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jaeger820
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Re: The mystery of Incidence
Reply #6 - yesterday at 6:00pm
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Sky9pilot and MKelley...appreciate your insight very much.  I'm going to take a slight pause on the R5 build to construct the incidence meter.  I've completed the vertical and horizontal stabilizer and was just about to dive into the fuselage so now is the perfect time to shift.

  

Sam
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MKelly
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Re: The mystery of Incidence
Reply #5 - Jan 25th, 2021 at 7:55am
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Jaeger820,

I highly recommend making the incidence meter Tom linked in the previous post - it's a very useful tool.  On my last three builds I used the incidence meter to check and set the incidence difference between wing and stab to about 2 deg before first flight attempts.  I also carefully checked the CG position.  All three models were very close to proper trim at first glide tests, and were well-behaved when starting powered trimming.  The combination of adjustable stabs and noseblocks, good measuring instruments, and careful pre-flight checks for warps, cg and incidence has dramatically decreased my frustration factor when trimming new models.

Before adopting these practices I broke several models on their first day out due to having either too much or too little differential incidence between wing and stab.

Mike
  
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Sky9pilot
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Re: The mystery of Incidence
Reply #4 - Jan 24th, 2021 at 7:09pm
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Most model plans that have been produced by various companies past and present have already considered incidence in drawing up their plans.  The problem with incidence usually comes with not following the plans in these areas: main wing incidence when making the wing saddle or if not a one piece wing getting both wing panels as attached to the fuselage correctly aligned and of the same incidence on each side.  The second area is the empennage (horizontal stab and vertical stab) aligned correctly.
So yes an incidence meter is quite helpful, just to check we've met the requirements of the plan we are building from.  We have a easy to make incidence meter that MKelly found and I posted a cleaned up picture for it.  Just make it from balsa and print up the meter portion and glue it to the backing correctly and you're set. 

The plans I draw I usually have the main wing set at a positive 2 degrees and the horizontal stab at zero degrees.  I also leave enought room to shim the horizontal stab leading edge or trailing edge if trimming is required.  I suggest Duco glue for affixing the horizontal stab because it can be dissolved with acetone for changing the incidence as needed.  Remember just a drop of glue on the leading edge and trailing edge is all that needed until you've worked out the trim.  Then add a bit more glue.  Don't over do the glue you don't want to upset the CG!!!

Here's the link to the "Incidence Meter" : Click Here
  

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Your attitude will determine your altitude!- John Maxwell
And ye shall know the truth, and the truth shall set you free. Jn 8:32
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jaeger820
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Re: The mystery of Incidence
Reply #3 - Jan 24th, 2021 at 12:38pm
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Sky9pilot...new to S&T and am pouring through site gathering all of the information on angle of incidence that I can.  I am an entry level builder and I am getting close to starting my next project (Jackrabbit R5).  I appreciate your and MKelley's information in this thread and am wondering if you could point me in the right direction on whether an incidence meter is something that I should have as a rule? Based on my reading it seems that would be the best way to know the angle for certain.  To be honest, I haven't paid much attention to it in any of my previous builds and I am certain it has bitten me.  Looking forward to your thoughts.
  

Sam
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Sky9pilot
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Re: The mystery of Incidence
Reply #2 - May 30th, 2020 at 9:23pm
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To clarify for some of our modelers...
TLAR= That Looks About Right
WRT= With Regards To
Wing Incidence = On fixed-wing aircraft, the angle of incidence (sometimes referred to as the mounting angle) is the angle between the chord line of the wing where the wing is mounted to the fuselage, and a reference axis along the fuselage (often the direction of minimum drag, or where applicable, the longitudinal axis).
AOA= Angle of Attack
For Airfoil terminology check pictures below
Hope this is helpful...
Sky9pilot
  

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If God is your Co-pilot...switch seats...
Your attitude will determine your altitude!- John Maxwell
And ye shall know the truth, and the truth shall set you free. Jn 8:32
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MKelly
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Re: The mystery of Incidence
Reply #1 - May 30th, 2020 at 8:20pm
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Here's the rambling stream of thought I posted in Tom's P-63 dimer thread.  Analysis, discussion and/or ridicule is welcome.

The big difference between our models and fullsize aircraft is that we have to fly across the full powered and unpowered speed range with all the incidence and thrust angles fixed - we can't trim in flight to adjust for changes in forces and moments due to different airspeeds.  Any aerodynamicist or stability and control guy could quickly make a fool of me, but here's my thoughts:

We want the fuselage to fly so that it presents minimum frontal area - that defines the horizontal reference line, and we shoot to have that parallel to the flight path as much as possible.  As Neal said, the wing's mean camber line has to be at a positive AOA to make lift, so the wing incidence is set positive WRT the horizontal reference line.  How much incidence depends on what AOA provides enough lift for level flight in the speed range we expect to fly - way too many variables here, so as Tom said TLAR is the approach.

The stab needs a bit of negative incidence WRT the wing to counter the wing's pitching moment and generate recovery from upsets.  Again, lots of variables including CG position, wing downwash, propwash, stab placement etc, so we set up the stab so it can be shimmed or adjusted experimentally to get a good glide and sufficient stability.  How much adjustment range?  TLAR says somewhere around -5 to 0 deg WRT wing incidence.  Stab incidence setting (by shim or other adjustment) gets tuned for best unpowered glide during trimming.

Up-and-down thrustline adjustment helps control flight attitude under power.  We've trimmed unpowered glide by setting the difference between wing and stab incidence, but the forces and moments are balanced only at glide speed.  As speed increases under power the model will most likely pitch up, so we add downthrust until the model climbs in a nearly-level attitude under power.  Again, lots of variables affecting things here, so ~2-3 deg downthrust is the TLAR rule of thumb for initial setup, and adjustments are made as required during trimming.

Side thrust adjustments are primarily to counter torque effects.  Torque causes the model to roll left, so we start with a couple of degrees right thrust and add/remove as required during trimming.

What's really been catching my interest during trimming lately is trying to understand and optimize the relationships between CG position, the resulting wing-stab incidence differential to get a steady glide, and the downthrust necessary to hold the nose down under power.  I think in several of my models I had CG too far forward, which led to higher wing-stab incidence differential, which led to too much downthrust, resulting in lost performance in both powered and unpowered flight, which I then compensated for with more rubber cross-section, which made torque effects worse and made the model heavier.  To me this is what Don Deloach is driving at in his excellent article.

I'm now trying to measure the incidences on the model and if it seems like (TLAR again) there's too much differential I'm moving the CG aft and retrimming the glide until I end up with a stable glide at a reasonable incidence setting.  This should lead to less downthrust - early experiments with the Broussard are encouraging.

Anyway, that's what I've rationalized after thinking through your comments.  Was that long and rambling enough (grin)?

Cheers,

Mike
  
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The mystery of Incidence
May 30th, 2020 at 12:30pm
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There's been some discussion on incidence lately on several build threads.  I thought a spot to spend some time and share some insights would be helpful and hopefully not too confusing.  The National Free Flight Society has a library of technical information on the various aspects of model aviation:  NFFS TECH LIBRARY click here
Kerak added these articles by Bill Hannan and Charles Hampson Grant.
  

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If God is your Co-pilot...switch seats...
Your attitude will determine your altitude!- John Maxwell
And ye shall know the truth, and the truth shall set you free. Jn 8:32
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