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Very Hot Topic (More than 25 Replies) Bell P-63F Kingcobra Air Racer (Read 496 times)
Sky9pilot
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Re: Bell P-63F Kingcobra Air Racer
Reply #10 - May 26th, 2020 at 2:26pm
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When I draw up a plan from the research I've done I usually try to draw the plan with a minimum of +2 degrees of incidence on the main wing and 0 degrees incidence on the stab.  There are lots of theories on incidence settings for bipes and the difference between the top wing and the lower wing.  Not getting into the triplanes!!!  With these rubber powered planes and the FAC use of airfoils with flat bottoms unless the older plan called for under camber allowed.  I usually try to use a "Clark Y" or the "Neelmyer" (I forget which old timer designer who said it was his favorite airfoil for rubber powered models) I use the bottom of the airfoil to set the incidence so this gives me in real life +2-4 degrees of incidence depending on the size of the airfoil.  Because the incidence is suppose to be measured through the middle of the leading edge to the middle of the trailing edge.  I then try to leave at least a 1/16" slot for the horizontal stab for shimming adjustment of the stab.  I haven't progressed to MKelly's sophistication of adjustable stabs.  That's coming for me down the road.  This is what I shoot for in drawing my plans.  I try to use DeLoach's/McComb's TVO for determining CG .  These are my starting points.  If you read through the docs in the section on setting CG you'll find many of the view on incidence and decalage for model airplanes. 

William McCombs, "Making Scale Model Airplanes Fly for Sport or Contests", Aircraft Data, 1981, 1992.  This book is written by an aeronautical engineer and model designer.  I refer to it constantly.    $18.95+$6.00 from Susan Creamer (Bill's daughter I believe), 1925 Clark Trail, Grand Prairie, TX, 75052. I don't know if this is still a valid contact.  I don't personally have this publication.  I hope to find one.
« Last Edit: May 26th, 2020 at 11:56pm by Sky9pilot »  

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MKelly
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Re: Bell P-63F Kingcobra Air Racer
Reply #9 - May 26th, 2020 at 10:06am
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AK,

For what it's worth, I try to start trimming with about 2-3 deg incidence difference between the wing and stab.  When looking at a plan if I'm concerned about the relationship of wing and stab I'll draw lines through the LE and TE of each surface and measure the angles (I often do this in Powerpoint, but I'll do it by hand if I'm casually checking a printout).  If necessary I'll modify the tail slot to allow a range of about 0-4 deg negative with respect to the wing.

Once the model is assembled, I'll use a Schmidt incidence meter (https://www.stickandtissue.com/forum/cgi-bin/yabb2/YaBB.pl?num=1561867205) to set the stab for about 2 deg negative incidence as a starting point for glide trimming.  I keep checking the incidence as trimming progresses - if it gets to be over 4 or under 1 degree difference I'll adjust the cg and start trimming over.

This isn't a perfect method, but it seems to help avoid some of the initial trimming disasters I had with the Little Gem and T-28.

Tom, the P-63 is coming together nicely.  One of these days I'm going to build the Tucker Special P-63 racer with the radically clipped wings.  At 1/24th scale it'll be a ~50 sq in wing area peanut with over 12" hook-to-peg!

Mike
  
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Re: Bell P-63F Kingcobra Air Racer
Reply #8 - May 26th, 2020 at 7:54am
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Kerak wrote on May 26th, 2020 at 7:35am:
My favorite question has always had to deal with the sometimes differences in biplane wing dihedral..


Another interesting question, we have to wonder if or how these ever get answered! Speaking of biplanes, I saw a reference to the term 'decalage' that suggested the word originated with the attempt to describe the angular alignment of the upper and lower biplane wings and only later was extended to include the wing/tail settings.
  
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Kerak
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Re: Bell P-63F Kingcobra Air Racer
Reply #7 - May 26th, 2020 at 7:35am
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Hey Andy...thanks for that great question to get ones mind moving this morning!

I suspect the answer is very much the latter of your final statement...build it, then attempt to compensate during flight trimming, i.e., tailplane incidence, prop down thrust, etc.

But your question is very stimulating...and serious as far as design and construction.  I suspect scale-effect is a large factor in the equation, as well.

My favorite question has always had to deal with the sometimes differences in biplane wing dihedral...as in the Camel...0 degrees on top and pronounced on the lower wing.  We all know that torque was the real factor in the Camel's performance...but what was Sopwith's thought process for the dihedral?  Then take the Royal Aircraft Factory SE5a...lots of dihedral...lots of stability...and those huge ailerons on both wings...real dog-fighter!  All the exact opposite of the Camel!  I've posed that question many times to people I considered to be experts...and gotten nil for a response.  Why do modelers avoid the Camel (besides the short nose moment)?

Great question to start the morning with, Andy! Smiley

Neal
  
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Re: Bell P-63F Kingcobra Air Racer
Reply #6 - May 26th, 2020 at 5:14am
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One of the question marks I'm going through now is the issue of "decalage". Aside from the haggling over what that means and to what extent it's used, what are you doing here? I see very little attention given to this in most plans and yet it seems it's something that we very much need if things are going to fly well (or in some instances, at all).

On this question, it's always been murky for me to figure out how to go from the theoretical musings of model aerodynamics to the end game where balsa actually meets the road! Theory is fine but how do folks actually build in these angles in something as small as a Peanut or Dimer? Tools and jigs or maybe a hopeful dose of "that looks about right"?
  
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Sky9pilot
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Re: Bell P-63F Kingcobra Air Racer
Reply #5 - May 25th, 2020 at 6:45pm
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A little more work done on the KingCobra P-63F.  Planked the nose back to L.E. of wing so holding while place the noseplug won't crush the nose of the model..Used scrap wood left from other builds.  Also did some more work on sealing the canopy plug and intake scoop with tissue and yellow carpenter's glue.  This glue seams to harden and sand better than the white glue.

Still have some finish sanding before covering.
  

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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 #4 - May 21st, 2020 at 6:21pm
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Worked on the canopy plug today and the intake behind the cockpit.  I'm using pink insulation foam to carve these out.  I also have a balsa canopy plug from another build (never throw these out) that I think might work trimmed down to fit the P-63F.    Almost ready to start covering!  Then we'll see how things fit.  Still have to carve a spinner and fit a prop.  Dimer rules say must be a balsa prop or a cast plastic prop can't make a prop from blades and special hub of brass or aluminum tubing.
  

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 #3 - May 18th, 2020 at 5:57pm
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Got some more work done today.  Got the empennage done with the tall vertical.  I added a rudder that can be cut free if needed for a floppy rudder if it's too much vertical stab...Made the horizontal stab in the Walt Mooney style with angled ribs with 1/32"X3/32" outside spars top and bottom.
I finished the wing and added 1.5" dihedral which will bring the wingtips up to the bottom of the cockpit edge.   You'll notice the spacing of the ribs in the wing.  The plans call for the two closer ribs together because they anchor the balsa for the removable landing gear.  I plan to make mine as in flying mode and eliminated this extra wood in the wings.  Just didn't space out the ribs. 

Had a little of help from Goofy!  Thanks Goofy!
  

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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 #2 - May 16th, 2020 at 8:58pm
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Looks really great, Tom...always loved the Airacobra/KingCobra from the moment I saw a model of one when I was just a kid...think it was the car door...something I could relate with.  Grin Grin Grin

Know what you're saying about a desire to install additional stringers...and truth-be-known, most dimers really need just a bit more to "round things out."  Likewise, I've added four more stringers to my Vega...who's counting?  Smiley Wink

Watching you're build, Tom.  Smiley

Neal
  
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Re: Bell P-63F Kingcobra Air Racer
Reply #1 - May 16th, 2020 at 8:03pm
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Got the fuselage sides done and made the second side over the first.  I later split the sides after sanding and installed the cross members with the box inverted starting at the rear.  I should have waited to install the wing saddles till after the cross members were added and the formers were in place so the saddles align with the outside formers.  A little surgery and they now do.  Stringers tomorrow.  This is a pseudo-dimer but I really struggle with not adding a few more stringers at the nose.  Here's the pictures so far. As you can see, I'm using my homemade magnet board.

Got some gold tissue from Hallmark so I'm gonna go with the gold, white and black scheme.
  

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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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Bell P-63F Kingcobra Air Racer
May 14th, 2020 at 12:08am
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I don't usually build Air Racers but I'm tired of the silver finish of the only two P-63F's in military schemes that are accurate.  Seems this one was surplussed out and raced in the Thompson in Cleveland & Bendix Air Races.  Found this blurb online: Bell P-63F Kingcobra 43-11719 is one of only two F variants built, internally known as the Bell Model 43 with a different tail and uprated Allison engine. In 1946 it raced both the Bendix and for the Thompson Trophy in Cleveland. It bounced around a number of different private owners and had some stunning paint jobs. Pictured here around 1968 in gold and white. In the 70s it raced at Reno in near factory original configuration. In the 80s it was painted a faux Soviet scheme. Today the CAF keeps this Kingcobra airworthy, having completed an authentic wartime restoration and respray just this year. (sometime in the late 70's or mid 80's)   Here's two of the possible schemes and the Reno scheme.
Drew my own plan.
« Last Edit: May 16th, 2020 at 8:08pm by 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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