85
votes
Accepted
Why does the B-52 take off without rotating?
The reason was to give the bombs the place close to the center of gravity.
Wing sweep (for high cruise Mach numbers) in combination with a high aspect ratio of the wing (for low induced drag) made it ...
- 225k
57
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Why does the B-52 take off without rotating?
Adding to the excellent Peter's answer who explained why for this particular model the wheels are placed far behind the centre of gravity (CG), I would like to clarify why this makes it impossible to ...
- 1,166
37
votes
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Why does my wind tunnel experiment give two different stall angles of attack?
This is called stall hysteresis. You have two different situations and the flow reacts differently in each of them.
When increasing the AOA
The flow is attached to the wing and the boundary layer is ...
- 1,837
37
votes
Why do planes fly at a high angle of attack when flying slow?
Lift is (among other things) a function of the airspeed and the angle of attack of the wing. Hence, if you reduce your speed, you have to compensate the associated lift loss by increasing the angle of ...
- 10.6k
34
votes
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What is the stall AoA for the space shuttle?
As far as I can tell, NASA has never given a straight-forward answer to this question. The theoretical stall AoA is likely in the 33-40° range (see reason for that guess below). However, at ...
- 9,731
22
votes
Accepted
Can a taildragger land tail first?
It's possible to land by touching the tail first, but this is undesirable as it puts a lot of stress on the tail, so it's not trained for and there are no circumstances where you would do this ...
- 12.6k
22
votes
Why do planes fly at a high angle of attack when flying slow?
In order to maintain level flight, an airplane must generate lift equivalent to its weight. The lift generated by a wing diminishes rapidly as the airspeed decreases. To compensate, the angle of ...
- 2,438
20
votes
Why are the angle-of-attack sensors on most airplanes mounted on the forward fuselage rather than at the wing roots?
Would installing the AOA vane (or, in general, sensor) at the wing root be more accurate than installing it near the nose? No, it wouldn't. In fact, it may be slightly worse due to the larger upwash ...
- 10.9k
18
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Why doesn't critical angle of attack increase with airspeed?
Your scepticism is justified. The critical angle of attack (cAoA) can grow as well as shrink with increasing airspeed. Growth is generally due to a higher Reynolds number while a reduction in the cAoA ...
- 225k
17
votes
Why is angle of attack information not displayed in the cockpit?
Many aircraft do include them, because it's very useful.
For example a modern 737:
A Garmin G1000 (one of the more common General Aviation glass cockpits):
Specifically the part that looks a bit ...
- 10.4k
16
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Why does stall speed decrease when flaps are deployed?
Your stall speed decreases because the $C_{L_{MAX}}$ of the wing increases with flaps deflected.
Deploying flaps increases wing camber and increases both $C_L$ and $C_D$ at the same AoA and airspeed. ...
- 60.7k
16
votes
Why does RPM for a fixed-pitch propeller change with an aircraft's pitch?
The driving variable here is airspeed, not pitch attitude per se. Your airplane naturally goes faster when you put the stick forward and decrease the wing's angle-of-attack, and this changes the prop ...
- 19.9k
16
votes
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Why angle of attack is always shown against the relative wind parallel to horizon?
I think this has been at least hinted at in other answers, but, just to put it succinctly, the diagrams aren't actually intended to show the airflow as always being parallel to the horizon. The ...
- 19.2k
15
votes
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What does it mean when pitch numbers move all over the HUD screen?
Most modern military aircraft HUDs have two primary methods of displaying the velocity vector (or flight path vector) relative to the pitch ladder.
Uncaged Mode
When the HUD is operating in uncaged ...
- 8,698
14
votes
Why are the AOA sensors placed outside the aircraft?
No, it is not possible to place the sensors inside the aircraft, because the angle-of attack sensors need exposure to the surrounding air, in order to measure, as you mention yourself, "relative ...
- 12.7k
13
votes
Why is angle of attack information not displayed in the cockpit?
Slightly off-topic, but I recently came across the "side string" which gives a direct reading of AoA. Every gilder pilot is familiar with the yaw string, but in two years of learning to fly gliders, ...
- 401
13
votes
Accepted
Does lift coefficient vary with the wind velocity for a given angle of attack?
Yes, it does vary slightly due to viscous effects.
In inviscid flow, the flow speed would not affect the lift coefficient - angle of attack relation. However, increasing the flow speed will result in ...
- 225k
13
votes
Accepted
How many AOA sensors does the 737 MAX have?
The Boeing 737 MAX has 2 AoA sensors.
However, MCAS only takes input from 1 AoA sensor at a time.
Sources:
The black box flight recorder data for Lion Air JT610 shows the data from 2 AoA sensors, ...
- 366
13
votes
Can induced drag be negative?
Can induced drag be negative?
Not for the full configuration, but for parts of it.
Induced drag is part of the reaction force when a stream of air is deflected. This reaction force is split into one ...
- 225k
13
votes
Accepted
Will the autopilot raise the AoA above the critical AoA to maintain altitude?
Ralph wrote a nice answer for the Boeing 737, which is a conventionally controlled aircraft, meaning the yokes are mechanically linked to the flight control actuators.
Airbus A320
The Airbus A320 ...
- 51.6k
12
votes
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How does flight control surface authority change with AOA?
Control authority is a function of the local dynamic pressure (the product of air density and speed squared), geometry (the product of control surface area and lever arm), local angle of attack $\...
- 225k
12
votes
Accepted
Will an Airbus A320 plane go into an unrecoverable nosedive and crash if a single angle of attack sensor fails, like on Lion Air JT610?
if a single angle of attack sensor fails
No.
An A320 has three AoA sensors. If one reports a different value to the other two, the avionics will ignore values from that AoA sensor and use the values ...
- 26k
11
votes
Accepted
Why do airliners on autopilot climb during cruise?
Climbing further while cruising is saving fuel, not wasting it.
First, every airplane has an optimum angle of attack for most economical cruise. If it loses mass (due to fuel consumed), it needs to ...
- 225k
11
votes
Accepted
Is it possible to get into a spin situation while coordinated?
Yes, what you describe is perfectly possible. A stall in banked flight can result in a spin, given you fly the "right" plane.
My first flight in an ASW-20C was late in the afternoon, when most ...
- 225k
11
votes
Accepted
Does airspeed affect the critical angle of attack on an airfoil?
Yes, through the Reynolds number
The Reynolds Number is a function of airspeed and is used to predict the transition from laminar to turbuluent flow:
$$
{Re} = \frac{\rho v L}{\mu} = \frac{v L}{\nu}...
- 8,509
11
votes
Accepted
Is drag coefficient lowest at zero angle of attack?
Obviously, drag should be smallest for symmetrical airfoils at zero angle of attack.
However, most airfoils have camber, and then the lowest drag is at positive lift coefficients in case of positive ...
- 225k
11
votes
Accepted
Could airliners fetch data like AoA and speed from an INS?
An INS gives your speed & orientation in space, but without reference to what the airmass that you're flying in is doing.
The Pitot tube and AOA sensors give speed and pitch orientation in ...
- 45.3k
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