64
votes
Accepted
Why force the nose of 737 Max down in the first place?
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 ...
- 57.8k
44
votes
Why do most biplanes have their top wing slightly forward of the lower wing?
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 ...
- 218k
33
votes
Accepted
Are helicopters aerodynamically stable?
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 ...
- 30.8k
33
votes
Would a helicopter with the blades on the bottom fly the same, if at all?
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 ...
- 57.8k
30
votes
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Why do aircraft need a vertical tailfin, but birds don't? (and lots of fish do?)
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 ...
- 218k
27
votes
Accepted
Why do blimps have fins?
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. ...
- 218k
25
votes
What does it mean when an aircraft is statically stable but dynamically unstable?
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 ...
- 218k
24
votes
Accepted
What does it mean for a plane to be aerodynamically stable?
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 ...
- 218k
24
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Would a helicopter with the blades on the bottom fly the same, if at all?
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
- 5,951
23
votes
Accepted
How does wing sweep increase aircraft stability?
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 ...
- 218k
23
votes
Why force the nose of 737 Max down in the first place?
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 ...
- 3,150
20
votes
Accepted
Why do low wing aircraft have higher dihedral than high wing aircraft?
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\...
- 218k
19
votes
Accepted
Why don't single-engine jets suffer severe gyroscopic effects?
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 ...
- 5,238
19
votes
Accepted
What does it mean when an aircraft is statically stable but dynamically unstable?
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 ...
- 1,813
17
votes
Accepted
Why do some fighter jets have movable horizontal stabilizer instead of elevators installed on the stabilizers?
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) ...
- 218k
17
votes
Accepted
Do any airplane designs exist that don't involve a flight surface that provides downforce?
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 ...
- 98.6k
16
votes
Accepted
Why are aircraft designed so that the wing stalls before the tail?
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 ...
- 218k
15
votes
Would a stationary WWII fighter fall on its nose with full throttle and brakes on?
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 "...
- 2,967
15
votes
Accepted
Why does directional stability decrease at supersonic speeds?
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 ...
- 218k
15
votes
Is a trimmed flight not always an unstable system?
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 ...
- 17.1k
14
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Longitudinal stability of airships: How is the critical speed defined?
On page 43 of the Technical Manual of Airship Aerodynamics I found the following image:
Here $F_e$ is the resultant airforce on the hull, or the dynamical lift of the hull. As this is before the CoG, ...
- 6,133
14
votes
Would a helicopter with the blades on the bottom fly the same, if at all?
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 ...
- 10.4k
14
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Why force the nose of 737 Max down in the first place?
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.
...
- 18.9k
13
votes
Do any airplane designs exist that don't involve a flight surface that provides downforce?
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 ...
- 218k
13
votes
Longitudinal stability of airships: How is the critical speed defined?
Airships have a center of lift well above their center of mass, since engines and the gondola are mounted at the bottom. This gives them a strong static pitch stability: They will try to assume an ...
- 218k
13
votes
Accepted
What are the pros and cons of having canard control surfaces versus a horizontal tail control surface?
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 ...
- 218k
13
votes
Are there any fly-by-wire airliners with negative or near-neutral pitch stability?
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 ...
- 218k
13
votes
Why do most biplanes have their top wing slightly forward of the lower wing?
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 ...
- 669
12
votes
What does it mean for a plane to be aerodynamically stable?
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 ...
- 39.2k
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