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PrandtlLight: Pusher-Propeller Box Wing PrandtlPlane Plans

PrandtlLight: Pusher-Propeller Box Wing PrandtlPlane Plans 2020-07-06

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#1
RCPrandtlPlanes submitted a new resource:

PrandtlLight: Pusher-Propeller Box Wing PrandtlPlane Plans - Complete plans for a foam board Box Wing PrandtlPlane

This RC airplane features an unusual design known as a Joined Wing, Closed Wing, Box Wing, or PrandtlPlane configuration. Rather than a traditional horizontal stabilizer that creates a downward force, there is a second wing at the rear of the aircraft, providing additional lift. The winglet surfaces joining the two wings theoretically reduce induced drag, but more importantly they provide improved structural stiffness over a cantilever wing.

The difference in aspect ratio, sweep, and...
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#3
@dh04000 I haven't done any measurements of power draw myself, but according to these results it should be a little more than 570W. I'm using a 5.1x5x3 prop, which should be pretty close to the 5.1x4.9x3, just slightly more amps. Of course this is way overpowered for this plane-- I almost always fly it on low rates with the throttle limited to 75% max, and cruise at about one third to half of that reduced maximum (~25-37%).
 
#4
@dh04000 I haven't done any measurements of power draw myself, but according to these results it should be a little more than 570W. I'm using a 5.1x5x3 prop, which should be pretty close to the 5.1x4.9x3, just slightly more amps. Of course this is way overpowered for this plane-- I almost always fly it on low rates with the throttle limited to 75% max, and cruise at about one third to half of that reduced maximum (~25-37%).

WOW! Almost 600 watts? I have a 160 watt on my nearly 1100 gram plane! Holy overpowered Batman!

How many watts do you think would fly reasonably for your setup? I am leaning towards 200 watts.... But due to how the motor mount is attached, it'll be painful to remove it I am wrong about the watts needed.

I also want to size down to a 3S 1300 or 2200. Probs 2200, since I want to add an "extra power draw". I'm being cryptic on purpose. ;)
 
#5
Yes, it is rather absurd, but it does make hand launching a breeze-- at full throttle I can simply hold it over my head and let go, and it's in the air.

200 watts certainly should be enough power for normal flying, without needing to go full throttle all the time. The real reason mine has so much power in the first place is just that the Emax 2306 motor was so cheap ($12) that I couldn't resist. Plus the small-diameter 3-blade prop isn't the most efficient.

Going up to a larger two-blade prop would definitely help, as long as it's under ~7" so it doesn't extend below the descending vertical fin. Although if you have some extra foam board, making a longer descending vertical fin to protect a larger prop shouldn't cause any problems.

If you want to make the motor easier to remove, you could make two of the plywood motor mount plates, with one glued to the plane, and the other attached to the motor. Then, drill additional holes, and use wood screws to attach the pieces of plywood together. The additional weight should be negligible.

I'd recommend going with the larger 2200mAh 3S, largely because it will be closer to the weight of the 1300mAh 4S in mine, and you need the weight to balance out the motor. My battery isn't all the way at the front of the nose, so putting a lighter battery farther forward might be fine, or you could end up needing balance to get the right CG.

I have a theory about what you mean by "extra power draw." I can't wait to see it!
 

FoamyDM

Building Fool-Flying Noob
#6
With an estimate of 100w/# of plane.... how heavy? honestly this isn't a knock. even at the full acrobatic estimates of 200-300w/#, that brings this plane to about 2lb for acrobatic flying!
I know my Arrow is WAY overpowered. and that can be fun. I looks to be powered off of the F-pack setup.Should lead for Fun flying, yet be able to fly off the A-pack 1806 2300kv 3S setup! Nice work
 
#11
@speedy7385 Sorry to hear about the crash, it looks like a nice build. Where was your CG located? Did it seem stable without control inputs? The pitch sensitivity could just be due to the size of the control surfaces, and may need reduced rates depending on how your linkages are set up-- how much elevon deflection do you have?

I really appreciate you taking the time to try out the design, it's awesome to see someone else build one.
 
#12
@speedy7385 Sorry to hear about the crash, it looks like a nice build. Where was your CG located? Did it seem stable without control inputs? The pitch sensitivity could just be due to the size of the control surfaces, and may need reduced rates depending on how your linkages are set up-- how much elevon deflection do you have?

I really appreciate you taking the time to try out the design, it's awesome to see someone else build one.
Thanks for sharing your plane.
Cg was set as your recommendation, think it was like 8 1/4" from back end of canopy. Had 40% expo, throw was set at 100%. Size of elervon was cut as per plan. I probably need to reduce the control service travel. Thanks for the info. Will try again after rebuild.
 
#14
Thanks for sharing your plane.
Cg was set as your recommendation, think it was like 8 1/4" from back end of canopy. Had 40% expo, throw was set at 100%. Size of elervon was cut as per plan. I probably need to reduce the control service travel. Thanks for the info. Will try again after rebuild.
You're very welcome. I just checked my controller and I have it set to 70% rates, 30% expo. My pushrods also go through the third hole from the end in the servo horn, so you may be getting a lot more control surface throw, especially if you have the pushrods further out on the horns. I haven't flown mine at 100% rates in a long time, but I do remember it being pretty twitchy. Glad to hear she'll be rebuilt!

@dh04000 Looking forward to seeing yours as well when you get to it.
 
#15
You're very welcome. I just checked my controller and I have it set to 70% rates, 30% expo. My pushrods also go through the third hole from the end in the servo horn, so you may be getting a lot more control surface throw, especially if you have the pushrods further out on the horns. I haven't flown mine at 100% rates in a long time, but I do remember it being pretty twitchy. Glad to hear she'll be rebuilt!

@dh04000 Looking forward to seeing yours as well when you get to it.
Yes, my rates were set at 100% 30% expo, servo horn on second outer hole and control surface horn was on the outer hole.
 
#16
I just finished mine everything went fairly well in the build thanks for such good instructions. I wanna say though that this isnt the easiest build ever and that the picture makes it look smaller then it is. Mines around 40"WS and i was thinking in the pics it was more like 20ish. Then i saw the plans and i was like oh this is only a wing half :eek:. hoping to get it painted and flying after all this snow dries up.