Official Black Horse 1724mm Dornier DO335 Pfeil ("Arrow") Thread

[Twowingtj]

TJ, is this a visual you are observing from pics??

By going with 2 different sized kV motors turning different size/pitched props, I'm concerned that you are going to run smack into a possible engineering issue known as aerodynamic hysteresis.

Two examples for prop difference is first hand observation of the full scale at the Udvar-Hazy museum and a plastic scale model that my bride built and we still have.

While in no way an expert on aerodynamic hysteresis, the higher rpm and lower pitch(angle of attack) should reduce the likelyhood of stalling the rear prop. Might have to build a wind tunnel .

[OV10]

Two examples for prop difference is first hand observation of the full scale at the Udvar-Hazy museum and a plastic scale model that my bride built and we still have.

While in no way an expert on aerodynamic hysteresis, the higher rpm and lower pitch(angle of attack) should reduce the likelyhood of stalling the rear prop. Might have to build a wind tunnel .

Ah blimey, just boldly go where others haven't and hope that you don't run into any power issues
I know you remember the power issues with the D8

[Twowingtj]

Ah blimey, just boldly go where others haven't and hope that you don't run into any power issues
I know you remember the power issues with the D8

Awwww buddy, you just had to go there didn't ya. Power wasn't really the problem with the D8. It was clearly the absence of a tail.

[Oxotnik]

I received my much-anticipated Do-335 book today. I flipped through the variants and I never saw an instance of the tractor and pusher motors/props being different in any of the variants. The motor was always a variant of the DB603, and the tractor and pusher props were both 3300mm diameter.

[MANFRED]

Did you get your rear spinner yet? If so, what size prop and motor did you ask for with the tractor cut?

[MrSmoothie]

I received my much-anticipated Do-335 book today. I flipped through the variants and I never saw an instance of the tractor and pusher motors/props being different in any of the variants. The motor was always a variant of the DB603, and the tractor and pusher props were both 3300mm diameter.

That's a little scary. I ordered and received the very same book today. Excellent reference! But Chuck, refer to the chart on page 25. It lines up with the chart below -- 3.5m and 3.3m.

I'll post the chart from Monogram No. 9 and highlight "fore" and "aft" showing different props. Motor was the same except for one letter designation to indicate opposite rotation. Also, every good scale drawing I've collected shows the prop diameter difference.

[Oxotnik]

Did you get your rear spinner yet? If so, what size prop and motor did you ask for with the tractor cut?

I have not received my aft spinner yet. The TruTurn tractor spinner I found at my LHS. It’s 3 1/2”, Ultimate shape, and has wide cuts. It’s a little larger than scale, but looks fine. I special ordered a 3 1/4” Ultimate Shape with pusher slots for the MAS 16x10x3, for the rear. I’m really hoping that my pusher spinner arrives in the next day or so. I’d love to get my Pfeil in the air this weekend.

[MANFRED]

Thank you, can hardly wait for your feedback on power vs weight performance with the motors you are using. Good luck with your maiden.

[Oxotnik]

Thank you, can hardly wait for your feedback on power vs weight performance with the motors you are using. Good luck with your maiden.

I believe that I have already reported the wattmeter test results. Spitty 5055-390kv, MAS 16x10x3 (tractor), CHNL 6S 70C battery. 1345 W, 55A, for 2690W total. If the final flight weight of the 335 is 3.5 lbs, then the power-to-weight will be 199 Watts/lb. I'm expecting the Pfeil to perform very nicely with this power train.

BTW, when I had my Flightine Spit IX at the field this past weekend, I stuck a wattmeter on it. With the stock 16x10x4 prop, it pulled 59A and 1376 Watts, which is only 30W more than the motor pulls with the 3-bladed MAS 16x10.

[Oxotnik]

Having gotten an email this morning about a sale on HobbyKing batteries, i went to look. I spotted this 6S-12AH battery, but it's only 12C. I've never actually used a graphene battery, but I thought that their chemistry allowed them dump energy with less resistence. So, the 15C discharge rating (30c burst) confuses me a it. I also haven't checked dimensions to see of this 12AH Graphene would fit in the battery bay of the 335.

Any thoughts on the battery, relative to its use in the Do-335?

[downwind3zero]

I like the idea of this Lipo! Depending on if you can get the CG right with it, the Amp draw of your motors (55A x2) would be OK with 12C from this battery.

[Oxotnik]

I like the idea of this Lipo! Depending on if you can get the CG right with it, the Amp draw of your motors (55A x2) would be OK with 12C from this battery.

Yeah, last night as I was lying in bed, it dawned on me that I was thinking about the C rating wrong. I was hung up on the absolute value of the C rating, but not thinking about it in terms of current push. 12AH batt at 15C can provide 180A. As you noted, my 5055's only pull 55A (max, static), each. So, the battery could easily provide the power to both motors. I'll do some measuring today, to see if the battery would fit in the battery bay. I also saw a 10AH 6S battery, too, which I'm assuming would be a bit smaller.

[Alpha]

I'll throw in another dimension to your [wise] musings, Oxotnik: We should also consider the ampacity of the wire being used. Not only will your system not pull nearly as much current as the battery itself is rated to provide, but even if the system did pull 180A continuous, parts of a Power System run into the bottleneck caused by fusing current.

Industrial batteries (12Ah is getting there!) flown in agricultural drones for example, don't need exotic 50c batteries, because their systems don't pull 500+ amps, and even if they did, the wiring would melt!

Power System design is about optimizing everything in a SYSTEM. Prop, motor, ESC, wire gauge and type, battery ampacity (peak, burst, sustained, average unloaded), etc. In the context of an airplane we're also adding Dimensions, Weight, and CG-ability into the equation. If anything in that system changes, the equation outputs differently.

It is best to consider all factors at once to find the best balance of all factors that meets your target.

[Oxotnik]

I'll throw in another dimension to your [wise] musings, Oxotnik: We should also consider the ampacity of the wire being used. Not only will your system not pull nearly as much current as the battery itself is rated to provide, but even if the system did pull 180A continuous, parts of a Power System run into the bottleneck caused by fusing current.

Industrial batteries (12Ah is getting there!) flown in agricultural drones for example, don't need exotic 50c batteries, because their systems don't pull 500+ amps, and even if they did, the wiring would melt!

Power System design is about optimizing everything in a SYSTEM. Prop, motor, ESC, wire gauge and type, battery ampacity (peak, burst, sustained, average unloaded), etc. In the context of an airplane we're also adding Dimensions, Weight, and CG-ability into the equation. If anything in that system changes, the equation outputs differently.

It is best to consider all factors at once to find the best balance of all factors that meets your target.

Why can't anything just be simple???? LOL

Good points all. Thanks!

[Oxotnik]

Hey, Alpha, since you're checking in on us, an observation has been made about the thrust lines for the tractor and pusher motors. Can you comment on whether there was an intentional offset of the thrust axes for the fore and aft motors?

[Alpha]

Mine had its motors bolted directly to the stock factory firewall without any modifications in thrust line. The bird flew straight as an Arrow.

[MrSmoothie]

Alpha, I'll chime in here -- once I looked closely, it was clear that the front motor has built in right thrust and the rear motor built in left thrust. The firewalls are angled, and the pre-drilled mounting holes are offset laterally. Finally, both cowls have angled "faces" to correspond to the offset spinners. Glad to hear that you went stock. I assume you put a reverse rotating prop on the back end?

[Creative RC]

I do wonder whether the rear prop should have a higher pitch on account of it receiving high speed air from the front prop. The fellow who was flying 4s in a video reported on his link document lower amps on the rear motor at full throttle. Specifically 57 Amps up front and 45 Amps at the rear. He thought it might be due to a longer power wire for the rear motor, but I don't think that alone would account for a 12 Amp difference.

If the amps were made about equal (without exceeding limits of motors) with a higher pitch prop at the rear, then would it not stand to reason that the overall thrust would be higher or better this way?

[paladin]

the front spinner can be purchased here:

https://www.dolphinco.com.au/spinner-3-aluminum-2-blades/

joe

[MrSmoothie]

I've been busy working on my Pfeil, and spending time on the kind of stuff I probably wouldn't bother with except for the "stay at home" order (and not living in Canada or other cold-for-months locations). Details are on my RCG build log if you're looking for more information. I'm especially happy that I got the nose gear doors hooked up and working. They always look simple in the end (and not like they took hours to figure out)...

https://youtu.be/G5EwMS52JDU