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Incidence Questions - 4/15/2008 5:26:41 AM   
LS171Malibu


 

Posts: 316
Joined: 4/17/2003
From: FORT WORTH, TX, USA
Status: offline 		I scratch built a Quickie for our local Sport 25 racing. For the most part, we must use a 424 airframe with an OS 25 at 52oz min. weight. I have built many planes and scratch built many also, but I messed up the incidence on this one. I made a reference mark from the stab zero to the firewall, which hit the center of the firewall. The LE is 11/64" closer to that line than the TE, which confirms the horrible downward pitch issues during flight. I am setting up to straighten things out and attemp to make this model competitve. I am using a MH17 airfoil. What should I use for wing incidence and thrust line angles? Any suggestions?

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Blake
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RE: Incidence Questions - 4/15/2008 7:36:42 AM   
Kevin Matney



Posts: 156
Joined: 11/29/2003
From: Erie, MI, USA
Status: offline 		Zero-lift angle of attack is -0.15 degrees and Ideal of attack is 1.04 degrees according to CompuFoil 3D.
Find out were you are at like FW to wing then wing to tail, and then work out what is the best way to go is first before you do any thing.

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Kevin Matney 2 wrongs don't make a right But 2 Wrights make an Airplane
www.Matneymodels.com

(in reply to LS171Malibu)
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RE: Incidence Questions - 4/15/2008 11:40:03 AM   
luv to race


 

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From: Liquored, FL,
Status: offline
quote:

I made a reference mark from the stab zero to the firewall, which hit the center of the firewall. The LE is 11/64" closer to that line than the TE, which confirms the horrible downward pitch issues during flight.



Are you comparing the stab incidence to the firewall? More importantly… what is the stab incidence in relation to the wing? When you block the plane up on your bench, put your incidence meter on the wing and shim the plane so that the wing is zero (or what ever angle the wing should be flying at, which I wouldn''t think it would be more than 1 degree plus/minus). Then check the stab incidence to the wing. I can''t imagine you''d want to be more than 1/4 either way. That obviously depends on the airfoil, which I''m not familiar with.

RB

< Message edited by luv to race -- 4/15/2008 11:42:06 AM >

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RE: Incidence Questions - 4/15/2008 11:52:36 AM   
LS171Malibu


 

Posts: 316
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quote:

ORIGINAL: luv to race

quote:

I made a reference mark from the stab zero to the firewall, which hit the center of the firewall. The LE is 11/64" closer to that line than the TE, which confirms the horrible downward pitch issues during flight.



Are you comparing the stab incidence to the firewall? More importantly… what is the stab incidence in relation to the wing? When you block the plane up on your bench, put your incidence meter on the wing and shim the plane so that the wing is zero (or what ever angle the wing should be flying at, which I wouldn''''t think it would be more than 1 degree plus/minus). Then check the stab incidence to the wing. I can''''t imagine you''''d want to be more than 1/4 either way. That obviously depends on the airfoil, which I''''m not familiar with.

RB


That deminsion was comparing the wing to the horizontal. Roughly 3/16" lower at the LE. I had made a template out of .063 T3 for cutting the saddle, all I can figure is that it slipped. Oh well, I''ll straighten her out.



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RE: Incidence Questions - 4/15/2008 11:56:12 AM   
LS171Malibu


 

Posts: 316
Joined: 4/17/2003
From: FORT WORTH, TX, USA
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quote:

ORIGINAL: Kevin Matney

Zero-lift angle of attack is -0.15 degrees and Ideal of attack is 1.04 degrees according to CompuFoil 3D.
Find out were you are at like FW to wing then wing to tail, and then work out what is the best way to go is first before you do any thing.


Kevin, the cores were beautiful, thank you. If I am reading your post right, I need a positive wing incidence of 1.04 degrees, in relation to the stab?

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RE: Incidence Questions - 4/15/2008 4:35:00 PM   
StanDouglas


 

Posts: 112
Joined: 8/28/2003
From: Austin, TX, USA
Status: offline 		Every once in a while I build one that has pitch problems even though I use templates, a router and cut the wings from the same templates and a Feathercut system.

When it has happened I simply loosen the wing bolts and place 1/32" shims under the front bolts (given I want to give it more positive incidence) tighten the bolts and fly. If it''s better, but not great, land and add another shim and so on until it''s right. Of course if its getting worse we may need shims under the trailing edge bolts.

Once I''m happy, I re-pot the wing saddle with epoxy and filler with the shims in place. The shims become part of the saddle. Your going to find that getting the incidence to within .04 deg is a little precise. And anyway the actual incidence required will be slightly different depending on how well you cut the wing core; wire lag, sanding the skin, incidence of the tail and all the rest.

Depending on all the variables you might actually need to alter the engine thrust line as well. Usually Q500''s work well with 0 engine incidence. But some actually require a different set up. You can do all the usual things like for a pattern plane. It''s actually an art form, so it may help to get a local artist help out!

Ciao

(in reply to LS171Malibu)
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RE: Incidence Questions - 4/15/2008 6:01:56 PM   
LS171Malibu


 

Posts: 316
Joined: 4/17/2003
From: FORT WORTH, TX, USA
Status: offline 		I started by doing exactly that, shims under the LE. My only problem was that I did not have enough shim stock to complete the test. lol [:''(] I never would have guessed I needed more than 3/32". I am going to borrow a digital meter and set as close to 1 degree as I can, and go from there. I have the CG at 3.125" on a 10" cord, I figure that should be close.

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RE: Incidence Questions - 4/15/2008 7:05:23 PM   
diggs_74



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Joined: 1/20/2004
From: Emerald, WI, USA
Status: online 		That may be a little far back on the CG.. Remember, a tail heavy airplane may amplify any pitch issues.. Just a thought..


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Lee LaValley
NMPRA# 33w

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RE: Incidence Questions - 4/15/2008 9:18:53 PM   
LS171Malibu


 

Posts: 316
Joined: 4/17/2003
From: FORT WORTH, TX, USA
Status: offline 		What is a good method to determine a starting point on CG. I have always used 30% to 33% of the mean aerodynamic chord. BTW On the maiden the CG was at 2.5", and the move to 3.125" made absolutely no noticable difference in flight. Of coarse, if there was not a huge incidence issue I feel the CG change would have changed the flight characteristics.

< Message edited by LS171Malibu -- 4/15/2008 9:26:42 PM >

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Blake

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RE: Incidence Questions - 4/15/2008 9:33:19 PM   
diggs_74



Posts: 1257
Joined: 1/20/2004
From: Emerald, WI, USA
Status: online 		Wow, the incidence must be off a bunch to not notice a change that large.. I would make sure and go to a more forward CG after fixing the incidence, just to be on the safe side. Maybe 2.75" for starters..


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Lee LaValley
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RE: Incidence Questions - 4/15/2008 10:19:03 PM   
LS171Malibu


 

Posts: 316
Joined: 4/17/2003
From: FORT WORTH, TX, USA
Status: offline 		I''ll set @ 2.75" and work back from there. Thanks.

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RE: Incidence Questions - 4/15/2008 10:20:12 PM   
Kevin Matney



Posts: 156
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From: Erie, MI, USA
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quote:

ORIGINAL: Kevin Matney

Zero-lift angle of attack is -0.15 degrees and Ideal of attack is 1.04 degrees according to CompuFoil 3D.
Find out were you are at like FW to wing then wing to tail, and then work out what is the best way to go is first before you do any thing.

What you need is the Zero lift for all out speed, you get the other in a trun. what that means is how hard you can turn with the most speed

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Kevin Matney 2 wrongs don't make a right But 2 Wrights make an Airplane
www.Matneymodels.com

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RE: Incidence Questions - 4/15/2008 10:45:43 PM   
LS171Malibu


 

Posts: 316
Joined: 4/17/2003
From: FORT WORTH, TX, USA
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quote:

ORIGINAL: Kevin Matney


quote:

ORIGINAL: Kevin Matney

Zero-lift angle of attack is -0.15 degrees and Ideal of attack is 1.04 degrees according to CompuFoil 3D.
Find out were you are at like FW to wing then wing to tail, and then work out what is the best way to go is first before you do any thing.

What you need is the Zero lift for all out speed, you get the other in a trun. what that means is how hard you can turn with the most speed


Ah, I get it! THANK YOU!!! Kevin, what is your opinion on the optimum CG?

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Blake

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RE: Incidence Questions - 4/15/2008 11:02:06 PM   
Kevin Matney



Posts: 156
Joined: 11/29/2003
From: Erie, MI, USA
Status: offline
quote:

ORIGINAL: LS171Malibu


quote:

ORIGINAL: Kevin Matney


quote:

ORIGINAL: Kevin Matney

Zero-lift angle of attack is -0.15 degrees and Ideal of attack is 1.04 degrees according to CompuFoil 3D.
Find out were you are at like FW to wing then wing to tail, and then work out what is the best way to go is first before you do any thing.

What you need is the Zero lift for all out speed, you get the other in a trun. what that means is how hard you can turn with the most speed


Ah, I get it! THANK YOU!!! Kevin, what is your opinion on the optimum CG?

The best CG is 2.468 From Compufoil Pro

_____________________________

Kevin Matney 2 wrongs don't make a right But 2 Wrights make an Airplane
www.Matneymodels.com

(in reply to LS171Malibu)
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