Twin edf SU-27

[iwpublic]

@Flitedesign 3d It seems there is a minor issue in the plans.

Formers EF1/2 (both, foam cut and 3d printed version) do not fit into ES4. If EF1/2 alignment tabs are put into ES4 cutouts there is plenty of space at the bottom (see pic 1).
View attachment 241734
If the formers EF1/2 are aligned with ES4 then either side of the cutouts is too high.
(see pic 2+3)
View attachment 241735
View attachment 241736
First I thought I messed up the printing scale. But I measured 3x and reprinted @100% scale.
EF1 "circumference" is ≈17.3cm.
ES4 "circumference" at EF1 position start to end is ≈18.5cm

Same effect is at EF3. If aligned with the inner side of ES4, the radius of EF3 would be too small to have ES4 ending up correctly on the outer side...
View attachment 241737
Now I could move the cutouts 0.6cm lower on each side.
Alternatively the space at the bottom could be filled by additional material on EF1/2.
But I am afraid this has an impact in the thrust line, hence, your advice would be much appreciated!

Couldn't sleep thinking whether I have messed up sth.
But here are some more pics highlighting what I mean:

EF1 circumference from lower end of the alignment tab to the other side is 14.1cm.
ES4 distance between inner side of alignment tab cut out is 15.1cm (tried to incorporate the curve as good as possible).

Have not made the EFx formers "thicker" but removed about 1cm from the ESx middle sections.
Now it fits.

 

[Griet]

I recently found the YT videos, and thus this forum and build thread, about the SU30. Excellent work here!
I have a couple of Freewing 12-blade units lying around, albeit for 6S, and now I'm wondering whether I should use them for this project....
As I read here, it will need some strengthening for the extra weight, but I'm also curious about the static thrust on 6S, if still would be hand-launchable. I have some 6S5000 batteries available, but they might prove too heavy, and maybe I need to open up the battery bay to put them more aft.

 

[iwpublic]

I recently found the YT videos, and thus this forum and build thread, about the SU30. Excellent work here!
I have a couple of Freewing 12-blade units lying around, albeit for 6S, and now I'm wondering whether I should use them for this project....
As I read here, it will need some strengthening for the extra weight, but I'm also curious about the static thrust on 6S, if still would be hand-launchable. I have some 6S5000 batteries available, but they might prove too heavy, and maybe I need to open up the battery bay to put them more aft.

Welcome to the forum, Griet!
I am planning to go for 6S as well. 3500mAh do fit well. The battery bay is quite large. What's the measurements of your 5000mAh pack?
Concerning the static thrust there are cheat inlets (3d prints) which should help. Will place them left and right just in front of the EDF in order to overcome the limited nacelle air intake.
Whether it's still hand launch capable with 6S I can tell you when I am done. Anyways, I have thought of adding optional gears, too. Let's see.
Note that the EDF blades should be CW and CCW.

 

[Griet]

I don't have the battery dimensions at hand, but I'm having the plans printed, so I'll be able to see if they fit.- But the weight might be a bigger problem than the size.
Why do you think the EDF unit should be counter-rotating? There is no P-factor in play here, and for my Freewing SU-35 it didn't make any difference when I upgraded the EDF units with 2 identical ones.
I'll be very curious towards your 6S experience with this one!

 

[iwpublic]

Well I'd go for the CW+CCW setup because it feels right.
According to research there are positive effects CW+CCW which are, however, not outweighing the disadvantage of spare parts diversification.
Bottom line the 2xCW setup is widely accepted but there are plenty examples with counterrotating setups.

As far as the 6S weight is concerned I am going to reinforce the fuselage with carbon fibre - alone because the base material I am using (Depron) is not as stiff as the Flitetest foam board.

 

[Griet]

The only effect to counter here that I can think of, is the gyroscopic precession from the EDF units. When you have a counter-rotating set, this effect of both units cancels each other out. The question now is how big this gyroscopic effect really is, is it big enough to worry about, or to even notice?
But anyway, I have my units already, so no dilemma here

 

[Flitedesign 3d]

@iwpublic The problem you are having with misalignment is because you are using foam of a wrong thickness, looks something like 2mm? The reason behind this is a sneaky one and it's a bit difficult to catch. When you bend foam board, the outer surface, i.e. the paper surface, stays dimensionally correct, whereas the inner surface gets compressed together, hence reducing the "circumference" you measured to be too great. In other words, the outer surface in plans is stretched out compared to the inner surface which mates with the formers, with the assumption that 5 mm foam is used. To fix your issue, I would suggest recutting the formers with the correct offset on the outer contour.

@Griet If you build it stock but unlike, me don't remove the papers on critical surfaces such as wing spars, bottom wing surface and fuselage box section, I would guess it's plenty strong, assuming you don't fly completely mental But go ahead with the build and you tell us! For the fans, I don't use contra-rotating fans. When I hold the plane in my hand and quickly accelerate, I can feel the torque, but it's nowhere near enough to cause problems in practice

 

[Griet]

But that's the inertial reaction torque you're describing, that shouldn't even matter when launching, as the engines are at constant thrust setting then. I was wondering about the steady-state torque, the precession of the spinning rotors. They should show a reaction torque 90° displaced from the applied torque, so pitch moment should result in a yaw moment for instance. Just wondering....
But anyway, I'm still waiting for the printing of the PDF sheets, as the plotter at the office seems to be kaputt, won't initialize properly anymore. I tried the tiled A4 sheets, but there is a serious scaling/misalignment problem there.

 

[iwpublic]

@iwpublic The problem you are having with misalignment is because you are using foam of a wrong thickness, looks something like 2mm? The reason behind this is a sneaky one and it's a bit difficult to catch. When you bend foam ...

Gosh you're right. Changed from 2mm depon to 4mm as 2mm would need at least 3 layers of glassing (29g) and its still not safe to say it would cope with belly landings.
And with 4mm - surprise surprise - the holes are close to match. So what you said makes perfect sense to me. Thanks

 

[Flitedesign 3d]

A4 sheets, but there is a serious scaling/misalignment problem there.

What's wrong with them? Which sheet? I just printed sheet 1 just to check, it's fine. Are you printing them on "actual size"? Did you notice the cut marks to trim off the margins? Regarding the torque, I didn't mean reaction torque, it's minuscule compared to the planes roll inertia. Also I don't think precession will be an issue unless you mount a rotary engine to your plane. What I meant was the sudden change in propeller torque, but once again, according to my experience it's not really relevant regarding the models performance

 

[Griet]

If you just heard a hand slapping a forehead with a slight sonic boom, that's me. I didn't even notice that my printer defaulted to a "fit to page" scaling. I now printed sheet 2 with the "actual size" setting, and now it does align perfectly. Thanks for pointing it out to somebody who thinks she's really getting too old for this sort of thing