How To Find The Altitude For Stall Speed And The Weight At Stall

Does Stall speed change at altitudes?

Aerodynamic stalls depend exclusively on angle of attack. As altitude increases, air density drops, which causes the wings to generate less lift. To maintain level flight, either angle of attack or speed must be increased with increasing altitude. If speed is increased, AoA remains constant, and so no stall occurs. If AoA is increased, speed remains constant, but there is a risk of stalling.

Speed in these cases means true airspeed, not indicated airspeed. Because indicated airspeed closely tracks air density, the indicated airspeed at which an aircraft will stall, all else being equal, remains fairly constant at all altitudes. The true airspeed increases.

Things become more complicated at transonic and especially supersonic speeds.

What is the stall out speed of a boeing 747-400?

At T-20 second, the N1 reads less than 25%, which is essentially idle thrust. Yet speed is at 350, ansd altitude just got up 200 ft in less than 5 seconds. And at T-15 second, thrust is back up to 86.7% N1, and speed is, what? A big fat zero? Unless you played a lot with the throttle and the elevator and put the plane in a stall, then I would blame it on MS. After all, they use to crash computers with amazing regularity, so why not expand this trend to aircraft? Just be thankful MS is not used as the OS inside real airplane. Edit: but Walter (below) 9 degree nose up means nothing if the flight path (gamma) is down, this could mean an alpha that is beyond stall. And besides, we have snapshots at 5 seconds interval; what happened between those?

Stall is not related to speed. Airplane stalls when critical angle of attack is exceeded. But speed at which the aircraft stalls is a reference for the pilots. It depends and varies on many factors:1) Aircraft Mass: The heavier the airplane is the higher the stall speed. As Weight opposes the lift, you need to give a higher angle of attack to maintain level flight. This gets you closer to the Critical angle. Lower the aircraft mass, lower the stall speed.2) Wing contamination: Ice on the wing surface also increases the stall speed. The ice makes the wings produce less lift, so to gain same lift as that of a clean wing the angle of attack has to be increased.3) Flaps: When you lower the flaps, the stall speed decreases. The lowering of flaps also tend to give decrease of Critical angle of attack There are many ways to explain this, one goes like this, when we deploy flaps, there is an increase in Coefficient of lift (Cl) and the wing tends to stall at a lower angle of attack. We know L=1/2*rho*Cl*A*V^2. If Cl increases, and we want the Lift to remain same, the V should decrease. Hence the stalling speed is lower. How the angle reduce is pretty simple to explain. Extending flaps, creates an effective angle of attack as it modifies the camber of the wing. However, it is conventional to plot the Lift curve using the clean wing chord line. This always give a reduced Critical angle of attack.

How do you find the stall speed of a plane?

You don't. A stall occurs when a certain critical angle of attack is exceeded by the wings; airspeed is irrelevant.

Because it is difficult to know the exact angle of attack when actually flying the aircraft, most pilots think in terms of stall speeds. But the speed at which an aircraft will stall varies considerably, because it only indirectly influences the angle of attack, and only the angle of attack matters for stalls.

Stall speeds can be calculated, although this is complicated and is done mostly only by manufacturers for certain specific configurations of the aircraft. Pilots can estimate stall speeds for other configurations by inference and interpolation, or can measure them by trial and error.

For example, the heavier an aircraft is, the more lift that is required to hold it in the air. This means that, at any given speed, a heavily-loaded aircraft needs a steeper angle of attack to the wings than a lightly-loaded aircraft. Thus, weight influences stall speed, but the stall actually depends only on the angle of attack. The angle of attack can also change in, say, a climb or a turn, so the stall speed also changes in climbs and turns.

The most accurate way to find a stall speed is to have an angle-of-attack indicator in the cockpit. The stall speed is then whatever speed that causes the angle of attack to reach the stall angle. With an angle-of-attack indicator, inadvertent stalls are much more easily avoided than by other methods; however, angle-of-attack indicators and sensors are more difficult to design than those for airspeed, so one sees them less often.

Airliners may have angle-of-attack indicators, as well as flight-path-vector indicators that can be used to check the actual angle of attack of the aircraft at any given moment. Some also have direct indications of the stall limit on the attitude indicator in certain configurations.

Does the stalling speed change with height?

yes, as the properties of the air change as your altitude increases or decreases, your stalling speed would change. If you go up, the stalling speed would increase as well, meaning you'd need to move faster to not stall out.

Edit:
Yes, I am an Aerospace engineer, and unfortunately, I am correct in this case. I was referring to True Airspeed (TAS). Please refer to Anderson's Aerodynamics book. Yes, stall is directly related to that separation of airflows. However, this ability is directly affected by the air properties, which do change with altitude. Please see the below equation to see how density is tied in.

Vstall= sqrt((W/S) / (CLmax × ½density))
Since decreases as you go up in height, Vstall will increase.

Also, thank you sparviero for weighing in. I appreciate your ability to understand the different opinions on this question and not just misunderstanding and attempting to rip me a new one.

First, the big surprise.Low-speed stall is not directly related to airspeed.It has everything to do with, and only with, the angle of attack.A wing of a given design stalls at a particular angle of attack. Only one angle of attack at all altitudes, all speeds, and all G-loadings. Period.Shocked?’Tis the truth.Pilots are given training to avoid stall using the airspeed indicator; even modern airliners refer to stall speeds in terms of an airspeed called VREF. This is shown on the charts in the Flight Manual, and is also shown on the modern PFD.Airspeed is a good measure of angle of attack in one-G flight. In accelerated flight—pullups, turns—it is no longer reliable. Pilot’s handbooks contain charts of stalling speeds under acceleration, but they are too complicated to memorize. Tabular stall-speed information is really quite useless, other than to suggest to pilots that they ought not to bank steeply at low speeds.Angle of attack, or alpha, is defined as the angle between the chord line of the wing and the relative wind. This is not the same as pitch (or body) angle, or flight-path angle. Airflow over the wing is essential: no wind, no lift; but when it comes to measuring lift, or more properly the coefficient of lift ($C_L$), angle of attack is the key. As alpha is increased, $C_L$ increases also, up to the critical angle. At that point, the airflow over the wing separates, lift is lost and the wing stalls.Under some conditions, critical angle of attack correlates fairly well with airspeed.In the flight regimes under which correlation is poor, however, we most often get bitten.Airspeed is used for stall management because putting in an angle-of-attack indicator separately in the cockpit will be asking too much of pilots. They should be able to ensure safe flight with the minimum of instruments to observe and monitor, so stall is associated with airspeeds for particular aircraft configurations.Assuming that the aircraft weight remains nearly constant, high altitude flight requires the same amount of lift as low-altitude flight.Lift is a function of the lift co-efficient, $C_L$, the density, $\rho$, the airspeed, and the wing area.For a given wing, the lift coefficient depends only on the angle of attack.During low-speed stall, all you need is a high $C_L$. The balance of forces at high altitudes is such that this occurs at a higher speed .

The Sr-71, as another person said here, is a delta wing, but an unconventional one. It relies heavily upon the “cheeks” that go all the way from the nose of the plane to the roots of the wings, making it a blended body. The Sr-71 is not equipped with any lifting movable surfaces like flaps, it's entire body is meant to create lift. The way the cheeks are set up, and engines and other structures on top of the wing, make it complicated for the airflow to flow as it's intended in high AOA scenarios, where (in the case of delta wings) the fluid is supposed to enter through the leading edge root of the wing and move diagonally outwards through a vortex. In our high AOA case, the front of the cheeks (LERX) will be wet, but there will be a separation of flow before hitting most of the delta root. The LERXs will take part of the flow coming into the root of the delta from the side, the little flow that can make the leading edge of the delta wet will not be able to flow very well because of the structures in the mid section of the wing, and barely no fluids will touch the outwards trailing edge of the wing. Thus, just creating most of thr lift in the forward section of the plane, making it rotate upwards. Pair this with a rear-heavy design and voilà. The Sr-71 had no stall indicator, but it had an Automatic Pitch Warning (APW), that let the pilot know he was flying with too much of an AOA. It had two modes, Shaker only and Pusher and Shaker: Shaker only mode would shake the pilot controls when he was approaching a prestablished angle, and Pusher Shaker would do the same but if no correction was made, the computer would take over and push on the stick to correct it automatically, kinda like newer planes that prevent pilots from stalling them. All in all, the Sr-71 was meant to hit super high speeds and minimize drag (and it was great at that) at the cost of having to be extra careful when maneuvering it. The stall angle figures are 18° for subsonic flight and 7–8° for supersonic cruise flight (source: Sr-71 Revealed).

Why do planes stall ay a certain altitude? ?

The equation for lift is

L = 1/2 Rho S V^2 Cl

Rho is the density of air, it drops as you gain altitude.
Cl is the lift coefficient, it increases when you have a larger angle of attack, but at some point, around 10 degree, you start experiencing flow detachment and stall; basically, the air is 'over stretched' and the wing stops working.
L, the lift, has to be equal to the weight of the airplane. You do not have much control over that value.
S is the aircraft wing area, which is also fixed.

So, in order to fly higher, you may want to fly faster. Except that, sooner or later, your engine will run out of reserve power (since he drag also varies as D = 1/2 Rho S V^2 Cd) and you will get uncomfortably close to the speed of sound, where the drag coefficient increases due to compressibility effect and shock wave generation.

As long as the aircraft is pressurized to a comfortable degree, nothing is supposed to happen to the pilot.

What causes a high speed stall in a rv-6?

The RV-6 is a fine aircraft. The stall speed is about 50mph in 1 G flight. A aircraft will stall when the wing does not generate enough lift to support the aircraft's weight (critical angle of attack exceeded).The faster aircraft goes, the more lift the wings provide. In straight and level flight, the wings have to support the weight of the aircraft and passengers. When an aircraft maneuvers, the aircraft is subject to G forces different from the 1 G it experiences in level flight. A common example given is that it takes 2 G's to maintain a level turn at a bank angle of 60 degrees. If an aircraft's stalling speed is 60 knots in 1 G flight, it's stalling speed will be over 85 knots (barely) in 2 G flight. At 4 G's the stall speed will about double from the 1 G stall speed (what we normally refer to as stalling speed). So you see that stall speed is not the same for all flight conditions. A high speed stall can will occur when the wing can no longer support the weight of the aircraft, or more correctly, when the critical angle of attack is exceeded. Most of us small aircraft pilots think of stall occurring at certain air speeds. In reality it occurs at a certain angle of attack, that is different for different wing designs.

For the high speed stall that your friend experienced in his RV-6 to have happened, he must have been pulling some G's. Was he recovering from a dive at low altitude? Or a high G pull up after a low pass?

Very sorry to here about the accident.