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Old May 29, 2015, 12:07 PM
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Build Log
Corsair Glider for SOS Build Off 2015

Here it is – my bowler being lobbed into the SOS Build Off ring; admittedly it was a long throw from the bench where the KK Bandit is still being produced but it is as they say, a peg in the ground.

I have chosen John Holt’s Corsair from the June 1953 Aeromodeller for reasons that I have mentioned before in the SOS ‘kick off’ thread – but the main ones are its simplicity and apparent robustness. According to the AM article, the original(s) were well placed in several competitions and so one is hoping for a sure flyer. The model is a fairly solidly built, relatively low aspect ratio design, as can be seen in the article header photo, where (presumably) Mr Holt holds it aloft whilst doing a passable impression of Frank Spencer of Some Mothers Do ‘Ave ‘Em.

Having downloaded the plan and article from the splendid Outerzone, I did realise that I actually have a full sized copy of the plan anyway. However, the Outerzone pdf enabled me to import the plan into DraftSight and trace it, making sure that the wing halves were identical, that the rib lengths matched the wing plan etc etc – ie correcting the little things that can either be the joy of or the irritation of building from Vintage plans, depending upon your outlook. I admit to falling into the latter camp….

Some changes will be made – it will be electric and R/C and I am also considering some other slight changes, and I intend to discuss these in the coming posts. But CAD design work is ongoing!!
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Old May 29, 2015, 01:55 PM
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Looking forward to following another of your entertaining build logs Colonel.
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Old May 30, 2015, 06:24 AM
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This ought to be a good one.
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Old May 30, 2015, 02:10 PM
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I agree. It will be a good one.

Dave-
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Old Jun 02, 2015, 01:48 AM
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So I'M meant to be in control?
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Let's hope so!!!

Now some points for discussion:

a) The aerofoil as drawn is a significantly undercambered one, which should be good for slow flight (which I like), but I understand that an Eppler 305 has a better glide performance. It also has the added advantages of being easier to build for a duffer like me - on the original, it looks to me like the lower spars could only be added after the wing has been lifted from the board. I feel this is likely to allow inaccuracies to creep in. But as can be seen from the pic below, the chord line of the E305 has a greater AoA with respect to the surface on which it sits - this will mean that the fus top will need a slight redesign in order to keep at least the incidences of the original. What are people's views - E205 or undercambered original?

b) Whilst we're on the subject, the original has the wing (chord line) set at 4° to the longitudinal datum, and the (lifting) tailplane at 1.8° (LE high - is that +ve or -ve for the tailplane?) giving a total of 2.2° difference - does that sound OK for it's intended use as a motor glider?

c) The other thing I have been mentally wresting with is the rudder and elevator design, and the proportions thereof. I rather fancy a single elevator for simplicity, but not sure if the elevator I have drawn is big enough. I realise that being primarily a glider, the control surface proportions need to be larger than on a power model due to the lack of propwash, but I'm not sure by how much. I'm sure George will suggest an AMT, but I would like to keep it as simple as possible.

d) The proportions of the model give approximately a 2:1 longitudinal ratio, nose to CG to tail. I think this means that I can mount 5g servos in the back end to counteract a fairly heavy motor and battery in the nose. I am wondering whether mounting the elevator servo in the tailplane could then mean that I could have a radio-operated D/T activated by one of the flick switches on the DX6i, which could be fun.....

I now invite comments from the floor (please get up, you're not that drunk....!)
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Last edited by Colonel Blink; Jun 05, 2015 at 08:01 AM. Reason: Corrected E305 to E205!
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Old Jun 02, 2015, 03:21 AM
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For the original aerofoil, could you prop the lower spars up on strips of wood/whatever to the correct height, or just glue the ribs to the bottom spars freehand and then pin the whole lot down to finish it.

Dave.
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Old Jun 02, 2015, 04:11 AM
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Interesting question Colonel, I feel your pain with the undercambered wing! For my one I'm going with an Eppler 205 its supposed to be a good aerofoil, and I don't think you can wrong there. As you say, it will be much easier to build accurately!
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Old Jun 02, 2015, 05:27 AM
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Quote:
Originally Posted by Colonel Blink View Post
Let's hope so!!!

Now some points for discussion:

a) The aerofoil as drawn is a significantly undercambered one, which should be good for slow flight (which I like), but I understand that an Eppler 305 has a better glide performance. It also has the added advantages of being easier to build for a duffer like me - on the original, it looks to me like the lower spars could only be added after the wing has been lifted from the board. I feel this is likely to allow inaccuracies to creep in. But as can be seen from the pic below, the chord line of the E305 has a greater AoA with respect to the surface on which it sits - this will mean that the fus top will need a slight redesign in order to keep at least the incidences of the original. What are people's views - E305 or undercambered original?

b) Whilst we're on the subject, the original has the wing (chord line) set at 4° to the longitudinal datum, and the (lifting) tailplane at 1.8° (LE high - is that +ve or -ve for the tailplane?) giving a total of 2.2° difference - does that sound OK for it's intended use as a motor glider?

c) The other thing I have been mentally wresting with is the rudder and elevator design, and the proportions thereof. I rather fancy a single elevator for simplicity, but not sure if the elevator I have drawn is big enough. I realise that being primarily a glider, the control surface proportions need to be larger than on a power model due to the lack of propwash, but I'm not sure by how much. I'm sure George will suggest an AMT, but I would like to keep it as simple as possible.

d) The proportions of the model give approximately a 2:1 longitudinal ratio, nose to CG to tail. I think this means that I can mount 5g servos in the back end to counteract a fairly heavy motor and battery in the nose. I am wondering whether mounting the elevator servo in the tailplane could then mean that I could have a radio-operated D/T activated by one of the flick switches on the DX6i, which could be fun.....

I now invite comments from the floor (please get up, you're not that drunk....!)
Right Colonel, let's get to it! Your Eppler section would be a perfectly viable choice in place of the original; as you say it would be easier to build (and cover), although as suggested packing up the bottom spar would be an easy solution to the building part. The model will fly fine with either section - just differently. A bit quicker with the Eppler and better penetration of a breeze, slower and more "floaty" with the original, your choice really. As Owl says I would personally prefer E205 (or E193 with the SLIGHT undercamber deleted) or RSG29 as a substitute.

The incidence settings sound perfectly reasonable, at least as a starting point, for either section.

Your suggested rudder looks perfect. With regard to the elevator, I wouldn't dream of pressurising you into an AMT, indeed the layout of the tail end would make it (unlike my Lulu) quite tricky, if going with an AMT on this one I would be looking to do a rocking cradle with a "as per" tailplane strapped on top. Your elevator proportions look OK, but I would make it a double sided elevator of the same proportions, simply a matter of a wire in tube joiner.

The nose is long enough so that with the motor up front and the ability to shift the lipo for balance there should be no problem with 5 gram - or even 7 or 9 gram - servos at the back. Don't forget you only need to run four wires, 2 signal plus common +ve and -ve.

A radio operated D/T might be fun, but it'll need to be reversible though, not just a one shot option.

All just personal views, just go with what feels right, enjoy the build and I'm sure all will be well.
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Old Jun 02, 2015, 06:17 AM
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So I'M meant to be in control?
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Dave / Al / George:

Thanks for the input!!!

I will be using the Eppler - partly because that's what has already been drawded up , but also because I would like some wind penetrating ability. Also, I believe that to build with the original aerofoil, it would not just be a case of packing up the main spar and lower secondary spars - if you look closely, they are positioned 'normal' to the undersurfacce - ie the mainspar leans forward and the secondary leans back - which apart from being awkward to pack correctly, would make it very difficult to slide the ribs on. Hence my comment about building ribs, LE, TE & upper spars, and adding the lowers after lifting from the board.

Re the D/T - I'll admit that I was indeed thinking of a one shot wonder just for the fun of showing off in still air & low altitude, rather than using it as its original intention - after all one could presumably drive it out of a thermal with the RC. But a reversible option would add weight and complication. so perhaps its better to just leave it!!!
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Old Jun 02, 2015, 06:25 AM
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A radio operated D/T is doable,when I get a bit of time I'll draw up the linkage and post a photo of a workable set up I've used.To use the D/T you may need to move the rudder servo from the tail to under the wing.The elevator servo is right where it needs to be for the D/T setup.
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Last edited by rabidrue1; Jun 02, 2015 at 06:36 AM.
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Old Jun 02, 2015, 07:43 PM
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Colonel I'm looking forward to your Corsair build !

Your choice of the E305 or 205 ? is sound, of course.

Best Regards,

Jeff
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Last edited by JeffMac; Jun 03, 2015 at 11:04 PM. Reason: airfoil
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Old Jun 03, 2015, 01:33 AM
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Quote:
Originally Posted by Colonel Blink View Post
I will be using the Eppler - partly because that's what has already been drawded up , but also because I would like some wind penetrating ability. Also, I believe that to build with the original aerofoil, it would not just be a case of packing up the main spar and lower secondary spars - if you look closely, they are positioned 'normal' to the undersurfacce - ie the mainspar leans forward and the secondary leans back - which apart from being awkward to pack correctly, would make it very difficult to slide the ribs on. Hence my comment about building ribs, LE, TE & upper spars, and adding the lowers after lifting from the board.
I would choose a modern Coupe d'Hiver airfoil: less penetrating but will hang your model in the air for a longer time. As you said, the E205 makes building a wing faster and easier. And is more penetrating. But there is no arguing about matters of taste.
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Old Jun 03, 2015, 02:13 AM
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Hi Colonel,
Nothing to add to the discussion E205 or E193 for me, would both be good choices.
KBO in you inimitable style.
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Old Jun 05, 2015, 07:01 AM
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I'll still post the D/T set up just in case.
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Old Jun 05, 2015, 07:54 AM
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So I'M meant to be in control?
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Please do, Scottie - one never knows when these nuggets of information will come in handy!
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