1/4 scale electric Bell P39 Airacobra build (designed by Jerry Bates)

[Craig Spence]

Hi all,

 

I will be building a 1/4 scale Jerry Bates Bell P39Q Airacobra very soon.

It will be electric powered with x4 6 cell batteries up front.

I have started a blog on Facebook as well if anyone wants to keep up on there https://www.facebook.com/Craigs-Bell-P39-Airacobra-build-102061535705394/

 

At the moment I’m just doing small bits whilst waiting for the laser cut kit to arrive.

 

I’ve started the 50cal guns, center line tank and also general hatch vinyl.

 

Ive also added some of the schemes I’m thinking about for finishing.

 

 

[Craig Spence]

Latest progress, centre line tank shape complete. Filler applied and will be re-sanded tomorrow for glass.

 

50 cal gun shrouds primed and will be finished with matte black tomorrow then guns glued inside.

[Craig Spence]

The center line drop tank was sanded this morning, refilled and sanded again.

I have now glassed both sides and will run down again tomorrow before adding a heavier filler.

 

I intend to build up 4 or 5 layers of glass cloth (78gm, 78gm, 38gm, 38gm and 38 gm to finish).

Once this has been done and sanded I will cut in half and gut it all out, this way I can add hard fixings inside for mounting and also add the scale joining lip.

[Craig Spence]

I’ve also finished the 50cals and cut some control horns for the rudder and ailerons from 1/16th G10.

[Stephen Belshaw]

Very neat work. What do you use to cut out small pieces such as the control horns from GF?

[Craig Spence]

Thanks Stephen. I use a cutting wheel on my Dremel, cut as close to the line as possible and then sand to the line.

[Graham Davies 3]

Some great skills here Craig. This is shaping up to be a fascinating and inspiring build. I've pulled up a chair and a bag of popcorn and shall watch with great interest!

[Craig Spence]

Thanks Graham, very much appreciated ?.

I’m hoping I get the laser cut kit soon & can then start the hard part. At present it’s looking like a month. So whilst waiting I intend to do all the fiddly bits.

[Nigel R]

This will be quite a beast! Watching with interest.

[Craig Spence]

Cheers Nigel, still undecided which motor set up but will make my mind up in the coming weeks.

I want to try and keep it as scale as possible so will be running a three blade prop anywhere between 25” & 27” diameter.

Edited January 19, 2022 by Craig Spence

[Craig Spence]

Ive got wood! ?

Arrived today from SLEC and all in good order.

 

Today I sanded the first layer of 78gm cloth & as suspected there was a lot of work to do.

I used P38 & sanded back to glass and in some areas through it (needed to be done and part of the plan).

I’ve now added the second layer of 78gm cloth and will be adding two layers 38gm tomorrow.

 

Once that’s done I can spray, sand, fill repeat. Then cut in half to hollow out.

 

I’ve also cut some more control horns, the plan indicated use of standard ones but I think I can get a better bond with G10 opposed to using screws through a finished surface. Stephen Belshaw, I’ve put photos up of how I do the G10, I use tissue paper to make patterns then glue to the G10, leaves a lovely line to cut to and just peels off after.

 

I’ve also cut the exhaust stack cylinders for the fuselage.

I’ll make the body’s for these in the near future.

 

I plan to start striping sheeting soon for the planking, it will save hours before I start.

 

Cheers all.

[Craig Spence]

I’ve added another 2 layers of glass today to the tank but won’t be posting that as it’s same/ same sort of thing until I get filler paint on it.

 

I have however done the first batch of exhaust stacks which is very time consuming.

I think I won’t glass these and will probably just seal the grain, sand and prime.

hopefully they turn out well.

 

cheers all.

 

 

[Craig Spence]

Oh well, one more before the weekend.

Tank has the final layer of cloth on and exhausts are first coat sealer (all twelve shaped now).

Should have manifolds done early next week.

[Craig Spence]

Small updates tonight.

Started on cockpit panels but then found a guy who does them so I will be getting him to do once I can find some decent photos of a P39 cockpit.
Exhaust stacks have been given a coat of finishing resin with micro balloons, sanded back and filler primed.

The center line tank has been filler primed, filler applied and will sand back tomorrow for ? hopefully finishing coats.

In other news I’ve decided I will be running a Turnigy Rotomax 100cc (167kv), 200a speed controller with x4 6 cell packs at 6200mah.
I will have to go with a two blade prop (27x12 Falcon) opposed to the three blade ?, this is mainly to do with motor cooling.

I am having a back plate made up with an impeller which will take air in through the cannon opening in the spinner and push it across the motor.

Other cooling options have been considered but will not be as effective.

 

Retracts have finally been sourced, CenturyJet in the US should have the mains on sale at the end of the month so I better put something aside for them

[Craig Spence]

Gave the stacks a coat of grey primer this morning & then black.
They’ve turned out ok so decided to glue them to their backing. The plans say use 1/8” ply but I’ve used 1/8” balsa, there’s not structural requirement for ply that I can see & worst case I’ll laminate it with 1/16” ply.

I’ve also do finished the center line tank finishing work. Next step is to cut it in half & hollow out.

Then, there’s a load of work detailing it.

Edited January 28, 2022 by Craig Spence

[Jon H]

Big fan of the P39 and my YT example always seems to gather a crowd around it as most have no idea what it is. While it has some strange handling characteristics its great fun to fly. 

 

[Nigel R]

30 minutes ago, Jon - Laser Engines said:

some strange handling characteristics

 

how so?

 

always interested in how some types have odd handling / behaviour

[Craig Spence]

Looks great Jon, what size was this one and what power were you using?

Im interested in the handling issues to, the full size had plenty but I think that was mainly due to the engine issues.

 

I know they had a habit of tucking out of loops and going into a flat spin but again I think this was due to not enough speed/ power in turn inducing the stall.

[Craig Spence]

4 minutes ago, Nigel R said:

 

how so?

 

always interested in how some types have odd handling / behaviour

I’m the same Nigel, I think the full size had a lot of engine/ power issues resulting in stalls.

 

The models shouldn’t suffer the same issues though if the power is right.

[Jon H]

9 minutes ago, Nigel R said:

 

how so?

 

always interested in how some types have odd handling / behaviour

 

Its got some odd coupling going on. Yaw-pitch, pitch-yaw, and the normal yaw-roll/roll-yaw. The reason its fun is rolling causes yaw, which causes pitch, so you are always working all 3 control surfaces to keep things together. 

 

I suspect this is gyro precession caused by the big spinner being so far ahead of the c/g due to the unusually long nose. The tail is also unusually short. This is all scale and stems from the full size having the engine in the middle. 

 

Looking at in cockpit footage of stalling and other testing on the full size the model is flying more or less as the full size did. 

 

Its also never in trim in pitch or yaw, which again seems to mimic the full size. All in all its easy enough to fly but a challenge to fly well and this, for me anyway, is what keeps it interesting. 

 

 

My model is 80 inch and 21lbs, laser 300v for power. Its over powered in a straight line and becomes really over sensitive on the elevator. Scale fast passes are done at a shade over 1/2 throttle i suppose, maybe 2/3? The power is perfect in the vertical though. 

Edited January 28, 2022 by Jon - Laser Engines

[Jon H]

Just for interest, this video shows some stall/spin testing 

 

 

 

This one has some nice footage in it as well 

 

 

[Craig Spence]

30 minutes ago, Jon - Laser Engines said:

 

Its got some odd coupling going on. Yaw-pitch, pitch-yaw, and the normal yaw-roll/roll-yaw. The reason its fun is rolling causes yaw, which causes pitch, so you are always working all 3 control surfaces to keep things together. 

 

I suspect this is gyro precession caused by the big spinner being so far ahead of the c/g due to the unusually long nose. The tail is also unusually short. This is all scale and stems from the full size having the engine in the middle. 

 

Looking at in cockpit footage of stalling and other testing on the full size the model is flying more or less as the full size did. 

 

Its also never in trim in pitch or yaw, which again seems to mimic the full size. All in all its easy enough to fly but a challenge to fly well and this, for me anyway, is what keeps it interesting. 

 

 

My model is 80 inch and 21lbs, laser 300v for power. Its over powered in a straight line and becomes really over sensitive on the elevator. Scale fast passes are done at a shade over 1/2 throttle i suppose, maybe 2/3? The power is perfect in the vertical though. 

That’s great info for me Jon.

I’ve drawn a few conclusions from it as well for my set up.

 

The full size had a huge prop! That coupled with the long nose would cause quite a bit of gyroscopic action as you said.

 

The scale size prop for my model would be 34”!! I initially dropped to a 27”x12” two blade but found the best efficiency with a 23x12.

Hopefully this reduces the issue. I’ve heard the electric foamies of the model fly on rails and I’m hoping this is due to a smaller more efficient prop on the front.

 

im also going with a straight thrust line to start with no side or down thrust to start with. I will then adjust as required.

 

Your model looks to be a great set up, is there much room up front with the engine in there?

[Craig Spence]

23 minutes ago, Jon - Laser Engines said:

Just for interest, this video shows some stall/spin testing 

 

 

 

This one has some nice footage in it as well 

 

 

Those spins are horrific! ?

[Nigel R]

Must be tight up front with retracts onboard, it's a narrow nose. Obviously the full size didn't have to squeeze the motor in that small space.

 

The pitch/yaw/roll coupling must keep things interesting in the air...

[Jon H]

I run a 20x10 2 blade on mine and torque swing on takeoff is an issue with a castering nose leg. The full size had left right brakes to help with this but i do not. 

 

My model also has a very heavy load on the nose wheel as its not quite scale in its main leg position so it tends to leap into the air if im not careful. It will also pitch down with power on a go around...which is nice. 

 

Fitment of the 300v is tight but its pretty neat. The retract unit is a little far back for scale and sits just behind the carbs. I made a custom nose leg and new retract mount as the YT examples were rubbish both for scale and functionality. The engine is mounted dead straight and in the air this seems fine. 

 

All the control coupling is interesting but becomes 2nd nature after a while. I was talking to someone while flying it a while back as they were complimenting my nice large loops and how effortless it all looked. i said it wasnt and talked them through a loop as i flew it.

 

running in with down elevator held in to keep the nose down and a tiny amount of left rudder at the high run in speed, release a little forward pressure to raise the nose but still holding down, start to add power, model rolls left so counter it, let the rudder off, elevator neutral, full power by now and nearly vertical, start adding right rudder, add left aileron, start pulling elevator, more right rudder, more elevator, more rudder (1/2 - 2/3 total deflection by now), ailerons as required and pull hard over the top probably with more left aileron applied. Into the dive, start rolling the power off, adjust all 3 controls to cover for the rising airspeed and watch for the elevator biting as i cross a certain speed. recover with down elevator applied while still pitching up.

 

I think that is a basic summary. Anyway my spectator was horrified it took so much work to get round a 'simple' loop. I just responded that it was more or less standard for warbirds and not a big issue. 

 

I would not consider the performance of a foamy as a basis for comparison as they are so light. The loadings and forces acting on them are just not quite the same. 

 

As a side note, how big/heavy is the 1/4 scale? About 105 inch i would guess and somewhere around 30lbs? I have my eye on the jim pepino 1/5 scale for a future project but that will be after my current cobra bites the dust.