Ronnin
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I heard a pilot saying that too much flap in this situation could lead to a tailstall. I thought extended flaps anytime you need more lift.
Did the airspeed decrease before or after he pulled back on the stick? With the aerodynamics destroyed, there is a lot of room to speculate on what the plane would do, but the ice could have contributed to the violence of the pullup and if the pullup caused an immediate stall (a stall is caused by a too-high angle of attack, not a low speed), the airspeed would drop rapidly and the plane would go from flying to falling like a brick in just a few seconds. I'm interested to see how much detail we get about what the plane was doing those few seconds - it may be quite revealing.Astronuc said:I wonder why the air speed would have been too low?
The stick-shake stall warnign is presumably based on the design stall speed of the plane, not the stall speed of the new ice+aerofoil wing.With the aerodynamics destroyed, there is a lot of room to speculate on what the plane would do,
mgb_phys said:The stick-shake stall warnign is presumably based on the design stall speed of the plane, not the stall speed of the new ice+aerofoil wing.
Is there any way for the system sense how close you are to stall from the real time response of the plane to controls?
William Rieke, a NASA research pilot and chief of aircraft operations at NASA Glenn, says to consider the 180-degree turn only if you have no other options -- like an airport in front of you. "If you are getting ice and you have to do a 180 to get out of it and in the process go back through [the ice] and accumulate more -- or worst case, if the air mass is moving in the direction you just turned to, you may be in it even longer than your initial encounter." If terrain clearance or clouds were not an issue, Rieke would prefer to descend 3,000 feet or, if the airplane was powerful enough to climb with the weight and drag of accumulated ice, climb 3,000 feet in an effort to get out of the icing conditions. (Most training aircraft would not be able to climb if they were carrying an appreciable quantity of ice.)
Listen to the chat in the actual test part of the video. The recovery procedure was to retract the flaps which resulted in near instant recovery. The issue wasn't speed (not with a pitch down attitude more than -10 degrees), but that the deployed flaps were interfering with the tail control at that air speed for that particular aircraft. This could be due to the air flow at the tail being interfered with, or because increasing camber by deploying flaps increases the pitch down torque on the wings.FredGarvin said:http://aircrafticing.grc.nasa.gov/courses/inflight_icing/related/3_2_3f_RI.html
The gist with a tail stall is related to flap usage because the wings are still producing lift with the decreased speed but the tail can not.
Umm...the issue is somewhat about speed. All aircraft with flaps have Vne's with the flaps deployed in each position which is usually quite a bit slower than most cruising speeds. The fact that the flaps are deployed means that you are operating at reduced speeds.Jeff Reid said:Listen to the chat in the actual test part of the video. The recovery procedure was to retract the flaps which resulted in near instant recovery. The issue wasn't speed (not with a pitch down attitude more than -10 degrees), but that the deployed flaps were interfering with the tail control at that air speed for that particular aircraft. This could be due to the air flow at the tail being interfered with, or because increasing camber by deploying flaps increases the pitch down torque on the wings.
FredGarvin said:Umm...the issue is somewhat about speed. All aircraft with flaps have Vne's with the flaps deployed in each position which is usually quite a bit slower than most cruising speeds. The fact that the flaps are deployed means that you are operating at reduced speeds.
Did you listen to the interview at the beginning? It was said that the corrective action was applied in about .2 seconds and it too "a few seconds" to recover. I counted about 3 seconds between pitch down and recovery and that's with someone with their hands on the flap controls. That is not instantaneous. They lost 300 feet in altitude and they knew exactly when they were going to induce the stall. Now put someone in the situation where they don't know it's coming and add a much delayed reaction time plus, probably, a lot worse condition of icing.
The one main point of confusion to groundhogs is that the tailplane produces a negative lift to counteract the aeroplane's natural tendency to pitch down. Most people assume that it acts in the same lift fashion as a main wing. In a wing stall, you want to drop the nose and/or add power in order to recover because your angle of attack is too high. In a tail stall, you have to pull up to regain proper attitude because the angle is too low.WhoWee said:What is the difference between a "wing stall" and a "tail stall" and how should you correct each... ...I thought he said the correction of one is the opposite of the other?
Jeff Reid said:Listen to the chat in the actual test part of the video. The recovery procedure was to retract the flaps which resulted in near instant recovery. The issue wasn't speed (not with a pitch down attitude more than -10 degrees), but that the deployed flaps were interfering with the tail control at that air speed for that particular aircraft. This could be due to the air flow at the tail being interfered with, or because increasing camber by deploying flaps increases the pitch down torque on the wings.
Update - you're correct, I thought "tail stall" mean a true stall of the elevator, but it doesn't. See my post below.Danger said:In a wing stall, you want to drop the nose and/or add power in order to recover because your angle of attack is too high. In a tail stall, you have to pull up to regain proper attitude because the angle is too low.
Jeff Reid said:update - The issue was speed related in the sense that the elevator could not provide enough sufficient negative lift (upwards force) to prevent the aircraft from pitching downwards, even at maximum non-stall deflection. The situation is made worse by the pilot increasing the elevators angle of attack so that it stalls (due to angle of attack, not air speed). In the video, the flaps are deployed while the aircraft has a pitch of -13 degrees. The pilot continously pulls back on the elevator to reduce the amount of negative pitch, which decreases the air speed. The flaps produced enough drag that even with pitch around -9 degrees, the airspeed continues to decrease as the pilot feeds in more up elevator until the elevators angle of attack becomes excessive and stalls (which is independent of air speed), severely reducing the negative lift at the tail and allowing the aircraft to quickly pitch downwards to -30 degrees. Retracting the flaps reduces the camber related pitch down torque on the wings, and allows the aircraft to pick up speed more quickly, restoring control authority to the elevator. The downwards diversion of air from the wings with fully extended flaps could also be reducing the air speed at the tail. This is why some aircraft use high "T" tails to keep the control surfaces out of the downwash from the wing.
The main point of my previous post is that a stall is the result of excessive angle of attack, not airspeed.
http://en.wikipedia.org/wiki/Stall_(flight)
In both cases, the angle of attack is too high. Tail stalls only occur when the elevator is excessively deflected upwards (or downwards if inverted flight, I'm ignoring inverted flight for the rest of this) quite a bit. Pulling up further will just worsen the situation. The recovery procedure is to feed in down elevator and/or increase air speed, reducing the angle of attack.
An extreme example of wing stall is a snap roll induced through excessive up elevator at speed without any aileron input. I flown some radio control models that do this, you peg the elevator stick back and instead of climbing, the model just rolls, a true snap roll (one wing stalls a bit before the other, resulting in a very fast roll). A not so obvious case occurs when ground towing a glider. The glider will snap roll if excessive up elevator is used. The first response to any roll reaction during a ground tow, both real and model gliders, is down elevator and then aileron to correct since there isn't enough time to figure out if the issue is a normal roll or snap roll. It's more of an issue for model gliders since there are contests (F3J) where minimum time during launch is a goal, so very high towing forces are used (over 20 g's). Snap rolls can also unintentionlly result from pylon racing models, when the high g turns cause the main wing to stall.
I wasn't aware that the term "tail stall", isn't the same as a true elevator stall. After doing a web search, a "tail stall" refers to the case where external forces deflect the elevator downwards, yanking the yoke forwards, generally coinciding with deployment of flaps at high speed (probably related to the pitching torque I mentioned before). The pilot has to pull back on the yoke with a lot of force, to overcome the external forces on the tail, to recover the elevator back to a normal position. Although this is not a true "stall" situation (excessive angle of attack on the elevator) it's called a "tail stall".Danger said:A tail stall refers to the inability of the horizontal stabilizer to provide proper trim. That results in a nose-down pitch attitude.
That would certainly explain the original disagreement. My apologies if I seemed condescending.Jeff Reid said:I wasn't aware that the term "tail stall", isn't the same as a true elevator stall.
Jeff Reid said:I wasn't aware that the term "tail stall", isn't the same as a true elevator stall.
No problem, my fault for not looking up tail stall before posting. I never heard the term "tail stall" before, I have read a lot about aerodynamics (I also fly radio control gliders). I was aware of how icing can badly affect airfoil shapes, but was totally unaware that deploying flaps at speed, combined with icing effects, could result in tail stall. After looking up more web pages about this, some aircraft are subceptible to tail stall while others aren't (those others still have icing issues, but not the pitch down issue related to tail stall).Danger said:That would certainly explain the original disagreement. My apologies if I seemed condescending.
They only work for a certain rate of ice build up, they don't handle ice forming on the tail. Some pilots turn the wing boots on and off in the belief that it's more effectiveNick M said:You can't make gear like de-icing boots automatic, because they only work when activated with ice build-up (you can actually make things worse by cycling them without a sufficient ice layer).
It appears that it was on autopilot. As with most of the low altitude turboprop+ice crashes - the pilot didn't know anything until the plane became unflyableI would also question why the pilot didn't notice a change in the flight characteristics of the aircraft if it was indeed icing related.
Looking at the diagrams, the leading edges of the vertical and horizontal stablizer also have de-icing boots. The news reports claims that tail icing isn't an issue for this particular aircraft.mgb_phys said:deicing bootsThey only work for a certain rate of ice build up, they don't handle ice forming on the tail.
That isn't known. The aircraft pitched up to 31 degrees, generating 2g's of lift during the process so the wings were able to generate a lot of lift and the elevator generate sufficient downforce to pitch the noseup. What isn't known is why the aircraft pitched up to 31 degrees.pilot not aware of problemIt appears that it was on autopilot. As with most of the low altitude turboprop+ice crashes - the pilot didn't know anything until the plane became unflyable
pilot not aware of problemIt appears that it was on autopilot. As with most of the low altitude turboprop+ice crashes - the pilot didn't know anything until the plane became unflyable
It engages before the plane reaches it's design stall speed, if the ice changed the aerodynamics so that the stall speed of the new wing shape was higher it could have stalled before there was any warning.edward said:The stick pusher engages before a plane is actually in a stall situation.