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Jan 25, 2020, 04:07 PM
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Tim Cullip's Avatar
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All four boom half shells are now done and I've managed to carve and sand one of the boom's front cowls from blue foam. I think I've also figured out the layout of the R/C equipment but am taking my time (aka procrastinating) about actually installing it. The combination of routing all the wiring and the fact that nothing will be accessible once the half booms are glued together leaves me feeling a little queasy.

The R/C gear is:
Dasmikro Rx (0.28 gms)
Two Racerstar 08028 brushless motors (2.15 gms each)
Two Pesky brushless ESC's I've never used before but am happy with them on the bench tests (0.16 gms each)
One HK 1.9 gm rotary servo for the ailerons
One unknown 1.9 gm linear servo for the elevator
One 5v booster (0.3 gms) to keep the voltage up so the Rx doesn't brown out
Still deciding on the battery but probably a 250mah 1S battery (6gms, don't like going that big on a Peanut but might have to to handle current draw of the two motors). Also still deciding where to put the battery (last equipment location decision to make).

I have managed to rewrap the two Racerstar 08028 motors down from 12000KV to something close to 7000KV (from 7 wraps up to 13 wraps). This produces 18000rpm on a 65mm prop (don't know the pitch but somewhere around 40mm?) at 0.85amps (so it will be a combined 1.70 amps for the two motors). I have one running clockwise and one running counterclockwise. The nice thing about using props from a quad is it is trivial to find both cc and ccw props. I won't be going with three bladed props though initially (maybe I'll switch to them later if this thing actually flies).

Any suggestions on which one (cc or ccw) should be the left motor and which the right motor? I seem to recall the real P-38 started with one configuration and then they switched it to the opposite - guess I should look that up again (though probably the reason they settled on one orientation might not be a factor in this small Peanut model).

I'm also trying to decide how much of the plane to cover in tissue before I glue it all together and how much to do after things like the booms have been glued in place.


Tim
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Jan 25, 2020, 04:29 PM
Go small or go home
ruzam's Avatar
I recall reading about the prop rotation direction on this plane. Something about the wash over the wings. One direction favoured lift. The other caused it to need twice as much runway distance to take off. Wait, sorry, that was a different WW2 twin engine. Seems the p38 suffered from prop wash turbulence over the elevator. Final configuration was props turning out on the top.
Jan 25, 2020, 07:43 PM
Registered User
Correct. You want them spinning down on the outside. Otherwise you will enter a roll but never leave it. And yes... experience.
Jan 25, 2020, 10:14 PM
DFS#000178
Rampage's Avatar
Full-scale P-38 started with the props turning inboard at the top. This created a turbulence over the h-stab and elevator that made the nose wander vertically ever so slightly and threw off the accuracy of the guns so the directions were switched to swing outboard at the top and the problem went away.
Jan 26, 2020, 09:10 PM
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Tim Cullip's Avatar
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Thanks for the information on the prop directions, I'm going to go with what the real P-38 ended up using.

I've started mounting the equipment, all the pieces are in place now except the elevator servo and the starboard motor.
The port boom halves have been glued together, it was the easy one since the only thing inside of it is the ESC, I've also mounted the motor to that boom and tested that it works (sure hope I never have to cut it open to get to the ESC or its wiring).

The starboard boom will be next, it will contain the elevator servo, Rx, voltage booster, and the other ESC (all but the servo have been put in place).


Tim
Jan 27, 2020, 09:05 AM
Registered User
Looking good.
Do you have a link for the voltage booster? I have ordered several already, but they are all at least one gramm. Fortunatly they cost almost nothing...
Jan 27, 2020, 01:42 PM
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Tim Cullip's Avatar
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Quote:
Originally Posted by JustAHuman
Looking good.
Do you have a link for the voltage booster? I have ordered several already, but they are all at least one gramm. Fortunatly they cost almost nothing...

Here is a link to the one I got (or very similar to the one I got, Pololu makes several versions):

https://www.robotshop.com/en/5v-step...r-u3v12f5.html


Tim
Jan 27, 2020, 03:22 PM
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Tim Cullip's Avatar
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Got the starboard boom closed up and tested that both motors and both servos are functioning as expected (relief).

Also made some more good progress on the various foam parts (lots of sanding, lots of foam dust).


Tim
Jan 27, 2020, 04:18 PM
free flight or micro RC ???
g_kandylakis's Avatar
and... subscribed...
Jan 28, 2020, 05:00 AM
Balsa, depron addict.
Hi Tim,

I think you build very fast.
Congrates for the foam parts.

Stef.
Jan 29, 2020, 03:17 AM
Registered User
Wao wao
Jan 29, 2020, 08:22 AM
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Thanks for the link.
Jan 29, 2020, 02:26 PM
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Tim Cullip's Avatar
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I've started the covering process (even though I still have two radiator scoops to go on the port boom). The top is Easy Build green tissue and the bottom is Esaki "white" (which is pretty transparent). Once fully covered I will then paint over both with what hopefully will be close to the colors for a Pacific theater P-38 (dark green on top, sky blue or aqua on the bottom). I used the Easy Build green so that hopefully I can get by with fewer/lighter coats of paint to get the desired result.

Note: I redid the stab/elevator and rudders as solid sheeting since the plane was looking somewhat nose heavy.

The current AUW without battery is 19.8gms. Keeping the whole thing under 28gms should be easy.
Also the two motors at full throttle will carry the current AUW (with battery) vertical so I don't think I'm going to be lacking in power, it's just not going to be a slow plane given the weight to wing area ratio.

As a bonus note, I've found a 150mah 1S battery that can handle the current draw of the two motors (even after 4 minutes on the bench at mostly full throttle). I just hope I don't have a spectacular crash on the first maiden launch.


Tim
Last edited by Tim Cullip; Jan 29, 2020 at 02:37 PM.
Jan 29, 2020, 02:55 PM
Go small or go home
ruzam's Avatar
I think you made the right call on the rudders. With all that beautiful sheeting, they would have stood out and then the nagging regrets.

Looks really good!
Jan 29, 2020, 06:24 PM
Registered User
Good to know things are working out for you.
Looks really neat. Bit jealous...

If you want a tip for the maiden flight. Don't try low throttle until you really know what the thing is doing. Sounds a bit generic, but I found the P38 especially twitchy when slow.


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