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Old Mar 31, 2011, 07:55 PM
ein flugel schplinterizer
seanpcola's Avatar
USA, FL, Pensacola
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Question
Motor/battery/prop suggestions for scale motorglider

I asked this up in the IC to EP conversion thread without a single response. Should have come here first.

I am about to start a conversion from IC to EP on a 1/4 scale Grob G-109 Motorglider. I would appreciate any suggestions and advice as to a reasonable EP setup.

Specs are:

4M wingspan, roughly 1132 square inches.

RTF weight as it sits with motor but fuel tank empty is 16 lbs.

Needs enough power to ROG. I want reasonably good performance but this is NOT a screamer. Sufficient power to take off and climb in a relatively scale manner is what I am looking for.

Motor run time: Maybe 10-15 minutes per battery charge would be fine. Take off and climb to thermaling altitude is all I need. Glide to landing.

It has been flown as is with a 2 cylinder 160 and 16 x 10 prop. I did not see it fly in this configuration. It was used to aerotow other scale gliders so I am assuming it had pretty decent pull.

I don't think there is any way that a folding prop could be used as the cowl is pretty large. The full scale 109 (my boss has two of them) uses a full feathering setup so that when the engine is shut off for soaring the blades pitch to perfectly parallel to air flow. Not gonna happen here of course. I expect that there will be a lot of penalty to pay for the prop drag when shut off but can see no way around that. This is more of a novelty flyer and certainly will never be a competition airframe.

On the prop subject: Looks like prop diameter is limited to 18" maximum for ground clearance in level takeoff. I may be able to run larger diameter wheels and gain 1/2 to 1" additional clearance but would rather try to stay in the 16-17" diameter range.

Room for batteries and such is not an issue at all. Plenty of space in the cowl for the motor and with the scale inlet openings I don't see a cooling problem.

I know this is going to be a relatively expensive project but I would like suggestions on motor brands other than the big name expensive contest/ performance line. This model won't be flown that often and when it does get to the field it's not gonna see a lot of run time. In other words, I want pretty cheap (relatively speaking) components.

I have had a few EP sailplanes and always ask someone else to tell me what I need. In other words, I'm too lazy to do a lot of research and figure you guys will have the answers, i.e pounds- watts and such.

Thanks in advance,

Sean
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Old Mar 31, 2011, 08:02 PM
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Sean,

Is that the Blue Airlines Grob 109? I am SO jealous! PICTURES!!!!!!!

tom
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Old Mar 31, 2011, 08:08 PM
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Corner Ketch, DE
Joined Sep 2005
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Sean You need 1600 watts to fly it rog so an Eflite power 90 would be perfect
Is there a reason that You can't use a folding prop
You could look at HC for some cheap outrunners
I would go with the power 90 and 8 cell's , Two 4000 mah packs in series would Be perfect

I could motor calc the airframe and motor for You to give ROC and Duration if You need me to do it its no problem

Charlie
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Old Mar 31, 2011, 08:19 PM
ein flugel schplinterizer
seanpcola's Avatar
USA, FL, Pensacola
Joined Sep 2004
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Hey Tom,

I need to send you a message anyway so I'll PM you this weekend.

No, this model was originally built somewhere in the late 90's or so. I'm going to try to get those details. Not sure of Mfg. but I'm sure it's one of the usual German scale outfits. No pics tonight but I'll get some before I rip it all apart and start over.
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Old Mar 31, 2011, 08:22 PM
ein flugel schplinterizer
seanpcola's Avatar
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Quote:
Originally Posted by creeve View Post
Sean You need 1600 watts to fly it rog so an Eflite power 90 would be perfect
Is there a reason that You can't use a folding prop
You could look at HC for some cheap outrunners
I would go with the power 90 and 8 cell's , Two 4000 mah packs in series would Be perfect

I could motor calc the airframe and motor for You to give ROC and Duration if You need me to do it its no problem

Charlie
Hi Charlie, Just can't see how a folder would work around the cowl. Come to think of it maybe I should get some photos of it up so you can see how the airframe is configured.

I really don't want to be any trouble but if you get bored and want to calc it I will be eternally grateful!
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Old Mar 31, 2011, 08:27 PM
ein flugel schplinterizer
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I'll get some pics of the model on here this weekend but here's the only good shots on my PC of one of our FS 109's taken at work. You can see the cowl and how a folder wouldn't work (I don't think).
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Old Apr 01, 2011, 06:51 AM
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Grob 109 is one of my all-time favorites!

tom
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Old Apr 01, 2011, 07:16 AM
ein flugel schplinterizer
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Yeah Tom, they are cool. My boss owns a LOT of full scale aircraft but the 109 is his go-to fun plane on great soaring days. With the prop feathered it's soaring performance is better than one might think. You gotta watch it on a really hot day though. With that 4 banger Limbach (modded Vdub) rated at 65 HP, flying out of a short grass runway tends to be............exciting.

I didn't know Blue Airlines had a G-109 so looked it up after you mentioned it. Sweet! It's slightly larger than the one in my shop. W/S on this one is a little over 161". I'm going to contact them and see what they cost.

I'd really like to come up with an answer to the folding prop issue. Maybe the right one would fold at least a little. I had a Goldberg Electra 25 years ago and the prop would only go back around 50-60 degrees due to the cowl but probably any little bit helps.
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Old Apr 01, 2011, 08:49 AM
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Posted a reply in glow to electric not knowing this thread existed
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Old Apr 01, 2011, 11:33 AM
ein flugel schplinterizer
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Just read it Sirocco. I really appreciate the input. I posted up there first and didn't seem to get any response so started this thread last night.

Looks like this will be a pretty straight forward conversion. Good news is I have a few NIP 35-50c 4S 4000 packs to play with and really good quality chargers up to that task. Also, glad to hear what you think on the prop size. I really was concerned over ground clearance.

I didn't think it was going to need tons of power for cruise and climb, so sounds like that part is covered. I'm focusing on the ROG aspect with regards to grass fields. Runway length is no problem. Wheel size has me wondering though, they are small and not really to scale in size either so I'll probably kick those up a bit, again to help with rolling resistance on grass with a side benefit of prop clearance. May have to run without wheel pants until I can mold new ones.

With the cowl made as it is I will put the ESC in with the motor. Not sure about battery cooling and I guess I'll have to experiment a little. Plenty of room for them behind the firewall if they have to go that far forward to balance everything. May have to provide cooling for them too.

I'll definitely disable any rx power feed if one is on whatever ESC I buy and run a separate pack for the radio/servos.
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Old Apr 01, 2011, 01:44 PM
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You don't need 100W/lb for ROG. You only need about 15-20W/lb for level flight on most glider type models, so 50W/lb would be a sensible minimum for ROG.

Although not scale for this a/c you could get the prop to fold forwards, like the Technoflug Carat, see http://www.scalesoaring.co.uk/MOTOR_...arat_Docs.html

Neil.
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Old Apr 01, 2011, 04:23 PM
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It occurred to me that another benefit of going with an apparently oversized understressed motor (if w&b suits) is that if you wanted more power in future, eg for towing, as an alternative to a larger diameter prop you could even use 10S and a bit less pitch.

Re the wheels, perhaps experiment with big wheels for test flying/confidence building then step back. I've been surprised by how little trouble some of the bigger models at our field have on little wheels, and it's literally a cow/sheep paddock with a bit of mowing - not smooth at all.

Not in the same weight class as your Grob, but re power for RoG, on 54W/lb my 4.5 lb trainer gets airborne in probably 30-40ft with gradual application of power and climbs away smartly.
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Old Apr 01, 2011, 06:15 PM
ein flugel schplinterizer
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Thanks Neil,

I email back and for with Noel (the author of the Carat article). Guess I can contact him in regards to a forward folding prop. Is something like that available for RC or can mods be made? I guess what I'm asking is this: For RC apps could the prop be spring loaded or pre-tensioned to go forward at shut down?

Scirocco, I agree. I prefer the idea of overpowering a little and lowering the demand/stress on the system. When I mentioned keeping the cost reasonable I meant that this model won't get flown that often so I can't see spending a lot more for the top of the line. My Ava E had the best Neu motor in it available (at the time) and I understand why it was good and why that airframe needed it. This project I think will be just fine with some of the lower end setups. I'd rather spend a little less on the motor and more on a quality ESC with more than adequate rating.

On the wheels, I have flown some pretty small power models off of it with no problem. I was wonder though about the wheel dia. v weight issue. I guess I should do as you say and see how everything works and then modify accordingly. The field is pretty smooth since it is a FS aircraft runway and I am the guy that is in charge of keeping it like a tabletop. Maybe the weight would actually be beneficial in this situation
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Old Apr 01, 2011, 07:29 PM
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This is a link to My friends motor glider
It fly's great and take offs are short and sweat!

Ck it out
http://www.rcgroups.com/forums/showt...indsong&page=2
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Old Apr 01, 2011, 08:06 PM
ein flugel schplinterizer
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Creeve,

I follow that thread too. Love it. He's doing an amazing job!

Solitaryman is Noel Rumors, the gent that wrote the Carat article Neil referenced.


OK, Here's the model in question as I retrieved it from 10 years of storage. Layers of dust and greasy film but the finish is 90% nice with a few nicks and cracked paint on the cowl. I'm not going to spend a lot of time on the cosmetics at this point, just wash off the filth, a little touch up and let it, as they say......fly!

As far as control systems go you can see the fuselage mounted servos for both the spoilers and ailerons. I do plan on moving the aileron servos out to the wings but will leave the spoiler setup as is since it works perfectly. All control surfaces except the rudder need hinging as they were done with tape. Plenty of room inside (not going with a scale cockpit) for batteries.
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