airxliban
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Rolling...do You Turn?

Sat Jun 05, 2004 4:02 pm

Got in an argument the other day with a Ph.D student in Mechanical Engineering. He said that when the plane banks....one aileron rotates up and the other rotates down, the plane rolls, BUT IT DOES NOT TURN it keeps on going straight.

I figured that the plane does actually turn, because when the plane banks, the force of lift has a component in the horizontal direction. On each wing they point in the same direction. Therefore, the plane also turns.

Also, in flight sim when you bank the plane turns damnit!!

Can anyone confirm? I thought I was relatively sure of my point, but I don't want to argue with a Ph.D candidate, especially since he knows just about everything else.
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liamksa
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RE: Rolling...do You Turn?

Sat Jun 05, 2004 5:22 pm

You are correct. It's the horizontal component of lift which 'pulls' the aircraft into the turn.
 
QantasA332
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RE: Rolling...do You Turn?

Sat Jun 05, 2004 5:23 pm

I'm afraid to say that the Ph.D student is right - roll by itself doesn't turn an aircraft, per se. As the plane banks, yes there's certainly a horizontal component of lift. But all it induces is sideslip (simply by virtue of the direction it's actually pulling in), not an actual direction-changing turn. If you make a little sketch it's easy to visualize why this is. In fact, if the aircraft's wing has high dihedral, there would be almost no motion due to that altered lift component at all (sideslip or other).

Having said that, roll does technically turn the aircraft - albeit in the 'opposite direction' of the roll. That's of course due to adverse yaw, as I'm sure you know already.

As for flight simulator, you probably have "auto-yaw" enabled, and that's why simply rolling turns your plane. Try flying without it and see what happens...

Cheers,
QantasA332

Edit: sorry, I was in a rush and left something out... The plane will roll if the 'net lift' is forward of the CG. However, all of the above is assuming even distribution. That is, transferring from level flight and just rolling, without any pitch-up or down, or anything else... Again, I'm in a bit of a rush, so please forgive me if I'm not making myself very clear...

[Edited 2004-06-05 10:28:55]
 
QantasA332
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RE: Rolling...do You Turn?

Sat Jun 05, 2004 6:26 pm

Sorry, I was in quite a rush and everything I said before was a bit of a jumble and not clear (particularly my edit!). Now that I have a bit more time to reply, I'll repeat myself, hopefully more accurately... Embarrassment

Assuming that the aircraft in question is trimmed longitudinally and everything else is very generally balanced, simply rolling an aircraft (one aileron up and one down, the usual) doesn't turn it. Sure there's a horizontal component of lift in the roll, but rather than "pulling" an aircraft into a turn, it induces sideslip instead. The aircraft is dragged laterally, but no actual "direction-changing" turn occurs. As I said before, sketching the situation might help to establish that.

Even regardless of the above, adverse yaw always turns the aircraft "opposite" its roll (in the absence of any countering yaw, of course). So, there's still no net turn.

Again, just roll - nothing else - won't really turn an aircraft.

Cheers (and sorry about the previous confusion!),
QantasA332
 
learpilot
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RE: Rolling...do You Turn?

Sat Jun 05, 2004 10:29 pm

QantasA332,

If banking doesn't turn an airplane, what does?
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SlamClick
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RE: Rolling...do You Turn?

Sat Jun 05, 2004 10:32 pm

It would be hard for anyone to give you a short answer. There are just too many variables. They include aircraft design and pilot inputs.

If you just roll a swept-wing jet into a bank, it will begin to turn (change heading) even with the yaw damper turned off. It would be a descending turn unless some back pressure was held. (exclude Airbus from this discussion) In these type aircraft, except during takeoff, landing and engine-out operations the rudder pedals are mostly just footrests.

In a straight-wing light plane the adverse yaw would be very noticeable. To begin a turn requires coordination of all three primary flight controls. Once a plane is in a sustained, coordinated turn, one might argue that the elevators change the heading because that is the only flight control that is displaced from neutral. But not so entering the turn.

I have not flown a light single in many years but I'd say that it would be difficult to roll without commencing a turn in that direction. It would require opposite rudder to hold the nose steady on the original heading.

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Bellerophon
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RE: Rolling...do You Turn?

Sun Jun 06, 2004 3:13 am

SlamClick

...I'd say that it would be difficult to roll without commencing a turn in that direction...

It is, I spend a lot of my spare time in a light aircraft trying not to change heading whilst rolling.  Big grin

It isn't as easy as it sounds, as anyone who has ever tried aerobatics will tell you!

...It would require opposite rudder to hold the nose steady on the original heading...

For moderate angles of bank this usually works. We call it side-slipping in the UK but I think it's known as forward-slip it in the US.

You can also maintain heading whilst rolling by applying forward elevator as you roll, provided the aircraft is approved for negative "g".

Other than these two actions, in general terms, if you apply bank to an aircraft trimmed for level flight, the heading will change, which means it is turning.

Regards

Bellerophon
 
320tech
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RE: Rolling...do You Turn?

Sun Jun 06, 2004 6:55 am

So, can I try to simplify this a bit now?  Smile

Adverse yaw is countered in most designs. The C172, for example, is designed so that the upgoing aileron protrudes slightly into the airstream, thus pulling the aircraft in the correct direction. Other aircraft use differential aileron travel, the upgoing aileron travelling further (IIRC) to create extra drag on that side. On the A320, the aircraft automatically applies the required rudder to provide a co-ordinated turn.

Although what others have said about adverse yaw is true, in practice, when you roll the airplane, you will turn in that direction. It just might not be a very nice turn.
The primary function of the design engineer is to make things difficult for the manufacturer and impossible for the AME.
 
QantasA332
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RE: Rolling...do You Turn?

Sun Jun 06, 2004 7:19 am

Thinking it over again before I went to sleep last night and reading these posts now, I realized I had left something else out: "weathercocking." I woke up early yesterday and took a test, so I must have been both tired and traumatized  Laugh out loud when I posted my (two!) replies. I'm very sorry to have made such a confusing mess, so I'll try one more time...

When you just roll (NO other control inputs, nothing), a few factors are present. There's sideslip, as I said before, there's adverse yaw, and there's "weathercocking." Whether you turn or not really is a question of balance between the latter two. Assuming everything else is kept trimmed, balanced, etc, in a roll, a plane will turn or not depending on how much adverse yaw there is and how much of a weathercock effect the tail 'produces'. As a plane rolls, there's obviously adverse yaw, which tends to turn the nose of the plane one way. However, sideslipping also induces a weathercocking effect, whereby air is hitting the tail in such a direction as to swing the nose around opposite the way it was going with adverse yaw. Basically, then, whether you turn or not depends on which of those two forces is greater, because they're really the only things that would turn the aircraft in the case of solely rolling...

Cheers,
QantasA332  Nuts
 
learpilot
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RE: Rolling...do You Turn?

Sun Jun 06, 2004 1:24 pm

What?  Confused There's no if's and's or but's about it. When you roll an airplane into a bank, it turns in the direction of the bank because of the Horizontal Component of Lift. PERIOD! This has worked in every airplane I've flown. As a matter of fact, I just landed about 2 hours ago. It was still working then.

There is adverse yaw present, but it is not enough to keep you from turning. It does cause a slip, however. Therefore, you use the rudder to counteract it.

To make sure I state this perfectly clear, Any time you bank an airplane, IT TURNS! The horizontal component of lift (resulting from a bank) turns the airplane, not the rudder. The rudder is there to counteract adverse yaw.

AirxLiban, YOU WERE RIGHT! You need to tell your student friend he needs to do a little more reading. He can start with the "Flight Training Handbook".
Heed our warnings or your future will be underpant free!
 
QantasA332
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RE: Rolling...do You Turn?

Sun Jun 06, 2004 2:08 pm

Okay...but I'm a bit confused. In level, unaccelerated flight, an aircraft is trimmed such that the net lift can be thought of as acting evenly along the whole length of the plane. Sure rolling produces a horizontal component of lift, but assuming the aircraft remains longitudinally trimmed, shouldn't it be pulled into a sideslip only? That is, if the net lift is effectively spread evenly across the length of the aircraft's fuselage, shouldn't the aircraft be pulled sideways only, with no nose-turning and thus direction-changing component? Sorry, it's hard to explain...

Cheers,
QantasA332
 
SUPRAZACHAIR
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RE: Rolling...do You Turn?

Sun Jun 06, 2004 4:44 pm

Just wondering what kind of turning tendency is found in planes with a symmetrical airfoil (equal lift up and down), i.e. military jets and aerobatic planes. These planes rely on angle of attack for lift (more Newton, less Bernoulli). Do they just rely on a lack of horizontal lift to roll without turning in aerobatic maneuvers?
 
FredT
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RE: Rolling...do You Turn?

Mon Jun 07, 2004 2:22 am

The roll does not turn the aircraft per se. All the roll does is to give the lift a horizontal lateral component.

This component accelerates the aircraft laterally.

However, an aircraft will always try to align itself with the direction of travel, through lateral and pitch stability, be they aerodynamically created of artificially created through a FBW system.

That is what causes the actual turning rather than the roll.

Of course, you could argue that since the stability is part of the system the roll makes the turn inevitable and thus the roll does cause the aircraft to turn... but by then we are getting into philosophy rather than aerodynamics.

Cheers,
Fred
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FredT
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RE: Rolling...do You Turn?

Mon Jun 07, 2004 2:28 am

SupraZach,
there is absolutely no truth to the oft-claimed distribution of lift between Bernoulli and Newton. All of it is created by Newtons third. Push air down and the air creates an equal but opposite force, manifesting itself as pressure gradients in the fluid which in turn is what creates the forces causing your aircraft to remain airborne.

Bernoulli's is valid, within the given limitations (which you are outside of in most of the envelope of many aircraft) but trying to use it to explain lift is a good way to shoot yourself in the foot.

Do a search, we've been over it before I'm sure.

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
QantasA332
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RE: Rolling...do You Turn?

Mon Jun 07, 2004 6:17 am

Aha! Thanks for that clarification, Fred! You said in a sentence what I was trying to say through however many daft posts it was: that solely rolling doesn't turn an aircraft per se - it only pulls it laterally - but it's 'weathercocking,' adverse yaw, and other resulting forces that actually turn the aircraft, if it turns at all. The lateral component of lift pulls only laterally, and so it's not what actually induces a turn. So I wasn't that far-off before, was I...!  Big thumbs up

Cheers,
QantasA332
 
SUPRAZACHAIR
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RE: Rolling...do You Turn?

Mon Jun 07, 2004 11:35 am

Jesus Christ, I don't know why I try and add anything to these discussions. I try and ask a legitimate question and get blasted because I mention Bernoulli, which is appearantly taboo around here. Try to stick the question instead trying to discredit me next time. Thanks.

[Edited 2004-06-07 04:38:40]
 
vikkyvik
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RE: Rolling...do You Turn?

Mon Jun 07, 2004 2:12 pm

Alright, first, SupraZachAir,
I can assure you that FredT or anyone else meant no offense. Opinions run strong, but (for the most part in this forum) they don't breed disrespect.

That said,
I had wanted to start my own thread on this question, but feared bringing it up again....I learned during my aerospace engineering training that you could calculate the lift by integrating the pressure coefficient on the lower and upper surfaces of the wing. Frankly, that sounds like Bernoulli exclusively to me; that is the major reason I have been confused about this issue.

Additionally, I have heard professors say, "additionally, you get the reaction force from the flow being directed downwards."

Completely off topic, and if a new topic must be started, well, go for it.

~Vik
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".
 
SUPRAZACHAIR
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RE: Rolling...do You Turn?

Mon Jun 07, 2004 2:31 pm

I know he prolly didn't mean anything by it, it just struck a nerve. I asked a legitimate question and what I got was lecture on why Bernoulli's contribution to lift is non-existant. I apologize for over-reacting.

there is absolutely no truth to the oft-claimed distribution of lift between Bernoulli and Newton. All of it is created by Newtons third.

I don't understand why you state this as though it's fact. One fact I will give you is that I don't want to argue about Bernoulli and Newton; not in this thread. If we're going to start a Bernoulli vs. Newton debate, lets give it a new thread, so as not to hijack this thread which is most decidely not about Newton, Bernoulli, or your mom.  Big grin

 
FredT
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RE: Rolling...do You Turn?

Mon Jun 07, 2004 3:14 pm

SupraZach,
absolutely no offense meant. I worded my reply strongly only since I know it is a can of worms which often causes much confusion if you leave any room for those advocating venturis, equal time of transit, peas bouncing off the wing or whichever ingenious but erroneous explanation of lift that they got spoonfed in flight training or pop science magazines. It was most definitely not aimed at you and I apologise if I gave you that impression!

That being said, I state what I said as a fact since it is indeed a fact. There’s no other reason for stating anything as a fact, is there?

Vikkyvik,
yes, there is a pressure difference and yes, you can integrate the pressures over all the surfaces to get the total force. However, I think you're making the common logical mistake to assume that if A means B, then if you have B it means that A is also true. Bernoulli's principle states that there is a pressure difference in accelerated air flows, but there being a pressure difference doesn't mean it has to be due to Bernoulli's... exclusively. Nor does it in any way correlate Bernoulli’s.

Furthermore, what you need to look at is not really what happens when the air is accelerated but why and how it accelerates. Eventually, you run into a chicken and the egg situation. The air accelerates since there is a pressure gradient and there is a pressure gradient since the air accelerates which causes a pressure gradient causing the air to accelerate which… whoa… I need to breathe! Big grin

This thread has been hijacked. I want all the coke and cheese sandwiches from the galley! Other than that, go on as usual…  Big grin Seriously though, we have been over it before I’m sure so a search should be more useful than covering it all once again.

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
airxliban
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RE: Rolling...do You Turn?

Mon Jun 07, 2004 3:30 pm

thanks for your responses guys.

anyway, regardless of the exact details, I get the point now and actually did try going into FlightSim and turning off the yaw and it seemed to produce the desired effect.

again, many thanks for your help.
PARIS, FRANCE...THE BEIRUT OF EUROPE.
 
soaringadi
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RE: Rolling...do You Turn?

Mon Jun 07, 2004 7:13 pm

I don't know how well I can explain this..... but I'll try

What I think is that the Phd student is **kind of right**...... Think of it as this way.... You've rolled the plane in a 90 degree bank. Now even though it is perpendicular to our actual horizon, it is Also parallel to some imaginary horizon. Now if you pull the nose up, the airplane will pitch up (In ref. to our Imaginary horizon), and climb if looked from That perspective. And when it climbs, thats when your heading changes......

Hope I didn't confuse anyone... ha ha...... coz I'm not good at explaining, but I Think you guys will get the PoInt.

enjoy  Smile
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QantasA332
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RE: Rolling...do You Turn?

Tue Jun 08, 2004 7:47 am

Newtonian versus Bernoullian lift explanations is a tough topic, quite obviously. Equal transit time theories can be thrown away immediately - windtunnel tests with smoke-injected airflow indicates that the air above the wing certainly doesn't always reach the wing trailing edge at the same time as its 'partner' - as well as some other silly and unfounded explanations. Of course, there's still Newton and Bernoulli to duke it out.

I think one of the key factors in deciding what actually creates lift is to make sure the definitions and concepts involved are concrete. For one, Bernoulli's equation (and Euler's) is really just a restatement of Newton's principle of conservation of energy/momentum/etc.; many things are not as mutually-exclusive as they seem. Pressure differences don't always indicate the 'presence' of Bernoulli, as Fred said - air striking the bottom of the wing and being turned (Newton) certainly exerts a higher pressure on that surface. Thus, the pressure-integration sort of method for calculating lift should hold true.

So which is it then, Bernoulli or Newton? Personally, I read too much conflicting info all from respectable sources to decide. Besides, it's more fun to sit back and watch.  Big grin However I must admit that, for the most part, I agree with John Anderson's explanation (below), but I feel that the boundaries between terms and equations and priciples are too thin and malleable to be just one thing or just the other.

John Anderson, author of many textbooks on aerodynamics (one of which I'm the proud owner of  Smile), discusses this issue at length, and I think I agree with him for the most part. He first examines how air is accelerated when passing over a wing. Simply due to the shape of wings, air passing over the top 'senses' an obstruction and its cross-sectional area reduces accordingly (windtunnel smoke tests confirm this). Through mass continuity, we can see that because of that reduction in area, it's velocity increases. That's how the acceleration occurs. Anderson then claims that it is due to the Bernoulli effect that the increase in velocity on the upper surface of the wing lowers the pressure there (and, of course, creating a lifting force). As I said, though, Bernoulli's equation is really just a restatement of Newtonian principle, so any boundaries between the two are thin.

Anderson does go on to detail the "air-turning" (Newton) explanation of lift, but he says that it's not so much the fundamental reason why lift is produced but it's more a result of lift. Higher pressure on the lower surface of the wing still produces an action-reaction sort of situation, and the turning-down of the departing airstream still exists as well (higher-pressure air pushes the wing up and the wing in turn pushes the departing airstream down, completing the action-reaction pair). I think that just reinforces how blurred the differences between the two theories are.

There's also circulation which, again, is not so much a theory of how lift is produced, but another mathematical means for calculating it.

Well, I've rambled on long enough - and off-topic, as well! Sorry! I suppose sometimes it's just better to accept that "it flies," and get on with things. Quite interesting stuff, though...

Cheers,
QantasA332

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