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Bilbo_Baggins

262 yaw

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Posted (edited)

Gents,

 

So, obviously this new jet has been a very interesting machine to get our hands on the past few days. One thing I haven't seen talked about yet is the rudder. Of course, it's a different machine in all facets, but I'm interested to learn why the ME262 rudder is/was so effective. Of all the machines in game this airframe has the most effective rudder. Quite noticeable and it's also very handy for course correction at 800kmh.

 

Could anyone please help explain why this airframe has such outstanding yaw control authority?

 

Regards

Edited by Bilbo_Baggins

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Not sure if this is what you looking for 

This is an attempt to answer the frequent question "Why is my aircraft turning left all the time?".

This occurs only in aircraft with propellers at the front of the aircraft. And yes, it does occur in real life. Four distinct phenomena cause the effect, all causing the aircraft to turn in the same direction. They are:

Prop wash

A propeller pushes air not just horizontally to the back, but more in a twisting helix around the fuselage (clockwise as seen from the cockpit). As the air whirls around the fuselage it pushes against the left side of the vertical tail (assuming it is located above the propeller's axis), causing the plane to yaw to the left. The prop wash effect is at its greatest when the airflow is flowing more around the fuselage than along it, i.e., at high power and low airspeed, which is the situation when starting the takeoff run.

Propeller torque effect

Torque effect is the influence of engine torque on aircraft movement and control. It is generally exhibited as a left turning tendency in piston single engine propeller driven aircraft.

According to Newton's law, "for every action there is an equal and opposite reaction," such that the propeller, if turning clockwise (when viewed from the cockpit), imparts a tendency for the aircraft to rotate counterclockwise. Since most single engine aircraft have propellers rotating clockwise, they rotate to the left, pushing the left wing down.

Typically, the pilot is expected to counter this force through the control inputs. To counter the aircraft roll left, the pilot applies right aileron.

It is important to understand that torque is a movement about the roll axis. Aileron controls roll. Prop torque is not countered by moving the rudder or by setting rudder trim. It is countered by moving or trimming the aileron.

This correction induces adverse yaw, which is corrected by moving or trimming the rudder (right rudder).

On aircraft with contrarotating propellers (propellers that rotate in opposite directions) the torque from the two propellers cancel each other out, so that no compensation is needed.

Further Reading: Propeller Torque Factor

P-Factor

P-factor is the term for asymmetric propeller loading, that causes the airplane to yaw to the left when at high angles of attack.

Assuming a clockwise rotating propeller it is caused by the descending right side of the propeller (as seen from the rear) having a higher angle of attack relative to the oncoming air, and thus generating a higher air flow and thrust than the ascending blade on the left side, which at the other hand will generate less airflow and thrust. This will move the propellers aerodynamic centre to the right of the planes centreline, thus inducing an increasing yaw moment to the left with increasing angle of attack or increasing power. With increasing airspeed and decreasing angle of attack less right rudder will be required to maintain coordinated flight.

This occurs only when the propeller is not meeting the oncoming airflow head-on, for example when an aircraft is moving down the runway at a nose-high attitude (in essence at high angle of attack), as is the case with tail-draggers. Aircraft with tricycle landing gear maintain a level attitude on the takeoff roll run, so there is little P-factor during takeoff roll until lift off.

When having a negative angle of attack the yaw moment will instead be to the right and and left rudder will be required to maintain coordinated flight. However negative angles of attack is rarely encountered in normal flight. In all cases, though, the effect is weaker than prop wash.

Gyroscopic Precession

This is the tendency of a spinning object to precess or move about its axis when disturbed by a force. The engine and propeller act as a big gyroscope. However, gyroscopic precession is likely to be minimal in a typical aircraft.

Gyroscopic precession is frequently confused with p-factor.

One author maintains p-factor is caused by a combination of factors unrelated to gyroscopic precession http://home.earthlink.net/~x-plane/FAQ-Theory-PFactor.html

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That’s very comprehensive.  Although being a jet, I don’t know how much the 262 would be subjected to prop wash.  The very effective rudder characteristics of each aircraft were something I always figured to be a global carryover from RoF.  I just dampen my rudder inputs until it feels about right.   

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I'm not sure about the rudder effectiveness, but what the FM does seem to have got right (from my understanding of aerodynamics) is the pronounced yaw-roll coupling you'd expect to find with a swept-back wing: any sideslip produces a strong roll.

 

Incidentally, sideslipping is very useful if you need to dump energy: as Mary Poppins almost sung, a boot full of rudder helps the Messerschmitt slow down. :biggrin:

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1 hour ago, Bilbo_Baggins said:

 

I'm interested to learn why the ME262 rudder is/was so effective.

 

Developer comment in FM forum that "Plane has low stability in rudder channel because of swept wing design, not a bug)"

 

 

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1 hour ago, CUJO_1970 said:

 

Developer comment in FM forum that "Plane has low stability in rudder channel because of swept wing design, not a bug)"

 

 

 

Thank you, I missed that Dev comment. 

 

What is it about the swept wing that gives it such effective rudder? 

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@CUJO_1970: I saw that, though I'm not entirely clear what it was supposed to mean. All other things being equal, I'd expect sweepback to increase stability in yaw, but as I said earlier, it may also cause strong yaw-roll coupling. Which can sometimes in turn result in Dutch roll, itself a form of instability, though I've not seen this in the IL-2 GB Me 262, or seen it in any flight test reports for the real thing. There are other factors to consider though, like the potential blanking of the vertical tail surfaces by the engine pods at large sideslip angles, so it would be unwise to make any generalisations about what we should expect the Me 262 to do.

 

In practice, it rarely seems necessary to me to use the rudder in flight anyway, as long as both engines are running...

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Posted (edited)

Its a characteristic of a swept wing.  Very basically, the rudder input pushes one wing forward into the path of flight.  Maximum lift on a straight wing, as the other wing is swept further back lift is lowered.  If you increase lift on one wing and lower it on the other, you get a roll moment.

Edited by [CPT]Crunch
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3 hours ago, Feathered_IV said:

Although being a jet, I don’t know how much the 262 would be subjected to prop wash.

 

Zero ;)

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3 hours ago, [CPT]Crunch said:

Its a characteristic of a swept wing.  Very basically, the rudder input pushes one wing forward into the path of flight.  Maximum lift on a straight wing, as the other wing is swept further back lift is lowered.  If you increase lift on one wing and lower it on the other, you get a roll moment.

 

Great explanation, thanks. 

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Maybe not relevant. But a RAF pilot was selected to fly ww2 planes and commented that it was strange flying actively with the rudders. 

I do not know for sure , but to me it sounded like

you do not use rudders that much flying jet. 

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Posted (edited)
4 hours ago, [CPT]Crunch said:

Its a characteristic of a swept wing.  Very basically, the rudder input pushes one wing forward into the path of flight.  Maximum lift on a straight wing, as the other wing is swept further back lift is lowered.  If you increase lift on one wing and lower it on the other, you get a roll moment.

 

This: at base, the ‘straightening’ of outside wing increasing lift relative to the inside wing which is effectively further ‘swept’, this causing positive roll in direction of the yaw.

Edited by EAF19_Marsh

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9 minutes ago, EAF19_Marsh said:

 

This: at base, the ‘straightening’ of outside wing increasing lift relative to the inside wing which is effectively further ‘swept’, this causing positive roll in direction of the yaw.

 

It also moves the distribution of lift outwards for the forward wing, and inwards for the backward one, meaning that the forward one has more leverage.

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1 hour ago, AndyJWest said:

 

It also moves the distribution of lift outwards for the forward wing, and inwards for the backward one, meaning that the forward one has more leverage.

 

Thanks. Also presume that unlike the earlier aircraft, over-enthusiastic rudder use causes stall from the tip and pitch-up owing to C/L vs CoG issues?

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9 minutes ago, EAF19_Marsh said:

 

Thanks. Also presume that unlike the earlier aircraft, over-enthusiastic rudder use causes stall from the tip and pitch-up owing to C/L vs CoG issues?

 

I'm not sure that any such effect would be any worse than what you'd see from yaw in a straight wing with significant dihedral. It depends on more than just wing planform, and twist (washout) plus the blanking effects of the engine nacelles I referred to earlier are all going to affect the way the Me 262 responds to yaw inputs. In a full-rudder sideslip the IL-2 GB Me 262 certainly feels unstable, though that may partly be down to it being easier to put into an extreme sideslip than other aircraft modelled. What it doesn't have (or not enough to be noticeable) is the gyroscopic precession and p-factor effects you get with a propeller, and of course the lack of gyroscopic input from a propeller makes it easier to yaw (and pitch) in the first place. If you want to see the other extreme, try sideslipping the Fokker Dr. 1. Not directionally stable in the first place, and with extreme gyroscopic precession. Apply full rudder one way (can't remember which) and it flicks straight into a spin. Makes a full-rudder sideslip in the 262 seem positively benign.

 

Actually, thinking about it some more, my statement about the distribution of lift moving outwards on the 'forward' wing in a sideslip may not be entirely true - one of the side-effects of the spanwise flow that sweepback causes is that the lift distribution moves outwards. 

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3 minutes ago, AndyJWest said:

Makes a full-rudder sideslip in the 262 seem positively benign.

 

Actually, thinking about it some more, my statement about the distribution of lift moving outwards on the 'forward' wing in a sideslip may not be entirely true - one of the side-effects of the spanwise flow that sweepback causes is that the lift distribution moves outwards.

 

Except is that not particular to configuration of rotary-engined multi-plane aircraft?

 

I would have thought that the rudder effect on a 262 might move lift inwards, so as AoA increases the CoL progresses forwards but you get a tip-stall first, potentially rendering less effective  the ailerons and tipping you over.

 

But I have no degree in this stuff, just layman undertaking 😎.

Sorry, not 'move lift inwards' , but focus lift relatively on the forward element of the wing between the fuselage and nacelles (ie the less high-aspect ration part)?

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I'm a layman too, just going off what I've read, and from what little physics I can remember. Not that I ever covered anything as complex as fluid dynamics. With the Dr.1, we at least have Chill31's input on his replica (apparently to be fitted with a rotary engine at some point), maybe we can persuade one of the wealthier forum members to build themselves a Schwalbe, and tell us how it responds to full rudder. Or maybe not, considering how similar input has pulled the tail off more than one aircraft, with fatal results. I sometimes think that we may be worrying too much about how the IL-2 GB aircraft respond to inputs that no sane pilot would put them through in the first place. 

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I assume that the turbines both turn the same way on each engine, 

 

How heavy a rotating  mass is there on these very early jet engines and would the high rpm produce any gyro force? 

 

A bit stupid/off topic but I was just wondering  

 

Cheers, Dakpilot 

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8 hours ago, [CPT]Crunch said:

Its a characteristic of a swept wing.  Very basically, the rudder input pushes one wing forward into the path of flight.  Maximum lift on a straight wing, as the other wing is swept further back lift is lowered.  If you increase lift on one wing and lower it on the other, you get a roll moment.

 

 

^This right here. Short and simple.

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8 minutes ago, Dakpilot said:

I assume that the turbines both turn the same way on each engine, 

 

How heavy a rotating  mass is there on these very early jet engines and would the high rpm produce any gyro force? 

 

A bit stupid/off topic but I was just wondering  

 

Cheers, Dakpilot 

 

See this discussion here:

https://aviation.stackexchange.com/questions/49499/why-dont-single-engine-jets-suffer-severe-gyroscopic-effects

 

In summary, jet engines do produce significant gyroscopic forces - but they are relatively small in comparison to other forces encountered by the aircraft.

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11 hours ago, F/JG300_Faucon said:

 

Zero ;)

Unless attacking from the rear =D

 

I find yaw on the 262 pretty useless for target correction at the speeds it flies, outside of wild 'shotgun blasts' though with the 30mm any single tag and the target is out.  Mostly it seems better to try to use as little rudder input as possible and fly into the target on two axes.

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On planes with swept wings, a sideslip will increase the span of the wing that is being pushed forward relative to the air flow. Accordingly, the opposite wing will loose some span. The asymmetric span leads to asymmetric lift leads to roll.

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10 hours ago, LuseKofte said:

you do not use rudders that much flying jet. 

 

Depends on the jet really.

Some jets are rolled with rudder ONLY above a certain AoA.

 

The F-4 and F-100 do come to mind.

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