mave311

My Feedback: (13)

Dude if you look at there website, that new 1\4 scale f-16 has the new system for the elevator stab.....you see the section were the airbrake is being shown and you see a slot were the servo pin goes through the skin directly to the stab. The stab is held in place by a collar and set screw...that's it. It is also used on the flap,,,,

FenderBean

Oh your talking about the direct drive. Its a reverse design, servo arm directly
moves the stab with a pin on the servo arm.

mave311

My Feedback: (13)

yes, direct servo on the stab....only difference is that you don't have that much throw....i mean you have just enough to do what you need to..

mave311

My Feedback: (13)

call me...sent you number through email....

Craig B.

Very good link and informative document. I suspect that skin compression failure of the bulkhead has been the primary cause of this accident and the fuselage flexing is secondary to that primary failure mode. IMO honeycomb formers should be backed on BOTH sides with plywood or glass board that will spread compressive loads across the whole bulkhead. Maybe that could be a fix for those kits already out there.

As for the excellent radiographs that ravill posted, I would ask for replacement stabs that had plywood formers supporting the ends of the anti-rotation bolts, not honeycomb. Those bolts will cut through the honeycomb formers like a hot knife through butter. The internal of those stabs shows inappropriate use of honeycomb.

IMO, honeycomb should not be used in servo mounts and load bearing bulkheads/ formers and where formers meet a honeycomb skin, there should be a band of carbon fibre tape to prevent skin compression failure at the junction of the bulkhead and the skin material.

I engineered a solid pivot mechanism on my Avonds f15 and suffered an impact to one of the stabs when one gear failed on landing. I managed to bend the 41-40 steel pivot rod without in any way damaging the foam core stab. You have to consider the weakest link in our systems. With the servo torques available to us and the flying loads we experience, often it is the servo mounts or control horn anchor points that are the weakest link.

Next time you glue some horns in, ask yourself if you would be happy to hang your 20litre jersey modeller fuelling can off that control horn. Our servos are capable of generating such forces on them and the structures supporting those control horns (in this case a honeycomb bulkhead) must be up to the task.

Springbok Flyer

Craig,

My sentiments exactly.

Cheers,

Jan

marc s

Sorry for the loss, and qudos for posting the info to allow a discussion around the event.

I have no structural experience through work or qualifications but have built like many a good few kits from various manufactures for clients and have a few comments based on good old CS.

In my opinion the 'honeycomb' type layup is more about the 'bling' than the actual practicality for rc jet manufacture, its one of those 'carbon fiber' moments which now we are all more relaxed about thankfully!

It may have its place in areas of engineering but for me its not in this role. An airex layup or simple glass cloth layup done well with good well positioned formers of ply or suitable wood is way plenty to provide stable and strong rc jet construction of any size.

The imposed weight restrictions we live with around the globe are in-fact causing issues I believe as these bigger jets are under pressure from the manufacturer to be super light to allow the builder to hit the weight numbers to allow him to fly without special certification. Big jets need to be the right weight to fly right and from watching shows and speaking to some very experienced pilots often the 'big' boys are too floaty and fly like kites simply because the weight is too low.

These crashes are horrible to see/hear about, I would be gutted for sure but firstly the rules need to change to allow 'proper' weight levels for these jets which keep getting bigger and second the manufacturers need to keep it simple and stop playing with 'high tech' materials that really dont have a place in rc jet/prop manufacture.

Next it will be GRAPHENE...................

marcs

Ron Stahl

My Feedback: (12)

Many of you have commented on the honeycomb usage issues in our models. I can tell you from decades of composite fabrication and design experience in the mid size (100-700 lb) UAV world were we test to FAA Far 23 regulations, that honeycomb like any other other light weight core material is there to provide stiffness to a structure by increasing the distance between the plies without adding significant weight to the structure not strenght. Any load transfer through a cored structure should have one of three methods; either fill the honeycomb with a core filling product like hysol or another high durameter epoxy, or remove the honeycomb locally and fill the space with additonal plies or solid wood with compressive strength like end grain balsa or aircraft plywood before you add the inner plies, or lastly locally remove the honeycomb and blend the edges so that the inner and outer skins are in contact with each other and then bond the structure to the solid composite areas. Sadly a lot of arf's are built pretty but not strong enough for the loads generated in flight. I am glad no one got hurt in this crash. My boss during the nineties was part of the Grumman design team for the F-14 and Bob Kress thier chief engineer worked as a consultant for a while for us at my company. I can tell you lots of war stories on what problems they had on the full size F-14 during flight testing and one of the biggest was the twisting of the aft section of the fuse during rolling manuvers to the point they were breaking planes in flight.

smchale

My Feedback: (67)

Dubd,

So sorry to see the loss of your aircraft and all that you had invested in it. There is a lot of good information and suggestions in this post and I'm encouraged to see that the manufacturer is following the thread. I'll be more encouraged to see them take the suggestions seriously enough to make changes to current and future airframes. I believe they have a responsibility to the client as well as to our hobby to do so!

I commend you for starting this thread and sharing your devastating experience with this community. It says a lot about your character and we need that. It would have been easy to stomach the loss and perhaps have a private dialog with FEJ off line, but by sharing this I'm certain you have saved many others from the same devastating fate. For that you are to be thanked.

VF84sluggo

My Feedback: (55)

Hmmm...sounds familiar

EDFJim

My Feedback: (2)

I am going to split the flaps and go to full time ailerons and use the stabs for pitch only; when /if the new wings arrive. This will limit twisting and control throw needed.. May not be scale but will reduce loads.

bevar

My Feedback: (27)

On Lukey's Big F-18F, we had to remove and change both the rudder and stab control methods because of flutter. The rudder Luke redesigned back to servos mounted in each vertical fin with an external linkage to the control surface.

The stabs now use 2 8711's each side mounted in an aluminum tray. When the tray from FEJ first arrived it was worthless. The aluminum was so weak it bent and twisted after installation with any pressure applied to the control surface fore or aft. Luke added maple hard rails and a ton of Hysol everywhere and it is OK now.

That Direct Drive System seemed to be a recipe for disaster in our opinion and had so much slop in it we removed it after one flight.

We also disabled the Leading Edge Slats after the first flight because they did not work properly in flight either.

Beave

VF84sluggo

My Feedback: (55)

What's interesting to me is that with all the beefing up, redesigning, reworking, hysoling, ad nauseum, that appears to be required, seems like the whole point of using honeycomb to save weight is rather moot.

One wonders if the jet would actually weigh less if honeycomb was not used - or, better yet, used appropriately - and such extensive "upgrades" were then not necessary? Granted, some fine-tuning can be expected, but not to the extent it seems folks are resorting to here so as to have an airworthy machine.

FalconWings

My Feedback: (57)

Honeycomb does have a purpose........keeps retail price higher.

Doolittleraider

My Feedback: (72)

+1 TRUE THAT! DOUBLE TRUE!

jetpilot

My Feedback: (48)

As Marc S mentioned. Use Airex! Its what all the Germans use. I told Anton years ago to use it. Dont know if he listened but its showing up in Skymaster planes now.
All my scale jets are From Germany, well most my jets period! Rediculously light and rediculously strong. All Airex.
Scott

jlmaviation

My Feedback: (54)

It is OK to use the honeycomb for the skin of the jet. But formers, bulkheads, gear mounts, wing roots, stab roots, fin roots and flight control spars , rods and wing spars they should use the honeycomb sandwiched structure. If that is too expensive than good old aircraft ply will do. The honeycomb sandwich structure if properly done provides 7 times more strength than of a structure of similar weight. The one picture that shows the honeycomb with partial ply on one of the primary stab bulkheads will fail in time.

jlmaviation

My Feedback: (54)

The reason I keep mentioning that the honeycomb is not correct is that when our models reach the sizes of these large jets you will need to do actual engineering of the structure to make is sound. The one link I posted shows some of the simple calculations that can be used. The German ARF manufacturers do the proper engineering calculations to make sure the airframe is sound.

JohnMac

Engineering, seriously? These guys do not even understand the concept of a lever. When I discussed the purchase of my F-14 (very early one - no honeycomb) I was amazed to see the use of thrust races for pivot point bearings. The "designers" assured me that all the lift forces from the wing worked directly up the centre of the pivot rod......Yeah, right sure they do. It took an hour and the use of sketches before the light went on and we managed to get them to understand that a cantilever wing is a lever, and that compression and tension forces would be apparent on the bearing, rendering even good quality (which these most certainly were not) thrust races quite useless.
Expecting an kind of engineering knowledge, let alone expetise from these guys is futile. Hell they do not know how to paint a model.

Sorry to hear of the loss of your model. I understand the investment of time and money in this model.
John

JohnMac

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jlmaviation

My Feedback: (54)

I never implied that Fly Eagle applied engineering in their designs. However, they should since these models are getting larger basic aero engineering knowledge is needed.

I implied that the German manufacturers did.

gjhinshaw

My Feedback: (303)

Sorry to hear this!!  The only thing that makes since to me is the left elev. grasp bracket came loose and allowed the elev. shaft to turn with no control of the elev.  I had this kind of set-up on my F18.  Do you know what and where I am talking about?? To me, this is a week link IF you do not get it correct on install. On every 10th flight take the covers off to make SURE that the screws have NOT backed out, nor has the shaft turned...... Understand?

Gerry

Airplanes400

My Feedback: (349)

You are correct in both statements!
The Chinese are self-imposed engineers ... and delusional, as well.
Building models of this size and weight takes REAL engineering. Like the Germans do.

dubd

Hey guys, thanks for all the posts and PMs. I attended the Spring Fling fly in this weekend to keep me motivated with the hobby. Once I get home I will respond to the replies and post some pictures of the destroyed stab.

Heres Jack Diaz and I with some "old faithful" birds. It was great to jump back on the horse again.

david surick

it is voltz