Aerodynamics - Yawing, Slipstream

[Krallu]

Hi there.

I am trying to get a grip of the aerodynamics. I understand what happens but not in detail why.

For example when I apply rudder and yaws a small aircraft I get a roll. Everywhere people speak about the outer wing goes faster and creates more lift. Ok. But how can that be, the wings are attached to the aircraft. They must go with exactly the same speed??

Also the slipstream. Everyone says it rotates around and hits the fin on the left side pushing the aircraft to the left. But if it is a rotating spiral. How come it won't rotate and someday hit it on the right side? Why only on the left?

Thanks!

[Heston]

The "outer wing goes faster" explanation of yaw-roll coupling is not the whole explanation (so you are right about that). Dihedral is more important - the wing that goes forward when the aircraft yaws has a higher angle of attack and so creates more lift, which causes the roll. You might need a model to look at if you can't easily visualise what I mean.

The spiral slipstream does always hit the fin on the same side. It goes round the fuselage like a screw thread. Again you need to visualise it carefully. Of course if the fin was below the fuselage it would push on the other side. And some engine/propeller combinations rotate the other way round so the spiral slip stream is the other way.

[dubbleyew eight]

ok.
you apply rudder and the aeroplane rolls. this is because during the yaw one wing is moving faster than the other until the yaw ceases. there after the aircraft will be at an angle to the direction of flight and both wings will be flying at the same speed.

the slipstream rotates around and hits the fin....
no it doesn't.
in reality the slipstream moves straight back past the aeroplane which is why fairings are aligned straight back and work.
of the rear edge of the propeller is a vortex sheet that curls at the prop tips. it is this vortex sheet off the prop that spirals around the fuselage and hits the tail. if you put a little piece of tape on the fuselage, in flight it will stream straight aft for most of the time but as the vortex sheet from the prop passes it will flick around in line with the vortex sheet. on a normal 2 bladed prop you will see 2 flicks per revolution of the prop.
where this impinges on the fuselage depends on the relative speed of the aircraft, the rpm of the prop and the fuselage length.

keep at the theory. it is what makes mastering it all possible.

[Heston]

you apply rudder and the aeroplane rolls. this is because during the yaw one
wing is moving faster than the other until the yaw ceases


But that explanation is what has confused the OP - he has noticed that the rolling force is still there once the yawing movement has stopped and the wings are again travelling at the same speed. Its dihedral that causes the yaw-roll coupling.

[dubbleyew eight]

the leading wing has it's full length in the airstream. the trailing wing is partially blanketed by the fuselage. there is a difference in the lift as a result.

[Heston]

Yes that's true as well...

[Kolossi]

Will there not also be a rolling force due to the airflow striking the horizontal stabiliser above the cofg, which in a slip (aka sustained yaw) is presented sideways to the airflow?

[dubbleyew eight]

immeasurably insignificant force from the edge of the horizontal stab.

[Krallu]

Ok, interesting.

Someone spoke about dihedral making the roll when applying rudder.
But in for example a PA28 aircraft. There is not that much of dihedral or back swept wings. Still the rolling moment continues when holding the rudder. ??

When you speak about the wing goes faster you mean the wing goes faster during the yaw? But when the yaw is finished and you just hold the yaw in place with rudder.What keeps the roll to continue now? When both wings move equally fast?

[Lord Spandex Masher]

the wing that goes forward when the aircraft yaws has a higher angle of attack and so creates more lift

Why would it have a higher angle of attack?

[Lord Spandex Masher]

What keeps the roll to continue now?

Side slip....

[worrab]

The rudder is a distance from the longitudinal centre-line of the aircraft, just as the ailerons are. The ailerons apply a rotational (moment of) force because they are a distance from the centre-line. When you apply rudder, you are similarly applying a rotational force. I suspect that if there were a fin and rudder below and as well as above the centre-line, that the instantaneous secondary roll would occur, but the continued rolling motion would not.

[phiggsbroadband]

Hi Krallu, in the model glider world, some gliders do not have ailerons but use dihedral wings and the rudder for turning... It works something like this...
Apply right rudder and the glider yaws to the right, pushing the tip of the left wing towards the incoming air. As the tip of the wing is at a high dihedral angle the air gets underneath it and gives it a lift. The right wing tip similarly gets pushed downwards by the airstream.
This method of control, which only uses the rudder, relies upon the wing having sufficient dihedral (say 15 degrees.) for it to work well. In a full size Cessna or Piper the dihedral is much less, so the effect is not so noticeable.

[Shaggy Sheep Driver]

the slipstream rotates around and hits the fin....
no it doesn't.
in reality the slipstream moves straight back past the aeroplane which is why fairings are aligned straight back and work.
of the rear edge of the propeller is a vortex sheet that curls at the prop tips. it is this vortex sheet off the prop that spirals around the fuselage and hits the tail. if you put a little piece of tape on the fuselage, in flight it will stream straight aft for most of the time but as the vortex sheet from the prop passes it will flick around in line with the vortex sheet. on a normal 2 bladed prop you will see 2 flicks per revolution of the prop.
where this impinges on the fuselage depends on the relative speed of the aircraft, the rpm of the prop and the fuselage length.


Prop vortex or not, surely a 'yaw string' cannot be used on a piston single precisely because it will not measure the yaw angle between the aeroplane and the relative wind, but will show the distorted airflow behind the prop (i.e. it will never stream aft in balanced flight as long as the engine is running). The (slightly - at cruise speed) rotating air mass behind the prop will impinge on the entire fuselage, wings, and fin. The net effect is to yaw the aeroplane and provide a slight anti-engine-torque roll effect as well.

[piperboy84]

I wish my engine was powerful enough to give me that torque roll effect on take off, i sit longingly waiting with my foot hovering over the right rudder but alas my old lycoming just keeps the aircraft pointing straight ahead down the strip.

[foxmoth]

But how can that be, the wings are attached to the aircraft. They must go with exactly the same speed??
if you hold a model stationary and "yaw" it one wing will move forward the other backwards, so they are NOT going exactly the same speed, especially at the wingtips there is a speed to be added/subtracted from the aircraft overall speed.

When you speak about the wing goes faster you mean the wing goes faster during the yaw? But when the yaw is finished and you just hold the yaw in place with rudder.What keeps the roll to continue now? When both wings move equally fast? If the yaw is finished there is no more roll - if you are "holding the yaw with rudder" you are still yawing!:rolleyes:

[Heston]

If the yaw is finished there is no more roll - if you are "holding the yaw
with rudder" you are still yawing


No. The aircraft is yawed ie not travelling in the direction it is pointing, but it is not yawing further (which it would have to be doing for one wing to be flying faster than the other). And the yawed aircraft experiences the rolling force that the OP is trying to understand. Go and try it sometime - you end up flying straight and level with crossed controls (side-slipping, as has been said).

The rolling force comes from a number of effects as has been explained, of which the dihedral effect is the greatest in a light aircraft - even though the amount of dihedral is only a few degrees.

[Shaggy Sheep Driver]

I wish my engine was powerful enough to give me that torque roll effect on take off, i sit longingly waiting with my foot hovering over the right rudder but alas my old lycoming just keeps the aircraft pointing straight ahead down the strip.

Piperboy, you are getting a torque roll effect, which is offset by the rotating airflow effect. With a weedy Lycoming there (should be) a slight yaw to the left on take off (or in any high power / low speed scenario). More yet on a soft field take off, as the torque-induced roll will put more pressure on the left main wheel which will drag in the mud a bit more than the right one.

Indeed, this torque-roll was the only way I could get the Yak52 to steer at a slippery, muddy, winter Barton. Differential brake had no effect whatever, but whacking power on and off sharply would make it turn left or right quite effectively as the appropriate wheel dug in.

[Pilot DAR]

but alas my old lycoming just keeps the aircraft pointing straight ahead down the strip.

Probably 'cause the Maule family put the fin on with an angle to the fuselage.

Add to the theme, what happens if flying a 150/2 or 172, and you open the left cabin door about 8 inches or so. Left door open, plane turns _____? Why?

[piperboy84]

Probably 'cause the Maule family put the fin on with an angle to the fuselage.


True, but I have flown both the fixed pitch and the CS version of the exact same model and the difference is quite noticeable, I guess that extra 150 to 200rpm the CS puts out on TO does make a bit of difference on the torque

Left door open, plane turns _____? Why?

Left, coz the door acts as a camber on the fuselage?

[Piper.Classique]

I seem to recall a right turn with left window open. I wonder if the window is acting as a lifting surface, or maybe a wee slat as it's fairly close to the wing.

[Silvaire1]

Left door open, plane turns _____? Why?

Left door open yaws the aircraft right... due to blanking flow over the vertical tail which being a long way aft has more moment on the aircraft than the door. That then couples to roll, as discussed above, because of dihedral. If you don't believe it, try it. A friend and I once flew a leg all the way to short final using only the throttle and the doors for control.

[fujii]

Extra drag left of centre of pressure.

[JAKL]

Could there be a more simplistic reason for the yaw induced roll? Maybe the 'undisturbed' airflow, under the wing going forward, that is angled toward the wing root reduces the span wise flow of high pressure air toward the wing tip, thus allowing the upper surface to produce lift with greater efficiency and the airflow under the lagging wing angled toward the tip, slightly increasing the span wise flow of air toward and over the tip causing a reduction in lift over the lagging wing. Hence a continued roll.

Thought I'd throw that into the debate for arguments sake, I'll duck down below the parapet and await the incoming :)

[dubbleyew eight]

you guys learning need to get things in proportion or you will go nuts.

weight of the aircraft is countered by the vertical component of lift. that's the big pair of forces.
thrust moving the aeroplane along is opposed by drag. that pair of forces is about a tenth of the magnitude of the lift.

all these other forces and influences you keep mentioning are quite small in comparison.

yaw forces on an aeroplane could be some proportion of all the things mentioned and then the could just be helped by a bit of turbulence.
when you are learning concentrate on the big things and don't get lost in in the little insignificant things or you'll never understand it.

[Heston]

when you are learning concentrate on the big things and don't get lost in in
the little insignificant things or you'll never understand it


That is good advice, but yaw-roll coupling (secondary effects of controls) is part of "Exercise 4 Effects of Controls" in the UK PPL syllabus, so anyone learning needs to understand it from almost the start...

[jxk]

http://upload.wikimedia.org/math/d/b/7/db7735d03f8de6082982164856a0d8ba.png is your answer! So if everything remains constant but 'v' increases (the outer faster moving wing) more lift will be developed thus the aircraft will roll.

[dubbleyew eight]

jxk can you actually put numbers into that formula to get valid answers?
it really helps if you know how to get the formula to work.


second thing. in training you are generally flying aircraft designed to FAR 23.
FAR 23 contains handling requirements and the design will have been tweaked so that it meets the handling requirements.
not all aircraft are designed to FAR23 (or the equivalent BCAR) so dont get set thinking that only one scenario is the real one.

in the yaw with rudder thing if you look back through all the posts you will find that all aspects of the issue have been mentioned. in different aeroplanes the relative importance of each mentioned issue will differ. sometimes it is just a guess as to which has prominence. you as a pilot dont have to worry about the exact proportions you just need to appreciate all the factors at play and know how to use them to get the aeroplane to do what you need.

[Krallu]

Yes I do understand the wings go faster when you are actually yawing the aircraft.

But when you hold the rudder still and the yawing moment have ceased and your nose is pointing to the left steadily. Even at this moment with an airplane that doesnt have dihedral, you still get a continous rolling moment if you do not use ailerons to counteract.

Why?

[dubbleyew eight]

have a look back at post number 5

[Krallu]

Ok, so that will be the only reason left for the roll to continue. With one wing in the full airflow and the other one is behind the fuselage. I see.

Thanks!

[worrab]

Not the only reason, but it is a reason.

Whenever there's a discussion about aerodynamics, I'm reminded of the episode of "Cabin Pressure" when Arthur asks how aircraft stay in the air and Douglas offers the standard explanation about the air having lower pressure on top of the wing than underneath. Arthur's response is "So how do aircraft fly upside down?"

Whatever flight control you operate on an aeroplane gives rise to one or more secondary effects. The rudder is itself a "wing" and gives rise to a rotational force on the aircraft. Through yawing the aircraft it forces one wing forward with a corresponding retrograde movement of the other to create a rolling motion. On a swept-wing aircraft, one wing now presents considerably more profile to the airflow than the other, and in all aircraft there is some blanking of the "trailing" wing. But of course the "lifting" wing has more drag than the other. All these factors and more (eg di/an-hedral) combine together to give the aircraft its own character and relative stability.

(Incidentally, many lives have been saved through the use of secondary effects after primary control was lost (eg Sioux City DC-10).)

[24Carrot]

Pilot DAR wrote:
Add to the theme, what happens if flying a 150/2 or 172, and you open the left cabin door about 8 inches or so. Left door open, plane turns _____? Why?

I don't know, and I'm not going to try it :), but my guess would be that the left wing rises because of the higher AoA on the left wing as the air tries to avoid the door. So you turn and slip right, and then I suppose it kind of self-corrects..

Now I'm not sure I can live with the suspense :eek:.
Will there be an answer?

[BigEndBob]

Surely when you yaw the aircraft, the aircraft turns, the outer wing is on a larger radius of turn than the inside wing, therefore more lift leading to roll.

[thing]

I'm not a theorist but yaw/roll coupling is very noticeable in gliders which have long thin wings that aren't blanked much by the long thin fuselage, so I would go with the one wing going faster than the other scenario. Seems to make intuitive sense.

Edit: I'm pretty sure that some radio controlled gliders don't even have ailerons, they just use the rudder for roll control. It's easily doable in a full size one.

[Heston]

I would go with the one wing going faster than the other scenario. Seems to make
intuitive sense.

But as with many things in flying your intuition would be wrong, or at least incomplete. At the risk of being boringly repetitive - the outer wing is only going faster while the aircraft is yawing, and yet the yaw-roll coupling still occurs after the aircraft is not yawing further but is in a yawed state ie going sideways through the air.

[thing]

the outer wing is only going faster while the aircraft is yawing, and yet the yaw-roll coupling still occurs after the aircraft is not yawing further but is in a yawed state ie going sideways through the air.

Blanking effect of the fuselage?

[Krallu]

Another thing.

When I increase RPM the nose goes up and when decreasing the nose goes down.

When I increase RPM there will be more speed over the wings and more lift for this angle of attack and so more lift than weight and therefore the nose goes up. And when decreasing the speed slows and I get less lift compared to weight therefore the nose goes down.


Correct? Or am I missing something?

[dubbleyew eight]

missing something.

the centre of mass of the aircraft is above the thrust line.

if you are flying along in trimmed equilibrium all the force couples balance.
if you then increase the rpm the propeller thrust increases. you now have an additional amount of force acting through the arm to the Centre of mass.
nose goes up until the force couples balance again.

the opposite occurs if you decrease thrust from a trimmed equilibrium.

in a high engine aircraft like the Lake LA4 amphibian the thrust line is above the centre of mass and increased prop thrust pushes the nose down.

[Desert185]

With regard to pitch/power, the thrust line of the mounted engine may be designed or adjusted for optimum desired performance. The Super Cub community, for one, plays with this. Sagging Lord mounts should also be taken into consideration.

Much to be learned by searching this subject.

[Piper.Classique]

You might also consider that the fuselage is straight. If it was banana shaped you would get a different result. The actual speed difference between the wings is quite small (yes,I know it's the square of the speed but the square of a small number is also small)

[BigEndBob]

If you apply rudder and hold it the aircraft yaws (apparent turn), then physically turns, then rolls due to difference in turn radius of inner and outer wing. This state continues as a spiral dive developes.
After that things get a bit more complicated.

If you oppose the roll with sufficient aileron, the aircraft flat turns, as can be used if you landing off the centreline and want to re-establish.

[Lightning Mate]

yaw-roll coupling.

Several people here have mentioned that.

It isn't. Roll-yaw coupling is something entirely different, but you would need to fly a fighter type aeroplane with a high B/A ratio to see the effects.

[Heston]

Yes well we could get into inertial cross coupling, but that's not really relevant to the OPs question which is to do with aerodynamic effects. Put your willy away - its the private flying forum after all.

[Steve6443]

Yes well we could get into inertial cross coupling, but that's not really relevant to the OPs question which is to do with aerodynamic effects. Put your willy away - its the private flying forum after all.

Well said, unfortunately some "Sky Gods" come on here to lecture to us weekend fly-boys whilst indulging in some "major league" willy waving - unaware that by doing so, they are slowly leading to the demise of this particular sub-forum.

Perhaps that's their ultimate goal, scare off all weekend fly-boys and close down the Private Flying forum on PPRuNe so that they only need blog with "Sky Gods" of their own calibre; I think I'm not alone to say I've noticed a significant decline in threads / contributions here because of the arrogance of a few......

[phiggsbroadband]

Hi, I find it amazing and a bit disconcerting that this question should have to be asked... Have you not done the BASIC TRAINING....
and I don't mean the PPL syllabus, but the real life training which starts at
the age of 10 with paper airplanes, then goes on to Balsa Bashing, with your fingers covered with balsa glue and nicks and cuts from very sharp modelling knives.
When you take your first model glider to the park, you soon realise that DIHEDRAL is required to make it fly in a stable straight line. Also lead weight in the nose will aid its stability. Then if you want it to control the turn, you need to tweak the RUDDER.
Then at the age of maybe twelve, you want a more aerobatic model plane, so you need a flat wing and AILERONS to make it turn, by BANK and YANK.
Only when you are near 16, and maybe in the Air Training Corps, shooting riffles, going camping, and seeing girls in a different light, will you get to be near a full size glider.
All this should come pre-PPL, so you should know the answer to your question, years before sitting in a C152 or PA 38.

[dubbleyew eight]

I think you guys are needlessly showing some envy there.

lightning mate wrote Roll-yaw coupling is something entirely different then he writes something you've mis interpreted.

yaw - roll coupling is something seen in bigger aircraft. it isnt seen in light aircraft.

I think you guys have misinterpreted what he was saying.

if we were talking about buffeting due to turbulence and someone mentioned mach buffet you would probably find Lightning saying that you were mistaken as well. mach buffet wasnt a part of the buffetting of a light aircraft.
can you see what I'm getting at?

[Steve6443]

All this should come pre-PPL, so you should know the answer to your question, years before sitting in a C152 or PA 38.


Your comment infers that it is a prerequisite to have been in the ATC, to have flown model planes as a child in order to become a PPL which, quite frankly, is a joke. My childhood was spent playing D&D, not with model airplanes.

What really winds me up is that you are practically demanding that we should all know the answer to this question before even starting PPL training. The initial poster posed the question because he does NOT understand and hoped to find enlightenment here, he doesn't need comments like above dumped on him but instead a reasonable answer.

Your post just underscores my previous statement, that this sub forum is rapidly going downhill because of know it all "Sky Gods" belittling those who do their flying as a hobby or are learning to fly......

[Lord Spandex Masher]

"Learning" requires listening to those who know.

It's not belittling to state a fact.

Don't be so defensive.

[Crash one]

When I read the original post referring to the fact that both wings are attached to the a/c therefore how can one travel faster than the other I had the mistaken notion that someone might mention that it is the WING TIPS that move at different speeds not the whole wing. But instead we are now being advised on the subject of Mach buffeting. What a bunch of utter garbage, no wonder this forum is going downhill. Someone asks a question & gets a load of gobbledygook Sky god utter pish from willy waving theoretical plonkers trying to make a mountain out of bugger all.Yes I did the model thing & the ATC but it is mot mandatory, & them what didn't will need help. Stop being so pompous & answer the question.