Adverse Yaw

[FLS]

Adverse yaw was discussed in another thread but there was a lot of confusion in the conversation so I want to clarify a few things.

Reviewing the basics, yaw refers to the aircraft nose moving sideways, this is controlled by the rudder. If you start to turn left and the aircraft nose moves to the right this is adverse, meaning acting against or in a contrary direction. So adverse yaw is yaw that is opposite your intended direction. You correct adverse yaw in a left turn with left rudder. This is referred to as a coordinated turn because the aircraft nose is now following the flight path.

To understand what happens in a turn it's useful to split the turn into 3 parts. The roll into the turn where you set your bank angle, the steady state turn itself, and the roll out of the turn. The roll into the turn can also be considered in 3 parts. The beginning of the roll, the steady state roll, and the end of the roll when you reach the desired bank angle.

When you start to roll the rising wing has more lift and hence more drag. The steady state roll has equal lift on the rising and descending wing but the lift vectors are twisted to different angles so the rising wing is lifted slightly back while the descending wing is lifted slightly forward. This is where you see the most adverse yaw, rolling into or out of the turn.

Once the bank angle is established the middle part of the turn has very little adverse yaw. Depending on the bank angle, the direction of turn, and the aircraft type, you might need left rudder, centered rudder, or right rudder to coordinate a left turn while in the middle, steady state, portion of the turn. So you push left rudder to coordinate a left roll and right rudder to coordinate a right roll but the turn itself, where you maintain the bank angle, has less adverse yaw and requires less correction.

The FAA doesn't go into this much detail in their basic flight information or test. Their position is that a simplified explanation still has the pilot doing the correct action. This doesn't mean that the FAA is unaware of twisted lift.

The book Aerodynamics For Naval Aviators states that the rising wing in a roll has more lift and hence more drag. This is also simplied. It doesn't mean the USN is unaware of twisted lift.

Simplistic explanations cause confusion when taken out of context. It can seem like people are disagreeing because they are talking about different parts of the turn or are using simplified explanations for complex phenomena.

[Sunka]

Nice read FLS ty.
Always nice to see wright up's from "real" trainers on the training  and help section.

[FLS]

Thanks Sunka. We are blessed with a very knowledgeable community. I have learned a lot here.

[Blagard]

It may be worth adding that adverse yaw is not always present so not all aircraft need rudder correction. Adverse yaw issues are sometimes corrected by wing design changes such as differential aileron movement, fries ailerons etc. So either eliminating or reducing the need for rudder correction.

[FLS]

It may be worth adding that adverse yaw is not always present so not all aircraft need rudder correction. Adverse yaw issues are sometimes corrected by wing design changes such as differential aileron movement, fries ailerons etc. So either eliminating or reducing the need for rudder correction.

How do you correct twisted lift or are you ignoring that? I already stated that adverse yaw might not be present after you set the bank angle.

[Blagard]

How do you correct twisted lift or are you ignoring that? I already stated that adverse yaw might not be present after you set the bank angle.

I think you are reading into my post something that is simply not there! The wing design changes I was referring to mainly balance out adverse yaw by inducing drag to the wing with the aileron in the up position when actually rolling the aircraft, not when roll input is removed. Drag will also counter the yaw due to the twisted lift vectors. But if you want to be picky I don't agree that,

The steady state roll has equal lift on the rising and descending wing

, unless the wing section is symmetrical.

[FLS]

I think you are reading into my post something that is simply not there! The wing design changes I was referring to mainly balance out adverse yaw by inducing drag to the wing with the aileron in the up position when actually rolling the aircraft, not when roll input is removed. Drag will also counter the yaw due to the twisted lift vectors. But if you want to be picky I don't agree that, , unless the wing section is symmetrical.

Thank you. That's what I was asking. It wasn't clear if you were referring to the roll. You can be picky, I should have said the difference is minimized.

[mtnman]

The steady state roll has equal lift on the rising and descending wing

I'm wondering about this too.  It wouldn't seem true if there was any dihedral involved?

I'm also wondering about the aileron deflection involved, and how it affects lift as compared between the two wings?  With one aileron deflected upward, and the other downward, can both wings be producing equal lift?

I'm looking at this from a slightly different perspective because I've been playing with these forces on a project I'm working on.  I'm building a radio-controlled glider based on a redtail hawk, and am configuring it to fly without a vertical stabilizer.  Yaw, adverse yaw, and roll are proving to be challenging, but I do think I have some workable options coming together.

[FLS]

As Blagard pointed out it's not really equal, my point was that adverse yaw at that point is more from twisted lift then from induced drag from unequal lift. If the lift is not close to equal the roll rate would be increasing or decreasing and not steady.

Project sounds cool. Reimar Horten had a theory that a bell shaped lift distribution, with negative lift toward the wing tips, would solve the adverse yaw problem with the flying wing design. Karl Nickel, who helped Horten with calculations in the 40's, flew Hortens gliders and said there was still plenty of adverse yaw. According to Nickel, model builders have used the theory for tailless aircraft but it actually increases induced drag and is not as efficient as an elliptical lift distribution. Nickel wrote a book with Michael Wohlfahrt, "Tailless Aircraft in Theory and Practice" which you might find useful. It was published in German, then translated into English by Capt Eric Brown who added so much to the book that he got co-author credit. It's published in the US by the American Institute for Aeronautics and Astronautics.

[Traveler]

Interesting side note, in real world flight training the FAA requires the Flight Instructor to use the term "Adverse Aileron Yaw" with a primary flight student. While other means of rolling an aircraft may be discussed the FAA wants the primary student to understand the role the aileron plays in adverse yaw during a turn and the relationship the rudder plays correcting it.  The first time I heard an instructor use the term "Adverse Yaw" was during orientation at Eastern on the 727-200. 

[Mace2004]

I'm looking at this from a slightly different perspective because I've been playing with these forces on a project I'm working on.  I'm building a radio-controlled glider based on a redtail hawk, and am configuring it to fly without a vertical stabilizer.  Yaw, adverse yaw, and roll are proving to be challenging, but I do think I have some workable options coming together.

You should consider using differential spoilers (also called spoilerons) or split ailerons (although these would be much more complicated to produce and control).  Differential spoilers induce roll by reducing lift on one wing and, by their fundamental nature, they also create profile drag and proverse yaw (yaw into the turn as opposed to adverse yaw which is against the turn).  The proverse yaw also leverages dihedral effect for roll without having to use a rudder to induce the yaw.  You will still see changes in the lift vectors for each wing because you can't change the fact that they will have different local AOA in a roll but since the natural action is proverse, spoilers would help manage the problem.  Spoilers do have a downside for gliders though in that they act by creating drag and killing lift on one wing without creating lift on the other wing. Also, because they just act on one wing, your potential roll rate may be lower (probably not an issue with a glider).  Just some trade-offs to consider.

BTW, it would be nice to see your design.

[mtnman]

You should consider using differential spoilers (also called spoilerons) or split ailerons (although these would be much more complicated to produce and control).  Differential spoilers induce roll by reducing lift on one wing and, by their fundamental nature, they also create profile drag and proverse yaw (yaw into the turn as opposed to adverse yaw which is against the turn).  The proverse yaw also leverages dihedral effect for roll without having to use a rudder to induce the yaw.  You will still see changes in the lift vectors for each wing because you can't change the fact that they will have different local AOA in a roll but since the natural action is proverse, spoilers would help manage the problem.  Spoilers do have a downside for gliders though in that they act by creating drag and killing lift on one wing without creating lift on the other wing. Also, because they just act on one wing, your potential roll rate may be lower (probably not an issue with a glider).  Just some trade-offs to consider.

BTW, it would be nice to see your design.

This is right about where I've settled too.  I've made a few test models with different wind and tail configurations, and have made some progress on full-scale drawings, but haven't begun building the final product yet.  Hopefully this winter...

At this point my intention is to build the "fuselage" with the ability to remove the tail and replace it with a different "flavor" or two.  The wings will also be removable, so that opens up the option to build different wings with different control surfaces as well.

I have the benefit of having access to "living models" and a photo-bug wife who can snap specific photos of a "to-order" basis, so that helps.  This makes it easy to get nice 3-views of my desired flight configuration, as well as set up opportunities for the birds to show the slight wing/tail changes they use to initiate turns.

My goal is to have a model that I can use for soaring (thermal and slope).  The slope option makes for easy launching, but I don't always have access to nearby slopes with favorable winds.  The thermal option is much easier for me day-to-day, but raises some concerns on launch (hi-start or winch) without a vertical stabilizer/rudder configuration of some sort.  Aileron-only control on a winch launch can be kind of sketchy.  I'm considering a launch "pod" with a rudder that would drop away as the winch line is released?  Not sure yet.

I have a few workable tail configurations; one option is more-or-less flat, but functioning as a type of v-tail (via angled hinge-lines) giving me a mix of rudder and elevator effect coupled with a computer radio.  Another option is a mostly flat tail that I can articulate up/down, left/right, and also rotate while making those other movements.  It's a challenge, but I've figured out (I think, lol) a ball-hinge coupling that will allow that.

A separate (but less appealing so far) idea is a mostly or completely stationary/static tail with some variety of pitcheron wings.  I'm less convinced this will work sans rudder/vertical stabilizer...

As far as the wings go, I'm definitely looking at inducing roll by reducing lift on one wing.  

One of the challenges I'm pursuing though is to remain as true as possible to "realistic" movement in regards to control surfaces.  I've looked at the F14 differential spoilers, and that would work apart from them being so completely "wrong" when it comes to realism...  Right now I'm strongly considering making the outboard sections of the wing (outer 1/2 of each wing) basically a giant aileron, or full-flying surface.  In general, I'm looking to reduce the lift on one wing while leaving the other wing neutral, similar to what my friends eagle is doing below...  If you look at where the arrows are pointing you'll see that the right outer "panel" is deflected upwards while the left is slightly down or close to neutral.  I'm not sure of the exact degree yet, but I'm anticipating doing this by programming the differential on my radio.



One concern I have is that by reducing lift on one wing, I'll also induce reduced drag on that same wing, and end up with some variety of induced yaw...  I'm considering using the inboard panels as "flaps" to induce drag on the side with reduced lift?  Essentially, the right aileron going up would be balanced by the right flap going down.  And again, I might make the flap extremely large in cord?  Another option would be to place spoilers where the median or greater secondary coverts lie, as on my birds it's common to see these feathers raise up as drag increases.  This would serve to create extra drag on the wing I want to drop.

As you can see in the photo below (one of my redtails) the bird is initiating a bit of a left turn correction (probably due to a bit of crosswind).  She has her tail deflected for a left bank, has the trailing edges of her left wing lowered to create drag, and her left wing is "pulled in" (reducing her span) to reduce lift.  The outer section of her left wing is also at a reduced AOA (very similar to what the eagle is doing in the photo above).  I can't do all of that, of course, but the goal is to get as close as possible...



I've built some hand-toss models with various hinged wing and tail configurations and have been able to get the models stable, predictable, and adjustable in pitch and roll.  Essentially, adjusting the model for a roll will cause the model to spiral in, but I can see that I've achieved the desired result...  I can also get the models to nose down so hard they "tuck", or nose up to the edge of stall where they just wallow and settle.

I've also played with adding some weight to one wing, which causes a steady bank and curving flight path.  With the weighted wing, I'm able to adjust the control surfaces on the opposite wing to get the model to glide straight or even bank/curve away from the weighted wing.  Promising, so far...

[Mace2004]

What an awesome project!  Have you considered taking a page from the Wright's book and trying wing warping?  They stole it from the birds themselves.

[FLS]

Nice pics Mtnman. In the book I mentioned Nickel says little models, even paper airplanes, are the best way to test design ideas. He also says that birds don't use their tails to fly except as drag devices to land. He has a paper on proper location of the CG in a flying wing at the Nurflugal site in the papers section if you're interested. http://www.nurflugel.com/Nurflugel/nurflugel.html

[mtnman]

Nice pics Mtnman. In the book I mentioned Nickel says little models, even paper airplanes, are the best way to test design ideas. He also says that birds don't use their tails to fly except as drag devices to land. He has a paper on proper location of the CG in a flying wing at the Nurflugal site in the papers section if you're interested. http://www.nurflugel.com/Nurflugel/nurflugel.html

Thanks for the link FLS, I've been looking it over.  I'm not sure yet how the cg will compare with his, as the sweep is completely different?

He's incorrect about birds not using their tails except for landing.  The tail is obviously used for braking, as he describes, but it's also used for steering (particularly by those species that live/hunt in forests) and for pitch stabilization.  As more flight agility is required, tails generally increase in size or length.  Soaring birds actually fan their tails for added lift.  Birds use their tails as an added "wing" for take-off too; if you watch slow motion footage of moderate to large-tailed birds you'll see that they pump their tails up and down in concert with their wings; essentially using the tail as a "third wing" for additional lift.  They'll close their tails, as well as allow air to spill between their tail feathers for reduced drag on the up beat, just as they do with the flight feathers on their wings.

There is a fantastic slow-motion segment showing this with a peregrine falcon in the "X-Ray Yellowstone" show on the Discovery Channel, but I cannot locate it for free viewing on the internet.  However, this eagle is displaying that same motion at around the 4:30 mark in this clip-

http://www.youtube.com/watch?v=aM0JMoGABgk#t=299
 
As complex as they are, and with their ability to morph their body, wing and tail shape for different conditions, I've basically chosen to copy one particular planform (for soaring).  The video below is very interesting, informative, and worth watching if you have the time, but in particular at around the 17:40 mark they have a reverse-mounted backpack camera showing how the redtail is using it's tail as a control surface while gliding.  The tail is less spread than it would be for soaring, and the bird is gliding rather than soaring, but you can still see that there is much more use of the tail then described by nickel.

http://www.youtube.com/watch?v=CxgFdXir6Dg#t=2705


Sorry about hijacking your adverse yaw thread.  If this conversation goes much further I'll create another thread.  Mace, I'll definitely share my project as I get further along with it.  I have high hopes for the winter building season...