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flase ribs

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kitfox555(at)sbcglobal.ne
Guest

Posted: Mon Mar 19, 2007 6:21 am Post subject: flase ribs

If I put false ribs in the stabs to increase lift, do I put them in the elevator and rudder or can I not put them in the rudder and elevator and just have them in the stabs?   Because I have the rudder already covered and not the stabs.



Jerry Evans 

kitfox 555 [quote][b]

n85ae

Joined: 14 Mar 2007
Posts: 403

Posted: Mon Mar 19, 2007 7:04 am Post subject: Re: flase ribs

[quote="kitfox555(at)sbcglobal.ne"]If I put false ribs in the stabs to increase lift, do I put them in the elevator and rudder or can I not put them in the rudder and elevator and just have them in the stabs?   Because I have the rudder already covered and not the stabs.



Jerry Evans 

kitfox 555  	  Quote: 	
		  [b] 	




The purpose of the false ribs is primarily to streamline the horiz/vert stab. The lift would be unchanged because the airfoiling is symmetric. The

rudder and elevator are still essentially flat (i.e. unchanged).



Regards,

Jeff

rliebmann(at)comcast.net
Guest

Posted: Mon Mar 19, 2007 7:09 am Post subject: flase ribs

Hi Jerry,

 The false ribs are not there to provide  any lift. If they are of a symetrical design, they will provide  stability.

 I have them in my fin but not the rudder  but they are in my stab & elevator. If you put them in the fin & stab  only, I think that that set-up would work fine.

  

 Take Care,    Ron       N55KF

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smokey_bear_40220(at)yaho
Guest

Posted: Mon Mar 19, 2007 7:10 am Post subject: flase ribs

I'll give a "stab" at this Jerry.  



If the front part, the stab, is a slab, it will have a

bit more drag and leave a larger boundry layer behind

it which will slightly reduce the control surface

effectiveness.  The control surface will have to move

a bit more to move out of the boundry layer and move

the slipstream.  But more importantly, the stab is

more effective and "hunts" less if it is not a slab. 

ie, ribs in the stab give more stability.



If the control surface has ribs, it will fill in the

boundry layer better behind the stab and be more

effective and less draggy whether it is behind a slab

or a ribbed stab.  So control surface ribs give more

elevator and rudder effectiveness.



Then there is appearance to consider.  Your choice

there.



Either surface ribbed will be an improvement.  Both

ribbed is better.  Ribs reduce drag just a little and

increase the surfaces effectiveness at its job.



Hope that helps as a first opinion.



Kurt S.  S-5

Retired at last, but somehow more busy.



--- jerry evans <kitfox555(at)sbcglobal.net> wrote:



 	  Quote: 	
		   If I put false ribs in the stabs to increase lift,

 do I put them in the elevator and rudder or can I

 not put them in the rudder and elevator and just

 have them in the stabs?   Because I have the rudder

 already covered and not the stabs.

 

 Jerry Evans 

 kitfox 555


	




 

Finding fabulous fares is fun.  

Let Yahoo! FareChase search your favorite travel sites to find flight and hotel bargains.

http://farechase.yahoo.com/promo-generic-14795097

Michel

Joined: 10 Jan 2006
Posts: 966
Location: Norway

Posted: Mon Mar 19, 2007 10:21 am Post subject: flase ribs

On Mar 19, 2007, at 4:09 PM, Ron Liebmann wrote:

 	  Quote: 	
		   The false ribs are not there to provide any lift.


	


Although I came to flying only four years ago and with my meagre 250 

hours, I consider myself still as a rookie, I will humbly object to the 

above statement.

A flight simulator I use allows me to select between several airfoils 

when constructing an aircraft. The simulator may not be reliable but 

the data coming from NACA surely is.

A symmetrical NACA airfoil has a much higher coefficient  of lift and 

drag than a flat plate. What is special with a symmetrical airfoil is 

that both lift and drag are symmetrical around 0 degrees of angle of 

incidence; while an asymmetrical airfoil may have a positive lift down 

to minus four degrees.



When the rudder is centred, false ribs won't do much of a difference. 

But when it is off-centred, it works like an asymmetrical airfoil (the 

MAC is no longer in the longitudinal section) and creates more lift 

(authority to sway) but also more drag (authority to return to the 

central position).



For the elevator and the horizontal stabiliser, it is slightly 

different. As I have learnt from this honourable list, the angle of 

incidence of horizontal stabiliser differs from the wing's angle of 

incidence by what is called, the decalage. If you are flying level at 

cruise speed, the angle of incidence of the wings' root may be e.g. 6 

degrees and the angle of incidence of the horizontal stabiliser, 3 

degrees. It may vary but the point is; the stabiliser must have a 

lesser angle than the wing to make sure it doesn't stall first, which 

could be catastrophic. Maybe the angle may be as low as 0 degrees, in 

which case, no lift nor induced drag is created. But if there is a 

slight angle - and there will certainly be at low speed - the use of 

false ribs should add some lift to the horizontal stabiliser.



Of course, I would welcome any objection to my above statement. It is 

mostly gathered from what I read here and I am avid to learn even more. 

My only request would then be: Please, don't spilt this list because of 

a flame! Thank you.



Cheers,

Michel

_________________
Kitfox 3 - Jabiru 2200

fox5flyer(at)idealwifi.ne
Guest

Posted: Mon Mar 19, 2007 10:53 am Post subject: flase ribs

Good post Michel.  You've gained a lot of aviation knowledge in a very short

time.

Thanks,

Deke

do not archive



---

Michel

Joined: 10 Jan 2006
Posts: 966
Location: Norway

Posted: Mon Mar 19, 2007 11:41 am Post subject: flase ribs

On Mar 19, 2007, at 8:53 PM, fox5flyer wrote:

 	  Quote: 	
		   Good post Michel.  You've gained a lot of aviation knowledge in a very 

 short time.


	


Thank you Deke but, reading again my post, I realise that I wrote sway 

when I meant yaw.

Roll, pitch and yaw are the three rotations while heave, sway and surge 

are the three movement, isn't it?



Michel



do not archive

_________________
Kitfox 3 - Jabiru 2200

Float Flyr

Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

Posted: Mon Mar 19, 2007 1:30 pm Post subject: flase ribs

Michel



Certainly no objection just a couple of  observations.



You said (at) 0 deg there would be no lift nor induced drag I'm not too sure

how you define induced drag.  One of the problems with a mass moving through

a fluid ( air ) is there is always drag induced by the movement.



The other observation I would like to make is that many planes e.g. the

CH701 actually have "upside down" airfoils on the tail to actually push the

tail down and apply load to the wings.  I'm interested in if you have tried

this design on your simulator.



BTW what sim are you using where you can use NACA foils??? 



Noel



[quote] --

_________________
Noel Loveys
Kitfox III-A
Aerocet 1100 Floats

Clem Nichols

Joined: 10 Jan 2006
Posts: 88
Location: Munfordville, Ky

Posted: Mon Mar 19, 2007 5:51 pm Post subject: flase ribs

Michel seems to be off line at the moment (what a fount of information!), so 

let me butt in and probably make a fool of myself.  As I understand it, the 

drag associated with a mass moving through air is parasitic drag unless lift 

is being produced in which case it is induced drag (plus the inevitable 

amount of parasitic drag).  The angle of attack of the horizontal stabilizer 

(either positive or negative) in level flight is determined by the relative 

positions of the center of lift and the center of gravity.  If the COG is in 

front of the COL then the tail of the plane has to exert a downward pull in 

order for the plane to be stable and fly level without constant back 

pressure on the stick.  This, I think, is the way most small planes are 

designed.  If, on the other hand, the COG is behind the COL, then the tail 

would have to produce lift to avoid a constant nose-high attitude.  At least 

this is how it's been explained to me.



Clem Nichols

Do Not Archive



---

MichaelGibbs(at)cox.net
Guest

Posted: Mon Mar 19, 2007 11:38 pm Post subject: flase ribs

Noel sez:



 	  Quote: 	
		  You said (at) 0 deg there would be no lift nor induced drag I'm not too 

sure how you define induced drag. One of the problems with a mass 

moving through a fluid ( air ) is there is always drag induced by 

the movement.


	


Clem is quite right.  Induced drag is the drag that comes from the 

production of lift.  You are referring to parasite drag, Noel.  No 

lift = no induced drag.



 	  Quote: 	
		  The other observation I would like to make is that many planes e.g. 

the CH701 actually have "upside down" airfoils on the tail to 

actually push the tail down and apply load to the wings.


	


With very few exceptions, all aircraft with conventional tails (those 

without canards) have downward lifting tails.  Having the center of 

mass forward of the center of lift is what provides stability in the 

pitch axis.



Mike G.

N728KF

Bob

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

Posted: Tue Mar 20, 2007 6:18 am Post subject: Re: flase ribs

Looks like it's mostly covered, just an engineer's input/side-track on airfoil moments.  To create lift, the wing airfoil tries to pitch it's nose downward (actually, the CL lifts the trailing edge) which engineers express as coefficient of moment (Cm). The tail is initially designed to offset that Cm and the download is typically roughly (note the combination of 2 weasel words) 10% of the wing lift. This is a drag that we pay for in fuel burn and it can be offset by using the cg in lieu of some tail downforce, or by adding a canard (which has it's own drag).  That's why cruise speed and fuel burn typically do a bit better at aft cg. We control the pitch of the plane by controlling how much or little the tail offsets the CM.  What we know as the ends of the cg range are just within the number of pounds of force that the elevator can make.  As we all know, bad things can happen when we exceed those limits of cg and elevator authority.  While we are on this side-track, wreckage of airplanes that lose part of the horizontal tail (like early Bonanzas) are easy to spot.  As soon as the tail starts to fail and not generate downforce, the nose snaps down and the wings break in a downward direction, rather than up. 



Back to the original question about adding ribs, though.  Airflow hates flat surfaces, especially if they can vibrate slightly.  It creates all sorts of drag to run air along a flat slab.  This is the not-so-secret of fish, birds, cars, hulls, etc. The early Kitfoxes and Avids flew fine without the ribs and they were lighter, but adding ribs in the later airplanes provided a curvature for the air to follow. The pressure of the leading edge of this slight airfoil shape will become positive and the aft portion will be negative, so there isn't the draggy turbulence created by vibrating the flat surface, either.  The net effect is that a slightly curved surface should reduce drag at cruise and delay boundary layer separation at the stalling point of that surface. 



Whether the speed differences at cuise and stall are enough for you to do the work is subjective and up to you.



Bob

Aero Eng (but work aircraft systems), A&P

_________________
Remember that internet advice may only be worth what you pay.

Michel

Joined: 10 Jan 2006
Posts: 966
Location: Norway

Posted: Tue Mar 20, 2007 9:08 am Post subject: flase ribs

A bit off-topic!



On Mar 19, 2007, at 10:30 PM, Noel Loveys wrote:

 	  Quote: 	
		   You said (at) 0 deg there would be no lift nor induced drag <SNIP>


	


Delayed by living in different time zones, you already go a very good 

answer from Clem, Noel, and the excellent explanation from Bob, which I 

enjoyed reading.



 	  Quote: 	
		   BTW what sim are you using where you can use NACA foils???


	


It is called X-Plane that you can find at www.x-plane.com. It runs on 

PC, Mac and Linux. You can download a free demo that will let you fly 

for six minutes with a joystick. After that, only the AP will work.

As the X in the name indicates, it is more a flight simulator for 

experimenting than Microsoft Flight Simulator. It comes with the tools 

to make you own aircraft and I would even say, any craft because I even 

modelled landsailers with it. After all, a sail is nothing more than a 

not-so-good wing profile.



The simulator is based on what is called the blade theory, where all 

elements are divided in a number of objects with eventual lift, drag 

and moment; the latter being what Bob talks about.



Of course, there are limitations. Airfoils data are tested in wind 

tunnels and go up to 20 degrees angle of incidence. That's because 

nearly all will stall beyond that. It means that when flying in a 

normal attitude envelope, the simulation is quite good. But beyond 

that, as e.g. in a spin; it may not be realistic at all. But then, no 

simulator will because such an attitude would request the use of CFD 

(Computational Fluid Dynamic) where each "parcel" of air is simulated 

and an individual object. That kind of simulation cannot happen on home 

computers at real-time speed, unfortunately.



But I have learnt a lot with X-Plane and I still enjoy changing small 

parameters and see how it affects the behaviour. My Kitfox is, of 

course, modelled as well as I could (with 3D cockpit) and is available 

at x-plane.org, the place where one distribute aircraft and scenery.



Cheers,

Michel



do not archive

_________________
Kitfox 3 - Jabiru 2200

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