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Muxje
11-07-2005, 10:06 PM
I'm currently planning to build Al Masters' Dornier Do335. This is a twin engine aircraft in a push/pull setup, with flaps and retracts (necessary because of the long landing gear legs). The design is for glow engines but I am considering using an electric setup to save weight. Lightening the rear pusher motor. firewall etc. comes with a 150% weight bonus because of reduction of counterweight in the nose to get the CG right.

Some data:
Engines:
- rear engine .25 (a simple OS 25 LA to save weight aft).
- front .25 In order to start from a grass strip, a .40 or a good .32 is recommended (OS 32 SX).
Wingspan is 56"
Acoording to the plans, dry weight is about 7,9lb, fueled 9lb. (A pretty heavy brid for the wingspan, so any weight reduction would be welcome http://207.44.250.58/forum/image/s45.gif )

A rough first estimate indicates that an electric setup of 7.75lb should be possible. That would result in a wing loading of 29 oz/sq feet which is not unreasonable.

Unfortunately I know very little of electrics. Applying rule of thumb and assuming 80W per lb (on someone's advice), I get a total power requirement of 600W. A bit less than another rule of thumb that comes to 700W to replace two (performance) .25 engines.

The first question would be: is 700W or 80W/lb reasonable for a fast plane with this wing loading and wing span? Too much? Too little? This is a scale bird, so I am not looking for unlimited vertical performance, just enough for a comfortable take off from grass.

The 2nd question would be what engines to use. Someone already recommended 2x Typhoon 29-18-10, nice and light engines but 300W seems to be the practical limit on them. I've taken a look at the Axi (http://www.rcuniverse.com/buynow/keywordclick.cfm?bid_id=223) 2820-10 or 2826 series as well. It would be great if someone has already built a twin with comparable parameters http://207.44.250.58/forum/image/s45.gif

Anyway, I hope you can help an electro newbie out. (Also posted this on the regular RCU electric forum)

rcers
11-07-2005, 10:10 PM
The first question would be: is 700W or 80W/lb reasonable for a fast plane with this wing loading and wing span? Too much? Too little? This is a scale bird, so I am not looking for unlimited vertical performance, just enough for a comfortable take off from grass.

Great project and you certainly have selected a challenge! But e-power will be great for this, as you could put a very light motor in the rear, and a beefy one in front to do most of the work.....

I would fly this plane with no less than 100w/lb. That will be a very safe number. It will fly well, but certainly won't go straight up forever. Throttle sticks are great, but you never want that feeling of "I need just a bit more power!".

I would shoot for the AXI 2826/10 in the nose and save the decision for the rear motor based on CG. You could even just use a very low power system (10-15amps) just to spin the rear prop for realism. Nobody has to know it is providing little of the motivation!

Mike

Dereck
11-07-2005, 10:46 PM
I'm currently planning to build Al Masters' Dornier Do335. This is a twin engine aircraft in a push/pull setup, with flaps and retracts (necessary because of the long landing gear legs). The design is for glow engines but I am considering using an electric setup to save weight. Lightening the rear pusher motor. firewall etc. comes with a 150% weight bonus because of reduction of counterweight in the nose to get the CG right.

Some data:
Engines:
- rear engine .25 (a simple OS 25 LA to save weight aft).
- front .25 In order to start from a grass strip, a .40 or a good .32 is recommended (OS 32 SX).
Wingspan is 56"
Acoording to the plans, dry weight is about 7,9lb, fueled 9lb. (A pretty heavy brid for the wingspan, so any weight reduction would be welcome http://207.44.250.58/forum/image/s45.gif )

A rough first estimate indicates that an electric setup of 7.75lb should be possible. That would result in a wing loading of 29 oz/sq feet which is not unreasonable.

Unfortunately I know very little of electrics. Applying rule of thumb and assuming 80W per lb (on someone's advice), I get a total power requirement of 600W. A bit less than another rule of thumb that comes to 700W to replace two (performance) .25 engines.

The first question would be: is 700W or 80W/lb reasonable for a fast plane with this wing loading and wing span? Too much? Too little? This is a scale bird, so I am not looking for unlimited vertical performance, just enough for a comfortable take off from grass.

The 2nd question would be what engines to use. Someone already recommended 2x Typhoon 29-18-10, nice and light engines but 300W seems to be the practical limit on them. I've taken a look at the Axi (http://www.rcuniverse.com/buynow/keywordclick.cfm?bid_id=223) 2820-10 or 2826 series as well. It would be great if someone has already built a twin with comparable parameters http://207.44.250.58/forum/image/s45.gif

Anyway, I hope you can help an electro newbie out. (Also posted this on the regular RCU electric forum)

I'll defer on the motors, but there's a lot you need to be considering if this is an 'early electric' for you.

I don't know the plan you're working from, but most glow models are colossally over-built by electrics standards.

For one example, the typical glow engine firewall of 1/4" ply can easily be replaced by 1/8" Birch ply - and even then I'd be adding holes for motor wires and air cooling into the fuselage. If that sounds flaky to you, I have hung 700W motors onto that - that's motors swinging 15 x 10 props at 7K RPM - onto that thickness of wood.

I've converted several 46-ish glow models to electric power and mostly, a kit is a pricey set of templates! A plan is a much more economical starting point.

Not knowing your modelling background, I'd suggest an intensive search, read and ask campaign to learn all you can about re-structuring glow models to make best use of electric power. If you haven't flown a decent size of model - and there is a world of difference between a parkflier and an e-powered 40-ish sized model that makes glow fliers wonder what they are doing wrong - I'd heartily recommend you build a simple electric sports model to learn how to operate this stuff in the field before you get too far along with what is a very challenging, yet rewarding, prototype to scale model.

I like Mike's idea of a lightweight prop-turner in the tail! No-one will know if you didn't tell them, and your weight and balance problems will be far easier to resolve. While electrics don't mind long wires carrying high powers running the length of an airframe, they are much better for being avoided.

Two idle thoughts to give you things to ponder - one, this prototype has very long, stalky UC legs. They'll be fun to build and could really lead to grief if you operate off a lumpy surface. Two - it will look far better without those ugly cylinder heads sticking out of the cowls. Your cooling issues will not go away with electrics, but will be far easier to deal with without destroying the model's good looks.

Good luck with your project

Dereck

Muxje
11-08-2005, 12:18 AM
Dereck, Thanks for your advice!

My modelling experience is mostly glow, from .20 combat up to .60 scale low wing models; my experience with electrics is limited to helicopters.

Al Masters' plan is indeed designed around glow engines, with heavier bulkheads and construction. However, the tail already looks like he designed it as light as possible; besides the bulkhead, I don't think there is much to be saved. But some more weight might be cut from the front, and lack of fuel-proofing will save some more. I'll take your suggestion to research this some more; any weight saved is good.

Mike's idea of getting a heavier workhorse engine in the front is good as well, in fact that's how the glow version was designed.

Dereck
11-08-2005, 12:50 AM
Dereck, Thanks for your advice!

My modelling experience is mostly glow, from .20 combat up to .60 scale low wing models; my experience with electrics is limited to helicopters.

Al Masters' plan is indeed designed around glow engines, with heavier bulkheads and construction. However, the tail already looks like he designed it as light as possible; besides the bulkhead, I don't think there is much to be saved. But some more weight might be cut from the front, and lack of fuel-proofing will save some more. I'll take your suggestion to research this some more; any weight saved is good.

Mike's idea of getting a heavier workhorse engine in the front is good as well, in fact that's how the glow version was designed.

SOme of the best info on structures, electric basics and so on is contained at:

http://members.aol.com/kmyersefo/page2.htm

There's several pieces by Keith Shaw, who is certainly one of the top electric scale and sport fliers of all time. Keith wrote the book on e-flight, before most of us had learned to spell 'battery'. He's too nice a guy to embarass, but one of his models is a 81" Bearcat from a gas plan - models from the plan have hit 18lb, Keith's 36 cell electric, with retracts, was under 14lb, under the gas spec lower weight limit.

Hope that helps

Dereck

rcers
11-08-2005, 02:21 PM
My recommendation may need a bit more power too. Dereck is right, this is a significant first e project. Not to discourage you! We can help too.

Great airplane choice however!

Mike

Muxje
11-23-2005, 01:43 PM
Well I've done some more research... Plenty of examples for motor and prop combos, but many seem geared towards providing lots of torque rather than providing a 'speed' setup. This plane is not a big winged floaty one, but has a higher stall speed and for that reason I need to keep the prop speed up.

After some playing around I came up with the following:
Front engine: Axi 2826/08 on a 3s lipo, 11x8 prop. The Drive Calculator spreadsheet reckons this combo will put out 365W (input 462W) at 9300 RPM.
Rear engine will be anything to make up the remaining power requirement, maybe an Axi 2820/10 on a 3s lipo, 9x6 prop. Drive Calculator indicates Pout 226W (input 315W) at 10300 RPM.

Is there any advantage in using more cells in series? The reason I picked a 3 cell battery is that I already have a good Lipo charger, but it will only do 3 cells max.

I've also gotten some good advice to modify the structure, and gone over the plan with a red pencil. I should be able to save a bit more weight.

rcers
11-23-2005, 02:21 PM
With the 8 turn motor you should be fine on 3s. Those sound like good numbers.

Mike

Dereck
11-23-2005, 02:29 PM
Well I've done some more research... Plenty of examples for motor and prop combos, but many seem geared towards providing lots of torque rather than providing a 'speed' setup. This plane is not a big winged floaty one, but has a higher stall speed and for that reason I need to keep the prop speed up.

After some playing around I came up with the following:
Front engine: Axi 2826/08 on a 3s lipo, 11x8 prop. The Drive Calculator spreadsheet reckons this combo will put out 365W (input 462W) at 9300 RPM.
Rear engine will be anything to make up the remaining power requirement, maybe an Axi 2820/10 on a 3s lipo, 9x6 prop. Drive Calculator indicates Pout 226W (input 315W) at 10300 RPM.

Is there any advantage in using more cells in series? The reason I picked a 3 cell battery is that I already have a good Lipo charger, but it will only do 3 cells max.

I've also gotten some good advice to modify the structure, and gone over the plan with a red pencil. I should be able to save a bit more weight.

MOre batteries in series? This was easier with 'big round jugs' as one cell extra only upped the volts by one-and-a-bit. Flatpacks add 3.something per extra cell, which changes things far more dramatically. Extra volts allows for more power without dragging more current into it, which can be a Good Thing if your motor is getting up near its redline current. The important sums involve how much extra power you get for the extra weight, and what does it do to the wingloading?

It's too early in the morning to do them again, but I recall that adding two cells to an 8 cell model - talking NiMh again - that weighed 3lb to start with, and re-propping to maintain a 30A current draw, upped the power by nearly 25%, while upping wingloading by only some 8% or so.

The snag with flatties is that you add one cell, suddenly your motor, ESC and charger may all need replacing :eek: This is not so good...

Still, despite the advances in technology, the cheapest power upgrade remains a lighter model :)

Regards

Dereck