Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: moot on July 16, 2009, 10:06:56 AM
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Film here. (http://dasmuppets.com/public/moot/OneWeekStuff/film18_rolls.ahf)
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?
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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.
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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?
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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.
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I don't know why it is faster inverted... It may be the wing camber allowing a deeper stall of the trailing wing.
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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.
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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.
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Film here. (http://dasmuppets.com/public/moot/OneWeekStuff/film18_rolls.ahf)
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.
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Moot: my good sir :salute
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
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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.
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Making my mark just I get notification when Tango starts laying out the plot...
:)
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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 :(
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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
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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.
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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 :aok. 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
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Ooh... ooh... I know. Can I play? :D
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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.
TY for explaination :salute
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Ooh... ooh... I know. Can I play? :D
It's an open discussion :). Jump on in, the water is fine!
Tango
412th FS Braunco Mustangs
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Roll rate reaches a steady state when the forces of the ailerons acting on air can no longer accelerate the roll against the forces of air acting on the wings, tail surfaces and, to a lesser degree, the fuselage itself.
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< concurs.. When the force of airflow against the ailerons is equal to the sum of forces resisting it. I guess you meant those two when you said two rolling moments.
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Brilliant guys ;). Yes that's exactly what I meant about rolling moments moot. OK doing good so far. So to my original question regarding the rolling moments in a steady roll, they are equal each other and therefore total sum is zero, implying another force at work opposite rolling aileron force. Of course the ailerons first create a force and acceleration into a roll but then there is another force that eventually cancels the rolling acceleration induced by aileron deflection out.
So what could be the main contributor that produces this opposing force in a roll that damps out aileron roll acceleration and also stops a roll when we neutralize aileron?
Tango
412th FS Braunco Mustangs
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The flow of air against the airframe whose surface is almost entirely aligned with it? Ignoring the natural lifting bias that allows a plane to neither climb nor sink in gravity.
If you've got an arm with a flat surface exposed to a strong flow of air at one end, the surface parallel to that flow of air.. and the other end of the arm attached to a freely rotating axis so that that's the only degree of freedom..
(http://dasmuppets.com/public/moot/OneWeekStuff/air.png)
Then you'd have the arm staying put in one position (provided the surface is perfectly regular or symetrical so that both faces receive an equal push from the airflow), and with a required force to make it rotate any given amount growing directly proportional to the airflow. This represents a wing without an aileron, and the force that opposes and limits ailerons' rolling moment.
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Hmmm, we need to step back just a step. When we talk about aircraft degrees of freedom we usually mean that to be degrees of freedom with respect to the airplane center of gravity. There are 6 degrees of freedom we have to deal with, but typically we usually talk about 3 of the 6 portrayed below longitudinal, directional, lateral respectively:
(http://thetongsweb.net/images/AircraftAxis.jpg)
To keep the analysis of roll simple we limit the analysis to a single degree of freedom in the stability axes (longitudinal, directional, lateral) and assume the airplane doesn't pitch or yaw but only allowed to roll along the longitudinal axis (while moving forward). The reason we do this is because pitch and yaw introduce a lot more complexity in analyzing roll. So this is what I mean by fixing it to a single degree of freedom. If we do so we get an idealized roll depicted like the following:
(http://thetongsweb.net/images/aileronroll.jpg)
Another way to look at is like this..
(http://thetongsweb.net/images/roll1.jpg)
Notice on this image I've included the airflow arrows. So let's go back to your guess at what is damping force against a roll - "The flow of air against the airframe whose surface is almost entirely aligned with it?". Let's assume that airflow against the airframe creates a roll damping force that opposes and stops roll acceleration caused by aileron input. Assuming the airplane is traveling at a constant velocity forward how could the airframe generate an opposing force against roll if the forward velocity is constant? To save us going down a rabbit hole the answer is that it can't.
So we're still left with our question: what causes the damping force that can cancel out the rolling moment from aileron input? I'll offer up a couple of hints. It has to do with the roll stability function of the wing. It also has to do with the rotational velocity experienced by the wing too.
Tango
412th FS Braunco Mustangs
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I'll take a stab at it: Disruption of airfow. In your example the left wing is going down so not only is the bottom of the wing pushing air, that very act is disrupting air over the top of the wing precisely where the aileron needs air to sustain roll. Of course the exact opposite is happening to the other wing (the top is pushing air and disrupting airflow over the bottom of the wing) but with the exact same result.
Not only that but if there were no forward momentum (i.e. introduction of clean air) there would be a partial vacume acting on the wing surface away from the roll which would try to "suck" the wing back creating the natural shock absorber effect you alluded to.
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Here's a hint towards one factor...
(http://i125.photobucket.com/albums/p61/stonewall74/db_0686_21.jpg)
The Corsair is an excellent example of it...
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Downward moving wing is at higher AOA than the upward moving wing. The higher AOA produces more lift which counters the roll.
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I'll take a stab at it: Disruption of airfow. In your example the left wing is going down so not only is the bottom of the wing pushing air, that very act is disrupting air over the top of the wing precisely where the aileron needs air to sustain roll. Of course the exact opposite is happening to the other wing (the top is pushing air and disrupting airflow over the bottom of the wing) but with the exact same result.
Not only that but if there were no forward momentum (i.e. introduction of clean air) there would be a partial vacume acting on the wing surface away from the roll which would try to "suck" the wing back creating the natural shock absorber effect you alluded to.
Paragraph #1 close, but no cigar. Paragraph #2,...not so much :).
The right answer is...
Downward moving wing is at higher AOA than the upward moving wing. The higher AOA produces more lift which counters the roll.
I'm plum out of time to type so I'll cut to the chase some time tomorrow to provide a fuller explanation!
Tango
412th FS Braunco Mustangs[/g]
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Let’s revisit some fundamentals of what is happening in an aileron roll. First how do ailerons cause an airplane to roll about it’s longitudinal axis? Ailerons are nothing more than plain flaps along a local section of a wing. Just like a flap when deflected they change the lift slope curve.
(http://thetongsweb.net/images/TH17G4.jpg)
As the above diagram shows, the lift curve and range of lift coefficient increases with flap deflection. With one aileron deflected down and the other deflected up in coordination these aileron deflections create changes in the lift curve slopes of the wings. The net effect is to create an imbalance between the lift produced by left and right wings. This imbalance in lift forces causes the airplane to roll. If we could see the net lift distribution due to aileron deflection across the span of the wings it would look something like this:
(http://thetongsweb.net/images/rolldist.jpg)
Notice how the greater lift distribution on the right compared to the lower lift distribution on the left results in the airplane rolling left. This is the rolling moment created by aileron deflection.
Of course we’ve by now also concluded in our above observations that there is another rolling moment that is produced which acts in the opposite direction of aileron rolling moment. This force is known as the roll damping moment. Conventional airplanes are designed to be laterally statically stable which means there is a roll damping force that naturally brings rolling moments back into equilibrium.
So what’s the main source of this roll damping moment? Just like the lift distribution differences of the wings cause a roll, the wing is also the main contributor in providing roll damping force. Colmbo pointed out that this occurs due to differences in angle of attack between up-going and down-going wings. What’s going on here?
Relative airflow with respect to a wing is not only a function of wing’s angle of attack to forward velocity but also a function of it’s rotational velocity as well. That means angle of attack of oncoming airflow to a wing is also affected by how fast an airplane is rolling as well.
(http://thetongsweb.net/images/rollaoa.jpg)
As the above diagram demonstrates as an airplane rolls the down-going wing’s attack of attack actually increases while the up-going wing’s angle of attack decreases. The faster an airplane rolls, the greater the magnitude of these changes in angle of attack. We know that lift is also a function of angle of attack. This means that the faster an airplane rolls, the down-going wing produces more and more lift. The opposite is true for the up-going wing. The faster we roll the less lift the up-going wing produces. At some rotational velocity this change in lift between the down-going and up-going wing produces an imbalance in lift forces that equals the imbalance created by aileron deflection.
Simply put, roll damping moment increases the faster an airplane rolls until at some point it equals the opposite rolling aileron moment.
When ailerons are brought back to neutral, the airplane continues to roll. Because of the roll the down-going wing continues to produce more lift than the up-going wing (roll damping) which now causes the roll to decelerate to the point it stops when there is no longer any rolling velocity to create an imbalance in lift distribution between the wings. You can see this is in the following roll time history diagram. As aileron input is brought to neutral, it's the roll damping moment that causes the roll to quickly subside.
(http://thetongsweb.net/images/rollresp.jpg)
So the wings are key to both the aileron rolling moment and the opposite wing roll damping moment. This is a key concept.
All the above discussion is a based on the idea that neither left or right wings are stalled. What happens if the down-going wing stalls but not the other during a roll? As we’ve seen roll damping moment to counter-act aileron rolling moment is a function of the lift of the down-going wing. If the down-going wing is stalled then it produces less lift resulting in lower roll damping. The deeper the stall the less the roll damping produced. The lift curve slope of a stalled wing has a steep downward slope which means roll damping moment falls off rapidly at stall. The effect is that there isn’t much force to counteract the rolling acceleration due to ailerons which means the airplane rolls around much faster. This is what’s happening in a snap roll.
So there you have it moot. The reason your third roll was so much faster is because the down-going wing was stalled and there was little roll damping moment to counteract aileron rolling moment :rock.
Now my fingers are worn out so I have to go give them a break!
Tango
412th FS Braunco Mustangs
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Oh one last thing. It was the Col. Mustard in the parlor with candlestick holder! :D
Tango
412th FS Braunco Mustangs
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You know WAY too much. :O
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That makes perfect sense. Dunno how I didn't figure it out.. I really like the second diagram. Thanks DTango :)
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But wait, we have not yet spoken of adverse yaw effects during steady state and roll onset. And the changes in roll rate based on 1g vs unloaded roll. And the effects of dihedral when rudder is pushed to create a rolling moment.
Btw first time I was attempting to do steady inverted flight in the RV, I was entering it from the top of a loop. Inverted stall speed in the RV turns out to be about 95 Knots,(unknown to me at this time) I made a very nice avalanche with out trying.
HiTech
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Inverted stall speed in the RV turns out to be about 95 Knots,(unknown to me at this time) I made a very nice avalanche with out trying.
That wicked 23000 airfoil on the RVs is completely unsuited to aerobatics. As smart as Van is, I never could understand his airfoil choice for those planes. I bet they'd all be at least 15% faster with a nicely tuned laminar airfoil...
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That wicked 23000 airfoil on the RVs is completely unsuited to aerobatics. As smart as Van is, I never could understand his airfoil choice for those planes. I bet they'd all be at least 15% faster with a nicely tuned laminar airfoil...
this i never knew....i thought the rv series were supposed to be capable of mild aerobatics?
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this i never knew....i thought the rv series were supposed to be capable of mild aerobatics?
They are extremely capable--I was commenting on the wing airfoil itself and not the aircraft. The 23000 airfoil possesses some really nasty characteristics that, I would have thought, would have disqualified it for use on the RV series of aircraft, but that's the one Van chose. HTs comment about an inverted stall speed some 40 knots faster than Vs would be one of those characteristics I was speaking of.
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But wait, we have not yet spoken of adverse yaw effects during steady state and roll onset. And the changes in roll rate based on 1g vs unloaded roll. And the effects of dihedral when rudder is pushed to create a rolling moment.
I'm all ears if you don't mind describing what the gist of each is. :)