BOT 3000 Spar Mod--Ollie?

[solo6796]

I had mentioned some time ago that although my Espritmodel Bird 3000 flew great, the wing was very weak. (Like some other ARF's we know...) I set mine up for winch launch and have specked out several times. The large ailerons make modest aerobatics possible and act as spoilerons with a switch on my xmtr for landing. Was able to get high launches but I had to really baby it up because the wing flexed a lot.

Well, I finally broke a wing on launch on a windy day. Could have been cumulative fatigue, but likely I just pushed it too hard. Turns out that the spar was only 1/8" lite ply with only the fiberglass D tube capping them.

I really loved the look and feel of this ship in the air so I bought another wing to see if I could stiffen her up a bit.

After removing the covering, I cut a notch in the ribs at the spar to accept a 1/4" X .030 CF laminate as a spar cap. I cut 1/4" balsa into shear webs and sistered the assembly to the existing spar with 30 min epoxy. Only took a couple hours to cut all the webs and finish one center section. When it's dry, I'll use my makeshift "weights" to laminate the other.

Ollie, do you have any suggestion for the outer panels? I think they MAY be strong enough, but will defer to your opinion on this.

Some pix are attached and if any are interested, I will keep you informed on the flight tests.

Alan

[solo6796]

Home made web cutting jig.....worked pretty good!

[solo6796]

shear webs sanded and ready to install...bottom spar cap in place

[solo6796]

dry fit...

[solo6796]

Final fit before mixing the epoxy...

[solo6796]

Weighted it down for about 45 min with a plastic barrier, Removed the masking tape and cleaned up any mess, then re-wrapped and weighted overnight for full cure. I'll glue the other one tomorrow when I get back from flying.

[solo6796]

Before the wing failure. I love the looks of this model.....

[Ollie]

Alan,
Congratulations on the very craftsman like job of reinforcing the inner wing panels. I feel sure they are now strong enough to withstand moderate winch launches.

The bending loads on the tip panels at the joint are probably only 10 to 15% of the bending load at the center of the wing and go to zero at the tip. You may be right that they can carry the associated load. I can't be sure because I don't know enough about the tip panel D-tube and associated shear webs which carry most of the bending load on the tip panels.

There are two more questions that occur to me. How are the panels joined at all three joints? Are the joiners and the way the joiners are tied into the spar strong enough?

Additional flight testing may reveal the weak points one at a time with the cost of new wing panels and reinforcement at each stage of testing. I think a better approach is to design the necessary strength into the wing structure before the first prototype is ever built. Finding a manufacturer that does this is a considersble challenge.

I see in one of the pictures that you have BOT plans. It may be that for a craftsman such as your self, it would be cheaper and less time consuming to use those plans to build a proper wing with carbon spar caps than to mess around trying to make the ARF wing strong enough.

[solo6796]

Ollie,

I'm in the process of building a BOT kit also. It's my first build, tho, and I started with the fuze on that one, nearly finished. There are enough differences in the wing mount and other dimensions between the two that I'm more comfortable trying this modification first, though.

The joiners are 2mm X 1.5cm fiberglass laminate and appear pretty tough. The joiner boxes are the lite ply all around filled with resin to the dimension of the joiner. The outer panels have the same Fiberglass D box/ lite ply spar construction as the inner ones.

Was thinking of having a machinist friend copy the main joiner in steel or brass and wrapping that box with kevlar or carbon. There is a little dihedral in each joiner. My Elegant 700 uses a flat steel
joiner and withstands hard zoom launches. I've been involved in flying since February and have never built a plane before.

My flying club has a project in scratch building a fine sailplane with carbon in the spar. We will even be laying our own fiberglass fuse in the mold. Jack Womack is our president and master builder, giving us instruction. Several of these ships will be constructed under his supervision.

The technique used on ths modification is from information gained in reading this forum and others, and talking with other builders. As I gain experience, I'll be more confident in my ability on the next build.

[Ollie]

Some of the ways that the next weakest link in the structure might reveal itself are:

The epoxy lining of the joiner box or the lite ply in the joiner box may be crushed by the bearing loads of the joiners trying to pry the box apart. The thinness of the joiners leads me to suspect this as a failure mode. This will probably begin to reveal itself as a loosening of the fit of the joiners rather than as a catistrophic failure.

The fiber glass joiners may break. It is hard to know what their strength is unless you know the orientation of the fibers and the fiber to resin ratio. Unfortunately this type of material breaks suddenly and without warning. When using brittle material it is good engineering practise to use a large safety factor in the design.

Jack Womack's classes are already equipping you with the building skills to build and repair your own models. That is time very well spent and the results will benefit you for the rest of your modelling career.

Beyond Jack's classes you can learn a lot about good design and advanced craftsmanship by studying the designs of Dr. Mark Drela as I have. Drela's work is at the pinacle of the designer's art and sets a standard to which all other designer's work should be compared.

As far as ARFs are concerned, it is very hard to know if you are buying a pig in a poke or a bag of goodies. Buying ARFs is not for the risk averse.

[solo6796]

I have read many of Dr. Drelas tips and several of my flying buddies have also referred to his instruction in flying technique as well as engineering and design. Found some great stuff on the Charles River site. I will have to find a way to beef up the main joiner box without destroying the air flow over that area. I have an idea that may work. It was tried on the fateful flight of #1 and the main joiner did not give. The break occurred just past the end of the joiner.

Ollie, I appreciate your comments on this. I'm just learning how to do this stuff and as I go, I pick up a few shortcuts and techniques, as well as the necessary equipment for success.

I see you're from Punta Gorda. I went through High School at Ft Myers Sr. Hi. Used to run up there every so often. Bought my first car in Punta Gorda. Bet it's grown up a lot since 1968.

Thanks again,

AJ

[Keith Johnson]

Interesting thread AJ. I hope you keep us informed about the building decisions, progress and results. I enjoy experimenting.

[solo6796]

A little more progress, Got some balsa for re-sheeeting the main wing section. Started getting a little artsy with it, though.......

So I thought I would ask... Seems to me that sheeting at a 45 degree angle might add some strength as well as looks. Will this arrangement work? Or would another layout be better? Or does it even matter?

Also got the bellcrank set up and most of the servo mount done.

[solo6796]

Equipment Bay

[solo6796]

I think I will reduce the size of the outer gusset a little...

[solo6796]

The spar is probably overkill, but it's my first try at it....

[solo6796]

Beefed up wing mount/towhook box. used 2" fiberglass tape laminated to fuze. Went ahead and strengthened a few other known stress points. ( from previous known rough landings....)

[solo6796]

Going well, Might get to hand toss it this Sunday.

ARF flyers beware! I'm finding a lot of butt joints in this wing with no sanding of very smooth mated parts; Very little adhesive used and very weak joints; spongy balsa used at stress points, etc.

I already made two new ribs for this section. Will go ahead and uncover the outer panels to put a little CA on those joints, just in case.

Ollie, you are right about a scratch built wing being so much better. However, this little modification is teaching me a lot about what not to do and little tricks for production and accuracy.

AJ

[Ollie]

AJ,

Planes come and go but, what you are learning and the skills you are developing will last you the rest of your life.

Investing in yourself is always a much better deal than buying things.

Best regards,
Ollie

[solo6796]

Finished this morning. Set control throws and trims. Came in at 58 oz which figures to be 8.35 oz per sq ft wing loading. Had to use 6 oz nose weight to get the cg within spec. Also had to add a little weight to the left wingtip to balance laterally.

May get to toss it today.....Hope it flies well!

A few pix are attached....

[solo6796]

On the floor

[solo6796]

Ailerons in Landing position...Still have some up travel left for landing control. This worked very well on my other one, bleeds off speed and increases glide angle. Also great for one switch de-thermaling. Mixed just a little down elevator with it to avoid stalling.

[solo6796]

My first try at sheeting.....

[solo6796]

Servo bay has lots of room...

[solo6796]

one more.....

[solo6796]

Decided to try making servo covers from balsa sheeting. Turned out light, but no protection for the servo arm on landing... I may try something else later...

[solo6796]

Hand tossed today... flys level with only a little trim. More responsive than the old one. Did a 360 degree turn and went way past me. Going to go ahead and epoxy the outer wing panels, mount the towhook, and button it up. Will be ready for the winch this Saturday......

AJ

[schrederman]

Hey.....

Between work and TNT, I've missed your progress. This looks great! I can't wait to see your BoT that you are building, and your Hawk project. Ollie is right. The time you spend investing in your skills and knowledge will payoff in years to come.

Jack Womack

[Ric Duley]

Aj,
Great job and a good thread.

[solo6796]

I appreciate the nice comments. I just found something that I really love to do....

My next project will be The Hawk. I/ve cleaned off my worktable (I can't believe how trashy it got!) and copying the Hawk Plans. If Shrederman is flying Saturday, I'll have my ribs. Already have my carbon. I'm scheduled to lay up the fuse later this month. The BOT kit will be after that. So I'm working at it.

Working on a little website, also...another little work in progress.....

http://home.houston.rr.com/rcsoaring/

Building is ok, but I'd rather be flying!

AJ

[solo6796]

Tried it out on the winch today. Wind was prob. 15-20 this morning, so I flew the Elegant till it died down a little.

Hand tossed again to final check. First launch was a pop-off, so I trimmed in a couple clicks of down. The next few launches were handled with ease by the wing. No noticable flexing was present, and the ship rotated pretty normally. Nice, high launches were achieved using moderate winch power.

The main joiner was weakened, however, so I plan to go ahead and wrap the joiner box with some kevlar. I will also get a steel joiner made.

All in all, she flew very well. Needs some ballast in the wind, and some tweaking of my landing set up (as well as my landing SKILLS).

It was good flying day. Kinda cool and windy this AM so Jack set up an LZ and practiced a bunch of timed landings. When it warmed up a little, a few of us found a thermal or two to ride. Dick stayed up a Looooong time.

This should finish this thread. Everybody is talking about and getting ready to build the "Houston Hawk". Got my ribs and webs today and Jack brought his partially assembled joiner box to discuss.

Keep tuned to that thread, and check out our website at:

http://www.houstonhawks.org/

Any of you Houston Pilots want to come and fly and meet some good folks, you're sure welcome!

Thanks,

AJ