ROB-x38

I'll take the plunge.

G'day Milt

In the banked, central rudder, steady, straight, unaccelerated, level flight case there must be a slip to balance the horizontal component of lift. No worries there.

In the banked, deflected rudder, steady, straight, unaccelerated, level flight case can't the balancing force to the component of lift come from the rudder. Why do we have to rely on the keel surfaces and directional stability of the aircraft to balance the forces?

Milt

ROB-x38 and all.


Welcome aboard. I think we need a few more than a quorum before we are through with this.

Ok lets go fly your twin.

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.

The R foot rudder plus a considerable contribution from the fin and rear right fuselage is balancing out the thrust from the R engine which would otherwise be yawing us strongly to the left.

We don't like that amount of rudder applied. It has reduced our options for lower speed flight down around Vmca and as flight testing has shown (and we may instinctively suspect) we are able to carve off some drag by banking a little into the live engine. A bit more aileron than the few degrees we already have applied you say. Well OK we suffer a little extra drag there but hope to lose more than that by a reducing rudder deflection.

So roll over by about 1 degree and retrim all round. What has happened is that we are now back to about 1/4 R foot rudder deflection. The small lift component is now trying to pull us sideways a bit causing additional side force on the fuselage, which has its greatest surface area to the rear, taking over some of the previous task of the rudder. That feels a bit better.

Roll on another 2 degrees. More lift component and greater tendency to slide to the right causing more side force on the rear fuselage.. Compensate for this by taking off some more rudder. Retrim all round for straight level flight.. Now we only have about 1/8 R rudder. But what about that crazy balance ball. Its now hanging off the centre of the instrument by the angle of bank. Still nothing except instinct to indicate we are sideslipping.

Now roll on another 2 degrees. More lift component, more sideways force, more sideslip, more compensating force from the rear fuselage. Retrim all round.
Hey - rudder is just a nat's whisker away from centre. That feels great but that crazy ball is now out further. Want to cover it? It's not much use to you now unless you reset in your mind its current position as being the new datum to use in turns.

Somewhere as we increased the angle of bank we went through the minimum drag point. Maybe we are right on it now in that we have a central rudder, the rear fuselage having taken over from the rudder. I would suspect that a little touch more bank to cause the rudder to be a little to the left would be the minimum but why worry as the shape of the drag curve in this area will be fairly flat.

What happens when we try more bank.?

OK roll on another 10 degrees R to a full 20 degrees. Now it’s a bit of a lead sled. We've had to trim nose up to compensate for the much larger portion of lift we are using out to the side and to keep us going straight you now have almost half right rudder out the other way. Drag has shot up considerably and we have now stabilised just above Vmca for the power we have.. The side slip is gross.

Roll on another 10 degrees. Hey is that full left rudder?? and come on keep your nose up. Don't let it yaw right like that. Hey you're losing it. Look at your speed.
TAKING OVER.

Nose down and wings level, power off , I'll get the power coming on gradually as our speed increases. Lets get that other engine going or will we check out your personal minimum control speed. You may even be able to get it back to the stall.

By the way do you know whether you can stall the fin on this type. Fin stall often has nasty consequences.


A bit simplistic to the experienced but we aren't all so endowed.

SR71

Milt,

The rudder doesn't have to be central though...

In fact, throughout this discussion, apart from the flight condition alf5071h's notes refer to, it won't be.

To my mind, it is quite possible to attain banked, zero sideslip flight when asymmetric with rudder deflection.

Regards the effect of CG location on V_mca...

We need force and moment equilibrium for an unaccelerated flight condition.

The point about which you consider these two forces to act makes a crucial difference to whether \sigma M = 0! It won't remain unaccelerated for long.

You can fix your axes system anywhere on the a/c and define things accordingly.

Agreed, for lateral force equilibrium, balance them about whichever origin you like on the a/c x-axis.

Having chosen this point, you then fix the required sideload required by the fin/rudder because presumably your airframe is not aeroelastic and the distance to the fin is fixed.

But, having chosen that point, if you then change it, whilst it does not change the size of the thrust related yawing moment, I cannot see how you won't change the required sideload that needs to be generated by the fin/rudder?

How do you otherwise propose to size your fin/rudder?

I see an analogous relationship between centre of pressure, aerodynamic centre, CG and AOA in the vertical sense and their lateral equivalents, except that, of course, you can't decouple lateral/directional stability.

My $0.02.

john_tullamarine

.. still here, Milt, with a smile and a tally sheet .....

ft

“2. The horizontal forces on a twin engined aircraft having asymmetric thrust flying in steady straight unaccelerated level flight, wings level, can be accumulated and represented by just three distinct forces familiar to us.

[...]

2.2 Total Drag - Acts parallel to an aircaft centre line. Equals Thrust. Vertical positioning of Total Drag line irrelevant. NO relationship to weight or cg.”

In my world, drag is always parallel to the direction of travel, no matter if it is horizontal or vertical. An academic point to be sure, you’re simply splitting up what I’d call total drag into the components parallel and perpendicular to the fuselage. The summed total will, for the horizontally unaccelerated case, be parallel to the direction of travel.



That said, here’s another version of what I posted above.

To fly straight and level with thrust asymmetry, you need to push rudder into the live engine to counter the moment generated by the thrust asymmetry which will otherwise turn the aircraft towards the dead engine and make it turn flat (at best).

This opposite rudder creates a lateral force towards the dead engine at the rudder. The opposing force to this lateral force will be generated by a side slip into the dead engine and the resulting aero forces on the fuselage. With the right amount of rudder and slip for the airspeed, the moment will be the same as the moment generated by the thrust asymmetry, the forces will cancel each other out and you will be flying straight.

Rather than having this balancing force created rather inefficiently by going sideways, you can bank the aircraft and have it generated by the wings. Generating forces is what airfoils do best as that is what they are built for.

The vertical component of lift from the wings and aero forces on the stabiliser will, together with m*g applied at the CoG (remember, we’re back in the vertical plane for a moment – pun unintentional), have to cancel out summed up as a moment about an arbitrarily chosen point (which never has to be the CoG as the moment is equal about any point on a rigid body... albeit it will begin rotating about the CoG when the summed moments don’t amount to zero). They also have to cancel out with their magnitudes summed up, unless you want to accelerate vertically. This dictates the amount of vertical force on the stabiliser for any angle of bank.

The horizontal force on the stabiliser has to equal the horizontal component of lift generated by the wings. The moment generated by the fixed moment arm between them has to equal the moment generated by the thrust asymmetry. The magnitude of these forces is set by the angle of bank. Alas, the thrust asymmetry dictates the angle of bank. We have very few options. Newton still has the upper hand on us!

We could center the rudder and bank enough to increase the slip into the live engine until the aero force generated on the fuselage equals the horizontal component of the lift and the angle of attack of the fin with no rudder (camber) generates all the torque to counter the thrust asymmetry. However, generating a force through slipping is not likely to be any more efficient now than when we were wings level.

No, keep that foot on the rudder to keep the nose pointed where we’re going. That slip would be creating loads of drag and pointing the remaining of thrust to the side instead of in the direction in which we want to go, just when we lost half the thrust available to begin with and are likely to be desperate for more of it!

Besides, we banked to get away from slipping into the dead engine. It’d be rather daft to start slipping the other way instead. It’s much better to let the airfoil things at the back take care of the force generation. As was pointed out before, that’s what they’re built for. They also have a longer moment arm, meaning they'll do more torque with less drag.

This conclusion valid as long as we’re going for performance. If we are going for maximum control authority, a bit of sideslip is probably preferable. That way, we’ll leave the rudder less deflected and have more travel to use for changing our situation.

If you are hard pressed to find any disagreement with Milt here, you’re right. I’m just looking at the same thing through a different window to broaden the perspective and perhaps get some feedback.

Regards,
Fred

Milt

ft and all temporary disbelievers.

Must take more care to avoid misunderstandings ft. My reference to the the vertical through which the total drag acts was to bring to attention that the horizontal forces we have been considering do not act along the same horizontal reference plane. They have vertical separations and most moments resulting are well looked after by longitudinal stability. For now forget I ever introduced that red herring.

All of us involved so far have been in agreement , I am presuming, with the superb logic in your first 6 paras after "That said, here's another version of what I posted above".

BUT then you continue to persist with the all too common proposition that an aircraft can fly straight and balanced with bank applied and not be side slipping. Old beliefs sure do die hard. Let me put it again as simply as I can.

The horizontally resolved portion of the total lift will be acting at right angles to the fore and aft axis emanating from the point through which total lift can be said to act. This will be close enough to the cg for our purposes.

What is the effect of this sideforce? There is ONLY ONE answer. The fuselage is of course forced to move sideways. 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.

If you are still somehow skeptical, take the bank angle to an extreme as I did in a simplistic description of flight in an asymmetric twin in a post above.
Eventually with enough bank you reach a point where you have full opposite rudder, to that with which we started, attempting to oppose the now gross tail force due to sideslip. Take it a smidgin further and you have lost control.

Now, having succeeded in convincing you, SR71, John Tullamarine's tally sheet and the all too silent visitors to the thread of the fact that we are sideslipping whenever we fly with bank and fly straight at the same time, perhaps we can spread the gospel, correct the training references and switch to other fascinating aviation myths.

Could I have missed an obscure situation when you can indeed fly straight and level with bank and not be sideslipping?

Our unfinished myth on this thread is that rediculous proposition that steady state directional stability has any relationship to the cg at a particular weight. This one is bound to exercise great minds further. Perhaps we should tidy up and start a specific thread and a new tally sheet for JT.

ft

Milt,
if there's a minute horizontal component of (main plane) lift, countered by a force at the (deflected) rudder (equal in magnitude, opposed in direction), why will anything go sideways?

If there is indeed slip, what will then balance the aero force on the slipping fuselage to avoid horizontal acceleration?

Regards,
Fred

Milt

ft

You have answered your own question!!

The minute sideways force is balanced by the minute sideslip caused by the rudder.

Think of it as the rudder pushing the tail around to cause a new position of the rear fuselage. It is now sideslipping.

I think you are overlooking the sideforce created by the sideslip.

And try thinking about rudders on boats!!

SR71

Let me get this straight...

Lets say I take a plank freely suspended in air in the horizontal plane and push sideways at the CG with a force, F.

Now I push in the opposite direction with a force, F, except that I exert this force at one of the extremeties.

Will the plank translate at all?

I think not.

Milt

SR71

Confused a bit by your use of translate - to change to another language or to change to another form.

Presume you mean rotate but if so the question is too simplistic.

What you have described is a perfect moment and if you keep the forces normal to the plank it will spin up like a prop.

If you maintain the direction of the forces then the plank will turn through 90 degrees and stop.

I think I must be wrong in interpreting what you mean.

Help

SR71

Milt,

I agree it will rotate but not translate i.e., move in a linear sense.

Assume the length of the plank is 2d, the applied moment is F*d whereupon you will get an angular acceleration.

Now if I apply an opposing moment to prevent any rotation, of magnitude F*d (Note: this doesn't affect the force equilibrium), I now have neither a rotation or a translation.

Agreed?

Isn't this case directly analogous to the one in question?

ft

Milt,
now you’re definitely the one confused here.

If the force on the rudder is opposed to the horizontal component of lift, any sideslip it creates will be in the opposite direction of the horizontal component of lift, adding to that horizontal force.

Either way, it doesn’t really pertain to the question I raised. My question was: Why would there have to be sideslip when the forces cancel out without it?

Look at the forces and moments involved.

The thrust asymmetry generates a moment. This moment is countered by the moment generated by the horizontal component of lift and the horizontal force at the stabiliser.

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. Why do you insist it has to be there?

I’ll see if I can dig up a good WWW reference for all of this.

Regards,
Fred

P.S. In physics, translate is the common term for when you get something moving, as opposed to merely rotating it. Cmpr “translational lift” for helos.

To be dull and hit the dictionary:

john_tullamarine

Isn't it rather wonderful the way threads wander off-topic into quite interesting areas ? Would anyone believe that the original question was along the lines of

" should a yaw string not be called a slip string since as far as I can tell it shows sideslip not yaw ? "


I think that we are all somewhat in the same ballpark ?

Milt, we probably need you to itemise several things to focus attention on some, if not all, of the main points of apparent difference throughout this thread.

Indeed, Milt's apparent heresy is quite thought provoking ...

(a) what is your definition of balanced flight ? Do you distinguish between the terms slip and sideslip ?

(b) could you characterise the aerodynamic slip vector direction as a function of bank angle ? From this follows a concern to see how the lateral fuse force varies with variation in bank angle. I don't think that anyone will dispute the observations that, if a sideslip is permitted to develop, the lateral drag will soon balance the accelerating force and the thing gets to a steady state or, indeed, that the lateral force component is a real factor generated in the presence of sideslip and has to be part of the equilibrium analysis.

(c) why should we want to operate with sufficient bank to achieve nil rudder deflection ?

(d) if we do see a need to do so, then what are the advantages and disadvantages of so doing compared to, say, wings level or some other angle of bank ?

(e) what sort of typical bank angle would you suggest is appropriate to achieve zero rudder deflection back in the region of published Vmca ? What sort of climb (or descent) performance might you expect for this case ? Some examples might go a long way to convincing us other heretics ... along with some indication of the generality of such examples ? Indeed, how do you see the climb (or descent) performance varying with bank angle .. for steady state cases ?

(f) you observe that the horizontal lift component generates a sideslip ? However, could you address the concern of the heretic members of the opposing camp who might opine that that force doesn't, itself, create any sideslip .. rather the motion resulting from the (unbalanced) set of forces does. If the said force is balanced by another (the lateral fin force, for instance), and presuming that the moment balance is taken care of along the way and that the equilibrium force/moment balance can be achieved at some point where the sideslip angle is zero, where is the lateral force imbalance necessarily needed to create the acceleration and velocity necessary to give rise to the sideslip which one needs to achieve some lateral flow circulation ? .. or is this scenario not achievable ? We should all agree that, in the presence of sideslip, there will be some balance involving whatever forces exist .. including the lateral fuse force. Re-reading the thread just now it may just be possible that you adopt both points of view in different posts ?


Score ? ... I still like catchup's first post the best ..... however, Milt's observation in respect of "yaw slip is showing" rates a very close second .. perhaps a tie ? Did you think I was scoring on technical matters ? .. not a chance .. life in the day job has been a bit torrid with workload this past fortnight .. I crave plain old hedonistic entertainment ...

I must say that this is all rather jolly good fun ... I'm just not entirely sure if Milt is on the level ... or winding us up for his own entertainment by having 2 bob each way on rudder deflection and varying the tale slowly throughout the thread ? ie the other camp's heretics are not overly concerned with the zero rudder deflection case ? Incidentally probably none of us have any difficulty with Milt's scenario in the case of undeflected rudder... it's just that we are probably not overly interested in that particular case ? .. as, in the real world, control in the low speed takeoff is paramount as is climb after the initial dynamic dramas have been sorted out ?

Either way .. all good, clean fun .. and it has challenged everyone to have a bit of a think about what might be going on ...

.. come to think of it ... Milt wins the entertainment stakes ... what say you, Fred ? I have not met Milt, although I know a little of his background .. and I suspect that he would make a very entertaining tutor in the classroom ... he certainly has provided sufficient fuel to the fire to get us all going.

ft

JT,
fun indeed. It is interesting how you know that you know things... and then when that knowledge is questioned, it turns out to be belief rather than knowledge until you have thought it over in more detail. It is always useful exercise! Sometimes, and this is my favourite, it turns out you were in fact wrong all along. Gotta love learning!

Milt, I still suspect you of pulling my leg. Are you bribed by JT to keep the discussion going for his amusement? Agreed, very high-scoring contribution and definitely a runner-up.

However, the opposition is fierce and albeit close, my final vote goes to Bellerophon, as brief as his participation has been. It is certainly no shame in coming a close second to a statement such as:

“The gliding fraternity use a piece of cotton, stuck to the outside of the canopy to achieve the same result, but on Concorde it tended to come off in the cruise, so a sideslip indicator it had to be!”

I must confess to perhaps being somewhat partial, having previously read very interesting posts by the abovementioned gentleman. Not to mentioned being corrected by him when my view of the world was wro... ahem... departed somewhat from the one considered reality by the majority of the population.

I think we should have established early on what position of the rudder we based our respective views upon though, to avoid a bit of confusion. Centered, towards dead engine or towards live engine.

I do think we'd all have a jolly good time arguing until we ran out of envelopes to scribble on if we could cancel the geographical inconveniences separating us all from a suitable common outlet for malt-based refreshment.

Cheers,
Fred

Milt

John Tullamarine and my faithful intriguing respondents.

Should have set up the ground rules up front to avoid the confusion which has been flowing back and forth. Perhaps they should preferrably be called air rules.
Thank you JT for your intervention. Yes I had my time as a QFI and RAAF CFS staffer plus other things.

Firstly what are we ON about. It's Asymmetric Handling and an examination of the forces acting..

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.

That should be a very simple determination, should be instinctive to a pilot and remain an uncomplicated fact provided my definitions agree with yours.

So herewith the definitions I am using and listed in response to JT's (a) to (c)..
These definitions are off the top of the head and almost from the heart. They should be consistent with text books.

(a) Balanced Flight. Any condition of flight when there is a continuing state with all forces acting on the aircraft in equilibrium.
Slip = Sideslip.

(b) My 'aerodynamic slip vector' (don't like the term) is that force normal to the line of flight generated by sideslip acting through the lateral centre of pressure.
Explanations. 1. Force Generated by Sideslip. The difference in the lateral aerodynamic forces resulting from differing airflows on each side of the fuselage.
2. Lateral centre of pressure. Similar to wing lift centre of pressure but instead the point through which horizontal side forces act.
No force without sideslip. Rudder force not considered seperately as it is included in total side force.

(c) Why would I bank to zero rudder? I wouldn't except when IAS is close to Vmca. In some cases there may be a trade off between the total drag wings level and banked.
Explanation. 1. Rudder central may be more efficient with rear fuselage only generating the sideforce.
2. Zero rudder allows for decreased Vmca.

(d) (e) Later coverage. Will only overly complicate at this time and further complicated by published Vmca having included a 2 second delay before taking recovery action.

(f) No definition here and we are splitting hairs which are all having a strong tendency to stand out straight! Simplest explanation I can offer is :-
Roll on bank to produce the horizontal sideforce. Immediately we have departed balanced flight. Initially the unbalanced sideforce is now overcoming aircraft inertia and slowly accelerating the aircraft sideways in the direction of the sideforce until the build up of the force due to sideslip balances that lift sideforce. Now we are back in balanced flight/equilibrium, having established a yawing couple by the two sideforces which balances out the opposing yawing couple by the offset thrust and the total drag. The resolved lift sideforce act through the total lift centre of pressure (almost coincident with the cg) and the sideslip force acts through the lateral centre of pressure which will have moved further aft as the sideslip angle increases. Incidently if the lateral centre of pressure moved forward of the total lift centre of pressure the aircraft would have a strong desire to turn around to go backwards.

Damn it I didn't want it to get that complicated again so soon but I don't know how to avoid it.

Are you having problems with the concept of a lateral centre of pressure? It doesn't generally get too much of a mention. Has more significance with boats than aircraft EXCEPT for our present subject and for those wanting to do 4 point slow rolls at low altitude !

Tear me to bits if you can or have I recruited a few disciples?.

bookworm

So what you're saying is that if the relative airflow is symmetric (i.e. parallel to the fore-aft axis of the aircraft) but the rudder is providing a lateral force because it is non-central, then you describe that lateral force as a side force and infer by definition that the aircraft is in a sideslip? Is that right?

Milt

Bookworm

Sorry no - not saying that.

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

If the airflow is symmetric then the rudder is central.

Think of the tail as a whole and imagine what happens when you wag the tail with the rudder!! Every wag results in a sideslip. If that doesn't compute nut out what happens when the captain of a ship commands "Right full rudder"

I feel a wag story coming on but will spare you!

Does that help?

ft

By having the forces cancel out anyway. Equilibrium is equilibrium regardless of how it is created. A horizontal component of lift to cancel the force contribution of the deflected rudder (which exists - see below). Thrust asymmetry to cancel the moment contribution.

That's the equilibrium situation I described above. Can you tell me why you consider it impossible?

No problems with lateral CoP or knife-edge flight. However, I do have a problem with:

That is only valid as long as the rudder is not deflected. Once the rudder is deflected, the crude airfoil made up of the fuselage/fin (as viewed from above) will go from a symmetric one to a cambered one. While the above quote holds true for a symmetric airfoil, it does not for the cambered case when there will be a force generated without any angle of attack (beta).

Heh, a thought goes out to newcrew who’s probably asking himself how he could create this monster through a simple question about a piece of yarn!

Regards,
Fred

Oktas8

Hi all, long time listener, first (or seventy-first) caller. I'd like to slip my 2c worth in too if I may, talk about this for a moment, although I don't want to force my viewpoint.

Milt several of your posts are worded carelessly, such as the one below. Since you have thought about this a great deal, is it carelessness or a wind-up? I can't tell...
No! R foot rudder gives a force to the LEFT. Due to LEFT slip that this causes, aerodynamic forces on the fin & rear fuselage are generated to the RIGHT, so we need even more rudder than we previously thought to maintain heading (i.e. prevent yaw). Change your plus to a minus above.

How can I stop the aircraft from slipping left? I need a force to the RIGHT. Some horizontal component of lift could provide the necessary.

Why can't I have
-Yawing moments balance by having rudder deflection moment opposing thrust / drag asymmetry moment, and
-Sideways (slip-inducing) forces balance by having horizontal component of lift force opposing rudder deflection force?

cheers,
O8

SR71

O8,

I agree.

In fact the paragraph you quote has been amended (in the original post - although slightly earlier than your own post - the plot thickens) by Milt because it represented an impossible flight condition.

It used to read "having shut down the left engine......rudder is trimmed left foot..."! I puzzled about this for a while...only a short while I hasten to add.

Maybe it was just a typo? Milt?

Still, all highly amusing.