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