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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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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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Sky9pilot
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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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