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Author Topic: Converting sailplanes to electric  (Read 1839 times)
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MarkSSC
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« on: April 26, 2014, 08:06:31 AM »

Hi.
I am thinking of building a sailplane and using an electric motor to get aloft. The configuration would have the folding prop at the front and I want to build something which replicates the lines of a modern glider eg. Duo Discus, DG 1000, ASK 21. I want to avoid fibreglass/epoxy construction and use predominantly balsa. Wingspan would be around the 2m mark.

Finding plans for such a beast is elusive. I am wondering if the build is feasible or if there are any undesirable flight characteristics inherent in a scale glider. Some of the modern ones also have complex wing forms which may challenge the builder. The electrical setup also needs thinking about as there is limited space in some machines. I am writing this in the assumption that someone has already been down this road and may have some tips to pass on. I value your interest. Thanks
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whitecrest
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« Reply #1 on: April 28, 2014, 12:22:17 PM »

I have found lots of interesting plans here:
http://www.outerzone.co.uk/index.asp

I built and flew a Gentle Lady years ago, and I believe it's still available as a kit. It's a two meter glider that is all balsa construction and should convert well to a nose-mounted electric motor. However, I don't think it has the look you are after.
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mjmccarron
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« Reply #2 on: April 28, 2014, 02:25:27 PM »

Years ago I had a Sophisticated Lady. I believe it's the same wing and control moments as the Gentle Lady but sports a T-Tail and a nice canopy. I use to haul it aloft with a baby bee .049 on a pod. I always thought it would be nice to have an 05 electric in the nose but never got around to it. The plans were destroyed in a flood a while ago. If anyone has a copy that they would be willing to share, I'd be happy to cover expenses to get a copy. Not sure if it's what you are looking for as far as looks but to me it's one of my favorites.
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Megowcoupe
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« Reply #3 on: April 28, 2014, 06:59:33 PM »

There are a couple of scale gliders that come to mind- the Sterling Diamant and Cirrus as examples.  Both were FF airplanes that had the option for RC and are built like rubber powered ships- i.e. stick and tissue construction.  Getting these airplanes to fly is quite challenging- the high aspect ratios means that the wings have to be lightly built- but have enough strength to withstand flight loads and hopefully catching a tip on landing.  On the other hand- there is some small RC foamie tow glider that does seem to work- so it can be done.  I have no idea how to integrate a prop into these airplanes though....

Sam
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ricardo
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« Reply #4 on: April 28, 2014, 08:40:46 PM »

Mark Drela's 2m Allegro has an electric version and can be balsa built.

This is SOTA stuff but there are instructions & a Yahoo Group too.
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« Reply #5 on: April 29, 2014, 08:59:26 AM »

My first conversion was a House of Balsa 2x2 (2 meter span x 2 channel) way back in the late 70's with a Graupner Speed 600 on six or seven cells (NiCd "C").  The plane had been built, stock (but with added spoilers), a few years earlier as a pure glider, but the conversion was not that difficult - if one is  adept (I think I needed two evenings) and was all balsa/lightply with a hardwood dowel leading edge.  It is NOT anywhere near being any kind of scale representation, but it is scalish in that it LOOKS like it could be real (have to squint a bit Roll Eyes).  Superb flyer in both modes.

The Sterling "Cirrus"* mentioned by Sam can be built to fly quite well under  power, but will need planking the fuselage (inset) at least to the rear of the wing trailing edge, stronger spars and added ailerons.  The Diamant"* (also by Sterling) could also work with the same mods.  There is another model of a "cirrus" by Ted Strader @ 72" span in the Plans Gallery, but it is not "scale" (Looks a bit like a Standard Libelle).

Finding an all wood modern glider (1960 onwards) in the 2 meter size will be next to impossible, but you might check with the Aerosente site. They produce many different gliders, but mostly in a larger format.  One "modern" glider that you might consider is the WOODSTOCK - an all-wood, man-carrying design (kit plane) from sometime in the late 60's - a very attractive aircraft.  I think there is a modelers plan for it out there, but it is not on Outerzone.  Other than that, you will likely have to settle with a plastic/glass fuselage.

*  Sterling kit E-7 (CIRRUS) and kit E-3 (DIAMANT) Click Here
« Last Edit: April 29, 2014, 09:32:53 AM by Pit » Logged

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Olbill
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« Reply #6 on: April 29, 2014, 09:03:02 PM »

Here's an interesting sailplane converted to electric. It was built by aero engineer Bob Parks. It's a 1.5x version of a Mirage and Bob named it the MegaMirage, I put in a huge old Astro motor running on a 6S pack. It climbs out at about 45 degrees.
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MarkSSC
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« Reply #7 on: May 01, 2014, 07:23:26 AM »

Thanks guys for the tips and links.  Smiley
I have continued my search, learning a bit I the process. My preference for the Duo Discus is based upon having been up in a real one. Clean lines and aerodynamic efficiency. As a modelling project, I would need to look closer at the wing construction...lots of interesting angles. Thought of drawing my own plans but have not tried that since I was 16. The compromise would be to work with a proven model such as the Cirrus. As another line of thought, does anyone know if the airfoil shapes are the same nowadays?
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Konrad
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« Reply #8 on: May 02, 2014, 10:00:50 AM »

Airfoil shapes today are vastly different than what are shown on the plans, even if you were staying with the open type structure.
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« Reply #9 on: May 02, 2014, 10:22:00 AM »

If size isn't an issue, you might want to think about the 3 meter Raven. More info
located here:
http://www.rcgroups.com/forums/showthread.php?t=1270125&highlight=raven+3m

I have done custom cutting of rib sets for the 3M Raven and also a 2M D box version and
I believe it could support the launch lift either through the nose or via pod. Just a suggestion.

Skyraider
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« Reply #10 on: May 02, 2014, 11:12:05 AM »

I remember that about 30 years ago [wow, THAT long ago?] I converted a slope glider , think it was called a "Bushwacker" [Huh] to a [url][url][url].20 4-stroke funny engine which had a rotary valve on the top of the head[http://www.mecoa.com/hp/vt/21.htm].... - Wow, they're still around, those engines!

Anyway, that was a VERY satisfying experience - just ticking over, I could get 20, 30 minutes of flight, using a large, low pitch, modified prop.  Needed to consume the entire tank so that the prop will not turn when landing.  Had a lot of fun with it, until the prop got stopped near the vertical...
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Konrad
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« Reply #11 on: May 02, 2014, 01:53:27 PM »

Guys,
As I read the OP's (MarkSSC) post, he is looking for  how to convert a model of a man carrying ship to Electric Assist. The conversion of gas bag, slope or F3B ships isn't the scope of the question (Yes all those have been done very successfully). I think Markssc is asking about models like the Graupner's ASW 22B @ 2.6 meter. Maybe a Fox?
http://www.motionrc.com/fms-fox-glider-2320mm-91-wingspan-pnp/

Now having don some of these stick and tissue conversions I will say that the smaller models have real issue with efficiency as the tip chords are very small. Also most of them have airfoils that are a close kin to a banana and have awful drag profiles. If you do one of the Sterling models get rid of the under chambered airfoils. They only work in still air and then there is a lot of debate about that. You want an airfoil that will let you cover ground looking for lift. Or just as important allow you to get out of sink! That old Graupner Cumulus with its flat bottom airfoil  taught us that the "Banana" (airfoil) found on say the Thermic series  wasn't helping us at all when it came to R/C.

When I first did these I was surprised at how little the power system added to the weight. These Sterling models needed a LOT of nose weight. So the motor and batteries took most of this dead weight out.

IPOwl
Are you talking about the Mark's Models Bushwacker? Mark Smith never claimed it was designed as a glider but rather as a sport/ trainer. Never did like the Bushwaker's built in muffler for those noisy 2 cycle. I did the same thing as you using a HP VT four cycle engine. I still have the .21, 0.25 2 each and 0.49 4 each . Great engines no valve float, but heavy.

All the best,
Konrad
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« Reply #12 on: May 06, 2014, 07:33:18 AM »


<snip>

IPOwl
Are you talking about the Mark's Models Bushwacker? Mark Smith never claimed it was designed as a glider but rather as a sport/ trainer. Never did like the Bushwaker's built in muffler for those noisy 2 cycle. I did the same thing as you using a HP VT four cycle engine. I still have the .21, 0.25 2 each and 0.49 4 each . Great engines no valve float, but heavy.

All the best,
Konrad

Konrad - WOW... looked it up - indeed, as you say [I erred] - yes, yhis is it and indeed NOT a converted glider...  [url][http://i.ebayimg.com/00/s/MTIwMFgxNjAw/z/0jEAAOxyYANTYonO/$_57.JPG]

After building that [converting the engine] I converted a Bird of Time [dug ot old photoss to remember], the BOT being one of my favorite shapes...

I remember that converting the bushwaker to $stroke was because the prop looked too small relative to the plane...

All the best,
R.
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TimWescott
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« Reply #13 on: May 07, 2014, 04:02:26 PM »

Konrad kinda said this already, so I'm really repeating it in different words:

You want 2m, and you want a scale model of a modern sailplane.  That means that you're going to have a teeny tiny chord.

2m model sailplanes tend to have lower aspect ratios than bigger ones, for the simple reason that you're already fighting issues with low Reynolds numbers.  When you go to make a model of that at 2m, your chord gets exceedingly small.

There are some vintage sailplane designs made for 10m class rules that would make suitable (in my mind) 2m-span scale gliders, but that's because they tend to have aspect ratios of around 10:1 or less, leading to wing chords that are sensible for a 2m span glider.

I used to get S&E Modeler magazine before it disappeared from the shop shelves -- to a design, nearly all the serious scale models you saw in there started above 3m span, and just got bigger from there.

If you want an electric motor glider, why not step up to scratch-building one of these: http://www.sonexaircraft.com/aircraft/xenos.html?  It should do well at 2m, it's realistic with a motor, and it should be pretty easy to build for a scale plane.
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Konrad
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« Reply #14 on: May 07, 2014, 05:19:56 PM »

Konrad - WOW... looked it up - indeed, as you say [I erred] - yes, yhis is it and indeed NOT a converted glider...  [url][http://i.ebayimg.com/00/s/MTIwMFgxNjAw/z/0jEAAOxyYANTYonO/$_57.JPG]

After building that [converting the engine] I converted a Bird of Time [dug ot old photoss to remember], the BOT being one of my favorite shapes...

I remember that converting the bushwaker to $stroke was because the prop looked too small relative to the plane...

All the best,
R.
I don't think I said that you or anybody else was in error. Just that the Bushwacker was not a glider, at least not as defined by the designer. But you sure can see the lineage it looks a lot like Mark's Windfree.

And yes there is a model, uh I mean toy airplane (Windfree) on the cover!

All the best,
Konrad
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« Reply #15 on: May 19, 2014, 03:28:43 AM »

   Hi Guys

                I'm currently looking to convert my scratch-built 78 inch Slingsby T31 tandem tutor to electric.  We  normally aerotow it and I will endeavour to retain that facility. It weighs about 5lb and I am unsure about motor requirement.  Any advice?
    I have a brand new JP Energ inrunner A3615-4 1600KV but have no info re prop size. ESC requirement.   Any knowledge out there re suitability?

                    regards to all.
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« Reply #16 on: May 19, 2014, 01:56:59 PM »

Sorry, but the info doesn't really say much.  Who is "JP Energ" (J. Perkins?)?  Not all 3615-4 1600KV's are alike, but I suppose it is 36mm stator diameter/14mm depth/4 wind.  What are the recommended cell counts, as they'll have a direct relation to the prop size (generally fewer cells = larger prop).  There must be a flyer that came with the motor, as that will have the cell count, recc'd ESC and sometimes the prop sizes.

The motor should be enough to fly the plane.  Wing loading is what you're most interested with here.
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« Reply #17 on: May 19, 2014, 02:18:26 PM »

What's the power rating of the motor?  Figure that 50W/lb will give you a sedate climb (50W/lb was the old rule of thumb for "high performance" electric, back when "high performance" meant "actually flies overhead before it runs out of poop").  250W/lb is probably enough for a rocket-like climb, but I don't fly hotliners, so take that with a grain of salt.

Use the most cells that the motor is rated for (that gets you the best power), and start with the flattest recommended prop that goes with that number of cells (unless you're building the 250W/lb rocket).

Figure on playing with propellers a bit to find the right one for the airframe.
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Konrad
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« Reply #18 on: May 19, 2014, 02:39:10 PM »

Wow, 1600Kv sounds a little too fast for a direct drive set up. Also that motor is starting to look very large like too large for a small 2 meter ship. Really a 5lb for a 2 meter ship without power system! If you stay with direct drive you will need to be running 9 inch props on a 3 cell set up. Going with a gearbox allows you to use a much more efficient size prop. Using as little as a 2:1 gear box and you can use 12 inch prop. But as said earlier we to know a lot more about the motor and ship.

All the best,
Konrad
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