Ice Contaminated Tailplane Stall

caliginousface

caliginousface

Frank N. Beans
Have a few questions if anyone can shed some light.

1. Why does adding airspeed reduce the "margin of safety"?

2. Why can increasing the flap setting initiate the stall? If flaps are extended, doesn't the downwash follow the deflection of the flaps? How would downwash be increased over the horizontal stab if this is the case?

I'd attach some pictures, but I'm running late. I'll post some when I get back. Most of my questions come from a text on icing and this link http://www.skybrary.aero/bookshelf/books/819.pdf

Thanks for any help!
 
I can help with your 2nd question. To start off, keep in mind that the horizontal stabilizer is an "upside down" wing, producing lift downward. Also keep in mind that when flap settings are increased in an airplane, the aircraft must be pitched down slightly to maintain the same airspeed. This actually INCREASES the angle of attack on the horizontal stabilizer! And since an airfoil which is contaminated with ice is said to stall at a higher airspeed, increasing the flap setting can trigger a tailplane stall.
 
If I understand you correctly, they both have to do with increasing the downward "lift" on the tailplane. Remember that in most conventionally configured aircraft (in most configurations), the horizontal stab is pushing the tail down. But it's a wing, just like the, uh, wing is, because the center of lift is typically behind the chord of the wing, meaning that the more lift, the more nose-down pitch moment must be counteracted by the horizontal stabilizer. When the stab stalls, there's nothing balancing the nose-down pitch moment, so down you go. Likewise, when you put the flaps down you A) increase the lift on the wing which again must be counteracted and B) Move the center of lift even further back, increasing the nose-down moment which must be corrected by the horizontal stab. And this translates to loading the horizontal stab up, which of course, just like a wing that points up, increases its susceptability to a stall.

Clear as mud?
 
Nitpick:
Regardless of the final report findings, the recent crash of a Bom bardier DHC8-Q400 near BUF (Buffalo NY) renewed widespread interest in the dangers of inflight icing and, more particularly, ICTS.
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Once we get over the fact that it wasn't a tailplane stall, but a wing stall. But anyway.

Regarding (2), it's largely simple lever physics. As the trailing edge flaps are extended the center of wing lift moves aft, which means that the tailplane has to generate more downward lift to maintain attitude...more lift = higher angle of attack to produce it. Like Boris Badenov said.

Flap extension is also prohibited for us (EMJ) while holding in icing conditions, and our minimum holding airspeeds are considerably higher in icing conditions. Best thing to do is get out. Don't loiter.

Somewhat related: This should be shouted from the rooftops.
Certification rules for aircraft operations in icing conditions were never intended to endorse flight of unlimited duration in severe icing conditions. The safest action is to avoid prolonged operation in moderate to severe icing conditions. Prolonged operations in altitude bands where temperatures are near freezing and heavy moisture is visible on the windscreen should be avoided.
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Severe icing has always meant, to me, "icing conditions beyond the capability of your aircraft's de/anti-icing systems to cope."
 
caliginousface said:
adding airspeed...increasing the flap setting i
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Remember critical AOA for tail occurs at a low AOA for the main wing. Hence the recovery procedure being to pull back, contrary to a main wing's stall recovery procedure. Adding airspeed and increasing flaps each reduce the main wings AOA, increasing the tail wings AOA. In other words, anything that reduces main wing AOA brings you closer to tail stall.
 
Wow I feel like an idiot. Thanks for the help, guys.

Now I read conflicting statements about downwash. One say it's increased, one says decreased when flaps are added. In relation to the horizontal stab, are general discussion about ICTS main wing downwash onto the stab, or downwash from the stab itself?
 
Flaps move the center of lift aft. And the center of lift is already aft of the CG. Therefore adding flaps would necessitate increased downforce from the tail for a given speed. In other words, if you're iced up in a 172, you're better off landing in a clean config. Sounds right to me but please correct me if I'm wrong.
 
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