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The yaw/slip thread (merged) aka Aerodynamics 101

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The yaw/slip thread (merged) aka Aerodynamics 101

Old 17th Sep 2004, 05:03
  #61 (permalink)  
 
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ft and all

"Ask yourself how you can have symmetric airflow around the fuselage when there is rudder applied."

Thought that to be a very simple proposition that could only have one answer. Tis fascinating how noticeably deep thinking minds introduce new angles on aerodynamic basics. Thanks for the challenges and just when I thought I had it all by the tail anyway when I started it wagging.

Please ignore the odd aircraft that has an offset fin by design. They are asymmetric to start with or perhaps screwed up would be a better term to use. Ohhh sorry I ever mentioned that.

Now I'll ask you to mentally turn our subject typical aircraft on to its side and assume that it is being carried along in balanced flight hanging on a giant sky hook suspended through its cg. Zero all the lift with a bit of negative wing alpha. Now we wag the tail again using the rudder to vary fuselage alpha. Doesn't the fuselage now act like a wing and take on what we can call lift and negative lift when we wag the other way.. What stops it doing the same thing horizontally when we fly it normally? Only difference is we call it all by different names and confuse the multitude, but we are now back with sideslip and the horizontal force resulting from that sideslip.

Maybe I can now stick my neck out with another motherhood statement which I believe is inviolate.

Sideslip of an aircraft having a symmetrical fuselage inevitably produces a sideforce acting through the total fuselage lateral centre of pressure PERIOD

ft I think that answers your second set of points also.

Anyone not enjoying this come join us. I promise to keep this going for a while. The red herrings are a hoot!

newcrew are you still there. Please don't blame yourself for having started all of this. Hang in there.

And I think I hurt the feelings of the ex Concord Captain when I called his sideslip indicator a con. Probably one of Brian Trubshaw's bright ideas.
Are you still there on the side Captain?

Standby Oktas 8 for another startling announcement
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Old 17th Sep 2004, 07:56
  #62 (permalink)  
ft
 
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Now I'll ask you to mentally turn our subject typical aircraft on to its side and assume that it is being carried along in balanced flight hanging on a giant sky hook suspended through its cg. Zero all the lift with a bit of negative wing alpha. Now we wag the tail again using the rudder to vary fuselage alpha. Doesn't the fuselage now act like a wing and take on what we can call lift and negative lift when we wag the other way.. What stops it doing the same thing horizontally when we fly it normally? Only difference is we call it all by different names and confuse the multitude, but we are now back with sideslip and the horizontal force resulting from that sideslip.

Maybe I can now stick my neck out with another motherhood statement which I believe is inviolate.

Sideslip of an aircraft having a symmetrical fuselage inevitably produces a sideforce acting through the total fuselage lateral centre of pressure PERIOD
The fact that a slipping fuselage creates a lateral aerodynamic force is not contested here. The requirement to have that lateral aerodynamic force to maintain straight unaccelerated flight with asymmetric thrust is.

As shown above, equilibrium can be maintained without that lateral force and thus without any slip.

Show me why you think the proposed equations of equilibrium do not hold water, and you’ll convince me. Until then, I think you’ll find it near impossible.

Regards,
Fred
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Old 17th Sep 2004, 08:24
  #63 (permalink)  
 
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Let's forget aerodynamics for a moment and get back to basic mechanics.

Consider a pendulum swinging from right to left and back again. At one point in time it is moving to the left. At a later point it is moving to the right. I order to change its condition from moving left to moving right it must (however briefly) have stopped moving. This requirement applies to all physical objects. It makes no difference whether the frame of reference against which we measure the motion is space or the air around us, the fundamental requirement remains the same.


Now let's consider our aircraft. We are initially flying straight and level in still air. We throttle back on the left engine and throttle up on the right. Because the thrust line has moved out onto the right wing, it generates a yawing moment to the left. So the aircraft starts to yaw to the left.

To arrest the left yaw we apply right rudder. This generates a force to the left acting at the tail of the aircraft. This has (at least) two effects. The first is a yawing moment to the right. If we have applied just enough right rudder the right yawing moment generated by the rudder will exactly balance the left yawing moment generated by our right engine, and the yawing will stop. (For purists, yes we will actually need a bit more rudder initially to actually stop the yaw). We now have a situation in which the aircraft is not yawing.

But the second effect of the rudder is a side force to the left. This will cause the aircraft to sideslip to the left.

If we now start to bank the aircraft to the right, we will tilt the lift towards the right. This initially small horizontal component of lift will cancel out part, but not all of the side force from the fin. The overall effect will be to reduce sideslip to the left. If we now gradually increase right bank in small increments the left sideslip will reduce to zero, then be replaced by right sideslip.

Now if we look at this sequence we will see that just like the pendulum, there was a point at which the aircraft was not moving to the right or left. It was not sideslipping at all. If we look a bit closer we will see that this did not happen with the wings level, but with a slight bank to the right. This is the situation in which the aircraft is banked to the right but not sideslipping.

Last edited by Keith.Williams.; 17th Sep 2004 at 11:34.
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Old 17th Sep 2004, 08:59
  #64 (permalink)  
 
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Oktas 8 SR71 ft and the tally sheet.

Have you had another look at your post Oktas8 ? Is that OK True Air Speed 8 or heavily overcast, the reverse of CAVU.

Don't want to embarrass you but this is a very good example of why so many get into trouble when presented with a sudden engine failure on a multi.

Even Milt will admit to some milliseconds of confusion until the direction of swing and the instruments tell him which side it's stopped happening It's not instinctive enough to provide for an automatic reflex. And that is after a lifetime of pushing and pulling all those levers. Do I see a lot of heads nodding?

One night I was in the left seat of a Gooney Bird with another very experienced operator in the right. We were determining the power settings to use on an engine which equated to one with its prop feathered for conditions soon after take off. Just after lift off and according to a pre brief I called 'Now" for a close down of the R engine. A Mixture control was pulled into cut off and the R prop feathering activated. Quite quickly I was engineless. A quick smack of the offending hand removed it from the LEFT mixture control which was rapidly reset to Rich to the accompaniement of the left engine going through some overspeed before the prop governor clawed back the revs. Incidently the power settings turned out to be close to 15 inches and 2,000 RPM.

So much room for error in this area.

That's why the TPs allow a 2 seconds do nothing delay in the determination of Vmcg and Vmca.

Oktas8 - I am back with you.

Come fly with me again in that twin and we'll do it more slowly.

We have climbed to a safe height, closed down the left engine and allowed the aircraft to yaw hard left to discover that the angle of Right sideslip is excessive and the rudder has trailed over to left foot. It is holding but we can't leave it like that. There is not enough tail sideforce to the left being produced to get us pointing in the direction close to where we want to go and all of that sideslip is giving us unbalanced lift from the wings so we have to hold level with a fair bit of aileron. Let us unscamble this a bit by helping the tail to do its job by pushing in some R foot rudder. Now we have given the tail some help and we have reduced the angle of RIGHT sideslip down to a lower angle so that we are now flying about 2/3 degrees Right of where the aircraft is pointing.

So you now go and change your plus to minus or explain some more where I might have goofed. It wouldn't be the first time!!

Oh and you'll have to be more specific with your last para. You have forces going every which way and can't get my mind around them.

Carelessness or wind -up? I must admit to a bit of wind up for a while when the directions of some of the functions/forces with the first flight in the twin but no-one picked it so after a couple of days I did an edit. I now see that SR71 was on to it.

Further responses pending to ft and welcome aboard Keith.Williams

Smooth landings.

Last edited by Milt; 18th Sep 2004 at 10:25.
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Old 17th Sep 2004, 09:16
  #65 (permalink)  
 
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Great discussion.

Now what do you say when the question pops up in an interview?

If you have asymmetric thrust, you will have to counter with rudder to stop the angular motion around the vertical axis going trough the CG.

Then if you want to fly in a straight line you will have to add up (after steady state condition has been achieved) all components of all forces involved in the horizontal plane.

If this sum in not zero, no way you will fly in a straight line.

How do you make this sum Zero?

The only thing you can do is to change the position of the CG and start all over again.

However, you may not be able to change the CG sufficiently enough to achieve the goal of straight flight.

Now is this a nutshell answer or not ?

Since it is not guaranteed that the CG will be in the longitudinal axis I see why Concorde trims the fuel in both directions lat and long.

Since in steady state and straight flight the only force on the slip ball is gravity, the ball can be anywhere depending on the amount of bank you need to set all equations to zero.

With straight line I mean straight direction, the plane could still be climbing or decending in a curved line.
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Old 17th Sep 2004, 09:38
  #66 (permalink)  
ft
 
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Cap,
how would the position of the CoG have any effect on the horizontal forces if you are flying in a straight line? There's no acceleration in any direction except the gravitational acceleration, which by definition is acting perpendicular to the horizontal.

If the moments don't add up, the angular acceleration will be about the CoG. However, the moments can be summed up about any point and will be the same regardless of the position of the CoG.

Often you use the CoG (if known) as the reference point for moment calculations. Since the weight is usually a factor, this means the moment arm from the reference point to the CoG is zero and the weight can be disregarded. The moment is the same regardless of reference point though.

Regards,
Fred
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Old 17th Sep 2004, 09:48
  #67 (permalink)  
 
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how would the position of the CoG have any effect on the horizontal forces
ft

In the case of asymmetric thrust, the position of the CG does affect the required forces does it not ?

If that is correct then while shifting the CoG you will have to change the forces.
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Old 17th Sep 2004, 10:24
  #68 (permalink)  
 
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Cap 56

Pan Pan Pan

Just when I thought we had all sent cg/CG back to longitudinal now we have Cap 56 trying to slip that interloper back in to steady state directional stab.

Heaven to Betsy

Forgotten now how we managed to get rid of it. I think it was with a few simple inspirational statements such as

"You cannot resolve weight acting through the cg/CG through 90 degrees to the horizontal."

and perhaps

"The only time that mass or weight or cg/CG comes into consideration in the horizontal is when we have acceleration.

But would you mind telling us why you think the rudder yaws an aircraft in steady flight around the CG and perhaps what is the alternative point of rotation?

Oh and by the way did you note the references to the so called slip indicator on Concord on page 1 of the thread?
Any comments particularly if you were/are a Concord pilot.

Just saw the post from ft and you can tell he now has similar pronouncements as above.
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Old 17th Sep 2004, 10:35
  #69 (permalink)  
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In the case of asymmetric thrust, the position of the CG does affect the required forces does it not ?
I’m afraid not. All that matters is that the moments (about the vertical) tally up to a grand total of zero.

If that is correct then while shifting the CoG you will have to change the forces.
Milt,
“now”? You’ll be hard pressed to find a post in which I disagreed with the irrelevance of the position of the CoG as far as unaccelerated lateral stability goes.

Any moment applied to a rigid body will, however, strive to rotate it about the CoG. Of course there tends to be numerous other constraints making sure that we get translation as well.

Regards,
Fred
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Old 17th Sep 2004, 10:41
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Milt

But would you mind telling us why you think the rudder yaws an aircraft in steady flight around the CG and perhaps what is the alternative point of rotation?
I didn't state that. I stated that if you consider the motion around the vertical axis going trough the CoG (that is take as a reference point the CoG ) this is NOT stating the rotation point is the CoG.

I realize that inertial forces and aerodynamic ones are quite different.

But unless you are capable to change the physical dimensions of the plane in mid flight the only thing you have to play with is the CoG and as soon as you bank you do not escape from the fact that gravity will have an influence in balancing all horizontal forces whatever the reference point you will use.

Maybe the way to walk around this problem is to assume the aircraft has no mass at all and after that introduce the concept of CoG. This will put all aero and engine forces on the front first.

All this in steady state of course.
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Old 17th Sep 2004, 11:16
  #71 (permalink)  
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But unless you are capable to change the physical dimensions of the plane in mid flight the only thing you have to play with is the CoG and as soon as you bank you do not escape from the fact that gravity will have an influence in balancing all horizontal forces whatever the reference point you will use.
Ah, here I think we have the crux of the matter. Gravity is always acting solely in the vertical. It will never be a factor when considering horizontal forces, whether you are level, in knife-edge flight or anywhere in between. As was stated before, the horizontal plane is per definition perpendicular to the gravitational force so there will never be any component of weight (gravity times mass applied at the CoG) in the horizontal.

If you consider lateral forces in the aircraft reference system, however, you will see a component of weight projected on the aircraft xy plane when you are in a bank or pitched up/down. That is why I have taken great care to be working in the horizontal plane rather than the aircraft lateral plane in this discussion.

Regards,
Fred
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Old 17th Sep 2004, 11:45
  #72 (permalink)  
 
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ft

Thanks, you are right 100 % and I appologize I should have made a drawing first.

So I will rewrite my first post as follows.

Now what do you say when the question pops up in an interview?

If you have asymmetric thrust, you will have to counter live engine(s) with rudder to stop the angular motion around the vertical aircraft axis.

Then if you want to fly in a straight line you will have to add up (after steady state condition has been achieved) all components of all aerodynamic and engine forces involved, in the horizontal plane.

If this sum in not zero, no way you will fly in a straight line.

How do you make this sum Zero?

The only thing you can do is to change the physical dimensions of the aircraft and/or bank the aircraft, so you can keep the moments balanced while at the same time change the forces in order to balance them horizontally.

Changing physical dimensions includes in this context "control deflection"

Since in steady state and straight flight the only force on the slip ball is gravity, the ball can be anywhere depending on the amount of bank you need to set all equations to zero.

With straight line I mean straight direction, the plane could still be climbing or descending in a curved line.
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Old 17th Sep 2004, 11:49
  #73 (permalink)  
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Cap 56



You could end up in straight flight without rudder I suppose... but it wouldn't be pretty!

Regards,
Fred
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Old 17th Sep 2004, 12:03
  #74 (permalink)  
 
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Time to introduce Vector thrust on commercial planes.
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Old 17th Sep 2004, 19:27
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Talking

Milt

...I think I hurt the feelings of the ex Concord Captain when I called his sideslip indicator a con...are you still there on the side Captain?...

Still here, feelings unhurt, but head now aching!

I'm with Keith Williams on this one.

On several occasions I have demonstrated, entirely to my own satisfaction if not that of my simulator instructor's, that my flying skills - so generously described by him during the subsequent debrief as unbelievable and unique - enabled me to fly a constant-heading engine-out climb, sideslipping in either direction, depending on the amount of bank and rudder applied.

If it is possible to slideslip in either direction in this condition, then it must be possible to find and maintain that position in which there is zero sideslip. Not me personally you understand, but perhaps a luckier pilot. It may be a difficult position to find, but that does not mean it does not exist!

Of greater concern to us was Concorde’s tendency, following an engine failure at high speed, to yaw one way (conventionally) but to roll the other (away from the failed engine); however that is a topic for another day!

Far more knowledgeable posters than I are now involved in this particular debate, and, as I said, my head hurts, so I shall now confine myself quietly to the sidelines.

By the way, do you have a licence to fish in this stream?

Regards

Bellerophon.
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Old 18th Sep 2004, 13:16
  #76 (permalink)  
 
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Bellerophon

If it is possible to slideslip in either direction in this condition, then it must be possible to find and maintain that position in which there is zero sideslip. Not me personally you understand, but perhaps a luckier pilot. It may be a difficult position to find, but that does not mean it does not exist!

Zero slip implies that the TAS vector is aligned with the longitudinal axis of the aircraft.

Hence, all aerodynamic forces from all non movable surfaces will cancel each other as far as their contribution to stop the rotation due to the engine failure is concerned.

The only thing remaining to stop the rotation (as far as I know Concorde), are the forces from the elevons, rudder and bank.

In order to fly without slip (and assuming the trust vector is parallel to the TAS vector) the aforementioned control surface forces will have to have to, when added together, create a couple (not a moment) equal to the moment created by the engines.

Assuming the horizontal component of the forces created by the elevons is small. The only option left over is to bank into the live engine and rudder.

The position that gives you a bank angle equal to the deflection of the slip ball should be zero slip straight flight on the condition tha your heading remains steady.
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Old 18th Sep 2004, 14:03
  #77 (permalink)  
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Cap 56,

I don't know that I'd get at all excited about distinguishing between couples (ie torques) and moments .... but I am intrigued by the suggestion that ..

"The position that gives you a bank angle equal to the deflection of the slip ball should be zero slip straight flight on the condition tha your heading remains steady."

It's a bit late at night here ..

.... but I would have thought that I could cause the bank/ball condition to occur at any bank angle of interest by simply banking .. and keep the heading straight by simply pushing on the appropriate rudder pedal .... and get all sorts of different slip angles depending on what bank I choose to use in the specific exercise ... provided that the end result is a steady state flight condition?

.. or is it just the case that it is a bit late at night here ?

I suspect that we are looking for a situation where -

(a) forces are in equilibrium (ie balance out) so that the aircraft stays at the same speed while not going sideways .. although a little bit of going up is desirable ...

(b) moments are in equilibrium (ie balance out) so that the aircraft doesn't spin slowly like a drunkard trying to find his way back from the pub to the marina late at night ...

And if, by some serendipitous sprinkling of pixie dust, that situation can end up with an aircraft under some modest measure of control with a tad spare performance after the initial surprise associated with the failure .. (excluding sim exercises where we are only surprised if the organist in the back forgets to fail the engine) .. then we ought to have a moderate content, assuming the hills and such are not an immediate concern to safety.

I, like B, have, as my first priority, staying blue side up ... and then worrying about whatever level of finesse I might be capable of on any given occasion to get some modest level of performance out of the wee beastie ..

Last edited by john_tullamarine; 18th Sep 2004 at 14:19.
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Old 18th Sep 2004, 15:16
  #78 (permalink)  
ft
 
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When in doubt, google.

Here are the pictures to go with the words. Hm, gotta put that as a bookmark methinks, it's a good site. And, I repeat, it has pictures. Little airplanes all over the place. You can never have too many little airplanes!

<stares off into the distance, dreaming of a world full of little airplanes, all with force and moment arrows attached>
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Old 18th Sep 2004, 15:16
  #79 (permalink)  
 
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john_tullamarine

Only a couple will create the condition where there is on force perpendicular to the speed vector, hence straight flight.
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Old 18th Sep 2004, 22:44
  #80 (permalink)  
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Cap 56,

Unable to link to Fred's reference at the moment ... perhaps that will lift the veil of confusion from my eyes in due course. And, of course, lots of little piccies are always a GOOD THING when trying to work out what's what .. unless, of course, you are Stephen Hawking and can do it all in your head .. what an incredible mind that man must have ..

Presuming your "on" is intended to be "no", that's fine for a simple torque ... no contrary view from me .... but an aeroplane in flight has a myriad of forces so the simple couple thing is a not-very-useful artefact of imagination, I suggest ? .. and, in any case, how does that relate to the suggestion in your previous post ?
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