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

[BigMacDaddy]

Not sure I will be able to make this work but figured I might as well start a thread about the attempt (posted in the half baked thread that I was thinkin about doing this)...

I started w/ OpenRocket because this is so much different than any other models I have designed before.


Made some progress on the design today -- learning how to use Fusion 360 to complement TinkerCAD. Maybe eventually I will do my designs 100% in Fusion. Looking at BT-60 main tube w/ BT-55 side tubes. I am thinking rear ejection although I worry the pop-pod might hit that rear tail section. Someone suggested drop pods for chutes which could also be possible although it certainly adds some complexity (but also coolness). Need to figure out what type of boat tail I am going to put on the main body section but depends a bit on recovery setup.

 

[Cape Byron]

I am thinking rear ejection although I worry the pop-pod might hit that rear tail section.

That would be what we call a 'bad thing'.

Rear eject would be cool on an MXY-7 Ohka, but not this one.

 

[KenECoyote]

VERY cool!

 

[bjphoenix]

It is very cool.

How do you keep from burning the horizontal stabilizer at the back?

 

[Daddyisabar]

Canted motors will miss tail boom! P61 Black Widow on Forum if interested.

 

[Daddyisabar]

Not sure I will be able to make this work but figured I might as well start a thread about the attempt (posted in the half baked thread that I was thinkin about doing this)...

I started w/ OpenRocket because this is so much different than any other models I have designed before.
View attachment 558685

Made some progress on the design today -- learning how to use Fusion 360 to complement TinkerCAD. Maybe eventually I will do my designs 100% in Fusion. Looking at BT-60 main tube w/ BT-55 side tubes. I am thinking rear ejection although I worry the pop-pod might hit that rear tail section. Someone suggested drop pods for chutes which could also be possible although it certainly adds some complexity (but also coolness). Need to figure out what type of boat tail I am going to put on the main body section but depends a bit on recovery setup.

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View attachment 558684

Get the motor a tad lower and still make it look good. Otherwise:


It looks like you will get substantial tail burn on the design above. Tail boom farther back good...farther up good. Not so good for the most important thing: Looks!

 

[Daddyisabar]

More asymmetry by lowering motor might require drop tanks, bombs or gear down to balance. Flying old warbirds is just a bit crazy! Prop blades are lousy for CP but look cool.



See rear end of Lancaster and Dornier for distance and offset to avoid burn from mighty D12.

 

[Daddyisabar]

Don't be scared to drop the gear!

 

[Daddyisabar]

Could go with a three motor mid cant if it looked good enough on P38. Motors hidden under wing. Hide through motor by building into canopy on top or bomb on the bottom.

 

[Daddyisabar]

I was thinking of the twin seat P38 with the radar pod up front. Three motor mid cant. Since I am very snobbish on the looks of my P38 (the purdiest twin engine of the war) I did not pull the trigger on the build.

 

[Daddyisabar]

The CG should be higher. It looks good for an airplane flight but the RSO hates that. It's a model rocket and should fly straight up like one! Gosh darn it! Why does it want to fly like an airplane?

Higher CG, more nose weight, the bigger motor(s), then more nose weight, then bringing heavy nose cone down on seperate chute. Then giving the nose cone in one hand and the rest of the rocket in the other to a Top Man. Then listen to him laugh out loud! Yikes!

 

[Daddyisabar]

Yes Mr. RSO, I am flying an airplane rocket they called the Fork Tailed Devil!

 

[BABAR]

Cool

Is there a way to transition to glider? Dump the motor and actuate the elevator?

Put 13mm motors in the pods, mainly to push out chutes.

 

[BigMacDaddy]

View attachment 558716
Canted motors will miss tail boom! P61 Black Widow on Forum if interested.

That P-61 is an awesome build -- I did not think to search for the P-61 in the forums when I was searching for past builds. I just saw a plastic model of the P-61 in Michaels on Friday and was tempted to purchase for inspiration but my kids were not letting me take the time to look at it.

Do you remember - how much nose weight did you need?
What size are your side pods (read that the main body is BT-60 from a Bertha)?
What was your wingspan?

I am always amazed at the building skills for the scratch-builders that do this stuff with balsa!

 

[BigMacDaddy]

The CG should be higher. It looks good for an airplane flight but the RSO hates that. It's a model rocket and should fly straight up like one! Gosh darn it! Why does it want to fly like an airplane?

Higher CG, more nose weight, the bigger motor(s), then more nose weight, then bringing heavy nose cone down on seperate chute. Then giving the nose cone in one hand and the rest of the rocket in the other to a Top Man. Then listen to him laugh out loud! Yikes!

I have definitely thought about doing a separate chute for nose cone and plane body. I even wondered about a second hidden-inside nosecone that would push out the first weighted one (with its chute) and would then allow the glider to recover. Not sure it is worth it to do anything that complex here - maybe just 2x chutes.

After seeing your build, I went ahead and extended the nose tube forward and the tail tubes back 1.5cm to make the whole thing longer (push CP back and CG forward).

 

[BigMacDaddy]

I was thinking of the twin seat P38 with the radar pod up front. Three motor mid cant. Since I am very snobbish on the looks of my P38 (the purdiest twin engine of the war) I did not pull the trigger on the build.

Twin-seat version does give a bit more space for the top mounted canted motor. I'd really like to do this with a single 24mm D engine but may need to consider other options if it gets too heavy or if I cannot get it to fly straight.

 

[BigMacDaddy]

It is very cool.

How do you keep from burning the horizontal stabilizer at the back?

Not sure yet

I have not designed the boat tail portion of the front section and one thing I would do is use 3D printed ABS to direct the engine blast a bit (i.e., I can protect that horizontal fin some). It sounds like canting the motor some is also critical but I'd like to avoid having to do two engines so I would need to get the angle just right to push it forward instead of pushing it into loop-the-loops.

 

[BigMacDaddy]

Cool

Is there a way to transition to glider? Dump the motor and actuate the elevator?

Put 13mm motors in the pods, mainly to push out chutes.

Thanks!

My glider mindsim is ill-equipped!

Loading recovery into the side pods is a good idea. I was contemplating attaching the main nose cone to the pod nosecones to pull them off on ejection.

 

[BigMacDaddy]

Haha... the drop tanks / pods were also used to carry people, for things like medical evacuations...

 

[KenECoyote]

Haha... the drop tanks / pods were also used to carry people, for things like medical evacuations...

View attachment 558740

Just hope that the pilot doesn't forget and drops the tank!

 

[Cape Byron]

Been musing over this in the doctor's waiting room...

Wouldn't it be advantageous to build the P-38 as is. Test as a glider and get those niggling things like, I dunno, gliding sorted and then work out propulsion? The glider may need some revisions in terms of wingspan, dihedral, chord, noseweight, etc before trying to punt it into the sky.

Just my $0.02 I have a lot of gliders in notebooks and very few of my own designs in the air, but that's how I'd approach it.

Edit: Sorry if that sounded snarky, it wasn't meant to be. I just don't want you to give up on the gliding bit. You know I want to sell a glider. Yours or mine, I really don't care.

 

[Daddyisabar]

That P-61 is an awesome build -- I did not think to search for the P-61 in the forums when I was searching for past builds. I just saw a plastic model of the P-61 in Michaels on Friday and was tempted to purchase for inspiration but my kids were not letting me take the time to look at it.

Do you remember - how much nose weight did you need?
What size are your side pods (read that the main body is BT-60 from a Bertha)?
What was your wingspan?

I am always amazed at the building skills for the scratch-builders that do this stuff with balsa!

I will go dig it out of storage and weigh the nose cone now that I have a scale. Build your model, then engine up and add nose weight to taste and more thrust if needed. The amount of nose weight needed on a warbird is not pretty. Flew it on two AT D21s and it ate up all that E42 power! All dope sealed balsa as plywood would have been too heavy.

 

[Daddyisabar]

Twin-seat version does give a bit more space for the top mounted canted motor. I'd really like to do this with a single 24mm D engine but may need to consider other options if it gets too heavy or if I cannot get it to fly straight.

View attachment 558739

A custom printed twin or triple cant motor mount would be super cool. Canted motors are the best way to go. A single motor in a boat tail is going to burn the tail plane unless it is lifted up higher, causing nasty asymmetry. You don't want it to look like a Sea Vixen or have to offset asymmetric drag with scary gear down or ordinance beneath the wing. Yikes!

Top motor hidden by the second cockpit and the bottom two would be hidden under the wing on a three motor cluster. See my JU87 build on the forum. On a two motor cluster you could hide the bottom motor with a bomb, like I did on my Henschel HS 132, also on TRF. Hiding rocket motors is TIGHT! Or just let it stick out.

On a Bt 60 it would be 3 X18mm, just like a Flis Kits TRES! So many composite 18mm motor options these days. Two 18mm and just like a Flis Kits Duces Wild!

The body of my P 61 kinda glides but would crash without a chute. Keep it simple. Gliding old warbirds (sports scale) would be very hard. We do these things not because they are easy, but because they are hard.

Get those motors as far forward as possible in the print with just enough room for recovery and nose weight. Build light and strong every where else. Desperation to add weight to the front and reduce weight in the back is a sure fire sign you are building an old school warbird oddroc.

 

[BABAR]

The body of my P 61 kinda glides but would crash without a chute. Keep it simple. Gliding old warbirds (sports scale) would be very hard. We do these things not because they are easy, but because they are hard.

Get those motors as far forward as possible in the print with just enough room for recovery and nose weight. Build light and strong every where else. Desperation to add weight to the front and reduce weight in the back is a sure fire sign you are building an old school warbird oddroc.

Thinking outside of the box, as usual. Any way to drop nearly the complete main body with the motor, all coming down on a chute. Leave the canopy and the bridging pylons. May need to enlarge the elevator to give more surface area.

 

[BigMacDaddy]

So I dry fit everything together tonight for the 1st time. It is difficult to get the assembly accurate on this since the two tail portions have to be precisely equally spaced on all measurements. Interesting challenge!

Not worrying about details like the canopy at this point. Just wanted to see what happens with a single D12 engine (need to figure out weight next to try to shift CG as far forward as I can). The wings are canted so I am hoping this drops the main body down enough that it does not burn the cross-member in rear too badly (I think I will put a bit of metal tape on the wood just to give myself a bit more hope). Really just want to see how much this arches over the top so I can gauge how aggressively I need to tweak design..

 

[BigMacDaddy]

Thinking outside of the box, as usual. Any way to drop nearly the complete main body with the motor, all coming down on a chute. Leave the canopy and the bridging pylons. May need to enlarge the elevator to give more surface area.

I think for my first test I will put two chutes in here with one just for the nose cone and the other for the body). I think the body might tumble recover although it might get some damage depending on how and where it lands (my field is pretty soft and damp unless it is frozen).

 

[BigMacDaddy]

I also went back to Michaels and bought this...

 

[Daddyisabar]

Thinking outside of the box, as usual. Any way to drop nearly the complete main body with the motor, all coming down on a chute. Leave the canopy and the bridging pylons. May need to enlarge the elevator to give more surface area.

You have flown silly airplane rocket oddrocs before! OR As from Dune prophecies; "He will know your ways as if born to them..." Your oddroc is fitted desert style.

 

[Daddyisabar]

So I dry fit everything together tonight for the 1st time. It is difficult to get the assembly accurate on this since the two tail portions have to be precisely equally spaced on all measurements. Interesting challenge!

Not worrying about details like the canopy at this point. Just wanted to see what happens with a single D12 engine (need to figure out weight next to try to shift CG as far forward as I can). The wings are canted so I am hoping this drops the main body down enough that it does not burn the cross-member in rear too badly (I think I will put a bit of metal tape on the wood just to give myself a bit more hope). Really just want to see how much this arches over the top so I can gauge how aggressively I need to tweak design..

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My advice. Be sure to have the nose cone chute pull out the body chute. Put the CG at least at the leading edge of the wing. Good launch equipment and conditions.

Hibachi effect on the rear will be interesting. Good luck!

 

[cwbullet]

Hibachi effect on the rear will be interesting.

That will be the challenge, but no one will know without trying.