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Old May 30, 2013, 02:52 PM
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Question
TF Redbox P-39 engines

I have fond memories of the man that taught me to fly RC back in 1988 and one of his favorite planes to fly was a TF P-39. Being that it was back in he late eighties it had to have been a yellow or redbox kit that he had. He always raved about what a great flier it was. I believe that he had a Kraft 61 2 stroke in it.

Well I have have wanted one of these planes for a long while so yesterday on a whim I posted a wanted thread for a TF P-39 kit of any vintage. To my amazement I have had 3 responses thus far with kits for sale. I really did not expect to get ANY responses, but now that I have I need to do some research.

I read through several build logs for the kit I found here, BUT they are quite old and I did not want to post on those threads; thus this thread.

The specs on the red box are:
Wing span 60"
Wing area 650 sq inches
Weight 5.5 to 6.5 lbs
Engine .40 to .60

Now one of the build logs had the final weight of the red box at 10lb, but the gent had retracts, painted it, full cockpit, the works.

My question is IF built as light as possible, no retracts or flaps would a Saito 65, OS 70 Surpass, or a Tower Hobbies 61 2 stroke fly this plane? At 6.5lb and 650 sq in that gives a very high wing loading of 40oz/sq in.

Having to buy a different engine to power this plane might just put it out of reach at this point. If one of the engines I listed would power it adequately then I may go forward with the purchase.

The Tower 61 would be good for it, but the huge muffler that comes with that engine is .... well HUGE. The red box uses an upright engine installation too so there is no hiding the muffler.

Thoughts??? Input?
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Old May 31, 2013, 03:24 PM
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The Tower .61 would be fine for a Red Box P-39. The muffler could be replaced with an aftermarket muffler like the Macs muffler that is much smaller and performs the same, or a JettStream muffler that will increase performance at certain rpm.
To build the model that light one must use light weight wood which will generally not be found in the kit. I think typically they come in at 8 lbs. If you have a Dremel and use weight savings type lightening techniques, hollow every block to 1/8th inch thickness after shaping, removing bulkhead and wing rib internal materials, replace sheeting with 4 to 6 lbs quality balsa, etc you might get it under 7 lbs.
Using lightweight hardware will help when assembling the model and creating the pushrods. The wing's aileron surfaces should be lighter balsa with mini servos, one for each aileron. Don't use the wing mounted bellcranks as designed. Using CA hinges for all control surfaces. Then covering the model with film will be the lightest finish.
Good luck,
Chris...
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Old May 31, 2013, 04:51 PM
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So my questions is how in the hell do they list the weight on the box a 5.5 - 6.5 lbs???? I would obviously rather put a 4-stroke on it; not much of a 2-stroke person. I really would love to have this plane but trying to lighten every piece of wood in an already difficult to build kit might be a bit more than I am up for.

Of course IF I could keep it under or at 7lb perhaps either the 70 Surpass or the Saito 65 might work alright for scale type performance. Maybe???

I wonder if it would really detract from the appearance if the wing was NOT sheeted, and cap strips were used instead? I am not looking to build a scale model, just a nice looking warbird in flight.
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Old Jun 01, 2013, 08:13 AM
Navy Retired
Dixfield, Maine
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Build it as designed. Put the tower .60 in upright. If my memory is correct the engine is mounted on wood beams, to side mount the engine would require re-engineering the front end. Cover with flat ultracote nor monocote. The large canopy would have to be painted. Put in a dummy pilot, instrument panel, and call it good to go. Spend the time to insure a straight and aligned model well built and sanded smooth and enjoy a flying a model that
looks like a P39 and is a good flying model.
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Old Jun 01, 2013, 12:00 PM
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I have a Saito .65 and have little doubt that it would be more than adequate power for a 7 pound red box P-39. My .02 cents of course.

PS - Either my math is wrong or yours is dmrcflyr2 as I get a wing loading of ~25 oz/sq ft on a 7 pound airplane with 650 squares. (I'm assuming you meant per square foot and not per square inch as you posted!!!) At 6.5 pounds I get a wing loading of just over 23 oz/sq ft. Either one of those can be considered feather light for a warbird. I regularly fly a 65" span Hellcat that is about 11 3/4 pounds and has a wing loading around 45 oz/sq ft and it flies just fine.
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Old Jun 01, 2013, 01:40 PM
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Quote:
Originally Posted by Chad Veich View Post
I have a Saito .65 and have little doubt that it would be more than adequate power for a 7 pound red box P-39. My .02 cents of course.

PS - Either my math is wrong or yours is dmrcflyr2 as I get a wing loading of ~25 oz/sq ft on a 7 pound airplane with 650 squares. (I'm assuming you meant per square foot and not per square inch as you posted!!!) At 6.5 pounds I get a wing loading of just over 23 oz/sq ft. Either one of those can be considered feather light for a warbird. I regularly fly a 65" span Hellcat that is about 11 3/4 pounds and has a wing loading around 45 oz/sq ft and it flies just fine.
Duh... I used 28 grams instead of 16 oz in the calculation. Yes much better wing loading! Thank you for that. I did pull the trigger on the purchase and really want to put the Saito in it anyway. I'm still trying to finish up a SIG Citabria so it may be many months before work is started on this plane. At least I will have it in inventory now.
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Old Jun 03, 2013, 10:58 AM
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If the bell-crank system is anything like the P-40, ... I would suggest doing a little research on changing that design. Since you are building this airplane from a kit anyway ... you could take just a little extra time to upgrade (substantially) ... to build a much better set-up. Put individual servos at each surface control. Use a smaller and lighter servo, ... there are plenty of high output / lightweight servos available for the upgrade, .. and they are affordable and very effective. The weight can be matched up pretty close and should not be a big factor if you choose a good servo for the job. There are slim-line and flat servos as well as many others. This is where you need to spend time and research ... but it will be well worth the time.

I would recommend the flaps and ailerons in the wings. There will be more work for you there ... but the pay-off is worth the time. I would also move the rudder and elevator servos back to the rear to eliminate the long push-rods. It does affect the C/G a little ... but that can be corrected if you are careful to keep the tail as light as possible during the build. You may need to move the battery forward and other things such as the radio gear ... but you can do this during the build.

The newer radios .... with better programming adjustments, and also better servo equipment ... are by far more superior these days compared to the days when the original kit was produced. You may spend a little more money for the extra servos / or you might even have them already? You will have more work to do on the build .., but that little bit of expense would have a great return in dialing in your controls.
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Old Jun 03, 2013, 11:07 AM
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Thank you for the advice. I am sure I will be taking much of it and re-engineering it a bit. I still think not on the flaps. I really do not think the benefit is there as I have see it fly without and heard it doesn't need them; that extra weight and design off the table.
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Old Jun 03, 2013, 07:12 PM
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Quote:
Originally Posted by dmrcflyr2 View Post
So my questions is how in the hell do they list the weight on the box a 5.5 - 6.5 lbs????
I wonder if it would really detract from the appearance if the wing was NOT sheeted, and cap strips were used instead? I am not looking to build a scale model, just a nice looking warbird in flight.
I believe the balsa weight in the kit was unable to be held to the same level of quality and maintain the kit pricing.

The best place to start in keeping it lightweight is the tail. Since the surfaces of the tail are sheet balsa, the replacement of those components to proper weight and grain would go a long way towards light weight.

The ailerons are sheet and the same can be done there.

4 to 6 lbs 3/32nd wing sheeting with open bays and cap strips would be fine and look cool.

The fuselage and wing are heavily designed, so lightening those structures and using replacement wood where necessary. The plastic is not lightweight, so some way of building a lightweight cowl hatch as a replacement of it would be required.

I use a digital gram scale to weigh my wood, and assemblies. Having spent 40 years building control line Stunt models where we typically have 700 sq in models to 55 oz makes me critical in my building when it comes with weight. I have every confidence that you can make great savings in your build using the same tools and with the proper attitude toward weight savings.

Smaller Mini servos can be used for these small sized 60 ships, too. metal geared servos with 40 to 50 oz in torque values will work beautifully with 2 stroke engines. 4 strokes need more strength because of the pounding of the engines power pulses through the airframe. The lightest a model can be built is using a 2 stroke engine, before going to electric where your structure can be made amazingly lightweight. One for rudder and elevator in the tail would be fine. One for each aileron is preferable.

Using fixed gear, light weight wheels like Lite Flites would be weight savers. LiPo rx battery would save a lot. Anywhere where weight can be thought to be saved, do it and you will succeed in making the lightest model possible.

Chris...
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Old Jun 03, 2013, 07:36 PM
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Some of the wood in the SIG Citabria kit I am building now was somewhat unacceptable for its intended use so I replaced them. You are suggesting quite a bit of wood replacement making it seem like I just spent a bunch of money for a set of plans and heavy plastic.

At this time I don't have any of the mini servos, just standards, but maybe by the time I get around to building the kit I'll have some.
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Old Jun 03, 2013, 08:33 PM
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You will want to go to the smaller servos for weight and size consideration. Standard size are jus too big and heavy for that wing and tail section modification. You will need to shop around ... but with a coupon code and free shipping, ... you will find something affordable that will handle the job. Metal gear servos are the way to go with that 2c.
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Old Jun 03, 2013, 09:57 PM
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Quote:
Originally Posted by dmrcflyr2 View Post
Some of the wood in the SIG Citabria kit I am building now was somewhat unacceptable for its intended use so I replaced them. You are suggesting quite a bit of wood replacement making it seem like I just spent a bunch of money for a set of plans and heavy plastic.

At this time I don't have any of the mini servos, just standards, but maybe by the time I get around to building the kit I'll have some.
Hi DM,
If you replaced wood for the tail and ailerons you'll save a lot of weight forward because of the long moment arm of the tail from the CG. That is of course if the stock in the kit needs replaced. The scale will tell you what you need. For instance; if when a horizontal stab, elevators, control horn connector and horn and hinges weigh more than 3.5 oz I look for lighter wood.

If you have a motor tool of the Craftsman, Dremel, or even Ace Hardware chinese import style, you can reduce the weight of a lot of the provided parts after assembly.

As far as the servos, there are many ways to save dough on good servos and get the right kind for the application. When one is looking to make a model a certain way, one must go that direction. I merely offer suggestions for your stated desire, making a TF Red Box P-39 lightweight.

You cannot build a TF P-39 from the plans, because the parts are not all drawn on the plan/instruction sheet. So you needed to buy the kit anyway!

I'm sure it'll fly great as long as you try to save weight, and then assemble it true and properly aligned. I have one and am re-engineering it as an electric with full competition Scale operational features and it is a very difficult subject because of it's small size (for a Scale model) and the kit's Stand-Off Scale origin with heavy construction.

I often re-use components, use older equipment, and heavier stuff myself. In this case the Citabria is the place for those things, and I'd think of the Airacobra as like a Classic Pattern model of the 70's with high speed, high performance and fine handling qualities that you would want to exploit with higher quality servos, horns, pushrods so no slop or delay interferes with your control.

It's a good looking model that when properly built is a good flyer, I expect mine to weigh ten pounds with all of the necessary components including retracts, flaps, internal horns, four bladed scale sized propeller (mine will be Cobra II) and sound system including speakers which are very heavy, so you'll have a blast even if you don't use every lightweight item suggested here.

Have fun,
Chris...
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Old Jun 04, 2013, 02:13 PM
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I built a Redbox P-39 and it was 8.5 pounds. It was glassed and painted with springair retracts, full cockpit, opening door panel lines. I regretted not putting flaps on it, but it was my first warbird build. I had a K&B .61 in it and it had plenty of power. Somebody wanted it worse than I did, and I sold it.
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Old Jun 04, 2013, 02:17 PM
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That's good to hear, 8.5 is a good weight with retracts and paint.
Chris...
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Old Jun 04, 2013, 03:10 PM
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I still have my Redbox P-39. It's right around 8 pounds and flies really well with an O.S. 61 two-stroker. Flaps really slow the landing speed down and makes short-field landings a breeze. A GREAT flying plane.
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