Aerodynamics - Yawing, Slipstream

[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]

Left door open.

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]

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 .
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.