Greetings:

I've had this question for a number of years and it occurred to me that perhaps some of the folks that fly akro could shed some light on it:

We all know what adverse yaw is (the yawing tendency towards the downward deflecting aileron during a rolling maneuver). Rudder/roll coupling (which is a tendency for aircraft to roll towards the side where rudder is applied) has been attributed to the reduced lift experienced on the side being yawed towards due to reduced airflow (fuselage blanking, one wing moving slower than the other, etc.) causing the aircraft to roll in that direction.

Well, my initial thoughts are that the forces that produce adverse yaw should be similar to the application of rudder (mentally turn an airplane 90 degrees in bank and the rudder works in the directions of an aileron and vice versa). However, the effects of roll-coupling seem not to fit adverse yaw model. Instead of a pitch up when rudder is applied in this attitude as what should happen using the adverse yaw model (it helps to draw a diagram to picture this), we experience a rolling motion when the aircraft is upright.

Of course I have my theories about why this happens, but I was wondering if anyone had some thoughts on the subject.

Thanks!

Ben

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• RE: Adverse yaw and roll coupling,Tarmack, 11:19z, 11-18-02
  • RE: Adverse yaw and roll coupling,Ben_Chiu, 17:42z, 11-18-02
    • RE: Adverse yaw and roll coupling,djpacro, 00:29z, 11-19-02
      • RE: Adverse yaw and roll coupling,Ben_Chiu, 23:35z, 11-19-02
    • RE: Adverse yaw and roll coupling,Mike_Greenwood, 04:49z, 11-19-02
      • RE: Adverse yaw and roll coupling,DHamblin, 12:39z, 11-19-02
      • RE: Adverse yaw and roll coupling,DHamblin, 12:46z, 11-19-02
        • RE: Adverse yaw and roll coupling,Ben_Chiu, 23:36z, 11-19-02
      • RE: Adverse yaw and roll coupling,Ben_Chiu, 23:35z, 11-19-02
        • RE: Adverse yaw and roll coupling,djpacro, 02:26z, 11-25-02
          • RE: Adverse yaw and roll coupling,Ben_Chiu, 22:37z, 11-25-02
• RE: Adverse yaw and roll coupling,DHamblin, 12:32z, 11-18-02
  • RE: Adverse yaw and roll coupling,Ben_Chiu, 17:43z, 11-18-02
    • RE: Adverse yaw and roll coupling,DHamblin, 17:55z, 11-18-02
      • RE: Adverse yaw and roll coupling,Mike_Greenwood, 06:25z, 11-19-02
        • RE: Adverse yaw and roll coupling,AlanParkinson, 21:37z, 11-22-02
          • RE: Adverse yaw and roll coupling,Mike_Greenwood, 19:36z, 11-23-02
      • RE: Adverse yaw and roll coupling,Ben_Chiu, 23:36z, 11-19-02
• RE: Adverse yaw and roll coupling,IFLYExtra300S, 13:15z, 11-18-02
  • RE: Adverse yaw and roll coupling,Ben_Chiu, 17:43z, 11-18-02
    • RE: Adverse yaw and roll coupling,Tarmack, 22:56z, 11-18-02
      • RE: Adverse yaw and roll coupling,Ben_Chiu, 23:37z, 11-19-02
    • RE: Adverse yaw and roll coupling,ghmorris, 02:12z, 12-01-02
      • RE: Adverse yaw and roll coupling,ghmorris, 03:14z, 12-01-02
      • RE: Adverse yaw and roll coupling,Ben_Chiu, 23:37z, 12-01-02
        • RE: Adverse yaw and roll coupling,ghmorris, 01:55z, 12-02-02
          • RE: Adverse yaw and roll coupling,Ben_Chiu, 19:51z, 12-02-02
            • RE: Adverse yaw and roll coupling,ghmorris, 21:07z, 12-02-02

Ben,

Just came across your question and want to be sure that I understand completely just what you are asking.

Are you only questioning the behavior of the airplane in knife edge flight and/or are you asking about the behavior in inverted flight, or during a complete 360 degree roll?

I have limited experience in any of those categories. Grin.

However, I have a son that is flying the heck out of his Extra 300 and I could use his judgement and info to try and produce an answer to your question.......by being sure just what you are asking!

Regards,

Cap'n Tarmack

Greetings Mel:

My apologies for the muddied question.

> Are you only questioning the behavior of the airplane in knife edge
> flight and/or are you asking about the behavior in inverted flight, or
> during a complete 360 degree roll?

My question is given the reaction of adverse yaw with the application of aileron, why doesn't an airplane react in a similar way when rudder is applied (with a pitch up)?

> I have limited experience in any of those categories. Grin.

Rgr. I wouldn't expect you'd do much knife edge stuff in a 747 without getting some folks at Boeing upset!

> However, I have a son that is flying the heck out of his Extra 300 and I
> could use his judgement and info to try and produce an answer to your
> question.......by being sure just what you are asking!

Rgr, I saw his reply. Thanks!

Ben

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This is standard engineering textbook stuff. Main parameters are:
1. The wing is lifting - so, the effect of the aileron is magnified and varied by the basic wing lift carried. As some-one else mentioned the geometry is also different.
2. The wing is symmetrical left and right. The centre of pressure of the rudder is above the cg. The tail is affected by local flow conditions - sideslip angle, roll rate and yaw rate.
I suggest that you grab one of the basic textbooks, Ben, or even skim through this website:
http://www.monmouth.com/~jsd/how/

REGARDS,
Dave Pilkington
_______________
>My apologies for the muddied question.
>
>My question is given the reaction of adverse yaw with the
>application of aileron, why doesn't an airplane react in a
>similar way when rudder is applied (with a pitch up)?
>
>Ben

Greetings Dave:

Thanks for the site URL. I'll have to look around there more when I have some time. I'm not sure yet what I'm asking for is covered there.

Ben

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Hi Ben,

>>My question is given the reaction of adverse yaw with the application of
aileron, why doesn't an airplane react in a similar way when rudder is
applied (with a pitch up)?<<

This is from a non-pilot as you know, so bear with me <g>. My understanding is that the adverse yaw condition is caused (assuming a left bank) by the right wing generating more lift (aileron down) hence more induced drag than the left wing. That induced drag makes the plane yaw to the right....correct?

Now, in a knife edge configuration, the rudder acts alone to pitch the airplane "up." Since it's a purely vertical move, I wouldn't expect yaw (along the defined "pitch" axis) at all. Similar to moving the elevator in level flight to pitch up. However, I *would* expect a roll tendancy to the upper side of the low wing. In other words if you were in knife edge flight with the left wing pointed to terra firma, and pitched up with rudder, I'd expect the airplane to tend to roll to the right due to the increased airflow over the lower wing.

Just my common sense thoughts...

--Greenie
**6 miles SSE KSJC**

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Actually, it has to do with the fact the (in a left turn) right aileron is down farther into the windstream, causing more drag than the upward deflected left aileron.

One way to combat this (full scale or RC model) is to have more upward throw on the up aileron and less downward throw on the down aileron.

The shape of the airfoil also is a factor, which is why symetrical airfoils don't suffer as much and more traditional flatbottom airfoils suffer more.

Dave Hamblin
(formerly DaveH on old site!)

Another thought, most airplanes DO have a tendancy to pitch the nose up or down (in relation to the plane, not the ground in knife edge) with large rudder inputs.

Again, this is affected by the placement and relationship to the rudder and horizontal stab/elevator and is likely due to some aerodynamic flow changes over the stab when the rudder is deflected.

Most all RC airplanes I have flown will also display this. Most seem to slightly pitch the nose down (one would do so violently, even in level flight, not a good thing close to ground!).

In fullscale you would likely have to be paying real close attention to notice whether you have some elevator pressure in to counteract it.

Dave Hamblin
(formerly DaveH on old site!)

Greetings Dave:

> Another thought, most airplanes DO have a tendancy to pitch the nose up
> or down (in relation to the plane, not the ground in knife edge) with
> large rudder inputs.

I suppose with some CG locations or flight configuration you might pitch up, but I think most may pitch down as a result of roll-coupling; in a bank, you reduce your vertical component of lift. Or are you talking about immediate pitch down or pitch up with rudder introduction?

> Again, this is affected by the placement and relationship to the rudder
> and horizontal stab/elevator and is likely due to some aerodynamic flow
> changes over the stab when the rudder is deflected.

That's what I was thinking as well. Roll-coupling has a great deal to do with higher pressure on the horizontal stab when the air molecules start bunching up with rudder application. Of course, but it's not completely responsible for roll-coupling because dihedral and vertical CG relationships play into roll stability as well, but I agree it's definitely a factor.

Well, to further the question, does the tail configuration have any bearing on whether there's any pitch up or down change with rudder application? Anyone fly an airplane with elevons not located near the rudder?

> Most all RC airplanes I have flown will also display this. Most seem to
> slightly pitch the nose down (one would do so violently, even in level
> flight, not a good thing close to ground!).

Agreed!

> In fullscale you would likely have to be paying real close attention to
> notice whether you have some elevator pressure in to counteract it.

Yep. Was hoping someone had noticed and noted either.

Ben

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Greeting Mike:

> This is from a non-pilot as you know, so bear with me <g>.

No worries. There are plenty of pilots out there that don't understand as much aerodynamics as perhaps they should.

> My
> understanding is that the adverse yaw condition is caused (assuming a
> left bank) by the right wing generating more lift (aileron down) hence
> more induced drag than the left wing. That induced drag makes the plane
> yaw to the right....correct?

Correct.

> Now, in a knife edge configuration, the rudder acts alone to pitch the
> airplane "up." Since it's a purely vertical move, I wouldn't expect yaw
> (along the defined "pitch" axis) at all. Similar to moving the elevator
> in level flight to pitch up. However, I *would* expect a roll tendancy
> to the upper side of the low wing. In other words if you were in knife
> edge flight with the left wing pointed to terra firma, and pitched up
> with rudder, I'd expect the airplane to tend to roll to the right due to
> the increased airflow over the lower wing.

I would agree with that. This is roll-coupling. If the rudder did truly act as an elevator (discounting left-turning tendencies in single-engine props), you wouldn't get that roll to the right.

I just used the knife edge example to place the rudder in a "position" where it's similarity to an aileron could be more easily pictured in the mind's eye. Let's try this. What happens if an airplane is in a dive and rudder is applied? Does roll-coupling increase, decrease, or remain the same (assuming the same airspeed)? And to get to my question, is there any pitch (rotation along the horizontal axis parallel with the wing) change when rudder is applied? Based on the adverse yaw model, there should be a pitch change, but is there?

I suppose we need to define a couple of things here: Adverse yaw is a rotation/movement along a perpendicular axis (vertical axis--yaw) from the originating aircraft movement (longitudinal axis--roll). Given this relationship, I would expect that if the rudder was placed in the same orientation, it would have a similar reaction (rotation on a perpendicular axis). Well, we see a reaction that fits this perpendicular axis, but it's along the longitudinal/roll axis, but it's not what you'd expect. Enter the knife edge flight example. If you applied upward or downward rudder, shouldn't induced drag affect pitch (as in pitch along the horizontal axis) the same way that induced drag creates adverse yaw? To my thinking it should (but the roll-coupling issues leads me away from this), but I have never had the opportunity to test it and see for myself. This is why I was hoping one of the folks that flies akro could tell me.

> Just my common sense thoughts...

And good ones!

Ben

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Ben,
You seem intent in going into this subject in more detail than I had anticipated.

My favourite report on aileron characteristics is online:
NACA TR 868, Summary of Lateral Control Research
http://naca.larc.nasa.gov/reports/1947/naca-report-868/

The most relevant to your question is, however,
NASA TN D-6946
Lateral-Directional Aerodynamic Characteristics of Light, Twin-Engine, Propeller-Driven Airplanes.
Its a cookbook of how to estimate the stability and control derivatives.
I wasn't able to find it quickly online - it may be there at
http://techreports.larc.nasa.gov/cgi-bin/NTRS
or
http://www.sti.nasa.gov/RECONselect.html

I have these on a CD - the NASA Legacy collection, from the Langley Research Centre. For some reason this did not include my other favourite report:
Riding and handling qualities of light aircraft: A review and analysis, NASA-CR-1975, Mar 01, 1972

The next step is the effect of rudder, sideslip and roll/yaw rate on pitching - most reports and textbooks ignore the cross-derivatives. ie they separate life into either lateral-directional or longitudinal characteristics.
This was the cause of an in-flight tail failure on a Tucano - during flight testing of rudder deflection and sideslips at high speed the downward pitch resulted in overloading the horizontal stabilizer. You often see tailplane inboard leading edge strakes on single engine turboprops to reduce the asymmetric effects on tailplane lift.

Then there's the gyroscopic effects of the propeller. eg applying left rudder briskly results in a pitch up.

From here - next time you're in Australia this is a good topic for discussion over a bottle of Long Flat Red.
Or, online real time discusison sometime.

Regards,
Dave Pilkington

>
>No worries.
>I just used the knife edge example to place the rudder in
>a "position" where it's similarity to an aileron could be
>more easily pictured in the mind's eye. Let's try this.
>What happens if an airplane is in a dive and rudder is
>applied? Does roll-coupling increase, decrease, or remain
>the same (assuming the same airspeed)? And to get to my
>question, is there any pitch (rotation along the
>horizontal axis parallel with the wing) change when rudder
>is applied? Based on the adverse yaw model, there should
>be a pitch change, but is there?
>Ben

Greetings Dave:

Thanks for the links. It seems I have a lot of reading to do!

> The most relevant to your question is, however,
> NASA TN D-6946
> Lateral-Directional Aerodynamic Characteristics of Light, Twin-Engine,
> Propeller-Driven Airplanes.
> Its a cookbook of how to estimate the stability and control derivatives.
> I wasn't able to find it quickly online - it may be there at
> http://techreports.larc.nasa.gov/cgi-bin/NTRS
> or
> http://www.sti.nasa.gov/RECONselect.html
>
> I have these on a CD - the NASA Legacy collection, from the Langley
> Research Centre.

I'll have a look for it. If I can't find it, and if it's not too large, perhaps you could email the report to me.

> The next step is the effect of rudder, sideslip and roll/yaw rate on
> pitching - most reports and textbooks ignore the cross-derivatives. ie
> they separate life into either lateral-directional or longitudinal
> characteristics.

That's what I've found as well (although I've only searched as far as the aviation books and aerodynamics texts found in pilot shops and local libraries). There are many factors involved and I suspect that due to these complexities (and relative low interest level from most pilots) have contributed to the lack of available text on the topic specifically.

> This was the cause of an in-flight tail failure on a Tucano - during
> flight testing of rudder deflection and sideslips at high speed the
> downward pitch resulted in overloading the horizontal stabilizer.

I never heard of this. It looks like a standard tail configuration and aileron setup. Interesting that there's a downward pitch in this case... I suspect that effects seen would vary based on the bank angle and angle of attack at the time of rudder input.

> You
> often see tailplane inboard leading edge strakes on single engine
> turboprops to reduce the asymmetric effects on tailplane lift.

> Then there's the gyroscopic effects of the propeller. eg applying left
> rudder briskly results in a pitch up.

Rgr. I think this can be eliminated (effects reduced) from our discussion if we apply rudder slowly.
My thoughts are that reasons why we see roll-coupling rather than pitch changes is due to four factors. Although please note that without some input from real world or model experience, this is all purely theoretical.

1. The proximity of the horizontal stabilizer causing a build up of pressure on the side that the rudder is applied. This increasing downward pressure on the half of the horizontal stabilizer causes the aircraft to roll. So, it's my belief that the placement of the horizontal stabilizer will change roll coupling tendencies (reduce or increase, but not eliminate).

2. The airfoil shape and AOA. Both are interrelated in level flight (e.g. a symmetrical airfoil requires a positive AOA to to to to fly level). The higher the AOA at the application of rudder, the more likely there'll be a pitch change.

3. The roll stability of the aircraft. I believe as Dave H pointed out that dihedral and other roll stabilization designs will affect the tendency to roll with the application of rudder.

4. Related to #2, the relative direction of gravity. Which goes back to my diving towards the ground scenario. I think that if lift is not being generate, we would see a adverse yaw type situation with the application of rudder, but that's just a guess. Anyone willing to test out my theory? I think it would be possible to test this in knife edge flight as well (the application of top and bottom rudder should produce pitch changes and less roll-coupling as compared to level flight).

Again, there are all just guesses. If anyone has thoughts contrary to mine, please speak up.

> From here - next time you're in Australia this is a good topic for
> discussion over a bottle of Long Flat Red.
> Or, online real time discusison sometime.

I haven't made over to Australia yet, but it's one of few places that I've have on my "someday" list. I would greatly enjoy such a discussion in person. However, it may be quite a long time before that might happen, so an online discussion (we do have regularly scheduled chats here at the VPC) or continuation of this thread is most welcome.

Best!

Ben

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The same forces that do create adverse yam likely do exist on the rudder as well. However, the vertical stab(rudder) is not as long as the wing, so the moment arm cannot exert the same force on the plane as the longer moment arm wing/aileron can. If you extended the vertical fin to a length similar to the wing, applying rudder would likely cause a pitch up.

The Luscombe I got my first 14 hours on (1946 8A) has an "impressive" amount of adverse yaw; a great tool to learn about it whereas our C150H has very little (I like to fly the Luscombe every so often to prevent lazy feet).

In my RC model experience, I've seen that roll coupling on the models is tied also into the airfoil shape and dihedral of the wing. "Flat bottom" wings tend to roll more than symetrical with rudder. My sons RC trainer can be almost rolled with the rudder (large wingspan, clark "Y" airfoil etc) where as my RC Extra rolls very little with rudder (symetrical airfoil etc).

So my two cents are the forces are there, but not in an amount you can percieve.

Now to really muck up the discussion; I have had several RC models that would either pitch up (or down) violently with full rudder applications. In reseraching this, it has to do with the placement of the horizontal stab, but that gets into another complicated subject!!!!

Dave Hamblin
(formerly DaveH on old site!)

Greetings Dave:

> The same forces that do create adverse yam likely do exist on the
> rudder as well. However, the vertical stab(rudder) is not as long as the
> wing, so the moment arm cannot exert the same force on the plane as the
> longer moment arm wing/aileron can. If you extended the vertical fin to
> a length similar to the wing, applying rudder would likely cause a pitch
> up.

OK, I'll buy that. (But I'd also counter that the length of the fuselage length should provide an "arm" to work with and the surface area of the rudder is comparable to that of the ailerons--at least in the aircraft that I fly day-to-day--and these factors should offset what you're saying.)

> The Luscombe I got my first 14 hours on (1946 8A) has an "impressive"
> amount of adverse yaw; a great tool to learn about it whereas our C150H
> has very little (I like to fly the Luscombe every so often to prevent
> lazy feet).

Very interesting. I have a friend, Jonathan Baron (Producer of the Air Warrior series of games), flies a Luscombe and he only has one leg!


> In my RC model experience, I've seen that roll coupling on the models is
> tied also into the airfoil shape and dihedral of the wing. "Flat bottom"
> wings tend to roll more than symetrical with rudder. My sons RC trainer
> can be almost rolled with the rudder (large wingspan, clark "Y" airfoil
> etc) where as my RC Extra rolls very little with rudder (symetrical
> airfoil etc).

Ah, yes. Very interesting! My thoughts were that airfoil shapes would make a difference (a vertical stabilizer is usually symmetrical, and they can be offset in singles to counter left-turning tendencies).

> So my two cents are the forces are there, but not in an amount you can
> percieve.
>
> Now to really muck up the discussion; I have had several RC models that
> would either pitch up (or down) violently with full rudder applications.
> In reseraching this, it has to do with the placement of the horizontal
> stab, but that gets into another complicated subject!!!!

Yes! That's what I was thinking as well. Would a T-tail have as much roll coupling? It's been a number of years since I've flown a T-tail, so I haven't been able to test this.

Thanks, Dave!

Ben

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My Extra300 RC model (sorry, can't afford a full scale one unless Patty or someone decides to donate me a used one) I can pretty much put full right or left rudder and it will just skid with a bit of roll coupling (which I programmed out on the computer radio I use). I had another plane which wasn't a scale model of a full size that when full rudder was applied it would tuck its nose under (pitch down) so fast it wasn't funny.

I'd have to actually lay it out but my gut level is that the fuselage doesn't really give the rudder much help (as far as its pitch moment) to the plane. The force the rudder puts on the fuselage is more a drag to the rear than a straight down force on the fuselage (its pulling backward on the rudder, not pushing straight down). Technically this puts a downward force but very little.

It would be interesting to see how you friend flies the Luscombe (does he move his foot from one rudder to the other??) In the air, move the stick to the left (or right) without any coordinating rudder and it really doesn't turn, just kinda skids along. In fact, you can use the rudder to bank into the turn and only use some back pressure on the stick (no aileron) and make some very nice turns. If you drop a wing in a stall you basically have to pick it up with rudder.

Not sure how a T tail reacts, haven't flown a full size or RC model of that variety.

Dave Hamblin
(formerly DaveH on old site!)

Hi Dave,

>>It would be interesting to see how you friend flies the Luscombe (does
he move his foot from one rudder to the other??) <<

If It were my airplane, I'd have a strap fitted to the pedal of my (only) foot. That way, I could push/pull for rudder input, much like many bicycle pedals. Braking would be a different story though.

--Greenie
**6 miles SSE KSJC**

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Hi Greenie,

My thoughts, too (on the one legged pedal).

This thread really fascinates me - I studied aerodynamics as part of a BSc in aero engineering many years ago - I will chip in with some possibly controversial statements shortly, but in the meantime I have a question for you specifically. I know you have the old FS Forum files library stored, but do you also have the forum messages archived? This question cropped up there some time ago and I calculated the relative sizes of "dihedral effect" and "one wing moving faster than the other" in a rudder-only turn, to try to demonstrate my point. If you still have that message it could be helpful.

I'm off to the gathering in Birmingham tomorrow with some of the guys, so I'll not be able to compile my views until Sunday.

Alan

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**** OLR.PL Build 1.73 ****

Hi Alan,

>>I will chip in with some
possibly controversial statements shortly<<

I look forward to it!

>>I know you have the old FS Forum files
library stored, but do you also have the forum messages archived?<<

Unfortunately, no. At least most aren't. I do have some purge files that cover a span of time, here and there, but unless you can remember approximately when that thread happened, it would be the "needle in a haystack" syndrome. I seem to remember that thread as well.

>>I'm off to the gathering in Birmingham tomorrow with some of the guys,
so I'll not be able to compile my views until Sunday.<<

Have fun, and give the gang my best!

--Greenie
**6 miles SSE KSJC**

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Greetings again, Dave:

> I'd have to actually lay it out but my gut level is that the fuselage
> doesn't really give the rudder much help (as far as its pitch moment) to
> the plane. The force the rudder puts on the fuselage is more a drag to
> the rear than a straight down force on the fuselage (its pulling
> backward on the rudder, not pushing straight down). Technically this
> puts a downward force but very little.

Well, I would think that since rudders are usually comparable in size to elevators and usually located in the same distance from the CG, it would be at least 1/2 or more as effective as the elevator.

> It would be interesting to see how you friend flies the Luscombe (does
> he move his foot from one rudder to the other??)

You know, it's been a while since I asked him about that (I've never flown with him) so I don't recall him saying exactly how he did it, but I do remember that he flew an unmodified Luscombe, so he must have been doing the rudder dance in double-time. I also remember that he had to do a demonstration of capability with an FAA inspector inspector (he passed on the third or fourth attempt). I don't know, I think it's wonderful that Jonathan was able to him a lot of credit for making it work, but I know folks that have problems flying with two feet and can only imagine what goes on in his cockpit when there's a gusty crosswind!

> In the air, move the
> stick to the left (or right) without any coordinating rudder and it
> really doesn't turn, just kinda skids along. In fact, you can use the
> rudder to bank into the turn and only use some back pressure on the
> stick (no aileron) and make some very nice turns. If you drop a wing in
> a stall you basically have to pick it up with rudder.

I guess that illustrations how effective the rudder really is. In fact I remember when that Airbus lost it's tail in NY, the FAA was quick to clarify that Va (maneuvering speed) does not include full rudder deflection!!

Ben

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Hello Ben,

As my old flight instructor would say;"Whatever it takes it takes" in other words how much rudder action is required on landing in a Pitts? "Whatever it takes"

I am not too clear on the question, But when sitting in the airplane, I roll it soo fast that the small amount of rudder that I use in a full deflection aileron roll at 400 degrees per second is so small it is almost a small pressure on the rudder (Of course this is in an Extra and a Citabria would be much different.

Basically for us "Redneck" pilots who sometimes fool Big corporations into hring us to fly Multi-million dollar jets....WHAT EVER IT TAKES IT TAKES to get a 10 on a fiqure in IAC competition or making sure the lady in 2B doesn't spill her coffee on a windy day landing in ORD.

Or you could say, Why ask Why?

Hope this helped a little...but do clear the air on the question?

Randy Ott (Son of Mel)
Extra 300S

Greetings Randy!

> I am not too clear on the question, But when sitting in the airplane, I
> roll it soo fast that the small amount of rudder that I use in a full
> deflection aileron roll at 400 degrees per second is so small it is
> almost a small pressure on the rudder (Of course this is in an Extra and
> a Citabria would be much different.

Understood. My question (as muddy as it was) was about why does the application of rudder cause the airplane to roll as opposed to pitch up (following the movement along the axes seen in adverse yaw). I was thinking that folks that fly knife edge (with symmetrical and unsymmetrical airfoils) could shed some light on what happens in pitch when top and bottom rudder is applied (mimicking aileron movement in normal level flight).

> Basically for us "Redneck" pilots who sometimes fool Big corporations
> into hring us to fly Multi-million dollar jets....WHAT EVER IT TAKES IT
> TAKES to get a 10 on a fiqure in IAC competition or making sure the lady
> in 2B doesn't spill her coffee on a windy day landing in ORD.

Good advice!

Thanks, Randy!

Ben

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Ben,

If you are in knife edge flight.......I would presume that kicking top or bottom rudder would more replicate the elevators, than the ailerons?

At that same point of flight(knife edge) would not the elevators then act as the rudder.....pulling would give you a left turn....pushing a right turn?

I can't figure out what you would use for ailerons in knife edge flght.......cept for me in my open cockpit biplane. Seems to me if I stuck out my tongue at just the right time...some sort of a bank angle might result. Grin.

Course, my son will tell me that I am wrong...but I had to jump in here before he did. Grin.

Regards,

Mel

Greetings Mel:

> If you are in knife edge flight.......I would presume that kicking top
> or bottom rudder would more replicate the elevators, than the ailerons?

Yes, but the induced drag and for the lack of a better term, "single-sided-ness" of its location (at least on aircraft that have rudders that only extend above or below the fuselage) should, based on our adverse yaw example, produce some pitch change (in relation to the aircraft).

> At that same point of flight(knife edge) would not the elevators then
> act as the rudder.....pulling would give you a left turn....pushing a
> right turn?

Correct again. Hmm, another question: In this situation does the elevator now produce roll-coupling when applied now that it acts as a rudder?

> I can't figure out what you would use for ailerons in knife edge
> flght.......cept for me in my open cockpit biplane. Seems to me if I
> stuck out my tongue at just the right time...some sort of a bank angle
> might result. Grin.

LOL! In a 152 you still need to open the doors or lean way over to create some roll without using the ailerons.

> Course, my son will tell me that I am wrong...but I had to jump in here
> before he did. Grin.

In insights have been time-stamped, noted, and as always greatly appreciated.

Ben

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The question seems to be (correct me if I've got it wrong), when one applies rudder in normal, upright flight, why doesn't the nose pitch up since when ailerons are deflected in normal, upright flight the lifting wing drags. Right?

Adverse yaw (or aileron drag) is caused because the wing is lifting, as the aileron is deflected down the chord line of that section of the wing changes, increasing the angle of attack, thus increasing the both the lift and induced drag of the wing. Since drag is increased on one side of the airplane, it yaws towards the lifting wing. The higher the angle of attack, the more pronounced the tendency for the lifting wing to drag.

In normal flight, the rudder is not lifting much, if at all (low or zero angle of attack). When the rudder if deflected, there is a force pushing the rudder to one side, but this force is ABOVE the longitudinal axis of the airplane, causing it to roll in the direction of the rudder deflection.

In knife edge, the rudder acts like the elevator, causing pitch up and down, however since it is asymmetric some aileron must be used to keep the airplane from rolling, since all the rudder's force is still not on the logitudinal axis.

-Greg Morris-
CFI Sunrise Aviation
http://www.sunrise-aviation.com/aerobatic.html

As I re-read my post, unfortunately after putting it up, I realized I had made a fundamental error in my diagram and explanation, the force arrow is drawn the wrong way for the tail, it instead represents the movement of the nose, thus my explanation for rudder-induced yaw is also incorrect. It is caused by the speed difference in the two wings.

-Greg-

Greeting Greg:

(We received your email about removing your previous message, but I see you replied already.)

Thanks for the insights. I'm not very clear on the knife edge comments. When you refer to pitch, it sounds like you're talking about "pitch" in relation to the horizon. My original question is whether rudder application affects pitch (on the aircraft's horizontal axis). It seems to me that rudder application should produce some pitch movement (if we follow what happens with aileron adverse yaw). However, I believe that lift generation and aircraft design have a much greater influence on this theoretical behavior. I suspect the only way to know for sure is to put an airplane in a dive straight down at a constant speed and apply rudder and note what happens.

Thanks!

Ben

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The adverse yaw portion of my first message is correct, the roll-yaw coupling portion isn't.

You are correct, as the rudder is deflected at knife edge it should change the amount of induced drag created by the surface, and thus cause the nose of the aircraft to move in pitch. However, this will be quite minimal, and usually in knife edge you have top rudder (if you are standing on your left wing, right rudder, the one on 'top' of the other) deflected, which is similar to the upwards deflected aileron.

If you want to find out, come out to LA and we'll give it a shot.

-Greg-

Greetings Greg:

> You are correct, as the rudder is deflected at knife edge it should
> change the amount of induced drag created by the surface, and thus cause
> the nose of the aircraft to move in pitch.

Aha! That's the bit of info I was looking for. Thanks!

> However, this will be quite
> minimal, and usually in knife edge you have top rudder (if you are
> standing on your left wing, right rudder, the one on 'top' of the other)
> deflected, which is similar to the upwards deflected aileron.

Rgr that. That's why I was thinking flying straight down at a constant speed would provide the best indication because the effects of gravity would be minimized.

> If you want to find out, come out to LA and we'll give it a shot.

I'm on the central coast (KSBP), so that's a possibility. Very tempting.

Thanks again!

Ben

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I'm based at KSNA with Sunrise Aviation (http://www.sunriseaviation.com).

-Greg-