ravill

My Feedback: (11)

Yeah Dantley, WHY OH WHY DID YOU DUMB THUMB YOUR BAD ARSE F-14 INTO THE GROUND????!!!!

(For those who lack common sense, that sentence is DRRRIIIPPPINNGG with sarcasm)

Wap4life, I'm giving you a virtual slap.

dubd

Rei, I hear you brother.

Erh7771, thanks for sharing your thoughts on FEJs's use of honeycomb. I wish the guys at FEJ will come into this thread and address that concern.

Gunradd, no manual. I had to go to a password protected site just to get the CG.

jetpilot

My Feedback: (48)

The guys trying to get this thread removed or closed are something else.
I wonder how many of these F14's are out there or on order?
I see in another thread a guy has ordered a P300 and a FEJ big F16. I just dont get it!
I nearly bit on one of those big F16's. Thank god I didnt!
Scott

Wap4life

My Feedback: (3)

Ravil- Why i do i deserve a virtual slap, by the way  thats funny.

I never said the Dubd crash his plane,  i stated that i hope he gets his just compensation. 

David Searles

My Feedback: (49)

Dantley,

FEJ still has your thumbs up photo and video of your F-14 flying on their website promoting this jet. Considering your current viewpoint, you might want to have them remove it. Just a thought.

David S

DiscoWings

My Feedback: (76)

Have you checked there main page of the site?

http://www.flyeaglejet.com/en/index%20jet.html

it has warning about the elevator and the install on the F-14 they are saying people aren't installing the bracket properly, seriously?[:@]

Maybe they are wanting to blame the crash on the elevator bracket install.

JohnMac

You are right!
You are right! It is only one of several measures that I outlined in a previous post that you need to take. However, mass balancing IS necessary as one of these measures IF you wish to ensure that flutter does not occur.
John

Reinol

My Feedback: (22)

Take note that it is not a honeycomb version they show in the picture!!!

Interesting.

lov2flyrc

My Feedback: (24)

I am amazed to see how many here believe FEJ will actually respond to this issue directly. Personally, I would be shocked if they did....
This is a company that has shown no conscience for years....Why would that change now?
Keep in mind, this is the same company that has knocked off whatever product they could get their hands on (flaws and all!). From Numerous Airpower products such as the EV1U, EV2U and EV5U; Skymasters F-15's, F-16's, F-18's ect... Hell, they even stole Antons latest sport jet design the moment he posted some CAD images, proudly displayed on their home page [X(]

You expect these guys to step up to the plate? [&o] Doubtful.....

I haven't looked closely at one of these FEJ kits in quite a while but I remember clearly when the F-86 was displayed at FJ a few years ago; A buddy wanted the kit and asked my opinion, I told him it was really poorly made (structure, I could flex it everywhere). He bought the kit anyway... it survived first flights but he didn't have it very long.....
Was tempted by the Hawk after seeing it at BITW! Thankfully, after thinking about the structure design I had seen from them in the past, Didn't take long for that temptation to fade away

JohnMac

Hi Shaun,
Yes you are correct. Mass balancing means that the surface is weighted at the leading edge so that it balances on the pivot. The reason for doing this is simple. If the tail surface encounters some turbulence that forces it up or down until it reaches it's elastic limit, and if it is balanced then it should arrive at it's elastic limit with little or no twisting moment. With the turbulence removed, the surface (if it is stiff enough i.e. correctly designed) will snap back past the nominal postion and bend in the opposite way, but to a lesser extent, and after a few cycles it will dampen out and become stable again. However. if the surface is not balanced on the pivot then when the surface reaches it's elastic limit, the side with the higher mass will continue on and impose a twisting moment on the surface. Normally an unbalanced surface is MUCH heavier at the trailing edge.
So as well as reaching the elastic limit, we now have a twisted surface which will now impose a large opposite force on the surface, drving it in the opposite direction with significant force. The surface now is driven to it's opposite elastic limit and the process repeats, it does not dampen out. In short we have aeroelastic flutter. This can quickly cause some kind of failure (in seconds or less) by either overstressing the structure itself or the linkages or the hinges can fail.
As I understand it a perfectly balance surface has a tendency to "float" around neutral and have a rather soft feel, so they are left a little bit tail heavy to overcome this problem. I hope that this helps,
John

JohnMac

That is not enough. Just adding doublers to the surface will not work. This stuff (honeycomb) cannot deal with point loading. If there are to be compression loads from bolts being tightened up on the skins then a laod bearing member MUST be inserted in place of the honecomb that will be sufficiaent to resist the force of the bolt being nipped up plus any subsequent loads impose on this point in use. Then you can add doublers to reinforce the point and spread the load. You are quite correct in your observation that the honeycomb is delaminating under the skin. If you only add doublers, then this will still occur (perhaps more slowly) under the skin and out of sight.
John

dubd

I have asked, but they don't respond to my emails.

GR7Racer

I'm coming into this thread pretty late in the game, but I've read the entire thing. First off, my condolences to Dantley.... this whole thing is just.....bad. I didn't know about it last weekend at the Willow otherwise I would have talked to you about it. I'm sorry.

I don't have any skin in this game at all, and I'm not especially impressed with the way FEJ is handling it.

I do think that "honeycomb" is getting a very bad rap here however. It's my impression that most of the people in this thread would avoid buying a kit if they saw honeycomb used in the layup/construction, and this bothers me. I've used this material in layups for my Giant Scale racers and when used properly, it offers great strength/rigidity at great weight savings. As our aircraft get larger, the benefits of using a core material like honeycomb or herex or rohacell are important in producing a strong airplane that is also lightweight. If we cultivate the idea that an airplane that includes honeycomb is automatically bad, then we are shooting ourselves in the foot.

When I first started flying jets about 8 years ago, the first thing I started asking was why the manufacturers weren't using vacuum-bagged core materials like honeycomb in their fuselage layups. It seemed like an obvious way to reduce the airframe weight by a significant amount, and probably ADD strength. I also observed that most of the airplanes flew like they could stand to lose a few pounds.

Now that being said, I think there are some obvious problems with the way FEJ has attached some vital components to that honeycomb as seen in the photos, and as has been pointed out by some of the folks in this thread. You cant just drill a hole through a honeycomb sandwich panel and bolt something to it....even with a plywood doubler on one side...even with a plywood doubler on both sides. Same goes for that pivot rod inside the stab. You just can't run bolts through the rod and the honeycomb sandwich panel on either side. You will compress the sanwich panel which offers little in compressive strength. This will cause the bolt to become loose and you will get some slop in the rod/stab interface. You will also eventually wear out the edges of the hole in the hc sandwich panel because you will be pressing that bolt against the very thin edges of the outer fiberglass skin. In a very high load area like the stab, that's going to happen pretty quickly. If that stab starts to flutter, it will happen instantly. There needs to be hardpoints built into the sandwich panel where these holes are drilled to stop the panel from compressing and spread the lateral load. This could be done by drilling a much larger hole and filling it with hysol, letting it cure, and then drilling the bolt hole. You can also machine oversized bushings and glue them into the hc sandwich panel. In addition to this, there really should be some other support on those bolts that will stop them from rotating.

The use of honeycomb in the layup of the fuse skin seems totally appropriate to me, assuming the bond between the hc and fiberglass is good. I'm also not alarmed about the use of honeycomb as a former inside the fuse, and believe it could be up to the task at hand, again assuming that the sandwich panel skins are properly bonded, and the former is bonded well to the fuse skins. The attachments of the servo mounting/stab rotating mechanism to that former does bother me. There absolutely needs to be hardpoints placed in the hc sandwich panel to drill and bolt through.

I don't know if anyone will ever know what failed to let the flutter get started, but it could have been one of the above items. It also might have been prevented by a better balanced stab. It might have also been prevented by better servo linkage geometry. But it seems to me the pivot rod/stab interface and the servo mounting bracket bolted to the hc sandwich panel could very well have been the cause. I would examine the wreckage and see if you can determine if the pivot rod is loose in either stab... assuming there is something left to examine. I'm hoping you can also go into your transmitter and give us the values on your Elevator travel and also the value of the dual rates that you were flying on. This might give us enough information to evaluate the linkage geometry and see how it was performing.

In further defense of honeycomb and honeycomb sandwich panel, I'm going to try to post some pictures of one of my racers. In the first picture, you can see a Zenoah GT80 engine mounted to a honeycomb sandwich panel firewall that is 3/8" thick. There are 3/4" diameter hardpoints glued into the panel where the 1/4-20 bolts go through it to mount the engine. You can see that only the bottom 2/3rds of the firewall actually bond into the fuse. the upper portion of the firewall is only supported by a couple of carbon rods and the cooling ducts. The fuse layup is about 4.5 oz of glass cloth on the outside skin, then 1/8" honeycomb, and then another 2.5 oz of glass cloth on the inside. The fuse is about 96" in length and only weighs 3 lbs when it comes out of the mold. It is also very rigid and needs no formers other than the firewall. That is what honeycomb can do for you when it is used properly. The wingspan is 131" and the model weighs only 25 lbs. When we dive for the start and pylon 1, we routinely go over 200 mph and then pull 20+ Gs. The engine weighs 6.5 lbs, so that honeycomb firewall has to support 130 lbs hanging off the front of it!

In the second picture, I'm holding the completed model. (sorry for the hero shot) The other model in the picture also has a honeycomb sandwich panel firewall that supports a 13 pound 290cc engine. It goes over 235 mph straight and level and pulls about 16 Gs... so you can do the math on that if you want.


KennyMac

rwwinter

Dubd sorry for the loss of your wonderful Tomcat!

I have now 44 flights on my Tomcat (no honeycomb). I fly my plane gently including loops, rolls and split S. The complete rear bulkhead was stiffened with glass before the first flight. Servos are Futaba BLS152. Engines are two Frank Raptor with more than 200NM each.

I found that the Tailerons developed more slop after each flight. I checked the complete system and was not able to find the error. Finally I put the Taileron pivot into a bench vise and BINGO... Both Tailerons had developed play. I was able to move the surface up and down by hand using medium force approx. 1.5degrees.

Perhaps this could led to the behavior shown in the video?

Rudi

ianober

My Feedback: (12)

Kenny Mac!!! I dont think anyone is slamming honeycomb here, just the negligent way that FEJ tries to use it. A bunch of composite folks have come in and defended honeycombs use when used properly so I dont think you have to worry about people getting a bed taste in there mouth. I think its great for the outer skin of the plane, just should not be used in load bearing structural bulkheads.

GR7Racer

Thanks Ian... but I think you might have missed my point. It is ok even for load bearing structural bulkheads... as long as you attach that load properly and the loads are in the proper axis. It's most likely that the hc former in the -14 would have worked fine if there were hardpoints in it to bolt the servo frame to. Same goes for the pivot rod in the stab. I've supported 200 lbs of engine load on a hc firewall with no problems (16 Gs X 13 lbs). It takes a little more work to get it right, and maybe that is beyond there capability so they should stick to plywood.... Just don't blame the hc, or say it shouldn't be used like that!! Maybe just not used that way by FEJ.

KMc

dbsonic

My Feedback: (3)

The pictures also show they(FEJ) use it as a spar material.. which is really not correct either(to put it kindly). The "strong direction" of the honeycomb is orthogonal to the plane of bending moment so its the wrong application again. Much better is a classic balsa/carbon laminate/shear web "box" or I-beam design. Can also be done well with spider foam(oriented correctly) laminated on all sides with carbon in the extreme fiber.

lov2flyrc

My Feedback: (24)

Actually, It can be used as load bearing bulkheads quite effectively....IF you choose the correct cell type, cloth, layup and as I mentioned earlier, hard points! However, since a large portion of the center of the F-14 bulkhead is open for pipe clearance, this is not the appropriate application for honeycomb structure. Aircraft grade Plywood would have added minimal weight gain and substantial strength to the structure.
Here is a perfect example of a honeycomb bulkhead done right..... This bulkhead supports the main landing gear AND the wing spars on a 400lb+ UAV (fully loaded). Add my weight, I can still jump up and down on the airframe without any flex or stress of the bulkhead! Layup and hard-points are KEY! This airframe uses honeycomb structure for every bulkhead.


BTW...if you look closely, you can even see the outline of the G-10 hard points in the picture.

jetnuno

Hi Dantley

Sorry for your loss. Been there and know how it feels.

I read most of the thread and saw the videos. In my opinion, this is clearly a structural fault. I have owned FEJ planes in the past a long way ago, but SMALL planes. Remember when they came to the market, they only had F16,F15 and AT3. I had an 1/8 F16 that I flew for arround 40 flights with no problems. When they came up with bigger planes, I always thought that they were going to places they DON'T UNDERSTAND. A twin F14 or other 20-30Kg plane, clearly demands for engineering they don't have. Neither in the frames or gears.

What really makes me sick, is how they have treated you and other customers. Their negligence behaviour also makes me sick. They just don't care for our safety.

I really stand by your position regarding safety issues and something should be done before something serious happens. You may loose a 27% discount or a F18, but something higher will arise... I hope.

THANK YOU

Nuno

rcjets_63

My Feedback: (4)

Ian, good response. I would add the clarification that honeycomb can be used successfully in load bearing bulkheads provided that it is done correctly. The firewall in Kenny Mac's racer (a gorgeous plane BTW) is a good example.

There are a few rules that must be followed when using sandwich panels (skin-core-skin) and they vary slightly depending on the material used. A major mistake is to apply a point load or compressive load normal to the panel without providing some reinforcement. In the case of Kenny's firewall, he mentions the inserts which 1) spread out the load and distribute it over a wider area to the rest of the panel, and 2) prevent the relatively softer core material from being crushed.

In the case of the FEJ design (as seen in the photos), inadequate reinforcement has been done and the core can be locally crushed by the thru-bolts for mounting the elevator pivot mechanism as well as the bolts through the pivot rod with the stabs (as seen in the X-rays).

One also has to take into account the structural load path. This is similar to the expression "a chain is only as strong as its weakest link". Assuming that the design has properly reinforced the regions where the load is applied to the bulkhead, you next must ensure that the load is properly transferred through the bulkhead and into the surrounding structure (fuselage skin). In the photos, we see lots of oversized and jagged holes for the rudder pushrods as well as the thrust pipes. Obviously, a hole is necessary, but it being oversized weakens the bulkhead and their irregular cut (poor workmanship) results in significant local stress concentrations even further weakening the bulkhead as well as introducing a site for cracking to start.

We also see in the photo that edges of the cutouts have not been filled. Once again, this is a serious no-no and another example of poor workmanship. The edges must be stabilized either by bonding a strip of cloth from one side of the panel to the other or by removing the core material around the periphery of the cutout and injecting the area with filler. Failure to stabilize the edge impedes stress transfer through the panel, more easily allows the core to be damaged, and will more easily allow any delamination which occurred during the hole cutting process to propagate across the panel.

Finally, one must consider how to transfer the load out of the bulkhead and into the surrounding structure (fuselage skin). The skin is generally thinner and more flexible then the bulkhead. The design must spread the load out from the bulkhead into the skin. This is typically done by applying multiple tapering layers of cloth to the joint. Another (considerably weaker) way of doing it is to apply a fillet of glue around the bulkhead. We often use the glue fillet method when installing plywood formers. It has it's limitations and some folks have likely seen cracks in the glue fillets in high stress areas. It's much worse when using this technique, as FEJ does, with honeycomb panels as shown in the photos. The panels are softer in compression strength and the stiff glue fillet will locally crush the core weakening the joint between the bulkhead and the fuselage skin.

To summarize, honeycomb bulkheads can be a great thing if done correctly. Examining the photos of the FEJ honeycomb bulkheads shows inadequate design in transferring loads into the bulkhead, transferring loads through the bulkhead, and transferring loads out of the bulkhead and into the fuselage.

In short, it's pretty obvious that FEJ has literally no idea what they are doing when using honeycomb bulkheads.

Regards,

Jim

dubd

Kenny, Thanks for jumping in. For the most part, we are saying the same thing. Nothing wrong with honeycomb if used properly. With so many people in this thread believing FEJ IS NOT using honeycomb correctly, what are we going to do about it? Money talks and if we keep buying their products they will keep delivering this garbage.

Rudi, it's ridiculous that you have to modify your F-14 so much and still can't fly it similar to the fullscale aircraft.

Shaun Evans

John,

Thanks for the explanation. It 'helps' in the sense that I now know that some people mean 'balanced on the pivot' when they say "mass-balanced", but it leaves me more confused because it seems to ignore everything I wrote after "...does 'mass-balanced' mean..." You're saying they're balanced on the pivot, then un-balanced to make it feel better for the pilot? Not only do I not get that, but it seems a demonstration that the surface is, in fact, not balanced on the pivot. Anyone who's ever pulled on the yoke of a piper aircraft knows that the stab is QUITE tail-heavy until the prop-wash aerodynamically balances it. In fact, that aero-balancing seems effective enough that they put a tab on it that resists your input and gives it some force-feel.

It may not seem like it, but I'm really trying to understand this.

VF84sluggo

My Feedback: (55)

That is literally, and figuratively, the bottom line here...

Sluggo

tamjets

My Feedback: (2)

+1
The first thing FEJ should do is to see if the pivot of the stabs are at the right location.
This might cause the failure.
It ashame they get this far in the market and lack of engineering to find out what wrong to fix it.
At the end I hate too see another model manfacture fold because of fail to made it success.

ianober

My Feedback: (12)

Thanks guys, I realize hc can be used as bulkheads when used PROPERLY, my point is that I would not trust FEJ to do it correctly, they have already shown this on obvious pictures. They have not removed any core structure to ad hard points for attachment reinforcement. I understood what you were saying KMac, I guess my main point again is that I don't trust FEJ to get that done with any kind of quality and I dont think they ever will. Like Todd said, just plain old aircraft ply would have been fine.