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FTFC'18 WWII Junkers EF-112 designed by DamoRC

DamoRC

Well-known member
Mentor
#1
I should be finishing the Reichenberg (plans are finalized and final build will get painted this weekend) but I had to get this prototype out of my system.

First came across this on the original draft competition thread when willsonman posted a few suggestions for entries. One of them was the Junkers EF-112. Although the three view on the Luft46 site looked interesting, it was the following image by Gino Marcomini that really made me want to try this.

JU EF112-9.jpg

So I used this image as the basis for a model in Sketchup (dodecagons rule!)

Sketchup.jpg

And here are some pics of the first rough prototype build. Some issues here, but nothing that can't be easily fixed. Will hack in some servos (motor mounts front and back are already installed) and might get her up this weekend.

IMG_2944.JPG IMG_2945.JPG IMG_2946.JPG
 

DamoRC

Well-known member
Mentor
#4
Quick update. Finished up the hack job on the servos and populated the battery bay with two ESCs and a parallel battery harness (one battery driving both ESCs). Running 2 x NTM 2826 1200kV motors swinging 9x6 props (pusher prop on the rear so I can run the motors in opposite directions). Not sure what these equate to in Flite Test power pack motors (probably C-Pack?)

1-hatch.JPG 2- hack job.JPG

I don't want to wing launch the plane so the rear ESC is connected to the gear channel and I am using two mixes on a Dx6i so that with gear switch off, the rear motor won't turn (for launching) and with gear switch on, the rear motor will track the throttle.

DamoRC
 
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#5
Really cool project.

If you have two power meters you might want to try bench testing the power draw of the motors running in tandem. Since the air is compressed ahead of the rear propeller it effectively reduces the pitch and you'll probably see the rear motor drawing less power. If that's the case you could get more speed/efficiency out of it by putting the 9x6 pusher on the front and a 9x8 or 9x9 on the rear if you wanted to.
 

DamoRC

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Mentor
#6
Thanks Joe - I hadn't thought about it that way - I'll do some testing per your suggestion.

Unfortunately, I didn't get to try the current twin motor setup today. I don't have a vid, but it would have been a very short one. Launched her with front motor running only and never got a chance to turn on the rear. Flew like a rock with a prop stuck to the front. Seemed tail heavy, pitch sensitive, and twitchy in the roll axis. All three combined meant that I fought with her for about 10 seconds, then ploughed her into the field.

post maiden.JPG

Oh well, back to the drawing board. I think I will scale her up from the current 1/15 to 1/12 or 1/10 to make life a little easier.

DamoRC
 

willsonman

Builder Extraordinare
Mentor
#7
Ouch! I remember doing this one in RealFlight years ago (2011). Never could get it to fly quite right. I looked though my old files and can no longer find the files for it. I DO remember that I ended up using and inverted airfoil on the tail for some reason.... which escapes me. Keep going... don;t let your failure, or mine, get you down!
 

Namactual

Well-known member
#8
Doh! :black_eyed:

Sorry about the maiden, but that is one cool looking aircraft. That will be a good contender once you get it dialed in. Looks straight forward to build eye catching as well.
 

rockyboy

Skill Collector
Mentor
#9
Hmm... looks like the nose aerodynamics are off a little on that last picture. :black_eyed:

Did you get a chance to do any glide testing with it before the powered launch? It can really help to confirm if you've got a fighting chance with CG if it'll actually glide when thrown.
 

DamoRC

Well-known member
Mentor
#10
Ouch! I remember doing this one in RealFlight years ago (2011). Never could get it to fly quite right. I looked though my old files and can no longer find the files for it. I DO remember that I ended up using and inverted airfoil on the tail for some reason.... which escapes me. Keep going... don;t let your failure, or mine, get you down!
Now I'm worried - if you couldn't get to fly in silico - :(

Did you get a chance to do any glide testing with it before the powered launch? It can really help to confirm if you've got a fighting chance with CG if it'll actually glide when thrown.
Nope - I pretty much never glide test. I typically go pretty nose heavy to start and dial it back.

I think she'll be fine at a slightly larger scale - I need to do the calcs, but the wing loading with two motors, 2 ESCs and a 2200mAh battery was probably way to high for the wing. Also a larger scale should help me make her look a little leaner and de-emphasize the nose ring which kinda makers her look like a duck mid-quack.

Thanks for the suggestions and encouragement. Should be good to try again in a couple of weeks.

DamoRC
 
#11
Ouch, that's an unfortunate maiden.

Looking at the design I wonder if the aerodynamic center (center of drag) of the aircraft is too far forward of the center of pressure for stable forward flight. Sorta like shooting an arrow with no fletching. This would make it unstable regardless of CG.

The more I look at this project the more I want to build one of these, it's a really cool plane. Good luck with the next one.
 

DamoRC

Well-known member
Mentor
#12
Ouch, that's an unfortunate maiden.

Looking at the design I wonder if the aerodynamic center (center of drag) of the aircraft is too far forward of the center of pressure for stable forward flight. Sorta like shooting an arrow with no fletching. This would make it unstable regardless of CG.

The more I look at this project the more I want to build one of these, it's a really cool plane. Good luck with the next one.
Okay - now I am going to have to do some reading!

Seriously, thanks for the comment Joe. When you look at the plane can you tell me what aspects of it make you think this? Do you think it's something I can design out or around?

DamoRC
 
#13
So I'm sure there are plenty of people way more qualified than myself on these forums when it comes to this stuff, but here I go.


Basically the aerodynamic center (AC) is the central point at which you could quantify all of the combined forces of drag and lift of the airplane. Sort of like how the CG is a point at which you could quantify all of the combined forces of the weight of the airplane.

You calculate it much the same way that you'd calculate the center of pressure except that you take everything into account: the wing(s), fuselage, empennage, landing gear, radiators, etc; All of the surfaces, cross-sections, and transitions.

Like how you want to keep CG forward of the center of pressure to maintain longitudinal stability you also want to keep the AC aft of the center of pressure.

The arrow analogy is probably the easiest way to conceptualize it. An arrow with fletching flies straight while and unfletched arrow tumbles. The fletching moves the AC of the arrow back behind the center of pressure which makes it dynamically stable. It just sort of "wants" to fly straight.

This was a really common problem on experimental performance aircraft from the 30's and 40's. The first example that pops to my mind is the Gee Bee Model R. I suspect that the Bugatti 100p was also designed with an unstable AC.

There are a bunch of easy ways to move the AC aft and increase stability. You can reduce the cross-section of the fuselage or remove/ease surface transitions forward of the CG, or increase the cross-section of the fuselage/booms aft of the CG, or shorten the nose, or lengthen the tail booms, or enlarge the tail assembly.

Hope that helped!
 

willsonman

Builder Extraordinare
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#14
Just to make a comment on the Bugatti...

The AC, CoP, and CG are very close but the CG is forward. The elongated nose moves the AC forward as well as the forward sweep of the wings. The aerodynamics of these points is very tight, which leads you to believe it would be very unstable in flight. What aids in this is the large tail surface areas in relation to the main wing area. Lots of surface area there and then factor in the inward flow of air from wing tip to root induced by the forward sweep... this directs are over the tail surfaces more efficiently. Its a complicated bird for sure but I'll give my $0.02 on the EF-112...

My personal opinion (recall from my work on the RF physics) is that the nose is just too long. The elongated nose moves the AC so far forward that is forward of the CG where typically in this planform would be around 33% of the MAC of the wing. In the real-world there would be quite a bit of weight in the rear with the second motor that could offset the weight of the front motor but that weight distribution is contrary to the aerodynamic design of the views. My previous mention of an inverted airfoil on the tail plane was to serve as two purposes... create higher conflicting drag between the main wig upward lift and downward lift of the tail and this increase in drag would move the CP rearward.

My gut feeling is that if this subject were to go into more analysis for a mockup the designers would have realized this. Seeing the issue they would have done one or both of two things: 1) increase the tail volume [increase tail stabilizer area] 2) move the entire fuselage aft in respect to the main wing. Looking at the views... I would move the fuselage aft no less than where the canopy front edge meets the fuselage. More if you dare place the rear prop that close to your H-stab. I would add V-stab area to the front, not the rear and see if you can add some additional H-stab chord accordingly (again depending on how close you dare place the rear prop to it).

It is an interesting aerodynamic subject for sure.
 

DamoRC

Well-known member
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#18
Thanks for the suggestions JoeM and Willsonman - I think I understand what you are getting at - the arrow analogy helps a lot. So basically I need to reduce drag in front of the CG or/and increase it behind the CG so I am not trying to fire this arrow feathers first.

I think there is a lot I can do to increase the rear drag. Not sure I want to give up on the overall look (long nose) just yet, at least not for the next prototype but there are probably a couple of tweaks that I'll try.

Thanks again for the advice!

DamoRC