Accelerated stalls/steep sturns

JordanD

Honorary Member
Getting a slight bit confused on accelerated stalls in steep turns. From what I gather, if you're say, in a left slipping turn, the right wing will stall first. However, with dihedral effect and a sideslip condition, wouldn't the low (left) wing have a greater AoA and thus exceed its critical AoA and stall first? I tend to get things mixed up and overthink them and that's where my confusion comes in. :mad:
 

Hammertime

Well-Known Member
I would tend to think that the slip condition would overcome the dihedral effect. But, why would this question even come up? Accelerated stalls should be done coordinated. If you're studying that hard, take a break!
 

JordanD

Honorary Member
I would tend to think that the slip condition would overcome the dihedral effect. But, why would this question even come up? Accelerated stalls should be done coordinated. If you're studying that hard, take a break!
Thinking about the crazy things students might do! There's a couple questions about this kind of thing in the FIA written bank. The best explanation I've been given is that in a situation like the one I gave, the fuselage disturbs the airflow over the right wing and causes it to stall first, but I can't find an explanation like that in the FAA publications, they just say which one stalls first without any real rhyme or reason.
 

CFI A&P

Exploring the world one toilet at a time.
Getting a slight bit confused on accelerated stalls in steep turns. From what I gather, if you're say, in a left slipping turn, the right wing will stall first. However, with dihedral effect and a sideslip condition, wouldn't the low (left) wing have a greater AoA and thus exceed its critical AoA and stall first? I tend to get things mixed up and overthink them and that's where my confusion comes in. :mad:
You should try it yourself- find a Citabria, Super D, Pitts with CFI and ask them to demonstrate the base to final stall - spin scenario. It's really fun, I ask CFI prospects if they want to try it while doing spin training.
 

JordanD

Honorary Member
I'll try to, looking forward to it actually. I like to make sure I understand the theory behind it though, especially in case they want me to go into detail on the CFI ride.
 

shdw

Well-Known Member
However, with dihedral effect and a sideslip condition, wouldn't the low (left) wing have a greater AoA
Dihedral effect isn't a separate condition. If you are in a coordinated level turn, dihedral effect wouldn't exist. Dihedral effect exists because of the sideslip condition and, as you noted, would give the lower wing a higher AOA. The lower wing would have an even greater AOA difference if you were descending, or less difference if climbing.

Beyond that there is much more going on that it would be hard to accurately speculate. If you tried you would probably wind up talking yourself in circles trying to analyze it beyond what was mentioned in the first paragraph. And you would certainly confuse any student, yourself, and likely most examiners.

I agree with CFI A&P, go test it. As for the theory, I'd keep it to what I presented in the first paragraph. Maybe even ignore the effects of climb/descent and just stick with level flight slips. Unless you can draw and explain the vector, and the audience is someone with the background to grasp what you're presenting.
 

shdw

Well-Known Member
I guess I can leave it at one of those things that just "is because the book says so." :D
Well if it's an FAA publication trying to teach you about aerodynamics I wouldn't trust it as far as my infant nephew could throw it. If you want to understand that sort of theory turn to aero for naval aviators which you an find free at faa.gov/library. Or purchase the book Illustrated Guide to Aerodynamics, which is a bit easier to read and equally as accurate. Both are great resources for the CFI IMO.
 

JordanD

Honorary Member
Well if it's an FAA publication trying to teach you about aerodynamics I wouldn't trust it as far as my infant nephew could throw it. If you want to understand that sort of theory turn to aero for naval aviators which you an find free at faa.gov/library. Or purchase the book Illustrated Guide to Aerodynamics, which is a bit easier to read and equally as accurate. Both are great resources for the CFI IMO.
I will definitely look into those. Thanks for the suggestions!

Sent from my PC36100 using Tapatalk 2
 

seagull

Well-Known Member
Would add "flightwise" by Chris Carpenter to the list. Also, "Stalls, spins and safety" by Sammy Mason, if you can find it. It will answer your question about why the outer wing stalls first, and it has nothing to do with slipping.
 

shdw

Well-Known Member
Would add "flightwise" by Chris Carpenter to the list. Also, "Stalls, spins and safety" by Sammy Mason, if you can find it. It will answer your question about why the outer wing stalls first, and it has nothing to do with slipping.
Thank you seagull, I'll need to add those to my library as well!
 

shdw

Well-Known Member
Get both volume 1 and 2 of flightwise, well worth it.
Ordered them all, thank you again for the suggestion.

JordanD

Well I had to consult someone who used to post here quite often to be able to understand this. Since Seagull was so vague with his reply and I just couldn't wait for the books to get here.

For a picture visit my facebook page: http://www.facebook.com/brian.konsko

As for the discussion, I'll do my best to summarize. First, the AOA difference is quite small, calculated to 0.69 degrees difference when using a 10 degree sideslip with 4 degrees of dihedral. Which is made even smaller by the aileron deflection used to counter the over banking tendency.

Now to keep the discussion uniform: We consider a left slipping turn where the lowered wing, the left wing, is the upwind wing. The upper, right wing, is the downwind wing. They will be referred to as such from here on.

For it to make sense to me I had to consider stall propagation of an aft swept (delta) wing versus a forward swept wing. If you review how a stall propagates on each you'll find that the forward swept wing will stall from the root outward, where as the delta wing will stall from the trailing edge tip forward and inward.

If we apply this to the slipped condition, our upwind wing is just like the forward swept delta wing. It will stall at the root, which further exacerbates the stall propagation of your typical rectangular wing. Recall a rectangular wing stalls at the leading edge root outward and rearward. In other words, the upwind wings tip will be even further protected from stall.

The downwind wing, on the other hand, will now act like our aft swept delta wing. That is, the stall will propagate from the aft tip, forward and inward. This occurs because the extended chord line lengthens the boundary layer at the trailing edge tip, allowing it to separate more readily. All in all the end result is the upwind (top wing in our scenario) wing stalling first, with the stall propagation being a combination of both that of a rectangle and delta wing.

Finally, and what really nailed this home for me, was the correlation of this condition to what is experienced during a power on stall. Since a power on stall nearly always induces a left side slip due to the engines yawing tendencies, it can be expected that it will break to the right.
 

seagull

Well-Known Member
Yes, not going to argue with tgray on that one! Actually, I had read it that it was slipping the other direction, and my answer as to why the outer wing stalls first was applied to a non-slipping condition.
 

shdw

Well-Known Member
Also, "Stalls, spins and safety" by Sammy Mason, if you can find it. It will answer your question about why the outer wing stalls first.
Correct you are mate:

Stall said:
Sweptwing configurations without modifications such as slats, leading edge flaps, and wing fences possess rather severe tip stall characteristics. The outboard sections of a swept wing trail the inboard sections. The outboard low-pressure areas tend to draw the boundary layers toward the tips. This results in thickening the boundary at the tips, which easily separates at high angles of attack, inducing a stall.

....

It is interesting to note that when pilots become sloppy in their rudder coordination and permit adverse yaw to develop, the same conditions that prevail in a swept wing can occur in any other wing configuration. For example, even the docile rectangular wing can become hostile when sloppy flying permits a cross flow across the wing, resulting in boundary layer separation and tip stall.

I think that wraps it up folks! Thank you Seagull for giving me the heading to fly and tgray for helping me before the book arrived.
 

seagull

Well-Known Member
I really liked Sammy's book, nice approach to the topic. Tony LeVier recommended the book to me, forgot that he wrote the forward, though!
 

Pilotforhire587

Lycra Man
on a sad note, couple of local guys down in Moontown AL crashed were killed a couple of weeks ago, they were flying in formation, aircraft in question was a yak, pulled a tight turn, stalled and spun in killing both pilots... . Its sad when it is some one local.
 
Top