3D Printed WWII B-17G

[DeepEndofPool]

Math day:

Let's start with the good news. I wasn't calculating wing area correctly (I didn't include the wing root) so the wing area is a little bigger and the wing loading a little smaller than I thought.

Here is the wing area calc from ecalc.ch/cgcalc.

And here is the wing loading, cubic loading and stall speed assuming the same weight, 2600g, as estimated in an earlier post.

Wing loading is down from 78 to 73.

The next thing to tackle (numbers wise) is the props. The distance from the inboard nacelle center line to the outside of the fuselage is 95.6mm (3.75"). The distance between the nacelles CL to CL is 181.2mm (7.13"). So the max prop diameter is 7". For authenticity's sake I'd like to use a three bladed prop. MasterAirscrew has 3-blade props in 5, 6 and 7 inches. If anyone has other suggestions, please speak up.

Assume the 3-blade 6x4 for now. eCalc says that if those props spin at 18,000 RPM it will generate a total of 4100g of thrust and will pull the plane through the air at 61 mph.

I would like to use a 3S battery setup if possible. The reason is that a 3S battery of a given mAh rating will be smaller and lighter than a 4S or 6S battery of the same mAh capacity. I'm not expecting size problems, but this bird is already heavy. If I can save some battery weight that would help.

Fiddling with these values in eCalc it looks like the motors need to be around 350 watts maximum and around 250 watts continuous and have a KV of about 2200 with a 3S battery.

One other motor parameter that's a non-issue is the motor diameter. The inside of the nacelle is 59 mm in diameter so a wide selection of motors will fit. Depth is currently 29 mm but that can be easily modified if necessary.

So I have motor power, motor size (max) and KVs for different number of cells. Using the db built into eCalc I found the RCinPower QAV 2206-2200 which seems to have the best performance balance.

The last parameter to look at is the total drive weight. Again, fiddling with eCalc different choices swing the drive weight anywhere from 700 g to over 1600 g. My earlier estimate of total weight was based on a drive weight of about 1200 g, but tuning the setup in eCalc that number comes down to 723 g, a savings of 477 g.

Reducing the weight by 477 g also reduces the wing loading. The new wing loading is now 60 g/dm2 which is less than 3DPrintLabs' P-38.

Interestingly enough if the props are changed to 2 blade (still 6.0x4.0) the plane picks up 4 MPH on the top end and the motors run 40 W less power, but thrust to weight drops from about 1.9 to 1.7.

Of course this is all just pushing numbers around in a model. I'd be very interested in people's feedback of where I've done something that won't work in reality.

Thanks.

 

[DeepEndofPool]

I was curious about putting the motor in the nacelle. So I found the motor specs on the web and modeled it in Fusion. Here is the result.

Well if the model is under powered we can always add a second motor to each nacelle.

 

[wilmracer]

So I'm crunching your numbers and that CG looks about right.

Looks like you're on the right path with the power setup too. I will say that she won't NEED 1.9:1 or 1.7:1 thrust to weight to fly. We all want power to get out of trouble, but even 1:1 is overkill on a bomber It might be worth taking a second glance at the power setup to see of you can go with something with a little less punch that would save some weight. I think she would fly with tons of power and be fun on your setup, but if you're worried about weight and wing loading you're packing extra umph that might be able to translate to weight savings. Just a thought... FWIW I always overpower my models too ;-)

 

[DeepEndofPool]

she won't NEED 1.9:1 or 1.7:1 thrust to weight

Yeah, I thought the same thing. I wasn't pushing for thrust/weight as much as for top end speed. I read somewhere that top should be 2.5 to 3 times stall speed. So with the initial stall calculated at 24 mph I was pushing for a minimum of 60 at the top end. Given that the prop was only available in one pitch that lead me to the 2200 KV motor selection. The motor current drives the choice of ESC and battery.

motors: 4x 29 g = 116 g
ESCs: 4x 22g = 88 g
Battery = 453 g

The weights of individual motors and ESCs seem reasonable. It's the fact that there are four of them that push up motor and ESC weight as well as required battery capacity. Flight time is already marginal so I didn't want to go any lower mAh. If anything, I'd like to push that up a couple of minutes.

I suppose you could run it with only two motors, put them in the outer nacelles and run bigger props, but that compromises the fidelity to the original B-17.

With the reduced weight, the stall speed is now down to 22 mph. I will look at dropping the top end to 55 mph to see if that will help. I will also go back and run the numbers with the 5 and 7 inch props and see if either comes out lighter.

Another idea would be to drop back to 2-blade props. That might put the motors closer to their optimum efficiency which will reduce current and extend flight time or allow a lighter battery. I could design for that and still put on 3-blades for show.

The nice thing is that none of this impacts the airframe so we can try different ideas without derailing the 3D design process.

Thanks for the coaching!

 

[wilmracer]

Another idea would be to drop back to 2-blade props. That might put the motors closer to their optimum efficiency which will reduce current and extend flight time or allow a lighter battery. I could design for that and still put on 3-blades for show.

Loads of good math there and it makes sense. And I'm with you on the desire to use 3 blade props. That always complicates things but it REALLY adds to the look of the model. One option, that is a PITA, is to cut down larger 3 blades. It opens up more options for props to start with. With a 4 engine model it will be a LOT of work... 3 blades per prop, 4 props, and balancing. I wouldn't worry about that for your prototype, but for a v2 or v3 where you want to go nuts with detailing. 2 blades for the prototype make sense to me, but its up to you!

 

[DeepEndofPool]

to go nuts with detailing

Retracting gear. Working bomb bay. Swiveling gun turrets. FPV. I think I better concentrate on making it fly first.

 

[foamtest]

This is a very interesting thread and I can't wait to see what comes of it. I will be watching for sure.

 

[localfiend]

I think what's more likely to drive motor choice is weight for balancing. 3d printed planes are going to have a more aft CG and you'll need weight up in front. I say plan for heavyish motors until you get a prototype printed and see where your CG ends up.

Also, if you need more prop choices, consider 3D printing.

Here's an 8x5ish 6 blade I 3D printed out of polycarbonate for use with a C-pack motor. It's incredibly strong with this material, and a bomber would be a good choice for scale type printed props. Nice when you can make whatever specs you want, and be able to get counter rotating versions with no hassle.

 

[DeepEndofPool]

Wow! That 6-blade is wicked! I was wondering if printed props were a possibility. Are there designs around or do I need to put something together myself? Fusion's parametric design capability seems like a perfect fit for a prop template where you just fill in the form.

For balance I was thinking of pushing that big battery as far forward as possible, but heavier motors are a good suggestion as well. If you (or anyone) has specific motor recommendations, please pass them along.

Thanks!

 

[DeepEndofPool]

Disclaimer: Some people like 3D printed props. Others feel that they are dangerous. I’m not qualified to speak to that or give any sort of recommendation. Therefore use anything here (idea, design, STL) AT YOUR OWN RISK. I will accept no liability whatsoever.

But for your amusement…

Okay, I admit it. I got distracted. The talk about 3D printed props made me wonder if the parametric design in Fusion 360 could be used to build a prop from just four numbers, prop diameter, pitch, shaft diameter and number of blades.

I started trying to translate this design into Fusion.


This is a wooden prop design, but it shows the airfoil shapes along the length of the blade. Unfortunately it didn’t translate very well. First, Fusion didn’t want to loft the collection of shapes correctly. The loft tool kept throwing errors. It took a couple of restarts, but finally I got it to work.

But when changing the prop diameter Fusion did some pretty weird stuff.

Part of the problem is that Fusion did not seem to rotate the splines correctly when changing the pitch. For my third prototype I went back to a much simpler design using only three profiles, root, middle and tip, for the blade. It gives up on splines and uses ellipses and arcs to approximate the airfoil shape. This seems to work better but not perfectly. I’ll come back to that in a bit. But first, here’s a 6x4 prop from the basic design.

Need that to be a 9 in pitch with a quarter inch shaft? Just change the numbers in the table.

How about a 3 blade 6x4 with a 3mm shaft?

Need that in a seven blade with a quarter inch shaft? No problem.

Each design generates both CW and CCW versions.

But there are still issues. The base design is 6x4. If you jump that to 12x6 or even 12x4 Fusion will either throw an error or distort the design. Interestingly enough, if you go from 6x4 to 8x4 to 10x4 to 12x4 there doesn’t appear to be a problem. I don’t know if this is because there is some error in my equasions or some bug in Fusion. Also, if you change from 6x4 to 8x4 and back repeatedly, it will eventually distort the design.

If that’s not enough, the STLs show up micro-sized in the slicer, probably having to do with the parameters being in inches. You just have to scale them back up (2540%), but it’s annoying.

The attached STLs are for you to play with since I don't have a printer. Totally untested and not well placed in the slicer. USE AT YOUR OWN RISK. But if you do print them, please share a photo. Thanks.

I think I'm done with props for a while.