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Michel

Joined: 10 Jan 2006
Posts: 966
Location: Norway

Posted: Sat Jan 06, 2007 3:03 pm Post subject: Norwegian Stalls

On Jan 6, 2007, at 7:02 PM, Michael Gibbs wrote:

 	  Quote: 	
		   I think we need a picture of Michel in his cockpit with his Viking 

 helmet on.  


	


Ok, I will provide that.

Meanwhile, I went flying today, did some stalls (mush down) and 

listened to 'the sound.'

Unfortunately, I'll need to have someone else to fly the plane, in 

order to find the cause but, so far, I wonder if it is not my elevator. 

But first, a question: Is this correct?: When we stall gently (mush 

down), the plane starts stalling the wings first then, if we keep the 

stick in the stomach, and provided we get enough elevator authority to 

get the nose up and the speed down, the horizontal stab (and the 

elevator) will stall too, and the nose will drop.

If this is correct, then I think the noise I experience is happening 

when the horizontal stab stalls and the elevator shakes.

Am I getting anywhere?



Cheers,

Michel

_________________
Kitfox 3 - Jabiru 2200

dave

Joined: 22 Sep 2006
Posts: 1382

Posted: Sat Jan 06, 2007 5:45 pm Post subject: Norwegian Stalls

Michel



Did  you get that post i sent in on this earlier ?  Same thing bascially 

that I found was a thud from the rear.

- Disturbed air from the wing hitting the horiz stab ?



I only heard this while doing deadstick stalls and headset off.

Dave

---

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Guest

Posted: Sat Jan 06, 2007 7:36 pm Post subject: Norwegian Stalls

every time I drink hot chocolate I get a sharp pain in my  eye??

  [quote][b]

Michel

Joined: 10 Jan 2006
Posts: 966
Location: Norway

Posted: Sun Jan 07, 2007 4:13 am Post subject: Norwegian Stalls

On Jan 7, 2007, at 2:43 AM, dave wrote:

 	  Quote: 	
		   Did  you get that post i sent in on this earlier ?  Same thing 

 bascially that I found was a thud from the rear.


	


Indeed, I read your post, Dave, and that's what led me to think about 

the horizontal stab. But, at first, I thought that it couldn't be that 

because, if it was, I would feel a vibration in the stick, which I 

don't.



But yesterday, I noticed that when I stall, the stick is pull back to 

the extend that it rests against the edge of seat (in my model 3) and 

that may damp vibrations.



Cheers,

Michel



do not archive

_________________
Kitfox 3 - Jabiru 2200

CrownLJ(at)verizon.net
Guest

Posted: Mon Jan 08, 2007 5:46 am Post subject: Norwegian Stalls

"....the horizontal stab (and the elevator) will stall too, and the nose  will drop."

  

   I don't think that this is correct.  If the  stab was to stall, the nose will pitch up. aka F101 Voodoo.  The stab  provides a downward force.  The elevator will loose authority, but again  can't stall as the wings do.  

  

 "...Is this correct?: When we stall gently (mush down), the plane starts  stalling the wings first then, if we keep the stick in the stomach, and provided  we get enough elevator authority to get the nose up and the speed down, the  horizontal stab (and the 

elevator) will stall too, and the nose will  drop..."

  

   You are correct that the wings "starts" to stall.  Most modern  planes are designed so that the wing will stall in stages.  Actually, only  part of the wing stalls, the other part is still flying.  The plane will  mush because the weight of the plane now exceeds the amount of lift that the  wings can produce at that angle of attach.  If you lob the nose up and  gently keep 0 G on - not requiring the wings to provide any lift - it is  possible to fly at 0 airspeed.  Actually, you are not flying since you will  not be overcoming gravity, but the airplane will not stall.  You can hold 0  airspeed and still roll the wings to lower the nose letting it drop and now  gently obtain airspeed.  Just a hair more G than 0 and more lift is  required out of the wing and it would stall at the angle of attach.

  

   To see how the wing stalls all at once as you approach your level  flight stall speed lob the nose up about 20 degrees and release the back  pressure.  You will notice that the IAS will be less than "normal" and the  nose will drop more abruptly than "normal".  This will occur because all of  the wing is stalling at about the same time, instead of a part of the wing  stalling.

  

 I know that this doesn't help you find you thud, but.....

  

  

 larry

   [quote][b]

Bob

Joined: 24 Oct 2006
Posts: 89
Location: Damascus, Maryland, USA

Posted: Mon Jan 08, 2007 6:07 am Post subject: Re: Norwegian Stalls

You've written about this for quite a while, so if fabric was coming off, I'd think you would've seen something by now.  As the attitude and loads change, stuff moves around quite a lot.  I chased a grinding in the tailcone noise something kind of like this in a Cessna and it turned out the ELT cable rubbed on a frame as the instructors would do stalls and turns.  Are you sure it's not something simple like an item rubbing the fabric?  Maybe the cargo liner?

Bob

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Guy Buchanan

Joined: 16 Jul 2006
Posts: 1204
Location: Ramona, CA

Posted: Mon Jan 08, 2007 9:12 am Post subject: Norwegian Stalls

At 05:45 AM 1/8/2007, you wrote:

 	  Quote: 	
		  "....the horizontal stab (and the elevator) will stall too, and the nose 

will drop."



   I don't think that this is correct.  If the stab was to stall, the nose 

 will pitch up. aka F101 Voodoo.  The stab provides a downward force.  The 

 elevator will loose authority, but again can't stall as the wings do.



Sorry Larry; the stab "lifts" down, particularly with the stick in your 
	


lap. It's most definitely possible to stall it too, after all it's only a 

lifting airfoil. Most stabs don't stall because they're designed not to. 

Certificated aircraft are designed to stall their wings long before the 

horizontal stab stalls because were it to be otherwise you would have 

really horrific landing behavior. (Picture a severe pitch over at flare 

followed by a negative-g drive into the ground.) Anything's possible in 

experimental land since many designers push the performance envelope 

severely. All my experience with my Kitfox, however, indicates quite 

normal, "certificated" behavior at all angle of attacks. I can even do the 

"falling leaf" with the stick in my lap. (It really is a falling leaf, 

since I have very little dihedral and my plane tends to fall off the 

"bubble" to one side or the other.)

Guy

_________________
Guy Buchanan
Deceased K-IV 1200
A glider pilot too.

Michel

Joined: 10 Jan 2006
Posts: 966
Location: Norway

Posted: Mon Jan 08, 2007 2:32 pm Post subject: Norwegian Stalls

On Jan 8, 2007, at 5:49 PM, Guy Buchanan wrote:

 	  Quote: 	
		   Sorry Larry; the stab "lifts" down, particularly with the stick in 

 your lap.


	


Wow, this is interesting stuff, Larry and Guy! Now, I have a friend 

building a Velocity and he told me that a canard doesn't stall the 

wings because the canard wings stalls first and the nose drops, hence 

gaining speed.

With our Kitfoxes, the wings stalls before the horizontal stab. Makes 

sense. And when our nose drops, it's because the  wings stalls first 

and the horizontal stab "holds" the tail up.

But when we mush down, then, what happens? Honestly, I am not sure I 

know what I am talking about because "mush down" is a word I came to 

know from its context, but from a proper definition.

Say, when I put the engine on idle, then gently pull the stick, trying 

to keep the same altitude and watch the speed decrease to near stall 

speed. At the end of it, I have the stick to the most backward 

position, I feel the plane trembling, a wing or the other drops, which 

I correct with a bit of rudder and aileron, to keep the ball centered, 

then I can't get any slower because I sink now maybe 500 fpm. This is 

mushing down, right? At that moment, the wings are partially stalled, 

right? And the elevator is not? But why doesn't the nose drops then?

By the way, does anyone know the relative angle between the wings and 

horizontal stabs on our Kitfoxes?



Cheers,

Michel



do not archive

_________________
Kitfox 3 - Jabiru 2200

dave

Joined: 22 Sep 2006
Posts: 1382

Posted: Mon Jan 08, 2007 3:11 pm Post subject: Norwegian Stalls

Michel,  I can measure the differance if you like in degrees if you let me 

know.



Also when my floats go on , I drop the front of the horiz.stab about one 

inch an I fly hands free.

the angle does change but the elevator still has the same authority.  As a 

matter if fact i can drag the tails of the floats down the runway when 

landing with gear down.  The wear on bottom of the floats shows this 

clearly.



I think the disturbed air from the stalled wing is hitting my tail section 

some how and causing a thud.  I will do further testing this week and let 

you know the results - deadstick and headset off so I an hear it .

Dave

---

kerrjohna(at)comcast.net
Guest

Posted: Mon Jan 08, 2007 3:22 pm Post subject: Norwegian Stalls

Michael, the maneuver you have described is called "falling leaf".  It is not so much a stall as operating at a point of minimum lift.  My instructor made me practice it with a Super Cub at idle to ingrain in me the notion of lifting the wing with rudder rather than aileron.

 John Kerr

 [quote]-------------- Original message -------------- 

From: Michel Verheughe <michel(at)online.no> 

[quote] --> Kitfox-List message posted by: Michel Verheughe 

 On Jan 8, 2007, at 5:49 PM, Guy Buchanan wrote: 

 > Sorry Larry; the stab "lifts" down, particularly with the stick in 

 > your lap. 

 Wow, this is interesting stuff, Larry and Guy! Now, I have a friend 

 building a Velocity and he told me that a canard doesn't stall the 

 wings because the canard wings stalls first and the nose drops, hence 

 gaining speed. 

 With our Kitfoxes, the wings stalls before the horizontal stab. Makes 

 sense. And when our nose drops, it's because the wings stalls first 

 and the horizontal stab "holds" the tail up. 

 But when we mush down, then, what happens? Honestly, I am not   sure I  

 know what I am talking about because "mush down" is a word I came to 

 know from its context, but from a proper definition. 

 Say, when I put the engine on idle, then gently pull the stick, trying 

 to keep the same altitude and watch the speed decrease to near stall 

 speed. At the end of it, I have the stick to the most backward 

 position, I feel the plane trembling, a wing or the other drops, which 

 I correct with a bit of rudder and aileron, to keep the ball centered, 

 then I can't get any slower because I sink now maybe 500 fpm. This is 

 mushing down, right? At that moment, the wings are partially stalled, 

 right? And the elevator is not? But why doesn't the nose drops then? 

 By the way, does anyone know the relative angle between the wings and 

 horizontal stabs on our Kitfoxes? 

 Cheers, 

 Michel 

 do not archive 

  [quote][b]

Guy Buchanan

Joined: 16 Jul 2006
Posts: 1204
Location: Ramona, CA

Posted: Mon Jan 08, 2007 7:32 pm Post subject: Norwegian Stalls

At 02:28 PM 1/8/2007, you wrote:

 	  Quote: 	
		  At the end of it, I have the stick to the most backward

position, I feel the plane trembling, a wing or the other drops, which

I correct with a bit of rudder and aileron, to keep the ball centered,

then I can't get any slower because I sink now maybe 500 fpm. This is

mushing down, right?


	


Yes. You can call it that.



 	  Quote: 	
		  At that moment, the wings are partially stalled,

right?


	


At least.



 	  Quote: 	
		  And the elevator is not?


	


Usually no. (Flat spins excepted.)



 	  Quote: 	
		  But why doesn't the nose drops then?


	


It does. The reason it doesn't drop straight down is because you've got the 

stick in your lap, the horizontal stab is not stalled, so that when the 

plane moves forward the tail is driven down and the nose up. Your mush may 

seem to be at zero airspeed, but it's not. It is an equilibrium balance 

between forward airspeed and the concomitant elevator effectiveness, and 

the stall of the wings and their associated drop. If you ever achieved zero 

airspeed, (say you pull nose high with no thrust,) you'll find the wings 

fall through quite perpendicularly. I can't remember what the aerobatic 

term for it is as the Decathlon I used wasn't allowed to back-slide at all. 

My instructor would never let me achieve "zero" airspeed.

Guy

_________________
Guy Buchanan
Deceased K-IV 1200
A glider pilot too.

CrownLJ(at)verizon.net
Guest

Posted: Tue Jan 09, 2007 9:06 am Post subject: Norwegian Stalls

 	  Quote: 	
		                      subject:                  Re: Re: Norwegian          Stalls            From:                  Guy Buchanan (bnn(at)nethere.com ([email]bnn(at)nethere.com?subject=Re:%20Re:%20Norwegian%20Stalls&replyto=6.2.1.2.0.20070108082934.0299a698(at)pop3.nethere.net[/email]))            Date:                  [b]Mon Jan 08 - 9:12          AM[/b]    	  Quote: 	
		   	  Quote: 	
		  At 05:45 AM 1/8/2007, you wrote:

 	  Quote: 	
		  "....the horizontal stab (and the elevator) will stall too, and the nose 

will drop."



   I don't think that this is correct.  If the stab was to stall, the nose 

 will pitch up. aka F101 Voodoo.  The stab provides a downward force.  The 

 elevator will loose authority, but again can't stall as the wings do.



Sorry Larry; the stab "lifts" down, particularly with the stick in your 
	


lap. It's most definitely possible to stall it too, after all it's only a 

lifting airfoil. Most stabs don't stall because they're designed not to. 

Certificated aircraft are designed to stall their wings long before the 

horizontal stab stalls because were it to be otherwise you would have 

really horrific landing behavior. (Picture a severe pitch over at flare 

followed by a negative-g drive into the ground.) Anything's possible in 

experimental land since many designers push the performance envelope 

severely. All my experience with my Kitfox, however, indicates quite 

normal, "certificated" behavior at all angle of attacks. I can even do the 

"falling leaf" with the stick in my lap. (It really is a falling leaf, 

since I have very little dihedral and my plane tends to fall off the 

"bubble" to one side or the other.)



Guy,  I think that we are saying the same thing - the stab provides downward lift,it is design NOT to stall.  If it were to stall, there would be no way to get it flying again, short of a parachute.  The F101 Voodoo had a stab placement wherebyit was quite easy to stall.  The result was usually the crew ejecting, as the nose would pitch up and the was no way to recover it with the airplane controls.  The method was topull the landing drag chute in hope that would point the airplane down again toget air over the tail to get it flying again.  Usually there was not enough altitudeto successfully recover.  I totally agree with you that most though it is possible to stall the stab, most airplanecan not because of the design.  It is impossible to stall the stab in the Kitfox, asit is design NOT to stall.  If it were to stall, there would be no way to get it 	
0 	  Quote: 	
		  it is design NOT to stall.  If it were to stall, there would be no way to get it 	
1 	  Quote: 	
		  it is design NOT to stall.  If it were to stall, there would be no way to get it 	
2 	  Quote: 	
		  it is design NOT to stall.  If it were to stall, there would be no way to get it 	
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		  it is design NOT to stall.  If it were to stall, there would be no way to get it 	
4 	  Quote: 	
		  it is design NOT to stall.  If it were to stall, there would be no way to get it 	
5 	  Quote: 	
		  it is design NOT to stall.  If it were to stall, there would be no way to get it 	
6 	  Quote: 	
		  it is design NOT to stall.  If it were to stall, there would be no way to get it 	
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		  it is design NOT to stall.  If it were to stall, there would be no way to get it 	
8 	
   

	
  

   [quote][b]

Guy Buchanan

Joined: 16 Jul 2006
Posts: 1204
Location: Ramona, CA

Posted: Wed Jan 10, 2007 7:14 am Post subject: Norwegian Stalls

At 09:01 AM 1/9/2007, you wrote:

 	  Quote: 	
		    Is this how you see it?


	


Yep.

Guy Buchanan

K-IV 1200 / 582-C / Warp / 100% done, thanks mostly to Bob Ducar.



Do not archive

_________________
Guy Buchanan
Deceased K-IV 1200
A glider pilot too.

Michel

Joined: 10 Jan 2006
Posts: 966
Location: Norway

Posted: Thu Jan 11, 2007 11:27 am Post subject: Norwegian Stalls

On Jan 9, 2007, at 12:08 AM, dave wrote:

 	  Quote: 	
		   Michel,  I can measure the differance if you like in degrees if you 

 let me know.


	


Yes, please. For your information, in my flight simulator, the 

difference between the AoA of the wing and the horizontal stab is 5 

degrees i.e. +4 for the wing at the root, and -1 for the stab. The wing 

has wash-out, the stab has not. Thanks, Dave!



On Jan 9, 2007, at 12:19 AM, kerrjohna(at)comcast.net wrote:

 	  Quote: 	
		   Michael, the maneuver you have described is called "falling leaf". 

Hum, I am not sure my stall goes all the way to "falling leaf," John, 
	


because I still have much forward speed but, thank you, "falling leaf" 

is a nice and poetic term. 



On Jan 9, 2007, at 4:26 AM, Guy Buchanan wrote:

 	  Quote: 	
		   The reason it doesn't drop straight down is because you've got the 

 stick in your lap, the horizontal stab is not stalled,

It makes sense, Guy, but then ... the sound can't be my elevator and 
	


horizontal stab stalling, right?



On Jan 9, 2007, at 6:01 PM, Larry Martin wrote:

 	  Quote: 	
		   From Micheal's description, I had the impression that he

 thought that the nose dropped because the stab stalled.  This is not 

 correct.  The

 nose drops because the center of pressure on the wing moves forward.

.. err, you mean 'move aft‚' Larry, right? If the center of pressure 
	


moves forward and the CoG stays hopefully the same, the moment is then 

to create a nose-up attitude.

This is interesting because years ago, before I started flying for 

real, I had that discussion with a flight simulator group where I meant 

to center of pressure (a.k.a center of lift) moved back as the plane 

slows down, hence the need to pull the stick. My assumption was based 

on the fact that a sailboats attempting to sail close to the wind (i.e. 

small AoA) needed to pull something called 'Cunningham hole' in order 

to make the leading edge stiffer and move the deepest hollow of the 

sail more forward - a sail being a "soft" airfoil.

But I was told that the center of lift never moves on a wing, and that 

it is usually at about 1/4 of the cord, from the leading edge.



Cheers,

Michel



do not archive

_________________
Kitfox 3 - Jabiru 2200

dave

Joined: 22 Sep 2006
Posts: 1382

Posted: Thu Jan 11, 2007 7:09 pm Post subject: Norwegian Stalls

Michel ,



I did measure it a few days ago and off the top of my head  both 

measurements were the same .

I measured  the horiz stab in a few places and measured the bar between the 

2 wing struts that runs parallel  to the bottom of wing approx.

Both were about the same -wing strut and bottom of the wing as well as the 

horizontal stab.



Now Like I said when floats go on  I drop the front of the horiz.stab about 

a inch at the front and that seems to compensate for the extra drag of the 

floats.  I would only guess that that would be a few degrees difference then 

between the wing and horiz.stab.



Understand that mumble jumble ?



I can measure better tomorrow.

Dave

---

Guy Buchanan

Joined: 16 Jul 2006
Posts: 1204
Location: Ramona, CA

Posted: Thu Jan 11, 2007 8:24 pm Post subject: Norwegian Stalls

At 11:27 AM 1/11/2007, you wrote:

 	  Quote: 	
		  It makes sense, Guy, but then ... the sound can't be my elevator and

horizontal stab stalling, right?


	


No. I think the sound is fabric slapping against the wing tanks.

 	  Quote: 	
		  On Jan 9, 2007, at 6:01 PM, Larry Martin wrote:

 From Micheal's description, I had the impression that he

thought that the nose dropped because the stab stalled.  This is not

correct.  The

nose drops because the center of pressure on the wing moves forward.



... err, you mean 'move aft‚' Larry, right? If the center of pressure

moves forward and the CoG stays hopefully the same, the moment is then

to create a nose-up attitude.

This is interesting because years ago, before I started flying for

real, I had that discussion with a flight simulator group where I meant

to center of pressure (a.k.a center of lift) moved back as the plane

slows down, hence the need to pull the stick. My assumption was based

on the fact that a sailboats attempting to sail close to the wind (i.e.

small AoA) needed to pull something called 'Cunningham hole' in order

to make the leading edge stiffer and move the deepest hollow of the

sail more forward - a sail being a "soft" airfoil.

But I was told that the center of lift never moves on a wing, and that

it is usually at about 1/4 of the cord, from the leading edge.


	


Yes and No. You were correct in stating that the SYSTEM Cl moves back, 

whereas the wing Cl remains constant before stall. (It moves after stall, 

quite dramatically.) The reason the SYSTEM Cl moves back when you slow is 

that down force on the tail diminishes as you slow, (remember lift is 

proportional to the square of the velocity,) whereas the pitch moment due 

to mass remains constant. You balance the mass pitch moment by increasing 

the tail's down force by pulling the elevator up, increasing the tail's 

negative Cl in direct proportion to the reduction in the square of the 

velocity.



Too confusing. Try this. I have a "wing" with 40 units of lift. 10 length 

units behind that "wing" I have a "tail" flying along with -4 units of 

lift. (It lifts down.) To balance this system my "airplane" weighs 36 units 

and it's CG is 1.11 length units in front of the "wing". Now I 

instantaneously slow by half. My "wing" and "tail" lift reduce to 10 and -1 

units respectively. (Lift is proportional to the square of the velocity.) 

However the weight is still 36 at 1.11 in front of the wing. Now there is a 

30 unit moment pitching down. To counteract that I have to increase the 

"tail" lift back to -4 units. I therefore have to quadruple the Cl by 

pulling the elevator up. I then have 10 units of lift on the "wing", -4 

units of lift on the "tail", and 36 units of weight 1.11 in front of the 

wing. The system balances about the wing, but I have 36 units of weight 

versus 14 units of lift and so am accelerating down quite rapidly. (I'll 

accelerate down until the increased wing angle of attack (Cl) and the 

increased velocity bring the "wing" lift back to 40 units.) Note that with 

a "wing" lift of 10 and a "tail" lift of -4, the SYSTEM Cl is 

instantaneously quite far aft of what it was at equilibrium, when the 

"wing" lift was 40.



There. Clear as mud.

Guy

_________________
Guy Buchanan
Deceased K-IV 1200
A glider pilot too.

Michel

Joined: 10 Jan 2006
Posts: 966
Location: Norway

Posted: Fri Jan 12, 2007 1:08 am Post subject: Norwegian Stalls

 	  Quote: 	
		   From: Guy Buchanan [bnn(at)nethere.com]

 There. Clear as mud.


	


Thank you, Guy. Clear as spring water. What you wrote is exactly what I was told, years ago. My posting was only as a comment on Larry who says the center of pressure of the wing moves forward.



Dave, if you can be bothered to measure accurately the AoA, thank you very much. The reason I don't do it myself is that I should rig something with a level and an angle measuring device. Then, we also need to refer to a common definition. I think AoA are always measured from the cord, which is an imaginary line going from the leading edge to the trailing one. But ... how do you get then on a built wing? My idea was to use a water hose and measure the difference at the two points. arctan(difference/cord) should then give me the AoA. My problem is that I always think about doing that in the winter, when it is sub-zero temperature, here! 



Cheers,

Michel



do not archive

<pre><b><font size  color="#000000" face="courier new,courier">

_________________
Kitfox 3 - Jabiru 2200

dave

Joined: 22 Sep 2006
Posts: 1382

Posted: Fri Jan 12, 2007 2:59 am Post subject: Norwegian Stalls

Michel,

I only compared the two with a inclinometer.



I possibly could attach a straight edge to the horiz stab and run it 

forwards under the wing and

note the differences at the front and rear of the wing.

Would this help you ?

Dave



PS how about a wingwalker tied on to top of plane?   Maybe he/she could have 

bird's eye  view to what is causing this mystery noise ?    How about a 

camcorder on tail?

---

lcfitt(at)sbcglobal.net
Guest

Posted: Fri Jan 12, 2007 7:00 am Post subject: Norwegian Stalls

 	  Quote: 	

 No. I think the sound is fabric slapping against the wing tanks.

Guy,  when I was building I was watching a couple of guys that were ahead of 

me in the process and the tops of their tanks looked marbelized because of 

the occasional fabric contact with the wing tanks.  Where it touched and 

where it didn't looked entirely different in paint - sort of ugly.

I went to lots of trouble ensuring that there was no paint adhesion on the 

tank top fabric.  I have had others fly close by and evaluate ballooning and 

they said they saw none.  But the real reason for the post is that my stalls 

are virtually silent.  I do, however have the IV wing while Michel has the 

III.

Lowell

 	  Quote: 	
		  >On Jan 9, 2007, at 6:01 PM, Larry Martin wrote:

> From Micheal's description, I had the impression that he

>thought that the nose dropped because the stab stalled.  This is not

>correct.  The

>nose drops because the center of pressure on the wing moves forward.

>

>... err, you mean 'move aft,' Larry, right? If the center of pressure

>moves forward and the CoG stays hopefully the same, the moment is then

>to create a nose-up attitude.

>This is interesting because years ago, before I started flying for

>real, I had that discussion with a flight simulator group where I meant

>to center of pressure (a.k.a center of lift) moved back as the plane

>slows down, hence the need to pull the stick. My assumption was based

>on the fact that a sailboats attempting to sail close to the wind (i.e.

>small AoA) needed to pull something called 'Cunningham hole' in order

>to make the leading edge stiffer and move the deepest hollow of the

>sail more forward - a sail being a "soft" airfoil.

>But I was told that the center of lift never moves on a wing, and that

>it is usually at about 1/4 of the cord, from the leading edge.

 Yes and No. You were correct in stating that the SYSTEM Cl moves back, 

 whereas the wing Cl remains constant before stall. (It moves after stall, 

 quite dramatically.) The reason the SYSTEM Cl moves back when you slow is 

 that down force on the tail diminishes as you slow, (remember lift is 

 proportional to the square of the velocity,) whereas the pitch moment due 

 to mass remains constant. You balance the mass pitch moment by increasing 

 the tail's down force by pulling the elevator up, increasing the tail's 

 negative Cl in direct proportion to the reduction in the square of the 

 velocity.

 Too confusing. Try this. I have a "wing" with 40 units of lift. 10 length 

 units behind that "wing" I have a "tail" flying along with -4 units of 

 lift. (It lifts down.) To balance this system my "airplane" weighs 36 

 units and it's CG is 1.11 length units in front of the "wing". Now I 

 instantaneously slow by half. My "wing" and "tail" lift reduce to 10 

 and -1 units respectively. (Lift is proportional to the square of the 

 velocity.) However the weight is still 36 at 1.11 in front of the wing. 

 Now there is a 30 unit moment pitching down. To counteract that I have to 

 increase the "tail" lift back to -4 units. I therefore have to quadruple 

 the Cl by pulling the elevator up. I then have 10 units of lift on the 

 "wing", -4 units of lift on the "tail", and 36 units of weight 1.11 in 

 front of the wing. The system balances about the wing, but I have 36 units 

 of weight versus 14 units of lift and so am accelerating down quite 

 rapidly. (I'll accelerate down until the increased wing angle of attack 

 (Cl) and the increased velocity bring the "wing" lift back to 40 units.) 

 Note that with a "wing" lift of 10 and a "tail" lift of -4, the SYSTEM Cl 

 is instantaneously quite far aft of what it was at equilibrium, when the 

 "wing" lift was 40.

 There. Clear as mud.

 Guy

---

rjdaugh

Joined: 30 Aug 2006
Posts: 195

Posted: Fri Jan 12, 2007 9:31 am Post subject: Norwegian Stalls

Michel,

I don't think you mean AOA, angle of attack.  AOA is measure relative to the

air.  Hard to measure without the right instruments.



I think you mean angle of incidence?  Someone will step in with the right

term..



Randy



.           



--

_________________
Randy
Kitfox 5/7 912S
Black Hills, South Dakota

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