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challgren(at)mac.com
Guest

Posted: Wed Jan 16, 2008 9:15 am Post subject: Holding Nose up

Andy Shontz:



You asked;



"One thing, it appears as though on landing the nose comes down pretty  

quick. I think other people have noted this one their XLs as well. Is  

it fairly difficult to hold the nose up once the mains hit?"



I assume the XL is similar to the HDS and it was impossible to hold  

the nose up on ours after touchdown.  We solved that problem by  

placing VG's on the bottom side of the horizontal stabilizer.  With  

the VG's there, you could aerodynamic brake down to about 20 mph.



Your other question:



" I was thinking a tail skid of some sort might be a good idea when I  

get around to building the fuselage, to not only protect the fuselage  

but also the rudder."



The HDS had a very effective tail skid in the tie down ring.  I  

discovered how effective it was when I failed to ask my BFR check  

pilot how much he weighed.  I had planned on 170# and after grinding  

down the tail skid on takeoff I found he weighed 240#.



Stan



p.s. Let the flames began.

professor71(at)HOTMAIL.CO
Guest

Posted: Wed Jan 16, 2008 1:06 pm Post subject: Holding Nose up

I have an HD which has a more similar wing to the XL and find that holding the nose up is not difficult if you come in at a little higher speed. 



Stan I am interested in the HDS wing and wondered if you also put vgs on the wing and what kind of differences did you notice in climb and stall speeds.



John

Make distant family not so distant with Windows Vista® + Windows Live™. Start now!  [quote][b]

daveaustin2(at)primus.ca
Guest

Posted: Wed Jan 16, 2008 3:59 pm Post subject: Holding Nose up

John,

 You'll find the details of my flight test with VGs  on my 601 HDS in the archives.  They are well worth the money.

 Dave Austin  601HDS - 912, Spitfire Mk  VIII

    [quote][b]

amyvega2005(at)earthlink.
Guest

Posted: Wed Jan 16, 2008 5:12 pm Post subject: Holding Nose up

Stan,

it depends on how you place the main gear position, flat side forward and what engine weight you have.  I put my mains flat side forward and can hold the nose off at 25 mph.  I powered it with the 180lb jabiru so it works out well that way. if you endine is lighter say a 912 ULS, they recommedn the mains at flat side rear facing.



Juan



--

vgstol(at)bigpond.net.au
Guest

Posted: Wed Jan 16, 2008 7:36 pm Post subject: Holding Nose up

For info on the HDS wing with  VGs have a look at http://www.stolspeed.com/content.php?id=57

  

 JG

 [quote]   ---

Terry Phillips

Joined: 11 Jan 2006
Posts: 346
Location: Corvallis, MT

Posted: Wed Jan 16, 2008 9:57 pm Post subject: Holding Nose up

Dave



 I have searched the Matronics Zenith-list archives for this w/o success. I guess that I just cannot find the right search string. Or perhaps I should be searching some other archive. Could you post a URL that would direct me to the thread containing the details cited below? 



 Thanks for your help.



 Terry



 

 At 06:53 PM 1/16/2008 -0500, you wrote:

  	  Quote: 	
		  John,

 You'll find the details of my flight test with VGs on my 601 HDS in the archives.  They are well worth the money.

 Dave Austin  601HDS - 912, Spitfire Mk VIII 	
 

 

 Terry Phillips

 ttp44~at~rkymtn.net

 Corvallis MT

 601XL/Jab 3300 slow build kit - Tail is finished; working on the wings

 http://www.mykitlog.com/N47TP/    [quote][b]

_________________
Terry Phillips
Corvallis, MT
ttp44<at>rkymtn.net
Zenith 601XL/Jab 3300 slow build kit - Tail feathers done; working on the wings.

ashontz

Joined: 27 Dec 2006
Posts: 723

Posted: Thu Jan 17, 2008 2:15 pm Post subject: Re: Holding Nose up

Thanks. Interesting adaptation. I'll keep that in mind when I get to the fuselage. I'd like to be able to hold nose off and be in better control of things and aerodynamically brake the way I learned in a Cessna 152. Just seems right to me. Plus, if I ever NEED to keep the nose up, like a short or soft-field landing, I'll be able to do it.



 	  challgren(at)mac.com wrote: 	
		  Andy Shontz:



You asked;



"One thing, it appears as though on landing the nose comes down pretty  

quick. I think other people have noted this one their XLs as well. Is  

it fairly difficult to hold the nose up once the mains hit?"



I assume the XL is similar to the HDS and it was impossible to hold  

the nose up on ours after touchdown.  We solved that problem by  

placing VG's on the bottom side of the horizontal stabilizer.  With  

the VG's there, you could aerodynamic brake down to about 20 mph.



Your other question:



" I was thinking a tail skid of some sort might be a good idea when I  

get around to building the fuselage, to not only protect the fuselage  

but also the rudder."



The HDS had a very effective tail skid in the tie down ring.  I  

discovered how effective it was when I failed to ask my BFR check  

pilot how much he weighed.  I had planned on 170# and after grinding  

down the tail skid on takeoff I found he weighed 240#.



Stan



p.s. Let the flames began.

amyvega2005(at)earthlink.
Guest

Posted: Thu Jan 17, 2008 5:39 pm Post subject: Holding Nose up

i am able to hold the nose off up to apprx. 20 mph. with full back stick.  tail skiid for a cessna works well if you tend to flare too much.



Juan

--

Terry Phillips

Joined: 11 Jan 2006
Posts: 346
Location: Corvallis, MT

Posted: Sat Jan 19, 2008 8:51 pm Post subject: Holding Nose up

Stan



Thank you very much for your post. No flames here. I think you're onto something good.



It really got me to thinking about the aerodynamics of the horizontal stabilizer. Please indulge me while I ramble about this. 



ZAC told me that the stabilizer airfoil is symetrical. Since the stabilizer exerts a downward force to balance the moment of the engine around the aircraft cg, the symetrical airfoil must be angled slightly downward to generate a downward force with neutral trim. After touchdown, with the nose up, the stabilizer would be angled upward, generating an upward force that would rotate the A/C around the mains, pushing the nose down.  So, to hold the nose up after touchdown, you would need to pull the stick aft, forcing the elevator up. And, what Stan is saying, is that VGs on the underside of the stabilizer will increase up-elevator effectiveness (apparently) by reducing boundary layer separation on the bottom of the elevator. That all makes sense to me now. 



So, I would like to ask Stan the usual questions, how many VG's? Where are they on the stab? Do you have any photos? Thank you for posting your experience.



This is a very interesting subject.



Terry

-----Original message-----

From: Stanley Challgren challgren(at)mac.com

Date: Wed, 16 Jan 2008 10:33:29 -0700

To: Zenith List zenith-list(at)matronics.com

Subject: Holding Nose up



 	  Quote: 	
		   

 

 Andy Shontz:

 

 You asked;

 

 "One thing, it appears as though on landing the nose comes down pretty  

 quick. I think other people have noted this one their XLs as well. Is  

 it fairly difficult to hold the nose up once the mains hit?"

 

 I assume the XL is similar to the HDS and it was impossible to hold  

 the nose up on ours after touchdown.  We solved that problem by  

 placing VG's on the bottom side of the horizontal stabilizer.  With  

 the VG's there, you could aerodynamic brake down to about 20 mph.

 

 Your other question:

 

 " I was thinking a tail skid of some sort might be a good idea when I  

 get around to building the fuselage, to not only protect the fuselage  

 but also the rudder."

 

 The HDS had a very effective tail skid in the tie down ring.  I  

 discovered how effective it was when I failed to ask my BFR check  

 pilot how much he weighed.  I had planned on 170# and after grinding  

 down the tail skid on takeoff I found he weighed 240#.

 

 Stan

 

 p.s. Let the flames began.

 


	




Terry Phillips

ttp44~at~rkymtn.net

Corvallis MT

601XL/Jab 3300 slow build kit - Tail is finished; working on the wings

http://www.mykitlog.com/N47TP/

_________________
Terry Phillips
Corvallis, MT
ttp44<at>rkymtn.net
Zenith 601XL/Jab 3300 slow build kit - Tail feathers done; working on the wings.

planecrazydld(at)yahoo.co
Guest

Posted: Sun Jan 20, 2008 4:18 am Post subject: Holding Nose up

do not forget that the stabilizer's primary job is balancing the pitching moment of the airfoil. This why trim changes occur with loading and speed.



Terry Phillips <ttp44(at)rkymtn.net> wrote:[quote] --> Zenith-List message posted by: "Terry Phillips" 



Stan



Thank you very much for your post. No flames here. I think you're onto something good.



It really got me to thinking about the aerodynamics of the horizontal stabilizer. Please indulge me while I ramble about this. 



ZAC told me that the stabilizer airfoil is symetrical. Since the stabilizer exerts a downward force to balance the moment of the engine around the aircraft cg, the symetrical airfoil must be angled slightly downward to generate a downward force with neutral trim. After touchdown, with the nose up, the stabilizer would be angled  upward, generating an upward force that would rotate the A/C around the mains, pushing the nose down.  So, to hold the nose up after touchdown, you would need to pull the stick aft, forcing the elevator up. And, what Stan is saying, is that VGs on the underside of the stabilizer will increase up-elevator effectiveness (apparently) by reducing boundary layer separation on the bottom of the elevator. That all makes sense to me now. 



So, I would like to ask Stan the usual questions, how many VG's? Where are they on the stab? Do you have any photos? Thank you for posting your experience.



This is a very interesting subject.



Terry

-----Original message-----

From: Stanley Challgren challgren(at)mac.com

Date: Wed, 16 Jan 2008 10:33:29 -0700

To: Zenith List zenith-list(at)matronics.com

Subject: Holding Nose up



[quote] --> Zenith-List message posted by: Stanley Challgren 

 

 Andy  Shontz:

 

 You asked;

 

 "One thing, it appears as though on landing the nose comes down pretty  

 quick. I think other people have noted this one their XLs as well. Is  

 it fairly difficult to hold the nose up once the mains hit?"

 

 I assume the XL is similar to the HDS and it was impossible to hold  

 the nose up on ours after touchdown.  We solved that problem by  

 placing VG's on the bottom side of the horizontal stabilizer.  With  

 the VG's there, you could aerodynamic brake down to about 20 mph.

 

 Your other question:

 

 " I was thinking a tail skid of some sort might be a good idea when I  

 get around to building the fuselage, to not only protect the fuselage  

 but also the rudder."

 

 The HDS had a very effective tail skid in the tie down ring.  I  

 discovered how effective it was when I failed to ask my BFR check  [quote][b]

amyvega2005(at)earthlink.
Guest

Posted: Sun Jan 20, 2008 8:44 am Post subject: Holding Nose up

Dave,



the VGs may be all well and good, i dont know much bout the effects of the VG in back to comment on that ,   however the hold of nose is mostly a CG effect.  not just the weight but where the main wheels are.  There are two ways the mains can go on an XL mians forward or back.  if the mains are on and fix in position, just put some weight in tail.  Does not have to be much.  

I suggest trying that first.  put a small 2 lb weight to the rear, first, then cruise down the runway, and adjust the weight until front nose wheel comes off at 25 mph plus.



Juan

--

amyvega2005(at)earthlink.
Guest

Posted: Sun Jan 20, 2008 8:47 am Post subject: Holding Nose up

to clarify, I do not have the nose sticking issue on my XL.  Mine comes off at 20 mph approx.  It is a weight issue.  you need to adjust weight to the rear or adjust the main wheels forward.



Juan



--

dredmoody(at)cox.net
Guest

Posted: Sun Jan 20, 2008 9:24 am Post subject: Holding Nose up

Your idea of adding a bit of weight far aft to  help un-stick the nose wheel needs to be factored into weight and balance to see  how much you are moving the CG. However, if it makes the plane un-stick  more appropriately the chances are that the CG will be well inside the envelope  since the mains are still in the design location.

  

 A second thought is that if someone needs to add  any weight that far back, it would be useful to do so in the form of a more  robust tail skid, particularly since the operational affect would be to make a  tailstrike more likely. 

  

 Just thinking out loud,

  

 Dred

 [quote]   ---

tonyplane(at)bellsouth.ne
Guest

Posted: Sun Jan 20, 2008 11:27 am Post subject: Holding Nose up

Speaking ONLY for my XL; N493TG. (Serial Number 6-5342) - I can not hold the

nose up after landing.



1.  Particulars:

     A.  Empty weight :799; empty  CG 290 mm  (My XL is probably one of the

heaviest empty - It has aux tanks, lighting/strobes all around, 6x600

tires/wheels/brakes with an 801 nose fork; dual sticks, and a lot of "stuff"

on the panel (EFIS, EIS, AP, T&B,Comm,Xpndr, AS/Atl steam gauges etc)..  It 

has a

purring Jab3300 and a ground adjustable Sensenich.



      B.  To "slightly" exceed aft cg (without baggage) would require zero

fuel and crew weight of about 540 lbs. To exceed fwd CG would require about 

a 110 lb

pilot with full fuel (48 gal) at take-off.  To exceed the CG limits, I would 

have

to be extremely negligent.  ((I did refuse to fly a 280# teen age "Young

Eagle".  I weigh about 200/210 and still had a lot of fuel on board (over 

gross) and an OAT of 94F

-  We got him a ride in a twin that day)).



      C.  Trim required during cruise - a little nose up with one SOB, no

wing heaviness, roll trim slight to either side depending on fuel state,

passenger wt.



       D.  My landing gear was installed with the then build instructions,

which put the wheels further aft than the current build instructions, which

reverses the landing gear relative to mine, and places the wheels closer to

the CG. (GOOD!)



        E. My elevator trim tab is the smaller one, not the current almost

full elevator span one.  My full flaps can go to around 27/28 deg - I 

believe I

read that the current flap actuator only provides 20 deg.  I can not hold

approach speed with trim (I like to use 65 kts on final until over the

fence/trees) with flaps (flaps result in a significant nose down pitching 

moment).



        F.  Total flight hours on 3TG to date: 343.  Total: TO/LDG: 672



2.  On 3TG's first flight at zero trim, full fuel,.zero flaps, I was going

to just let the airplane lift off, but when I was still stuck to the ground

at 70 kts, I applied back pressure and was airborne.  I believe that 3TG

could possible use all the runway and not lift off without some "up" 

elevator.

First landing was zero flaps -3TG flew like.... an airplane- no surprises.



3.  As soon as 3TG touches, the nose comes down even with full aft stick.

As the nose is coming down, I release back pressure. If I do not release

back pressure, the nose will sometimes bounce- height of bounce depends on 

sink rate.

 If my speed is a "little" high at touch down and if I keep full up stick, 

the nose

will bounce up higher and come down again and bounce again.

I usually use partial to mainly full flaps (depending on

cross wind/soft field), but landing without flaps is easier, in my opinion.



4.   In my experience, with my XL, with its gear arrangement:  THING TO 

AVOID DURING LANDING!!

AVOID a high sink rate landing - low or high airspeed.  The nose will bounce 

you up dratimatically.

GO AROUND is recommended or the second bounce will be nasty.

If you get too slow close in, you increase your sink rate and pitch 

authority will not help much in the flare -

recommend squeeze in some power to arrest sink and also so you have some 

energy to flare



Tony Graziano

N493TG

--------------------------



      Subject:      Re: Holding Nose up

      From:      Edward Moody II (dredmoody(at)cox.net)

      Date:      Sun Jan 20 - 9:24 AM



Your idea of adding a bit of weight far aft to help un-stick the nose

wheel needs to be factored into weight and balance to see how much you

are moving the CG. However, if it makes the plane un-stick more

appropriately the chances are that the CG will be well inside the

envelope since the mains are still in the design location.



A second thought is that if someone needs to add any weight that far

back, it would be useful to do so in the form of a more robust tail

skid, particularly since the operational affect would be to make a

tailstrike more likely.



Just thinking out loud,



Dred

  ---

planecrazydld(at)yahoo.co
Guest

Posted: Sun Jan 20, 2008 1:18 pm Post subject: Holding Nose up

a while back there was a lot of discussion about main gear spring placement. Zenith advised back then that there are 2 correct placements: straight edge aft and straight wedge forward, depending on the installed engine weight. If your CG is correct and the nose falls through after touchdown - and the straight edge of the gear spring as aft, then reverse it to put the straight edge forward. If it already is straight edge forward then be very careful in adding weight aft and be certain to do all the possible W&B trade studies to be absolutely certain that you are going aft of the limit.



Aft CG (even within the limits) makes it VERY easy to overload ANY aircraft as they are a lot more responsive in pitch when CG is aft. 



All a fwd CG usually does is cause the plane to stall faster - and it makes the plane "groove" more therefore less likely to respond to enthusiastic pitch control inputs.



Edward Moody II <dredmoody(at)cox.net>  wrote:[quote]       Your idea of adding a bit of weight far aft to  help un-stick the nose wheel needs to be factored into weight and balance to see  how much you are moving the CG. However, if it makes the plane un-stick  more appropriately the chances are that the CG will be well inside the envelope  since the mains are still in the design location.

  

 A second thought is that if someone needs to add  any weight that far back, it would be useful to do so in the form of a more  robust tail skid, particularly since the operational affect would be to make a  tailstrike more likely. 

   

 Just thinking out loud,

  

 Dred

 [quote]   ---

challgren(at)mac.com
Guest

Posted: Mon Jan 21, 2008 9:38 am Post subject: Holding Nose up

On Jan 19, 2008, at 9:48 PM, Terry Phillips wrote:



 	  Quote: 	
		   



 Stan



 Thank you very much for your post. No flames here. I think you're  

 onto something good.



 It really got me to thinking about the aerodynamics of the  

 horizontal stabilizer. Please indulge me while I ramble about this.



 ZAC told me that the stabilizer airfoil is symetrical. Since the  

 stabilizer exerts a downward force to balance the moment of the  

 engine around the aircraft cg, the symetrical airfoil must be angled  

 slightly downward to generate a downward force with neutral trim.  

 After touchdown, with the nose up, the stabilizer would be angled  

 upward, generating an upward force that would rotate the A/C around  

 the mains, pushing the nose down.  So, to hold the nose up after  

 touchdown, you would need to pull the stick aft, forcing the  

 elevator up. And, what Stan is saying, is that VGs on the underside  

 of the stabilizer will increase up-elevator effectiveness  

 (apparently) by reducing boundary layer separation on the bottom of  

 the elevator. That all makes sense to me now.



 So, I would like to ask Stan the usual questions, how many VG's?  

 Where are they on the stab? Do you have any photos? Thank you for  

 posting your experience.



 This is a very interesting subject.


	


Terry:



How many VG's?  I believe there were 42 VG's on each side.  I used the  

same spacing and template on the underside of the stabilizer as I did  

on the wing so there would have been about 2.5" between them.  They  

were placed at 10% of the chord.  The below photos are of the wing.   

Guess my neck was already stiff from working upside down applying them  

so I skipped the photos.



Stan



Any Photos?

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Terry Phillips

Joined: 11 Jan 2006
Posts: 346
Location: Corvallis, MT

Posted: Tue Jan 22, 2008 10:57 pm Post subject: Holding Nose up

Hi Paul

'Sorry it has taken so long to answer. I've been visiting my daughter in 

Seattle. I forgot to take the charger for my laptop, so I've been 

incommunicado for the past 4 days.

The way I visualize this, is that, ideally, the horizontal stabilizer 

should be installed so that, at cruise with neutral trim and elevator, it 

exerts a downward force that just balances the moment exerted by the heavy 

weight of the engine at the front of the aircraft. I think that would be 

the minimum drag design. ZAC told me that the zodiac stabilizer was 

symmetrical--an inspection of the drawings would tell me the same thing, if 

I was clever enough to look. So, I think, that, if the stabilizer was 

parallel to the airstream flowing aft of the wings at cruise, then it would 

not produce a force up or down. Since, in tractor designs, the function of 

the stabilizer is to balance the weight of the engine up front, then the 

stabilizer must be angled slightly upwards w.r.t. the airstream. Then, just 

like when you angle your hand upward out the window of a moving car, it 

would generate the required upward force. Since an airplane in flight will 

rotate around the cg, the balancing moment must be w.r.t. the cg.

When the airplane touches down on the mains, the nose (hopefully) will be 

upward. If the nose is high (relative to cruise angle of attack) then the 

tail will be low, and the horizontal stabilizer will be making an upward 

angle with respect to the airflow past the airplane. Thus (I think) that 

the air flowing past the stabilizer will be pushing the stabilizer upward, 

driving the nose wheel to the ground.

The elevator, of course, is designed to bend the air flow so that, by 

pulling back on the stick, the force exerted by the stabilizer/elevator can 

be changed from the upward force that would exist with a neutral elevator, 

to a net downward force. But, if I interpret Stan's results correctly, the 

unmodified zodiac elevator is incapable of holding the nose up after touch 

down. But, Stan's addition of VGs to the underside of the stabilizer, 

increased the effectiveness of the elevator on Stan's airplane. Then he 

could hold the nose up until he slowed to 20 mph.

So that's what it looks like to me.

My understanding of VGs is that they function by causing the laminar 

boundary layer over an airfoil to transition to a turbulent boundary layer. 

Turbulent boundary layers  (for reasons I do not understand) stay attached 

better than laminar boundary layers. And by staying attached, the airflow 

over the airfoil exerts a net lower pressure on the bottom of the 

stabilizer/elevator (think Bernoulli's law), thus effectively pulling the 

tail down (and the nose up).

There are a couple of other effects in play here.

One is that, on the ground, the plane will rotate around the mains, rather 

than around the cg. The cg is forward of mains, so the moment arm of the 

stabilizer around the center of rotation is shorter when the plane is on 

the ground. So moment from the force produced by the stabilizer will be 

less than the moment produced by the same force when the plane is flying. 

So, on the ground, a greater downward force is needed to raise the nose 

than when flying.

The second is that, when the plane is flying, the wings must cause the 

airstream aft of the wings to deflect downward (to balance the upward lift 

on the wings).  So, the horizontal stabilizer sees an airstream that has a 

net downward velocity. If the horizontal stabilizer is to generate a net 

upward force at cruise, then it must be inclined upward at a great angle 

than would be necessary if the airflow were horizontal. On the ground, the 

wings still try to deflect the air flow downward. But the ground is in the 

way. So the airstream the horizontal stabilizer/elevator sees will be 

flowing nearly parallel to the ground. The horizontal air flow on the 

ground should further increase the upward force generated by the horizontal 

stabilizer.

So, why is Stan's result important to me, since zodiac's have been landing 

just fine for years, plunking their nose down on the runway right after 

touch down? One reason would be soft field takeoff and landing capability. 

In a 152, a skilled pilot can land on the mains, and keep the nose up, 

until, maybe 20 mph or less. And with full up elevator one can taxi a 152 

with the nose way up in the air, thus reducing the nose wheel loading. I'd 

like to be able to imitate that with my zodiac on some of the back country 

airfields here in western Montana. So, I think Stan's result is pretty 

exciting.

First I have this airplane to build!

Terry

At 04:09 AM 1/20/2008 -0800, you wrote:

 	  Quote: 	
		  Hi Terry,

I'm confused.  Why does touching down change the force generated by the 

stabilizer from downward to upward?  Do you experience a huge difference 

in control feedback when the wheels touch down?

If the VGs have any impact at all on this phenomenon, it must be because 

the VGs generate lift in the negative direction for the stabilizer.  That 

way the lift would be reduced as the airspeed and ground speed are reduced 

during landing.  I don't see how this is possible since the stabilizer has 

a positive angle of attack after touchdown.  However, the combined 

stabilizer and elevator must have a negative total force on the airplane's 

tail for the nose to be elevated on landing.

Maybe I'll just stay confused.

Paul

At 08:48 PM 1/19/2008, you wrote:

>Since the stabilizer exerts a downward force to balance the moment of the 

>engine around the aircraft cg, the symetrical airfoil must be angled 

>slightly downward to generate a downward force with neutral trim. After 

>touchdown, with the nose up, the stabilizer would be angled upward, 

>generating an upward force that would rotate the A/C around the mains, 

>pushing the nose down.

Terry Phillips

ttp44~at~rkymtn.net

Corvallis MT

601XL/Jab 3300 slow build kit - Tail is finished; working on the wings

http://www.mykitlog.com/N47TP/

_________________
Terry Phillips
Corvallis, MT
ttp44<at>rkymtn.net
Zenith 601XL/Jab 3300 slow build kit - Tail feathers done; working on the wings.

Terry Phillips

Joined: 11 Jan 2006
Posts: 346
Location: Corvallis, MT

Posted: Tue Jan 22, 2008 11:50 pm Post subject: Holding Nose up

Whoops--got that backwards. The stabilizer generates a downward force, so 

must be angled downward at neutral elevator and trim. What can I say--I'm a 

geometrically challenged chemical engineer--that's why I'm not an ME. I 

think I'll go to bed and try to rethink my reply in the morning.



Terry

At 11:46 PM 1/22/2008 -0700, you wrote:

 	  Quote: 	
		  Hi Paul



'Sorry it has taken so long to answer. I've been visiting my daughter in 

Seattle. I forgot to take the charger for my laptop, so I've been 

incommunicado for the past 4 days.



The way I visualize this, is that, ideally, the horizontal stabilizer 

should be installed so that, at cruise with neutral trim and elevator, it 

exerts a downward force that just balances the moment exerted by the heavy 

weight of the engine at the front of the aircraft. I think that would be 

the minimum drag design. ZAC told me that the zodiac stabilizer was 

symmetrical--an inspection of the drawings would tell me the same thing, 

if I was clever enough to look. So, I think, that, if the stabilizer was 

parallel to the airstream flowing aft of the wings at cruise, then it 

would not produce a force up or down. Since, in tractor designs, the 

function of the stabilizer is to balance the weight of the engine up 

front, then the stabilizer must be angled slightly upwards w.r.t. the 

airstream. Then, just like when you angle your hand upward out the window 

of a moving car, it would generate the required upward force. Since an 

airplane in flight will rotate around the cg, the balancing moment must be 

w.r.t. the cg.



When the airplane touches down on the mains, the nose (hopefully) will be 

upward. If the nose is high (relative to cruise angle of attack) then the 

tail will be low, and the horizontal stabilizer will be making an upward 

angle with respect to the airflow past the airplane. Thus (I think) that 

the air flowing past the stabilizer will be pushing the stabilizer upward, 

driving the nose wheel to the ground.



The elevator, of course, is designed to bend the air flow so that, by 

pulling back on the stick, the force exerted by the stabilizer/elevator 

can be changed from the upward force that would exist with a neutral 

elevator, to a net downward force. But, if I interpret Stan's results 

correctly, the unmodified zodiac elevator is incapable of holding the nose 

up after touch down. But, Stan's addition of VGs to the underside of the 

stabilizer, increased the effectiveness of the elevator on Stan's 

airplane. Then he could hold the nose up until he slowed to 20 mph.



So that's what it looks like to me.



My understanding of VGs is that they function by causing the laminar 

boundary layer over an airfoil to transition to a turbulent boundary 

layer. Turbulent boundary layers  (for reasons I do not understand) stay 

attached better than laminar boundary layers. And by staying attached, the 

airflow over the airfoil exerts a net lower pressure on the bottom of the 

stabilizer/elevator (think Bernoulli's law), thus effectively pulling the 

tail down (and the nose up).



There are a couple of other effects in play here.



One is that, on the ground, the plane will rotate around the mains, rather 

than around the cg. The cg is forward of mains, so the moment arm of the 

stabilizer around the center of rotation is shorter when the plane is on 

the ground. So moment from the force produced by the stabilizer will be 

less than the moment produced by the same force when the plane is flying. 

So, on the ground, a greater downward force is needed to raise the nose 

than when flying.



The second is that, when the plane is flying, the wings must cause the 

airstream aft of the wings to deflect downward (to balance the upward lift 

on the wings).  So, the horizontal stabilizer sees an airstream that has a 

net downward velocity. If the horizontal stabilizer is to generate a net 

upward force at cruise, then it must be inclined upward at a great angle 

than would be necessary if the airflow were horizontal. On the ground, the 

wings still try to deflect the air flow downward. But the ground is in the 

way. So the airstream the horizontal stabilizer/elevator sees will be 

flowing nearly parallel to the ground. The horizontal air flow on the 

ground should further increase the upward force generated by the 

horizontal stabilizer.



So, why is Stan's result important to me, since zodiac's have been landing 

just fine for years, plunking their nose down on the runway right after 

touch down? One reason would be soft field takeoff and landing capability. 

In a 152, a skilled pilot can land on the mains, and keep the nose up, 

until, maybe 20 mph or less. And with full up elevator one can taxi a 152 

with the nose way up in the air, thus reducing the nose wheel loading. I'd 

like to be able to imitate that with my zodiac on some of the back country 

airfields here in western Montana. So, I think Stan's result is pretty 

exciting.



First I have this airplane to build!



Terry


	




Terry Phillips

ttp44~at~rkymtn.net

Corvallis MT

601XL/Jab 3300 slow build kit - Tail is finished; working on the wings

http://www.mykitlog.com/N47TP/

_________________
Terry Phillips
Corvallis, MT
ttp44<at>rkymtn.net
Zenith 601XL/Jab 3300 slow build kit - Tail feathers done; working on the wings.

psm(at)att.net
Guest

Posted: Wed Jan 23, 2008 3:46 am Post subject: Holding Nose up

Hi Terry ,



Thanks for writing.  I too am a retired challenged engineer, but my 

field was electronics rather than those funny little chemical 

things.  (Actually, I started majoring in chemistry but managed to 

screw that up  -- but that is another story for another 

day.)  Fortunately, the area I am challenged in is not geometry -- 

especially when it comes to visualizing airplane dynamic 

balances.  (When I broke my relationship with the chemistry 

department, I changed my major to Mathematics.)



Yes, the forces exerted by the stabilizer and elevator are generally 

downward.  Only when you push on the stick is there an upward force 

from the elevator, and you don't tend to do that often.  I figure 

that means the leading edge of the stabilizer should be slightly down 

compared to the trailing edge.  I haven't yet set my stabilizer in 

place so I am not sure how the design calls out this angle.



If you use tricycle gear configuration then the main wheels must be 

behind the airplane's CG.  If not, the tail will slide along the 

runway rather than the nose gear meeting the runway.  When landing, 

the transition from controlling rotation around the CG to rotation 

around the main wheels happens as you described nicely in your fist 

message yesterday.  Since the forces were in balance and probably 

nicely trimmed out just before touching down it is the amount of 

nose-down force caused by the relationship between the CG and the 

main wheels that makes this whole story work.  If the main wheels are 

too far behind the CG then the nose will come crashing down as the 

weight of the plane is transferred from the wings to the 

wheels.  This problem is more pronounced if the CG is at the front 

end of the envelope.



You can spend a lot of time getting the CG to work for flying 

conditions, and this has little impact on the behavior of the plane 

when the wheels are holding the plane up.  It is more important that 

the flying stability is correct than the landing dynamics are 

comfortable.  That means (to me) that the way to correct the landing 

situation is to move the wheels rather than moving the CG.  For this 

tendency that some (not all) Zodiacs seem to have to force the nose 

down quickly on landing the solution seems to be to move the main 

wheels forward.  I don't know how much movement is needed, but the 

first step is to mount the main gear with the flat edge forward to 

make the wheels move a little forward of the position they would have 

if the flat edge is mounted facing aft.  If this isn't enough to fix 

the landing difficulty then it starts getting a lot harder to change 

the plane to get nicer landing behavior.



The other side of the main wheel vs. CG question comes in when you 

have 250 pound folks standing on the step behind the wing to get into 

the plane.  If the wheels are positioned nicely for the landing 

dynamics to work out then the plane will tend to drop to its tail 

when the big guy (or gal) tries to get in.  As with chemical and 

electrical engineering, airplane design is a game of trade-offs.



I hope this discussion helps us understand the landing issue.  My 

plan is to use the flat side of the main landing gear mounted forward 

and hope for the best.



Good luck,



Paul

At 11:47 PM 1/22/2008, you wrote:

 	  Quote: 	
		  Whoops--got that backwards. The stabilizer generates a downward 

force, so must be angled downward at neutral elevator and trim. What 

can I say--I'm a geometrically challenged chemical engineer--that's 

why I'm not an ME. I think I'll go to bed and try to rethink my 

reply in the morning.



Terry

Terry Phillips

Joined: 11 Jan 2006
Posts: 346
Location: Corvallis, MT

Posted: Wed Jan 23, 2008 10:44 am Post subject: Holding Nose up

Paul



Thank you for the post. This is all starting to make much more sense to me. 

And the discussion has proven to me, once again, a truism from my working 

days. Only by explaining something, giving a talk, whatever, do I really 

begin to understand it. I learn even more when I get it wrong the 1st time. 

I have read many posts on this forum about which way the landing gear 

should face. Now, finally, I'm beginning to understand what the discussion 

is all about.



And, now I see that VGs on the underside of the stabilizer, can delay the 

low speed stall of the stabilizer airfoil that occurs at high angle of 

attack (i.e., nose up upon landing). Thanks Paul, Stan, Tony, and all who 

responded. I think I can build a better airplane now, and, stabilizer VGs 

will be part of it. I think I'll head for the shop and see if it has warmed 

up from this morning's low of 34°.



Terry

At 03:40 AM 1/23/2008 -0800, you wrote:

 	  Quote: 	
		  Hi Terry ,



Thanks for writing.  I too am a retired challenged engineer, but my field 

was electronics rather than those funny little chemical 

things.  (Actually, I started majoring in chemistry but managed to screw 

that up  -- but that is another story for another day.)  Fortunately, the 

area I am challenged in is not geometry -- especially when it comes to 

visualizing airplane dynamic balances.  (When I broke my relationship with 

the chemistry department, I changed my major to Mathematics.)



Yes, the forces exerted by the stabilizer and elevator are generally 

downward.  Only when you push on the stick is there an upward force from 

the elevator, and you don't tend to do that often.  I figure that means 

the leading edge of the stabilizer should be slightly down compared to the 

trailing edge.  I haven't yet set my stabilizer in place so I am not sure 

how the design calls out this angle.



If you use tricycle gear configuration then the main wheels must be behind 

the airplane's CG.  If not, the tail will slide along the runway rather 

than the nose gear meeting the runway.  When landing, the transition from 

controlling rotation around the CG to rotation around the main wheels 

happens as you described nicely in your fist message yesterday.  Since the 

forces were in balance and probably nicely trimmed out just before 

touching down it is the amount of nose-down force caused by the 

relationship between the CG and the main wheels that makes this whole 

story work.  If the main wheels are too far behind the CG then the nose 

will come crashing down as the weight of the plane is transferred from the 

wings to the wheels.  This problem is more pronounced if the CG is at the 

front end of the envelope.



You can spend a lot of time getting the CG to work for flying conditions, 

and this has little impact on the behavior of the plane when the wheels 

are holding the plane up.  It is more important that the flying stability 

is correct than the landing dynamics are comfortable.  That means (to me) 

that the way to correct the landing situation is to move the wheels rather 

than moving the CG.  For this tendency that some (not all) Zodiacs seem to 

have to force the nose down quickly on landing the solution seems to be to 

move the main wheels forward.  I don't know how much movement is needed, 

but the first step is to mount the main gear with the flat edge forward to 

make the wheels move a little forward of the position they would have if 

the flat edge is mounted facing aft.  If this isn't enough to fix the 

landing difficulty then it starts getting a lot harder to change the plane 

to get nicer landing behavior.



The other side of the main wheel vs. CG question comes in when you have 

250 pound folks standing on the step behind the wing to get into the 

plane.  If the wheels are positioned nicely for the landing dynamics to 

work out then the plane will tend to drop to its tail when the big guy (or 

gal) tries to get in.  As with chemical and electrical engineering, 

airplane design is a game of trade-offs.



I hope this discussion helps us understand the landing issue.  My plan is 

to use the flat side of the main landing gear mounted forward and hope for 

the best.



Good luck,



Paul


	




Terry Phillips

ttp44~at~rkymtn.net

Corvallis MT

601XL/Jab 3300 slow build kit - Tail is finished; working on the wings

http://www.mykitlog.com/N47TP/

_________________
Terry Phillips
Corvallis, MT
ttp44<at>rkymtn.net
Zenith 601XL/Jab 3300 slow build kit - Tail feathers done; working on the wings.

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