplane.
During certification of a passenger aeroplane, many tests are ...
44
Placing the top wing ahead of the bottom wing in biplanes is called (positive) stagger. It is mostly used in small biplanes and improves pilot vision.
In order to accommodate a variation of pilot weights and to reduce accelerations in maneuvers, it is advisable to place the pilot very close to the center of gravity. If the upper wing were in the same ...
33
It's complicated ;)
There are two types of stability; dynamic and static.
If an aircraft is disturbed by, say, a gust of wind, it will deviate from its attitude but then will immediately and without control inputs return to its original attitude. This is positive static stability. If it remains in the disturbed attitude unless corrected, it has neutral ...
30
Not all of them need a fin:
This is the Horten IV, a flying wing glider that did not need a fin (picture source). Instead, it used spoilers at the wingtips to create yawing moments, and the swept wing helped in improving its weak directional stability. It could afford to do so because it was a glider. The second prototype of a jet-powered flying wing, the ...
30
Yes that is possible, like the Hiller flying platform demonstrated. It had two counter-rotating propellers inside a shroud and the pilot controlled his craft by shifting his body weight, like on a Segway. There is no law of physics that prohibits a helicopter from flying upside down.
The Hiller flying platform was one of several types built in the 1950s, ...
27
What if the engines fail? In early airships this happened frequently, and many ships limped home on a reduced number of engines.
Note that all airships were both vertically and laterally unstable. The helmsman had to continuously adjust the rudder angle to keep the ship on course. In NACA TN 204, Frank Rizzo concluded that enlarging the fins would be ...
25
(Wikimedia; arrows and CG icon are my additions) P-47 side drawing.
The thrust has a bigger moment arm around the tires contact points compared to the weight. It won't nose over below a certain RPM (it will be specified in the training). Below that RPM:
The weight will counter the thrust
The propeller downwash will produce downward force on the horizontal ...
25
Static stability means that a deviation from a trimmed state produces forces which return the system to this trimmed state.
If these forces produce an overshoot which increases over time, such that the system oscillates around this trim point with increasing amplitude, the system is dynamically unstable. The long period oscillation (phygoid) of gliders is ...
23
Static stability
is the tendency of a system to return to its initial state after a disturbance. Typical disturbances in case of airplanes are:
Flying into a vertical or horizontal gust
A jerk on the stick
The classic explanation is with a ball sitting in a pit. Whenever its position is changed by a disturbance, it will roll back towards the center. This ...
23
We spend all our weekends mowing lawn with these guys.
Courtesy: Helifreak.com
You can find thousands of Youtube videos showing how comfortably they can do that
23
Counter-intuitively, lowering the nose of an aircraft is not done for the purpose of "going down". Climb/descent is managed with throttle, and speed is managed with the control column/stick. The logic of this makes sense when you consider that going up or down involves the addition or removal of potential energy which is sourced by the engines and sunk into ...
22
Directional stability
When a swept wing is flying in a sideslip, the windward side behaves like a wing with less effective sweep $\varphi_{eff}$ and the leeward side like one with more effective sweep. Wing sweep causes a flattening of the lift curve slope for two reasons:
The effective angle of attack is reduced by the cosine of the sweep angle.
Only the ...
19
There are several sources for a sideslip-induced rolling moment:
The dihedral angle $\nu$ of the wing, which will increase the local angle of attack $\alpha$ on the windward wing according to $\Delta\alpha = \beta\cdot sin\nu$; $\beta$ being the angle of sideslip,
The sweep angle $\varphi$ of the wing, which in a sideslip causes a de-sweeping of the airflow ...
19
Well, jet engines do have gyroscopic effects. It is a major concern in the design of the turbomachinery. When the plane pitches/yaws, the resulting gyroscopic moment causes the compressor and turbine blades to move closer to the case. If excessive, this can cause the blades to rub into the case, causing loss of performance.
As to why the plane is not ...
19
If an aircraft is statically stable, it will always return to equilibrium after a disturbance. But what happens after can either show instability or stability. This is where the dynamic stability comes in.
You can think of an aircraft at equilibrium at a particular speed, altitude and angle of attack and it is suddenly faced with a disturbance which changes ...
17
All parts of an airliner's horizontal surface move, not just the rear part. The rear part, called an elevator, can move much faster and is for maneuvering. The forward part, called a (trimmable) stabilizer, is for trimming and moves slowly. It is moved in response to changes in loading, speed or flap settings and positions the tail surface such that only ...
16
There are multiple configurations which are possible with tail or canard, which based on their locations and whether they produce lift or down-force, results in a stable or unstable aircraft (taking the aircraft center of gravity into account). The figures below show some of the possible configurations.
Source: f-16.net
In the most general case, there is ...
16
You are missing the weight force. The wing should stall first because then it will produce less lift and the weight will make the aircraft pitch down.
In attached flow, the lift from wing and tail is balanced such that the combined resulting force is acting exactly at the longitudinal position of the center of gravity. If the wing stalls, the balance of ...
15
This was also an issue when doing maintenance. For engine ground running at full power, the tail of the aircraft was weighted down to stop the aircraft tipping over the front wheels.
You can see the "saddle bag" and weights hung over the fuselage in front of the tailplane in this still from a video of a WWII Spitfire ground run:
(YouTube)
15
It is a combination of several effects:
Aeroelasticity: With the higher forces at high speed, the structure deforms such that the effective flow angle at the tail surface is reduced.
The supersonic lift curve slope of the tail surface decreases with Mach while that of the fuselage stays roughly constant, so with higher Mach the tail contribution to ...
15
But assume, there comes a wind gust, and the angle of attack will
increase. This causes the center of pressure to move forward in front
of my gravity location. So the aircraft will become instable because
the angle of attack will further increase (nose up). The only thing
the pilot could do is to use the elevator to trim in pitch.
This is not accurate. Are ...
14
One of the first helicopters that really flew (c. 1918) was the 'Petróczy-Kármán-Žurovec', intended to be used by the Austro-Hungarian Army as a tethered observation platform. The observer stood above the contra-rotating rotors...
(Image source)
http://www.aviastar.org/helicopters_eng/petroczy.php
14
To add a bit to the existing answers, the reason for the unexpected pitch-up moment on the 737 MAX, as far as I understand it, had to do with the flattened portion on the bottom of the engine cowling.
The root of the problem is that the 737 was designed back in the day of low-bypass turbofans (specifically, the Pratt & Whitney JT8D.) Due to the low ...
13
No.
Modern FBW airliners use less static stability than what the early jets were used to, but stability is still positive. The negative camber at the root airfoil of sweptback horizontal tails might indicate predominantly negative tail loads, but is also used to keep the isobars on the swept surface parallel. Also, the bent-up leading edge delays separation ...
13
It's called a staggered wing and is done to reduce aerodynamic interference between wings in certain circumstances. A wing with positive (forward) stagger is most common because it improves both downward visibility and ease of cockpit access for open cockpit biplanes.
12
Of course, just put the center of gravity back to its rear limit and fly slowly. Then all of them will produce positive lift on their tails.
Stability is not produced by a downforce at the tail. The newest book I read which claimed this was from 1911 (I happened to read the 1913 edition). Stability is produced by making the lift per area of the forward ...
12
For statically stable aircraft, the canard is a spoiler in disguise. It will create a strong downwash right behind itself, coupled with an upwash outward of $\pi$/4 of its semispan. With changing angle of attack and lift coefficient, the vertical position and the strength of both up- and downwash will change, so the wing incidence variation over wingspan can ...
12
Your guesses are pretty correct - an "aerodynamically stable" aircraft tends to stay (relatively) straight and level if the controls are let go.
Pitch
Let's say the aircraft's elevator is trimmed to fly level (maintaining the same altitude). You push on the yoke to lower the nose, then release the pressure on the control. The nose-down altitude allows the ...
12
Effect on Aerodynamic Coefficients
It is possible to locate a vertical fin, as well as a control surface, ahead of the center of gravity. $c_{n_r}$ (yaw-induced yawing moment or yaw damping) will not likely change much, all other things being equal. $c_{n_\beta}$ (sideslip-induced yawing moment or directional stability), on the other hand, will be reduced, ...
12
No, static longitudinal stability does not necessarily imply a download on the tail.
Static longitudinal stability requires that the Centre of Gravity is in front of the Centre of Lift, indicated as n.p.$_{fixed}$ in the drawing. Only then will an increase in Angle of Attack d$\alpha$ result in an opposing pitching moment: if d$\alpha$ > 0 then $dC_N$ > 0, ...
Only top voted, non community-wiki answers of a minimum length are eligible
