Aerodynamics question - aileron rolls

[moot]

Film here.
First roll is without any rudder.  Second with some rudder keeping it more or less straight.  Third rolls are with the nose slightly above horizon, full stick forward and full right rudder.   What happens that makes the third "roll" that much faster? 

[Die Hard]

Snap-rolling. Stalling one wing.

http://www.youtube.com/watch?v=B3eTcXXqNzM

http://www.youtube.com/watch?v=W45ZCJaVLhM

http://www.youtube.com/watch?v=Uy0KkqFf_bU That one was insane.

[moot]

Why is it so much faster and sustainable compared to regular "upside" snap-rolls?  Top wing/airplane surface's vs bottom's lift or lack thereof?

[Die Hard]

Rotation is induced by a rapid pitch input followed by rapid yaw input, thus stalling one wing further than the other. This imbalance in lift causes the high speed roll.

[Die Hard]

I don't know why it is faster inverted... It may be the wing camber allowing a deeper stall of the trailing wing.

[moot]

Not exactly.  That roll in the .ahf happens even if you input slowly.  The Ta152 does the same thing under somewhat different circumstances.  Other planes do it as well, IIRC the 51 does it.  From roughly the same inputs, with a clear threshold regardless of how quick you cross it.  Why is it so much faster than positive elevator snaprolls?  Is it only the difference in lift profile between the wings' top and bottom surface?

nm - forum took forever to process reply, didn't see the above.

... It's not just a funny behavior.  It's useful as an airbrake and distracts the other guy when you need to make him overshoot as you're going conveniently at the right speed and angle.

[Die Hard]

Slowly or quickly really doesn't matter much; when the angle of attack exceeds the wing's limit the wing will stall. When you have yaw input as well one wing will stall before the other inducing a spin.

[CAP1]

Film here.
First roll is without any rudder.  Second with some rudder keeping it more or less straight.  Third rolls are with the nose slightly above horizon, full stick forward and full right rudder.   What happens that makes the third "roll" that much faster? 

i think the rudder helps to speed up the roll by "pulling" the nose in the direction of your roll. too much, and you'll end up in a snap roll as mentioned just above, when you stall a wing. time it right, and use just enough, and you'll get a nice fast roll.

[dtango]

Moot: my good sir 

What kind of reader of mystery novels are you?  Are you the kind that likes to skip to the end to find out what happens, or do you like discovering and following the clues to see where they lead?   I'd be more than happy to respond to your "mystery" here either way if you'd like!

Tango
412th FS Braunco Mustangs

[moot]

If you write it, I'll read it. I'm busy studying most of the time, but it's a flexible schedule and I thrive on diversity so I'd have read even a lengthy novel within a day or so, at most.

[Stoney]

Making my mark just I get notification when Tango starts laying out the plot...

 

[JunkyII]

Not exactly.  That roll in the .ahf happens even if you input slowly.  The Ta152 does the same thing under somewhat different circumstances.  Other planes do it as well, IIRC the 51 does it.  From roughly the same inputs, with a clear threshold regardless of how quick you cross it.  Why is it so much faster than positive elevator snaprolls?  Is it only the difference in lift profile between the wings' top and bottom surface?

nm - forum took forever to process reply, didn't see the above.

... It's not just a funny behavior.  It's useful as an airbrake and distracts the other guy when you need to make him overshoot as you're going conveniently at the right speed and angle.

What do you mean by this? Are you suggesting using this if you need alittle more to force an overshoot?  Im confused

[dtango]

OK then!  You’ll be our super-sleuth, Sherlock Holmes.  I’ll play the role of the evil Professor Moriarty leaving diabolical clues for you .   We’ll see if this is a fun way to solve this mystery by starting out with answering your question with…a few questions of course!

Die-hard is right.  Your third roll is faster because it’s a snap roll where one of the wings is stalled.  That of course is our first clue.  But the question of course is “Why?  Why  my dear Watson?”.

Let’s simplify our mystery for a moment and ponder the case of a steady roll with only one degree of freedom (roll axis only).  We know for a roll to occur there have to be rolling moments (forces) involved.   Here’s a riddle to cogitate on:  What are the rolling moments and in a steady roll is there an imbalance of these rolling moments?

Tango
412th FS Braunco Mustangs

[moot]

Junky - Yep. It's a small amount of airbraking and, in the situation I'm talking about (tiny margin of error, but only 1 solid hit required - easy at low speed & point blank), every bit counts. And it can easily be made to end with wings nearly level which is a great position at that point, because you're most likely flying very slow and need all the lift you can get, to set up for the snapshot that's coming up.  It also works as a distraction that (in my experience) usually numbs the other guy's reactions.. They keep a good look at you to figure out why you're snaprolling like that, and tend to loosen up their reflexes.



Prof - In a 1D freedom situation, it's the ailerons working in accord to spin the plane around the longitudinal axis.  If the ailerons are instantly moved back to neutral, the rolling stops.  To test the balance of these two rolling moments we'd have to move the ailerons independently, and compare the effect of each one.  But we can't and the 1D freedom situation forbids anything else that I can think of, so I'm stumped at this point.

[dtango]

Prof - In a 1D freedom situation, it's the ailerons working in accord to spin the plane around the longitudinal axis.  If the ailerons are instantly moved back to neutral, the rolling stops.

Good observations .  Let's put our magnifying glasses on these observations a bit shall we? 

So the ailerons help spin the airplane around it's roll axis.  Usually that means an airplane at some point has no rolling, but when we apply aileron, rolling begins.  What can we conclude about the forces at work here to go from no rolling to rolling?  Obviously the application of aileron creates a force (or moment) that causes the roll.  We can also conclude that this rolling moment is also created in the direction of the roll.  If there is a force we know from newtonian physics that Force = Mass * acceleration.  Thus there must be some acceleration in the direction of the roll.

But here's our next riddle.  If there is acceleration in the roll direction caused by the deflection of aileron, why doesn't an airplane continue to roll faster and faster and faster the longer we hold aileron deflection?  Instead the roll rate hits some maximum and then stays there even as we continue to apply aileron.  EDIT: for clarification I should rephrase "continue to apply aileron" to "continue to hold aileron input".

But that's not all .  Let's also examine your other observation "when we bring ailerons back to neutral the rolling stops".  Why would an airplane stop rolling after we bring ailerons back to neutral?

Tango
412th FS Braunco Mustangs