Why does the aircraft stall when it loses airspeed ?

[flame_bringer]

i know that the flow seperation point comes forward when the airspeed reduces, but i have no idea why does the aircraft tend to pitch up when it loses airspeed ?
anybody explain please

[TheOptimist]

*imagines*

Does it pitch up? I thought it pitched down....

Only a student so I don't know really.

[poss]

Swept wing aircraft, when they stall at the tip, have a tendency to pitch up due to a combination of airflow over the tail changing and the C.G moving forwards. Other than this aircraft have a tendency to pitch down.

“The aeroplane may be considered stalled when the behaviour of the aeroplane gives to the pilot a clear and distinctive indication of an acceptable nature that the aeroplane is stalled”

One of these distinctive indications is a pitch down that cannot be immediately arrested and which is sometimes accompanied by a rolling motion that cannot be initially controlled.

[Neptunus Rex]

An aircraft, or more correctly the wing, does not necessarily stall when it loses speed. It stalls when the wing exceeds the stalling angle of attack. In straight and level unaccelerated flight, ie at 1 g, that angle is proportional to Indicated Air Speed and is called Vs. In a 60 degree bank level turn, you would be pulling 2g. In that case the stalling speed would be Straight and Level Vs√g = Vs x 1.44. In a vertical climb the g would be zero, so the stalling speed would be zero knots, which explains why you can still roll in the vertical when below the Straight and Level stalling speed. Get your insrtuctor to give you a few hours in a Cap 10 or a similar aerobatic aircraft and it will all fall into place. It's a lot of fun too!

http://www.augk18.dsl.pipex.com/Smileys/flyer.gif

[sprthompson]

I think the pitch up you might get when you get close to the stall is caused by the centre of pressure moving forward. As the airspeed gets slow the air across the surface loses energy/momentum and the higher pressue air behind it will push forward making it seperate from the wing, so the front of the wing is generating the lift and thus creating a moment pitching up...
(just a student also but fascinated by this stuff)

(also I suppose when you stall you're normally pulling back on the column more than you ought to be...)

[flame_bringer]

thanks everyone for the replies i owe u a big time
but im still not convinced as to why would the aircraft be stalling, perhaps getting confused and mixing between airspeed and thrust because as far as i know when we reduce thrust the aircraft sinks , and a thrust reduction must be accompanied with an airspeed reduction so the aircraft is supposed to be pitching down not up .
i just need someone to expalin it to me aerodynamically so i can be persuaded i mean it doesnt make sense really that if thrust reduces the aircraft picths down but if airspeed reduces the aircraft piches up when they both happen simultaniously ( thrust reduction and airsped reduction ) .

[poss]

A thrust reduction does not mean an airspeed reduction... if you idle the engine and put your nose down does the airspeed still drop off? As airspeed drops off, for you to maintain straight and level you must pitch upwards... it is this upwards pitch that will then, eventually, cause the wing to exceed its critical alpha (around 15 degrees on a clean wing, less with more trailing edge flap) thus the wing produces no lift. The aircraft will then do as designed and pitch downwards.

[sprthompson]

Have you flown a stall yet? I think you might be mixing cause and effect and this will probably be solved when you fly and intentionally stall the aircraft.

[Pelikanpete]

The aircraft does not pitch up as you loose airspeed. If in straight and level flight you reduce power, the nose will pitch down. If the pilot wants to maintain level flight at a lower speed he must increase the angle of attack of the wings as the amount of lift is proportional to speed (V squared).

As has been correctly pointed out already - as the angle of attack is increased to maintain the lift the centre of pressure or lift moves closer to the leading edge of the wing. Eventually as speed is reduced further and angle of attack increased to compensate the moving air will no longer be able to flow over the wing surface due to low speed/energy and surface/skin drag and so separate causing loss of lift and eventually stall. In straight wing aircraft the nose tends to pitch down during the stall. In swept wing aircraft the wing tip may stall before the wing roots causing an undesirable nose up pitch (unless design features have been incorporated into the wing to ensure the root stalls first - such as a twist in the wing that means the tips have a lower angle of attack than the roots, wing fences to prevent spanwise flow, vortex generators on the tip surfaces to re-energise the boundry layer of air flow and Krueger Flaps on the roots to encourage root stall first).

[flame_bringer]

Thanks i understand now
so by varying the airspeed we're varying the centre of pressure positiion ( bringing it forward towards the leading edge ) and the aircraft tends to pitch up . and by reducing thrust drag increases and it pulls the aircraft down and make it pitch down.
took me ages to understand it , i had to stall a couple of aircrafts in flight simulator X on purpose to understand lol

[Pelikanpete]

The aircraft does not pitch up when you reduce power. It's the pilot/autopilot who has to provide the control inputs to cause the pitch to increase or the aircraft will descend after the power is reduced (because the aircraft is slowing down and so producing less lift). By increasing the angle of attack to compensate for the loss of airspeed level flight can then be maintained.

[SNS3Guppy]

Swept wing aircraft, when they stall at the tip, have a tendency to pitch up due to a combination of airflow over the tail changing and the C.G moving forwards.

You may be thinking of the center of pressure moving, rather than the center of gravity. The CG doesn't change unless the load switches.

The aircraft does not pitch up when you reduce power.


Some aircraft pitch up, some pitch down with a reduction in power.

The location of the thrust line in relation to the longitudinal axis of the aircraft in part determines how the application of thrust affects pitch.

An example is a Lake Amphibian. In this aircraft, the engine is located above the aircraft, facing aft. Application of power causes the aircraft to pitch down, and reduction in power causes the aircraft to pitch up.

In aircraft with underslung engines below the long axis of the airplane, application of power typically causes an upward pitching moment.

In aircraft with centerline thrust, or thrust along the long axis of the aircraft, a change in power may mean an pitch up, or down, or no pitch change.

Loss of airflow over the horizontal stabilizer may lead to a pitch up or down, depending on the aircraft, configuration, trim, etc.

Likewise, a change in airspeed causes a change in airflow over the horizontal stab. Most aircraft utilize a negative load, or download on the horizontal stabilizer in flight. As airspeed decreases and angle of attack increases, the angle of downwash or downflow aft of the wing alters. Where download on the horizontal stab decreases, the aircraft tends to pitch down.

Likewise, on some aircraft in which a measure of download in created by propeller thrust or effects of the aircraft power, then a loss or reduction of power means a change in download, and a change in pitch.

[TheOptimist]

Guppy I imagine that if you lose concentration for a moment that's a very quick speed builder! Imagine someone who didn't know that lumping on full power and pushing in the yoke to keep the nose down. That'd be a shock and a half.

[mikebravo787]

any object..even if it is a paper chucked it swings up and slows down... and stalls into the new direction.. and same reverses.. kinda.. a falling leaf.. or a kite.. falling out of the skies..

on a serious note.. the swept back wing planforms at higher wing loading angles..
these could be straight and level, turning paths, climbs or descends, if the angle of attack increases beyond the stalling angle, the CP shifts forward giving a pitch up nose tendency..co the effect.. on your type of aircraft..

conversely on a dihedral, conventional, supercritical wingplanform in a straight and level flight, if we maintain the thrust/ power settings, as we wash off speed, the point at which you cannot maintain height even if you increase angle of attack ids the point of stall.. ie; the question and its answer.

cheers

[SNS3Guppy]

Guppy I imagine that if you lose concentration for a moment that's a very quick speed builder! Imagine someone who didn't know that lumping on full power and pushing in the yoke to keep the nose down. That'd be a shock and a half.


I'm not sure I understand what you're saying.

Not all aircraft pitch up with application of power, and not all aircraft pitch down with reduction in power, either.

Change in pitch with stall depends on the aircraft in question and other variable inputs ranging from configuration to center of gravity to application of power to the type of aircraft to the nature of the stall.

Some airplanes pitch up, some down, and some not at all.

Some aircraft with a T-tail configuration at high angles of attack, particularly in a stalled condition, will continue to pitch up. They may lack elevator authority to push the nose over or reduce angle of attack, in fact. Other aircraft may pitch down substantially, and some T-tailed aircraft may exhibit very different characteristics.

Blanket statements tend to be very inaccurate, particularly when attempting to define what one aircraft does based on what another exhibits.

An excellent example given previously for unconventional behavior with power application is the Lake Amphibian. Apply power and one has to increase back pressure on the controls because the aircraft wants to pitch nose-down. Nose-down trim is required. Conversely, reduce power and the aircraft wants to pitch nose-up.

What one can expect from an aircraft should be placed specifically in context for the aircraft in question, under a given set of circumstances.

[superliner]

Why does the aircraft stall as it looses airspeed?

To that, I'd say 'cuz the Lift decreases governed by L= C*(1/2*d*v^2), where C is the co-efficient of lift, d the density of air and v the velocity of the airplane through the air. So on reducing airspeed you've reduced the amount of lift produced by the wings but not quite the weight.

Not all aircraft pitch up with application of power, and not all aircraft pitch down with reduction in power, either.

Yes, it would depend on whether or not the "line of thrust" coincides with the "line of drag". Any separation between the two would create a pitching moment which would have to be accounted for during trimming. I, would like to think, the tendency might even differ with the same aircraft in different configurations, but I'm really not sure, I'm open to be corrected.

Swept wing aircraft, when they stall at the tip, have a tendency to pitch up due to a combination of airflow over the tail changing and the C.G moving forwards.


Aren't they supposed to do just the opposite of that? What with "wash-out" and all so that the wing root stalls before the tips. Again, just a student, would gladly appreciate being corrected.

[muduckace]

Gravity = Bad

Fast enough airspeed in ref to an airfoil that produces a higher air pressure under the wing than over the wing is the amazing effect that overcomes the forces of gravity.

A stall is simply the equalization of the speed of air traveling over the wing (is round so air flow is a lower pressure due to velocity) to the slower air under the wing (due to a flat surface and the drag induced by flaps).

Spoilers create a positive pressure over the wing to to decrease lift as flaps do the opposite of increasing pressure under the wing.

The aircraft only pitches up prior to a stall due to aircraft configuration to maintain positive lift, once equalized the CG of properly designed aircraft should pitch the aircraft nose down. Not to mention pilot input to regain a "positive" airspeed resulting in a positive pressure under the wing.

Trans-Mach airspeeds in aircraft not designed for sonic flight will create a stall at high airspeeds as the center of lift (pressure) travels aft of the wing while the horizontal stabilizer still maintains lift.

Hope this was a simple and accurate explanation as I understand it.

[DERG]

That'll do me!:ok:

[bcgallacher]

Most aircraft are designed to drop the nose with a reduction in power - the 4 forces lift, drag, thrust and weight tend to balance each other out in level flight and with a reduction in power the aircraft will naturally lower the nose.The last thing you need with an engine failure for example would be a pitch up.!

[mikebravo787]

bcgallacher (http://www.pprune.org/members/244490-bcgallacher)

Most aircraft are designed to drop the nose with a reduction in power - the 4 forces lift, drag, thrust and weight tend to balance each other out in level flight and with a reduction in power the aircraft will naturally lower the nose.The last thing you need with an engine failure for example would be a pitch up.!

I disagree on this as a rule.. it all depends on the type of role the aeroplane is used in. For Amphibians these aeroplanes are designed to pitch up with Engine failures for obvious reasons..
cheers..n' happy landings:)

[rapidshot]

to my opinion the best explanation is hidden in the definition of the neutral point and the static margin




Neutral point


A mathematical analysis of the longitudinal static stability of a complete aircraft (including horizontal stabilizer) yields the position of center of gravity at which stability is neutral. This position is called the neutral point. (The larger the area of the horizontal stabilizer, and the greater the moment arm of the horizontal stabilizer about the aerodynamic center, the further aft is the neutral point.)

The static center of gravity margin (c.g. margin) or static margin is the distance between the center of gravity (or mass) and the neutral point. It is usually quoted as a percentage of the MAC . The center of gravity must lie ahead of the neutral point for positive stability (negative static margin). If the center of gravity is behind the neutral point, the aircraft is longitudinally unstable (the static margin is positive), and active inputs to the control surfaces are required to maintain stable flight. Ultimately, the position of the center of gravity relative to the neutral point determines the stability, control forces, and controllability of the vehicleLongitudinal static stability

Longitudinal static stability is important in determining whether an aircraft will be able to fly as intended. - Static stability :As any vehicle moves it will be subjected to minor changes in the forces that act on it, and in its speed....

Static margin
Static margin is a concept used to characterize the static stability and controllability of aircraft and missiles.*In aircraft analysis, static margin is defined as the distance between the center of gravity and the neutral point of the aircraft....




Pitching Moment Curves

If we are given a plot of pitching moment vs. CL or angle of attack, we can say a great deal about the airplane's characteristics.

http://adg.stanford.edu/aa241/stability/images/cmcurve.gif