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Curtis Condor biplane twin


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Back in the late 80s I bought a Curtiss Condor plan from the USA. I had seen a model flown at Old Warden scale day and really liked the look if it.

I have been meaning to built it all this time. it has been a long-standing project. Now with lock-down looming it seems time to actually do it.

The model is an 82" span twin engine biplane model of the 30s airliner. Originally designed by R Schweizer in 1970 there is room for some design improvement and lightening, but basically it is a good plan. Originally designed for two .40  2-stroke engines. The version I saw fly in the 80s was "just about" powered by two 1st version OS 40 4 strokes. But how to power my version in 2020? All up weight should be around 5kg

I have 2x Saito 45 four strokes I could use. However, one is fitted in a model I regularly fly (Hawker Tomtit)

I do have an Enya 53 4 stroke that would be great to use. It is powerful and has a better profile to fit in the cowls - with rear mounted cams and pushrods. But I only have one, and they are no longer available.

I could go electric and have several 4S 3700 lipo packs that might be suitable. However what motor to use. I imagine a 42mm can with about 700kv would be about right with an 11" prop? My usual SK3 motor brand seems to have very limited availability at the moment. Choices choices!

So glow or sparks and which engine to use?

Edited By David Ovenden on 02/11/2020 08:24:35

Edited By David Ovenden on 02/11/2020 08:40:06

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While a bit OTT performance wise a pair of laser 70's would do it for sure. You could prop them quite large with a fine pitch and cruise around at half throttle i suspect.

If not, someone might have a spare enya 53 they would be willing to part with. The only snag with that is getting them matched and running together.

I did a google search for the model and its an interesting looking plane. Is your fixed gear or retract? both seem to have existed

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Posted by Jon - Laser Engines on 02/11/2020 10:15:35:

No 70's at the moment but i would build a matched pair for a twin anyway so it makes no difference really.

If you look on the laser site there is a dimension drawing for the 70. See if it will fit with say an 6-8oz tank. Should be plenty.

Hi Jon, why would it be necessary to build a matched pair if they are all built to the same high standard?

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Posted by Eric Robson on 02/11/2020 10:44:08:
Posted by Jon - Laser Engines on 02/11/2020 10:15:35:

No 70's at the moment but i would build a matched pair for a twin anyway so it makes no difference really.

If you look on the laser site there is a dimension drawing for the 70. See if it will fit with say an 6-8oz tank. Should be plenty.

Hi Jon, why would it be necessary to build a matched pair if they are all built to the same high standard?

Because every engine ever built is different to the next so for twin projects i always pick the two closest matched engines from a batch to make its easier to get them exactly right in a twin.

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Hi Jon, My question was a little tongue in cheek, as in my working days the company I worked for produced two machines for a dairy firm. One was a little troublesome and on asked by the project manager why, I replied these machines are like a Rolls Royce individually hand made and can vary. His reply was unprintable. We did not have the benefit of CNC machines at that time so the machined parts were mainly down to the skill of the machinist and the the fitter who had to make them fit.

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Ah ok Eric, the pun didnt make it through the internet.

As for the enya 53 they are really good engines. I have one in an airsail tomahawk and despite being completely clapped out it runs well. Finding a TN version might be tricky as most, including mine, were air bleed.

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Posted by Jon - Laser Engines on 03/11/2020 08:21:59:

Ah ok Eric, the pun didnt make it through the internet.

As for the enya 53 they are really good engines. I have one in an airsail tomahawk and despite being completely clapped out it runs well. Finding a TN version might be tricky as most, including mine, were air bleed.

True Jon,

But new TN carbs are available as a "spare part" so I could possible upgrade an older model to a TN carb.

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Posted by David Ovenden on 03/11/2020 19:49:08:
Posted by Jon - Laser Engines on 03/11/2020 08:21:59:

Ah ok Eric, the pun didnt make it through the internet.

As for the enya 53 they are really good engines. I have one in an airsail tomahawk and despite being completely clapped out it runs well. Finding a TN version might be tricky as most, including mine, were air bleed.

True Jon,

But new TN carbs are available as a "spare part" so I could possible upgrade an older model to a TN carb.

I actually upgraded my engine to a twin needle as the old air bleed was shot. It kinda ruined the engine.

It would knock like crazy and tuning it was like russian roulette. I lost count of the number of times it would backfire and shoot the prop across the field. The worst part was that once you got it running nicely, had a fly, and then went for a restart you were in a world of hurt as it would again throw the prop off as you went past half throttle when it was cold. If you held it as fast as you dare, let it warm up, creep the throttle open...bit more heat, bit more throttle...nearly at full power...BANG! prop gone and spinner exploded.

I got fed up with this in the end and decided to try and fix it as it didnt behave like that with the air bleed carb. it took me 5 minutes to diagnose and another 10 minutes to fix.

My airbleed carb had a choke of 5mm. The replacement TN was 7.2mm if memory serves. My solution was to knock up an ali sleeve on the lathe at work with a 5mm i/d and press it into the throttle barrel to reduce the choke.

Problem solved. The engine is still knocky at full power when cold but its not spat the prop at me since my mods so i count that as a win

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David,

The rational for Jon's comments with paired engines are reliability (its a Laser or two), easier to tune and more reliable/predictable pick up across the rev range. IMHO unlikely you will be flying WOT so having a pair working nicely together just adds to the enjoyment of flying ic twins.

I don't have a lot of experience, but I would always prefer to fly a twin on both engines especially one that may have a nasty bite.

Before anyone thinks ic are more likely to do this my experience is about 50:50 with electric on the one dead stick front.

Ok a pair of Laser 70's are not that cheap (and why would they be), but by the time you have bought everything else to make and kit out the model they will be a minor expense + they will out last many models.

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Chris, I am in no doubt about the quality of Laser engines, nor the huge advantage of having a matched pair. I am currently just investigating my options. I have not entirely ruled out using electric motors.Though I do love the sound of four strokes!

Jon's comments re the Enya have been read and taken on board.

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Been doing a bit more thinking about the project, partly inspired by finding the photos of the model I saw fly at OW in 1983. The model was also featured on the cover of RCM&E the same year. I found that too.

Given the balsa shortage situation I dug about and found I had more wood in stock than I thought, and have all I need for the project except some lite ply that I will use to replace birch ply for lightly stressed components.

The plan shows one piece wings which are bolted on to the fuselage. But my current thinking is to build separate centre sections with plug on panels each side. Like this it would be much quicker to assemble. It also led me to contemplate making interchangeable electric and glow lower wing centre sections. Best of both worlds? The idea would be to make 2 identical basic lower wing centre sections at the building stage . I would finish the electric version and test fly. If the model is a success I could build the nacelles and fit glow motors to the second centre section.

Or more wild still. Make interchangeable bolt on "cassettes" to fit either the electric motor and mount module or the i/c engine plus tank & throttle servo unit into the nacelle? I've done it once before on a Hawker Tomtit and it all worked fine. Model is still flying after 20 years. The advantage of this is that I only one centre section to make and one undercarriage unit to build. But how to make the engine nacelles suitable flexible to accomodate everything? I'm working on ideas for this.

Yes,  there would be issues of balance (without the 2 lipo packs) and trim (different power types and output characteristics) to factor in. But the ailerons are in the top wing only and one cold have a different model memory for each set up? And then  I could chose between the sweet sound of two four strokes in synch and the plug and fly convenience of electric.

At least at the moment we have time to work on new projects/ideas when we can't go flying.

Edited By David Ovenden on 08/11/2020 08:20:42

Edited By David Ovenden on 08/11/2020 08:22:08

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Here is my (very poor) photo of the model that inspired me at Old Warden on a grey overcast (and wet) day in 1983. And below how RCM&E featured it the same year in sunshine and with blue skies!

Anyone remember / know the builder of the model? I seem to remember he was Bob??

condor ow.jpg

condor rcm&e cover.jpg

There are some nice colour schemes to chose from: I like the American Airlines orange and blue, but am tempted by G-AEWE as its an British reg and I have silver and red Solartex left in stock!

colour schemes 2.jpg

There are plenty more to though!

colour schemes 1.jpg

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Hi Lorenz,

Yes, the colour scheme details are from a very old (1967) Flying Review magazine. The video I posted of the large RC model flying in Switzerland is finished in blue/orange with tail markings as you describe. So I'm sure you are right. And you are ex-Swissair!

I have started making templates today ready to cut out the wing ribs.

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Managed to get some work done on the Condor today. I have altered the wing design a bit from the plan which was drawn in the 70s and seems overengineered by today's standards. It was also designed to be a one-piece wing (82" span) but as I am making plug-on tip panels I decided to use carbon fibre tube instead of the many heavy ply wing joiner/dihedral braces on the plan.

I also knocked up a cutter for the 14mm holes in the ribs and got some ribs cut.

The balsa "shortage" is starting to change my thinking. One of the ribs is cut from 2 small pieces of balsa from the scrap box glued together rather than start a new sheet of 3/32"! The  templates come from the ply doublers "rescued" from a BT Kingfisher model. So 2nd time around.

20201110_170632.jpg

20201110_170641.jpg

Also did some gardening, went for a 1hr walk and fixed the fence!

 

Edited By David Ovenden on 10/11/2020 16:41:49

Edited By David Ovenden on 10/11/2020 16:44:05

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Finished cutting the lower wing ribs today. Waiting for the carbon tubes to arrive so I can start building the centre section. Will start on cutting out top wing ribs next.20201112_170505.jpg

Wing structure has been modified a bit from the original design. Will have plug on wing panels while the centre section remains attached to the fuselage. Though even they will unbolt to allow easier long-term storage.

plan 1.jpg

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