Structures

Hello all!

I find myself here for the next two weeks in Dnepropetrovsk Ukraine at the Grand Hotel and have been thinking about some things. Structures for one. There have been some structural failures of note both during and before the Nats in Muncie. I was hoping that there has been enough post mortem analysis conducted by those that were victims to brief the rest of us so that we won't repeat that experience. Thank you kindly in advance should you decide to share such information with the rest of us. I do know at least a portion of what happened to Mr. Fancher's airplane but could there have been more hidden from view? Is any of this linked to attempts to build lighter structures?

You need to get out more, Jim!

I was my experience that Soviet-era women cleaned up pretty nicely after the Commie Grime was scrubbed off. Aren't there some rather exciting examples of same hanging around entrance to your hotel?

As far as I know, PW is still without a clue as to complete failure of his structure. Being in Hawaii at the moment, I doubt if model aircraft are much of a priority for him at this time.

Dirt,

Yeah, the female species cleans up extremely well around here and there are numerous ones to drool over or behind, never in front of though! Paul mentioned to me at the Nats there was apparently a slight crack developing between a block and some moulded parts on the plane but there must be more than that at fault? Possibly too light with heavy .72 Fourstroke vibration? Gordon had a problem with vibration as well with his Tony and an .82 Fourstroke I think? It broke his flap horn, reportedly the horn was about .072 steel. On another note, I was watching a maroon SV-11 (non-fourstroke) fly Thursday or Friday at the Nats and the outboard wing broke right off during maneuvering.

> On another note, I was watching a maroon SV-11
>(non-fourstroke) fly Thursday or Friday at the Nats and the
>outboard wing broke right off during maneuvering.
>
>Jim Pollock

That was a take-apart structure that was improperly assembled (glue joint failed) and it took itself apart. There wasn't much damage since it was the outboard wing that went and he was able to land it OK. But the cause could be attributed to structural failure and wind (high stress). Some of the other losses mentioned could have been aggravated by the wind which imposte heavier stress loads on the planes.

LAST EDITED ON Aug-07-04 AT 08:53 PM (CDT)
 
Hey, hey, big Jim. How do you pronounce that long word? "Jest lak it sounds"?

Hi, Jim!
We missed you at the baseball stadium parking lot today. Beautiful day, except for the howling winds, but you were at Muncie so you're an expert on wind flying! So you're spending two glorious weeks at the "Grand Hotel"? Sounds as inviting as two weeks in a Motel 6. If I was there, I'd be thinking of ways to get the h*** out. Watch out for the local flora and fauna; they all want a ticket out, too. We're still on schedule to launch in March. You pick the year. Have fun (ha!)and take care.

RK

Hi Rollin and Ty too!

Ty, Geest likea eet zoounds!

Rollin,

I'm no expert wind flyer! Otherwise the big blue and grey pig wouldn't have tried to plow through the blacktop with the $50.00 Eather prop. Guess I had better use plastic props until I learn how to do an hourglass in the wind. Funny thing was that it went right through the wind O.K. in the vertical eights. Go figure.
Let's get that bucket of bolts and tiles they call a shuttle back in space before our astronauts forget what it's like out there. I'm sure there getting tired of riding in a Soyuz.

I notice from some other post here that no one listened when advised of the rotating cylinder engine shortcommings!

Also,

There is one other thing I noticed from this thread. That is no one really gives a flat squat about airplane structures. I guess everyone figures if they us some appropriate glue and wood then there's no way there magnificent creation will fall victim to excessive areodynamic loads. Maybe not! Oh well, consider this thread closed then! Sorry for wasting the time of those so inclined to read it.

Do you think the actual issue might be adequate stiffness vs. failure strength? Do typical stunter structures get too flexable before they get too weak?

Ron,

Yes, flexibility is a definite issue. That's why Brett uses foam wings for his Infinity to mitigate flexibility issues. Also, using polyspan can sometimes lead to flexibility issues. I have observed some flexibility in my Intrepid's wingtip structure although the area inside the 6th rib doesn't flex any because of the web shearing. Ted even had some flexibility issues with the dearly departed Pond Scum Trivial Pursuit after recovering it with Polyspan. I guess he's in the process of building his "Foam" wing Hughes racer as we type these words?

Jim,

There are also non-structure related issues. I know Pat Johnston planted his beautiful Bearcat at the Nats in high wind. We looked it over when I was in Idaho last week and it appears that the elevator horn either flexed or partially failed. I've always felt silly putting 1/8" horns on elevators, but I won't any more.

Another issue is not just stress on structures, but how the stress is distributed. I've only experienced one in flight structure failure. That was caused not by too weak a structure, but by a stress riser that was not adequately distrubuted. I cut the re-inforcement off short without tapering it out to pass the load longitudinally over a large area and it broke right at the cut. Duh!

Don't do that, Jim. At least we know why you weren't at the meeting last night.

My Time Machine fuselage split between the wing and the stab a couple of weeks ago. I think the cause was the cross grained sheeting on the bottom. I had always done my fuselages this way since my free flight days of yesteryear, but it looks like the sheeting simply split with the grain and transfered the stress directly to the fuselage sides. They couldn't handle the load and cracked. The Time Machine has a simple structure, so there are no longerons or diagonal formers to help distribute the load.

I noticed the airplane acting funny and was able to fly out the tank, but I had a two piece airplane after it landed. The carbon pushrod was the only connection between the nose and the tail.

She's all better now. I cut off the bottom sheeting, spliced the sides internally, and recovered the bottom (grain lengthwise this time). I'll figure out another way to reduce the fuse twisting.

Anyone else had this happen? What's your take on this?

Thanks,

Ron King

LAST EDITED ON Aug-13-04 AT 09:22 PM (CDT)
 
Ron,

Ya know, I used to think I just overbuilt things. It always seemed that no matter what I did, my planes were always heavier than other's comparable planes. Now I know why. I do a lot of things that it seems other's just don't do (or at least it looks like it). When I build fuse sides, I put 1/8 x 1/4 diagonal reinforcement along the inside of each side from the wing to the stab and usually use some sort of cross bracing scheme at the top and bottom of the box all the way from the motor mounts to the tail post. The top and bottom block are usually not particularly reinforced except for the occasional bulkhead to maintain shape (I use molded top and bottom blocks), but I generally reinforce the fuse box with this addional structure. I also reinforce the center of the wing, but nothing special. And I always use 1/64 ply doublers on the inside of the fuse sides to support the stab and wing. Not just the front, but all the way around the wing and stab.

I had thought this was probably way overkill and I was just hauling about 3 or 4 ozs that I could do without, but now I'm not so sure. My planes usually hold together in very stiff winds.

Hmmm... Maybe I'll rethink the new plane design. I had come up with a plan for considerably reduced structure but now I'm not so sure...

edit: typos

Ron,

How are things back in the USA. I will return next Thrusday.
Is the road to the Meadow Wood flying site any better than the
little used cow trail I saw before I left? You must be doing some really hard corners with the Time Machine to split it in half. I have never had a problem with planking the bottom on any of my planes cross grain. I guess you just have to remember to use 7-8 pound 1/8th or 3/16th and not use CA for the adhesive (too brittle!). I don't think Bob Palmer had any problems with bottom planking on any of his planes and they are all that way. Again, I bet he didn't use CA!

Jim and Randy,

Looking at the structure, there are some ways to beef these things up without adding too much weight. I know the Time Machine was designed using a minimalist approach and I may have sanded the bottom a little too much for my own good. I also know that I am still not as smooth a pilot as either of you. I sometimes get surprised at how fast the ground appears in my periphery coming down from the hourglass. Thus, the added jerks and stress (plane and pilot )

Anyway - I took a look at some of the older plans I have laying around and many of them used additional 1/8 x 3/16 or nominal size triangle stock longerons along the bottom edge of the fuselage. These not only gave you some sanding room, they offer longitudinal strength. My Legacy not only has a small longeron riding along the lower edge, Alan also designed diagonal formers which also help.

I'll probably go back to cross grained bottoms, but will make sure the internal structure can handle the load. I agree with you, Randy. Even though most of my planes do not survive a long time, I prefer to build them to last - just in case.

Jim - I haven't been to Meadowood, but I understand the road has been improved a lot. I don't know if you got my email, but I will also take my engine back from you when you get home. That way you can get that PA 40 Super Light job and get your Staris on the road.

Ron King

I think both PW and TF were the victim of someone hiding on a grassy Null. The statictics for people shooting at planes from a distance is Staggering! It accounts for some 99% of all structure failures. Its an AMA and FAI coverup.

PJ

What caliber do you suppose there using? I'm a fan of the 45-70 myself!