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Very Hot Topic (More than 25 Replies) Bell P-63F Kingcobra Air Racer (Read 643 times)
Sky9pilot
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Re: Bell P-63F Kingcobra Air Racer
Reply #25 - Jun 2nd, 2020 at 6:13pm
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Got the large stripe down the fuselage today using black tissue.  I made a paper pattern for the swoops at the nose and then angled strips for the remainder of the stripes on back to the tail.  I found the stripes weren't even on each side but only after I applied the nose swoops.  This is a dimer supposedly to be built in the fasion of the Ten Cent models of years past.  OOPs Embarrassed Cry
  

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Your attitude will determine your altitude!- John Maxwell
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MKelly
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Re: Bell P-63F Kingcobra Air Racer
Reply #24 - Jun 2nd, 2020 at 10:33am
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That's going to look really sharp Tom!

Mike
  
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Kerak
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Re: Bell P-63F Kingcobra Air Racer
Reply #23 - Jun 1st, 2020 at 4:36pm
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Lookin' fancy, Tom!  One plastic source I've considered a number of times comes from clear soda drink or water bottles...but...have never had any success getting it to lay out flat with the heat...something to do with "memory shape."  Figure it out for us.... Smiley Wink

Neal
  
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Re: Bell P-63F Kingcobra Air Racer
Reply #22 - Jun 1st, 2020 at 1:05pm
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Got some work done on the P-63F got the tissue on most of the plane.  Still have to close in the bottom of the fuselage.  But that won't happen till I finish the fuselage stripe and the noseblock and spinner.  Tryed to plunge mold a canopy but the plastic I had from a McDonalds big breakfast container just curled up over the heat.   So looking for more plastic at the moment...I know I had some before the move so it's packed away in a tub somewhere.  Time to dig through the boxes.  Here's the pics.
  

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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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Re: Bell P-63F Kingcobra Air Racer
Reply #21 - May 31st, 2020 at 7:11pm
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In my continued research noticing the scoop on the Kingcobra is larger than the P-39 Airacobra I found this picture of the panels removed showing the engine and intake scoop.
  

( 207 KB | 18 Downloads )
24e35ce95911b52c79b4ee13fe3d566d.jpg

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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Sky9pilot
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Re: Bell P-63F Kingcobra Air Racer
Reply #20 - May 30th, 2020 at 7:39pm
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Under the CG section I opened a new topic where we can discuss at length the tech issues of Free Flight aerodynamics.   Kerak posted a couple of articles one by Bill Hannan and one by Charles Hampson Grant on incidence.  I also posted a link to NFFS (National Free Flight Society) technical library.  I look forward to our discussions in the future.  I did some more work on the P-63F covered the wing in the Hallmark Gold tissue and covered the top of the fuselage and the enpennage with white Hallmark tissue out of the same pack.  It went on well wet.  Suprising strength.  I used some acrylic white thinned with airbrush medium and brushed all the white tissue to give it a more opaque look.  Still have to apply the gold tissue to the bottom half of the fuselage.  Then close the wing opening on the bottom of the fuselage.  Pictures tomorrow.
Sky9pilot
  

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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Re: Bell P-63F Kingcobra Air Racer
Reply #19 - May 29th, 2020 at 6:52pm
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I've enjoyed the discussion.  The DeLoach/McComb TVO formula has influenced my location of CG and I then trim from there.  Seems there are so many ways to approach trimming a free flight model.
  

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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: Bell P-63F Kingcobra Air Racer
Reply #18 - May 29th, 2020 at 3:10pm
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AK, I had a long and rambling set of paragraphs on incidences and thrust angles on Wednesday night, but just as I thought it was starting to make sense an approaching thunderstorm glitched our power and I lost it.  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)?  Tom, thanks for letting us clutter up your P-63 thread...

Cheers,

Mike
  
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Re: Bell P-63F Kingcobra Air Racer
Reply #17 - May 27th, 2020 at 10:28pm
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I tend to build in the "TLAR" engineering school of construction.  As Neal said with time and acetone to loosen some glue spots...she can be made to fly!  Smiley
  

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And ye shall know the truth, and the truth shall set you free. Jn 8:32
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Kerak
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Re: Bell P-63F Kingcobra Air Racer
Reply #16 - May 27th, 2020 at 7:05pm
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To confuse things even further...my Komet Me-163 has a symmetrical airfoil.  If it is launched with even the slightest negative AOA...what I get is a dramatic power impact into the ground directly in front of me.  My experience was that I had to be very careful to be certain to launch with a pronounced positive AOA.  So to my way of thinking (isn't this highly technical on my part)...a negative decalage is all an effort to ensure a positive AOA.  Of course...too much results in non-effective turbulence...but even if the AOA is set at 0 deg...the tail is going to be negative to influence the AOA.  Is that making any sense?  Shocked  I think is was stated previously...many aircraft in the past have had the ability to adjust decalage...not merely through trim tabs, but with the entire horz stab...probably relative to payload....  Makes one wonder just how exact this science is?  I know...so what's the formula...darned if I know.  Smiley

To paraphrase...if you build it, it will fly...if it is not first destroyed during the trimming process.  Cheesy

Neal
  
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Re: Bell P-63F Kingcobra Air Racer
Reply #15 - May 27th, 2020 at 2:52pm
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I believe that is the case, thrust line.  With positive aoa on the wing the natural down thrust is created which eleminates the need or reduces the amount of down thrust to be added later.  Depending on which designer you read they often try to eliminate the need for additional down thrust to be added to our models.  AOA doesn't affect the right side thrust that we often add to counter the prop's ( I believe it's called P-factor) tendency to cause the model to yaw to the left.  Found these two pics to illustrate P-factor.
  

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302px-Propeller_blade_AOA.png
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282px-Propeller_blade_AOA_versus_pitch.png

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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Re: Bell P-63F Kingcobra Air Racer
Reply #14 - May 27th, 2020 at 10:33am
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MKelly wrote on May 27th, 2020 at 8:56am:
The static wing incidence gives the airfoil enough AOA to produce lift in level flight.


Well, I'm a bit undecided on this. I'm not sure about the relationship of thrust line to wing AOA. For a given speed, the wing is going to fly at a certain AOA, that being the angle between the chord line and the relative wind. As speed varies and the wing AOA along with it, the angle between the fuselage longitudinal axis and the relative wind does as well. So if we arbitrarily increase the wing incidence, the result seems to be a lower pitch attitude for the fuselage. I'm assuming that the Cg is correct and the plane is balanced.

The Lockheed L-1011 airliner was flown slower than designed for fuel economy when fuel costs went up back in the day. As a result, the fuselage had a definite 'up slant' that caused much woe in the flight attendants who complained about having to push service carts "up hill".

The Navy F-8 had a unique feature of having a wing leading edge that was mechanically raised for landing. This resulted in the fuselage pitching down a little which improved pilot visibility. Wing AOA wasn't affected, only fuselage pitch angle.

We don't have to worry about flight attendants or pilot visibility, so why the positive wing incidence? Something to do with thrust line?

I don't know.


  
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Re: Bell P-63F Kingcobra Air Racer
Reply #13 - May 27th, 2020 at 10:15am
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I have used the Robart meter years ago.  When I was into RC.  It is a bit large for our rubber powered models.  Very nice instrument.  Mine sure didn't cost almost $60!!!
  

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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Re: Bell P-63F Kingcobra Air Racer
Reply #12 - May 27th, 2020 at 8:56am
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Good find AK!  The Robart meter is too big for most of our models but they've given some great ideas, may have to play around and see how functional the Schmidt meter is for some of these tasks.

The static wing incidence gives the airfoil enough AOA to produce lift in level flight.

Tom, I have the McCombs book and have found it very useful.  The key information (tail volume and cg) is captured in several of the articles linked in the "Finding CG..." forum here on S&T, but the book gives some additional useful information and rules of thumb for dihedral, fin area etc as well as a lot of empirical tables and graphs to put some of the information in context.  For the beginner I'd say Don Ross's book is a better starting place, but I find myself using McCombs more often now as I'm trying to decide what (if any) tweaks to make on models I'm building.

Mike
  
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Re: Bell P-63F Kingcobra Air Racer
Reply #11 - May 27th, 2020 at 7:29am
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I found this instruction manual for a commercial incidence meter. I think it is a pretty good explanation of the basics of using an incidence meter. Click on "How To Instructions".

https://robart.com/products/model-incidence-meter

Does anyone have a good explanation of why the wing should be set with a small incidence angle?
  
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