Thread Tools
Sep 10, 2019, 05:53 AM
Registered User
Same with my experience with the sullivans, support around where the bends are located is really important to not allow flexing of the exterior sheath.

shame about the 90mm F-117...

I've just bought a VERY high performance 90mm equivalent fan, that would give it sparkling performance...
Sign up now
to remove ads between posts
Sep 11, 2019, 10:47 PM
Registered User
Dirty Dee's Avatar
Thread OP
Working on the forward fuselage now, and thinking about the wing spar. Currently I am planning on having a single spar that cuts through the ducting in front of the fan. Another idea I am considering is doing a rigid "donut" bulkhead and not having the spars cut into the ducting.

I first will do some thrust tests and see if the effort is worth it. I'm hoping to have the remainder of the fuselage ready to test print by the weekend.
Sep 11, 2019, 11:11 PM
I fly, therefore, I crash!!!
SteveT.'s Avatar
That has always been the 'bugaboo' with designing a U-2. I have considered machining an aluminum yoke to go around the front of the fan. While I do have a mill, I don't have a CNC mill, but my good flying buddy does.

SteveT.
Latest blog entry: My shop....
Sep 12, 2019, 10:31 AM
Registered User
Dirty Dee's Avatar
Thread OP
I know that cetorca's scratchuild used a sandwiched former without cutting into the ducting, so It might not be too big of a problem unless high-g or aerobatic maneuvers are planned to be performed.

that spar cutting through the ducting is a bit of an eyesore, so I'm likely to try out the 3D printed donut former idea anyway.

https://www.rcgroups.com/forums/show...0-lockheed-U2R


If you also compare against the Phase3 U2 which is a tad bigger than my design, it only had (3) 3mm carbon spars that stuck out ~1-2inches. And it was just a friction fit to the fuselage.
Sep 12, 2019, 02:36 PM
Design.Build.Fly
phantomphan's Avatar
Hi Chris, I think you could use the former and blade spar design.
A simple mock up of the former/s and blade spar design is all thats really needed to test the strength of the mating parts before committing to printing full/complete fuselage parts.

Starting with what we know.
Aircraft 1200g (2.64 lbs) max target weight. Assume tube fuse creates zero lift.
600g (1.32 lbs) carried by each wing.
Approx. 300g (.66 lbs) by each spar. (When using two wing spars this is generally not a pure 50/50 split and varies depending on the X distance of the spars relative to the wing CL along the MAC.)

1. We need to know the distance from the wing root to the wing MAC.
With that number we can multiply by .66 lbs to see how much torsional force is on the mating surfaces at 1 g. ( F x D)

2. We can use that number multiplied by some g-force, like 9, to give a target for how much force the former/s and spar/s need to be able to support.

3. Next a test former and spar can be made up. I tend to lock down the former on a bench, mate the spar and then hang weight on the spar where the MAC would be.

This is about as simple as I know to make it without getting silly complicated.
If using carbon blades, plenty strong, this should show if the printed material tries to yield or not.
If the test parts are kept simple, additional structure from the wings and fuse generally add strength and thus add more safety factor.

Ref:
https://www.rcgroups.com/forums/show...-35-90mm/page5
Post 71.
Last edited by phantomphan; Sep 12, 2019 at 03:05 PM.
Sep 12, 2019, 02:51 PM
Registered User
Dirty Dee's Avatar
Thread OP
Oh wow, thanks for that technical detail! I had to skim a bit since I have my rcgroups set to 100posts/page, but I think I found your reference as post 71?
https://www.rcgroups.com/forums/show...3&postcount=71

I'll try your calculation tonight and post the data.
Sep 12, 2019, 02:53 PM
I fly, therefore, I crash!!!
SteveT.'s Avatar
I could also laser cut plywood formers for those particular ones if needed..

SteveT.
Latest blog entry: My shop....
Sep 12, 2019, 03:04 PM
Design.Build.Fly
phantomphan's Avatar
Quote:
Originally Posted by Dirty Dee
Oh wow, thanks for that technical detail! I had to skim a bit since I have my rcgroups set to 100posts/page, but I think I found your reference as post 71?
https://www.rcgroups.com/forums/show...3&postcount=71

I'll try your calculation tonight and post the data.
Your welcome. Yes, post 71. ( I meant to add the post number and forgot )
Sep 12, 2019, 06:02 PM
Registered User
Dirty Dee's Avatar
Thread OP
Steve, a nice tool to have in the arsenal . Since I will be releasing this design to the public, I may first attempt a 100% 3D printed bulkhead (integrated to the fuselage or glued in). But I may reach out to pay for some cutting services if I change my mind.


Here is my MAC calculation. So short and sweet, just hang weight from the MAC 317mm from centerline to failure or 9Gs?

With worst case of 1200g AUW, 600g/G

9G=5600g/12.34lb
Sep 12, 2019, 08:27 PM
Design.Build.Fly
phantomphan's Avatar
I have to convert to imperial.
317mm = 12.5in = 1.042ft
600g = 1.32 lb
1.042 * 1.32 = 1.375 ft*lbs
1.375 * 9G = 12.375 lbs

But yeah. Close enough. Just hang the weight from the 317mm mark
Sep 12, 2019, 08:49 PM
Design.Build.Fly
phantomphan's Avatar
Actually I just realized something... I should have looked at the drawing more closely.
For this calc there is no need to include the fuse width in the MAC calc. If we just do the MAC on the wing panel the amount of force on the structure should show a reduction.
The ref for hanging the weight would be from the wing root.
Sep 12, 2019, 08:56 PM
Registered User
Dirty Dee's Avatar
Thread OP
Ok, so (263.12mm) 0.863ft for the lever arm length.

which gets 10.25lbs.
Sep 13, 2019, 06:39 AM
Design.Build.Fly
phantomphan's Avatar
Close enough
If the structure can handle that I have very high confidence the U2 wont become a lawn dart.
Last edited by phantomphan; Sep 13, 2019 at 07:01 AM.
Sep 13, 2019, 10:39 AM
Design.Build.Fly
phantomphan's Avatar
Hi Chris, I need to try and clarify. For calculating only wing loads we dont need to include the fuse width in the diagram. In other words, dont extend the wing to the fuse center line. We are basically just looking for the point that represents the center of pressure on the wing panel.

Or. Imagine that the fuselage is rigid or fixed and the force on a wing panel wants to rotate it about a fulcrum, in our case that point being where the wing panel meets the fuse. We know that the force, lift, on the wing panel is distributed along the span of the wing panel, but by calculating or diagramming the MAC, of just the wing panel, we can simplify that force (CofP) to a single point and/or line of reference.

In post 54, your drawing above includes the fuse in the MAC diagram which is not needed for the purpose of calculating load on just the wing. However, because the fuse is relatively narrow compared to the overall wing span this isn't that big of a deal.
I think your aware also, given the AirBus project, that the way the MAC was drawn is a simple way to include a narrow low lifting fuse into the overall aircraft CG without having to calc/estimate both the wing and fuselage center of pressure. For aircraft that have wide lifting bodies the CG diagram in post 54 will typically fail us. Ive ran into this on an F4, 16, and 18 where model manufactures didnt fully test their products and I ended up with bad CG info.

If I may, lets use my F35 with its lifting body and stubby wings as an example. The wingspan is about 42in. The fuse width at the wing roots is about 16in. Yes, 16, no typo. A wing panel is about 13in wide, and they are plug in employing a .25in birch ply spar.
The MAC line for just the wing panel is about 4.5in from the wing root. This 4.5in is important for a couple of reasons. 1) It lets me estimate the CofP and CG just for the wing panel so I know where to put the wing spar & simplifying my design. 2) I could estimate the rotational lifting force just on the wing panel which let me build and test a much lighter wing.
In the first pic attached you can see the estimated CG point of just the wing, not the fuse or the whole aircraft. The CG of the whole aircraft is very close to where the anti-rotation pin is drawn on the wing root.
In the second pic you can see how the wing was actually built, eliminating the leading edge flaps. The wing is a monocoque design and at 9G its estimated to be only carrying about 10 lbs or so.

I know this is long winded, and I hope it makes sense, but if you get to building something like an F117 it can be very helpful having an idea of the forces involved.
Last edited by phantomphan; Sep 13, 2019 at 10:47 AM.
Sep 13, 2019, 04:13 PM
Registered User
Dirty Dee's Avatar
Thread OP
Ok, I think I understand what you are explaining.

For the structural design, I just take into account the wing panel with determining the MAC, and that CP point for the purpose of load testing?

Most of my designs have used the TLAR approach, so I may be a bit slow with understanding this stuff


Quick Reply
Message:

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion 3D printed 120mm EDF - 3D printed 1:6,5 scale L-39 Lynxman Large EDF Jets 1575 Today 02:36 PM
Discussion 3D printed 1:1 scale sub dll932 3D Printing 0 Jul 24, 2017 12:05 PM
Discussion 3D printed 1/18 scale parts srcampb Scale Boats 2 Aug 27, 2016 06:51 AM
Discussion U.K. Based 3D printing job Davefire 3D Printing 1 May 18, 2016 01:15 AM
Help! 3D printed 1/12 scale pan racer Jolijar On-road Cars 7 Mar 25, 2012 03:55 PM