# 3/4th Scale Replica Bell P-39 Airacobra

Discussion in 'Wood Construction' started by Eagle92lightning, May 13, 2019.

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1. May 14, 2019

### vhhjr

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A 75% P-39 is a little narrow so I added 4 inches to the cockpit width and to get drive shaft clearance 2 inches to the height of the mock-up I built many years ago. The doors were also enlarged a bit, but they turned out too small for safe access or rapid escape. I had planned on hinging the top of the canopy for better access. I started with a giant scale RC model plans by Joe Pepino and enlarged them. One can also have them digitized and do the rework in CAD.

The drive line would be the hardest part of the project. Casales makes angled drag boat "V" and "Z" drives that have been used as PSRUs. CV joints have better misalignment tolerance than universal joints.

Just putting a engine in the nose is problematic from a weight & balance standpoint. The original was designed for the weight of a nose cannon plus the rear engine. Putting the redrive, battery, etc. in the nose helps.

One possibility is to make a hybrid with the engine/generator in the back and use an electric motor prop drive to eliminate the driveshaft. That also allows the engine to be sized for cruise and use some battery reserve to get takeoff and climb power.

The P-39 was a fun project, but will most likely never get past the mock-up stage. So many project, so littl

Vince Homer

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2. May 14, 2019

### spaschke

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It's not nice to say "lomcevaks" in public.

3. May 14, 2019

### bmcj

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+1

Without the cannon, perhaps the engine can go up front. It might need a little adjustment on the fire-aft wing position to make balancing easier, but it could be done without being too noticeable.

The P-39 was not the most loved airplane to those who flew it. That’s why many of them were given to Russia and replaced with the P-63 King Cobra. I hear that one of the issues with the P-39 was that the CG varied greatly as the ammo was used up, making the plane tailheavy toward the end of a flight.

4. May 14, 2019

### Jerry Lytle

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If I recall correctly, and at my age that is problematical, the Army Air Corps limited P-39 pilots to about 5'8" in height due to the cramped cockpit. Where ar you going to find a 75% pilot? That is about 4'3" by my calculations.

5. May 14, 2019

### Victor Bravo

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Why a sub-scale Cobra anyway? You could just as easily build a full size one, a lot lighter, and use a built V8 and an automotive drive shaft system using mostly stock parts. Wood, foam and graphlite, maybe half the weight of the original. You'd still have to carry ballast in the nose, but an aluminum V8 amidships will be a lot easier to balance than an Allison V12

6. May 14, 2019

### pictsidhe

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The pilots were very limited in height.
The Russians absolutely loved the p-39s they got. They loathed the Spitfires, the Hurricanes were much more suited. Different planes suited different theatres.

7. May 14, 2019

### mm4440

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You have to like a fighter with roll down windows.Your legs can extend into the gun bay with a semi reclining seat. A challenging project. Carbon fiber drive shaft is a must for safety. An iron bird to test the entire engine, shaft, gearbox and prop system on the ground before you start building an airframe is a must. Getting the cg forward enough will be difficult. A light engine like the Aeromomentum would help. I recommend a steel tube fuselage from the gear box to motor mounts; it could be your iron bird. " Iron Bird" would be a good name. Good luck.

8. May 15, 2019

### ScaleBirdsScott

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I also like the idea of using an ICE power generation plant and then running a suitably powerful electric motor, with battery and fuel stores as appropriate.

9. May 15, 2019

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For someone with "little experience" this should be a piece of cake.

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10. May 15, 2019

### pictsidhe

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Personally, I think the tailwheel endorsement is the easier and cheaper option. But I'm just an armchair expert.

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11. May 15, 2019

### wsimpso1

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To make it look at all right on the ramp, you will have to sit semi-reclined with your feet and rudder system well forward. I am designing and building a conventional looking and engined airplane, and it is a HUGE task. I do urge you to look more closely at the Walter Mitty replica fighter as a build project and a purchased airplane to fly in the meanwhile...

I am a retired powertrain engineer who did vibe isolation as a big part of my work life. I even worked on a 100 hp airplane with a driveshaft, solving their torsional resonance problems. Here be dragons... Putting the engine in the back is not for anyone but a talented engineer experienced in vibration, with a vibe analysis, a dynamics FEA package, a torsional vibe measurement system all available and used. Even well designed for vibe, there will still be challenges...

That being said, I suspect that it is doable aiming for a fully "soft" system. That means 1st order resonance will have to occur more than half an octave below engine firing frequency at idle, and other powertrain vibration modes will have to be either below idle or more than half an octave above max firing frequency. I would investigate an elastomeric isolator at the engine and make the shaft as flexible as will still give good FOS in it, then break the shaft up along its length as needed to keep it below critical speed. The PSRU can then be at the front after a lot of the torsional vibe is gone. If any of this sounds at all foreign, you really should investigate other options.

This is the one place where I agree that the series hybrid approach makes sense. Its weight is not so much of an impediment in a fighter replica. A light engine- genset aft, an electric motor in the nose, an electric constant speed prop, and batteries way forward to adjust the CG could be doable in a P-39 type config. But then my ignorance of electrical system design may be minimizing the difficulties of this path. I personally would not go hog wild on the idea of engine sized for cruise plus batteries for takeoff and climb - the engine sized for cruise (at 75% power) is usually just about right for takeoff and climb at 100%. Also, if you are ever going to do any proficiency flying - touch and go to practice landings - well, the battery will will probably only be good for a couple, then you will need to land to recharge or do a recharge cruise at modest power for a while.

Billski

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12. May 15, 2019

### stanislavz

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+100% for going pure electric. - electric motor in front, battery in place of original motor. An 1hour with current batery tech from scrapped tesla.

+150% for going hybrid with 50% petrol power for ok cruise and compact enough to fit in front. And battery at the end.. like long/big battery booster.

13. May 15, 2019

### Derswede

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It would be a lot easier to find an LSA bird, start flying and once you have some experience, make the determination of what you want to build. There is a pretty Chief on Barnstormers now for $15,900, a recent rebuild that will get you in the air and teach you about having fun flying. That price is about what you would spend on just the tools you would need to build a plane, esp. one which will require considerable engineering. A MiniMax is a good start on the UltraLight level, I see them all the time for as cheap as$4000 for a flying airplane. My family rebuilt quite a few birds over the years, and it costs quite a bit just to get the workshop set up. We had to build lots of parts for several planes, so just the machinery you would need would be a chunk of change. Lots of good experience available here on HBA, it will save you time and grief.

Derswede

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14. May 15, 2019

### Victor Bravo

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I wasn't saying it would be easy by any stretch. I was saying that leaving it full size and making it a lot lighter would solve some of the powerplant cost and driveline development problems, as well as solving the small cabin size problem.

15. May 15, 2019

### hiroyoshi

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My suggestion to the 3/4 scale P-39 conundrum is this: build a 300 hp Barracuda.

The Barra has something of the 'look' of a P-39, and this could be enhanced through reshaping the rudder (which looks like a Messerchmitt 109 rudder, just a bit). That's a 'simple' job.

What you'd get would be a proven airrame that has some of the 'fight feel' of a fighter (and a good bit of the low-level performance, particularly in climb), without having to go through a long design/fabrication process (as in, you've got to design everything and mock up a lot to see that it all works).

You coud get closer to the mark by narrowing the fuselage to single-seat, and lowering the turtledeck to allow for a closer canopy profile. Narrowing the fuselage means redesigning the control system, but this isn't as hard as it might sound, since the elevator control circuit's on the centerline anyway. One issue would be where you'd put your feet, because the nosewheel compartment is between the 'rudder pedal tunnels'.

Yes, it has a cranked wing with 'hershey bar' outer panels, but it flies really, really well, is fast, and climbs beautifully...and the snappy acceleration and high rate of climb are the coolest things about flying a fighter anyway.

16. May 15, 2019

### CharlieN

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I do not spend much time around here but to me the driveline is not all that hard to make work, as in a V8 or such behind the pilot, a clutch with damper with shaft going forward to a reduction unit driving the prop. The tech for this is much more available today than even a decade ago. The clutches, Viscus dampening and carbon driveshafts were just not in our reach even a few years back.
But, and the big But this is not a task for a first project. I would not even try to attempt a hybrid electric on a home engineering scale until more resources are available to us. Working with high voltage DC current is not for the feint of heart.
My true advice though, do not even consider this project for your first one, DO consider it for when the knowledge, technology and skills have all come to you.
Do start building, I consider building planes to be a collage course worth of learning. I am going on 40 years of learning from building.
Get flying, do not even consider your achieving your private pilots certificate the end of your learning, it is just the beginning. Tail wheel, do it.
I am a major proponent of scratch building since the dedication and learning is steep, very steep at times. The skills and understanding you develop in time makes the next project seem quite easy. Nothing is ever easy.
Start with something achievable and climb the ladder from there. A scale P-39 to me is a great goal for the future, but not where one should start.

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17. May 15, 2019

### Jean Crous

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I have to agree with CharlieN, I have built , refurbished and rebuilt a number of aircraft over the past 22 years and can say , I am glad I started with a refurbish job. Someone that has never built a full scale aircraft from kit or plans .......tread carefully. It can turn into a bottomless financial pit and a morale breaker. As a full time aircraft builder, I have finished off many uncompleted projects for the first owners and many more for the second and third owners of a project. This being said , go for something achievable , and build up experience first.
Just my 2cents worth.
Jean.

18. May 16, 2019

### Tiger Tim

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One thing’s for sure in this thread: it sure is easy to commit to a plan that spends someone else’s time and money.

19. May 17, 2019

### hiroyoshi

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Some thoughts, kind of random, but the scaled P-39 is intriguing...

1) Get a set of Mini-Imp plans from Pat Hart; Molt Taylor did a good job in designing a shaft drive to work with an opposed 4, and while a vibe analysis should still be done, it's a good place to start.

2) Building a wooden primary structure might be a good idea from a vibe damping standpoint. Using a steel-tube fuselage, I'd expect to find a lot of cracking with this kind of system because it will shake.

3) split the driveshaft into two sections. I think this was mentioned before.

4) One of the 'weird fit' items will be the control stick, because the drive shaft tunnel will go where it would normally be. Might have to go with a side-stick.

5) The doors are going to be a lot harder to design in than one might think, keeping the bending strength of the fuselage in mind. I would choose a roll-down wind and a canopy 'hatch'.

6 And what Charlie said above...don't make this your first project. A scale P-39 is an order of magnitude more difficult than a Falco, and the Falco (for which plans are free, see www.seqair.com) is probably the most difficult wooen homebuilt arount.

I've been doing this stuff for about 30 years, and have a doctorate in structural engineering with a good bit of vibration analysis.

But that said, everything I said above might not be correct, so feel free to point out errors!

I would consider this as a project, but am looking at something else...a scaled Staggerwing with an M14P, two seats, and fixed gear (like the A17F).

20. May 17, 2019

### hiroyoshi

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OK, another few thoughts. Tell me if this gets boring.

- scale to 80%

- the main fuselage structure can be a pair of deep timber box beams, spaced to carry the engine bearers (for an assumed 351W, and built in a curve to match the fuselage lines. This has the advantage of providing very positive support for the engine and driveline, good tosional stiffness, and a solid basis for a bulkhead-and-skin structure. The beams would be dreadful to build, but it could be done. I would see them actually merging into one structure at the tail and where the reduction gear's mounted. (So, yes, one would need hard jigs for each side, and the 'longerons' in the beams would neeed to be steamed.)

- the 2-beam fuselage would allow a door, no problem, but since the pilot's feet would rest between the beams, emergency egress would be very had, so a canopy-hatch would be a must.

- use automotive driveshafts from a race shop, and accept the weight penalty.

- a very preliminary estimate sees an empty weight of about 2300#

This really can be done. It would be a monster of a project, but there are few deal-killers here.