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# The yaw/slip thread (merged) aka Aerodynamics 101

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# The yaw/slip thread (merged) aka Aerodynamics 101

19th Sep 2004, 01:08

Join Date: Jun 2001
Location: Australia
Posts: 890
Thanks for the clarification Milt. Please excuse my quoting you in big blocks - normally I hate that, but in this case the page is so long that scrolling up & down is tedious , so using quotes is easier.

We have climbed to a safe height, closed down the left engine, upped the power on the right engine and settled into a steady straight cruise with wings level and found that the rudder is trimmed right foot with about one third deflection. Balance ball is in the centre even though we know we are sideslipping by about 3 degrees. Hard to tell though.
In steady straight wings level flight with the LEFT engine shut down, you will be slipping LEFT with rudder force to the LEFT. Directional stability forces - those tending to make the aircraft yaw into a sideslip - will be to the RIGHT.

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. ... 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 ... push 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.
You describe a different situation here: it will not be straight wings level flight! If the wings are level, you'll be turning into the dead engine. If the flightpath is straight, you will be banking into the live engine. Using some R foot rudder to give the tail some help will do as you say, but the wings still won't quite be level, or the aircraft will not quite be keeping straight. Also of course the balance ball won't be centred in this case.

That's the last time I'll quote you - promise! - as I can see the discussion is moving on to other things. I won't clarify what I meant earlier, simply because that marvellous site bookmarked above says it all much better than I can, with pictures too!

cheers,
overcast8

19th Sep 2004, 11:52

Join Date: Dec 2003
Location: Dubai
Posts: 212
john_tullamarine

Bellerophon has given the discussion a bit of a puch, he wants to know if assymetric thrust straight flying is possible with zero slip.

It an ideal condition since it would give you min drag.

Straight flight is only possible if the sum of the projection of all forces on the horizontal plane has no componrent perpendiculal to the TAS vector. Hense in the case of zero slip, perpendicular to the longitudinal axis.

Somehow you will need to balance the moment of the live engine.

I think what Bellerophon is looking for is possible but only at one single point (or confined area around this point) and speed and it may well be somewhere verh high above sea-level.

A point where rudder and bank forces are able to cope with the assymetric thrust.

19th Sep 2004, 15:47

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without puching the TAS vector in another direction. For that you need a couple or something very close to it if you want to remain pragmatic.

19th Sep 2004, 17:34

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And believe me, I am not worried about the real exercise, just a touch of bank into the live is fine by me.

19th Sep 2004, 18:27

Join Date: Oct 2000
Location: N. Europe
Posts: 436
he wants to know if assymetric thrust straight flying is possible with zero slip
Cap56,
that's exactly the question that most of the thread so far has been concerned with and hardly a new angle.

(Just trying to avoid having the thread go around the same circle once more)

Regards,
Fred

19th Sep 2004, 20:36

Join Date: Jul 2003
Location: An Island Province
Posts: 1,080
Forgive me if I have lost the plot; I expect that many others are in a similar position.
However, to answer the question: “if asymmetric thrust straight flying is possible with zero slip – in that this condition would give you min drag”.

Surely, min drag will depend on the control surface positions required to give zero slip. Control surface deflections may give large increases in drag. In some aircraft, the resultant drag leads to a compromise of allowing some slip to minimise the value of total drag, seeking a minimum. A practical example (although not directly slip / drag related) was for the BAe 146 where asymmetric climbs are flown wings level (for ease of flight handling) and the small increase in roll spoiler / aileron drag is accepted; the large fin and effective rudder provides good lateral / directional control, but with slip.

Thus the solution sought depends entirely on the aircraft type; – the application of the preceding theory.

19th Sep 2004, 22:09
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Alf ... if, by going around in circles, the message eventually becomes clear ... then that is probably a reasonable thing .. ? .. and I concur with your thoughts entirely ..

(a) one does the sums with one's aerodynamicist hat on in the back room

(b) one confirms a range of things in the tunnel

(c) one goes out and flies the climbs to find just where the optimum configuration etc lies

(d) one considers what is a reasonable compromise considering the numbers, workload, etc., etc.

20th Sep 2004, 02:07

Join Date: Oct 2003
Location: Canberra Australia
Posts: 1,300
A review.

This voice in the wilderness is returning and reiterating.

For too long now there has been almost aviation wide belief that we pilots can eliminate sideslip and continue to fly an unbalanced aircraft straight down its fore and aft axis.
All I am saying is THAT IS ABSOLUTELY IMPOSSIBLE.

So I am seeking a concensus amongst a few thinkers and movers to change the belief. The books are mostly wrong so there is no point in referring to a bunch of little diagrams to prove your points. You have to go beyond those books.

Here are a few motherhood statements I have used so far.

The overriding fact to be determined initially is whether an aircraft having any angle of bank and in steady straight balanced flight can achieve that condition without sideslip. This is the area causing the greatest confusion. Please can anyone nominate a circumstance now or forever hold your peace.

ANY imbalance in thrust on a multi, not having centre line engines, which is then flown straight by either rudder or bank or a combination inevitably results in sideslip. It is unavoidable

What is the effect of any resultant sideforce on a fusealge? There is ONLY ONE answer. The fuselage is of course forced to move sideways until it returns to balance.. This inevitably causes sideslip. This sideslip causes weathercocking of the fuselage and if adequate takes over from the rudder and allows us to fly rudder central. The sideslip angle then will be very close to that for wings level straight flight only differing as a result of the "efficiency" of the particular aircraft's vertical tail when sideslipping with and without rudder. The advantage we are endeavouring to achieve is to bring that rudder close to central thus providing us with better options at Vmca.

Two new pronouncements.

Any dissimilarity of one side of an aircraft with the other with an aircraft in steady straight flight will result in that aircraft's direction of flight NOT being coincident with the fore and aft axis. OR

Any aircraft having total drag offset from the fore and aft axis will be unable to have a direction of flight along its fore and aft axis.

How can it be otherwise?

20th Sep 2004, 04:22

Join Date: Jan 2001
Posts: 420
As is common with these types of threads, definitions are vital. If we're using the same terminology to imply slightly different meanings, then the arguments are completely lost.

I take sideslip to mean an angle in the horizontal plane between the longitudinal axis and the flight path relative to the air.

If I'm flying a twin with the left engine out then in the normal rudder position, there will be dynamic pressure on the right side of the fuselage. If I have enough right rudder to stabilize at a point where there is even a small amount of dynamic pressure on the left side of the fuselage, then the obvious continuity of this process would imply that there is a rudder position in between where there is no dynamic pressure on either side of the fuselage. That point meets my definition of zero sideslip.

That point would probably be a minimum for profile drag, but if it takes a significant amount of rudder, then I'd be surprised if it were also a minimum for total drag.

Matthew.

20th Sep 2004, 05:34
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Zero slip (presuming that the heretics of the first part are able to convince the heretics of the second part that such might exist) may constitute minimum drag but there are other contributors to drag in addition to the slip angle and related airflow .. a case of establishing the story for the aircraft.

Definitions in this thread are certainly important and one of the values of the thread is that people are challenged to think about what is being said and what it all might mean ..

.. chaps who, like me, are bears of very little brain .. with the shape to match .. will probably just get confused ... but, hopefully, some folks out there will benefit.

Last edited by john_tullamarine; 20th Sep 2004 at 05:54.

20th Sep 2004, 08:06

Join Date: Aug 2001
Location: Dorset
Posts: 775
Hello again Milt,

You have previously accepted that it is possible to have left sideslip or right sideslip with the left engine out, depending upon what combination of rudder and bank angle you are using. As stated in my pevious post, this means that there must be a combination of rudder and bank angle at which the aircraft is not slipping in either direction.

It is all very well for you to repeatedly pronounce that bank without slip is impossible. But how do you account for the situation above?

Can you describe some process whereby the aircraft changes from left sideslip to right sideslip without actually passing through a zero sideslip condition?

If on the other hand you accept that the zero sideslip condition exists, do you believe it be with wings level or with some degree of bank angle towards or away from the live engine?

20th Sep 2004, 08:22

Join Date: Aug 2000
Location: UK
Posts: 3,646
For too long now there has been almost aviation wide belief that we pilots can eliminate sideslip and continue to fly an unbalanced aircraft straight down its fore and aft axis.
All I am saying is THAT IS ABSOLUTELY IMPOSSIBLE.
Second call for a twin with slip string and pilot in Canberra please...

20th Sep 2004, 09:03

Join Date: Dec 2003
Location: Dubai
Posts: 212
So in summary;

If the force from the rudder is equal then the one obtained from the bank, while at the same time you balance the moment from the live engine, you have zero slip straight flight.

Whether this is at all possible depends on the type of aircraft and/or you may have to do it with less than max/cont power/thrust.

Hereby you will have to neglect the small force contributions from ailerons and spoilers.

In my opinion the zero slip straight assymetric flight will have a deflection of the slip ball equal to the bank.

20th Sep 2004, 10:06

Join Date: Oct 2000
Location: N. Europe
Posts: 436
Alright, summing things up.

Milt:

The overriding fact to be determined initially is whether an aircraft having any angle of bank and in steady straight balanced flight can achieve that condition without sideslip.
...with asymmetric thrust.

ANY imbalance in thrust on a multi, not having centre line engines, which is then flown straight by either rudder or bank or a combination inevitably results in sideslip. It is unavoidable
That’s your hypothesis which is meeting with some opposition.

What is the effect of any resultant sideforce on a fusealge? There is ONLY ONE answer. The fuselage is of course forced to move sideways until it returns to balance..
This is, at least in my opinion, a fact.

Thus far, we agree. If there is such a resultant horizontal side force, a slip will develop until it is cancelled (straight flight being a given condition).

However, I showed above how you can achieve straight, balanced zero-slip flight with asymmetric thrust without such a resultant horizontal side force.

The disagreement boils down to whether this is possible or not. What you need to address is why the condition I gave, which fulfils all the requirements, is impossible. You claim it is impossible. Now tell us why this is so?

How can it be otherwise?
The horizontal component of lift is equal and opposed to the horizontal force at the stabiliser.

Thrust equals drag.

M_yaw_total = 0

F_lateral_total = 0

F_longitudinal_total = 0

We have equilibrium, sans any slip.
That’s how it can be otherwise.

Questioning common knowledge is fine and something which should always be done. Convention without consideration is a dangerous thing. But if you do think the given model is wrong, you have to point out an error in the reasoning used, or all you will have is an unsubstantiated claim. If you can’t substantiate your hypothesis, a hypothesis is all it will remain. If the calculations given match up, they are either a good representation of reality and thus valid, or there is an error or oversimplification in there somewhere.

As it stands, we have an explanation of why you can have straight, level, asymmetric thrust flight with zero sideslip. We have an alternative hypothesis that this explanation is not correct. We do not have the data to falsify the given explanation. Until we have this data, the given explanation will stand.

I repeat: Where is the error? If it can't be found, I think I'm ready to rest my case.

Regards,
Fred

20th Sep 2004, 10:45

Join Date: Oct 2003
Location: Canberra Australia
Posts: 1,300
heedm Keith.Williams ft and all.

Glad to have you join in heedm.

My definition would leave out 'in the horizontal plane' and would include 'normal to the vertical axis' and there are probably a few ways of saying the same thing.

Sideslip of an aircraft is the angle between the longitudinal axis and the relative airflow in the plane of reference. That is only me saying the same thing .

I cannot think of any occasion when there may be an angle between relative airflow and flight path. Can you or anyone else?

Now climb into your twin again with its left engine out. Keep wings level for simplicity. To stabilize in straight balanced flight, as you say, there is dynamic pressure on the right side of the fuselage to balance the tendency to yaw left. That pressure represents a finite force which MUST NOT be diminished, not even by a milligram, if you are to continue to maintain straight balanced flight. Don't you now have a hell of a problem in using rudder any which way to get anywhere near zero sideslip. There is no way to get there. Immediately you change the rudder angle by a nat's whisker you have changed that finite side force and you are no longer in straight balanced flight. Give the aircraft a very long tail and you could reduce that side force requirement considerably and hence the sideslip but you can't just wish it to go away.

I see confusion in appreciating a requirement for a balance of forces. An overly concentration on balances of moments and couples/torques.

Please anyone prove to me where I am wrong so that I can crave forgiveness for being so persistent.

Keith.Williams.

Cannot imagine accepting the possibility of having sideslip either way so I guess I have to apologise for not having refuted your contention. I do so now.

So back to square 1.

Any aircraft in straight and level balanced flight having total drag offset from the fore and aft axis will be unable to have a direction of flight along its fore and aft axis. This applys equally to wings level and banked flight.

Fred
Just saw yours and the response is so simple that I am surprised you missed it.

Your quote - don't know how to put it between lines yet like you pros.

"The horizontal component of lift is equal and opposed to the horizontal force at the stabiliser"
Where are you getting that "horizontal force at the stabiliser" if it is not being generated by the very sideslip you think you have made go away. Fred you can't make it go away else you will have a strong tendency to become a frisby.
Convinced yet?

20th Sep 2004, 11:21

Join Date: Oct 2000
Location: N. Europe
Posts: 436
"The horizontal component of lift is equal and opposed to the horizontal force at the stabiliser"
Where are you getting that "horizontal force at the stabiliser" if it is not being generated by the very sideslip you think you have made go away. Fred you can't make it go away else you will have a strong tendency to become a frisby.
Convinced yet?
Milt,
as you deflect the rudder, you turn the fin into a cambered airfoil. Thus it generates a force without any sideslip.

Coincidentally, that behaviour is also required if you are ever to get out of zero-slip flight using the rudder. I don't think we need to debate whether that is possible?

I shall continue waiting.

Regards,
Fred

20th Sep 2004, 12:09

Join Date: Aug 2001
Location: Dorset
Posts: 775
Hello again Milt,

It is of course quite possible that I have got it all wrong and I am quite happy to be swayed by a suitably logical argument.

You said in your most recent post "Cannot imagine accepting the possibility of having sideslip either way...." I, and I suspect many other readers, believe that the following situations are (at least theoretically) possible.

Situation 1.
Let's suppose our left engine fails and we put in right rudder to arrest the left yaw. We then increase rudder so that we yaw the nose to the right of our original flight path. If we get the rudder just right it appears to me that we will now be sideslipping down our original track. That is sideslipping toward our dead engine. Our lateral stability would tend to roll us away from this sideslip, but we should be able to hold the wings level by applying a bit of aileron towards the dead left engine

Situation 2.
Let's suppose that instead of doing the above we bank the aircraft hard towards the live engine. The tilted lift force will cause us to sideslip towards the live right engine.

Which of these situations do you consider to be theoretically impossible and why?

20th Sep 2004, 12:29
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Milt,

"My definition would leave out 'in the horizontal plane' and would include 'normal to the vertical axis' and there are probably a few ways of saying the same thing."

While one can resolve the vector to the aircraft plane, why bother for this qualitative discussion ? Depending on whether the bird is going up or down at the time, the vector elevation will follow the performance .. with a definite effect on forces developed by the fuselage. ... and, in any case, how does this alter the situation ?

"there is dynamic pressure on the right side of the fuselage to balance the tendency to yaw left."

Are we considering only the skidding zero rudder deflection situation here ? Why would not the pilot input rudder to reduce the skid (slip) ?

"Immediately you change the rudder angle by a nat's whisker you have changed that finite side force and you are no longer in straight balanced flight"

If we ignore minor effects (such as rolling moments) due to rudder deflection, surely the rudder input simply alters the lateral force system along with the yawing moment balance ? Why is this fundamentally different to the free/fixed rudder case ? Why can we not input an additional force in the opposite sense by playing with bank angle ?

"I see confusion in appreciating a requirement for a balance of forces. An overly concentration on balances of moments and couples/torques."

The two considerations (forces and moments) need to be balanced for steady flight (which would then be straight). It is implicit that varying the rudder input requires an opposing lateral force adjustment via a bank variation. Perhaps you can offer an explanation of how the observed airflow direction as rudder and bank is varied fits into your thesis ?

"Any aircraft in straight and level balanced flight having total drag offset from the fore and aft axis will be unable to have a direction of flight along its fore and aft axis. This applys equally to wings level and banked flight."

Why is this offset necessarily so ? Why can not one vary the drag components to alter the drag vector ?

"Where are you getting that "horizontal force at the stabiliser" if it is not being generated by the very sideslip you think you have made go away."

This would be quite OK if we were looking at a rudder free/fixed environment, but why is it that one cannot input rudder to achieve the force desired ? Indeed, the arguments suggests that ailerons and elevators, similarly, can't work ?

20th Sep 2004, 13:07

Join Date: Oct 2003
Location: Canberra Australia
Posts: 1,300
ft JT and Keith later after some more use of grey matter and recall from experience. Thanks for the prompts JT. Give me a day to get back.

Fred with your comments we get right back to the rudder wagging the tail.

Consider straight and level balanced non asymmetric flight. Rudder is centre. Now apply a little rudder and the aicraft must yaw into a sideslip as a direct result of the sideforce you have just applied to the tail by the rudder/fin force. Any change to rudder angle under any circumstances changes the side force. Slight pause there to quickly go through all of the circumstances that come to mind.?? . That is the very reason we have a rudder is it not? You should not be considering the rudder/fin as a seperate entity to the tail as a whole.

I am most intrigued that you could consider it to be otherwise.

Can you put that down as a little win for JT's tally sheet or does it still leave you puzzled, confused and mybe unconvinced?

20th Sep 2004, 15:20

Join Date: Oct 2003
Location: Canberra Australia
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Cap 56 Keith

Sorry I missed you last time round Cap 56.

Some rearguard action I seem to be fighting. !!

I think you are saying

"If the force from the rudder/fin (they work together) is equal to the the force from sideslip caused by the bank, while at the same time you balance the moment from the live engine, you have zero slip straight flight."

I have to presume, as you haven't spelt it out, that you have cancelled out the sideforce due to the bank caused sideslip. Now we have no sideforce to counter the yaw being rapidly developed by the live engine. We are turning into a frisby again. Do you really want to accelerate that frisby by taking the rudder further the WRONG way.

Grab a model and visualise.

Then you say " In my opinion the zero slip straight asymmetric flight will have a deflection of the slip ball equal to the bank."

Absolutely impossible to have zero side slip with bank applied. The very reason we put on bank in the first place was to generate that indispensable sideforce to counter the yaw and allow us to get the rudder close to centre. Apply rudder further the other way and we reduce that indispensable sideforce. How can we get it back. Only means if you insist with the rudder is to bank more and retrieve that same X pnds of sideforce with more sideslip. That force cannot be reduced any which way if we are to stay balanced. Take the rudder misapplication further to a conclusion and you auger in unless your speed is relatively very high, probably beyond performance capability.

Unless you can be convinced of those statements then we have to agree to disagree for now but don't let that stop you from contributing wherever you think you can add to the debate.. So many of our less initiated must be benefitting enormously from the repartee and arguements.

Oh and that slip ball. It always drops by gravity to the lowest point in its curved glass tube which in the case of balanced banked flight will result in the ball being out of centre by the bank angle...

Keith.

Hello again. Keep the propositions coming.

Your situation 1 is now real close to how it happens except for that increased use of rudder. Before you put on that extra rudder we were balanced having arrested the yaw.
So now you go and spoil it all by changing the rudder. The tiniest bit either way will unbalance us again. But if all you want to do is go along your original track/course then by all means use a little rudder to do a slow flat turn to do just that. It is only 2 or 3 degrees anyway. Having turned to the new heading you must return to balance again by returning the rudder to its precise original anti yaw position. Remember if you change CAS or thrust or drag you unbalance and there will be a changed rudder deflection to regain balance.

We have no means of accurately measuring the angle of sideslip so must make a guess for navigation purposes if we need to be so accurate.

Situation 2. Yes that is what we do to get the rudder back close to centre. Usually about 5 degrees thank you which gives us enough horizontal component of lift to cause us to sideslip to the desired extent..

Bank hard, say 30 degrees and it is unlikely that full left rudder will hold the right yaw resulting from a now very high left sideforce due to a gross sideslip angle to the right.. We will rapidly lose it rolling over inverted to the right as the slowing right wing stalls. A very nasty event usually ending in a spin - sometimes inverted.. Don't go there.

Hey - but weren't you trying to find a way to go to zero sideslip. Instead we augered in..

With apologies for having to regurgitate much of what has been stated previously.

I should have quit hours ago and it's now after mid night down here. The subject has become addictive. Just when I wanted to start an argument about rudders operating in the wrong instinctive sense, but that can wait a while.