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Author Topic: P 39 Airacobra  (Read 3041 times)
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Hillsboro Bob
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« on: August 24, 2012, 11:04:29 PM »

While waiting for the adjustable button I ordered for my Wildcat, I decided to start the Bell P39 peanut scale project.  I downloaded the Guillow plan (24” wingspan model) from the HP Plan Gallery and have reduced the plans down to give me the 13” wingspan.  I had already down sized a Stahl plan (my Wildcat was a reduced down Stahl plan), but I wanted to try the Guillow frame and former fuselage construction method verses the keel and former method in the Stahl plan.

I plan to alter a few things as follows:

1.  I plan to eliminate most if not all of the diagonals in the fuselage.  This will reduce weight and the
     remaining structure should have ample strength with all the stringers for a peanut scale.

2. The plan shows a symmetrical wing section with zero incidence.  The stab is shown with a slight
    negative  incidence but I can’t really determine how much.  To be safe, I will build in ½- ¾ degree of
    positive incidence into the wing.

3.  I will build in adjustable surfaces. (Rudder, elevator and ailerons) I plan to use some thin light foam here
     like on the Wildcat.

4.  I will re-design the nose / cowl area to accommodate an adjustable button.

5.  I will be making the landing gear as light as possible for a try at ROG.  But, the gear will still be
     removable just in case it needs to come off for better flying.

6.  I’ll be using mostly 1/32” balsa sheet and 1/32 sq sticks.  I ordered balsa from Specialized Balsa LLC and
     should be arriving soon.  It is suppose to be in the 6# to 8# per CF range.  I hope I am not disappointed.

7.  I will use 1/64 sheet balsa in most areas where the plan calls for card stock.

8.  Ribs will be lightened with hole cutouts.

9.  Wing tips, stab and rudder will have laminated perimeters.  This will be my first try a laminating and I
     plan to use (2) @ 1/64 x 1/32 for the tail pieces and (3) @ 1/64 x 1/20 for the wing tips.

10.  I will move the motor peg forward just enough to have room to install the rubber motor without a lot of
      trouble.

11.  The dihedral on the Guillows 24” span model is 2 3/8”.  I will reduce the dihedral  to 1 1/8” on my
      peanut.  This  should bring the wingtips slightly above the thrust line.


At this point I still plan to use thinned white glue with domestic tissue, but this may change as I move through the build.

Currently I am planning on a color scheme I found on the internet (second photo attached)  This too may change before I get to that point.

My goal weight is 9 grams for the completed P39 airframe less the prop and motor.  I think I can build it in a manner that will not require much if any ballast weight. 

Since returning to this hobby after a 30 year layoff, I have managed to get a little better with each peanut.  The third photo below shows the models completed to date and the all up flying weight that resulted for each.   Each model sports the name of one of my grand kids on the nose.  I have 8 grand kids and one on the way so by the time I get to the ninth plane it should fly right of the board.

If I can manage the 9 grams on the P39 I figure I can get a prop / motor combo @ 4 grams for a total all up flying weight of 13 grams.  This would give a wing loading just under 5 grams/ square inch.
Attached files Thumbnail(s):
P 39 Airacobra
P 39 Airacobra
P 39 Airacobra
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Bob
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« Reply #1 on: August 25, 2012, 07:16:29 PM »

You building the same fin/rudder config. as on plan?

Couldn't tell by the plan, but do you have all of the rib patterns?


Good luck with it, Bob  Smiley .


jym6aw6


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Hillsboro Bob
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« Reply #2 on: August 25, 2012, 09:41:39 PM »

There is another plan sheet that is not in the photos with rib patterns.  I am still looking into the fin/rudder setup shown on the plan.  I think I will need to modify it to allow full rudder movement. 

Thanks for checking in.

Bob
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« Reply #3 on: August 25, 2012, 11:21:21 PM »

Sorry Bob, I was unclear about the fin/rudder. Was wondering if you intended to change the outline/shape to make it more resemble an actual production a/c (compare plan with your color/markings thumbnail).

Fin/rudder on plan looks like prototype version (likely was) and heightened slightly, typical practice among model designers of the day. For the sake of looks and stability would be better to go with the scale version.

Speaking of stability, were you going to enlarge the stab just a bit?

Really looking forward to watching this build. Had lots of fun with your Seamew and Wildcat, each new one gets more interesting  Grin !


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« Reply #4 on: August 26, 2012, 12:40:07 AM »

Bob

There are plenty of photographs like this one http://en.wikipedia.org/wiki/File:P-39Q_Airacobra.jpg of a slightly differently painted real P39Q on the internet.

May be useful to you.

Paul
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Hillsboro Bob
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« Reply #5 on: August 26, 2012, 12:47:59 AM »

JMY6...........Thanks for mentioning the stab area.  I think your right.  The wing area is 27 sq in +/- and the stab/elev in the plan is around 4 1/2 sq in that is under 17% of the wing area.  I will increase to 20%.  I also think using the scale fin outline would be better like you said.  The fin in the plan is around 1 1/2 sq in as should be at least 1/3 of the stab/elev area.  THANKS AGAIN.

Bob
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« Reply #6 on: August 26, 2012, 01:03:47 AM »

Paul,

Thanks..I checked the  pic you linked.  It's similar to the thumbnail I posted.  I happen to have the card model of the the plane in the thumbnail which gives me all views of the planes color and panel lines for easy reference.  I will keep looking for a color three view.  I may check the plastic kits at the local hobby shop.

Bob
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« Reply #7 on: August 26, 2012, 04:44:59 AM »

 Cool  fyi the plan of this p 39 is the XP 39.  IT HAD THE CRAPPY RUDDER AND TURBO SUPERCHARGER INLETS ON SIDE.  TO MAKE IT MORE FAMILIAR YOU NEED TO ENLARGE THE RUDDER ELEVATOR COMBO.  see AIRNEWSYEARBOOK vol 1 which has excellent pics showing both the x version and later b/c model with the redesign of the tail and TSC .

jim Grin 
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Hillsboro Bob
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« Reply #8 on: August 26, 2012, 12:02:04 PM »

Thanks Jim...

I am glad I made this initial post before actually started the build.  You and the others here have certainly helped me get started in the right direction.

My other build I have going on right now is a little easier.  I am also building a Peck lazer cut J3 cub.  I like to build two models at the same time.  It helps me stay interested.  It seems as I build a model I always get to a point on it where I slow down and wish it was done.  When that happens I move over to the other model which is usually at a different stage of construction.  Seems a little odd to me that something I like to so much can get dull at times.  Does this happen to anyone else ?

Bob
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« Reply #9 on: August 27, 2012, 08:14:52 AM »

Quote
It seems as I build a model I always get to a point on it where I slow down and wish it was done.

Yes, I think you're not alone on this.
I have to say, you've got quite a nice collection Bob.

For a while back when, I reduced some models from 24" to 13", keeping all proportions the same as shown.
As a result, I have some nice looking boat anchors. Cheesy

Then, I did a couple where I would limit what I would alter. Hmmm.
Now, It doesn't matter what I feel I have to change. It's getting changed.
 I now refer to them as a " based on this or that " model. Grin I feel so ashamed. Cool

Let's understand, 1/32" square stringer/longerons are one fourth of 1/16" by volume. So you might be careful using those for longerons. I think you already know this, but there it is.
The airfoil of the 24" model is not quite what I would expect for a pnut. It's thick and symetrical? Maybe that's for scale appearance.
The stab and fin areas are most likely not ideal, but you seem to already be dealing with that.

I hope this is helpful, or at least gets the propellers spinning.

Reagrds
Tony


Tony
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« Reply #10 on: August 27, 2012, 08:18:57 AM »

Bob, I missed this post, sorry!

My first published plan was a peanut P-39, reduced from the '39 in the Model Builder Plans Book.  Published as "Ivan's Iron Dog" in Model Builder it flew great - sorta ugly plans by today's standards (rapidograph and as I recall horrible hand-lettering...).  I have no idea where my originals are, but perhaps a H'pocketer may have a set they can copy for you.  Probably pub'd ca. 1986 or so.
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As it is not at all likely that any means of suspending the effect of air-resistance can ever be devised, a flying-machine must always be slow and cumbersome. . . . But as a means of amusement, the idea of aerial travel has great promise.

— T. Baron Russell, 'A hundred Years Hence,' 1905
Greg Langelius
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« Reply #11 on: August 27, 2012, 11:53:39 AM »

There's a taildragger version, the Bell XFL Airbonita
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« Reply #12 on: August 27, 2012, 12:56:02 PM »

Bob, I missed this post, sorry!

My first published plan was a peanut P-39, reduced from the '39 in the Model Builder Plans Book.  Published as "Ivan's Iron Dog" in Model Builder it flew great - sorta ugly plans by today's standards (rapidograph and as I recall horrible hand-lettering...).  I have no idea where my originals are, but perhaps a H'pocketer may have a set they can copy for you.  Probably pub'd ca. 1986 or so.

It's in the Dec '88 Model Builder.  I have the plan, but not in electronic form.  I'll see if I can get my scanner ressurected and send it to whoever wants it (will need to be "pasted" together).  I don't think uploading to the Gallery would be OK, as Bill Northrup is still offering his plan service (as far as I know).

Pete
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« Reply #13 on: August 27, 2012, 01:04:10 PM »

Greg, to the extent that I am permitted to do so, I give you permission to scan, and distribute, gratis copies of my P-39 Plan published in the December, 1986 issue of Model Builder
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« Reply #14 on: August 27, 2012, 01:59:13 PM »

i luv a copy of thos plan please

jim Cool
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Hillsboro Bob
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« Reply #15 on: August 27, 2012, 11:47:44 PM »

Quote
It seems as I build a model I always get to a point on it where I slow down and wish it was done.

Yes, I think you're not alone on this.
I have to say, you've got quite a nice collection Bob.

For a while back when, I reduced some models from 24" to 13", keeping all proportions the same as shown.
As a result, I have some nice looking boat anchors. Cheesy

Then, I did a couple where I would limit what I would alter. Hmmm.
Now, It doesn't matter what I feel I have to change. It's getting changed.
 I now refer to them as a " based on this or that " model. Grin I feel so ashamed. Cool

Let's understand, 1/32" square stringer/longerons are one fourth of 1/16" by volume. So you might be careful using those for longerons. I think you already know this, but there it is.
The airfoil of the 24" model is not quite what I would expect for a pnut. It's thick and symetrical? Maybe that's for scale appearance.
The stab and fin areas are most likely not ideal, but you seem to already be dealing with that.

I hope this is helpful, or at least gets the propellers spinning.

Reagrds
Tony

Tony,

I have had the same issues with straight plan reductions.  I have several other models that look good enough but are marginal at best when it comes to flying.  Lately (actually since joining this forum) I have been trying harder to look at the whole picture including the flying capbilities of my models.  The first thing I started working harder on is building lighter.  To that end you and all the others on this website have been helpful more than you know.  I find myself reading every new post that comes out regarding scale flyers.  On my recently completed Wildcat (a scaled down plan) I adjusted the stab area & dihedral and I am feeling pretty good about it's potential flying capability.  With a 6" Peck prop hanging on the nose it balanced at about 30% back of the LE (pic 1) without any ballast.  That means it will be right at  0.5 gram/sq inch wing loading.  I will find out when the adjustable nose button I ordered gets here.

Back to the P39,  I have been tinkering with the Guillows plan (pic's 2 & 3) and I think I can make it work.  And you are right about the airfoil.  I will be revising to a Clark Y type and I will be using select 1/20 longerons and 1/32 sringers.  The balsa I ordered has not yet arrived so I have time to keep re-working the plans or find another plan.

Bob


Tony

Attached files Thumbnail(s):
Re: P 39 Airacobra
Re: P 39 Airacobra
Re: P 39 Airacobra
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« Reply #16 on: August 27, 2012, 11:52:59 PM »

Bob, I missed this post, sorry!

My first published plan was a peanut P-39, reduced from the '39 in the Model Builder Plans Book.  Published as "Ivan's Iron Dog" in Model Builder it flew great - sorta ugly plans by today's standards (rapidograph and as I recall horrible hand-lettering...).  I have no idea where my originals are, but perhaps a H'pocketer may have a set they can copy for you.  Probably pub'd ca. 1986 or so.

Let me know if those plans show up.  I would definitely be interested.

Thanks

Bob
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« Reply #17 on: August 28, 2012, 06:38:07 AM »

I have located my plan and article.  I'll try to get my scanner operating tonight or tomorrow.  Due to the screwy way MB chops up the articles, I'll try to re-formate it to save a HUGE amount of memory.

Hope to have it ready for dissemination by tomorrow evening (Centr. European Summer Time).

Pete
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« Reply #18 on: August 28, 2012, 09:23:27 AM »

Pete:  It sounds as if you are entirely too organized.

Hillsboro Bob:  Here's the '39 from out of the Vault of Time.  It's pretty well worn-out.  A bit distressing, because I discovered that I built quite a bit lighter 'way back when.  Need to re-think that, I suppose.
Attached files Thumbnail(s):
Re: P 39 Airacobra
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« Reply #19 on: August 28, 2012, 07:47:15 PM »

Your model has held up well Wordguy.   It looks great.  How did it fly ?

I have continued to work on the Guillow plan. My balsa order still has not arrived.

Recent revisions to original plan:

1  Revised tail fin to P39Q variant (enlarged slightly from scale).
2  Changed Air scoop and re-aligned main landing gear to match three view. 
3  Revised ribs to a Clark Y section.  I downloaded a Clark Y from the internet and then used my copier to scale each of
   the ribs.
4  Enlarged stab to a full 5 square inches ( approx 20% of wing area ).
5  Located MAC. 

Changing the airfoil will require modifying formers C, D and E.  I will fine tune these as I build.


Attached files Thumbnail(s):
Re: P 39 Airacobra
Re: P 39 Airacobra
Re: P 39 Airacobra
Re: P 39 Airacobra
Re: P 39 Airacobra
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« Reply #20 on: August 29, 2012, 07:35:40 AM »

Bob, as I recall, it flew great (it evolved from a C. Mather design, so I can claim no credit).  The obligatory skosh of right and down thrust.  Again, IIRC, I think this is the only Pnut (only airplane?) I've ever built that required no nose weight.
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« Reply #21 on: August 29, 2012, 03:27:33 PM »

I've gotten the P-39 plan scanned.  Two files in .pdf @ ~4.5 Mb that SHOULD print out at 100%.  I added a cm scale so that one can check, and the two sides can be directly joined at the edge of the  lines.  It will take a few days before I can get the article scanned (my scanner doesn't like my new confuser - need the new drivers for the OCR portion).

Those who wish a copy, send me a PM (private message) with your email address. 

Pete
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« Reply #22 on: August 29, 2012, 05:07:19 PM »

Pete,

I sent you an message with my email (I think##).  Let me know if it worked.  First time trying the personal message thing.

Thanks

Bob
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« Reply #23 on: August 29, 2012, 07:57:53 PM »

I've got the whole schmear ready.  Re-formatting the article went easier than I anticipated (once I got the programs to work Angry.
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« Reply #24 on: August 29, 2012, 08:30:34 PM »

I got the plans and article.  Thanks Pete.  Very helpful. 

Bob
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