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Author Topic: Advice for 20" Comet F4U Corsair RC conversion?  (Read 470 times)
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PatrickSurry
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« on: March 14, 2017, 07:47:44 PM »

Hi all, I recently discovered this site via google search for something like my subject line, though not a lot of luck. 

I've recently been building some of Peter Rake's smaller RC models (Sopwith Scooter, DH6, D.VII) but for some reason decided I needed a Corsair and managed to find an old 20" Comet kit to build off.

Wondering if anyone has advice or pitfalls to avoid?   I've read a few things about changing CG/dihedral/incidence/thrust line when converting from rubber FF to RC but not clear about the rationale.   Also looking for motor recommendations and whether ailerons are feasible with those gull wings - one servo with bendy torque rod, or two?   I guess I'll use a small 4ch DSM rx, maybe with stability control.

Gratuitous picture of the freshly completed D.VII (yet to maiden).

Thanks,
Patrick
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Konrad
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« Reply #1 on: March 14, 2017, 10:12:02 PM »

Wow, if you built that D.VII we will be coming to you looking for advice!

We change the force arrangement  because we will be offering control inputs. Free Flight model need to be very stable and self correcting ships, RC not so much.

I like to take out some of the declarage (raise the LE of the stab a bit) to cut down of the speed zoom we get with most free flight models.

As to dihedral I like to take out about 3/8 as much as we have in the free flight trim. I do this because I really like ailerons (I'd look at 2, 3gram servos). But if making the ship a rudder elevator type I'd add about another 3 degrees. Unlike a free flight ship you want the rudder to be powerful enough to actually turn the model. And that really is the job of the dihedral.

As to motor and radio gear what do you have to begin with? With this model I'd thing you would want to start with a 2 cell 350 mAh and "180" class outrunner. I'm thinking something like out of a brushless UMX model. But given a bit more detail as to what you want we can post more specific detailed recommendation. If you aren't already committed the Spektrum DSM2/DSMX radio I'd like to point you to the Fr-Sky radio as having a lot of programing power, RX options and VALUE!
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Bill G
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« Reply #2 on: March 14, 2017, 10:39:51 PM »

On ailerons, I add them to all my builds now. With 4 channel control, aileron/rudder mixing also provides options for much smoother and more controllable flight that rudder, with some subjects.  I can understand some using relatively high dihedral and RET on ultra micro subjects with 16" span or less or high wing civilian subjects, but 20" is plenty large enough for ailerons.  More recently I built a 20" Sterling DVIII with ailerons, which turned out to be a surprisingly good flyer. I'm convinced the basically 0-0-0 settings work well, where the horizontal stabilizer will appear to have the LE lifted up a hair, when compared with the flat bottom of the wing.  I've been gradually reducing positive wing incidence in general in more recent times, and discovered that many of my models had excessive positive incidence. The D8 wing was built precisely with a hair of washout, but outside of that, I believe the incidence settings have much to do with why it's rock solid, and another guy's who's build is lighter than mine (and tried to tell me "how things should be done") was unstable in flight.  
On torque rods, I've built a number of various spans, where the rods are quite thin and springy, but the models handle fine.  The forces are low on small models, and they don't need to be brick stiff, as some think.  The 16.5" Guillows FW190 below uses .032" music wire, which seems fine in flight.  I also like using .015" music wire with single servos, but the arrangement takes some time pre-shaping the wire, also with a few setup tricks to remove friction, for it to work well.  I used the setup in a Guillows Rumpler, with the aileron servo hidden in the radiator.  The Fokker D8 below uses micro linear servos.  Looking closely, the small "hold down" tab of balsa that is glued to the aileron header on the wing, is the only piece that would need to be cut away to remove the servo.  The pcb tabs on the servo fit into slots cut into the wing ribs, and there is also a small length of stringer glued to the underside of one of the wing stringers, that keeps the servo in place.

Flight videos
https://www.youtube.com/watch?v=w47tnwSghR0
https://www.youtube.com/watch?v=qCGnimb-FrI
https://www.youtube.com/watch?v=OjDRIw0e6Z4

One of the best flying warbirds I've flown, especially in terms of trim, is of all things a Guillow's P51.  The settings are very close to true 0-0-0, where the LE of the horizontal stab appears to be tilted slightly upward, compared with the flat bottom portion of the wing.  When I looked at it, I thought I had taken out too much positive wing incidence, but it flight it proved to be dead on.  It's rare to hit one dead on this perfectly, and when you do, you know it immediately.  It was an ancient build, with a new wing built for it.
https://www.youtube.com/watch?v=JNzCNA4SPZ8
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PatrickSurry
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« Reply #3 on: March 15, 2017, 07:23:04 AM »

Yes D.VII is mine - simple plan from a laser cut kit.  Very pleased with my first attempt building a balsa "fan method" prop (I wrote some notes on rcgroups if that isn't taboo to say here Smiley, and fun to experiment with printed lozenge tissue.

And thanks for all the advice, makes a lot of sense.  I'd like to build in ailerons and try a 4ch setup (or 5 w/ independent ailerons?).  I like the idea of small in-wing servos if I can find a spot to fit them (the single linear servos are a cool idea).  Torque rod seems like a challenge in the gull wing, tho perhaps a flexible rod supported in tubes both sides of the bend would work?  

 In terms of Tx I started with a Spektrum model, but recently pulled out the guts of it and built into a plugin module for a new Taranis, which I love.  So most of my Rx gear is still Spektrum (actually probably mostly OrangeRx), but I can definitely use the FrSky stuff.  I haven't seen good micro options there but probably not looking in the right place.  For the 14" builds I used PKZ 3ch bricks, and a micro OrX in the Scooter which is about the same size as the Corsair.  

I don't have a motor preference, but I might have a spare of what I used in the Scooter (Turnigy AX-2203C 1400KV which weighs about 20g, speced at about 24W draw and 148g thrust with a 7x3.8 prop on a 2S battery).  The 350maH 2S is the same battery I used there.

What about motor thrust line, is it ok to stick with the rubber orientation, or do I want something like 2deg down&right like on the Peter Rake models?
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Konrad
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« Reply #4 on: March 15, 2017, 08:45:06 AM »

It looks like you are on sound footing with the this stuff.

It appears that it is fine to reference RCGoofs here at HIP. HIP is not petty in that way. But "I" think it is very poor form to mention an organization that doesn't  have the best interests of the modeler at heart. RCG is more interested in protecting poor providers of products and services (if they are sponsors) than helping the membership get the most from their hobby experience. That is why most dead tree press has lost its credibility as they protect their advertisers. Now aren't you sorry you asked? Roll Eyes

Ok, I'd look more at the 10 gram motor myself. But as the F4U has a short nose the 20 gram motors might actually be better.

As to motor thrust I like to start with about 3 degrees down with the high cambered airfoils we use from these kits. I'd also start with about 2 degrees of right thrust if for no other reason than I don't want to have built in left thrust. I find that slow, low powered, heavy ships need more right thrust than fast, high powered, light ships. This is because of the "P" factor from flying at a higher angle of attack. This higher angle of attack gives the downward prop blade more bit (pitch) than the upward moving prop blade. This difference in relative pitch is what causes the model to pull to the left as power is added or the nose is pulled up into a climb.

You might want to take a look at my ordeal with a single linear servo and bowden tube for the ailerons here in my P6-E thread.
http://www.hippocketaeronautics.com/hpa_forum/index.php?topic=11737.0

Not that you need it but a lot of what I know about these Gummy Band to RC Conversions can be found here.
http://www.hippocketaeronautics.com/hpa_forum/index.php?topic=12443.0



All the best,
Konrad
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daveh
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« Reply #5 on: March 15, 2017, 04:26:40 PM »

Patrick,

There is a section on this site called Micro R/C that may have some points and ideas that could be of interest to you.

Dave
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PatrickSurry
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« Reply #6 on: March 15, 2017, 04:59:24 PM »

Thanks, I'll check it out.   

Also, Konrad mentioned the FrSky system earlier, which I'd love to try (native for the Taranis Tx) - but which Rx would you recommend to start with in a model like this?   I've tried to parse the descriptions of the receivers on their website but admit to being pretty confused.  Some of the tiny ones don't seem to offer any servo connectors, despite having 8ch - is that something you wire separately?  Any help on how to mentally navigate the options would be appreciated.
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Konrad
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« Reply #7 on: March 15, 2017, 05:16:20 PM »

Are you looking here?
http://alofthobbies.com/radio/frsky-telemetry-system.html

Are you confused between PWM vs Sbus?

PWM is the normal 50hz analog signal  S-Bus needs special digital servo that can pick out the proper data packet from a serial (one) data line.

If you are really concerned with weight you can hard wire your servo to this PWM RX
http://alofthobbies.com/frsky-xmr---micro-park-flyer-receiver.html

This might be a good RX if you want to use the Jr style servo connector.
http://alofthobbies.com/frsky-x4r-4-channel-receiver.html

If you are willing to use the old protocols (not suitable for telemetry).

I really like this RX
http://alofthobbies.com/frsky-v8r4-ii-4-channel-receiver.html

And if using the JST connector on the end of your Micro servo this RX is hard to beat. It even comes with the connectors you would want to crimp onto your servo leads.
http://alofthobbies.com/frsky-v8r4-ii-4-channel-receiver.html

I hope this helps.

BTW, I don’t like gyros in general and really don’t like them in these micro planes.

All the best,
Konrad
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PatrickSurry
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« Reply #8 on: March 15, 2017, 06:39:34 PM »

Aha, no I was looking at this http://www.frsky-rc.com/product/ where they seem to have a somewhat different list.  I find their categorization confusing too, rather than (say) size/weight/#-channels but that's probably just because I don't understand all the words yet.  SBus v PWM is definitely one of the confusions, but I'm getting there. 

Your recommendations are very helpful, I'm definitely tempted by soldering some servo leads but I imagine that might lead to cursing down the line somewhere Smiley   I guess the 300m range limit is not really a concern for a 20" span model since I suspect I'd lose sight of it before then!

Looks like most of the options are currently out of stock, would the D4R-II be a viable option too?  http://alofthobbies.com/frsky-d4r-ii-4-8-channel-receiver-with-27ms-cppm.html.

To use voltage (or other) telemetry with these, I need to add external sensors right, or is voltage telemetry built in?   That sounds like an interesting feature of the newer tech (X&D vs V?) - is anything other than voltage monitor useful for these small models?   And I guess X vs D adds S-Port which supports some new sensors that I don't care about for this application? 

Sorry, I guess I need to find a better primer on this tech...



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Konrad
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« Reply #9 on: March 15, 2017, 07:32:28 PM »

That should be a good RX. But note the voltage these receivers transmit is the internal voltage for the radio not the voltage for the motor/power system. For that you will want another sensor.

Most sites/importers have the manuals online. Take the time to down load what you might want and read up on them. I'm sure that what I want or think is important, there are few others that would agree.
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PatrickSurry
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« Reply #10 on: March 16, 2017, 10:16:58 AM »

Thanks again.  The conversion thread you pointed to has exactly the kind of practical advice I was looking for, and I feel like the electronics choices are making more sense.   Also your P6-E aileron travails and "pushing rope" got me thinking about a kind of pull-pull bellcrank setup.   I've got a bunch of spare servo discs from rotary microservos, so if I used a single servo with a disc in the body, and did pull-pull cables to same discs mounted as "bellcranks" in each wing, then they could drive a short pushrod, offset for aileron differential.   This should easily handle the bend in the gull wing via a couple of bits of cable guide tubing right at the bend point?   WDYT - I've used pull-pull for rudder and elevator but never for ailerons!
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Konrad
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« Reply #11 on: March 16, 2017, 10:40:38 AM »

I too have not used Pull Pull with ailerons, but if one has a way to adjust tension it should work. Your drawing is one way to do it. I fear that the guide tubes will wear rather fast as the contact area is small. And that the drag the lines will have against these will be higher than anticipated (poor centering). Bowden tubes might actually be better. Some can be made with 3 elements which really cuts down on the drag.  These elements might be a .015 wire inside a CA capillary tube  inside another plastic sheath that is glued to the ribs and airframe structure. Differential can be had at the aileron horn or at the servo wheel just like "normal" linkages.

But with some of the great 3 to 5 gram servos we now have, I'd go with the dual servo set up.

On the subject of ailerons do make them oversize. This has to do with Reynolds numbers. You might have noticed that the ailerons on the UMX planes are way oversized, this is why.
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PatrickSurry
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« Reply #12 on: March 16, 2017, 10:49:03 AM »

Yeah, I guess if I have space for linear servo in the wing itself that is probably safer.   Does anyone know if I mount one forward and one backward (rotated 180deg), can I just splice the two signal wires together and use a single channel to drive them both to get them moving in opposite directions?
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Konrad
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« Reply #13 on: March 16, 2017, 10:56:24 AM »

Yes, that would work for linear servos. But in an effort not to introduce any unwanted mechanical offset (differential) I would look for a reversed servo. Take a look at Horizon Hobbies for these. (sorry, I just looked at the HH site and didn't see reversed throw linear servos)

Now I really like rotary servos over the UMX type linear servo. Take a look at Hobby King for some of these real small servos.  The ones found at Horizon have worked really well for me in very high powered applications like the Edge 540QQ. 20 years ago I'd have been willing to pay over $100 USD for these. http://www.spektrumrc.com/Products/Default.aspx?ProdID=EFLR7100

I would mount these on their side to fit them inside the airfoil profile.

All the best,
Konrad
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« Reply #14 on: March 16, 2017, 01:12:22 PM »

Yes, that would work for linear servos. But in an effort not to introduce any unwanted mechanical offset (differential) I would look for a reversed servo. Take a look at Horizon Hobbies for these. (sorry, I just looked at the HH site and didn't see reversed throw linear servos)

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All the best,
Konrad

HH linear servos come with a nifty servo reverser cable in the pack. This is not the usual sort. You fit the reverser between the RX and servo, power the RX up, then switch it all off and remove the reverser, you now have a backwards servo! And you can undo it by repeating the process.
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Konrad
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« Reply #15 on: March 16, 2017, 01:13:55 PM »

Good to know.
Thanks!
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« Reply #16 on: March 16, 2017, 10:10:28 PM »

Interesting - I think I knew that at one point but forgot Smiley   

Re in-wing servo, I'm just nervous about wing chord, I'll measure some ribs once I have the kit in hand (shipping).  Exploring your suggestion found another interesting option only 6mm thick:  http://www.e-fliterc.com/Products/Default.aspx?ProdID=EFLRDS35

Also, I now have visions of rotating retracts after discovering this video https://www.youtube.com/watch?v=bghf7A50L80 - wow!   Might need all 5ch Smiley

Otherwise I'll probably build with removable gear since I think these warbirds look much better in flight without.
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Bill G
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« Reply #17 on: March 17, 2017, 02:17:20 AM »

I like the idea of small in-wing servos if I can find a spot to fit them (the single linear servos are a cool idea).  
That is very true, about finding a spot to fit servos in our small model's wings.  So far, the micro HK digital servos have worked well and become a favorite.  I came up with a method to fit them in the 24" Guillow's Cub wing ribs.  The tops of the ribs were reinforced with a laminate of thin music wire, I believe around .020", aligned with the bottom of the stringer notch.  The bottom of the wing rib was cut away, and replaced with a thin plastic strip, which was glued into notches cut into the bottom of the wing rib, with the depth cut equal to the thickness of the plastic sheet.
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« Reply #18 on: March 19, 2017, 06:50:48 PM »

Woohoo, the kit arrived from my ebay seller.   Here are a few photos of the unboxing.  Not sure that I will make much use of the contents other than the full-size plan (which is also here in the plan gallery http://www.hippocketaeronautics.com/hpa_plans/details.php?image_id=3117&mode=search). 

The wood is "rock-solid", 33g for 3 sheets of 3"x15" which I think works out at nearly 15lb/cu.ft.  And the 1/16" stick fan - remember that?  Wonder when it was boxed - kits were selling for $1 apparently.

Anyway, good news for ailerons: the chord at rib E is almost 9mm thick so I should be able to get some rotary servos in there.  But now I'm obsessed with making rotating retracts.  I think I've worked out a simpler design than the video above, still driven by linear servo, but will need a little 3d printing to confirm.   

Anyone know a clever way to copy the diecut parts directly onto lighter balsa, other than copying onto paper and cutting out from that?

Also, any ideas on a more scale-ish 3-bladed prop?  The included one for rubber is 6" dia, I'd guess 6x5 or 6x6.

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Konrad
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« Reply #19 on: March 19, 2017, 06:55:01 PM »

Your kit looks to be made from wood which is a closer kin to Oak than Balsa.

And the guys give Guillow kits a hard time.

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« Reply #20 on: March 22, 2017, 01:39:33 AM »

Apparently DPC Aerowerkes has both this Corsair kit, and the Sterling Fokker D8 that I built last year, in laser cut kits.  I'm at a toss up point with LC versus the old kits, as there still is some personal appeal to building the vintage kits from the original parts, albeit heavier wood.  I replaced the rear fuse formers, tail feather formers, wing spar, TE, and a few other parts with lighter stock.  It was literally the heaviest balsa I've ever seen, being heavier than much of the common basswood that I've used.  Turned out to be one of the best flying micros I've built at 100 grams, with no ballast required.  

As for cutting new parts from templates, I haven't done that for over 10 years. I position the wood sheet stock under the plan, and score on the plan lines with a dull exacto.  Done properly, it may slice the plan here and there, but it will still be intact. The method places ample score markings on the wood underneath, to cut new parts from.  I've scratch built numerous entire models also, using the method.  For parts such as tail feathers, you can often take better advantage of the grain strength, by changing the seam locations between the parts. As for accuracy, I'm convinced the method is at least as accurate as cutting from templates.

I inherited a few older Guillows kits from the early 60s.  I believe the price tags were in the $2 range.  I could easily see the small Comet kits being only $1 back then.

On the rotary aileron servos, I'll fit them in there, even when there isn't enough room. I reinforced the top of the ribs with a thin wire laminate on a Guillows Cub, and then replaced the cut away bottom portion of the rib with strips of clear plastic sheet.  That's all there was room for!  Edit: noticed that I posted that detail before.  I'll use the HK micro digital servos over micro linear servos, given a choice.
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« Reply #21 on: April 02, 2017, 08:30:31 PM »

Slowly getting started.   Scanned the balsa sheets to cut my own parts, tho decided to laminate the tip parts from 1/32" strips.  To reduce cutting and maximize symmetry I split all the fuselage formers in half to cut both at once.  That also allows a clamshell approach so I can build the fin and main longerons flat (laminated with 2 x e1/16 sq strips).   Added some bits to introduce moveable elevator and rudder, slightly larger than scale.    I ordered some μ-lite on Konrad's suggestion (they offer "Corsair blue" so it was a no-brainer!), tho haven't worked with it before - any tips?  Still toying with if/how to try retractable wheels so haven't started on the wings yet.
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Konrad
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« Reply #22 on: April 02, 2017, 08:38:06 PM »

Love laminations myself!

As to iron films I like to keep an ice tray full of ice near by. I'm always burning the tips of my fingers or thumb with these small models. R.A. microlite does use higher heat than the Nelson/Solite iron films
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