Weathercock effect in turns

[italia458]

I thought Italia's original point was that it was the elevator that makes the plane turn.

My point was that the elevator actually helps turn the aircraft. It's party due to the elevator and party due to the rudder. When the aircraft is banked, the rudder has a pitch component added and the elevator has a yaw component added, relative to the horizon and turn (assuming turning while straight and level).

[minstermineman]

You are suggesting that the down wing has increased drag. Yes, that is another way of saying that there is a moment created about the earth-z-axis. That must indeed happen if the aircraft is to yaw. But why that increased drag? The wing is going slower and has less lift; why should the drag have increased?

If you (pilot)shift yourself to the left so you hang further under the left wing, then you yourself will create more drag on that side of the aircraft than was there previously, plus you will alter the C of G to the left, so bank left.

[911slf]

Flexwings in turns - reply to #40

I am no aerodynamicist, and my experience is on hang gliders rather than microlights, but this is my understanding of what happens on hang gliders and I believe microlights are analogous.

A hang glider pilot is in a harness rather like a climbers harness, you can sit in it and the weight would be supported on your upper thighs and buttocks. But the pilot would normally, after take off, lean forward and some of the weight would then be supported by shoulder straps - this is for drag reduction and comfort, not aerodynamically mandatory, and not done with the very early harness models.

There is a strap connected from your harness via a carabiner to a loop which is permanently connected to the glider keel. Your weight is taken on this strap (breaking strain usually two ton, and there are usually two of them). This strap is usually near vertical, unless or until you start to pull a lot of G - not recommended. You are holding the bottom horizontal bar of an A frame rigidly connected to the glider. If you push the A frame to your right, you have effectively shifted your weight to the left. The strap is now dangling from the keel to a point a bit to the left of the centre of the horizontal bar, so no longer quite vertical, so your weight, about three feet below the centre of lift, is now a bit to the left of the centre of lift. If the glider is stiff you will have to hold this by muscle force until the wing starts to bank under the this offset load. Eventually you will turn left. If the glider is spirally neutral you will end up in a turn where the strap supporting your weight is once more perpendicular to the wing, but the wing is not parallel with the horizon, and the strap therefore, is no longer perpendicular to the ground. While many gliders are spirally neutral, some are spirally stable, some are spirally unstable, and some vary according to how you change the tension in flight. A modest degree of spiral instability (a tendency for turns to steepen), is desired for staying centred in small thermals - by hot ship pilots, not me. And spiral stability is definitely not a topic I can explain.

You ask why the increased drag on the left wing when you do this. The wing is effectively a sail, albeit somewhat stiffened by battens, the more you load it the more it will deform, and the increased deformation makes it less efficient and increases drag.

With a sailplane with a rigid wing, the greater the load, the more efficient the wing and the better the angle of glide - even though the actual descent rate increases. This is why in races they often carry water ballast.

If you put a heavier pilot on a hang glider,it will fly (and stall) faster, but at a worse angle of glide, as the wing deforms more and becomes less efficient. So if you shift your weight as above you are loading the left wing more, deforming it more, and increasing the drag more, on that side only.

Probably simpler to draw it but I am not sure how to attach a drawing.

[PBL]

italia,

Originally Posted by italia458

My point was that the elevator actually helps turn the aircraft. It's party due to the elevator and party due to the rudder.

You are now allowing that maybe some of the lateral surface is relevant to reacting to sideslip (or yaw, depending on which axes you are measuring in). That is progress of a sort.

Let me suggest you accept what ImbracableCrunk said right at the beginning, and what aerodynamicists have said in text books (which I have quoted), and what they say when one discusses it with them. The VS is there for a purpose. Someone put it there. They know why they put it there, and it works, over decades of experience. So when they say why, and how, I imagine there would - should - be a strong inclination to believe them, no?

Minstermineman,

Originally Posted by minstermineman

If you (pilot)shift yourself to the left so you hang further under the left wing, then you yourself will create more drag on that side of the aircraft than was there previously, plus you will alter the C of G to the left,

Thanks for responding to my request for explanations! Maybe you could phrase things in the usual terms of Newtonian dynamics?

There are two coordinate frames typically used when speaking about flight. One is earth-centered, in which the z axis passes through the CofG (and you, if you are hanging on a non-rigid line below a wing). So "alter[ing] the CofG to the left" makes no sense in this frame. The other is aircraft-centered, also passing through the CofG. Again, moving the CofG "to the left" makes no sense in this frame.

911slf,

Thanks also for responding to my request for explanations! I appreciate your attempt to explain what happens in discursive terms, but I think it suffers from the same issue as minstermineman, namely that you are using (let me say) hanggliderspeak which is at variance with the ways in which one expresses dynamics when talking aerodynamics.

You are hanging below some object which is not fixed to the earth. You push on a frame attached to that object. Except for very briefly (and very slightly) when you are moving the frame (equal and opposite reactions, and all that), you are hanging directly below the CofG. That's what Newton says happens when you don't want to be a pendulum (which, as you point out, is not recommended!)

I am not sure I can get much further with your explanation without trying to interpret in the usual way of balance of forces, and I am not sure I can do that as it stands.

[911slf]

reference for forces on hang gliders

PBL this

TURNING FLIGHT AND SIDESLIP IN HANG GLIDERS (Summary)

appears to be a fairly detailed treatment by a hang glider pilot. I have not read it all carefully, as it goes into more detail than I as a casual sporting flyer would want to read. So far as I can tell it does address your interests, and I will be interested to see what you make of it.

It does touch upon a point that has confused me in the past. Hang glider pilots are encouraged to let the nose up a bit in turns, which always seemed a bit counter intuitive to me, given we normally fly not much above the stall.

[italia458]

PBL...

You are now allowing that maybe some of the lateral surface is relevant to reacting to sideslip (or yaw, depending on which axes you are measuring in). That is progress of a sort.

Let me suggest you accept what ImbracableCrunk said right at the beginning, and what aerodynamicists have said in text books (which I have quoted), and what they say when one discusses it with them. The VS is there for a purpose. Someone put it there. They know why they put it there, and it works, over decades of experience. So when they say why, and how, I imagine there would - should - be a strong inclination to believe them, no?

What I've said from the start, in different ways, is that while in a banked attitude, the entire tail section (vertical stabilizer/rudder and horizontal stabilizer/elevator) controls yaw (and pitch) partially, either eliminating unwanted yaw (coordinated flight), or producing yaw (slip/skid). It's a function of your bank, the more bank, the more the rudder controls pitch and the more the elevator controls yaw. A V tail aircraft is a perfect example of what I mean.

I think we're talking about something else. The VS is there for stability. I'm not contesting what a VS is used for. Your second paragraph is quite confusing to me, I don't understand how it applies to the discussion on weathercocking while in a turn...

If it is correct that the aircraft is weathercocking while in a turn, then my reasons for why it does not should be false and I haven't seen anyone clearly disprove what I've been saying. I'm completely open to different ideas but I think firstly you have to clear away the false information and then start with a fresh slate to explain a concept or theory.

EDIT: PBL are you talking about this quote from IC?

Make an airplane with your right hand
Palm down
Fingers together
Bank your airplane 30 degrees to the left
Slide your airplane hand to the left due to the now horizontal component of lift
Take your left index finger and push on your vertical stab
Watch your airplane magically weathervane

I already addressed that. What he described was an aircraft in a slip and the tendency of the aircraft to "weathervane" into the wind because of the crosswind component on the tail section. I completely agree with this! But I don't see this happening while in coordinated flight!

[chris.dever]

1. Make an airplane with your right hand
2. Palm down
3. Fingers together
4. Bank your airplane 30 degrees to the left
5. Slide your airplane hand to the left due to the now horizontal component of lift
6. Take your left index finger and push on your vertical stab
7. Watch your airplane magically weathervane

My boss walked in .... probably to check out why I was making the noise of a radial engine ....'right hand' plane rolled onto its back and disappeared into thin air !

[italia458]

Haha that's awesome!

[Thomo91]

In my understanding -

"Weathercock" is in relation to the ground (an object). {compass heading}

"Slip" is in relation to air flow (relative airflow). {unbalanced flight}

[Thomo91]

while in a banked attitude, the entire tail section (vertical stabilizer/rudder and horizontal stabilizer/elevator) controls yaw (and pitch) partially, either eliminating unwanted yaw (coordinated flight), or producing yaw (slip/skid).

Yes that's right, well kinda.

It's a function of your bank, the more bank, the more the rudder controls pitch and the more the elevator controls yaw. A V tail aircraft is a perfect example of what I mean.

I'm not much of an aerodynamicist, but in my layman way of putting it I hope I'm on the right track.

Relative to the airflow:

Trimmed S/L, Aileron is applied, adverse yaw is stopped by rudder input, AoA is increased to maintain height (elevator). Bank angle is maintained at, say, 45 degrees. To do that aileron is neutralized, and so is rudder. What happens if you let go of the stick to the S/L trimmed position? Yes, the nose will fall off the horizon, and everything goes yuk.

Most aircraft I've flown (which is mostly small ultralight stuff, and your average Cessna Single engine types). Only require a little of top rudder past the 45 degree mark, if you want to maintain it for a while. Depending on the day, load, and of course the speed. (note: please ensure you know what the implications of what to much top rudder will do!!)

Back to your quote; Yes the more angle of bank, the more the rudder is used to maintain a nose up position, what is generally known as top rudder.

Elevator however is still only used to increase AoA. I'll leave you to decide what the aircraft will now do.

Another way; In relation to the aircraft, the elevator isn't doing anything to the yaw, rudder still controls yaw no matter what orientation the aircraft is in. Elevator still controls AoA no matter what orientation the aircraft is in.

I reckon you're on the money but it looks like you may be mixing it with a relation to the ground, (no offense intended) you may think, that, as it's now moving along the X axis using elevator instead of rudder (if the aircraft maintained wings level - using rudder only to turn without banking) it now controls yaw. Not the case relative to airflow.

Dunno If I've just confused you all, or if that actually makes sense, I hope it's a bit of the later.

[PBL]

911slf,



thanks for the link to Seibel's article. I understand why you haven't read it all. It is very long (Safari wants 89pp to print it!) and to my taste somewhat meandering. However, it seems Seibel has seriously tried to experiment and can describe what he found in terms mostly familiar to an aerodynamicist. I have read about a quarter of it so far.

Here his answer to my question, from Part IV, Section "What Makes an Aircraft Turn", second paragraph, first sentence: "In hang gliders, the wingtips provide the same function as would a vertical tail". He is talking about the action of the VS during sideslip.

Originally Posted by 911slf

It does touch upon a point that has confused me in the past. Hang glider pilots are encouraged to let the nose up a bit in turns, which always seemed a bit counter intuitive to me, given we normally fly not much above the stall.

That seems to me straightforward. Your lift vector is tilted by the roll, so there is a lower component of lift in the vertical (earth-negative-z) direction and so you need to increase total lift in order to match weight with this component if you wish to stay level, and you do that by increasing AoA. That's why you need elevator in a level turn in an aircraft, and why (I imagine) you need to "let the nose up" in a hang glider.

Seibel suggests that some instructional books suggest this has something to do with yaw/sideslip, and that they are wrong. I agree with Seibel.

PBL

[Basil]

I accept that it may be flippant to say so, but I am very glad that I did not read a thread such as this before learning to fly

[italia458]

Most aircraft I've flown (which is mostly small ultralight stuff, and your average Cessna Single engine types). Only require a little of top rudder past the 45 degree mark, if you want to maintain it for a while. Depending on the day, load, and of course the speed. (note: please ensure you know what the implications of what to much top rudder will do!!)

Back to your quote; Yes the more angle of bank, the more the rudder is used to maintain a nose up position, what is generally known as top rudder.

Please do not teach anyone this method of turning! A steep turn is probably one of the hardest manoeuvres to complete accurately. In the method you described using "top rudder" you're not flying in a coordinated turn, you're in a slipping turn, which can easily lead to a stall and possibly a spin if bank is increased enough or speed reduced enough. This is probably the most common mistake a flight instructor sees when teaching steep turns. DO NOT USE RUDDER TO KEEP THE NOSE FROM DROPPING!! Make sure all turns are coordinated. The nicely compiled graph on increase in load factor, stall speed, etc. with an increase in bank is only accurate if you are in a coordinated level turn at that bank angle. If you look at your turn coordinator while in a steep turn using "top rudder", the ball will not be centred.

Another way; In relation to the aircraft, the elevator isn't doing anything to the yaw, rudder still controls yaw no matter what orientation the aircraft is in. Elevator still controls AoA no matter what orientation the aircraft is in.

I reckon you're on the money but it looks like you may be mixing it with a relation to the ground, (no offense intended) you may think, that, as it's now moving along the X axis using elevator instead of rudder (if the aircraft maintained wings level - using rudder only to turn without banking) it now controls yaw. Not the case relative to airflow.

Yes, in relation to the aircraft that's true. I was talking in relation to the horizon, where your reference for manoeuvres is based. It's the same as your attitude indicator. You should be flying using this reference system and if you are using "top rudder" to control pitch in your turns, I know you are! So that part is good.

AoA is a different factor and yes, elevator controls AoA, but rudder also controls AoA while in a turn. If you entered a cross controlled turn, like you described using "top rudder", then the inside wing would have a higher AoA compared to doing a properly coordinated steep turn.

I'd highly recommend that if you do your steep turns using "top rudder" then go up and practice doing them properly. It'll feel weird at first but you need to break that habit, it will bite you at sometime!!

I accept that it may be flippant to say so, but I am very glad that I did not read a thread such as this before learning to fly

Good call Basil!

[italia458]

In my understanding -

"Weathercock" is in relation to the ground (an object). {compass heading}

"Slip" is in relation to air flow (relative airflow). {unbalanced flight}

Weathercocking is not relative to the ground, it's relative to the wind! If you enter a slip, there is a weathercocking tendency that is prevented by using opposite rudder to remain in the slip.

Dihedral is used to help bring the aircraft back to level flight if a slip is encountered.

[Thomo91]

Please do not teach anyone this method of turning! A steep turn is probably one of the hardest manoeuvres to complete accurately. In the method you described using "top rudder" you're not flying in a coordinated turn, you're in a slipping turn, which can easily lead to a stall and possibly a spin if bank is increased enough or speed reduced enough. This is probably the most common mistake a flight instructor sees when teaching steep turns. DO NOT USE RUDDER TO KEEP THE NOSE FROM DROPPING!! Make sure all turns are coordinated. The nicely compiled graph on increase in load factor, stall speed, etc. with an increase in bank is only accurate if you are in a coordinated level turn at that bank angle. If you look at your turn coordinator while in a steep turn using "top rudder", the ball will not be centred.

Thanks Italia458, And yes I certainly realize what you're saying (that's why I added the "note") It is more relevant in gliders I guess, but don't worry, the ball is always centered. Not much point in doing a side slip reducing your performance. Like you said some aircraft will really bite if you do it wrong. Keep the ball centered.

Weathercocking is not relative to the ground, it's relative to the wind! If you enter a slip, there is a weathercocking tendency that is prevented by using opposite rudder to remain in the slip.

Yes that's right, it has gone from one position, and 'weathercocked' into another. {compass heading} (ground wasn't a really good example, more a point on the horizon)

I'd highly recommend that if you do your steep turns using "top rudder" then go up and practice doing them properly. It'll feel weird at first but you need to break that habit, it will bite you at sometime!!

Thanks, I will certainly review how I go about it next time I'm up, as I couldn't really tell you what I do at the time as you do what ever you have to without much thought. The ball is always centered though, I make sure of that.

I accept that it may be flippant to say so, but I am very glad that I did not read a thread such as this before learning to fly

[italia458]

It is more relevant in gliders I guess

Same goes for gliders, don't use "top rudder"! I'm a glider pilot as well and can tell you they fly just the same as any other conventional airplane. They just don't have power so you're usually descending slowly unless in a thermal.

It sounds like you're doing the turn properly if you're keeping the ball centred. I was just mostly worried about people reading that and thinking that "top rudder" is OK to be used in a steep turn. Flight instructors already encounter many common problems in training and keeping this myth circulating will just make it worse!

[Thomo91]

I was just mostly worried about people reading that and thinking that "top rudder" is OK to be used in a steep turn. Flight instructors already encounter many common problems in training and keeping this myth circulating will just make it worse!

Agree, apologies for that.

Once again, do what your instructor says, not opinions on forums!

[italia458]

Agree, apologies for that.

Once again, do what your instructor says, not opinions on forums!

Cheers mate!

[Basil]

Digressing from the thread:
Like the 'top rudder' in a steep turn it is easy for a student to misunderstand a briefing or to transfer a technique which may be appropriate in one phase of flight to another where it is not.

Student Basil was told that going above or below best glide speed would be inefficient and dispose of energy. Sitting in JP with instructor, Bas decided that energy was too high and stuck the nose down and shortly afterwards applied some energy killing g to raise nose and regain original speed.
At the subsequent debriefing it was pointed out that, whilst the technique would have been acceptable as a substitute for speedbrake at 20,000ft, it was not a recommended procedure at 1000ft on the approach.

My RAF QFI sat unflinchingly through the whole event.