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 ...
Koyovis's user avatar
  • 61.6k
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 ...
Peter Kämpf's user avatar
38 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 ...
Koyovis's user avatar
  • 61.6k
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 ...
Simon's user avatar
  • 31.2k
32 votes

Without wind, would a plane go straight if the pilot let go of the controls?

If the airplane is properly trimmed, the airmass is smooth, and the aircraft is inherently stable in its design, then yes - many airplanes are capable of flying straight with only very light and ...
Michael Hall's user avatar
  • 25.8k
31 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. ...
Peter Kämpf's user avatar
30 votes
Accepted

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 ...
Peter Kämpf's user avatar
26 votes
Accepted

Is there an aerodynamic reason for airship fins to be at the tail?

Yes. With fins in the middle the airship would be impossible to control. In order to work, fins need a lever arm with respect to the center of gravity. The longer, the better. Almost all air vehicles ...
Peter Kämpf's user avatar
25 votes

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
Hanky Panky's user avatar
  • 6,021
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 ...
Peter Kämpf's user avatar
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 ...
Anthony X's user avatar
  • 3,280
20 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 ...
Daniel K's user avatar
  • 5,518
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 ...
Anas Maaz's user avatar
  • 1,953
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 "...
alephzero's user avatar
  • 3,030
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 ...
Peter Kämpf's user avatar
15 votes
Accepted

How is an aircraft designed to be longitudinally stable?

My short answer: Stability is controlled by moving the center of gravity (CoG). The handbook should give center of gravity limits. A more forward CoG means a more stable airplane. Shifting it past ...
Peter Kämpf's user avatar
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 ...
quiet flyer's user avatar
  • 22.3k
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 ...
xxavier's user avatar
  • 11.1k
14 votes

Does static longitudinal stability require download on the tail?

For the impatient reader: The answer is no. Let me explain it in detail. For this, it is helpful to simplify things as much as possible and then add the complications step by step so I can explain ...
Peter Kämpf's user avatar
14 votes
Accepted

Does static longitudinal stability require download on the tail?

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 ...
Koyovis's user avatar
  • 61.6k
14 votes
Accepted

Does "pendulum effect" apply to hang gliders or any aircraft?

For a proper discussion, we should first define what a pendulum is. Only then can be established if such an effect can exist in airplanes. Let's base the definition on Wikipedia. It says that A ...
Peter Kämpf's user avatar
14 votes

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. ...
reirab's user avatar
  • 19.4k
14 votes

Without wind, would a plane go straight if the pilot let go of the controls?

Yes. Most aircraft are designed to be inherently stable. That is, if you just let go of the controls, the aircraft will return to whatever it is trimmed to - typically stable and level flight. Trims ...
Harper - Reinstate Monica's user avatar
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 ...
Peter Kämpf's user avatar
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 ...
MadMarky's user avatar
  • 669
12 votes
Accepted

Are birds aerodynamically stable?

Birds have integrated active control systems that rely on several sensors to adjust the control surfaces to stabilize the flight. Since it is very difficult (perhaps impossible) to study the ...
DeltaLima's user avatar
  • 83.1k
12 votes

How does this RC helicopter keep itself upright?

The rotor is gyro stabilized. The balance bar is the gyro. If the machine rolls right, the balance bar wants to stay in a level plane and generates a correction by influencing the rotor blades to go ...
John K's user avatar
  • 130k
12 votes

Is a trimmed flight not always an unstable system?

I think you misunderstand how it works, and how you would respond to changes in wind. Increases in airspeed impact all flight surfaces, including the elevator, so a change in airspeed due to a gust ...
GdD's user avatar
  • 53.7k
11 votes

Is it possible to balance a tail heavy plane with a vertical prop on its tail?

Tryin' to cheat the laws of stability, aren't we? In both videos it is evident that the plane is unstable in pitch. Adding a lifting prop will not change this, because changes in the angle of attack ...
Peter Kämpf's user avatar

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