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


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6 minutes ago, Jon - Laser Engines said:

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. 

 

 

That’s a good insight Jon ?

 

The 1/4 scale is 102” wingspan and around 19kgs.

This may go up or down, it all depends on how well I can place the batteries.

 

I worked it out the other night that I will have 13lb of motor and batteries up front, not to mention a huge steel nose retract.

 

It may be the first warbird I’ve built that will need weight on the tail ?

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I was just looking up the plans and yea shes a porker. Quite surprised to see it that heavy. Shame its so hard to fit an engine in the front, it would probably save loads of weight. 

 

As for weight, mine has the rx battery and servos down at the back as it was nose heavy. I probably still is a bit nose heavy, but i already fly with significant down trim so im leaving it alone. 

 

One day, if it gets mega tatty but i dont want to bin it, i may hack mine apart and put the engine in the middle. It would be an interesting experiment for sure and the composite fuselage lends itself to this sort of thing due to its rigidity. It would be a mega project though.

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31 minutes ago, Jon - Laser Engines said:

I was just looking up the plans and yea shes a porker. Quite surprised to see it that heavy. Shame its so hard to fit an engine in the front, it would probably save loads of weight. 

 

As for weight, mine has the rx battery and servos down at the back as it was nose heavy. I probably still is a bit nose heavy, but i already fly with significant down trim so im leaving it alone. 

 

One day, if it gets mega tatty but i dont want to bin it, i may hack mine apart and put the engine in the middle. It would be an interesting experiment for sure and the composite fuselage lends itself to this sort of thing due to its rigidity. It would be a mega project though.

There’s a guy in the states who has put the engine behind the cockpit and it really does look great.

I have my doubts though, he has made a geared shaft extension with a geared reduction drive using a DLE 50.

 

Works great and I was considering it but I just saw too many issues which until you fly it you won’t know the effects.

 

The weight for one with the extension would be a lot, the cooling would be difficult even with the air intakes.

Remember you want 3 times going out what you’ve got coming in.

 

Plus the heat exchange to the inside of the airframe.

I really was considering it but too much risk for me.

 

The electric set up will be heavy but an electric motor will be much more efficient, instant power and torque when required.

 

Once I start the actual build I will be doing everything I can to make it as light as possible.

I’ve had to do it in the past where I basically remove formers & ribs. It doesn’t save a great deal but does help.

I'm also considering a water based finishing system opposed to resin, I’ve never tried it before but I’m guessing it will be easier.

 

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Do you have a link to the chap with the extension? curious to have a read up on what he did and how. I did toy with the idea of having a go at it when the model was new but decided not to. If i get to a point where the model is falling apart anyway i may have a go at it as i have nothing to really loose at that point. If it all works, i can then give it a cosmetic makeover. 

 

On the cooling, the 1 in 3 out thing is kicked around but is not actually true. 1:1 or even less on the exit is fine if you have a venturi style outlet on it. Provided you have a pressure differential you can use much smaller cooling outlets than you might think. The only snag is, as you say, you have no idea if it works until you are in the air as it is very reliant on the slipstream past the model giving you that venturi effect.

 

 

 

 

 

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40 minutes ago, Jon - Laser Engines said:

Do you have a link to the chap with the extension? curious to have a read up on what he did and how. I did toy with the idea of having a go at it when the model was new but decided not to. If i get to a point where the model is falling apart anyway i may have a go at it as i have nothing to really loose at that point. If it all works, i can then give it a cosmetic makeover. 

 

On the cooling, the 1 in 3 out thing is kicked around but is not actually true. 1:1 or even less on the exit is fine if you have a venturi style outlet on it. Provided you have a pressure differential you can use much smaller cooling outlets than you might think. The only snag is, as you say, you have no idea if it works until you are in the air as it is very reliant on the slipstream past the model giving you that venturi effect.

 

 

 

 

 

The one in three out is a general rule of thumb and you are right about the Venturi.
I will have to do something similar for the motor and ESC.

 

The Venturi effect on his build will be very good, but personally I don’t think it’ll be focussed enough.

Plus the turbulence inside could mess with the carb intake which I’ve had before and had to make a separate intake for the carb.

 

His build is on Facebook if your on there, it’s called “Lee’s P39 Airacobra build” I believe.

 

He is a really good builder ?

 

 

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Little bit of work done tonight, sprayed the exhaust stack mounts black and then went to work on the center line tank.

The tank is proving to be a pain but after hitting one shell it’s starting to feel like it’s getting there.
I have to make up a lip for the tank, I’ll probably do this with strips of 1/6th lite ply and then glue to the edge.

Once done I’ll tack the shells together for shaping and then break apart until I build the fuz and figure out a mounting method.

The laser cut kit will be arriving mid February now, going nuts waiting for it ?

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Small update. So I have acquired a Turnigy 100cc motor but still have to get a suitable speed controller - looking for a 200A one at present which should be ample.

I’ve hollowed out the exhaust stacks and made a light balsa former for the center line tank.
The former is only there so I can sand flat, I’ll add epoxy and micro balloons to make a strong former and take it back to about 1/4” thickness.

Once both shells have been done like this Ill add further strengthening and give it a light sand before adding the lip.
I’ll add some details in the coming weeks and this will be stored to one side along with the other pre made parts.

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I’ve done a bit more detailing to the center line tank but will need a lot of finishing work in future.
I’ve decided to store it away for now and concentrate on getting the workshop cleared again.

I will be getting plans on the table soon and may pin some stuff to it whilst I’m waiting on the kit.
I’ll also get ahead of time by starting to cut my planking which I’ll get on later tonight.

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Well, I’m starting to get impatient now whilst waiting for the kit.
I’ve laid the first part of the plan on the bench (& gave myself no working room! - I could’ve sorted it out but I’m too tight to use too much plan protector ?), put shallow cuts in the crutch and pinned the crutch to the board.

Side view of fuz on the wall for reference and all part patterns cut out and in box for reference.

I’ve also thrown together a 1/32nd plastic kit for reference. I won’t say the name of the kit as it is awful and I’ve had to cut, modify and sand to get it all together.
I’ve also left parts out in the box which I’ll use for reference as well.

Come on kit! Hurry up ?

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Haven’t posted for a while so here goes.
Made the first split flap and I must say I’ve rather enjoyed it.
I’ve never built a split flap and further more not from G10.

It’s built from .030 G10 for the main flap and the components are .020.
Only 1/4” at its thickest it’s quite fiddly to profile the balsa ribs (matchsticks!!!).

This will be getting attached with a 5/8” wide or 34mm continuous brass piano hinge with no 4 wood screws.

I think once it’s prepared & painted it’ll really look good.

Kit should be arriving any day and I can’t wait!

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  • 2 weeks later...

The kit finally turned up today ?
Only a short night building after a late return from work but good to get some parts out.

I’ve started on the first hatch and had to do some minor cuts as it’s slightly different from plan.
The hatch was later masking tapped to the fuz to hold it in place whilst building.

I’ve also noticed that mounting the motor will be tight for drilling but I decided against pre drilling the former to ensure it’s completely center.

Before this I also received my hinges from America, there from CB Associates and we’re recommended on the plan.
I’m quite keen on them as they seem robust and light weight, also easier to install.

The part I’m struggling with is piano hinges for the flaps. Everything in the UK is too bulky for the width of the surface and too heavy. 
I’ll probably adapt this part of the plan to accommodate another hinging method but we’ll see.

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1 hour ago, maurice northcott said:

Nice work Craig.

 

From where did you source your G10 please - the reason I ask is because you seem to describe thickness in terms of thousandths (of an inch, I guess) whereas most suppliers seem to describe thickness in mm? I apologise if you have already given this info in the thread.

 

Cheers

maurice

Thanks Maurice,

 

I got it from;

 

https://www.aiplastics.com/products/industrial-laminates/epoxy-4w-ep.html

 

It’s labelled as 4W/EP which is essentially the same thing.
 
These guys cut the material to your desired length as well. I referred to the thickness in imperial as it’s an American plan and that’s what’s detailed, I just converted imperial to metric as that’s what the company uses.

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2 hours ago, Craig Spence said:

Thanks Maurice,

 

I got it from;

 

https://www.aiplastics.com/products/industrial-laminates/epoxy-4w-ep.html

 

It’s labelled as 4W/EP which is essentially the same thing.
 
These guys cut the material to your desired length as well. I referred to the thickness in imperial as it’s an American plan and that’s what’s detailed, I just converted imperial to metric as that’s what the company uses.

Thats a pretty keen price too ?

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Took a hit to progress last week as I’ve been ill and only got out of bed today.
Anyway, I’ve made some progress today and will post some decent pics when the workshops had a good spruce up tomorrow.

I’ve encountered a few issues along the way, some formers are too big dispite being cut as per plan & some are too small (you can see this where the stringers are not flush with the formers).
It’s easily resolved but annoying and time consuming.
I’ve also had to butcher one of the formers as it’s significantly out.

The tail wing tube is tight against the fin post and the ribs are just cut for the ally tube and no the phenolic tube (same with stab).
On the fuz this isn’t a major issue as I’ll just cut the tube to suit.
On the stab it will be an issue and something I’ll have to address once the fuz construction is complete, I may be able to open them up with a step drill but some of the ribs haven’t got much meat to do this.

Lastly, the balsa stringers are playing game and with a few slots on the backside there meeting the curves lovely.

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Good progress today but I’ve had to stop early as still not feeling great.
A few issues dealt with which I’ll be passing back to the relevant people to make the next persons build easier.

A few parts are tongue to tongue instead of tongue and groove (easily solved).
Had to sand the phenolic tube to fit tight against the fin post.
Fin 1 former did not sit on the stringer as designed and the cut in the last former is 1/4” above it, again an easy fix.

Next week I should be able to start sheeting this part but it will be tricky. It will be a mix of planking and sheeting as it has some tight curves front and back, it’s also tricky around the fin and stab.

Can’t wait.

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  • 2 weeks later...

Haven’t posted for a while so here’s where I’m at.
I’ve planked the top fuz but left a gap as I have to make a frame to hold it upside down.

The nose guns have proven to be a pig! The recesses in the formers are not even close for a 5/8” tube so I ripped off the ply and started again.

I’ve added tubes in the fin for lights as this model will be fully lit.
The fairings for the tail are also tricky and I will be doing more work on these when the whole plane is sheeted.

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