dgiatr

does anybody know what should be the rpm-prop-power relation between front and rear engine at this plane;
thanks!

vertical grimmace

My Feedback: (1)

Should not matter being a push pull. Unless you want the two engines in sync, then you just match rpm. This thing does not need equal power to fly though. In fact the full scale could fly one one engine only, front or back.

Ameisenbar

As per the original, the two engines should be the same and would therefore run at the same RPM. For that to happen, the rear prop should then have a slightly higher pitch than the front one, since it is operating in air that the front prop has already accelerated.

However, practically for the model one, the rear engine might well need to be smaller, since it will be mounted further aft than the scale position. Being smaller,  the operating rpm will probably be a little higher, so you might not need to change the pitch compared to the front.

With an electric setup, you could put a Wattmeter on each motor and adjust things on the ground so each is taking a reasonable share of the load relative to its size. Loadings would change in the air, but probably not enough to matter. With IC it is a bit harder to measure the power going to each prop.
John

kahloq

As Vertical said...its a push pull design. It does not matter. The two motors do not need to match nor do both motors need to run the same pitch. The rear motor does not need to run a higher pitch...in fact if it were setup up that way, the prop would more then likely be stalling the higher the rpms because of the prop wash from the front motor.....but.....the rear motor doesn't get the full effect from the front motor anyway since the fuse and cowl blanks and also spreads the prop wash away from the fuse some.

jimkron

I run the same engine and prop on front and back. Plane flys great. Eflite 160s with Xoar 20-10 tractor and pusher prop. By the way, it took me months to get the pusher...very rare. I think it was from Steve's Hobbies in Florida. They are a US dealer for Xoar if I remember right.

jim

ps Love those 3-bladed props

vertical grimmace

My Feedback: (1)

You do not need a pusher prop if you can reverse direction. Mount the conventional prop backward (facing forward). I looked into this for a gas set up. I was going to reverse the direction of a gas engine just for this reason.
The lack of prop availability is a real pain. No point in making life tougher if there is an easy solution. Especially with an electric setup where you can essentially swap 2 wires.

Ameisenbar

It is true that they don't need to match...for any particular combination of motors there will be a prop combination that ensures that both motors are doing their fair share of the work...But the rear prop is working in air that has already been accelerated by the front prop. This will mean that if the props are the same the rear one is operating at a lower angle of attack, eg further from stall, and so will do less work. The presence of the fuselage will reduce this effect but not eliminate it.

So ideally the rear prop would have a slightly higher pitch, or be run at slightly higher rpm, for the motors to do equal work. Probably this does not matter in practice so long as the setup has enough power, since the difference would only be a few percent. After all, they will fly with an undriven dummy on the rear as long as the front has enough power. Or vice versa I guess, at least the full size could fly on either engine alone. If like the full size you were trying to get the best possible top speed, you would fiddle with the pitch for the best result.

John

TA152

The english testpilot Eric Brown wrote that the Dornier flies faster with the rear engine single than the front one, but it also had constantly a higher temperature than the front engine. The Dornier also suffered some problems with the rear engine cathing fire.

dgiatr

Thanks for the useful informations!

wphilb

My Feedback: (10)

Has anyone tried running both props off of a single engine, say in the front?  That would really help the balance issues but would require a very long rear propshaft and obviously a powerful engine.

kahloq

Some updates on mine.

As far I was able to understand instructions for the ESM electric retracts, they MUST be powered separately from the RX and only AFTER the RX has power. I presume this is to make sure the motors don't burn out or something. Maybe I read it wrong, but, going off that premise, it would seem that two rx batteries would be required with separate switches. One for the main rx to be switched on first, then another to switch on power for the retracts.
Well, it would be rather easy to accidentally flip on the wrong switch first(or turn off the rx switch first while the retracts are still powered) with a two switch. I can't imagine ESM actually designed it this way, ie....having to ensure the retract main controller box is never receiving power when the rx is off.

So.....just to make sure....what I've done is wired in a separate BEC to the rear motor's ESC. In any preparation for flight, the rx has to be switched on before the flight batteries are hooked up...and on powering down...flight batteries are always removed before the rx switch is turned off.

However, the trick here is I didn't want that BEC actually providing power to the RX as well. So, I had to use a y-harness with a male-to-male servo extension to still allow motor signal from the rx to get to the esc while not allowing the bec's power output to run to the receiver. Normally, a BEC is run inline with the servo wire coming from the esc to the receiver to power the rx. In this case, the y-harness is used to split off the signal from the esc towards the rx and only the branch of the y-harness towards the retract controller box has the bec hooked in.

The front main motor is run off of two 6s 8000's in series for 12s. The rear motor has a 6s 5000, and only when I plug in that battery will the retracts get power.

vertical grimmace

My Feedback: (1)

There's a balance issue?


Simply, not thinkin' this is a good idea.

kahloq

Well, I had originally decided not to put a full cockpit in and only use a pilot bust. However, the busts from hobbyking that are the same as aces of iron were too big to fit without cutting the floor out. If I have to cut the floor then light as well go the full nine yards. I am building the cockpit kit myself instead of getting the Ty Planes one. This area under the cockpit floor has no structure internally, so I am adding wood support rails from bulkhead to bulkhead. as such, I will integrate the cockpit I make with those to make this area a lot stronger.

Ameisenbar

I think he means the CoG issue that can arise with a heavy engine at the back. Looking back through this thread some guys have had to add quite large quantities of lead to the nose. Then you start to get into an all up weight problem. This is of course not such a problem for the guys who go electric, since the motor itself is quite light and the battery can be positioned well forward.
A shaft from a front engine to the rear prop would be about two metres long. The problems of a shaft go up dramaticaly with the length, probably at least with the square of the length. A longer shaft will deflect more for the same out of balance force. The German guys have shown that a shaft from a mid engine position is practicable, if not particularly easy, but a shaft the full length would have to be much larger diameter and very stiff. Add to this the fact that the rear prop shaft is not aligned with the front one and you have a potential engineering problem, requiring say gears to align the two centres.

The reason that a shaft is difficult is that any out of balance in the shaft will cause a force that deflects the shaft in a way that increases the out of balance, so tending to make matters worse. Add to this the fact that the power from the engine is coming in impulses every time it fires, and that the propellor is acting like a flywheel at the far end, so the shaft will be subjected to torsional vibrations. (Straight eight cylinder car engines used to bust crankshafts because of this sort of thing.)

Ithink if you are going to go with one engine at the front, it would be much simpler to just use the idler propellor at the back as per the plans. If you plan on two engines, then we know that it is possible to run a shaft from the mid engine position, but I would suggest that a careful development process with lots of ground testing would be in order. If a shaft fails in flight, it will take out the whole of the rear fuselage, and the law of gravity will shortly afterwards take out the rest of the plane.

John

kahloq

Here's a couple pics of the instrument panel I made. It its not a kit, not was it part of any cockpit kit such as a TY Planes cockpit. I merely made the thing myself out of thin plywood. The only parts I didn't make were of course the gauges. Everything else was scratch built from. Its not entirely scale as I didn't have enough room for the number of gauges the real plane had. I used 1/5 size dials as that is what I had. Looks really nice in my opinion, but the plane is actually a 1/6.5 scale. So 1/6th size dials would have given me more room to get a few more dials on the panel. Of course, had I not even said that, no one would have known.
I found a pic online of a real DO-335's cockpit and followed what it looked like reasonably close. Im now working on a flight stick...also hand made. The rest of the cockpit interior I am making myself as well. While I like TY's cockpit kits, this area of the fuse had no wood support or anything but the fuse itself. No formers, nothing. So....I put in some beefy wood rails and such and then a plywood floor to help with the structure in this area. So, since I was doing that, I figured I'd just make the cockpit myself.

Ameisenbar

Looks pretty cool to me...

I've just acquired an ASP 160 twin, it will almost fit inside the front cowling. Just the rocker covers will need a bit of an opening. With the 120 I already have at the back, that would give me a total of 2.8 cubic inches, or 47cc.

John

kahloq

That will sound really nice.

Ameisenbar

Yes, it should sound good...but I will probably need to put a muffler on the twin because it only comes with two short open pipes. A pair of flexipipes might be enough, and would also get the exhaust out of the cowling. I'll also need to make sure of enough airflow through the cowling.

Has anyone ever worked out what all the servos mentioned on the box are supposed to be for? They say 11 for one engine, 13 for two. My count seems to be:

elevator 2
rudders 2
ailerons 2
flaps 2
throttles 2 (for two engines)
retracts 1 (for air, effectively none for the new electric ones I think?)

That makes 11, but the box says 13 for a twin engine configuration. I wonder what the other two are for? Maybe they expect a separate servo for a choke on each engine? Iwould usually just block the exhaust to prime, or use a mechanical choke lever if necessary.
I haven't started assembly yet, I am quietly collecting bits so far.

regards
John

kahloq

Almost done. Still have to add the exhaust stacks front and back locations and then she'll be ready to balance. I shimmed the front motor down a decent amount....basically 4 washer widths on the top motor standoffs relative to none on the bottom motor standoffs. That effectively only gave me about 2 washer widths of down thrust. To my eye, with no washers and the motor standoffs flush top and bottom, it seemed to me like the motor was pointing up a couple degrees. Anyway....Im happy with the way it looks now for the thrust line to fuse.

Couple of pics...sorry for the clarity....they were taken with my phone.

vertical grimmace

My Feedback: (1)

Dang, that thing is so big, we are gonna need 3 lawn and leaf bags after the maiden! Lol JK

Let me know when you are going out. I will be free this weekend. Don't wanna miss seeing this beast fly.

kahloq

Since some people said they had flutter occur on their elevator(s) and at least one person thought that was the reason they lost theirs, ive added hingeline gap tape. May not do much good, or it may help. Just didn't want to take a chance and the hinge tape doesn't weigh much at all.

As for balancing, my EZ balancer from great planes can handle the weight of the plane, but, it cannot adjust far enough back. I need 8.5" from the LE against the fuse and the GP ez balancer only goes to 7.5" at absolute max. So....either I buy a bigger balancer or try to figure out something else. I don't have any where I can hang the plane to do the string/bob method. I also don't fancy paying $229 for that all aluminum balance rig from Southwest Systems. Sig has one, but again, I don't know if its capable of holding a 35 pound plane and how far the balancer can handle.

kahloq

Ok...well...this is one heavy ****. The plane itself WITHOUT the flight batteries weighs 30.6 pounds. The batteries add another 6.6 pounds. That comprises two 6s 8000mah packs for the front and one 6s 5000 pack for the rear. That's a total of 37.2 pounds. I hope to God that I don't have to add any weight to balance this thing. Simple wingloading calculations give 72 oz per sq ft.

Jimkrom said his weighed 35 pounds and it flew fine. That's a wingloading of 68.3

Not sure how I packed on all that extra weight aside from reinforcing the inside of the fuse with wood in several places.

Will definitely need to have vid cams going on the maiden flight...JUST in case......if weather holds, then possibly this weekend.

vertical grimmace

My Feedback: (1)

One of the main things to do to avoid flutter is to have all of your control rods very well supported. Flutter generally rears it's head when flying fast. Sealing the hinge line will help, but poorly supported control rods and bad hinging were most likely the cause of that other crash. Just think of the control rod on your throttle on the Gee Bee this past weekend, you want to avoid that kind of thing.

As far as the CG goes, why not fabricate a fixture using dowels? Measure and mark the wing with white or black paint to define the range, and set the plane on the sticks. I know it is heavy, but this will give you an indication.

kahloq

Due to the weight of the plane, the dowls would more then likely poke thru the wing skin.

jimkron

Bummer on the weight. I was hoping yours would be lighter than mine. Obviously you'll need full flaps, lots of speed, and a long grass runway. We landed mine on a 400 foot astroturf runway, but the pilot (not me) was very experienced at large warbirds. I also had Sierra brakes...that was a must. We flew at a contest with a grass field and the landings were perfect...barely used the brakes. Keep that speed up...or she'll drop like a rock. And please, take your time balancing this bird.

jim