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RE: UIUC AIAA Design/Build/Fly Project - 4/11/2004 8:57:27 PM   
nichanderson


 

Posts: 21
Joined: 11/9/2003
From: Pomona, CA, USA
Status: offline 		We were heavily considered using a core, and in essence used the Kevlar as a relatively heavy core material on the lower half of the fuselage. We picked up both 1/8" thick nomex honeycomb, and 1/2" thick nomex flexcore honeycomb, but both are too heavy for the type of strength we were needing. We tried bending balsa to fit the heavily contoured fuselage and weren't able to get it to work. We could have tried steaming it but didn't. We were also considering the "coremat" or Baltekmat that is a synthetic fiber embedded with glass microspheres but that was also to heavy. Now we are seriously considering drilling weakening holes, I meant lightening holes in our fuselage.

Buying the epoxy on site is a good possibility, but the availability of a 'good' laminating (i.e. non hobby shop epoxy) resin would be difficult to find.

Nick Anderson

(in reply to Tall Paul)
       Post #: 126

RE: UIUC AIAA Design/Build/Fly Project - 4/11/2004 11:18:08 PM   
valkyrie1983


 

Posts: 23
Joined: 8/2/2003
From: Urbana, IL, USA
Status: offline 		Well, we flew again today. Things went pretty well. First flight was unloaded to test out a couple of modifications to the airframe. 2nd flight carried 0.5 L of water. The dump system worked well and sloshing was not a problem. 3rd flight carried 1 L of water. The dump went well, but there were some strong gusts (Stupid Illinois winds) when we came in for landing. It was a pretty rough landing, we dinged up the plane a bit. Bent gearbox shaft and some broken sheeting around teh battery compartment. Its still in pretty good shape though, hopefully we'll be up again tomorrow if the weather is nice. Keep checking the pictures and video sites, there might be some put up of the dump sequences.

Geoff

(in reply to nichanderson)
       Post #: 127

RE: UIUC AIAA Design/Build/Fly Project - 4/12/2004 2:48:50 AM   
Jeremy Sebens


 

Posts: 91
Joined: 4/22/2002
From: Champaign, IL, USA
Status: offline 		Nick, you might look into segmented Rohacel cores. Rohacel is a really stiff, light foam that's used a lot for core material -basically, really strong air . By segmenting it, you gan get it to conform to your fuselage shape, and simply fill the gaps between segments with a resin/microballon slurry. Rohacel is by no means cheap, but if you were considering nomex, it shouldn't be too much of a shock... CST carries it, and they should be able to get it to you guys rather quickly.

(in reply to nichanderson)
       Post #: 128

RE: UIUC AIAA Design/Build/Fly Project - 4/12/2004 5:50:59 AM   
nichanderson


 

Posts: 21
Joined: 11/9/2003
From: Pomona, CA, USA
Status: offline 		Thanks, the problem is that any core automatically adds weight, and with only 2 layers of 5.8 oz carbon (and an equal amount of epoxy per layer) we had a fuselage that was plenty strong, and reasonable lightweight. All these roachell and honeycomb cores are meant for FULL SIZE applications, (i.e planes that carry people, cars, boats, etc...) not for small, light UAVs.

The tradeoff is simply to high between that extra weight vs. the extra strength involved.

Another issue encountered is the design mission, it doesn't have to fly for years on end with breaking or crashing, at most 20-30 flights, so if you are significantly overloading or fatiguing things it may not matter. After the contest is over, its going to sit around up high on some cabinet like most other engineering projects do after the competition is done.

Don't get me wrong, I really wanted to use the flexcore honeycomb, but once we got in a shape where the fuselage would hold its own form, its was readily apparent that it would be plenty strong enough for what we were doing we wouldn’t have gained anything out of the honeycomb expect, “oh look, we have a nomex-carbon fuselage” Someone needs to start making small UAV grade materials instead of adapting down full size UAV materials.

Where do you people buy your props ? We snapped a prop today, and would like to get one with more pitch than what we had (20x12) so that we can get some more static thrust ? Specifically 20x(14-18)

Thanks

Nick Anderson

(in reply to Jeremy Sebens)
       Post #: 129

RE: UIUC AIAA Design/Build/Fly Project - 4/12/2004 6:32:57 AM   
Mluvara



Posts: 1425
Joined: 1/14/2002
From: San Jose, CA, USA
Status: offline
quote:

ORIGINAL: nichanderson

Thanks, the problem is that any core automatically adds weight, and with only 2 layers of 5.8 oz carbon (and an equal amount of epoxy per layer) we had a fuselage that was plenty strong, and reasonable lightweight. All these roachell and honeycomb cores are meant for FULL SIZE applications, (i.e planes that carry people, cars, boats, etc...) not for small, light UAVs.

...

Don't get me wrong, I really wanted to use the flexcore honeycomb, but once we got in a shape where the fuselage would hold its own form, its was readily apparent that it would be plenty strong enough for what we were doing we wouldn’t have gained anything out of the honeycomb expect, “oh look, we have a nomex-carbon fuselage” Someone needs to start making small UAV grade materials instead of adapting down full size UAV materials.


Rohacell and honeycomb is used on a wide variety of aircraft in R/C and UAV's with very good results. The key is material selection.

Your fuselage could have been built with much lighter carbon, namely 2.5oz, albeit very expensive (~30$/foot). Core does not automatically add weight, and it depends on how you make your layup. A good choice for a fuselage your size (if you really wanted core) would be the core sandwiched between 2.5 oz carbon. However, your one-off plug does make layup interesting as far as the edges and a smooth surface goes. If it was done in a mold, then you could have used something like 2mm rohacell or spider foam (pretty cheap, just have to hot wire it) as a core, trapped between some light glass (2oz) as your inner layer and 2.5oz carbon outer layer. The key is placing reinforcement where you best need it. There are some giant scale racing planes that are 1/4 scale, use 1/8" nomex honeycomb as the core and have like 2-3oz glass on the outer and inner surfaces (Some other carbon reinforcement). The fuselages weigh 2.5 - 3 lbs and there is a lot of surface area. They also hold up to immense loads and vibration.

I wish you luck on your flights.

Michael

_____________________________

Michael Luvara
RCATS - http://www.rcatsystems.com
Luvara Air Racing - http://www.LuvaraAirRacing.com

(in reply to nichanderson)
       Post #: 130

RE: UIUC AIAA Design/Build/Fly Project - 4/12/2004 7:03:58 AM   
nichanderson


 

Posts: 21
Joined: 11/9/2003
From: Pomona, CA, USA
Status: offline 		I've seen it used quite a bit in non-monocoque fuselages in bigger planes, servo trays, bulkheads etc.. but not much of it in actually fuselage structure. The predominant material seems to be balsa covering with a thin, nearly veil layer of carbon or s-glass (or the standby monokote).

We looked into the 2.5 oz carbon, which is almost always woven from 1K fibers. The huge price jump between 3 k carbon (15-20$/yd) and 1 K carbon (50-100$/yd) shows that it has very little use in most applications. For UAV and RC applications it is superb, but the demand is so low that the price is always higher than that of 3 k. Which means that really the only people that use 1K carbon is the hobby industry. Maybe all the carbon fiber hood people will discover 1 K carbon some day and drop the price like they did with 3 and 6 K carbon.

Also unfortunate, if I would have known the amount of money we had available, I would have demanded that we purchase the 1 K carbon as opposed to the cheap 3 K.

We were certainly ready to add a core at the 1 lay-up stage off our unconventional male plug. The flexcore would have fit fine and all, but once we cut the piece and simply felt it with our hands, our minds rapidly changed. The again it was 1/2" thick, and we had no way to plane it down to the correct thickness.

Reinforcing critical areas with honeycomb is a good idea, but the terminology seems to indicate that honeycomb is significantly heavier than that of a wet lay up. Composite fabric units are oz/yd2 well honeycomb is lb/ft3. I think thin strips of carbon in key reinforcing areas is the way to approach it, and depending on the stress involved, honeycomb. Most of the strength of honeycomb comes from the thickness and utilization of mass in the outer skins where the stress is encountered, so you'll end up with a non uniform structure either on the inside or outside, with large patches strategically placed, which seems to me to be a large problem for completing the interior of the aircraft.

Until you start reaching person size airplanes, I can't see a full monocoque structure of carbon/honeycomb being of much use. The fuselage we mold next year will probably be 1 K carbon, with the sevelged tape 3 K carbon laid 2-3 layers thick in specific areas where stress is to be encountered. We have a tremendous stockpile of 150 g/sm pre-preg uni carbon, but so far have not figured out a way to construct molds that will withstand 250F and 14 PSI. (vac bagging).

When it comes down to it, Balsa is probably the best core material out there, but honeycomb is so much more "cool" ;-)

BTW, I posted most of our attempted flight pictures from today here:
http://calpolydbf.com/gallery/album18

We put the airframe through some pretty extreme loading/landing situations and it came out on top. Things were breaking off the inside and the fuselage still stayed intact.

Nick Anderson

(in reply to Mluvara)
       Post #: 131

RE: UIUC AIAA Design/Build/Fly Project - 4/12/2004 7:26:33 AM   
Jeremy Sebens


 

Posts: 91
Joined: 4/22/2002
From: Champaign, IL, USA
Status: offline 		Yeah, using a core only REALLY helps when you can go down on fabric weight as a result. Cores really get that whole y^2 term in the bending equation working for you. You can sometimes drop a lot of weight from the fabric IF you have lighter fabric available.

True, core densities are measured in lbm/ft^3, but when your dealing with foams in the range of 1-2 lbm/ft^3, and with 1/8 being a really thick core, the area density is still down in the 1-2 oz/yd^2 - beats the 4.9 oz/yd^2 of your kevlar, and adds a LOT more stiffness. However, if what you have is working, there's no need to change it up at this point. You're only going to gain a few ounces, an while you gain stiffness and a little strenght, you would sacrifice a bit of toughness.

That's a neat looking plane BTW. What are it's specs? AUW, wing area, drivetrain, etc.?

Were those flight videos loaded or unloaded attempts - just curious.

As to the plane I'm flying for these guys, a few quick specs:

Wing span: 7 ft.
Wing Area: 6 ft^2
AUW (unloaded): 9.5 lbf
Payload capacity: 8.8 lbf (4 l)
Battery: 30 cells, CP1300
Motor: Graupner 3300-7H
Gearbox: 3:1
Prop: RFM 18.5X12

_____________________________

Landing: Contest of strength between planet and aircraft.
NOTE: To date, planet remains undefeated.

(in reply to nichanderson)
       Post #: 132

RE: UIUC AIAA Design/Build/Fly Project - 4/12/2004 7:34:45 AM   
Jeremy Sebens


 

Posts: 91
Joined: 4/22/2002
From: Champaign, IL, USA
Status: offline
quote:

ORIGINAL: nichanderson


Where do you people buy your props ? We snapped a prop today, and would like to get one with more pitch than what we had (20x12) so that we can get some more static thrust ? Specifically 20x(14-18)



To get more static thrust, you're probably going to have to go up on diameter. You're already at a pitch/diameter ratio of .6, and static thrust seems to max out near there. Much more pitch, and the blades start to stall and static thrust starts to fall off. Going to a 22X12 or 22X14, would realy up the thrust (and the wattage), if you have an extra inch of clearance to play with.

IT's possible that an RFM 20X13 might get you a bit more oomph - blades are available here: http://www.aircraft-world.com/shopdisplayproducts.asp?page=2

Also, APC has several props in the 20-22 inch range. I think you'll have to get them directly from APC. You want the Thin-Electric series. Those weigh 3-5 oz, as opposed to 9-15...

(in reply to nichanderson)
       Post #: 133

RE: UIUC AIAA Design/Build/Fly Project - 4/12/2004 7:48:24 AM   
davidfee



Posts: 691
Joined: 6/21/2003
From: San Diego, CA, USA
Status: offline 		Nick,
At the risk of sounding out of place, I think your fuselage might be over-built for the task. I built the SDSU DBF fuselage last year from a single layer of 5.6oz carbon. No honeycomb, rohacel, balsa... etc. I used strips of cloth as doublers around the openings, but that was it. There were a couple minimal internal bulkheads to help stiffen the structure. The bulkheads were sandwich panels made from some 4(?) ounce carbon and 1/8" sheets of regular blue DOW styrofoam.

Buy 5 and 30-minute epoxy at a local hobby shop in Kansas. Don't worry about good laminating resin... if you need to make major repairs, you will not have time to wait for anything slower than 30-minute.

Good luck in Wichita
-David

(in reply to nichanderson)
       Post #: 134

RE: UIUC AIAA Design/Build/Fly Project - 4/12/2004 5:19:45 PM   
Tall Paul



Posts: 4630
Joined: 6/23/2002
From: Palmdale, CA, USA
Status: offline 		Your camera man mssed the good parts!

(in reply to nichanderson)
       Post #: 135

RE: UIUC AIAA Design/Build/Fly Project - 4/15/2004 4:53:11 PM   
valkyrie1983


 

Posts: 23
Joined: 8/2/2003
From: Urbana, IL, USA
Status: offline 		Another update:
We had to take a couple days off from flying due to high winds. Finally got back out yesterday. Things didnt' go so great The first flight was empty and went really well. The 2nd flight carried 1L of water and everything went as planned. For the next flight we went to 4L. We never made the takeoff due to some oscillatory effects of our plane. The tip skids would bend too much under the added inertia, so we were rocking back and forth down the runway until we were hitting the wing tips. The plane then veered into the grass and had did a rough flip. We snapped our carbon joiner tube and pretty much trashed the inside rib bays on our wings. Everything has been repaired (we are getting good at that) and will be ready to fly as soon as the weather cooperates (probably a couple of days, there are 20mph winds today and tomorrow). Stronger tip skids have been added that should be able to hold up to the higher loads with 4L of water on board.
How is everyone else coming along?

Geoff

(in reply to Tall Paul)
       Post #: 136

RE: UIUC AIAA Design/Build/Fly Project - 4/15/2004 5:36:56 PM   
davidfee



Posts: 691
Joined: 6/21/2003
From: San Diego, CA, USA
Status: offline 		Oh boy... sorry to hear about that! I hope your mods have fixed the problem... good luck and good weather!

-David

(in reply to valkyrie1983)
       Post #: 137

RE: UIUC AIAA Design/Build/Fly Project - 4/15/2004 6:16:15 PM   
nichanderson


 

Posts: 21
Joined: 11/9/2003
From: Pomona, CA, USA
Status: offline 		We took off and flew a few laps for the first time yesterday. Mighty fine to see the plane you've spent hours toiling over finally take to the air..

Heres some pictures:
http://calpolydbf.com/gallery/album19

The third flight we took off with a liter of water, plane flew fine, but apparently a little sluggish the pilot said. Next on the to-do list is get the thing painted, when its in the air you can barely tell the orientation of the airplane itself.

Nick Anderson

(in reply to davidfee)