Key Aerodynamics: Why is it considered safe to stall in a slip vs not safe in a skid?
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- čas přidán 18. 03. 2021
- In this clip from an Instagram live session, I discuss some Key Aerodynamics. Specifically, I answer this question: Why it is considered safer to stall while in a slip vs. while in a skid?
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Nice explanation of maneuvers that just don't get enough attention during training. I was looking at other videos from other instructors that also provided nice explanations, and I added some clarifications. I'll add those down below. Not to correct anything, because your explanation is perfect. But to add some depth so that anyone interested might gain some additional knowledge concerning the aerodynamics.
Many instructors explain that "fuselage blanking the wing" causes that wing to stall". Not true (OK, indirectly slightly true). Exceeding stall AOA is what causes the wing to stall, and blanking doesn't change the AOA. What actually happens in a SLIP is that the high wing usually has down aileron (to hold that bank angle due to yaw and, yes, "blanking". But mostly yaw-roll coupling). The down aileron changes the local chord line so that area of the wing now has a higher decalage angle (difference between chord line and longitudinal axis of the fuselage) and thus, a higher AOA. And the low wing has up aileron and a lower AOA.
Opposite with a SKID: low wing is usually kept from over banking (due mostly to yaw-roll coupling, and a little blanking) with down aileron, so the low wing stalls, and the plane rolls upside down.
Stalling in the pattern is bad, period. A slip is a bit more forgiving because the high wing stalls first and the plane rolls towards wing level, giving you a little more time to correct the situation. It also reduces the load factor and as a result, AOA. Stalling in a skid, everything gets worse real fast. You've seen airshows where the Piper Cub does a "flat turn" keeping the wings level and using full rudder to "skid" around a 360 degree turn? Well, that's about the only use there is for a skid, and leave that to the pros.
To clarify some of the above aerodynamics: Yaw-roll coupling, or "proverse roll" is a result of the wings dihedral. When you yaw the plane and cause the relative wind to shift from on the nose to one side or the other, the forward wing sees a higher AOA and the trailing wing sees a lower AOA, thus the plane will tend to roll in the same direction of the rudder input (proverse). If you had anhedral (wings angled down, opposite of dihedral), it would roll opposite the rudder input, or adverse roll. These terms should be somewhat familiar since pilots should be aware of "adverse yaw" caused by differential aileron drag when ailerons are displaced when rolling (that's why airplanes have rudders to correct this, provided the pilots actually use the rudder). Anyone who is still awake, sorry about getting technical.
Ben
26,000+ hrs
USAF/Major Airline Capt/Aerobatic & Tailwheel Instructor/ CFI/II/MEL/Movie/TV pilot/Sailplane racer/Aero engineering and flight test.
Thanks for the great info Ben! 🙌🏻
I think the camera feed is mirrored (see Jason's finger ring) which makes the video a little more tricky to watch than it needs to be (for me at least). :-)
This
Demystified? Check! Best explanation I've heard. Thanks for discussing this using the formula for lift. Helps to tie it together. Nice job, Jason!
my cfi used to call skidding turns to final "gravedigger turns". And we want none of that. Thanks for the upload. Good, informative video.
This is the best video on slips and skids I have seen. Thank you so much.
Another way to say it - when you stall in a slip, the roll can't couple with yaw. You can actually balance the two (roll/yaw), and float downward in a full stall (falling leaf). In a skidding turn, the rolling moment couples with yaw, and you'll spin.
Inertial forces do not really become active until the airplane leaves the incipient stage of a spin.
The best, most clear explanation I have heard. You have a skill for explaining these types of questions. Love the channel.. It has helped me tremendously in my PPL training.
Jason, did you catch the mirrored comment here? It does make it really hard to follow this one.
One day, my instructor asked me to do a Power off stall, and I kept on having the airplane spin each and every time i do it. Guess what? I was coordinated, but then i learned two lessons:
1- over coordinating (something that i did right when the nose was about to drop) can result in the airplane entering a spin, especially if the wings are banked to the direction you are over-coordinating towards.
2- that actually... the rudder ALSO banks the airplane as the video stated, and i could have prevented that spin with less rudder input (just enough for the ball to center) or slight opposite rudder to return to level wing attitude with corrections here and there.
Good info thanks
Here's a spin from a slipping turn: czcams.com/video/BK66OUptfJI/video.html
This is a typical pilot explanation and it is incorrect. The inside wing (it should be called the upwind wing because it’s not inside anything) is not seeing “more of the relative wind”. In fact the upwind wing is seeing a higher angle of attack (or maybe that’s what he means?) which is why the aircraft wants to roll requiring opposite aileron. And no you don’t want to stall in this configuration because the upwind wing will stall first (it’s at a higher geometric aoa because the wing has dihedral) and it is effectively a spin entry. In a skid you also have sideslip and a yaw rate so you also have an asymmetric flight condition and if you stall, again, one wing will stall before the other and you could roll off. Stalls need to be performed in coordinated flight to avoid an inadvertent spin entry. The ball needs to be centered.
So if we were to take an aircraft with 0 dihedral the upwind wing would see the same AoA as the other wing hence no stall and no spin entry?
@@Iboxx any wing will stall. Dihedral doesn’t cause stall, but in sideslip it will generate an asymmetry in the stall. But that is just one parameter. Sweep also will cause asymmetrical stall. Regarding 0 dihedral, it depends on whether it is high or low wing. A high wing is good for about 2 deg equivalent dihedral because there is upflow from the fuselage in sideslip. Low wing airplanes need more dihedral to overcome that effect - something like 7 or 8 deg in order to have some dihedral effect which is a certification requirement. Unless you are talking airliners where there is a ton of sweep and they need very little dihedral and in the case of high winged aircraft with sweep they could have anhedral (c5, c17, harrier etc). The only aircraft I can think of with no effective dihedral or sweep would be an aerobatic airplane- something like an extra or a sukoi or a giles etc. They obviously stall and spin and snap. I have not studied the exact mechanism by which they do it. I suspect that it is via massive control power and the ability to generate high roll and yaw rates.
In a skid a stall causes a spin in a slip it dont
Slip - sideways, but not yawing - no rotation, no spin
Skid - sideways and are yawing - stall while yawing -> spin