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Incidence setup with a full flying stab

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Old 07-25-2017, 04:05 PM
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ltc
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Question Incidence setup with a full flying stab

I have a quick setup question that I'm hoping someone can answer. I've never seen a trimming guide that discusses a full flying stab.

When setting up a conventional wing and tailplane, you normally set the incidence angle on the wing relative to the stab (ignoring thrust/datum line for this discussion). Say 0 deg reference on the stabs and then +0.7 deg positive on the wing.
Based on test flight trimming you then adjust the wing incidence, again relative to the stab, to minimize any elevator trim needed. (I know there is more to it than that, but I'm simplifying for this discussion)

However, on a full flying stab, if you initially set the incidence on the wing relative to the stab, and then have to add trim to the stab, you have adjusted your incidence by default. There is no elevator trim to adjust, since you are adjusting stab/elevator all at once.
Do you just leave it as is?

Am I missing something obvious?

Thanks in advance.
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Old 07-26-2017, 03:08 AM
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CW
 
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Because you can't use the stab as a reference you need to create a datum line - canopy flange or just sit the fuse in the flying attitude you want. Then set the wing incidence to your required + 0.7 deg to this datum line. In my case I have 0.5 deg down thrust to the datum. Set the all moving stab about 0.1 deg less positive than the wing ie +0.6 deg. On the first flight use "elevator" trim to get level flight. The first flight is the tricky one if the stab is wildly out of trim. My experience is that the stab usually ends up between 0.6 and 0.8 deg less than the wing incidence. This gets the stab incidence in the ballpark, then use wing incidence changes and CoG movement to achieve straight up lines, knife edges etc. as you would normally. Re trim "elevator" for hands off level flight each time you make a change.
Bill

Last edited by CW; 07-26-2017 at 03:51 AM. Reason: Correction to data
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Old 08-02-2017, 06:52 PM
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Dave Harmon
 
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Quote:
Originally Posted by ltc View Post
I have a quick setup question that I'm hoping someone can answer. I've never seen a trimming guide that discusses a full flying stab.

When setting up a conventional wing and tailplane, you normally set the incidence angle on the wing relative to the stab (ignoring thrust/datum line for this discussion). Say 0 deg reference on the stabs and then +0.7 deg positive on the wing.
Based on test flight trimming you then adjust the wing incidence, again relative to the stab, to minimize any elevator trim needed. (I know there is more to it than that, but I'm simplifying for this discussion)

However, on a full flying stab, if you initially set the incidence on the wing relative to the stab, and then have to add trim to the stab, you have adjusted your incidence by default. There is no elevator trim to adjust, since you are adjusting stab/elevator all at once.
Do you just leave it as is?

Am I missing something obvious?

Thanks in advance.
Just use Brian Hebert's Triangle Trimming method but ignore the stab setting.
Use his initial thrust and wing settings....sounds like you are setting up a BJ airplane....if so....use BJ's incidence jigs and start there....note that BJ likes to fly extremely tailheavy so be sure the battery is well fwd.
The Essence/Nuance/Etude should balance a bit fwd from the wing attach bolts....this is a good starting point then move the battery back and forth about 1/4" at a time because this point is right at the teeter point and a little battery movement makes a large change.
Your Prolog was trimmed using TT.......of course it has elevators but the procedure is the same.
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Old 08-03-2017, 10:53 AM
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ltc
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Quote:
Originally Posted by Dave Harmon View Post
Just use Brian Hebert's Triangle Trimming method but ignore the stab setting.
Use his initial thrust and wing settings....sounds like you are setting up a BJ airplane....if so....use BJ's incidence jigs and start there....note that BJ likes to fly extremely tailheavy so be sure the battery is well fwd.
The Essence/Nuance/Etude should balance a bit fwd from the wing attach bolts....this is a good starting point then move the battery back and forth about 1/4" at a time because this point is right at the teeter point and a little battery movement makes a large change.
Your Prolog was trimmed using TT.......of course it has elevators but the procedure is the same.
I've never owned a Prolog ....
This is a Fantasista (Akiba design produced by BJCraft) and it did not come with incidence jigs or any setup information.
I was just going over radio setup, subtrim, etc. and it got me wondering how to set the flying stab initially, again as there was no zero mark or jig to go by.
It sounds like perhaps the Fantasista (I have several different sizes of this airframe) is a bit different in that it doesn't have any jigs supplied.
Thank you for all the replies and support.
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Old 08-06-2017, 06:54 AM
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malibuflight
 
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I am getting ready to do this exact thing on my new essence. As mentioned, Herbert triangulation publication is where Iam starting. Down thrust is the point for all the rest. Negative 1 degree will give me the anchor point. Then i will check wing incidence and then ffs to make sure its at 0. I put it together making sure I used the BJ incidence gigs. We will see where it is. I also used a string around wing mounting bolts to get the cg close to know where the battery tray should be which ended up much further forward than the picture on BJ's website showed. Maybe because Im using direct link to rudder. When all is about right, I'll flip it over and use the cg gig to finish that part.
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