P-38 Lightning (BT-60/BT-55 + 3D printed Parts + Plywood Wings/Fins)

BigMacDaddy

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I loaded 2oz of nose weight in this and it pulls the CG nearly to the front of the wings. I think I am going to try canting the 24mm engine to aim through the CG (or a bit behind it) from below to avoid burning the stabilizer and to help make this fly straight. Hopefully it is not too much that the model gets hung up on the launch rod/rail.
 

Daddyisabar

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You could design a locking tab system so that burned tail plane could be replaced after every flight. Max out wing dihedral and raise tail plane to where it still looks good. A tiny bit more distance can make a huge difference.

On the Lancaster it needed about 8.5 inches and positioned at the top of the motor tube to avoid major cutting/hibachi damage on a D12. Note the button base was the first tail that was almost cut on the first flight.

20230125_071739.jpg
Lable papered 1/4" balsa. Still slightly hibachied and sooted up. Leading edge sealed with CA.

Burn will be slightly different on each flight do to conditions and motor used.
 
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BigMacDaddy

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You could design a locking tab system so that burned tail plane could be replaced after every flight. Max out wing dihedral and raise tail plane to where it still looks good. A tiny bit more distance can make a huge difference.

On the Lancaster it needed about 8.5 inches and positioned at the top of the motor tube to avoid major cutting/hibachi damage on a D12. Note the button base was the first tail that was almost cut on the first flight.

View attachment 559353
Lable papered 1/4" balsa. Still slightly hibachied and sorted up. Leading edge sealed with CA.

Burn will be slightly different on each flight do to conditions and motor used.
Thank you -- good advice.

I think now I might be canted enough to avoid scorching the back fin but I am worried about getting hung up on the launch rod (and spinning / arching backwards or forwards). I am thinking about doing a pair of launch rods through tubes on top of wings and bottom of tail fin. They should flex some to handle somewhat asymmetrical thrust but hopefully not enough to make this fly like a plane and not a rocket.

If it does not work I will go back to modify design for a pair of 18mm engines canted in opposite directions (and make a plug to hide the top motor hole for display). I can also extend the body a bit more and make the rear fins larger.

I wish that OR could simulate this type of rocket accurately so that I could trust what I see there.
 

Daddyisabar

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Thank you -- good advice.

I think now I might be canted enough to avoid scorching the back fin but I am worried about getting hung up on the launch rod (and spinning / arching backwards or forwards). I am thinking about doing a pair of launch rods through tubes on top of wings and bottom of tail fin. They should flex some to handle somewhat asymmetrical thrust but hopefully not enough to make this fly like a plane and not a rocket.

If it does not work I will go back to modify design for a pair of 18mm engines canted in opposite directions (and make a plug to hide the top motor hole for display). I can also extend the body a bit more and make the rear fins larger.

I wish that OR could simulate this type of rocket accurately so that I could trust what I see there.
You have left the comfort provided by the warm bosom of computer simulation. A new realm of silly airplane oddrocs awaits, a land mindsims and experimentation. Science is blurred, RSO's are frightened and olde tyme mysticism rules. This is not your Mama's 3-4FNC anymore. Welcome to the jungle! :)
 

Daddyisabar

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A single cant motor is risky. Probably not the best idea but who am I to say since I have never had the balls to try one. WD 40 on the rod is your friend! Do not fly in front of anyone who is triggered by rod whip. ;)
 

lakeroadster

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Why not simply make the rear elevator out of something non-flammable, perhaps carbon fiber :dontknow:. You know ... Improvise, Adapt and Overcome.

But my mind sim on this rocket screams "Weather Cocking", unless you launch with zero wind speed, and that's a tough nut to crack.
 

Daddyisabar

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The coating on the leading edge of the Lancaster's first tail boom was high temp J B weld. At 8.5 inches and just above being in line, the MIGHTY D12 flame cut through it like a hot knife through butter. NEVER UNDERESTIMATE THE POWER OF THE MIGHTY D12!
 

lakeroadster

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The coating on the leading edge of the Lancaster's first tail boom was high temp J B weld. At 8.5 inches and just above being in line, the MIGHTY D12 flame cut through it like a hot knife through butter. NEVER UNDERESTIMATE THE POWER OF THE MIGHTY D12!
A simple bench test is easy peazy. You could even use one of those cato prone E motors.
 

Daddyisabar

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A simple bench test is easy peazy. You could even use one of those cato prone E motors.
For accurate results the bench would need to be set up in a wind tunnel...or use a really big fan. The flame/heat will also lap over leading edge protection. Burning chucks and nasty sparks everywhere...a hellish inferno!
 

Daddyisabar

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The problem is not only the propellant flame but the heat from the delay grain as well. Like the big boys flying their quick burn N motors...more than three seconds of hell!

I don't need a rocket scientist. I need a materials developer!
 

Daddyisabar

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Lowering the motor or raising the tail just a tiny bit can make a huge difference. Find that sweet spot just outside the DANGER ZONE! Too close and your burned, too far and the asymmetric tail throws you out of control. Where is Kelly Johnson to solve the problems on the back end of the P38? Gotta call in the skunk works.
 

Daddyisabar

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Good idea -- I have a bunch of old D and E engines I bought cheap due to the likelihood of unexpected flight/burn events.
With those motors you have to worry about burn on both ends. Real fun to static test taped to a rod at the club. 3-2-1 kaboom! Fireball up, clay nozzle bits down. Until the RSO put a stop to it. Then out to the office parking lot by the airport.
 

K'Tesh

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Daddyisabar

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The thing about flying warbirds, no matter what else you are told, is that the science must be exact. Failure is not an option. You must demonstrate a through knowledge of the NACA(before NASA) aerodynamic reports from the late 30's and early 40's. You are pushing the envelope, on the razors edge between glory and disaster. Get your late day pad assignment to fly your new Beech Staggerwing oddroc, hook it up and return to flight line only find out the Sheriff and some top men from ULA have just joined the crowd. The heat is On! No fear, I trust the science, no need for prayer. Push the button and:
FB_IMG_1625457762436.jpg
Whew! It actually flew. :)
 

lakeroadster

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The thing about flying warbirds, no matter what else you are told, is that the science must be exact. Failure is not an option. You must demonstrate a through knowledge of the NACA(before NASA) aerodynamic reports from the late 30's and early 40's. You are pushing the envelope, on the razors edge between glory and disaster. Get your late day pad assignment to fly your new Beech Staggerwing oddroc, hook it up and return to flight line only find out the Sheriff and some top men from ULA have just joined the crowd. The heat is On! No fear, I trust the science, no need for prayer. Push the button and:
View attachment 559411
Whew! It actually flew. :)

How did it fly during the coast phase? Stable or erratic?
 

Daddyisabar

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How did it fly during the coast phase? Stable or erratic?
Nice and stable each flight, otherwise the RSO would chastise my tender sensitivities. Once that heavy nose starts to point down the short delay ejection pops the nose cone and chutes for baby soft landings. Nose cone on a seperate chute due to excessive mass.

Nose weight and power rule! I love biplane and triplane warbirds! They have always flown well when all motors light.
FB_IMG_1626024435673.jpg
Albatross DV going into the wind at Alamosa. Did a roll and then ejected, best flight ever!
 

BigMacDaddy

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Like Sgt. Oddball in Kelly's Heros: "No negative vibes!"

What is the motor's cant angle? Is it through or slightly above/below the CG? Where is the final CG? The answers will determine my fear factor. :)
CG is right at front of wings. Motor is angled more or less through CG (as best I can simulate / estimate). Motor cant angle is around 10 degrees.

I launch in a wet muddy big field so pretty low risk if it crashes and/or burns (I also bring fire extinguisher).
 

Imaknow

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@BigMacDaddy
I don't know if there is a reason for it or not, but the boom scoops appear to be not centered on the side centerline of the booms like they are on the real bird. They seem to be raised. Was this an engineering decision?

This is a super cool project, I certainly hope it works out!
 

BigMacDaddy

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@BigMacDaddy
I don't know if there is a reason for it or not, but the boom scoops appear to be not centered on the side centerline of the booms like they are on the real bird. They seem to be raised. Was this an engineering decision?

This is a super cool project, I certainly hope it works out!

Thank you -- I think it is just the picture. The scoops under the points on the BT-55 tubes are centered horizontally. Is that what you mean?

EDIT: Ah, you said vertically. I basically squeezed the oval profile into the round space so there are definitely some compromises.

I wanted to create the oval profile on the front of these parts even though this needed to be mounted on a circular BT-55 tube (so I needed to create an unusual profile for that part).

Do you think there is a way to make them better?

EDIT: There is a slight notch in the rear of the pointed front of the part since I am hoping to make propellers that can snap onto them for display.

1674733457638.png 1674733633208.png
 
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Daddyisabar

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Well, replacement parts will be needed for those of us who go, "These propellers are so cool I might just leave them on this time..."
Look at how those CRAZY scale guys replace escape towers and all kinds of other bits and pieces that fall off during flight! :)

Yes Mr. RSO. Propellers on oddroc airplane rockets are cool!
 
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