Turning forces

[N422NM]

this question came from the DPE in the CFI oral today.

"What causes an aircraft to turn ?"

My answer "The airplane must be banked to turn which causes a change in the horizontal component of lift..."

He says "ok, I'll agree with that in that it initiates the turn, but then what else is required to continue turning ?"

I racked my brain and searched my PHAK, AFH etc...and couldn't come up with anything else substantial. He then told me something I've never heard...I can see the theory of it but had never heard it before....

"Once the bank is established, the aircraft weather vanes into the relative wind and this is what keeps the aircraft turning, in addition to the change in horizontal lift..."

Did I sleep through something at some point ?

[flyguy]

Hmmmm.......I've never heard that either. The way you explained it is that way I learned, and the way I teach, and all the examiner's I've done checkrides with have never had a problem with it.

I flew with an examiner that asked me "what makes an airplane fly" I said "money". (I heard from the gouge that this was the answer he was looking for). He laughed hysterically and I earned major brownie points. He naver did ask me to explain what really makes an airplane fly. A few checkrides later a different examiner asked me the same question and I gave the same answer. He wasn't impressed. Not even a smile. But I did well on the rest of the oral so it didn't hurt. [/HIJACK]

[flyguy]

By the way, how did the checkride go? You're lucky you got a DPE. It looks like the FAA is really cracking down on that and requiring CFIs go with the FSDOs.

[BobDDuck]

I don't buy that theory at all. The relative wind is going to be coming straight over the front end of the aircraft. The tail is attached to the nose in a straight line which means the plane isn't going to weathervane anywere. It's sort of one of those which came first the nose turning due to the HCL or the plane "weathervaning" due to the new direction of the relative wind.

[88MPH]

Originally Posted by N422NM

"What
"Once the bank is established, the aircraft weather vanes into the relative wind and this is what keeps the aircraft turning, in addition to the change in horizontal lift..."

I'd have asked him what causes a slip.

[fish314]

I don't think this is correct. If the aircraft is flying in a circle (just imagine continuing that turn all the way around at a constant bank), and it is coordinated, then shouldn't the longitudinal axis be tangent to that circle. And for any object travelling in a circle, the velocity vector (the opposite of relative wind) would also be tangent to the circle. In other words, there should be no yaw, and therefore nothing to weathervane into.

Imagine a jet flying in a circle (rather than a prop, so there is no p-factor or other induced yawing forces from the engines). The forces acting on the jet are Thrust and Drag, acting tangentially to the circle, Weight, acting down, perpindicular to the circle, and lift. Lift would be acting Up, opposite the weight, with it's vertical component, and in towards the center with it's horizontal component.

Basically this is just a longer drawn out way of saying the same thing that gypsimac said. If the aircraft is not in a slip or a skid, then there shouldn't be any weathervaning to worry about.

[N422NM]

Originally Posted by BobDDuck

I don't buy that theory at all. The relative wind is going to be coming straight over the front end of the aircraft. The tail is attached to the nose in a straight line which means the plane isn't going to weathervane anywere. It's sort of one of those which came first the nose turning due to the HCL or the plane "weathervaning" due to the new direction of the relative wind.

That was my arguement to it as well. I have to finish the ride later as the oral went long and weather conspired against me...

[seagull]

The horizontal component of lift is one vector, the forward motion is the second vector. The combination leads the aircraft to track at an angle to the x axis in the direction of the bank. This is a sideslip, and the aircraft will yaw into the direction of the turn due to the sideslip on the vertical stab. It will also want to roll away from it due to dihedral effect.

[88MPH]

Theoretically, imagine there were no vertical lift lost in a bank. and there were no adverse yawing tendancies from the ailerons. The bank alone does not make the airplane change it's attitude to where the relative wind would affect one side of the vertical stab more than the other side.

Now realistically, there is adverse yaw, but the bigger player here is the loss of vertical lift. Back pressure sustains atlitude, i.e. sustains the verticle component of lift, and the new lift vectors create the turn.

[phoenixflyer]

Originally Posted by N422NM

this question came from the DPE in the CFI oral today.

"What causes an aircraft to turn ?"

My answer "The airplane must be banked to turn which causes a change in the horizontal component of lift..."

He says "ok, I'll agree with that in that it initiates the turn, but then what else is required to continue turning ?"

I racked my brain and searched my PHAK, AFH etc...and couldn't come up with anything else substantial. He then told me something I've never heard...I can see the theory of it but had never heard it before....

"Once the bank is established, the aircraft weather vanes into the relative wind and this is what keeps the aircraft turning, in addition to the change in horizontal lift..."

Did I sleep through something at some point ?

You must have spent most of your cfi groundschool watching videos on the internet.

[flyguy]

Originally Posted by phoenixflyer

You must have spent most of your cfi groundschool watching videos on the internet.

Hey you weren't in my groundschool were you? Oh, I guess I didn't teach a CFI groundschool, and you don't work for Air China. But I caught my students playing on the Internet while in class many a time.

Same thing happened on my CFI ride, except it wasn't weather that discontiued it. Actually I take that back, it was weather. It was raining oil under the airplane's cowling. Anyway, good luck on the rest of the ride!!

[N422NM]

Originally Posted by phoenixflyer

You must have spent most of your cfi groundschool watching videos on the internet.

That and comparing the smoking traits of american pilots vs. chinese pilots...

[flyguy]

Originally Posted by N422NM

That and comparing the smoking traits of american pilots vs. chinese pilots...

Don't get me started!

[seagull]

Not sure what you are talking about here. You bank and the vector is offset for lift. If you don't increase the AoA you will descend, but that has nothing to do with whether you are offsetting lift. The offset lift creates sideslipe, which is why dihedral effect works!

Originally Posted by gypsimac

Theoretically, imagine there were no vertical lift lost in a bank. and there were no adverse yawing tendancies from the ailerons. The bank alone does not make the airplane change it's attitude to where the relative wind would affect one side of the vertical stab more than the other side.

Now realistically, there is adverse yaw, but the bigger player here is the loss of vertical lift. Back pressure sustains atlitude, i.e. sustains the verticle component of lift, and the new lift vectors create the turn.

[88MPH]

Originally Posted by seagull

Not sure what you are talking about here. You bank and the vector is offset for lift. If you don't increase the AoA you will descend, but that has nothing to do with whether you are offsetting lift. The offset lift creates sideslipe, which is why dihedral effect works!

Correct me if I'm wrong, but doesn't the "direction" of lift determine the turn, and not the yaw?

[seagull]

You are only partially right. It takes both to turn. Direction of lift without yaw will yield a straight path that is at an angle to the x-axis of the aircraft. That is sideslip, and the vertical stab will constantly work to prevent that by aligning the x-axis with the direction of flight. If the lift is still offset (from a bank), then it never is able to achieve that alignment, so the x-axis will be continually changing direction in the direction of the bank.

At a certain amount of bank, the directional stability (from the vertical stab) exceeds the roll stability from dihedral effect, so the aircraft will sustain the bank angle. In fact, with enough bank, the sideslip will be enough to increase the angle of bank into the turn (up until 90 degrees, obviously).

[88MPH]

Ok, I see. It makes a little more sense now, put like that!

[CoBuilder]

I believe something like what seagull stated was also in "Stick and Rudder" but in different terms. Plane goes into a bank through horizontal component of lift but that only results in sideslip by deflection of ailerons itself. The tail has to weathercock around to complete the turn and this is achieved through somewhat by back pressure on the stick.

Look at a turn backwards. If a turn goes awkward you decrease AoA to keep plane under control and there'll likely be some sideslip.

[perpetual]

I'll simplify / detail all this and hopefully get it right for easy understanding.

I'll include the really basic version in bold with the more detailed info of interest mixed in.

Information in [[ ]] are just for clarification to those who may better understand or want clarification and should be skipped for the basic understanding.

Maybe you all can picture this.

You're flying straight and level at a constant power setting and holding altitude. Why is the airplane even flying in the first place?

Well, you're wing is at some positive angle into the on-comming airflow [[greater than the zero lift angle for a given airfoil reguardless of AOA ]]

Because of this angle into the airflow, [[ AOA + AOI ]] higher pressure is felt on the bottom of the wing while lower pressure is felt on the top. This pressure difference results in a lifting force [[ airflow below the wing meeting higher velocity flow from above giving cause to downwash effects spanwise throughout the wing + bernoulli effects chordwise ]]

So now that we're flying, picture this.

You're flying straight and level and then pull back on the stick or yoke.

What happens?

Well, your wing's angle of attack increases upwards, [[ the elevator/stabilator/ruddervator/canard etc deflects airflow and causes a moment change about the cg as a result of downward or upward force, causing the wings' angle of attack to change ]] increasing the pressure difference giving you lift and you go up.

The whole "up" direction is really just relative to the wing and not the "earth" etc.

You can think of this "up force" as the Lift vector or "lift" which is being created vertically upwards and outwards from the top of the wing. [[ignoring effects of centre of pressure change throughout angle of attack for simplicity]]

Now.. picture this:

You are flying straight and level and simply bank the airplane to the left.

What happens?

Well remember how there is that little angle that into the on-comming airflow? Well the angle still exists [[excluding controll inputs that would otherwise decrease the AOA and rely upon lift provided principally by the vertical tail ]] and because of this what do we get? We get a lifting force, right?

Remember that the lifting force is pointed upwards and outwards from the wing, so if the wing is banked (as in a turn) well, the lifting force is also banked the same amount.

What happens? The lifting force created by the wings will cause the aeroplane to "turn" or be lead in the direction of bank. [[not taking stability factors into account nor the effects of an/dihedrial or wing sweep]]

What happens if the bank is too steep? Will the airplane turn?

Yes, up to a point.

There is a point in time when the angle of bank will exceed the lifting force being provided by the airplanes' wing angle [[AOA +AOI]] to maintain its' level flight.

What happens is that the wieght of the aeroplane is not being "lifted" by enough force because of the smaller angle of attack. Thus the aeroplane slips / loses altitude.

So how do we maintain a constant continuous turn for various bank angles?

Well, we need to increase the angle of the wing into the on-comming airflow. So, we pull back the stick/yoke and increase the "angle of attack" of the wing.

The increased "angle of attack" creates more lift, and just enough to keep the aeroplane in level flight.[[ ignoring lateral thrust vectoring, or other extreme methods or speeds utilizing a vertical tail for a source of lifting during 'knife edge flight"]]

Comments welcome

-Perpetual

[meritflyer]

Perfecto! Too many people try to be PhDs in aerodynamics. Your explanation was very well said!

[88MPH]

Originally Posted by perpetual

...

If this thread is still alive, not to beat a dead horse, but you mentioned nothing about rudder/coordination required to "turn" which was the original question. Just so I understand, you're saying it has nothing to do with the rudder?

Which is what I think, cause simply put, you can be in a slip (albeit not a pretty one), and still be turning. So the weathervaning mentioned above should have nothing to do with it. Right?

[seagull]

Well, this discussion was more about coordinated turns, I thought.

Rudder in itself may have some small inputs once the turn is established due to various factors, however, it is the vertical stab in more shallow banks, or the horizontal stab in very steep banks, that is actually moving the nose of the aircraft around.

If you are just using rudder, and using the aileron to prevent any bank, the change in direction of the nose is due to the constant force from the rudder applied, the change in direction of the aircraft due to the lift from the AoA you create on the inboard side of the fuselage.

It's actually a lot easier to explain this with just text than what actually makes a car turn. Now THAT can be a challenge, although it seems it should be just intuitive, you have to come up with the actual forces that are resulting in the change of direction.