kahloq

BCRC.......

I havent started mine yet, but have a 65cc rimfire on 12s 8000 for the front and have a power 60 on 6s for the rear. Prop up front is a 20x14 Biela and the rear prop is a pusher 16x10c3 mas. Since you already know how it flies with the 55cc gas up front, Id say you'd get just as good or probably even better performance with the setup Im going with.

The way Im wiring up the two 6s 8000 batteries allows the front motor to see a voltage into the esc as 12s, while the rear motor esc will see 6s. That is unless I still need nose weight to balance, then I can put in another 6s 6000 for the rear.

Ameisenbar

Hi Guys, The local shop just discounted all their ESM stuff so I succumbed to the temptation. It is likely to be a while before this one flies since I have a few other projects that ought to come first. So far I have the big box plus two spinners. I also have a 120 on the way to go in the front. There will be a  power plant in the back too. The guy in the shop suggtested electric there, I can see the advantage since the motor itself would be quite light and the heavy battery can go well forward. However I can also see a hybrid plant being a pain.

I have idly wondered about the possibility of using IC but putting the aft engine where the big one did, and using an extension shaft  to drive the rear propellor, just like the big one did. The aft engine would have to be inverted and would pick up its cooling air from the convenient scoop provided. Access could be a problem, as could getting enough air in to keep things cool.  If the rear engine can be further forward there might be less need for nose weight. Anyway there will be time to think about this.

Can anyone tell me why the booklet says 11 servos for one engine and 13 for two? I can see that the extra engine needs one for the throttle, what does the other one do? They could also have been a bit more specific about the type of servos. I've read some of this thread, I see there are some servo choices mentioned. I don't want to add more weight than is needed but on the other hand I want good enough servos for the job.

John

vertical grimmace

My Feedback: (1)

Maybe choke or igniition kill. Not really a need for a second servo for an engine though.

TA152

Hi John.
I think your Dornier may be a little underpowered with a 120 in The nose. I used a Laser 160v in The nose,and a Laser100 aft.
It balanced with 2000gr of lead in the nose

Ameisenbar

Well, the box says 120 in the front and 90 in the back, which is what I had in mind, not just the 120 on its own. Is that not enough? I have an ASP radial but I don't think it would fit in the cowl!

You could of course connect both throttles to the same servo using a snake. With glow engines we have usually done a kill by having a mix to a switch that shuts the throttle completely. I can see some fun making sure both engines have a good idle set as well as both reaching full at the same time with that approach

John

kahloq

Is the 120 a 4 stroke or 2 stroke? A 120 4stroke up front and a 90 in the back is going to be very marginal. The box weights ESM usually gives are real low and dont account for retracts and other things. A 120 up front and a 120 in the tail would be ok. But if your thinking 2 strokes, you'll be fine.

Ameisenbar

Ok, you are making me think a bit. I am thinking in terms of four strokes, two strokes just don't sound right. I have had a good look at the fuselage, there is heaps of room inside just behind the hatch. So it seems to me that it should be possible to mount the 120 in there, with a shaft to the rear airscrew. I know shafts are not common in model aircraft, but they are quite usual in model boats so given a bit of engineering it should be possible. I am thinking in terms of  an aluminium tube shaft, and a ball race mounting for the prop hub. That shifts the weight of the engine from about 1100mm behind the CoG to about 400mm. (44 inches down to 16 inches for the Americans) There would be the weight of the shaft and bearings in between, which might easily be a few hundred grams or so, (say half a pound) but overall the gain of getting that engine further forward should mean that less nose weight is needed. Ok, so then I get another engine for the front, maybe a 160 or so to give a bit of extra grunt.  If I can reduce the takeoff weight it is all gain from the flyability point of view.

So the main question apart from the shaft itself becomes cooling. The exhaust arrangements are not such a problem, except of course the muffler is inside with the engine, radiating heat so not exactly helping with the cooling. There are two ways to get air in, one being the big scoop underneath, and the other the carb air intake on the starboard side. The latter is blanked off but need not remain so. A bit of ducting would be needed to direct the incoming air to where it is needed. The next problem would be getting the air out, which tends to be harder than getting it in. Should be possible to get it to go out via where the aft engine would have gone. The exhaust stacks can also provide some air exit without detracting from their appearance, although the exhaust would be coming out one of course.

I might make up a trial shaft and engine mount and give it a bench run to see how it works out.  I haven't told my wife about the need for retracts yet!
John

invertmast

My Feedback: (23)

The least of your problems will be cooling with an extension shaft that long.


Look into rotational vibration of propeller shafts and plan to spend allot of time reading, designing, building and redesigning and rebuilding. Long prop shaft extensions are difficult to pull off

hellcat56

My Feedback: (21)

the rear shaft has already been done
go here to see how he built it
master builder Evo

http://evoshangar.homestead.com/Do335.html

jimkron

The biggest challenge I faced with the electric conversation was CG. My plane weighs 35 pounds with 2x5s lipos for each motor. All 4 batts are stuffed in front of the cockpit after I made a big hatch. One Castle ESC sits in the cowl, the other midship with retract stuff. That long distance between the cg and rear prop makes for a very tail heavy plane if your not careful.
Taking off and flying is relatively easy, but landing a heavy plane is tricky. You need to reinforce all the landing gear areas with carbon fiber. I find this area weak on most heavy planes, especially arfs from China. I wish mine were 5 pounds lighter. I'm using Sierra retracts that are stout but heavier than the cheap ESM ones (at least the older retracts).
Watch your weight...and watch the bottom fin on landings.

Jim

kahloq

Thank you for the cautionary words. What setup did you run again? What mah size 5s packs did you use? Since I'll be using two 6s 8000 packs up front for 12s on the forward motor, those should weigh roughly the same as 4x 5s 5000's(if thats what you used.).
My two 6s 8000's weigh 2800 grams(98.7oz) by themselves....... 4x 5s 5000's weigh 2900 grams(102oz)....for same brand and discharge ability as the 6s 8000's I have....so basically a 3 oz difference.
I can also put in a separate 6s battery for the rear motor up front if needed as its there really only to spin the prop(16x10x3).

I do have the ESM gear, but it's the newer v3(on route from troybuilt as they did an exchange for the version 2's I had bought awile ago). So...we'll see how things go. Hopefully I can keep the plane lighter then 35 pounds. Good thing is, I have an 800 ft paved runway to land on. Thought, friend of mine has the 95" esm b-25 and his weighs 29 pounds. He uses a good portion of the runway to land and he even has brakes installed....although he's running glow motors .

jimkron

I used 4 Hyperions 5000mah with a total of 20s...so 10s for each motor. Not sure the weight. I have them stacked all together in front of the cockpit. More than enough speed for takeoff and about 7 minutes of flight time before landing. My Sierra retract have brakes...a must for me...astroturf runway is only 400 feet. Grass runway would be to die for.
I chose to put the same engine (Eflite 160) and prop size (Xoar 20X10) on both ends of the Dornier. I got it in my head that symmetry was important, but have no engineering expertise to say one set-up is better than another. If results mean anything, this engine/prop configuration was simple and efficient as seen in the video.
Whatever batt config you use will have to be next to one another in front of the cockpit. If not, cg and additional nose weight problems will follow.

Jim

Ameisenbar

Hi Guys, I see some correspondence way back about the way some ESCs can blow up if the battery leads are too long. I'm not planning to go electric myself, but I am an electrical engineer so maybe a few comments on what is happening and why could be helpful, especially in a case like this where the rear motor is going to be a long way from the battery.

What happens when the ESC is running is that it draws pulses of current rather than a steady flow. The result of this is that the resistance and inductance in the battery leads causes the voltage at the ESC to fall with every pulse. The ESC has capacitors, which store a bit of energy to help reduce this effect. However ESCs are built to a price, so the capacitors are only big enough for the normal usage. Where the battery is close this is fine, since the battery itself acts a bit like a large capacitor so tends to keep the voltage steady.

If the leads are too long, they have more resistance and inductance, so the voltage at the ESC will tend to vary up and down more. This tends to upset the electronics inside, including the little microcontroller that makes it all happen. If things get upset too much, the circuit will fail, and since these devices are handling a lot of current, they tend to fail in a pretty spectacular fashion.

So the way to avoid this is:
1/ Keep the battery side wires short. The motor side is not so critical since extra length here tends to smooth the current out.

2/ Add extra capacitance across the battery lines near the ESC. These should be good quality capacitors similar in rating to the ones in the ESC itself. Adding more than is needed will do no harm, so be generous. (The spark when you connect up will get bigger, but that is not usually a problem. ) The capacitors will be electrolytic, so the polarity needs to be correct.

3/ Use heavier wire that you might tend  to use if they were shorter. This will reduce the resistance, although it will not affect the inductance significantly,  and that is part of the problem. It will help keep the maximum power up.

You can also use heavier than normal wires on the motor side, and this would be a good idea if they are getting quite long. The downside is the copper is heavy stuff, but on balance you will save more by having the batteries as far forward as possible than the extra copper will cost in weight. Don't ever add capacitors on the motor side!

Hope this is helpful...I have tried to avoid getting too far into the technicalities without oversimplifying too much.

Rule 1: Don't let the magic smoke out.
Rule 2: Don't let rule one scare you.

regards
John

invertmast

My Feedback: (23)

Yea, he built it. But he didnt design the prop drive shaft system, it was designed by the manufacturer of that kit. Doing an extended prop drive shaft isnt as simple as supporting a tube with some bearings, the. Connectig one end to the motor and the other to the prop. Its much more complicated, and the vibrations from an improperly designed assembly can lead a very hazardous situation, where any part of the assembly could fail.

BobH

My Feedback: (2)

All good advice and information John, thank you.

Ameisenbar

Back on the shaft idea, I am well aware that a shaft 700mm long turning at around 10,000 rpm is not going to be trivial. On the other hand, what man has done man can aspire to, and the Engel guys have obviously done it, for a larger engine than I need too. The potential problem with a shaft like this is whirling. If the shaft is the least bit out of balance or non straight, then the forces tend to make the out of balance worse, so it is unstable. It is also in a fuselage which is slightly flexible, so the engine and propeller shafts cannot be guaranteed to be perfectly aligned at all times. The Engel kit is using flexible couplings, these have definite limits on the misalignment that can be permitted. I used some couplings on a thirty foot boat that are only allowed to be out of alignment by a couple of degrees..not even easy to measure in the bilges of a boat. So yes, it is something that would want to be bench tested in a rig with some safety precautions, eg means to contain a shaft that abruptly failed, before trying it in a plane. If the shaft or the couplings let go in the air the model would be a write off. It sounds like Dornier themselves experienced trouble with the shafts too.

regards
John

invertmast

My Feedback: (23)

John,
Its not just the misalignment of the shaft/engine and the balance. It is the rotational vibrations caused from power pulses of the engine, the tube will actually deflect and deform and twist causing all sorts of havoc and frustration.

Most all aircraft that involve an extended propellor shaft like this experience the problems and have to deal with the issue.

The BD-5B had such horrendoush rotational vibration problems from its extended prop shaft that they were going through airframes because the vibration was so bad that the rivet holes were enlarging after only a few hours of engine run time. You could also visually see the entire tail section of the thing trying to shake itself apart as the engine was turned over by the electric starter, but as soon as the plugs were removed from the cylinders, the vibrations were gone. During its testing, a solid drive system was used to test out what the problem was and the vibrations were so severe, the engine couldnt accelerate above an idle due to the vibrations absorbing all of the engines power

There is a way for it to be accomplished, but its going to involve some head scratching and engineering. There is a hap in australia who is designing his own extended drive shaft for his large DO335 and last i saw, he was still having problems with the design.

hellcat56

My Feedback: (21)

Invertimast
your right he didnt design it
just like we dont design the electric motors and stuff
but
he built the plane and installed this unit and it worked to perfection
he has numerous flawless flights and as far as I know the unit worked great
this unit did NOT fail
The plane did later crash but it was not because of the rear engine shaft

the design work has been done
the unit does work
instead of trying to re invent the wheel
take was has been proven to work
and
learn without all the theory of why it wont work

invertmast

My Feedback: (23)

Im not saying it wont work, im saying that unless you know EXACTLY what material, tube diameter, thickness, etc was used, your likely not going to have the same success with doing the design and engineering portion.

Engel's extension shaft was designed for use with and ONLY with the zenoah G45 as it was designed around the power pulses of that engine. When you now use that same design with an engine it wasnt designed for, you are operating outside of its design and could be slapped in the face with issues i have posted about previously.

The reaaon i bring this all up, is bc most people are completely clueless when it comes to being able to design and construct something reliable bc they are to lazy to do any research on te matter. I will be starting a 100" DO335 project within the next year, and feel that in order to do anything "right", one must know as much information about any bumps in the road as possible, especially when it comes to a spinning blender at 7000rpm!

kahloq

Started working on mine today. Cut a top hatch out on the front of the nose just behind the gun ports. Followed the panel lines basically. The fuse former under it will have to be modified/mostly cut away to make room for the batteries, but I will reinforce the nose area all around it, so should be good to go. I ordered a few mini piano hinges from Lee Valley Tools online. They are located in Canada, but decent prices for the hinges. The place in Englewood CO(norton hobbies) isnt accepting orders currently.

http://www.leevalley.com/US/hardware...=3,41241,41263

kahloq

Ok...here's some pics so far.
First two pics show the batt hatch I cut out along with the fuse former that needs to be cut away some. Batts will be placed on a tray right behind the firewall for maximum nose weight.
Next up is the rimfire 65cc mounted and then the 3 blade spinner. Finally, the last pic shows the amount of clearance I will have for the prop blades. NOT much at all....but that cant be helped unless I can find longer then 60mm length M6 hex bolts. Those were the longest that ACE hardware has. Anyone got a palce I get longer one? Only need like 65 or 70mm length...just tog et another washer's space to get the prop a bit further away from the cowl.

jimkron

This is how mine finally was configured.

kahloq

I'm planning on having the batts sit flat....forward to back right behind the firewall.......I cant put them like you did standing up cuz my batteries are too long. Im using the 6s 8000mah size. If I need a separate battery for the rear motor, I can stand a 6s 6000 up like you did behind the two 6s 8000's...of on top of them...either way

kahloq

rough picture of placement. Will need to put in a flat wood tray, but you get the idea. Two of these batts will sit side by side. If a batt for the rear motor is needed separately, then it cant sit on top.
Thats a bit far off to tell yet though.

vertical grimmace

My Feedback: (1)

Invert, are you planning on using a scale located rear engine, and if so, which one? I will be interested in any results you have on this issue.